Multi-Stimuli Responsive Ionic Liquid Crystals: The Fluorenoviologens

Abstract

Materials capable of tunable optical absorption and fluorescence properties in response to multiple external stimuli, while providing a readable signal, have garnered significant scientific interest. Such materials hold promise for applications in wearable electronics, anticounterfeiting technologies, self-powered light sources and displays, human–machine interfaces, and intelligent sensing systems. A highly effective approach to achieving multi-stimuli optical responsiveness is to integrate various functionalities into a single structure, such as reversible electrochemistry, ion and electronic charge transport, photoluminescence, and supramolecular organization (e.g., mesomorphism). Here, we introduce a new class of thermotropic smectic ionic liquid crystals, composed of the bistriflimide salts of π-conjugated fluorenoviologen dications. The dications feature a central fluorene core functionalized in position 2,7 with two pyridine moieties, whose nitrogen atoms are alkylated with promesogenic alkyl chains of varying lengths. In their bulk liquid crystalline phases, these materials exhibit ON/OFF electrofluorochromism (under UV photoexcitation), with voltage-triggered fluorescence quenching and a shift from yellow to dark electrochromism. Additionally, they display thermofluorochromism, showing a striking fluorescence color change from green to blue on going from the crystalline solid phase at room temperature to the liquid crystalline phases at high temperatures.

Keywords

electrochromism / electrofluorochromism / ionic liquid crystals / smart materials / stimuli-responsive / thermofluorochromism

References

[1] S. Wang and M. W. Urban, “Redefining Polymer Science via Multi-Stimulus Responsiveness,” Chem 9, no. 6 (2023): 1362–1377.

[2] M. Mrinalini and S. Prasanthkumar, “Recent Advances on Stimuli-Responsive Smart Materials and Their Applications,” ChemPlusChem 84, no. 8 (2019): 1103–1121.

[3] N. Gao and C. Pan, “Intelligent Ion Gels: Design, Performance, and Applications,” SmartMat 5, no. 1 (2024): e1215.

[4] D. Yan, Z. Wang, and Z. Zhang, “Stimuli-Responsive Crystalline Smart Materials: From Rational Design and Fabrication to Applications,” Accounts of Chemical Research 55, no. 7 (2022): 1047–1058.

[5] J. Cui, Y. Li, D. Chen, T. G. Zhan, and K. D. Zhang, “Ionic Liquid-Based Stimuli-Responsive Functional Materials,” Advanced Functional Materials 30, no. 50 (2020): 2005522.

[6] J. Uchida, B. Soberats, M. Gupta, and T. Kato, “Advanced Functional Liquid Crystals,” Advanced Materials 34, no. 23 (2022): 2109063.

[7] Y. Huang, L. Ning, X. Zhang, Q. Zhou, Q. Gong, and Q. Zhang, “Stimuli-Fluorochromic Smart Organic Materials,” Chemical Society Reviews 53, no. 3 (2024): 1090–1166.

[8] X. Chen, J. Chen, X. Song, et al., “Bioinspired Mechanochromic Liquid Crystal Materials: From Fundamentals to Functionalities and Applications,” Advanced Materials 36, no. 50 (2024): 2403766.

[9] X. Wen, J. Zhang, J. Li, et al., “Bio-Inspired Cholesteric Phase Cellulose Composite With Thermochromic and Circularly Polarized Structural Color for Multilevel Encryption,” Advanced Functional Materials 34, no. 2 (2024): 2308973.

[10] Y. Wang, Y.-M. Zhang, and S. X. A. Zhang, “Stimuli-Induced Reversible Proton Transfer for Stimuli-Responsive Materials and Devices,” Accounts of Chemical Research 54, no. 9 (2021): 2216–2226.

[11] M. Blanke, T. Neumann, M. E. Gutierrez Suburu, et al., “Tuning the Fluorescence in Dynamic Covalent Bonded Liquid Crystals,” ACS Applied Materials & Interfaces 14, no. 50 (2022): 55864–55872.

[12] D. M. Agra-Kooijman, M. Mostafa, M. Krifa, L. Ohrn-McDaniel, J. L. West, and A. Jákli, “Liquid Crystal Coated Yarns for Thermo-Responsive Textile Structures,” Fibers 11, no. 1 (2023): 3.

[13] A. Seki and M. Yoshio, “Multi-Color Photoluminescence Based on Mechanically and Thermally Induced Liquid-Crystalline Phase Transitions of a Hydrogen-Bonded Benzodithiophene Derivative,” Chemphyschem 21, no. 4 (2020): 328–334.

[14] Y. Ai, Y. Li, M. H. Y. Chan, G. Xiao, B. Zou, and V. W. W. Yam, “Realization of Distinct Mechano- and Piezochromic Behaviors via Alkoxy Chain Length-Modulated Phosphorescent Properties and Multidimensional Self-Assembly Structures of Dinuclear Platinum(Ii) Complexes,” Journal of the American Chemical Society 143, no. 28 (2021): 10659–10667.

[15] K. Goossens, K. Lava, C. W. Bielawski, and K. Binnemans, “Ionic Liquid Crystals: Versatile Materials,” Chemical Reviews 116, no. 8 (2016): 4643–4807.

[16] K. V. Axenov and S. Laschat, “Thermotropic Ionic Liquid Crystals,” Materials 4, no. 1 (2011): 206–259.

[17] M. Mansueto and S. Laschat, “Ionic Liquid Crystals.” In Handbook of Liquid Crystals, eds. I. Dierking. (John Wiley & Sons, Ltd, 2014, 1–52).

[18] A. Beneduci, G. A. Corrente, and G. Chidichimo, “Electrochromic and Electrofluorescence Liquid Crystals.” In Electrochromic Smart Materials: Fabrication and Applications, eds. J. W. Xu, M. H. Chua and K. W. Shah. (The Royal Society of Chemistry, 2019, 261–292).

[19] S. Yazaki, M. Funahashi, J. Kagimoto, H. Ohno, and T. Kato, “Nanostructured Liquid Crystals Combining Ionic and Electronic Functions,” Journal of the American Chemical Society 132, no. 22 (2010): 7702–7708.

[20] A. Beneduci, S. Cospito, M. La Deda, L. Veltri, and G. Chidichimo, “Electrofluorochromism in π-conjugated Ionic Liquid Crystals,” Nature Communications 5, no. 1 (2014): 3105.

[21] A. Beneduci, S. Cospito, M. L. Deda, and G. Chidichimo, “Highly Fluorescent Thienoviologen-Based Polymer Gels for Single Layer Electrofluorochromic Devices,” Advanced Functional Materials 25, no. 8 (2014): 1240–1247.

[22] A. Beneduci, S. Cospito, A. Crispini, et al., “Switching From Columnar to Calamitic Mesophases in a New Class of Rod-Like Thienoviologens,” Journal of Materials Chemistry C 1, no. 11 (2013): 2233–2240.

[23] I. Pibiri, A. Beneduci, M. Carraro, et al., “Mesomorphic and Electrooptical Properties of Viologens Based on Non-Symmetric Alkyl/Polyfluoroalkyl Functionalization and on an Oxadiazolyl-Extended Bent Core,” Journal of Materials Chemistry C 7, no. 26 (2019): 7974–7983.

[24] L. Veltri, G. Cavallo, A. Beneduci, et al., “Synthesis and Thermotropic Properties of New Green Electrochromic Ionic Liquid Crystals,” New Journal of Chemistry 43, no. 46 (2019): 18285–18293.

[25] S. Cospito, A. Beneduci, L. Veltri, M. Salamonczyk, and G. Chidichimo, “Mesomorphism and Electrochemistry of Thienoviologen Liquid Crystals,” Physical Chemistry Chemical Physics 17, no. 27 (2015): 17670–17678.

[26] L. Veltri, V. Maltese, F. Auriemma, et al., “Mesophase Tuning in Discotic Dimers π-Conjugated Ionic Liquid Crystals Through Supramolecular Interactions and the Thermal History,” Crystal Growth & Design 16, no. 10 (2016): 5646–5656.

[27] G. A. Corrente, G. Di Maio, M. La Deda, et al., “The Rainbow Arching Over the Fluorescent Thienoviologen Mesophases,” Nanomaterials 12, no. 23 (2022): 4284.

[28] A. Boultif and D. Louër, “Powder Pattern Indexing With the Dichotomy Method,” Journal of Applied Crystallography 37, no. 5 (2004): 724–731.

[29] A. Altomare, C. Cuocci, C. Giacovazzo, et al., “EXPO2013: a Kit of Tools for Phasing Crystal Structures From Powder Data,” Journal of Applied Crystallography 46, no. 4 (2013): 1231–1235.

[30] D. Louër and A. Boultif, “Some Further Considerations in Powder Diffraction Pattern Indexing With the Dichotomy Method,” Powder Diffraction 29, no. S2 (2014): S7–S12.

[31] J. R. Blanton, R. J. Papoular, and D. Louër, “PreDICT: A Graphical User Interface to the DICVOL14 Indexing Software Program for Powder Diffraction Data,” Powder Diffraction 34, no. 3 (2019): 233–241.

[32] G. A. Corrente, D. A. González, E. Aktas, et al., “Vis–NIR Electrochromism and NIR–Green Electroluminochromism in Dual Functional Benzothiadiazole-Arylamine Mixed-Valence Compounds,” Advanced Optical Materials 11, no. 1 (2023): 2201506.

[33] J. C. de Mello, H. F. Wittmann, and R. H. Friend, “An Improved Experimental Determination of External Photoluminescence Quantum Efficiency,” Advanced Materials 9, no. 3 (1997): 230–232.

[34] G. A. Corrente, D. A. González, E. Aktas, et al., “Reversible Vis-NIR Electrochromic/Electrofluorochromic Switching in Dual-Functional Devices Modulated by Different Benzothiadiazole-Arylamine Anodic Components,” Journal of Materials Chemistry C 11, no. 48 (2023): 17115–17127.

[35] M. A. Qaddoura and K. D. Belfield, “Probing the Texture of the Calamitic Liquid Crystalline Dimer of 4-(4-pentenyloxy)benzoic Acid,” Materials 3, no. 2 (2010): 827–840.

[36] A. Nesrullajev, M. Tepe, and N. Kazanci, “Surface-Induced and Oriented Textures of Smectic Liquid Crystals: Effect of Thin Films,” Journal of Physics D: Applied Physics 35, no. 22 (2002): 2994–3001.

[37] J. Cornil, D. Beljonne, J. P. Calbert, and J. L. Brédas, “Interchain Interactions in Organic π-Conjugated Materials: Impact on Electronic Structure, Optical Response, and Charge Transport,” Advanced Materials 13, no. 14 (2001): 1053–1067.

[38] W. W. Porter, T. P. Vaid, and A. L. Rheingold, “Synthesis and Characterization of a Highly Reducing Neutral ‘Extended Viologen’ and the Isostructural Hydrocarbon 4,4' " '-Di-n-octyl-p-quaterphenyl,” Journal of the American Chemical Society 127, no. 47 (2005): 16559–16566.

[39] R. F. Winter, “Half-Wave Potential Splittings ΔE_1/2 as a Measure of Electronic Coupling in Mixed-Valent Systems: Triumphs and Defeats,” Organometallics 33, no. 18 (2014): 4517–4536.

[40] G. A. Corrente, E. Fabiano, F. Manni, et al., “Colorless to All-Black Full-NIR High-Contrast Switching in Solid Electrochromic Films Prepared With Organic Mixed Valence Systems Based on Dibenzofulvene Derivatives,” Chemistry of Materials 30, no. 16 (2018): 5610–5620.

[41] J. Rault-Berthelot and J. Simonet, “The Anodic Oxidation of Fluorene and Some of Its Derivatives,” Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 182, no. 1 (1985): 187–192.

[42] A. Beneduci, S. Cospito, D. Imbardelli, B. C. De Simone, and G. Chidichimo, “n-Type Columnar Liquid Crystal Combining Ionic and Electronic Functions,” Molecular Crystals and Liquid Crystals 610, no. 1 (2015): 108–115.

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