Phenanthroimidazole as Molecularly Engineered Switch for Efficient and Highly Long-lived Light-Emitting Electrochemical Cell.

Phenanthroimidazole as Molecularly Engineered Switch for Efficient and Highly Long-lived Light-Emitting Electrochemical Cell. | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Phenanthroimidazole as Molecularly Engineered Switch for Efficient and Highly Long-lived Light-Emitting Electrochemical Cell. Babak Nemati Bideh, Majid Moghadam, Ahmad Sousaraei, Behnoosh Shahpoori Arani This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2182062/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 09 Feb, 2023 Read the published version in Scientific Reports → Version 1 posted 8 You are reading this latest preprint version Abstract Although, light-emitting electrochemical cells (LECs) based on Ir(III) complexes owing to the superior advantages exhibit high potential for display and lighting applications, they still suffer from relatively low stability and sluggish response time. To mitigate this challenge, herein, a series of Ir(III) complexes based on phenanthroimidazole (PI) as ancillary ligand were functionalized to achieve efficient, highly stable yellow to orange LEC devices with fast response. These complexes exhibit appropriate electrochemical stability and significant suppression of concentration quenching in the thin films compare to archetype complex. Concerning, the fabricated LECs showed remarkable long device lifetime over 1400 and 2100 hours and EQE of 2 and 3% for yellow and orange-LECs, respectively, in which obtained t 1/2 for yellow LEC is among the highest value for cationic iridium (III) complexes based yellow-LECs reported so far. Subsequently, incorporation of ionic tethered functional group on PI, improved the mobility of emissive layer, reducing the device turn-on time around 75–88%. This study represents facile functionalization and characterization of PI ligand and its potential application in optoelectronic devices (OLED). Physical sciences/Chemistry/Materials chemistry/Optical materials Physical sciences/Optics and photonics/Applied optics/Optoelectronic devices and components Physical sciences/Physics/Electronics photonics and device physics/Photonic devices Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction Light-emitting electrochemical cells (LECs), as simple, low-cost and efficient emissive thin-film devices represent great potential for next generation of optoelectronic devices. 1 These devices exhibit many attractive features over traditional organic light-emitting diodes (OLEDs), such as single emissive layer consist of ionic luminescent species which can be easily fabricated from solution process and compatibility with an inert-metal cathode (e.g., Ag, Al, and Au) which allowing the nonrigorous encapsulation of devices. 1 , 2 , 3 The fluorescent and phosphorescent materials used as emitter in LECs generally consist of small molecules, conjugated polymers, ionic transition metal complexes (iTMCs), thermally-activated delayed fluorescence (TADF) molecules, quantum dots, and luminescent perovskite nanoparticles. 4 , 5 Among them, iTMCs have been received more attention due to their promising advantages. First, unlike organic compounds, iTMCs are inherently ionic and do not require additional ionic groups. Second, while in fluorescent emitter only 25% of the singlet excitons can be accessible for luminescence, the triplet relaxation pathway in form of the radiative deactivation, accelerated through spin-orbit coupling in phosphorescent iTMCs that allows these emitters to reach internal quantum efficiency (IQE) up to 100%, which is requisite to obtain LEC devices with high external quantum efficiency (EQE). Third, the iTMCs show stable redox properties that are required to achieve a device with high optical stability 6 , 7 . Amid all iTMCs, phosphorescent cationic cyclometalated iridium (III) complexes are widely exploited for optoelectronic applications, thanks to their unique properties such as, high phosphorescence quantum yield, chemical inertness and relatively good photochemical/thermal stability, short and tuneable excited-state lifetimes, and finally high versatility in tuning the emission colour of the emission through modification of ligand. These favourable characteristics arise from the high ligand-field splitting energies (LFSEs) that is a consequence of the large size of d orbital (5d), high electric charge of the iridium ion (Ir 3+ ) and high field strength exerted by anionic cyclometalating ligands (C^N) endowed by strong spin-orbit coupling. 7 – 10 However, there are two significant obstacles that restrict their practical application as emitter in LEC devices, that is, low stability (defined as half-lifetime, t 1/2 , time to reach one-half of the maximum brightness) and long response time (expressed as turn-on time, t on : time required to reach a maximum brightness) of devices. 11 , 12 The inherent instability of iTMC is ascribed to a ligand exchange reaction with a residue solvent or water molecules during the device operation, forming a non-luminescent complex. 5 , 13 , 14 Therefore, an efficient way to increase the stability of iTMC can be limiting the access of foreign substances to the metal center. Accordingly, it is indicated that the device stability can be significantly improved by introducing peripheral bulky aromatic groups that increase the hydrophobicity of the complex preventing water molecules to approach the metal center. Furthermore, supramolecularly-caged which come from intraligand π-π-stacking of aromatic rings, minimizes the expansion of metal-ligand bonds in the excited state, protecting the ligand exchange by surrounding molecules such as water and solvent. 10 , 15 , 16 , 17 It has been shown that turn-on time of the LEC devices is effectively improved by increasing the conductivity of the emitting layer. 18 , 19 Accordingly, several studies based on addition of ionic materials to the emitter layer and chemical modification of iTMCs have been done to improve response time of iTMC-LEC devices. For example, the addition of ionic salts (LiPF 6 ) and ionic liquids to the emitter layer and attachment of ionic moiety (such as imidazolium and triethylammonium) to ancillary ligand, lead to decrease the turn-on time of LECs. 12 , 18 – 20 , 21 – 23 Although an improved t on is achievable by these strategies, in some cases, other important parameters such as lifetime, luminance, and external quantum efficiencies (EQE) can be significantly reduced. 20 , 21 Another concern about iTMC-emitters is that they exhibit severe phosphorescence concentration-quenching in the solid state as a result of their relatively long triplet lifetimes which remarkably suppresses the phosphorescence efficiency of the emissive layers and thereby the LEC performances. It has been shown that enclosing of bulky groups (such as phenyl) to peripheral ligands can provide steric hindrance and effectively reduces self-quenching of iridium complex emitters in the solid-state form. 24 , 25 , 26 Phenanthroimidazole (PI) molecules with a specific structure and supreme photophysical features are used frequently as small molecules, ancillary ligands, and as a host material in optoelectronic devices, containing an imidazole moiety fused with strong metal-binding heterocycle in which by changing the substituents at the N1 and C2 imidazole positions, can adjust the electronic properties of PI and simply attaches to tethering groups. 27 , 28 Therefore, PI ligands with high chemical modification potential can be employed as a molecular engineered switch to achieve an efficient, stable and fast response time LECs. However, based on our best knowledge no reports has been published on the effect of chemical modification of these ligands and their complexation with Ir(III) as well as their potential application in LEC device. 8 In light of all the aforesaid facts, we designed and synthesized three novel cyclometalated iridium (III) complexes as orange emitters based on the phenanthroimidazole (PI) (L1, L2 and L3) ligand containing electron donor/acceptor and ionic substitutions (Chart 1 ), namely, Ir1, Ir2, and Ir3 + . In this work, a combinational approach such as bulky structure, electron donor/acceptor substitutions and ionic moiety of PI ligand are demonstrated to optimize the device efficiency, stability and response time of LECs based on cyclometalated iridium (III) complexes as emitter. Accordingly, the prepared solution of complexes exhibits high yellow to orange phosphorescence and reversible red/ox properties. Meanwhile, the complexes were exploited as emitter for solution-processable LECs, which afford efficient and fast response electroluminescence (EL). 2. Results And Discussion Synthesis and characterization. The complete synthesis procedure of the PI ligands and their cyclometalated iridium (III) complexes are given in the electronic supplementary information (ESI) and they were fully characterized by 1 H/ 13 C NMR, elemental analysis and TOF-mass spectrometry. Briefly, the PI ligands were synthesized, and easily purified without the need of column chromatography. The prepared ligands (PI) were reacted with chloro-bridged Ir(III) dimers of [Ir(ppy) 2 Cl] 2 in a mixture of methanol/dichloromethane to synthesize Ir1, Ir2, and Ir3. Subsequently, Ir3 + was obtained by reacting Ir3 with methyl iodide in acetonitrile (See scheme 1, ESI). Photophysical Characterizations. Figure 1 a depicts the room temperature UV–visible absorption (UV-Vis) and photoluminescence (PL) spectra of the complexes in acetonitrile. Detailed photophysical characteristics of these complexes and archetypal complex [Ir(ppy) 2 (phen)] + 29 are given in Table 1 . All the complexes showed intense absorption bands (ε > 8 × 10 4 M − 1 cm − 1 ) in the UV region of spectrum with a maximum in the range of 260 to 300 nm, which is attributed to the ligand centered (LC) spin-allowed 1 π-π* transitions involving both the cyclometallating (ppy) and ancillary (PI) ligands. The broad and less intense absorption bands between 300 and 430 nm are ascribed to spin-allowed metal-to-ligand ( 1 MLCT) and ligand-to-ligand charge transfer ( 1 LLCT) transitions, while the low-intensity bands beyond 430 nm correspond to the spin-forbidden 3 MLCT, 3 LLCT, and LC 3 π-π* transitions of the complexes. The spin-forbidden triplet transitions ( 3 MLCT, 3 LLCT) are partially allowed owing to the strong spin–orbit coupling of heavy iridium (III) atom to occur at a lower molar absorptivity than the corresponding singlet-allowed excitation ( 1 LLCT/ 1 MLCT). 30 The absorption spectra in the lower-energy region for Ir2 are significantly red-shifted by 16 nm compared to those of Ir1 which is directly resulting from changing the HOMO/LUMO energy gap because the presence of electron withdrawing (Br) and electron donor groups (OCH 3 ) on the PI ligand of the complexes. The complexes Ir1, Ir2 and Ir3 + show broad structureless PL spectra corresponding to yellow-orange emission centered at 580, 592 and 602 nm, upon 350 nm photoexcitation in an argon-saturated dichloromethane solution, demonstrating the emissions in the solution arise dominantly from 3 CT states. 32 Obviously, introduction of electron-withdrawing and electron donor groups on the aryl ring of the PI ligand exerts a negligible influence on the maximum luminescence. Complex Ir2 shows a red-shift of 19 nm in maxima emission, compare to that of the archetypal complex [Ir(ppy) 2 (phen)]PF 6 . 31 This red-shift can be explained by the electron-deficient nature of the fused imidazole moiety with Br as electron withdrawing group (L2), and π-expanded structure of the PI ligand, leading to stabilization of the LUMO and a smaller energy band gap, consequently (see optical band gap, E g Opt , in Table 1 ). For the same reason, Ir1 exhibits lower maxima emission in this series, because of destabilization of the LUMO by two electron donor groups (OCH 3 ) on the L1. Degassing solution of Ir1, Ir2 and Ir3 + complexes cause high PL quantum yield (PLQY) values of 0.46, 0.42, and 0.40 and excited-state lifetimes (τ) of 880, 970, and 950 ns, respectively, which exhibit slightly higher PLQY compare to the archetypal complex [Ir(ppy) 2 (phen)] + (Φ pL = 0.39) 31 , which could be attributed to the rigid structure of PI ancillary ligand. 33 It is noteworthy that, PLQY of these complexes are the highest values for yellow to orange phosphorescence among their parent cationic iridium complexes (See table S2, ESI). 7 , 8 Time-resolved phosphorescence decays were found to be monoexponential, indicating the presence of a single emissive species. Furthermore, radiative (k r ) and nonradiative (k nr ) decay rates were calculated from PLQY and τ, and listed in Table 1 . All complexes Ir1, Ir2 and Ir3 + show relatively long excited-state lifetimes and smaller k r values compare to benchmark complex which suggests that their emitting triplet states should contain considerable ligand-centered 3 π − π* character relative to parent complex [Ir(ppy) 2 (phen)] + , and shielding effect of the periphery phenyl groups around the iridium core in these complexes that prevents nonradiative intermolecular charge recombination. 26 , 34 In order to see the potential use of the complexes as emitter in LEC devices, their emission properties were measured in neat films (pristine complexes without IL, see Table 2 .). All complexes showed featureless emission spectra (Fig. 1 b), indicating that the emission of both solution and neat film arises from the 3 CT state. 30 , 35 PL emission spectra of the solutions in comparison with the neat films of Ir1, Ir2 and Ir3 + are red-shifted by 4, 4 and 11 nm, respectively, that indicates the lower intermolecular interactions of PI-based complexes (Ir1, Ir2) compare to parent complex [Ir(ppy) 2 (phen)] + . Nevertheless, Ir3 + is the most affected complex that 11 nm red-shift was observed probably due to increasing the intermolecular interaction in its neat film. All complexes as neat film showed lower PLQY compare to their solutions (See Table 1 ). The major reason is associated to the close packaging of the complexes in the neat films, which promotes the self-quenching processes. 26 However, in the neat films, the PLQYs of Ir1 and Ir2 (0.32, 0.26) are about 2.5 to 3-fold higher than that of [Ir(ppy) 2 (phen)]PF 6 (0.11), even though their PLQYs in the solution are slightly different, demonstrating that the bulky phenyl groups at the PI ligands significantly suppress the phosphorescence concentration quenching. 25 , 26 Furthermore, by adding ionic liquid (IL) to the neat-films, the values of PLQYs significantly increased up to 0.53 and 0.38, due to decreasing the self-quenching of the emission. 36 It is noted that Ir3 + complex presented a much lower PLQY in film (pristine and mixed) respect to Ir1 and Ir2, exhibiting higher aggregation tendency of ionic compounds in comparison with neutral ones. 28 Electrochemical Characterizations and DFT calculations. According to the LEC working principle, the transport of electrons and holes in metal complex based LEC devices take place through consecutive oxidation and reduction of metal complex during device operation. Therefore, the redox behaviour of iTMCs play an important role in understanding the overall performance of LEC devices. Hence, the electrochemical properties of complexes were investigated using cyclic voltammetry and differential pulse voltammetry techniques. Figure 2 shows the cyclic voltammograms and the electrochemical data are presented in Table 2 . As shown in Fig. 2, at the positive potential, all complexes exhibited a main reversible oxidation process, which attributed to the oxidation of Ir(III) to Ir(IV) with a strong contribution from the cyclometalating ligand, ppy, . 37 At the negative potential, all complexes possess one reversible or quasi-reversible reduction peak which is considered to be caused by phenanthroimidazoles as ancillary ligand with minor Ir(III) center involvement. 16 It is noteworthy that for most of this type of cyclometalated complex along with N^N ancillary ligand, the highest occupied molecular orbital (HOMO) has been reported to be a mixture of the d π (Ir) orbitals of iridium and the π orbitals of the C^N ligand while lowest unoccupied molecular orbital (LUMO) localized at N^N ancillary ligand. 38 Therefore, To shed light to the effect of various functional groups of ancillary ligands on the electronic properties of emitter, the HOMO and LUMO levels and electrochemical band gaps of complexes were derived from their corresponding redox potentials using empirical formula (footnote of Table 1 ). 25 Table 1 Photophysical data for Ir1, Ir2, and Ir3 + . absorption \({ \lambda }_{abs }\left[nm\right] \left(\epsilon /{10}^{3}\right)\) a PL (solution) b PL (neat film) c E g Opt d (eV) \({\lambda }_{em}\) [nm] Φ p [%] \(\tau\) [ns] \({k}_{r}\) \([\times {10}^{5}{s}^{-1}]\) \({k}_{nr}\) \([\times {10}^{5}{s}^{-1}]\) \({\lambda }_{em}\) [nm] Φ p [%] Ir1 269 (84.2), 294 (58.3), 334 (22.1),376 (10.8), 396 (10.0), 466 (1.4) 580 46 880 5.20 6.14 584 32 (53) 2.54 Ir2 274 (82.0), 302 (50.5), 331 (35.7), 383 (12.4), 402 (11.4), 482 (1.6) 602 42 970 4.33 5.98 606 26 (38) 2.45 Ir3 + 272 (78.0), 303 (4.89), 318 (32.8), 341 (16.0), 383 (7.8), 395 (7.5), 452 (0.6) 592 40 950 3.66 6.42 603 8 (15) 2.52 [Ir(ppy) 2 (phen)] + - 583 e 39 e 230 e 17.3 e 2.7 e 591 e 11 e - a In air-equilibrated CH 3 CN at 298 K (10 − 5 M). b In degassed CH 3 CN at 298 K (10 − 5 M); the emission quantum yields (Φ p ) were calculated by comparison with quinine sulfate (Φp = 0.545 in 1 M H 2 SO 4 (estimated error of ± 5%.); Lifetime were calculated based on a mono exponential decay model. k r and k nr were calculated based on the equations τ = 1/ (k r + k nr ) and Φ p = k r / (k r + k nr ). c Solid-state absolute quantum yield was measured by employing an integrating sphere system (estimated error: ± 5%), Values in parentheses were obtained for films with a composition similar to LEC (iTMC:IL, 4:1). d Optical bandgap, from the intersection of absorption and emission spectra. e Data from ref. 31 Table 2 Electrochemical properties of complexes Ir1, Ir2, Ir3 + . E 1/2 ox (∆E) a (V) E 1/2 red (∆E) b (V) HOMO c (eV) LUMO d (eV) E gap Elc . e (eV) Ir1 0.87 (81) -1.81 (70) -5.67 -2.99 2.68 Ir2 0.91 (86) -1.61 (74) -5.71 -3.19 2.52 Ir3 + 0.89 (112) -1.70 (106) -5.69 -3.10 2.59 a Half-wave potential, E 1/2 = ½ (E pa +E pc ); 0.1 M acetonitrile/TBAP versus Ag/AgCl at scan rate of 100 mV/s, Values in parentheses: difference between the anodic and cathodic peak potentials, ΔE = E pa -E pc (mV). b The half-wave potential of reduction peak for complexes. c From E HOMO = -(4.8 + E ox ) eV. d From E LUMO = -(4.8 + E red ) eV. e Electrochemical band gap from E gap = E HOMO - E LUMO . Table 3 Device Performance of the LEC: ITO/PEDOT:PSS/iTMC(Ir1, Ir2, Ir3 + ):[Bmim][PF 6 ]/Al. Device. λ max, EL (nm) CIE [x, y] a V (V) b t 1/2 (h) c t on (h) d L max )cd.cm − 2 ( e efficacy (cd.A − 1 ) f EQE (%) g Ir1 581 [0.519, 0.480] 2.6 2130 0.65 870 8.60 3.1 Ir2 605 [0.648, 0.351] 2.7 1450 1.30 563 5.52 2.5 Ir3 + 596 [0.608, 0.391] 2.6 2.25 0.15 45 0.38 0.24 [Ir(ppy) 2 (phen)] + h 578 - - 73 6.4 63 - 2.1 a Commission Internationale de l’Eclairage color coordinates, 1931. b Driving voltage (after 15h). c Lifetime: Time to reach one-half of the maximum luminance (Values obtained from extrapolation). d Turn-on time: time to reach maximum luminance. e Maximum luminance. f Maximum efficacy: ratio luminance/average current. g Maximum external quantum efficiency. h Data from ref. 36 First of all, the reversible oxidation and reduction processes determine the good electrochemical stability of complexes which are beneficial to achieve stable iTMC-LECs. Since, Ir3 + has the highest ∆E value compared to other complexes, thus, it can be concluded that the electrochemical stability of this complex is lower than the others. The oxidation potential of complexes is nearly identical (0.87–0.91 V) which indicates that peripheral groups on the PI ligand hardly alter the HOMO level of the complexes. However, the reduction potential of Ir1 (-1.81 V) is significantly cathodically shifted (ca. 0.2 V) respect to that of Ir2 (-1.61 V), which attributes a significant destabilized LUMO for Ir2. The presence of peripherals electron-donating (OCH 3 ) and withdrawing groups (Br) affect the reduction potential of PI ligands that are destabilized and stabilized the LUMO levels of cyclometalated complexes, respectively. In the meantime, the energy gap increased for Ir1 (2.68 eV) compare to Ir2 (2.52 eV) and also led to gradual blue-shift for the emission (22 nm). These data are further supported by DFT calculations. The computed HOMO, LUMO energy levels and electron density contour plot (Fig. 3 ), revealing that; first, the substitution groups on the PI ligands have only a minor influence on the HOMO levels, but pronounced on the LUMO levels, which expressed in the reduction potentials (Table S1, ESI), and second, the HOMO is mainly localized on the iridium and the cyclometalating ligand (ppy), whereas the LUMO is localized on the PI ligand (Fig. 3 ). These results reinforced the important role of functionalization of PI ligands in the electronic properties of these complexes. Electroluminescent Properties of LECs. To confirm the excellent phosphorescence of complexes Ir1, Ir2, Ir3+, LEC devices were fabricated on ITO glass in a double-layer architecture consisting of PEDOT:PSS layer, in order to increase the reproducibility of the devices, and emissive layer using a solution process. The active layer was contained Ir-iTMC and ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF 6 ]) at a 4:1 (iTMC:IL) molar ratio. The incorporation of IL to the films contribute to accelerate the LEC device response (reduce the turn-on time) by increasing the concentration of ionic molecules and ionic mobility in the emitter layer. In addition, IL increases the efficiency of devices by decreasing the concentration quenching as discussed earlier. At the end, aluminium metal (Al) was deposited on emissive layer as cathode electrode contact. The EL data of LECs were collected at pulsed current densities 102 mA.cm - 2 over time (1 kHz block wave and 50% duty cycle). More details concerning the LEC devices fabrication and characterization methods can be found in the supplementary Information. At the beginning, the effects of bulky structures of complexes on surface morphology of the spin-coated films were investigated by top-view SEM (See Figure S9, ESI). The good solubility of the ionic complexes in the acetonitrile (specially Ir3 + ), led to pinhole-free films with smooth and good morphology quality for LEC fabrication. Figure 4 displays the EL spectra of the iTMC-LECs. EL spectra and maximum emission are closely similar to PL spectra of the complexes in thin-film, meaning that the nature of the emission is identical and come from the same excited state in both excitation method (Tables 1 and 3 ). The maximum EL emission of LEC based Ir1, Ir2, and Ir3 + were centred at 581, 605, and 596 with CIE coordinates of (0.52, 0.48), (0.64, 0.35), and (0.61, 0.39), respectively, that are corresponding to yellow to orange emission. As already mentioned, the shorter wavelength emission of Ir1 respect to others is due to influence of electron-donor and electron-withdrawing groups on the PI ligand that effectively destabilize and stabilize the LUMO levels in this series. Notably, the LECs presented similar EL spectrum during the operation with time which is a desirable feature for LECs 7 , 39 The time-dependent luminance, average voltage, and efficacy of LECs based on Ir1, Ir2 and Ir3 + are demonstrated in Fig. 5 . As shown in Fig. 5 a-c, luminance gradually increases with time until a maximum is reached and then starts to decrease which is a typical characteristic for LEC devices. 1 , 2 , 40 Upon biasing the device, because of high initial injection barriers of electrons and holes, an operating voltage about 5.8 V is first observed. However, at this high voltage the ions dissociate and migrate faster towards the respective electrodes, leading the formation of p and n-doped regions near the electrodes and the following construction of p-i-n junction within the layer. It facilitates the electrons and holes injection from the inert electrodes thereby lowering the barrier for charge injection. 40 , 41 When, the electrochemically doped layers are well-formed, the devices reach the maximum luminance values (L max ) and the initial voltages drop to minimum stable values (2.6–2.7 V) that are nearly equal to the electrochemical band gap, 42 indicating no charge injection barrier. Additionally, the minimum average voltage of devices remains close to the steady-state value along the device operation time, depicting that there are no signs of charge transport issues or chemical degradation in all LECs (Fig. 5 a-c). 43 Interestingly, the initial voltage for LECs based on Ir1 and Ir2 decreases to 3 volts after about 60 minutes, while for Ir3 + this occurs after 10 minutes which might be related to the enhancement of ionic mobility of emissive layer that is induced by tethered methyl pyridinium moiety (PyCH 3 + ) on the Ir3 + complex. 19, 23 Under the constant current, the LECs based on Ir1, Ir2 and Ir3 + give maximum luminance of 870, 563 and 45 cd.m - 2 and external quantum efficiency (EQE) of 3.1, 2.5 and 0.24%, respectively. These results are in good consistent with PLQY recorded in the films for complexes. The higher L max and EQE of LEC device based on Ir1 with respect to those of other were attributed to the their higher PLQY in the film. Regarding, the Ir3 + based LEC device shows the lowest luminance and efficiency, revealing the high tendency of ionic iridium complex (Ir3 + ) to quench the excitons in the solid-state (See PLQY of the films in Table 1 ). Moreover, the L max and EQE afforded by Ir1 based-LEC device are among the highest values reported in the literature for [Ir(ppy) 2 (N^N)] + type emitter in the wavelength range of 570-585nm (see ESI table S2). 6 , 8 , 12 The response time of an LEC device is defined as turn-on time (t on ) which is corresponding to the time required to reach the maximum luminance. The Ir1, Ir2 and Ir3 + based LEC devices showed fast response with t on of 0.65, 1.3 and 0.15 h, respectively. The significant improvement of response time of the Ir3 + based LEC device can therefore be attributed to the ionic nature of the emissive layer, due to the accelerated formation of the doped regions. This further proves the significant role of ionic methyl pyridinium moieties in reducing the t on of the LEC devices up to 75–88% in this series and great potential of the modification of PI ligand with ionic groups. Remarkably, the Ir1 and Ir2 based LEC devices illustrated relatively high extrapolated half-lifetimes of 2130 and 1450 h, respectively in which obtained t 1/2 for Ir1 based LEC is among the highest value for cationic iridium (III) complexes based yellow-LECs reported so far (See Table S2, ESI). Take into account, it can be ascribed to their good electrochemical stability (see section 2 ) and the presence of hydrophobic phenyl rings on their periphery of positions that limits the occurrence of water induced substitution reaction. 10 , 13 , 17 Therefore, the higher stability of Ir1 based LEC device respect to Ir2 might be attributed to the higher hydrophilicity of substitutions group on the Ir2 complex (Ph-OH, Ph-Br) respect to Ir1 (ether moiety, Ph-O-CH 3 ) that it already demonstrated the effective influence of methoxy groups on the LEC device performance. 44 It is worth highlighting that lifetimes of the LECs are obtained by linear extrapolation of the time dependence of luminance and are in the same range as other very stable and efficient yellow/orange LECs which are mostly fabricated based on iridium complexes with sterically hindered N^N ligands (ranging over 1400 hours) . 6 – 8 , 16 , 45 Compared to the parent archetype [Ir(ppy) 2 (phen)] + , replacing phenanthroline by phenanthroimidazole ligand, leads to an impressive improvement in the EL properties of LEC devices with same structure. As an illustration, t 1/2 , L max , and EQE of Ir1 based LEC device compared to [Ir(ppy) 2 (phen)] + increase in turn 28, 14, and 1.5 times, respectively, (see Table 1 ) 36 which can be attributed to the bulky and specific structure of PI ligand as discussed earlier. Moreover, Ir1 based LEC device demonstrates almost a ten times shorter t on than archetype based LEC. It further indicates that complex Ir1 is more mobile in thin film despite of its larger size, perhaps due to the suppression of intercomplex π-π stacking interactions between cation molecules. Although, the Ir3 + based LEC represents much lower t on in comparison with Ir1/Ir2 based LECs but it suffers from relatively short half-lifetime (2.25 h). It has been shown that in general, the concentration of ionic species has a considerable effect on lifetime and t on values; the higher concentration of the ionic species leads to faster response but lower stability for LECs. 21 , 46 The different mobility of anions and cations give rise unbalanced charge injection/transport and movement the recombination zone in the active layer which increases the quenching of the excitons in the recombination zones (off-centered recombination zones) and thus deteriorating the device efficiency and lifetime. 19 , 47 However, further modification of complexes such as selection of bulky counter anion will contribute to create LECs based on extra ionic complexes with acceptable stability and performance. 48 Furthermore, the current efficiency (Fig. 5 d) and luminance versus time plots also follow the similar trends that emphases the high stability of yellow LEC. Overall, these data indicate the high potential of Ir1, Ir2, and Ir3 + complexes for use in display and lighting applications, confirming the advantages of employing the phenanthroimidazole as ancillary ligand for modification of the Ir(III) metal-based emitter toward achievement of efficient, stable and fast response LECs. 3. Conclusions In conclusion, three novel Ir(III) complexes Ir1, Ir2 and Ir3 + were designed and successfully synthesized based on phenyl pyridine and phenanthroimidazole (PI) as cyclometalated and ancillary ligand, respectively, in which PI was functionalized with various functional groups. The complexes exhibit yellow to orange emission with PLQY up to 38% in both solution and mixed thin film, as well as good electrochemical stability. Meanwhile, the experimental data were corroborated by computational study of complexes that reveals the significant effect of the ligand functionalization with electron-donor and electron-withdrawing groups on the electronic properties of complexes, leading the emission ranging from yellow to orange hue. Moreover, the fabricated yellow to orange LEC devices by these new bulky phosphorescent complexes, accomplishing superior half-lifetime over 2100 h, EQE over 3%, luminance exceeding 800 cd.m - 2 and improvement of the device turn-on time up to 75 to 88%. Eventually, the incorporation of phenanthroimidazole as N^N ancillary ligand was confirmed as an efficient and easy strategy to obtain iTMC based LECs with long half-lifetimes, short turn-on times, and high luminance opening new door(s) in opto-electronic application. Declarations ACKNOWLEDGMENT The authors acknowledge Bu-Ali Sina university for financial support and acknowledges the research council of the university of Isfahan for financial support. References Costa, R. D. Light-Emitting Electrochemical Cells: Concepts, Advances and Challenges ; 2017. Matsuki, K.; Pu, J.; Takenobu, T. Recent Progress on Light-Emitting Electrochemical Cells with Nonpolymeric Materials. Advanced Functional Materials 2020, 30 (33), 1908641. Schlingman, K.; Chen, Y.; Carmichael, R. S.; Carmichael, T. B. 25 Years of Light‐Emitting Electrochemical Cells: A Flexible and Stretchable Perspective. Advanced Materials 2021 , 33 (21), 2006863. Fresta, E.; Costa, R. D. Applying Ionic Transition Metal Complexes to Light-Emitting Electrochemical Cells. In Springer Handbook of Inorganic Photochemistry , Springer, 2022; pp 1849–1877. Nannen, E.; Frohleiks, J.; Gellner, S. Light-Emitting Electrochemical Cells Based on Color‐Tunable Inorganic Colloidal Quantum Dots. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-2182062","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":145861039,"identity":"dfc7b9fc-80b5-4e27-a60d-243ce2c5c390","order_by":0,"name":"Babak Nemati 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University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Majid","middleName":"","lastName":"Moghadam","suffix":""},{"id":145861041,"identity":"788b2e0a-87db-4298-a477-e236b54cbc66","order_by":2,"name":"Ahmad Sousaraei","email":"","orcid":"","institution":"PSL University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Ahmad","middleName":"","lastName":"Sousaraei","suffix":""},{"id":145861042,"identity":"6d868226-a40a-4f91-9d7d-0740dcdd013b","order_by":3,"name":"Behnoosh Shahpoori Arani","email":"","orcid":"","institution":"Isfahan University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Behnoosh","middleName":"Shahpoori","lastName":"Arani","suffix":""}],"badges":[],"createdAt":"2022-10-19 08:59:22","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-2182062/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-2182062/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41598-023-29527-7","type":"published","date":"2023-02-09T18:44:06+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":28232682,"identity":"d17492a1-4c20-4bed-a1b2-95de564d56b2","added_by":"auto","created_at":"2022-10-25 14:40:16","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":167305,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Electronic absorption and emission spectra of iridium complexes Ir1, Ir2, Ir3\u003csup\u003e+\u003c/sup\u003e in ACN solution, (b) PL of complexes in solid-state, inset: Ir1 (left) and Ir2 (right) powder under 405 nm irradiation.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-2182062/v1/081ef823fa9b3480e663c711.png"},{"id":28232123,"identity":"93b93f0e-5c7e-4c7d-8aa5-d5b2998e33ed","added_by":"auto","created_at":"2022-10-25 14:35:16","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":79590,"visible":true,"origin":"","legend":"\u003cp\u003eCyclic voltammograms of Ir1, Ir2, and Ir3\u003csup\u003e+\u003c/sup\u003e in degassed ACN (10\u003csup\u003e-3\u003c/sup\u003e M), measured at a scan rate of 0.1 V/s and differential pulse voltammograms of Ir1 (dot line).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-2182062/v1/cf8e92443c423a999bd82baf.png"},{"id":28232129,"identity":"c442cbc2-e426-495f-b15d-99e38a137596","added_by":"auto","created_at":"2022-10-25 14:35:16","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":356811,"visible":true,"origin":"","legend":"\u003cp\u003eEnergy levels and surface distributions of HOMO/LUMO orbitals at optimized S\u003csub\u003e0\u003c/sub\u003e geometries computed for complexes I1-Ir3.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-2182062/v1/d186c38e7f166743d31bfd63.png"},{"id":28232127,"identity":"f3636f44-4ad4-46ce-b9c2-ef8815043401","added_by":"auto","created_at":"2022-10-25 14:35:16","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":226312,"visible":true,"origin":"","legend":"\u003cp\u003eElectroluminescence spectra of LECs Ir1, Ir2 and Ir3\u003csup\u003e+\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-2182062/v1/405431baebed914091922ab2.png"},{"id":28232130,"identity":"650cb2d4-cfe7-4018-9050-8eaa0a2b538a","added_by":"auto","created_at":"2022-10-25 14:35:16","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":303918,"visible":true,"origin":"","legend":"\u003cp\u003eLuminance and average voltage versus time for iTMC-LEC devices, (a) iTMC: Ir1, (b) iTMC: Ir2, (c) iTMC: Ir3\u003csup\u003e+\u003c/sup\u003e. (d) Efficacy vs time\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-2182062/v1/609f42761083a84c645892bc.png"},{"id":44719055,"identity":"58d9a848-75a1-4dfd-b669-6d69fd6c03af","added_by":"auto","created_at":"2023-10-16 18:52:42","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1269830,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2182062/v1/449c3f57-8f73-426c-b64e-66050c4c5fec.pdf"},{"id":28232128,"identity":"c916741a-6050-4b15-9357-ef9272902913","added_by":"auto","created_at":"2022-10-25 14:35:16","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2550194,"visible":true,"origin":"","legend":"","description":"","filename":"ESISci.Rep..docx","url":"https://assets-eu.researchsquare.com/files/rs-2182062/v1/ac8c9c851850ec861dce85b2.docx"},{"id":28233599,"identity":"45c0f732-239d-45df-8dd3-406844102201","added_by":"auto","created_at":"2022-10-25 14:45:16","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":164784,"visible":true,"origin":"","legend":"","description":"","filename":"SYNOPSISTOC.docx","url":"https://assets-eu.researchsquare.com/files/rs-2182062/v1/6f5e5414018909fadd75331c.docx"},{"id":28232125,"identity":"0a10b36c-c935-44c7-86b8-47aba09b734c","added_by":"auto","created_at":"2022-10-25 14:35:16","extension":"png","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":108431,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eChart 1.\u003c/strong\u003e Molecular structures for emitter Ir1, Ir2, Ir3\u003csup\u003e+\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"CHART.png","url":"https://assets-eu.researchsquare.com/files/rs-2182062/v1/735708ffe1d27a2aa997e872.png"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003ePhenanthroimidazole as Molecularly Engineered Switch for Efficient and Highly Long-lived Light-Emitting Electrochemical Cell.\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eLight-emitting electrochemical cells (LECs), as simple, low-cost and efficient emissive thin-film devices represent great potential for next generation of optoelectronic devices.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e These devices exhibit many attractive features over traditional organic light-emitting diodes (OLEDs), such as single emissive layer consist of ionic luminescent species which can be easily fabricated from solution process and compatibility with an inert-metal cathode (e.g., Ag, Al, and Au) which allowing the nonrigorous encapsulation of devices.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e The fluorescent and phosphorescent materials used as emitter in LECs generally consist of small molecules, conjugated polymers, ionic transition metal complexes (iTMCs), thermally-activated delayed fluorescence (TADF) molecules, quantum dots, and luminescent perovskite nanoparticles.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e Among them, iTMCs have been received more attention due to their promising advantages. First, unlike organic compounds, iTMCs are inherently ionic and do not require additional ionic groups. Second, while in fluorescent emitter only 25% of the singlet excitons can be accessible for luminescence, the triplet relaxation pathway in form of the radiative deactivation, accelerated through spin-orbit coupling in phosphorescent iTMCs that allows these emitters to reach internal quantum efficiency (IQE) up to 100%, which is requisite to obtain LEC devices with high external quantum efficiency (EQE). Third, the iTMCs show stable redox properties that are required to achieve a device with high optical stability\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. Amid all iTMCs, phosphorescent cationic cyclometalated iridium (III) complexes are widely exploited for optoelectronic applications, thanks to their unique properties such as, high phosphorescence quantum yield, chemical inertness and relatively good photochemical/thermal stability, short and tuneable excited-state lifetimes, and finally high versatility in tuning the emission colour of the emission through modification of ligand. These favourable characteristics arise from the high ligand-field splitting energies (LFSEs) that is a consequence of the large size of d orbital (5d), high electric charge of the iridium ion (Ir\u003csup\u003e3+\u003c/sup\u003e) and high field strength exerted by anionic cyclometalating ligands (C^N) endowed by strong spin-orbit coupling.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e However, there are two significant obstacles that restrict their practical application as emitter in LEC devices, that is, low stability (defined as half-lifetime, t\u003csub\u003e1/2\u003c/sub\u003e, time to reach one-half of the maximum brightness) and long response time (expressed as turn-on time, t\u003csub\u003eon\u003c/sub\u003e: time required to reach a maximum brightness) of devices.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e The inherent instability of iTMC is ascribed to a ligand exchange reaction with a residue solvent or water molecules during the device operation, forming a non-luminescent complex.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e Therefore, an efficient way to increase the stability of iTMC can be limiting the access of foreign substances to the metal center. Accordingly, it is indicated that the device stability can be significantly improved by introducing peripheral bulky aromatic groups that increase the hydrophobicity of the complex preventing water molecules to approach the metal center. Furthermore, supramolecularly-caged which come from intraligand \u0026pi;-\u0026pi;-stacking of aromatic rings, minimizes the expansion of metal-ligand bonds in the excited state, protecting the ligand exchange by surrounding molecules such as water and solvent.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e It has been shown that turn-on time of the LEC devices is effectively improved by increasing the conductivity of the emitting layer.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e Accordingly, several studies based on addition of ionic materials to the emitter layer and chemical modification of iTMCs have been done to improve response time of iTMC-LEC devices. For example, the addition of ionic salts (LiPF\u003csub\u003e6\u003c/sub\u003e) and ionic liquids to the emitter layer and attachment of ionic moiety (such as imidazolium and triethylammonium) to ancillary ligand, lead to decrease the turn-on time of LECs.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e Although an improved t\u003csub\u003eon\u003c/sub\u003e is achievable by these strategies, in some cases, other important parameters such as lifetime, luminance, and external quantum efficiencies (EQE) can be significantly reduced.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eAnother concern about iTMC-emitters is that they exhibit severe phosphorescence concentration-quenching in the solid state as a result of their relatively long triplet lifetimes which remarkably suppresses the phosphorescence efficiency of the emissive layers and thereby the LEC performances. It has been shown that enclosing of bulky groups (such as phenyl) to peripheral ligands can provide steric hindrance and effectively reduces self-quenching of iridium complex emitters in the solid-state form.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e Phenanthroimidazole (PI) molecules with a specific structure and supreme photophysical features are used frequently as small molecules, ancillary ligands, and as a host material in optoelectronic devices, containing an imidazole moiety fused with strong metal-binding heterocycle in which by changing the substituents at the N1 and C2 imidazole positions, can adjust the electronic properties of PI and simply attaches to tethering groups.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e Therefore, PI ligands with high chemical modification potential can be employed as a molecular engineered switch to achieve an efficient, stable and fast response time LECs. However, based on our best knowledge no reports has been published on the effect of chemical modification of these ligands and their complexation with Ir(III) as well as their potential application in LEC device.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eIn light of all the aforesaid facts, we designed and synthesized three novel cyclometalated iridium (III) complexes as orange emitters based on the phenanthroimidazole (PI) (L1, L2 and L3) ligand containing electron donor/acceptor and ionic substitutions (Chart \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e), namely, Ir1, Ir2, and Ir3\u003csup\u003e+\u003c/sup\u003e. In this work, a combinational approach such as bulky structure, electron donor/acceptor substitutions and ionic moiety of PI ligand are demonstrated to optimize the device efficiency, stability and response time of LECs based on cyclometalated iridium (III) complexes as emitter. Accordingly, the prepared solution of complexes exhibits high yellow to orange phosphorescence and reversible red/ox properties. Meanwhile, the complexes were exploited as emitter for solution-processable LECs, which afford efficient and fast response electroluminescence (EL).\u003c/p\u003e"},{"header":"2. Results And Discussion","content":"\u003cp\u003e\u003cstrong\u003eSynthesis and characterization.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe complete synthesis procedure of the PI ligands and their cyclometalated iridium (III) complexes are given in the electronic supplementary information (ESI) and they were fully characterized by \u003csup\u003e1\u003c/sup\u003eH/\u003csup\u003e13\u003c/sup\u003eC NMR, elemental analysis and TOF-mass spectrometry.\u003c/p\u003e\n\u003cp\u003eBriefly, the PI ligands were synthesized, and easily purified without the need of column chromatography. The prepared ligands (PI) were reacted with chloro-bridged Ir(III) dimers of [Ir(ppy)\u003csub\u003e2\u003c/sub\u003eCl]\u003csub\u003e2\u003c/sub\u003e in a mixture of methanol/dichloromethane to synthesize Ir1, Ir2, and Ir3. Subsequently, Ir3\u003csup\u003e+\u003c/sup\u003e was obtained by reacting Ir3 with methyl iodide in acetonitrile (See scheme 1, ESI).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePhotophysical Characterizations.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea depicts the room temperature UV\u0026ndash;visible absorption (UV-Vis) and photoluminescence (PL) spectra of the complexes in acetonitrile. Detailed photophysical characteristics of these complexes and archetypal complex [Ir(ppy)\u003csub\u003e2\u003c/sub\u003e(phen)]\u003csup\u003e+ 29\u003c/sup\u003e are given in Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. All the complexes showed intense absorption bands (\u0026epsilon;\u0026thinsp;\u0026gt;\u0026thinsp;8 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e M\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) in the UV region of spectrum with a maximum in the range of 260 to 300 nm, which is attributed to the ligand centered (LC) spin-allowed \u003csup\u003e1\u003c/sup\u003e\u0026pi;-\u0026pi;* transitions involving both the cyclometallating (ppy) and ancillary (PI) ligands. The broad and less intense absorption bands between 300 and 430 nm are ascribed to spin-allowed metal-to-ligand (\u003csup\u003e1\u003c/sup\u003eMLCT) and ligand-to-ligand charge transfer (\u003csup\u003e1\u003c/sup\u003eLLCT) transitions, while the low-intensity bands beyond 430 nm correspond to the spin-forbidden \u003csup\u003e3\u003c/sup\u003eMLCT, \u003csup\u003e3\u003c/sup\u003eLLCT, and LC \u003csup\u003e3\u003c/sup\u003e\u0026pi;-\u0026pi;* transitions of the complexes. The spin-forbidden triplet transitions (\u003csup\u003e3\u003c/sup\u003eMLCT, \u003csup\u003e3\u003c/sup\u003eLLCT) are partially allowed owing to the strong spin\u0026ndash;orbit coupling of heavy iridium (III) atom to occur at a lower molar absorptivity than the corresponding singlet-allowed excitation (\u003csup\u003e1\u003c/sup\u003eLLCT/\u003csup\u003e1\u003c/sup\u003eMLCT).\u003csup\u003e30\u003c/sup\u003e The absorption spectra in the lower-energy region for Ir2 are significantly red-shifted by 16 nm compared to those of Ir1 which is directly resulting from changing the HOMO/LUMO energy gap because the presence of electron withdrawing (Br) and electron donor groups (OCH\u003csub\u003e3\u003c/sub\u003e) on the PI ligand of the complexes. The complexes Ir1, Ir2 and Ir3\u0026thinsp;+\u0026thinsp;show broad structureless PL spectra corresponding to yellow-orange emission centered at 580, 592 and 602 nm, upon 350 nm photoexcitation in an argon-saturated dichloromethane solution, demonstrating the emissions in the solution arise dominantly from \u003csup\u003e3\u003c/sup\u003eCT states.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e Obviously, introduction of electron-withdrawing and electron donor groups on the aryl ring of the PI ligand exerts a negligible influence on the maximum luminescence. Complex Ir2 shows a red-shift of 19 nm in maxima emission, compare to that of the archetypal complex [Ir(ppy)\u003csub\u003e2\u003c/sub\u003e(phen)]PF\u003csub\u003e6\u003c/sub\u003e.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e This red-shift can be explained by the electron-deficient nature of the fused imidazole moiety with Br as electron withdrawing group (L2), and \u0026pi;-expanded structure of the PI ligand, leading to stabilization of the LUMO and a smaller energy band gap, consequently (see optical band gap, E\u003csub\u003eg Opt\u003c/sub\u003e, in Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e). For the same reason, Ir1 exhibits lower maxima emission in this series, because of destabilization of the LUMO by two electron donor groups (OCH\u003csub\u003e3\u003c/sub\u003e) on the L1. Degassing solution of Ir1, Ir2 and Ir3\u003csup\u003e+\u003c/sup\u003e complexes cause high PL quantum yield (PLQY) values of 0.46, 0.42, and 0.40 and excited-state lifetimes (\u0026tau;) of 880, 970, and 950 ns, respectively, which exhibit slightly higher PLQY compare to the archetypal complex [Ir(ppy)\u003csub\u003e2\u003c/sub\u003e(phen)]\u003csup\u003e+\u003c/sup\u003e (\u0026Phi;\u003csub\u003epL\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.39)\u003csup\u003e31\u003c/sup\u003e, which could be attributed to the rigid structure of PI ancillary ligand.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e It is noteworthy that, PLQY of these complexes are the highest values for yellow to orange phosphorescence among their parent cationic iridium complexes (See table S2, ESI).\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e Time-resolved phosphorescence decays were found to be monoexponential, indicating the presence of a single emissive species. Furthermore, radiative (k\u003csub\u003er\u003c/sub\u003e) and nonradiative (k\u003csub\u003enr\u003c/sub\u003e) decay rates were calculated from PLQY and \u0026tau;, and listed in Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. All complexes Ir1, Ir2 and Ir3\u003csup\u003e+\u003c/sup\u003e show relatively long excited-state lifetimes and smaller k\u003csub\u003er\u003c/sub\u003e values compare to benchmark complex which suggests that their emitting triplet states should contain considerable ligand-centered \u003csup\u003e3\u003c/sup\u003e\u0026pi;\u0026thinsp;\u0026minus;\u0026thinsp;\u0026pi;* character relative to parent complex [Ir(ppy)\u003csub\u003e2\u003c/sub\u003e(phen)] \u003csup\u003e+\u003c/sup\u003e, and shielding effect of the periphery phenyl groups around the iridium core in these complexes that prevents nonradiative intermolecular charge recombination.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eIn order to see the potential use of the complexes as emitter in LEC devices, their emission properties were measured in neat films (pristine complexes without IL, see Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e.). All complexes showed featureless emission spectra (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb), indicating that the emission of both solution and neat film arises from the \u003csup\u003e3\u003c/sup\u003eCT state.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e PL emission spectra of the solutions in comparison with the neat films of Ir1, Ir2 and Ir3\u0026thinsp;+\u0026thinsp;are red-shifted by 4, 4 and 11 nm, respectively, that indicates the lower intermolecular interactions of PI-based complexes (Ir1, Ir2) compare to parent complex [Ir(ppy)\u003csub\u003e2\u003c/sub\u003e(phen)] \u003csup\u003e+\u003c/sup\u003e. Nevertheless, Ir3\u003csup\u003e+\u003c/sup\u003e is the most affected complex that 11 nm red-shift was observed probably due to increasing the intermolecular interaction in its neat film. All complexes as neat film showed lower PLQY compare to their solutions (See Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e). The major reason is associated to the close packaging of the complexes in the neat films, which promotes the self-quenching processes.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e However, in the neat films, the PLQYs of Ir1 and Ir2 (0.32, 0.26) are about 2.5 to 3-fold higher than that of [Ir(ppy)\u003csub\u003e2\u003c/sub\u003e(phen)]PF\u003csub\u003e6\u003c/sub\u003e (0.11), even though their PLQYs in the solution are slightly different, demonstrating that the bulky phenyl groups at the PI ligands significantly suppress the phosphorescence concentration quenching.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e Furthermore, by adding ionic liquid (IL) to the neat-films, the values of PLQYs significantly increased up to 0.53 and 0.38, due to decreasing the self-quenching of the emission.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e It is noted that Ir3\u0026thinsp;+\u0026thinsp;complex presented a much lower PLQY in film (pristine and mixed) respect to Ir1 and Ir2, exhibiting higher aggregation tendency of ionic compounds in comparison with neutral ones.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectrochemical Characterizations and DFT calculations.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAccording to the LEC working principle, the transport of electrons and holes in metal complex based LEC devices take place through consecutive oxidation and reduction of metal complex during device operation. Therefore, the redox behaviour of iTMCs play an important role in understanding the overall performance of LEC devices. Hence, the electrochemical properties of complexes were investigated using cyclic voltammetry and differential pulse voltammetry techniques. Figure 2 shows the cyclic voltammograms and the electrochemical data are presented in Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eAs shown in Fig.\u0026nbsp;2, at the positive potential, all complexes exhibited a main reversible oxidation process, which attributed to the oxidation of Ir(III) to Ir(IV) with a strong contribution from the cyclometalating ligand, ppy, .\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e At the negative potential, all complexes possess one reversible or quasi-reversible reduction peak which is considered to be caused by phenanthroimidazoles as ancillary ligand with minor Ir(III) center involvement.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e It is noteworthy that for most of this type of cyclometalated complex along with N^N ancillary ligand, the highest occupied molecular orbital (HOMO) has been reported to be a mixture of the d\u003csub\u003e\u0026pi;\u003c/sub\u003e(Ir) orbitals of iridium and the \u0026pi; orbitals of the C^N ligand while lowest unoccupied molecular orbital (LUMO) localized at N^N ancillary ligand. \u003csup\u003e38\u003c/sup\u003eTherefore, To shed light to the effect of various functional groups of ancillary ligands on the electronic properties of emitter, the HOMO and LUMO levels and electrochemical band gaps of complexes were derived from their corresponding redox potentials using empirical formula (footnote of Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003ctable border=\"1\" id=\"Tab1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003ePhotophysical data for Ir1, Ir2, and Ir3\u003csup\u003e+\u003c/sup\u003e.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eabsorption\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({ \\lambda }_{abs }\\left[nm\\right] \\left(\\epsilon /{10}^{3}\\right)\\)\u003c/span\u003e\u003c/span\u003e\u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"5\"\u003e\n \u003cp\u003ePL (solution) \u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003ePL (neat film) \u003csup\u003ec\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eE\u003csub\u003eg Opt\u003c/sub\u003e \u003csup\u003ed\u003c/sup\u003e\u003c/p\u003e\n \u003cp\u003e(eV)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\lambda }_{em}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\n \u003cp\u003e[nm]\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u0026Phi;\u003csub\u003ep\u003c/sub\u003e [%]\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\tau\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\n \u003cp\u003e[ns]\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({k}_{r}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\n \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\([\\times {10}^{5}{s}^{-1}]\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({k}_{nr}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\n \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\([\\times {10}^{5}{s}^{-1}]\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\lambda }_{em}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\n \u003cp\u003e[nm]\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u0026Phi;\u003csub\u003ep\u003c/sub\u003e\u003c/p\u003e\n \u003cp\u003e[%]\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eIr1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e269 (84.2), 294 (58.3), 334 (22.1),376 (10.8), 396 (10.0), 466 (1.4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e580\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e46\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e880\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5.20\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6.14\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e584\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e32 (53)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.54\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eIr2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e274 (82.0), 302 (50.5), 331 (35.7), 383 (12.4), 402 (11.4), 482 (1.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e602\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e42\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e970\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4.33\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5.98\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e606\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e26 (38)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.45\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eIr3\u003csup\u003e+\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e272 (78.0), 303 (4.89), 318 (32.8), 341 (16.0), 383 (7.8), 395 (7.5), 452 (0.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e592\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e40\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e950\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.66\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6.42\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e603\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e8 (15)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.52\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e[Ir(ppy)\u003csub\u003e2\u003c/sub\u003e(phen)]\u003csup\u003e+\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e583\u003csup\u003ee\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e39\u003csup\u003ee\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e230\u003csup\u003ee\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e17.3\u003csup\u003ee\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.7\u003csup\u003ee\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e591\u003csup\u003ee\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e11\u003csup\u003ee\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colspan=\"10\"\u003e\n \u003cp\u003e\u003csup\u003ea\u003c/sup\u003e In air-equilibrated CH\u003csub\u003e3\u003c/sub\u003eCN at 298 K (10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003eM). \u003csup\u003eb\u003c/sup\u003e In degassed CH\u003csub\u003e3\u003c/sub\u003eCN at 298 K (10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e M); the emission quantum yields (\u0026Phi;\u003csub\u003ep\u003c/sub\u003e) were calculated by comparison with quinine sulfate (\u0026Phi;p\u0026thinsp;=\u0026thinsp;0.545 in 1 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e (estimated error of \u0026plusmn;\u0026thinsp;5%.); Lifetime were calculated based on a mono exponential decay model. k\u003csub\u003er\u003c/sub\u003e and k\u003csub\u003enr\u003c/sub\u003e were calculated based on the equations \u0026tau;\u0026thinsp;=\u0026thinsp;1/ (k\u003csub\u003er\u003c/sub\u003e + k\u003csub\u003enr\u003c/sub\u003e) and \u0026Phi;\u003csub\u003ep\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;k\u003csub\u003er\u003c/sub\u003e/ (k\u003csub\u003er\u003c/sub\u003e + k\u003csub\u003enr\u003c/sub\u003e). \u003csup\u003ec\u003c/sup\u003e Solid-state absolute quantum yield was measured by employing an integrating sphere system (estimated error: \u0026plusmn; 5%), Values in parentheses were obtained for films with a composition similar to LEC (iTMC:IL, 4:1). \u003csup\u003ed\u003c/sup\u003e Optical bandgap, from the intersection of absorption and emission spectra. \u003csup\u003ee\u003c/sup\u003e Data from ref.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003ctable border=\"1\" id=\"Tab2\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eElectrochemical properties of complexes Ir1, Ir2, Ir3\u003csup\u003e+\u003c/sup\u003e.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eE\u003csub\u003e1/2 ox\u003c/sub\u003e (∆E) \u003csup\u003ea\u003c/sup\u003e (V)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eE\u003csub\u003e1/2 red\u003c/sub\u003e (∆E) \u003csup\u003eb\u003c/sup\u003e (V)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eHOMO \u003csup\u003ec\u003c/sup\u003e (eV)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eLUMO \u003csup\u003ed\u003c/sup\u003e (eV)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eE\u003csub\u003egap Elc\u003c/sub\u003e. \u003csup\u003ee\u003c/sup\u003e (eV)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eIr1\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.87 (81)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-1.81 (70)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-5.67\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-2.99\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.68\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eIr2\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.91 (86)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-1.61 (74)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-5.71\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-3.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.52\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eIr3\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e+\u003c/strong\u003e\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.89 (112)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-1.70 (106)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-5.69\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-3.10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.59\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colspan=\"6\"\u003e\n \u003cp\u003e\u003csup\u003ea\u003c/sup\u003e Half-wave potential, E\u003csub\u003e1/2\u003c/sub\u003e = \u0026frac12; (E\u003csub\u003epa\u003c/sub\u003e+E\u003csub\u003epc\u003c/sub\u003e); 0.1 M acetonitrile/TBAP versus Ag/AgCl at scan rate of 100 mV/s, Values in parentheses: difference between the anodic and cathodic peak potentials, \u0026Delta;E\u0026thinsp;=\u0026thinsp;E\u003csub\u003epa\u003c/sub\u003e-E\u003csub\u003epc\u003c/sub\u003e (mV). \u003csup\u003eb\u003c/sup\u003eThe half-wave potential of reduction peak for complexes. \u003csup\u003ec\u003c/sup\u003e From E\u003csub\u003eHOMO\u003c/sub\u003e = -(4.8\u0026thinsp;+\u0026thinsp;E\u003csub\u003eox\u003c/sub\u003e) eV. \u003csup\u003ed\u003c/sup\u003e From E\u003csub\u003eLUMO\u003c/sub\u003e = -(4.8\u0026thinsp;+\u0026thinsp;E\u003csub\u003ered\u003c/sub\u003e) eV. \u003csup\u003ee\u003c/sup\u003e Electrochemical band gap from E\u003csub\u003egap\u003c/sub\u003e = E\u003csub\u003eHOMO\u003c/sub\u003e - E\u003csub\u003eLUMO\u003c/sub\u003e.\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003ctable border=\"1\" id=\"Tab3\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eDevice Performance of the LEC: ITO/PEDOT:PSS/iTMC(Ir1, Ir2, Ir3\u003csup\u003e+\u003c/sup\u003e):[Bmim][PF\u003csub\u003e6\u003c/sub\u003e]/Al.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eDevice.\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u0026lambda;\u003csub\u003emax, EL\u003c/sub\u003e (nm)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCIE [x, y] \u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eV (V) \u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003et\u003csub\u003e1/2\u003c/sub\u003e (h)\u003csup\u003ec\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003et\u003csub\u003eon\u003c/sub\u003e (h) \u003csup\u003ed\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eL\u003csub\u003emax\u003c/sub\u003e )cd.cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e( \u003csup\u003ee\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eefficacy (cd.A\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) \u003csup\u003ef\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eEQE (%) \u003csup\u003eg\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eIr1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e581\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e[0.519, 0.480]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2130\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.65\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e870\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e8.60\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eIr2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e605\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e[0.648, 0.351]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1450\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e563\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5.52\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eIr3\u003csup\u003e+\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e596\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e[0.608, 0.391]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.15\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e45\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.38\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.24\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e[Ir(ppy)\u003csub\u003e2\u003c/sub\u003e(phen)]\u003csup\u003e+ h\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e578\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e73\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e63\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colspan=\"9\"\u003e\n \u003cp\u003e\u003csup\u003ea\u003c/sup\u003e Commission Internationale de l\u0026rsquo;Eclairage color coordinates, 1931. \u003csup\u003eb\u003c/sup\u003e Driving voltage (after 15h). \u003csup\u003ec\u003c/sup\u003e Lifetime: Time to reach one-half of the maximum luminance (Values obtained from extrapolation). \u003csup\u003ed\u003c/sup\u003e Turn-on time: time to reach maximum luminance. \u003csup\u003ee\u003c/sup\u003e Maximum luminance. \u003csup\u003ef\u003c/sup\u003e Maximum efficacy: ratio luminance/average current. \u003csup\u003eg\u003c/sup\u003e Maximum external quantum efficiency. \u003csup\u003eh\u003c/sup\u003e Data from ref.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eFirst of all, the reversible oxidation and reduction processes determine the good electrochemical stability of complexes which are beneficial to achieve stable iTMC-LECs. Since, Ir3\u003csup\u003e+\u003c/sup\u003e has the highest ∆E value compared to other complexes, thus, it can be concluded that the electrochemical stability of this complex is lower than the others. The oxidation potential of complexes is nearly identical (0.87\u0026ndash;0.91 V) which indicates that peripheral groups on the PI ligand hardly alter the HOMO level of the complexes. However, the reduction potential of Ir1 (-1.81 V) is significantly cathodically shifted (ca. 0.2 V) respect to that of Ir2 (-1.61 V), which attributes a significant destabilized LUMO for Ir2. The presence of peripherals electron-donating (OCH\u003csub\u003e3\u003c/sub\u003e) and withdrawing groups (Br) affect the reduction potential of PI ligands that are destabilized and stabilized the LUMO levels of cyclometalated complexes, respectively. In the meantime, the energy gap increased for Ir1 (2.68 eV) compare to Ir2 (2.52 eV) and also led to gradual blue-shift for the emission (22 nm). These data are further supported by DFT calculations. The computed HOMO, LUMO energy levels and electron density contour plot (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e), revealing that; first, the substitution groups on the PI ligands have only a minor influence on the HOMO levels, but pronounced on the LUMO levels, which expressed in the reduction potentials (Table S1, ESI), and second, the HOMO is mainly localized on the iridium and the cyclometalating ligand (ppy), whereas the LUMO is localized on the PI ligand (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e). These results reinforced the important role of functionalization of PI ligands in the electronic properties of these complexes.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectroluminescent Properties of LECs.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo confirm the excellent phosphorescence of complexes Ir1, Ir2, Ir3+, LEC devices were fabricated on ITO glass in a double-layer architecture consisting of PEDOT:PSS layer, in order to increase the reproducibility of the devices, and emissive layer using a solution process. The active layer was contained Ir-iTMC and ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF\u003csub\u003e6\u003c/sub\u003e]) at a 4:1 (iTMC:IL) molar ratio. The incorporation of IL to the films contribute to accelerate the LEC device response (reduce the turn-on time) by increasing the concentration of ionic molecules and ionic mobility in the emitter layer. In addition, IL increases the efficiency of devices by decreasing the concentration quenching as discussed earlier. At the end, aluminium metal (Al) was deposited on emissive layer as cathode electrode contact. The EL data of LECs were collected at pulsed current densities 102 mA.cm\u003csup\u003e-\u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e over time (1 kHz block wave and 50% duty cycle). More details concerning the LEC devices fabrication and characterization methods can be found in the supplementary Information.\u003c/p\u003e\n\u003cp\u003eAt the beginning, the effects of bulky structures of complexes on surface morphology of the spin-coated films were investigated by top-view SEM (See Figure S9, ESI). The good solubility of the ionic complexes in the acetonitrile (specially Ir3\u003csup\u003e+\u003c/sup\u003e), led to pinhole-free films with smooth and good morphology quality for LEC fabrication. Figure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e displays the EL spectra of the iTMC-LECs. EL spectra and maximum emission are closely similar to PL spectra of the complexes in thin-film, meaning that the nature of the emission is identical and come from the same excited state in both excitation method (Tables \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e). The maximum EL emission of LEC based Ir1, Ir2, and Ir3\u0026thinsp;+\u0026thinsp;were centred at 581, 605, and 596 with CIE coordinates of (0.52, 0.48), (0.64, 0.35), and (0.61, 0.39), respectively, that are corresponding to yellow to orange emission.\u003c/p\u003e\n\u003cp\u003eAs already mentioned, the shorter wavelength emission of Ir1 respect to others is due to influence of electron-donor and electron-withdrawing groups on the PI ligand that effectively destabilize and stabilize the LUMO levels in this series. Notably, the LECs presented similar EL spectrum during the operation with time which is a desirable feature for LECs \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eThe time-dependent luminance, average voltage, and efficacy of LECs based on Ir1, Ir2 and Ir3\u0026thinsp;+\u0026thinsp;are demonstrated in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea-c, luminance gradually increases with time until a maximum is reached and then starts to decrease which is a typical characteristic for LEC devices.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e Upon biasing the device, because of high initial injection barriers of electrons and holes, an operating voltage about 5.8 V is first observed. However, at this high voltage the ions dissociate and migrate faster towards the respective electrodes, leading the formation of p and n-doped regions near the electrodes and the following construction of p-i-n junction within the layer. It facilitates the electrons and holes injection from the inert electrodes thereby lowering the barrier for charge injection. \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e When, the electrochemically doped layers are well-formed, the devices reach the maximum luminance values (L\u003csub\u003emax\u003c/sub\u003e) and the initial voltages drop to minimum stable values (2.6\u0026ndash;2.7 V) that are nearly equal to the electrochemical band gap,\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e indicating no charge injection barrier. Additionally, the minimum average voltage of devices remains close to the steady-state value along the device operation time, depicting that there are no signs of charge transport issues or chemical degradation in all LECs (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea-c).\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eInterestingly, the initial voltage for LECs based on Ir1 and Ir2 decreases to 3 volts after about 60 minutes, while for Ir3\u0026thinsp;+\u0026thinsp;this occurs after 10 minutes which might be related to the enhancement of ionic mobility of emissive layer that is induced by tethered methyl pyridinium moiety (PyCH\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e) on the Ir3\u0026thinsp;+\u0026thinsp;complex.\u003csup\u003e19, \u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eUnder the constant current, the LECs based on Ir1, Ir2 and Ir3\u003csup\u003e+\u003c/sup\u003e give maximum luminance of 870, 563 and 45 cd.m\u003csup\u003e-\u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e and external quantum efficiency (EQE) of 3.1, 2.5 and 0.24%, respectively. These results are in good consistent with PLQY recorded in the films for complexes. The higher L\u003csub\u003emax\u003c/sub\u003e and EQE of LEC device based on Ir1 with respect to those of other were attributed to the their higher PLQY in the film. Regarding, the Ir3\u003csup\u003e+\u003c/sup\u003e based LEC device shows the lowest luminance and efficiency, revealing the high tendency of ionic iridium complex (Ir3\u003csup\u003e+\u003c/sup\u003e) to quench the excitons in the solid-state (See PLQY of the films in Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e). Moreover, the L\u003csub\u003emax\u003c/sub\u003e and EQE afforded by Ir1 based-LEC device are among the highest values reported in the literature for [Ir(ppy)\u003csub\u003e2\u003c/sub\u003e(N^N)]\u003csup\u003e+\u003c/sup\u003e type emitter in the wavelength range of 570-585nm (see ESI table S2).\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eThe response time of an LEC device is defined as turn-on time (t\u003csub\u003eon\u003c/sub\u003e) which is corresponding to the time required to reach the maximum luminance. The Ir1, Ir2 and Ir3\u003csup\u003e+\u003c/sup\u003e based LEC devices showed fast response with t\u003csub\u003eon\u003c/sub\u003e of 0.65, 1.3 and 0.15 h, respectively. The significant improvement of response time of the Ir3\u003csup\u003e+\u003c/sup\u003e based LEC device can therefore be attributed to the ionic nature of the emissive layer, due to the accelerated formation of the doped regions. This further proves the significant role of ionic methyl pyridinium moieties in reducing the t\u003csub\u003eon\u003c/sub\u003e of the LEC devices up to 75\u0026ndash;88% in this series and great potential of the modification of PI ligand with ionic groups.\u003c/p\u003e\n\u003cp\u003eRemarkably, the Ir1 and Ir2 based LEC devices illustrated relatively high extrapolated half-lifetimes of 2130 and 1450 h, respectively in which obtained t\u003csub\u003e1/2\u003c/sub\u003e for Ir1 based LEC is among the highest value for cationic iridium (III) complexes based yellow-LECs reported so far (See Table S2, ESI). Take into account, it can be ascribed to their good electrochemical stability (see section \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e) and the presence of hydrophobic phenyl rings on their periphery of positions that limits the occurrence of water induced substitution reaction.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e Therefore, the higher stability of Ir1 based LEC device respect to Ir2 might be attributed to the higher hydrophilicity of substitutions group on the Ir2 complex (Ph-OH, Ph-Br) respect to Ir1 (ether moiety, Ph-O-CH\u003csub\u003e3\u003c/sub\u003e) that it already demonstrated the effective influence of methoxy groups on the LEC device performance.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e It is worth highlighting that lifetimes of the LECs are obtained by linear extrapolation of the time dependence of luminance and are in the same range as other very stable and efficient yellow/orange LECs which are mostly fabricated based on iridium complexes with sterically hindered N^N ligands (ranging over 1400 hours) .\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e Compared to the parent archetype [Ir(ppy)\u003csub\u003e2\u003c/sub\u003e(phen)] \u003csup\u003e+\u003c/sup\u003e, replacing phenanthroline by phenanthroimidazole ligand, leads to an impressive improvement in the EL properties of LEC devices with same structure. As an illustration, t\u003csub\u003e1/2\u003c/sub\u003e, L\u003csub\u003emax\u003c/sub\u003e, and EQE of Ir1 based LEC device compared to [Ir(ppy)\u003csub\u003e2\u003c/sub\u003e(phen)]\u003csup\u003e+\u003c/sup\u003e increase in turn 28, 14, and 1.5 times, respectively, (see Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e) \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e which can be attributed to the bulky and specific structure of PI ligand as discussed earlier. Moreover, Ir1 based LEC device demonstrates almost a ten times shorter t\u003csub\u003eon\u003c/sub\u003e than archetype based LEC. It further indicates that complex Ir1 is more mobile in thin film despite of its larger size, perhaps due to the suppression of intercomplex \u0026pi;-\u0026pi; stacking interactions between cation molecules.\u003c/p\u003e\n\u003cp\u003eAlthough, the Ir3\u003csup\u003e+\u003c/sup\u003e based LEC represents much lower t\u003csub\u003eon\u003c/sub\u003e in comparison with Ir1/Ir2 based LECs but it suffers from relatively short half-lifetime (2.25 h). It has been shown that in general, the concentration of ionic species has a considerable effect on lifetime and t\u003csub\u003eon\u003c/sub\u003e values; the higher concentration of the ionic species leads to faster response but lower stability for LECs.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e The different mobility of anions and cations give rise unbalanced charge injection/transport and movement the recombination zone in the active layer which increases the quenching of the excitons in the recombination zones (off-centered recombination zones) and thus deteriorating the device efficiency and lifetime.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e However, further modification of complexes such as selection of bulky counter anion will contribute to create LECs based on extra ionic complexes with acceptable stability and performance.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eFurthermore, the current efficiency (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ed) and luminance versus time plots also follow the similar trends that emphases the high stability of yellow LEC.\u003c/p\u003e\n\u003cp\u003eOverall, these data indicate the high potential of Ir1, Ir2, and Ir3\u0026thinsp;+\u0026thinsp;complexes for use in display and lighting applications, confirming the advantages of employing the phenanthroimidazole as ancillary ligand for modification of the Ir(III) metal-based emitter toward achievement of efficient, stable and fast response LECs.\u003c/p\u003e"},{"header":"3. Conclusions","content":"\u003cp\u003eIn conclusion, three novel Ir(III) complexes Ir1, Ir2 and Ir3\u003csup\u003e+\u003c/sup\u003e were designed and successfully synthesized based on phenyl pyridine and phenanthroimidazole (PI) as cyclometalated and ancillary ligand, respectively, in which PI was functionalized with various functional groups. The complexes exhibit yellow to orange emission with PLQY up to 38% in both solution and mixed thin film, as well as good electrochemical stability. Meanwhile, the experimental data were corroborated by computational study of complexes that reveals the significant effect of the ligand functionalization with electron-donor and electron-withdrawing groups on the electronic properties of complexes, leading the emission ranging from yellow to orange hue. Moreover, the fabricated yellow to orange LEC devices by these new bulky phosphorescent complexes, accomplishing superior half-lifetime over 2100 h, EQE over 3%, luminance exceeding 800 cd.m\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e and improvement of the device turn-on time up to 75 to 88%. Eventually, the incorporation of phenanthroimidazole as N^N ancillary ligand was confirmed as an efficient and easy strategy to obtain iTMC based LECs with long half-lifetimes, short turn-on times, and high luminance opening new door(s) in opto-electronic application.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eACKNOWLEDGMENT\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors acknowledge Bu-Ali Sina university for financial support and acknowledges the research council of the university of Isfahan for financial support.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003e\u003cspan\u003eCosta, R. D. \u003cem\u003eLight-Emitting Electrochemical Cells: Concepts, Advances and Challenges\u003c/em\u003e; 2017.\u003c/span\u003e\u003c/li\u003e\n \u003cli\u003e\u003cspan\u003eMatsuki, K.; Pu, J.; Takenobu, T. 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Circumventing Dedicated Electrolytes in Light-Emitting Electrochemical Cells. \u003cem\u003eAdvanced Functional Materials\u003c/em\u003e \u003cstrong\u003e2020\u003c/strong\u003e, \u003cem\u003e30\u003c/em\u003e (33), 1906715. DOI: https://doi.org/10.1002/adfm.201906715.\u003c/span\u003e\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Chart 1","content":"\u003cp\u003eChart 1 is available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-2182062/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-2182062/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAlthough, light-emitting electrochemical cells (LECs) based on Ir(III) complexes owing to the superior advantages exhibit high potential for display and lighting applications, they still suffer from relatively low stability and sluggish response time. To mitigate this challenge, herein, a series of Ir(III) complexes based on phenanthroimidazole (PI) as ancillary ligand were functionalized to achieve efficient, highly stable yellow to orange LEC devices with fast response. These complexes exhibit appropriate electrochemical stability and significant suppression of concentration quenching in the thin films compare to archetype complex. Concerning, the fabricated LECs showed remarkable long device lifetime over 1400 and 2100 hours and EQE of 2 and 3% for yellow and orange-LECs, respectively, in which obtained t\u003csub\u003e1/2\u003c/sub\u003e for yellow LEC is among the highest value for cationic iridium (III) complexes based yellow-LECs reported so far. Subsequently, incorporation of ionic tethered functional group on PI, improved the mobility of emissive layer, reducing the device turn-on time around 75\u0026ndash;88%. This study represents facile functionalization and characterization of PI ligand and its potential application in optoelectronic devices (OLED).\u003c/p\u003e","manuscriptTitle":"Phenanthroimidazole as Molecularly Engineered Switch for Efficient and Highly Long-lived Light-Emitting Electrochemical Cell.","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-10-25 14:35:14","doi":"10.21203/rs.3.rs-2182062/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major revision","date":"2022-11-21T10:11:03+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2022-11-07T08:47:13+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"3e056315-739b-4779-8fb0-986c5c64e73b","date":"2022-10-31T07:56:24+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2022-10-31T07:10:13+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2022-10-31T07:08:43+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2022-10-20T19:15:48+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2022-10-20T19:10:18+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2022-10-19T08:47:34+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"8b4a3ab8-39b3-4483-877b-3680431e62ff","owner":[],"postedDate":"October 25th, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":16484164,"name":"Physical sciences/Chemistry/Materials chemistry/Optical materials"},{"id":16484165,"name":"Physical sciences/Optics and photonics/Applied optics/Optoelectronic devices and components"},{"id":16484166,"name":"Physical sciences/Physics/Electronics photonics and device physics/Photonic devices"}],"tags":[],"updatedAt":"2023-10-16T18:48:34+00:00","versionOfRecord":{"articleIdentity":"rs-2182062","link":"https://doi.org/10.1038/s41598-023-29527-7","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2023-02-09 18:44:06","publishedOnDateReadable":"February 9th, 2023"},"versionCreatedAt":"2022-10-25 14:35:14","video":"","vorDoi":"10.1038/s41598-023-29527-7","vorDoiUrl":"https://doi.org/10.1038/s41598-023-29527-7","workflowStages":[]},"version":"v1","identity":"rs-2182062","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2182062","identity":"rs-2182062","version":["v1"]},"buildId":"WrCJVZZCHTDjtuVLN7oU0","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}