Bridging the gap between traditional and nontraditional luminogens with strong non-aromatic through-bond conjugation and through-space conjugation

Bridging the gap between traditional and nontraditional luminogens with strong non-aromatic through-bond conjugation and through-space conjugation | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL Aggregate This is a preprint and has not been peer reviewed. Data may be preliminary. 20 March 2025 V1 Latest version Share on Bridging the gap between traditional and nontraditional luminogens with strong non-aromatic through-bond conjugation and through-space conjugation Authors : Xiaomi Zhang , Yunhao Bai , Junwen Deng , Xuanshu Zhong , Jinsheng Xiao , Wendi Xie , and Huiliang Wang 0000-0001-7964-0809 [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.174243681.18698720/v1 Published Aggregate Version of record Peer review timeline 283 views 167 downloads Contents Abstract Supplementary Material Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract The development of nontraditional luminogens (NTLs) with superior photoluminescence (PL) properties is of great scientific and practical significance and has drawn rapidly growing interests in recent years. An extremely important but unresolved question is that are there any distinct differences in the structures and PL mechanisms between traditional luminogens (TLs) and NTLs. In this work, four dihydropyridine derivatives with strong non-aromatic through-bond conjugation (TBC) were designed and synthesized, and the influence of strong non-aromatic TBC and through-space conjugation (TSC) effects on their PL behaviors was studied. These compounds in solutions show significant concentration-dependent and excitation-dependent emissions, which are typical PL behaviors of NTLs. In solid state, the compounds show wide excitation spectra while narrow emission spectra, with high quantum yields up to 57.4%, but they do not show significant excitation-dependent emissions, similar to TLs. And very impressively, two kinds of crystals also exhibit optical waveguide property, which is the first report in NTLs. The UV-vis spectra, crystal structures and theoretical calculations prove the presence of large non-aromatic TBC interactions in these NTLs and strong non-aromatic TSC can be formed among the molecules which are in a planar conformation and stacked into layers through intermolecular hydrogen bonding and π⋅⋅⋅π interactions. The combined effect of strong non-aromatic TBC and TSC endow the compounds unique PL behaviors that are between those of TLs and NTLs, thus bridging the gap between TLs and NTLs. Full paper Bridging the gap between traditional and nontraditional luminogens with strong non-aromatic through-bond conjugation and through-space conjugation Xiaomi Zhang, Yunhao Bai, Junwen Deng, Xuanshu Zhong, Jinsheng Xiao, Wendi Xie, Huiliang Wang* Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, China Correspondence: [email protected] (H.W.) Keywords: photoluminescence, nontraditional luminogens, traditional luminogens, through-space conjugation, through-bond conjugation The development of nontraditional luminogens (NTLs) with superior photoluminescence (PL) properties is of great scientific and practical significance and has drawn rapidly growing interests in recent years. An extremely important but unresolved question is that are there any distinct differences in the structures and PL mechanisms between traditional luminogens (TLs) and NTLs. In this work, four dihydropyridine derivatives with strong non-aromatic through-bond conjugation (TBC) were designed and synthesized, and the influence of strong non-aromatic TBC and through-space conjugation (TSC) effects on their PL behaviors was studied. These compounds in solutions show significant concentration-dependent and excitation-dependent emissions, which are typical PL behaviors of NTLs. In solid state, the compounds show wide excitation spectra while narrow emission spectra, with high quantum yields up to 57.4%, but they do not show significant excitation-dependent emissions, similar to TLs. And very impressively, two kinds of crystals also exhibit optical waveguide property, which is the first report in NTLs. The UV-vis spectra, crystal structures and theoretical calculations prove the presence of large non-aromatic TBC interactions in these NTLs and strong non-aromatic TSC can be formed among the molecules which are in a planar conformation and stacked into layers through intermolecular hydrogen bonding and π⋅⋅⋅π interactions. The combined effect of strong non-aromatic TBC and TSC endow the compounds unique PL behaviors that are between those of TLs and NTLs, thus bridging the gap between TLs and NTLs. 1. INTRODUCTION Photoluminescent materials have wide applications in many fields including optoelectronic devices, information storage and anti-counterfeiting, chemical and biological sensing, and bioimaging. [1-5] Organic photoluminescent materials receive more attention due to their diversity in structures and photoluminescence (PL) properties, easy preparation, low toxicity, etc. [6-8] Organic photoluminescent materials are generally classified into traditional luminogens (TLs) and nontraditional luminogens (NTLs), based on the type of chromophores present in them. An extremely important but unresolved question is that are there any distinct differences in the structures and PL mechanisms between TLs and NTLs. We do believe that there should not be a strict boundary between them, as conventional and nonconventional chromophores coexist in the chemical structures in many organic luminogens. Traditional organic luminogens generally contain large π-conjugated aromatic structures like benzene rings and/or heterocycles. Many efforts have been made to regulate the photophysical properties (e.g., emission wavelength and quantum yield) of the luminophores through ingenious structural design like adjusting the extent of aromatic through-bond conjugation (TBC) and introducing donor-acceptor groups. In fact, intra/intermolecular through-space conjugation (TSC) effect also has a significant impact on the PL emission of TLs. The presence of spatial electron transfer between adjacent benzene rings (i.e., intramolecular TSC) in [2.2]paracyclophane (22PCP) has been noticed by Steinberg in 1952, [9-10] and in 2012 Collard’s group found that the intramolecular TSC in 22PCP derivatives leads to their significantly red-shifted fluorescence emissions than the compounds without intramolecular TSC. [11] Very recently, Zhang and coworkers studied the effect of intermolecular TSC on the emissions of AIEgens, and they found that intermolecular TSC of isolated benzene rings leads to enhanced and red-shifted fluorescence emissions in aggregate state. [12-14] Strong aromatic TBC and TSC effects generally endow TLs with excellent photophysical properties, such as photoluminescence emissions in long-wavelength ranges even up to red and near-infrared ranges as well as high quantum yields. [15-17] In recent years, more and more natural and synthetic organic macromolecules and low-molecular-weight compounds without significant π-conjugated structures have been found to be intrinsically fluorescent and/or phosphorescent emissive in aggregate states. [18-22] These NTLs usually only contain non-conjugated functional groups such as amino, amide, carbonyl, ester, anhydride, hydroxyl, urea, ether, sulfonic acid, cyanide, etc. and in some cases isolated benzene rings and/or heterocycles. [23-32] NTLs possess many intrinsic advantages over TLs, such as wide ranges of sources, excellent hydrophilicity, biocompatibility and processability. Clusteroluminescence (CL) [33] and clustering-triggered emission (CTE) [34-35] mechanisms have been proposed to explain the PL of NTLs. In discrete states (e.g., dilute solutions), NTLs are generally non-emissive or very weakly emissive in low-wavelength range, due to the lack of strong TBC. On the contrary, in the aggregate states, the NTLs become emissive or exhibit largely enhanced and red-shifted emissions, due to the clustering of nonconventional chromophores (NCCs) and hence the formation of TSC through the sharing and/or overlap of the n and π electrons of adjacent heteroatoms and/or unsaturated bonds. Moreover, rigid molecular conformations as well as the strong chemical and physical interactions between NCCs are beneficial to suppress non-radiative decay and hence enhance PL emissions. When two or more consecutive unsaturated bonds and/or n-electron carrying atoms are present in one NTL, non-aromatic TBC (e.g., π-π, p-π, or multiple conjugation interactions) can be formed. Unfortunately, less attention has been paid to the effect of non-aromatic TBC on the PL of NTLs. In our very recent work, we found that the respective isomers of cyclohexanedione and dimethyl-1,3-cyclohexanedione exhibit very different fluorescence emission behaviors. [36] Some of the cyclohexanedione isomers undergo keto-enol tautomerism in concentrated solutions and in solid state to form a conjugated O=C−C=C−OH structure, i.e., π-π-p non-aromatic TBC. The isomers with a non-aromatic TBC generally show stronger and more red-shifted fluorescence emissions than those without. By studying the chemical and aggregate structures of the compounds and their relationship with the fluorescence emissions, we proved that non-aromatic TBC plays an important role in the PL of NTLs, and the combinational effect of non-aromatic TBC and TSC determines the PL of NTLs. Based on the above understanding, we propose that the combinational effect of TBC and TSC, no matter aromatic or non-aromatic, determines the PL of both TLs and NTLs. The contribution of TBC and TSC to PL varies with the chemical and aggregate structures of the luminogens. It is reasonable to predict that increasing non-aromatic TBC in NTLs may lead to PL emissions similar to those of TLs, and hence bridge the gap between TLs and NTLs. In this work, we designed and synthesized four dihydropyridine derivatives with multiple NCCs like C=O, C=C and N−H adjacent to each other, which form strong non-aromatic π-π-n-π-π TBC. The compounds exhibit some very impressive and interesting photophysical properties like high quantum yields, excitation-independent emission and optical waveguide properties. By analyzing the UV absorption spectra and single crystal structures of the compounds, combined with theoretical calculations, the effect of non-aromatic TBC and TSC effects on the PL of NTLs was further elucidated. 2. RESULTS AND DISCUSSION 2.1. Photophysical Properties P1 P2 P3 P4 SCHEME 1 The chemical structures of P1-P4. The chemical structures of P1-P4 are shown in Scheme 1. 1 H NMR, 13 C NMR and FTIR characterization results of the compounds are provided in Supporting Information (Figure S1). Four Chinese characters “一帆风顺”, which means “have a favorable wind throughout the voyage” or “everything is going smoothly”, were written with the dichloromethane solutions of the compounds P1-P4, respectively, and then dried. Under 365 nm UV light irradiation, the character P1 emits green fluorescence and the characters written with P2-P4 emit blue fluorescence (Figure 1a). The excitation and emission spectra of the compounds in solid powders were measured. As shown in Figure 1b, the excitation spectra of these compounds cover a wide wavelength range from 250 nm to 450 nm (or 500 nm) and the intensity within this entire range is relatively high. Multiple excitation peaks are present in the spectra, the maximum excitation wavelength (\(\lambda_{ex}^{\max})\ \)for P1 is 477 nm, and for P2-P4 two main peaks appear at about 340 nm and 430 nm, respectively. The emission spectra of these compounds all show a main peak and a shoulder peak (Figure 1c). The maximum emission wavelength (\(\lambda_{em}^{\max}\)) of P1 is at 516 nm and the shoulder peak is at around 560 nm, and the\(\lambda_{em}^{\max}\) of P2-P4 is around 470 nm and the shoulder peak is at 500 nm. The PL lifetimes of these compounds are all in nanoseconds (Figure 1d), indicating the fluorescence nature. The quantum yields (QYs) of P1-P4 in solid state under the excitation of 440 nm are 14.9%, 57.4%, 16.4% and 15.2%, respectively. FIGURE 1 (a) Photos of P1-P4 in solid state under 365 nm UV light. (b-c) Fluorescence excitation spectra (b) and emission spectra (c) of P1-P4 in solid state. (d) The lifetime profiles of P1-P4 in solid state. The compounds in solutions show significant concentration-dependent emission (CDE). As shown in Figures 2a and 2b, both P1 and P2 solutions emit very weak blue fluorescence at low concentrations (e.g., 2 × 10 -7 mol L -1 ) under 365 nm UV light irradiation, and as the concentration increases, the fluorescence emission of the solutions significantly increases, emitting blue-green and blue fluorescence, respectively. The fluorescence spectra of P1-P4 solutions with different concentrations are shown in Figure 2c, 2d and Figure S2. Their PL intensity firstly increases significantly with increasing concentration, and when the concentration is higher than 2 × 10 -4 mol L -1 , the PL intensity increases slightly or even decreases. The \(\lambda_{em}^{\max}\) of P2-P4 solutions remains constant at 444 nm, while that of P1 solutions red-shifts significantly from about 440 nm at the concentration of 2 × 10 -7 mol L -1 to 486 nm at a higher concentration of 2 × 10 -6 mol L -1 and then keeps constant. FIGURE 2 (a, b) Photos of P1 (a) and P2 (b) in dichloromethane with different concentrations (increasing from left to right) under 365 nm UV light. (c, d) Fluorescence excitation spectra (dashed lines) and emission spectra (solid lines) of P1 (c) and P2 (d) in DCM with different concentrations. It is very interesting to note that when the concentration is low, the excitation spectra of P1-P4 solutions have only one broad peak, with the\(\lambda_{ex}^{\max}\) at around 387 nm (P1) and 365 nm (P2-P4), respectively. However, when the concentration increases to 2 × 10 -4 mol L -1 , two peaks are shown in the excitation spectra on both sides of the \(\lambda_{ex}^{\max}\)position in the low-concentration solutions. When the concentration increases to 2 × 10 -3 mol L -1 , the excitation spectra show two independent and narrow peaks. The two emission peaks of the P1 solution are located at 301 nm and 437 nm, and those of P2-P4 solutions are around 299 nm and 408/410 nm, respectively. To our knowledge, this phenomenon of emission spectra splitting with increasing concentration has never been reported in NTLs. The fluorescence emissions of the compounds are also solvent-dependent. Taking P2 as an example, its PL intensity and emission wavelength vary with solvents with different polarities. In solvents with low polarities like toluene, DCM, ethyl acetate, THF and acetone, it emits weak blue fluorescence, while in solvents with high polarities, such as ethanol, methanol, NMP, DMF, and DMSO, it emits strong blue white fluorescence (Figure S3a, S3c). The fluorescence spectra of these solutions are consistent with this observation. The \(\lambda_{\text{em}}^{\max}\) of P2 red-shifts from 444 nm in solvents with low polarities to 460 nm in solvents with high polarities. In mixed solvents of water/ethanol, as the volume percentage of the poor solvent-water increases, the fluorescence emission intensity of the P2 solution increases and the \(\lambda_{\text{em}}^{\max}\) slightly red-shifts (Figure S3b, S3 d), exhibiting AIE behavior. FIGURE 3 Fluorescence emission spectra of P1 (a), P2 (b) and P3 (c) in solid state and P1 in a DCM solution (d) excited with different excitation wavelengths. Most NTLs exhibit excitation-dependent emissions (EDE), that is, their PL intensity and \(\lambda_{em}^{\max}\) change significantly with the excitation wavelength. [37-38] However, for P1, P2, and P3 solids, their \(\lambda_{em}^{\max}\) remains constant at different excitation wavelengths, and the PL intensity of P1 and P2 only increases slightly or even remains almost constant with the increase of excitation wavelength, while the PL intensity of P3 increases significantly as the excitation wavelength is increased from 260 nm to 300 nm, but gradually decreases thereafter with increasing excitation wavelength (Figure 3a-c). This is different from the common feature of NTLs, but similar to the properties of TLs. The EDE behavior of the compounds in solutions is similar to those of common NTLs. As shown in Figure 3d and Figure S4, the PL intensity of the solutions firstly increases significantly with increasing excitation wavelength till the \(\lambda_{\text{ex}}^{\max}\) and then decreases, and the \(\lambda_{\text{em}}^{\max}\) red-shifts slightly with increasing excitation wavelength. 2.2 Optical waveguide properties Some traditional organic luminogens have been reported to exhibit active optical waveguide properties, [39-40] but no NTLs have been reported to possess such properties. We prepared rod-shaped P1 single crystals and sheet-shaped P4 single crystals, and found that they possess optical waveguide properties. The optical waveguide behavior of the rod-shaped P1 single crystal was studied using a two-photon laser beam (\(\lambda_{1}+\lambda_{2}\)=800 nm). When the excitation light source is generated by two-photon collision, the excitation spot is small, which is beneficial for exciting crystals with smaller sizes and also reduces damage to the crystals. When the two-photon laser beam is focused on the midpoint in the width direction of the micrometer sized rod-shaped P1 crystal, bright spots appear at both ends of the crystal in the width direction (marked as 1 and 2), while weaker spots appear at both ends in the length direction (positions 3 and 4) (Figure 4a, b). When the excitation light source is precisely moved along the length direction, the light intensity at the end close to the excitation light source gradually increases, while the light intensity at the other end gradually decreases (Figure 4b). The optical waveguide behavior of the sheet-shaped P4 single crystal was studied using a single-photon laser beam with a wavelength of 420 nm. When the laser beam is concentrated on the midpoint in the width direction, one bright and one dark light spot appear at the symmetrical ends of the crystal in the width direction (regions 1 and 2), and two light spots also appear at the two ends in the length direction (Figure 4c). When the excitation light source is precisely moved along the length direction, the emission point remains at the tip, and the light intensity at the end close to the excitation light source gradually increases, while the light intensity at the other end gradually decreases (Figure 4d). This asymmetric light propagation makes the crystal sheet suitable as optical planar diodes. FIGURE 4 (a-d) The micrographs of P1 (a, d) and P4 (c, d) single crystals when the excitation light was on the center in the width direction (a, c) and moving along the length direction (b, d). (e, f) The ratio of the intensity I tip / I body against the distance D b-t of P1 (e) and P4 (f), curves were fitted by an exponential decay function I tip / I body =Aexp(-R D ). The light intensities at the excitation end ( I body ) and the emission end ( I tip ) of the two crystals were quantitatively measured, and it is found that the ratio of the two intensities, I tip / I body , and the distance from the excitation end to the emission end ( D b-t ) shows a single exponential decay relationship (Figure 4e and 4f). The obtained propagation loss coefficients of the two crystals are 16.09\(\text{dB\ }\text{mm}^{-1}\) and 16.85 \(\text{dB\ }\text{mm}^{-1}\), respectively. 2.3 PL mechanism To understand the effect of the chemical structure and aggregation structure of P1-P4 on their electronic transitions, we measured the UV-vis absorption spectra of the DCM solutions of these compounds with different concentrations. FIGURE 5 (a-d) UV-vis spectra of P1 (a), P2 (b), P3 (c) and P4 (d) in DCM with different concentrations. As shown in Figure 5, at concentrations lower than 2 × 10 -6 mol L -1 or 2 × 10 -7 mol L -1 , the UV absorption spectra of P1-P4 solution show only one absorption peak or shoulder peak at 273, 250, 255 and 255 nm, respectively. The absorption peaks attributed to π-π* electronic transitions are red-shifted compared to those of compounds containing isolated C=C or C=O bonds (around 200 nm), while similar to the benzenoid band (230-270 nm) of benzene, indicating the formation of strong non-aromatic TBC within the P1-P4 molecules. As the concentration increases, new absorption peaks appear at 390, 358, 367 and 367 nm, respectively, and the absorbance of these peaks gradually increases. The long-wavelength absorption peaks occurring at higher concentrations are attributed to the TSC effect induced by the aggregation of the compounds. The UV absorption peaks of P1 solutions at both low and high concentrations are red-shifted with comparison to those of P2-P4 solutions, indicating P1 has both higher non-aromatic TBC and TSC than the others. And this is the main reason for the red-shifted emission of P1. The red-shifted emissions of all compounds in solid state than in solutions are due to the stronger TSC formed in solid state. FIGURE 6 Conformations and intermolecular interactions of P1 (a), P2 (b), P3 (c) and P4 (d) in crystals. Note that the ethyl group connected to the ester group in P4 rocks even in crystals, so two conformations of the ethyl group of each P4 molecule are shown. To determine the aggregation structures of P1-P4 molecules, single crystals of these compounds were obtained, and the crystallographic data of the single crystals are listed in Table S1. The crystal structures plotted with Olex 2 software are shown in Figure 6. The dihydropyridine ring in P1-P4 molecule is coplanar with two carbonyl groups, and the C=C bond length is longer than that in ethylene (1.330 Å) and the single and double bond lengths tend to be averaged, due to the intramolecular delocalization effect of neighboring carbonyl and amino groups, which implies the presence of strong non-aromatic TBC effect within the molecules. The P1-P4 molecules are interlaced and stacked into a layered structure, with interlayer distances mostly less than 4 Å. Intermolecular hydrogen bonds are formed between the co-planar amino groups and carbonyl groups, with bond lengths ranging from 2.020 to 2.130 Å. The tight interlayer stacking and intralayer hydrogen bonding not only stiffen the conformations of the molecules and hence inhibit their movements, but also shorten the distances between inter- and intralayer chromophores in the aggregate, leading to expanded TSC and hence the strong fluorescence emissions of these compounds. The shape and size of crystal structures have a significant impact on the constraint and guidance of photon flow. Asymmetric light propagation is also related to the dipole moment of molecular transitions. [40-41] The optical waveguide properties of P1 and P4 crystals originate from the highly ordered arrangement of molecules driven by interlayer hydrogen bonding and TSC interactions in the large-sized 1D/2D crystals. To understand the influence of aggregation on the electronic transitions of the compounds, the HOMO-LUMO electronic structures and energy levels of the single molecules and consecutive molecules with increasing molecule numbers from 2 to 4 were calculated based on the conformations and molecular arrangement in the crystal structures. The HOMO-LUMO energy gaps of P1-P4 single molecules are relatively small, with that of P1 being the smallest (3.95 eV) and those of P2-P4 very similar (4.12 eV-4.19 eV). These results also prove that P1 has a stronger non-aromatic TBC than the others. FIGURE 7 The molecular orbital surfaces of HOMO-LUMO and energy gaps of single molecular P1-P4. Due to the presence of different types of intra- and interlayer short-distance contacts between chromophores in the aggregates of the P1-P4 compounds, we firstly used P1 as an example and selected their interlayer and intralayer dimers, trimers, and tetramers for theoretical calculations (Figure 8). Due to the unequal interlayer spacing of P1 (3.658 Å and 3.499 Å), two types of dimers were selected for the calculation. For the two interlayer dimers, the one with a shorter interlayer distance has a smaller HOMO-LUMO energy gap (3.77 eV vs. 3.82 eV) (Figure 8a, left side and middle). For the interlayer trimer and tetramer, the HOMO-LUMO energy gap decreases slightly with increasing molecule number (Figure 8b and 8c, left side). On the other hand, for the intralayer dimer, trimer and tetramer, their HOMO-LUMO energy gaps are significantly lower than those of the interlayer ones (Figure 8, right side), and the decrease in HOMO-LUMO energy gap with increasing molecule number is also more significant. Note that there is a significant decrease in HOMO-LUMO energy gap from the P1 monomer (3.95 eV) to the intralayer dimer (2.90 eV), but only a very small decrease is observed for the interlayer dimers. These results suggest that intramolecular electron clouds transfer is easier to occur among intralayer molecules closely contacted by hydrogen bonding. This should be the fundamental reason for the optical waveguide properties of P1 and P4. FIGURE 8 The HOMO-LUMO surfaces and energy gaps of interlayer (left) and intralayer (right) P1 dimer (a), trimer (b) and tetramer (c). The molecular orbital surfaces and energy gaps of the intralayer dimers, trimers and tetramers of P2-P4 are shown in Figure 9. Similar to P1, a significant decrease in HOMO-LUMO energy gap from monomer to dimer is observed for P2-P4 compounds, and the HOMO-LUMO energy gap decreases with increasing molecule number. The HOMO-LUMO energy gaps of P1 monomer and aggregate are always lower than those of P2-P4. This is the reason for the red-shifted emission of P1 than P2-P4. FIGURE 9 The molecular orbital surfaces of HOMO-LUMO and energy gaps of the dimers, trimers and tetramers of P2 (a), P3 (b) and P4 (c). The above characterization and theoretical calculations confirm the presence of strong non-aromatic TBC and TSC in the chemical structures and aggregates (crystalline structures) of P1-P4. The more red-shifted emission of P1 than the others can be well explained with the combinational effect of non-aromatic TBC and TSC. 2.4 Discussion The dihydropyridine derivatives exhibit PL behaviors between TLs and NTLs. In other words, some of their PL behaviors are similar to those of NTLs, while some are similar to those of TLs. The dihydropyridine derivatives in solutions show significant concentration-dependent and excitation-dependent emissions, which are typical PL behaviors of NTLs. They can be explained with the aggregation and the formation of diverse clusters of NCCs and hence different extents of TSC in the aggregates in solutions. The splitting of emission spectra into two independent and very narrow peaks at higher concentrations is a new phenomenon found in NTLs. The possible reason is that the irregular aggregates of molecules at low concentrations gradually turn into uniform ordered structures (e.g., microcrystals) at high concentrations, and hence the appearance of a very narrow excitation peak at a longer wavelength, and the remained discrete molecules lead to the excitation peak at a shorter wavelength. In solid state, the dihydropyridine derivatives exhibit PL behaviors very similar to those of TLs. They all show very wide excitation spectra while narrow emission spectra, but no significant excitation-dependent emission. The occurrence of wide excitation spectra can be explained with the presence of different extents of TSC induced by the intra- and interlayer interactions of NCCs, and hence a wide range of energy levels (wavelengths) suitable for excitation. Due to the ordered and consecutive arrangement of the NCCs and the lower HOMO-LUMO energy gaps of the ”clusters” formed by intralayer hydrogen bonding than those of the ”clusters” formed by interlayer π-π interactions, the absorbed energy can be transferred through intralayer TSC and released through radiative transitions, resulting in the narrow emission peak and excitation-independent emission behavior. 3. CONCLUSION In this work, we designed and synthesized four dihydropyridine derivatives with strong non-aromatic TBC and studied the effects of non-aromatic TBC and TSC on the photophysical properties of the NTLs. The compounds exhibit PL behaviors between those of TLs and NTLs. The multiple NCCs adjacent to each other form strong non-aromatic π-π-n-π-π TBC similar to aromatic TBC, as evidenced by the red-shifted absorption band in UV-visible spectra and the averaged single and double bond lengths. Moreover, strong TSC is also formed in the aggregates (crystals) of the compounds through intralayer hydrogen bonding and interlayer π-π interactions. The combinational effects of strong non-aromatic TBC and TSC determine photoluminescence behaviors of the compounds. Therefore, this study bridges the gap between traditional and nontraditional luminogens and provides a deeper understanding of the PL mechanism of organic luminogens. 4. EXPERIMENTAL SECTION 4.1 Materials 2,4-Pentanedione (99.0%), 1,3-cyclohexanedione (98.0%), methyl acetoacetate (99.0%) ethyl acetoacetate (99.0%), ammonium acetate (99.0%), formaldehyde (AR grade), deuterated chloroform (CDCl 3 ), deuterated dimethyl sulfoxide (DMSO- d 6 ), anhydrous ethanol (HPLC grade), and benzene (AR grade) were all purchased from InnoChem Technology Co. Ltd. (China). Dichloromethane (DCM), toluene, ethyl acetate, tetrahydrofuran (THF), acetone, ethanol, methanol, dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) were all of AR grade and purchased from Beijing Tongguang Fine Chemical Co., Ltd. (China). N -methylpyrrolidone (NMP, 99.0%) was purchased from TCI Co. Ltd. (Shanghai, China). 4.2 Synthesis Formaldehyde (1 mmol), ammonium acetate (1 mmol) and the diketone(s) and/or acetoacetate (2 mmol, or 1 mmol each) were mixed with ethanol (5 mL) and added into a round bottom flask equipped with a condenser, and then the solution was kept at 40°C for 3 h or 0.5 h under magnetic stirring (Scheme 2). Products P1 and P2 were directly precipitated from the solutions, while P3 and P4 were precipitated by adding the reaction solutions into ice-water, and the precipitates were filtered and collected, and then recrystallized with dichloromethane to obtain the products P1-P4. The yields of P1-P4 were 84.6%, 97.2%, 83.0%, and 78.7%, respectively. SCHEME 2 Synthesis route of dihydropyridines P1-P4. 4.3 Characterization 1 H and 13 C NMR spectra were measured on Bruker Avance III 600 MHz nuclear magnetic resonance (NMR) instrument (Bruker BioSpin GmbH, Rheinstetten, Germany) at ambient temperature, using CDCl 3 and DMSO- d 6 as the solvents. Fourier transform-infrared (FTIR) spectra were recorded on a IRAffinity-1 FTIR spectrometer ( Shimadzu, Japan) at ambient temperature. Mass spectrometry was performed with a 5975C MSD mass spectrometer (Thermo Scientific, Germany). UV-vis spectra were recorded with a UV-vis spectrophotometer (UV-2450, Shimadzu, Japan). Single crystal data were collected on an Agilent Technologies SuperNova single-crystal X-ray diffractometer (Santa Clara, USA) using graphite monochromated Mo Kα and Cu Kα radiation ( λ = 1.5418 Å) . 4.4 Fluorescence spectroscopy Fluorescence spectra were recorded on FS5 and FS980 fluorescence spectrometers (Edinburgh instruments, UK) at room temperature. The excitation and emission slit widths for solid powders were 0.2-0.3 nm, the excitation and emission slit widths for solutions were 3 nm. The lifetime was measured with an FS980 fluorescence spectrometer and the luminescence quantum yield was measured with an absolute quantum yield tester (Quantaurus-QY, Hamamtsu, Japan). Photographs of solutions and solid powders under UV light (365 nm) were taken with a digital camera (Cannon, Japan) in a dark room. The exposure time was 1/30 s and the ISO was 400. The optical waveguide behavior was tested using a Tsunami Spitfire OPA-800C ultrafast optical parametric amplifier (Spectra-Physics, USA), and the excitation light sources for P1 and P4 are single photons at a wavelength of 420 nm and two-photons at a wavelength of 800 nm, respectively. 4.5 Computational methods The calculation model was based on the crystal structure data obtained from the X-ray single crystal diffraction of representative molecules. All electronic structures and energy levels of these compounds were calculated on the basis of Gaussian 09 program (D.01 version) [42] with the B3LYP [43] functional and a 6-31G(d, p) base set [44-47] , and D3 dispersion corrections were included to improve the accuracy of calculations. ACKNOWLEDGEMENTS This research was funded by National Natural Science Foundation of China (grant no. 22472010) and the Program for Changjiang Scholars and Innovative Research Team (PCSIRT) in University. AUTHOR CONTRIBUTIONS Xiaomi Zhang and Yunhao Bai contributed equally to this article. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. CONFLICT OF INTEREST STATEMENT The authors declare no conflict of interest. DATA AVAILABILITY STATEMENT The data that support the findings of this study are available in the supplementary material of this article. ORCID HuiliangWang https://orcid.org/0000-0001-7964-0809 REFERENCES 1. 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Collection Aggregate Keywords nontraditional luminogens photoluminescence through-bond conjugation through-space conjugation traditional luminogens Authors Affiliations Xiaomi Zhang Beijing Normal University View all articles by this author Yunhao Bai Beijing Normal University View all articles by this author Junwen Deng Beijing Normal University View all articles by this author Xuanshu Zhong Beijing Normal University View all articles by this author Jinsheng Xiao Beijing Normal University View all articles by this author Wendi Xie Beijing Normal University View all articles by this author Huiliang Wang 0000-0001-7964-0809 [email protected] Beijing Normal University View all articles by this author Metrics & Citations Metrics Article Usage 283 views 167 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Xiaomi Zhang, Yunhao Bai, Junwen Deng, et al. 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