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 capacitors1,2 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: [1] Pan, X.; Wang, M.; Fang, X.Q.; Zhang, C.N.; Huo, Z.P.; Dai, S.Y.; Sci. China: Chem., 2013, 56, 1463–1469. [2] Sasi, R.; Chandrasekhar, B.; Kalaiselvi, N.; Devaki, S.J.; Adv. Sustainable Syst., 2017, 1, 1600031. [3] Axenov, K.V., Laschat, S.; Materials, 2011, 4, 206. [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.