Thermotropic properties of new electrochromic viologen-based ionic liquid crystals.

Thermotropic ionic liquid crystals are considered very promising functional electrolytes in DSSCs, batteries, fuel cells and capacitors^1,2 for their unique combination of anisotropic ion conductivity and high polarizability.³ Moreover, they are interesting in the optoelectronic field as electrooptical functional unit.^3-5 Many efforts have been devoted to the design of liquid crystals salts based on the viologen unit as organic cation, due to their interesting electrochromic and electrochemical properties.⁶ Most of the reported structures contain flexible chains directly linked to the nitrogens of the bipyridine rings.⁶ Their thermotropic behavior depends on the nature of the anion and of the N-substituents. A suitable choice of the latter makes access to different molecular orbital energy levels allowing different colors of the corresponding radical cation. N-Aryl substituted viologens, are of great interest because they show green colored radical cations due to the delocalization of the radical-cation over an extended p-system.⁷. Herein, we present a series of new N-aryl substituted liquid-crystalline viologens based on mono(4-alkylphenyl) and bis(4-alkylphenyl) bipyridinium cations with the same counterion but different alkyl chains (n=8, 10,12, 14), reporting their thermal behavior, their mesomorphic properties as a function of the chain length and their electrochromic properties in solution and in the LC phase.⁷

References:

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[2] Sasi, R.; Chandrasekhar, B.; Kalaiselvi, N.; Devaki, S.J.; Adv. Sustainable Syst., 2017, 1, 1600031.

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[4] Bhowmik, P.K.; Kareem, M.; Al-Karawi, M.K.M.; Killarney S.T.; Dizon, E.J.; Chang, A.; Kim, J.; Chen, S.L.; Principe, R.C.G.; Ho, A.; Han, H.; Mandal, H.D.; Cortez, R.G.; Gutierrez, B.; Mendez, K.; Sharpnack, L.; Agra-Kooijman, D. M.; Fisch, M.R.; Kumar, S.; Molecules 2020, 25(10), 2435.

[5] K. Goossen, K. Lava, C. W. Bielewski, K. Binnemans, Chem. Rev., 2016, 116, 4643.

[6] Monk, P.M.S., The viologens: physico-chemical properties, synthesis and application of the salt of 4,40-bipyridine, John Wiley and Sons, Inc., Wiley – VCH, New York, Weinheim, 1998.

[7] Veltri, L.; Cavallo, G.; Beneduci, A.; Metrangolo, P.; Corrente, G.A.; Ursini, M.; Romeo, R.; Terraneo, G.; Gabriele, B.; New J. Chem., 2019, 43, 18285-18293.