The effect of positional isomerism of cyano group of the rod-like mesogens on the liquid crystalline and optical characteristics

The effect of positional isomerism of cyano group of the rod-like mesogens on the liquid crystalline and optical characteristics | 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 Research Article The effect of positional isomerism of cyano group of the rod-like mesogens on the liquid crystalline and optical characteristics Zhigao Liu, Xiaotong Liu, Yurun Liang, Shunbo Zhang, Xiuning Hu, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4089030/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Two novel cyanostilbene-based rod-like mesogens named BI-CN-4 and BI-CHO-4 consisting of one terminal pyrene and one terminal alkyl chain were prepared by Suzuki coupling and Knoevenagel reactions. The influence of the positional isomerism of cyano group of cyanostilbene unit on the liquid crystalline characteristics, photophysical characteristics, and mechanochromism characteristics is explored by using POM, DSC, XRD, UV spectra, PL spectra, DFT and TD-DFT calculations. BI-CN-4 in which the cyano group is adjacent to the phenylpyrene unit exhibits monotropic smectic C phase, whereas BI-CHO-4 in which the cyano group is far away from the phenylpyrene unit exhibits enantiotropic smectic C phase. BI-CN-4 exhibits more distinct positive solvatochromism due to more distinct intramolecular charge transfer. Both compounds exhibit AIE or AIEE behavior and reversible mechanochromism behavior due to twisted molecular configurations. These results demonstrated that the distinct molecular design could endow organic molecules with multifunctional properties and potentials. Liquid crystal Aggregation-induced emission Mechanochromism Cyanostilbene Pyrene Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Organic luminescent materials with tunable emission intensity and emitting color in different have obtained much attention and also have been widely applied in various fields including organic light-emitting diodes (OLEDs), [1,2] anti-counterfeiting technology [3,4] and biological imaging [5,6] and others. Among these materials, mechanochromic luminescent materials showed distinct emitting-color changes as a result of the change in molecular packings, intermolecular interactions, morphological structures and molecular conformations induced by mechanical stimuli. [7] In general, the mechanochromic luminescent materials always exhibit strong solid-state emission. However, the conventional organic luminescent materials suffered aggregation-caused quenching (ACQ) due to the strong intermolecular π-π stacking interactions in aggregated state, which hindered the development of mechanochromic luminescent materials. Fortunately, the opposite phenomenon of ACQ called aggregation-induced emission (AIE) or aggregation-induced enhanced emission (AIEE) was discovered by Tang et al. and Park et al. in 1-methyl-1,2,3,4,5-pentaphenylsilole and cyanostilbene molecules. [8,9] Due to the good performance, the AIE- or AIEE- active units are regarded as promising candidates for constructing mechanochromic luminescent materials. Therefore, various mechanochromic luminescent materials containing AIE- or AIEE- active units including cyanostilbene, tetraphenylethylene, dibenzofulvene and β -diketonate and others have been developed. [10,11,12,13,14,15,16,17] Molecular engineering plays an important role in constructing novel photoelectric materials and self-assembly materials. Tuning the chemical structures including the aromatic units, linking groups, appending groups and even the position of lateral/terminal groups of the organic molecules, especially the organic liquid crystalline molecules would induce the great difference of self-assemblies, phase transition temperature, functional properties and others. [18,19,20] For instance, Tschierske and Cheng reported some bolapolyphilic liquid crystals and the changes of the type of rod-like aromatic units and the length of lateral chains led to various mesophases including lamellar phases, honeycomb columnar phases and different cubic phases. [21,22,23,24] In addition, we also reported some rod-like molecules in which the change of the position of aromatic units led to the transition of mesophases, different AIE behaviors and stimulus-response behaviors. [25,26] Therefore, the exploration of the influence of chemical structures of liquid crystalline molecules on the properties and the understanding of structure-property relationships would be important for the construction of different materials. Pyrene and cyanostilbene are important functional groups having the good characteristics of high fluorescence quantum efficiency, easy modification and electronic properties to be employed to construct functional materials. Recently, some liquid crystalline materials or/and luminescent materials containing both pyrene and cyanostilbene have been constructed and usually the cyanostilbene was appended to the 1- position, 1,6- positions or/and 1,3,6,8- positions. [ 26 ,27,28,29,30] These materials exhibited wonderful self-assembly properties and luminescent properties. Inspired by these good performances, we employed pyrene and cyanostilbene as aromatic units to fabricate two rod-like liquid crystals. And the influence of the positional isomerism of cyano group on the liquid crystalline self-assembly, solvatochromic properties, AIE behaviors and mechanochromic behaviors was investigated. Such investigations have benefits on the understanding of the relationship between chemical structure-properties and also have a great effect on novel multifunctional materials. Results and Discussion 2.1. Synthesis The synthesis of the rod-like mesogens mainly involves three key steps including Suzuki coupling reaction, etherification reaction and Knoevenagel condensation reaction as shown in Scheme 1. Firstly, Suzuki coupling reaction pyren-1-ylboronic acid and 4-bromobenzaldehyde or 2-(4-bromophenyl)acetonitrile took place to yield compounds 1 . [31,32] In addition, 4-hydroxybenzaldehyde and 2-(4-hydroxyphenyl)acetonitrile were o -alkylated with bromotetradecane in the presence of base to yield compounds 2 . Finally, compounds 1 were reacted with compounds 2 under the Knoevenagel condensation condition to yield the rod-like liquid crystals BI-CN-4 and BI-CHO-4 . The synthetic details and corresponding spectrum analysis results were provided in the Supplemental Materials. 2.2. Liquid-crystalline properties The thermal behavior and optical textures of compounds BI-CN-4 and BI-CHO-4 were studied by using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). On heating process of compound BI-CN-4 under POM, the crystal transformed into the isotropic state at about 175 ℃. On first cooling process from isotropic state of compound BI-CN-4 , irregular schlieren texture could be observed indicating that compound BI-CN-4 could self-organize into smectic C phase (Fig. 1a). [33] On heating process of compound BI-CHO-4 under POM, the crystal gradually transformed into mesophase at about 144 ℃ and then transformed into isotropic state at about 148 ℃, indicating that the mesophase range is very narrow. On cooling process from isotropic state of compound BI-CHO-4 , distinct schlieren texture could be observed indicating that compound BI-CHO-4 could also self-assemble into smectic C phase (Fig. 1b). [ 16 ] The phase transition temperatures were further detected by DSC (Fig. 1c and 1d). In the DSC curve of compound BI-CN-4 , two endothermic peaks at 140.5 ℃ and 170.5 ℃ could be observed in the second heating cycle which corresponded to the phase transition from crystalline state to crystalline state and from smectic C to isotropic state, respectively. Interestingly, there were two exothermic peaks at 89.7 ℃ and 144.8 ℃ which could be attributed to cold-crystallization transition peaks in the second heating cycle. Such exothermic peaks appeared in the second heating cycle might be due to the appearance of metastable structure cooling from isotropic state, which could transform into more stable well-ordered structures via endothermal recrystallization process (Fig. 1c). Only one exothermic peak at 131.9 ℃ in the first cooling cycle could be detected which corresponded to the phase transition from isotropic state to smectic C phase (Fig. 1c). In the DSC curve of compound BI-CHO-4 , four endothermic peaks at 77.3 ℃, 127.8 ℃, 140.5 ℃ and 148.4 ℃ could be observed in the second heating cycle. These peaks at 77.3 ℃ and 127.8 ℃ could be ascribe to the phase transition from crystalline state to crystalline state. The peaks at 140.5 ℃ and 148.4 ℃ could be assigned to the phase transition from crystalline state to smectic C phase and from smectic C phase to isotropic state (Fig. 1d). In the second heating cycle, two exothermic peaks at 139.1 ℃ and 73.1 ℃ could be observed which corresponded to the phase transition from isotropic state to smectic C phase and from smectic C phase to crystalline state, respectively (Fig. 1d). The DSC data were well in line with the observations under POM further demonstrating that compound BI-CN-4 is monotropic liquid crystal whereas compound BI-CHO-4 is enantiotropic liquid crystal (Fig. 1a-1d). The difference in thermal behavior and self-assembly behavior of both mesogens could be attributed to the position of cyano group. The cyano group which is adjacent to the phenylpyrene unit could increase the molecular conjugation and finally lead to close molecular stacking. Therefore BI-CN-4 exhibited much higher melting temperature than that of BI-CHO-4 and BI-CHO-4 was more likely to form lamellar structure. 2.3. Solvatochromic properties The solvatochromic behavior of the pyrene-based liquid crystals was examined in some common organic solvents with different polarities by using absorption and emission spectra (Fig. S1, Fig. 2 and Table S1-2). Both compounds BI-CN-4 and BI-CHO-4 display only one absorption band at about 345-359 nm and 362-370 nm, respectively and the absorption bands of BI-CN-4 and BI-CHO-4 display little transformation with the change in solvent polarities indicating that the electronic configurations in the ground state are very stable. Both compounds BI-CN-4 and BI-CHO-4 also display one emission band at 450 nm and 465 nm in non-polar cyclohexane, respectively. The fluorescence color of compound BI-CN-4 is relatively weak, in contrast, the fluorescence color of compound BI-CHO-4 is blue (inset of Fig. 2a-2b). The emission bands of compounds BI-CN-4 and BI-CHO-4 gradually red-shift, the fluorescence color of compound BI-CHO-4 gradually turn yellow-green, but the fluorescence color of compound BI-CN-4 is hardly detected due to the weak emission in solutions with the increase in solvent polarities (from cyclohexane to dimethyl formamide). These observations indicated that positive solvatochromic behavior could be achieved in BI-CN-4 and BI-CHO-4 , which might be assigned to the intramolecular charge transfer. The highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) were obtained by density functional theory (DFT) to visualize the intramolecular charge transfer effect. The HOMOs of compounds BI-CN-4 and BI-CHO-4 were primarily situated on the pyrene unit and only a little situated on the cyanostilbene unit. On the contrary, the HUMOs of BI-CN-4 and BI-CHO-4 were primarily situated on the cyanostilbene unit and only a little were situated on the pyrene unit. By comparing the emission spectra of both BI-CN-4 and BI-CHO-4 , compound BI-CN-4 exhibited more distinct solvatochromism behavior, which could be assigned to the more distinct charge transfer. The hole-electron examination was employed to explore the differences of charge transfer between BI-CN-4 and BI-CHO-4 by using Multiwfn (Fig. 2c). [34] The hole-electron distributions at the excited state demonstrated that the negative charges of BI-CN-4 and BI-CHO-4 primarily situated on cyanoethylene and the positive charges of BI-CN-4 and BI-CHO-4 primarily situated on pyrene. Therefore, both compounds exhibited distinct charge separation at the excited state. The C hole - C ele diagrams demonstrated that the charge centroids of BI-CN-4 and BI-CHO-4 were individual and the D indexes that represented the distance between the centroids of C hole and C ele showed big difference. The D indexes were simulated to be 6.61 Å for S 0 →S 1 excitation of BI-CN-4 and 4.09 Å for S 0 →S 1 excitation of BI-CHO-4 , respectively (Fig. 2d). The big difference of D indexes indicated that different charge transfer after excitation for both compounds. [35] Therefore, a smaller distance between electron acceptor (cyano group) and electron donor (pyrene) had more greater effect on the charge transfer. 2.4. AIE and AIEE behaviors Cyanostilbene a typical of AIE and AIEE materials on account of distorted conformation which could restrict intramolecular rotation. In order to explore the AIE and AIEE properties of BI-CN-4 and BI-CHO-4 , the absorption and emission spectra in THF-H 2 O mixtures with varying proportions of water ( f w : 0%-90%) were studied. The absorption spectra in pure THF exhibited two peaks at 278 nm and 345 nm for BI-CN-4 and also two peaks at 280 nm and 362 nm for BI-CHO-4 (Fig. S2). The position and intensity of absorption spectra displayed little change with increasing the f w from 0% to 90%. The level-off tails of the long-wavelength region emerged when the f w was higher than 30%, indicating that the aggregates were formed in the mixtures. [36] The emission spectrum of BI-CN-4 in pure THF exhibited an extremely weak band at 484 nm and the emission spectra displayed little fluctuation when the f w was lower than 30% (Fig. 3a-b). The position of the emission spectra displayed a little red shift due to the polarity and the intensity showed a little increase due to the formation of aggregates, when the f w was ranged from 30% to 70%. The emission intensity further increased with increasing the f w and finally reached to the maximum when the f w was 90% (Fig. 3a-b). In contrast, the emission spectrum of BI-CHO-4 in pure THF exhibited a strong band at 493 nm. The position of the emission spectra also showed a little red shift and the intensity showed a little decrease due to the increased polarity of the mixture, when the f w was ranged from 0% to 60% (Fig. 3c-d). The emission intensity further increased with increasing the f w and finally reached to the maximum when the f w was 90% (Fig. 3c-d). These observations in absorption and emission spectra in THF-H 2 O mixtures of BI-CN-4 and BI-CHO-4 demonstrated that BI-CN-4 and BI-CHO-4 exhibited AIE and AIEE behaviors, respectively. 2.4. Mechanochromic behaviors The mechanochromic behaviors of BI-CN-4 and BI-CHO-4 in of the pristine, ground, and fumed states were also explored by the absorption and emission spectra, fluorescence lifetimes, powder X-ray diffraction analysis and differential scanning calorimetry curves. Both compounds BI-CN-4 and BI-CHO-4 display a blue-emission band at 472 nm and 474 nm in the pristine state, respectively (Fig. S3 and Fig. 4). The fluorescence lifetimes of BI-CN-4 and BI-CHO-4 were about 1.89 ns and 2.40 ns, respectively (Fig. 4e-f). After BI-CN-4 was ground in mortar for several minutes, the emission spectrum showed obvious red shift to 498 nm, the fluorescent color turned into blue-green and the fluorescence lifetimes showed little fluctuation to 1.87 ns (Fig. 4a, 4c and 4e). For BI-CHO-4 , the emission spectrum showed more obvious red shift to 512 nm, the fluorescent color turned yellow-green and the fluorescence lifetimes showed obvious increase to 4.26 ns (Fig. 4b, 4d and 4f) after grinding in mortar for several minutes. After fuming with n -hexane for several minutes, the emission spectra of BI-CN-4 and BI-CHO-4 recovered to the original state. The reversible changes could be attributed to the change of molecular stacking and molecular conformation caused by the external mechanical force and further solvent fuming. In order to explain the mechanism of mechanochromic behavior of BI-CN-4 and BI-CHO-4 , powder X-ray diffraction and differential scanning calorimetry investigations were conducted. The powder X-ray diffraction patterns of BI-CN-4 and BI-CHO-4 in the pristine state clearly showed several intensive and sharp reflection peaks at 5°-40° indicating well-ordered molecular stacking (Fig. 5a-b). After BI-CN-4 and BI-CHO-4 were ground in mortar for several minutes, some characteristic peaks became weak and even disappeared demonstrating that the molecular stacking became disordered. Further, after fuming with n -hexane for several minutes, the peaks appeared again demonstrating that the well-ordered molecular stacking recovered by fuming (Fig. 5a-b). Additionally, the DSC curves of the pristine solid of BI-CN-4 and BI-CHO-4 for the first heating cycle only showed one endothermic peak at 173.4 ℃ and 150.7 ℃, respectively, which could be attributed the phase transition from solid state to liquid state (Fig. 5c-d). After BI-CN-4 and BI-CHO-4 were ground in mortar for several minutes, the DSC curves of the ground solid of BI-CN-4 and BI-CHO-4 for the first heating cycle appeared one new exothermic peak at 53.6 ℃ and 54.1 ℃, respectively, which could be attibuted to thermal crystallization peak, demonstrating that the amorphous state could be formed by grinding (Fig. 5c-d). Therefore, the change in molecular stacking and formation of amorphous state by grinding would be responsible for the mechanochromic behaviors. Conclusion In summary, two novel rod-like liquid crystalline molecules with distinct self-assembly, solvatochromic properties, AIE behaviors and mechanochromic behaviors were designed. The positional isomerism of cyano group induced the transition of from the monotropic smectic C phase to enantiotropic smectic C phase. The cyano group which was adjacent to pyrene induced more distinct solvatochromic behaviors due to the charge transfer. The distorted molecular conformation endowed the rod-like liquid crystalline molecules AIE or AIEE behaviors. The mechanochromic luminescence investigation demonstrated that both liquid crystalline molecules exhibited reversible mechanochromism natures with high contrast change in fluorescent color due to the change in molecular stacking and the state of solid. These investigations give a new strategy for the design and synthesis of multifunctional materials. Declarations Conflict of interest: There are no conflicts of interest to declare. Consent to Participate : All authors approved their consent in the participation of their work in the manuscript. Consent for Publication : All authors read the manuscript and given their consent to publish the work. Data Availability : All data generated and analyzed during the current study are included in the article and Supplementary Materials file. Ethical Approval : This article does not contain any studies with human or animal subjects. Authorship contribution Zhigao Liu : Data curation; Investigation. Xiaotong Liu : Investigation; Writing-original draft; Writing-review & editing. Yurun Liang : Investigation. Shunbo Zhang : Investigation. Xiuning Hu : Investigation. Yulong Xiao : Conceptualization; Funding acquisition; Methodology; Project administration; Resources; Writing-original draft; Writing-review & editing. Funding This work was supported by China West Normal University Doctor Startup Fund (No. 412821), Major project funds of Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province (No. CSPC202101), The Science and Technology Department of Sichuan Province (No. 2022NSFSC1237) and Innovation and entrepreneurship training program for college students (No. S202310638056). References S. Tan, K. Jinnai, R. Kabe, C. Adachi. Long-persistent luminescence from an exciplet-based organic light-emitting diodes, Adv. Mater. 33 2008844 (2021). https://doi.org/10.1002/adma.202008844 R. De, S.K. Pal, Self-assembled discotics as molecular semiconductors, Chem. Commun. 59 3050-3066 (2023). https://doi.org/10.1039/D2CC06763C M.Q. Li, Q. Zhang, J.R. Wang, X.F. Mei, Mechanochromism triggered fluorescent color switching among polymorphs of a natural fluorescence pigment, Chem. Commun. 52 11288-11291 (2016). https://doi.org/10.1039/C6CC04958C G. Huang, Q. Xia, W. Huang, J. Tian, Z. He, B.S. 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Supplementary Files SupplementaryMaterials.doc Scheme1.png Scheme 1 Synthetic route of unsymmetric pyrene-based mesogens BI-CN-4 and BI-CHO-4: Reagents and conditions : ( i ) Pd(PPh 3 ) 4 , THF, H 2 O, K 2 CO 3 , 78 °C; ( ii ) Acetonitrile, K 2 CO 3 , 80 °C; ( iii ) t -BuOK, t -BuOH, 70 °C. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-4089030","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":279074992,"identity":"cd0e26ed-d743-41cc-9979-efb948ad09de","order_by":0,"name":"Zhigao Liu","email":"","orcid":"","institution":"China West Normal University","correspondingAuthor":false,"prefix":"","firstName":"Zhigao","middleName":"","lastName":"Liu","suffix":""},{"id":279074993,"identity":"cf906139-608c-422d-b7f6-a0e9fd8005c0","order_by":1,"name":"Xiaotong Liu","email":"","orcid":"","institution":"China West Normal University","correspondingAuthor":false,"prefix":"","firstName":"Xiaotong","middleName":"","lastName":"Liu","suffix":""},{"id":279074994,"identity":"436e94e1-72e7-4c26-8137-254ca47b2de7","order_by":2,"name":"Yurun Liang","email":"","orcid":"","institution":"China West Normal University","correspondingAuthor":false,"prefix":"","firstName":"Yurun","middleName":"","lastName":"Liang","suffix":""},{"id":279074995,"identity":"71513f6b-c218-40d8-b509-188ef8916af0","order_by":3,"name":"Shunbo Zhang","email":"","orcid":"","institution":"China West Normal University","correspondingAuthor":false,"prefix":"","firstName":"Shunbo","middleName":"","lastName":"Zhang","suffix":""},{"id":279074996,"identity":"e4544651-d7a5-4fb4-a1ae-481c0ce846c3","order_by":4,"name":"Xiuning Hu","email":"","orcid":"","institution":"China West Normal University","correspondingAuthor":false,"prefix":"","firstName":"Xiuning","middleName":"","lastName":"Hu","suffix":""},{"id":279074997,"identity":"dfa25379-4736-456d-a4f0-c1975457813b","order_by":5,"name":"Yulong Xiao","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA2UlEQVRIie3QMQrCMBSA4UggXVK7tlqsR2gpFI8TEDq1u4OUTpmKs8dwck4JxsUDdHPo6uIiLoppcHJIOjrkXx6EfJAXAGy2f8wBgA3Tg2r4oZnALwkoUQSPIqpYKALMJOY4Yc9NFaUCilm3XWHg8NNBTxBpmwtPjgKt00LIh+E87/QEMuZSRrJrk/QFksTHmYFM6vZFK5JSHPPiPYpAxl0KSYxw0pd0BAnkLjyUu+yHXcqdj5Fpl+m5Se83+WMehWJePKqF53ChJUv2e4J014ei2nTDZrPZbB9qrUW4kWEjDgAAAABJRU5ErkJggg==","orcid":"","institution":"China West Normal University","correspondingAuthor":true,"prefix":"","firstName":"Yulong","middleName":"","lastName":"Xiao","suffix":""}],"badges":[],"createdAt":"2024-03-13 04:23:19","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4089030/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4089030/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":52788508,"identity":"11c568ed-2483-4302-8d1c-d808a78a4190","added_by":"auto","created_at":"2024-03-15 19:34:10","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1713824,"visible":true,"origin":"","legend":"\u003cp\u003ePOM textures of (a) \u003cstrong\u003eBI-CN-4\u003c/strong\u003e cooling at 120 ℃ and (b) \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e cooling at 100 ℃; DSC thermograms of (c) \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and (d) \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e in the first cooling and second heating cycles.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4089030/v1/70df6c65372a53e68fe7ad1d.png"},{"id":52788509,"identity":"6fb00bd6-8e10-4194-af85-a60864d50a1f","added_by":"auto","created_at":"2024-03-15 19:34:10","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1574199,"visible":true,"origin":"","legend":"\u003cp\u003eEmission spectra of (a) \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and (b) \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e in different solvents, the insets of (a) and (b) show the corresponding fluorescence images; (c) the optimized molecular configuration, HOMO and LUMO of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e; (d) distributions of hole and electron, and \u003cem\u003eC\u003c/em\u003e\u003csub\u003ehole\u003c/sub\u003e and \u003cem\u003eC\u003c/em\u003e\u003csub\u003eele\u003c/sub\u003e of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e.\u0026nbsp;\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4089030/v1/0f91f485404a915edbb05672.png"},{"id":52788507,"identity":"464df32f-d1ff-402e-b3b3-b4a74ee09620","added_by":"auto","created_at":"2024-03-15 19:34:10","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":871897,"visible":true,"origin":"","legend":"\u003cp\u003eEmission spectra of (a) \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and (c) \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e in the mixture of water and tetrahydrofuran with alterative \u003cem\u003ef\u003c/em\u003e\u003csub\u003ew\u003c/sub\u003e; (b) and (d) plot of relative emission peak intensity \u003cem\u003evs\u003c/em\u003e \u003cem\u003ef\u003c/em\u003e\u003csub\u003ew\u003c/sub\u003e, the insets of (b) and (d) show the corresponding fluorescence images.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4089030/v1/f0f5f3ee62effb8b32d908ad.png"},{"id":52788510,"identity":"08040c38-767e-418e-9c7e-a726568fc97b","added_by":"auto","created_at":"2024-03-15 19:34:11","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1049120,"visible":true,"origin":"","legend":"\u003cp\u003eNormalized emission spectra of (a) \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and (b) \u003cstrong\u003eBI-CHO-4\u003c/strong\u003ein solid states (as-prepared, ground and fumed); the fluorescence images of (c) \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and (d) \u003cstrong\u003eBI-CHO-4\u003c/strong\u003ein solid states before being ground and fumed; Fluorescence decay profiles of(e) \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and (f) \u003cstrong\u003eBI-CHO-4 \u003c/strong\u003esolid states before/after being ground.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4089030/v1/f4b09284cb4558f2d4bd8dea.png"},{"id":52788506,"identity":"c8c68de0-197a-40ca-81cc-c6de999308aa","added_by":"auto","created_at":"2024-03-15 19:34:10","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":519807,"visible":true,"origin":"","legend":"\u003cp\u003epowder X-ray diffractions of (a) \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and (b) \u003cstrong\u003eBI-CHO-4\u003c/strong\u003ein solid states (as-prepared, ground and fumed); DSC curves of(c) \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and (d) \u003cstrong\u003eBI-CHO-4 \u003c/strong\u003esolid states before/after being ground.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4089030/v1/e03a090395c4ce0e27b7aa7e.png"},{"id":53094890,"identity":"bdff314d-f79e-4464-a588-3421bff81d7f","added_by":"auto","created_at":"2024-03-20 13:44:47","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4452125,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4089030/v1/a9d6037e-4f7f-4f91-96d4-a86c36ac1d79.pdf"},{"id":52788511,"identity":"f8efb7ee-8842-4366-98ad-6fdbeb5cd970","added_by":"auto","created_at":"2024-03-15 19:34:11","extension":"doc","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":8035840,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterials.doc","url":"https://assets-eu.researchsquare.com/files/rs-4089030/v1/5d4d39afab0e5f12db15b2f0.doc"},{"id":52788505,"identity":"0db4939a-dff5-4c18-af2f-577a1f458585","added_by":"auto","created_at":"2024-03-15 19:34:09","extension":"png","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":19479,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eScheme 1\u003c/strong\u003e Synthetic route of unsymmetric pyrene-based mesogens \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e: \u003cem\u003eReagents and conditions\u003c/em\u003e: (\u003cem\u003ei\u003c/em\u003e) Pd(PPh\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e4\u003c/sub\u003e, THF, H\u003csub\u003e2\u003c/sub\u003eO, K\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e, 78 °C; (\u003cem\u003eii\u003c/em\u003e) Acetonitrile, K\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e, 80 °C; (\u003cem\u003eiii\u003c/em\u003e) \u003cem\u003et\u003c/em\u003e-BuOK, \u003cem\u003et\u003c/em\u003e-BuOH, 70 °C.\u003c/p\u003e","description":"","filename":"Scheme1.png","url":"https://assets-eu.researchsquare.com/files/rs-4089030/v1/2b8a1030f13f19e948534135.png"}],"financialInterests":"No competing interests reported.","formattedTitle":"The effect of positional isomerism of cyano group of the rod-like mesogens on the liquid crystalline and optical characteristics","fulltext":[{"header":"Introduction","content":"\u003cp\u003eOrganic luminescent materials\u0026nbsp;with tunable emission intensity and emitting color in different have obtained much attention and also have been widely applied in various fields including organic light-emitting diodes (OLEDs),\u003csup\u003e[1,2]\u003c/sup\u003e anti-counterfeiting technology\u003csup\u003e[3,4]\u003c/sup\u003e and biological imaging\u003csup\u003e[5,6]\u003c/sup\u003e and others. Among these materials, mechanochromic luminescent materials showed distinct emitting-color changes as a result of the change in molecular packings, intermolecular interactions, morphological structures and molecular conformations induced by mechanical stimuli.\u003csup\u003e[7]\u003c/sup\u003e In general, the mechanochromic luminescent materials always exhibit strong solid-state emission. However, the conventional organic luminescent materials suffered aggregation-caused quenching (ACQ) due to the strong intermolecular \u0026pi;-\u0026pi; stacking interactions in aggregated state, which hindered the development of mechanochromic luminescent materials. Fortunately, the opposite phenomenon of ACQ called aggregation-induced emission (AIE) or aggregation-induced enhanced emission (AIEE) was discovered by Tang et al. and Park et al. in 1-methyl-1,2,3,4,5-pentaphenylsilole and cyanostilbene molecules.\u003csup\u003e[8,9]\u003c/sup\u003e Due to the good performance, the AIE- or AIEE- active units are regarded as promising candidates for constructing mechanochromic luminescent materials. Therefore, various mechanochromic luminescent materials containing AIE- or AIEE- active units including cyanostilbene, tetraphenylethylene, dibenzofulvene and \u003cem\u003e\u0026beta;\u003c/em\u003e-diketonate and others have been developed.\u003csup\u003e[10,11,12,13,14,15,16,17]\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eMolecular engineering plays an important role in constructing novel photoelectric materials and self-assembly materials. Tuning the chemical structures including the aromatic units, linking groups, appending groups and even the position of lateral/terminal groups of the organic molecules, especially the organic liquid crystalline molecules would induce the great difference of self-assemblies, phase transition temperature, functional properties and others.\u003csup\u003e[18,19,20]\u003c/sup\u003e For instance, Tschierske and Cheng reported some bolapolyphilic liquid crystals and the changes of the type of rod-like aromatic units and the length of lateral chains led to various mesophases including lamellar phases, honeycomb columnar phases and different cubic phases.\u003csup\u003e[21,22,23,24]\u003c/sup\u003e In addition, we also reported some rod-like molecules in which the change of the position of aromatic units led to the transition of mesophases, different AIE behaviors and stimulus-response behaviors.\u003csup\u003e[25,26]\u003c/sup\u003e Therefore, the exploration of the influence of chemical structures of liquid crystalline molecules on the properties and the understanding of structure-property relationships would be important for the construction of different materials.\u003c/p\u003e\n\u003cp\u003ePyrene and cyanostilbene are important functional groups having the good characteristics of high fluorescence quantum efficiency, easy modification and electronic properties to be employed to construct functional materials. Recently, some liquid crystalline materials or/and luminescent materials containing both pyrene and cyanostilbene have been constructed and usually the cyanostilbene was appended to the 1- position, 1,6- positions or/and 1,3,6,8- positions.\u003csup\u003e[\u003c/sup\u003e\u003csup\u003e26\u003c/sup\u003e\u003csup\u003e,27,28,29,30]\u003c/sup\u003e These materials exhibited wonderful self-assembly properties and luminescent properties. Inspired by these good performances, we employed pyrene and cyanostilbene as aromatic units to fabricate two rod-like liquid crystals. And the influence of the positional isomerism of cyano group on the liquid crystalline self-assembly, solvatochromic properties, AIE behaviors and mechanochromic behaviors was investigated. Such investigations have benefits on the understanding of the relationship between chemical structure-properties and also have a great effect on novel multifunctional materials.\u0026nbsp;\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003e\u003cstrong\u003e2.1. Synthesis\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe synthesis of the rod-like mesogens mainly involves three key steps including Suzuki coupling reaction, etherification reaction and Knoevenagel condensation reaction as shown in Scheme 1. Firstly, Suzuki coupling reaction pyren-1-ylboronic acid and 4-bromobenzaldehyde or 2-(4-bromophenyl)acetonitrile took place to yield compounds \u003cstrong\u003e1\u003c/strong\u003e.\u003csup\u003e[31,32]\u003c/sup\u003e In addition, 4-hydroxybenzaldehyde and 2-(4-hydroxyphenyl)acetonitrile were \u003cem\u003eo\u003c/em\u003e-alkylated with bromotetradecane in the presence of base to yield compounds \u003cstrong\u003e2\u003c/strong\u003e. Finally, compounds \u003cstrong\u003e1\u003c/strong\u003e were reacted with compounds \u003cstrong\u003e2\u003c/strong\u003e under the Knoevenagel condensation condition to yield the rod-like liquid crystals\u003cstrong\u003e\u0026nbsp;BI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e. The synthetic details and corresponding spectrum analysis results were provided in the Supplemental Materials.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.2. Liquid-crystalline properties\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eThe thermal behavior and optical textures of compounds \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e were studied by using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). On heating process of compound \u003cstrong\u003eBI-CN-4\u003c/strong\u003e under POM, the crystal transformed into the isotropic state at about 175 ℃. On first cooling process from isotropic state of compound \u003cstrong\u003eBI-CN-4\u003c/strong\u003e, irregular schlieren texture could be observed indicating that compound \u003cstrong\u003eBI-CN-4\u003c/strong\u003e could self-organize into smectic C phase (Fig. 1a).\u003csup\u003e[33]\u003c/sup\u003e On heating process of compound \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e under POM, the crystal gradually transformed into mesophase at about 144 ℃ and then transformed into isotropic state at about 148 ℃, indicating that the mesophase range is very narrow. On cooling process from isotropic state of compound \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e, distinct schlieren texture could be observed indicating that compound \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e could also self-assemble into smectic C phase (Fig. 1b).\u003csup\u003e[\u003c/sup\u003e\u003csup\u003e16\u003c/sup\u003e\u003csup\u003e]\u003c/sup\u003e The phase transition temperatures were further detected by DSC (Fig. 1c and 1d). In the DSC curve of compound \u003cstrong\u003eBI-CN-4\u003c/strong\u003e, two endothermic peaks at 140.5 ℃ and 170.5 ℃ could be observed in the second heating cycle which corresponded to the phase transition from crystalline state to crystalline state and from smectic C to isotropic state, respectively. Interestingly, there were two exothermic peaks at 89.7 ℃ and 144.8 ℃ which could be attributed to cold-crystallization transition peaks in the second heating cycle. Such exothermic peaks appeared in the second heating cycle might be due to the appearance of metastable structure cooling from isotropic state, which could transform into more stable well-ordered structures \u003cem\u003evia\u003c/em\u003e endothermal recrystallization process (Fig. 1c). Only one exothermic peak at 131.9 ℃ in the first cooling cycle could be detected which corresponded to the phase transition from isotropic state to smectic C phase (Fig. 1c). In the DSC curve of compound \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e, four endothermic peaks at 77.3 ℃, 127.8 ℃, 140.5 ℃ and 148.4 ℃ could be observed in the second heating cycle. These peaks at 77.3 ℃ and 127.8 ℃ could be ascribe to the phase transition from crystalline state to crystalline state. The peaks at 140.5 ℃ and 148.4 ℃ could be assigned to the phase transition from crystalline state to smectic C phase and from smectic C phase to isotropic state (Fig. 1d). In the second heating cycle, two exothermic peaks at 139.1 ℃ and 73.1 ℃ could be observed which corresponded to the phase transition from isotropic state to smectic C phase and from smectic C phase to crystalline state, respectively (Fig. 1d). The DSC data were well in line with the observations under POM further demonstrating that compound \u003cstrong\u003eBI-CN-4\u003c/strong\u003e is monotropic liquid crystal whereas compound \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e is enantiotropic liquid crystal (Fig. 1a-1d). The difference in thermal behavior and self-assembly behavior of both mesogens could be attributed to the position of cyano group. The cyano group which is adjacent to the phenylpyrene unit could increase the molecular conjugation and finally lead to close molecular stacking. Therefore \u003cstrong\u003eBI-CN-4\u003c/strong\u003e exhibited much higher melting temperature than that of \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e was more likely to form lamellar structure.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.3. Solvatochromic properties\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe solvatochromic behavior of the pyrene-based liquid crystals was examined in some common organic solvents with different polarities by using absorption and emission spectra (Fig. S1, Fig. 2 and Table S1-2). Both compounds \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e display only one absorption band at about 345-359 nm and 362-370 nm, respectively and the absorption bands of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e display little transformation with the change in solvent polarities indicating that the electronic configurations in the ground state are very stable. Both compounds \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e also display one emission band at 450 nm and 465 nm in non-polar cyclohexane, respectively. The fluorescence color of compound \u003cstrong\u003eBI-CN-4\u003c/strong\u003e is relatively weak, in contrast, the fluorescence color of compound \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e is blue (inset of Fig. 2a-2b). The emission bands of compounds \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e gradually red-shift, the fluorescence color of compound \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e gradually turn yellow-green, but the fluorescence color of compound \u003cstrong\u003eBI-CN-4\u003c/strong\u003e is hardly detected due to the weak emission in solutions with the increase in solvent polarities (from cyclohexane to dimethyl formamide). These observations indicated that positive solvatochromic behavior could be achieved in \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e, which might be assigned to the intramolecular charge transfer. The highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) were obtained by density functional theory (DFT) to visualize the intramolecular charge transfer effect. The HOMOs of compounds \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e were primarily situated on the pyrene unit and only a little situated on the cyanostilbene unit. On the contrary, the HUMOs of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e were primarily situated on the cyanostilbene unit and only a little were situated on the pyrene unit. By comparing the emission spectra of both \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e, compound \u003cstrong\u003eBI-CN-4\u003c/strong\u003e exhibited more distinct solvatochromism behavior, which could be assigned to the more distinct charge transfer. The hole-electron examination was employed to explore the differences of charge transfer between \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e by using Multiwfn (Fig. 2c).\u003csup\u003e[34]\u003c/sup\u003e The hole-electron distributions at the excited state demonstrated that the negative charges of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e primarily situated on cyanoethylene and the positive charges of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e primarily situated on pyrene. Therefore, both compounds exhibited distinct charge separation at the excited state. The \u003cem\u003eC\u003c/em\u003e\u003csub\u003ehole\u003c/sub\u003e-\u003cem\u003eC\u003c/em\u003e\u003csub\u003eele\u003c/sub\u003e diagrams demonstrated that the charge centroids of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e were individual and the D indexes that represented the distance between the centroids of \u003cem\u003eC\u003c/em\u003e\u003csub\u003ehole\u003c/sub\u003e and \u003cem\u003eC\u003c/em\u003e\u003csub\u003eele\u003c/sub\u003e showed big difference. The D indexes were simulated to be 6.61 \u0026Aring; for S\u003csub\u003e0\u003c/sub\u003e\u0026rarr;S\u003csub\u003e1\u003c/sub\u003e excitation of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and 4.09 \u0026Aring; for S\u003csub\u003e0\u003c/sub\u003e\u0026rarr;S\u003csub\u003e1\u003c/sub\u003e excitation of \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e, respectively (Fig. 2d). The big difference of D indexes indicated that different charge transfer after excitation for both compounds.\u003csup\u003e[35]\u003c/sup\u003e Therefore, a smaller distance between electron acceptor (cyano group) and electron donor (pyrene) had more greater effect on the charge transfer.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.4. AIE and AIEE behaviors\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCyanostilbene a typical of AIE and AIEE materials on account of distorted conformation which could restrict intramolecular rotation. In order to explore the AIE and AIEE properties of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e, the absorption and emission spectra in THF-H\u003csub\u003e2\u003c/sub\u003eO mixtures with varying proportions of water (\u003cem\u003ef\u003c/em\u003e\u003csub\u003ew\u003c/sub\u003e: 0%-90%) were studied. The absorption spectra in pure THF exhibited two peaks at 278 nm and 345 nm for \u003cstrong\u003eBI-CN-4\u0026nbsp;\u003c/strong\u003eand also two peaks at 280 nm and 362 nm for \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e (Fig. S2). The position and intensity of absorption spectra displayed little change with increasing the \u003cem\u003ef\u003c/em\u003e\u003csub\u003ew\u003c/sub\u003e from 0% to 90%. The level-off tails of the long-wavelength region emerged when the \u003cem\u003ef\u003c/em\u003e\u003csub\u003ew\u003c/sub\u003e was higher than 30%, indicating that the aggregates were formed in the mixtures.\u003csup\u003e[36]\u003c/sup\u003e The emission spectrum of \u003cstrong\u003eBI-CN-4\u0026nbsp;\u003c/strong\u003ein pure THF exhibited an extremely weak band at 484 nm and the emission spectra displayed little fluctuation when the \u003cem\u003ef\u003c/em\u003e\u003csub\u003ew\u003c/sub\u003e was lower than 30% (Fig. 3a-b). The position of the emission spectra displayed a little red shift due to the polarity and the intensity showed a little increase due to the formation of aggregates, when the \u003cem\u003ef\u003c/em\u003e\u003csub\u003ew\u003c/sub\u003e was ranged from 30% to 70%. The emission intensity further increased with increasing the \u003cem\u003ef\u003c/em\u003e\u003csub\u003ew\u003c/sub\u003e and finally reached to the maximum when the \u003cem\u003ef\u003c/em\u003e\u003csub\u003ew\u003c/sub\u003e was 90% (Fig. 3a-b). In contrast, the emission spectrum of \u003cstrong\u003eBI-CHO-4\u0026nbsp;\u003c/strong\u003ein pure THF exhibited a strong band at 493 nm. The position of the emission spectra also showed a little red shift and the intensity showed a little decrease due to the increased polarity of the mixture, when the \u003cem\u003ef\u003c/em\u003e\u003csub\u003ew\u003c/sub\u003e was ranged from 0% to 60% (Fig. 3c-d). The emission intensity further increased with increasing the \u003cem\u003ef\u003c/em\u003e\u003csub\u003ew\u003c/sub\u003e and finally reached to the maximum when the \u003cem\u003ef\u003c/em\u003e\u003csub\u003ew\u003c/sub\u003e was 90% (Fig. 3c-d). These observations in absorption and emission spectra in THF-H\u003csub\u003e2\u003c/sub\u003eO mixtures of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e demonstrated that \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u0026nbsp;\u003c/strong\u003eexhibited AIE and AIEE behaviors, respectively.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.4. Mechanochromic behaviors\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe mechanochromic behaviors of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e in of the pristine, ground, and fumed states were also explored by the absorption and emission spectra, fluorescence lifetimes, powder X-ray diffraction analysis and differential scanning calorimetry curves. Both compounds \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e display a blue-emission band at 472 nm and 474 nm in the pristine state, respectively (Fig. S3 and Fig. 4). The fluorescence lifetimes of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e were about 1.89 ns and 2.40 ns, respectively (Fig. 4e-f). After \u003cstrong\u003eBI-CN-4\u003c/strong\u003e was ground in mortar for several minutes, the emission spectrum showed obvious red shift to 498 nm, the fluorescent color turned into blue-green and the fluorescence lifetimes showed little fluctuation to 1.87 ns (Fig. 4a, 4c and 4e). For \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e, the emission spectrum showed more obvious red shift to 512 nm, the fluorescent color turned yellow-green and the fluorescence lifetimes showed obvious increase to 4.26 ns (Fig. 4b, 4d and 4f) after grinding in mortar for several minutes. After fuming with \u003cem\u003en\u003c/em\u003e-hexane for several minutes, the emission spectra of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e recovered to the original state. The reversible changes could be attributed to the change of molecular stacking and molecular conformation caused by the external mechanical force and further solvent fuming.\u003c/p\u003e\n\u003cp\u003eIn order to explain the mechanism of mechanochromic behavior of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e, powder X-ray diffraction and differential scanning calorimetry investigations were conducted. The powder X-ray diffraction patterns of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e in the pristine state clearly showed several intensive and sharp reflection peaks at 5\u0026deg;-40\u0026deg;\u0026nbsp;indicating well-ordered molecular stacking (Fig. 5a-b). After \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e were ground in mortar for several minutes, some characteristic peaks became weak and even disappeared demonstrating that the molecular stacking became disordered. Further, after fuming with \u003cem\u003en\u003c/em\u003e-hexane for several minutes, the peaks appeared again demonstrating that the well-ordered molecular stacking recovered by fuming (Fig. 5a-b). Additionally, the DSC curves of the pristine solid of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e for the first heating cycle only showed one endothermic peak at 173.4 ℃ and 150.7 ℃, respectively, which could be attributed the phase transition from solid state to liquid state (Fig. 5c-d). After \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e were ground in mortar for several minutes, the DSC curves of the ground solid of \u003cstrong\u003eBI-CN-4\u003c/strong\u003e and \u003cstrong\u003eBI-CHO-4\u003c/strong\u003e for the first heating cycle appeared one new exothermic peak at 53.6 ℃ and 54.1 ℃, respectively, which could be attibuted to thermal crystallization peak, demonstrating that the amorphous state could be formed by grinding (Fig. 5c-d). Therefore, the change in molecular stacking and formation of amorphous state by grinding would be responsible for the mechanochromic behaviors.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn summary, two novel rod-like liquid crystalline molecules with distinct self-assembly, solvatochromic properties, AIE behaviors and mechanochromic behaviors were designed. The positional isomerism of cyano group induced the transition of from the monotropic smectic C phase to enantiotropic smectic C phase. The cyano group which was adjacent to pyrene induced more distinct solvatochromic behaviors due to the charge transfer. The distorted molecular conformation endowed the rod-like liquid crystalline molecules AIE or AIEE behaviors. The mechanochromic luminescence investigation demonstrated that both liquid crystalline molecules exhibited reversible mechanochromism natures with high contrast change in fluorescent color due to the change in molecular stacking and the state of solid. These investigations give a new strategy for the design and synthesis of multifunctional materials.\u003c/p\u003e\n"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eConflict of interest:\u003c/strong\u003e There are no conflicts of interest to declare.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Participate\u003c/strong\u003e: All authors approved their consent in the participation of their work in the manuscript.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp; \u003cstrong\u003eConsent for Publication\u003c/strong\u003e: All authors read the manuscript and given their consent to publish the work.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e: All data generated and analyzed during the current study are included in the article and Supplementary Materials file.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval\u003c/strong\u003e: This article does not contain any studies with human or animal subjects.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eAuthorship contribution\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eZhigao Liu\u003c/strong\u003e: Data curation; Investigation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eXiaotong Liu\u003c/strong\u003e: Investigation;\u0026nbsp;Writing-original draft;\u0026nbsp;Writing-review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eYurun Liang\u003c/strong\u003e: Investigation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eShunbo Zhang\u003c/strong\u003e: Investigation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eXiuning Hu\u003c/strong\u003e: Investigation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eYulong Xiao\u003c/strong\u003e: Conceptualization; Funding acquisition; Methodology; Project administration; Resources; Writing-original draft; Writing-review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by China West Normal University Doctor Startup Fund (No. 412821), Major project funds of Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province (No. CSPC202101), The Science and Technology Department of Sichuan Province (No. 2022NSFSC1237) and Innovation and entrepreneurship training program for college students (No. S202310638056).\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eS. 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C. 2 2243-2250 (2021). https://doi.org/10.1039/C3TC31638F\u003c/li\u003e\n\u003c/ol\u003e\n"},{"header":"Scheme 1","content":"\u003cp\u003eScheme 1 is available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Liquid crystal, Aggregation-induced emission, Mechanochromism, Cyanostilbene, Pyrene","lastPublishedDoi":"10.21203/rs.3.rs-4089030/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4089030/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTwo novel cyanostilbene-based rod-like mesogens named \u003cb\u003eBI-CN-4\u003c/b\u003e and \u003cb\u003eBI-CHO-4\u003c/b\u003e consisting of one terminal pyrene and one terminal alkyl chain were prepared by Suzuki coupling and Knoevenagel reactions. The influence of the positional isomerism of cyano group of cyanostilbene unit on the liquid crystalline characteristics, photophysical characteristics, and mechanochromism characteristics is explored by using POM, DSC, XRD, UV spectra, PL spectra, DFT and TD-DFT calculations. \u003cb\u003eBI-CN-4\u003c/b\u003e in which the cyano group is adjacent to the phenylpyrene unit exhibits monotropic smectic C phase, whereas \u003cb\u003eBI-CHO-4\u003c/b\u003e in which the cyano group is far away from the phenylpyrene unit exhibits enantiotropic smectic C phase. \u003cb\u003eBI-CN-4\u003c/b\u003e exhibits more distinct positive solvatochromism due to more distinct intramolecular charge transfer. Both compounds exhibit AIE or AIEE behavior and reversible mechanochromism behavior due to twisted molecular configurations. These results demonstrated that the distinct molecular design could endow organic molecules with multifunctional properties and potentials.\u003c/p\u003e","manuscriptTitle":"The effect of positional isomerism of cyano group of the rod-like mesogens on the liquid crystalline and optical characteristics","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-03-15 19:34:03","doi":"10.21203/rs.3.rs-4089030/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"313aa422-35f5-4ca8-a022-4ec3b413f240","owner":[],"postedDate":"March 15th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-03-20T13:36:39+00:00","versionOfRecord":[],"versionCreatedAt":"2024-03-15 19:34:03","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4089030","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4089030","identity":"rs-4089030","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}