Photoisomerization and Mesophase Formation in Azo-Ionic Liquids

• Published in: Crystal growth & design 2020-01, Vol.20 (1), p.214-225
• Main Authors: Renier, Olivier, Bousrez, Guillaume, Stappert, Kathrin, Wilk-Kozubek, Magdalena, Adranno, Brando, Pei, Hanwen, Spielberg, Eike T, Smetana, Volodymyr, Mudring, Anja-Verena
• Format: Article
• Language: English
• ISSN: 1528-7483; 1528-7505; 1528-7505
• DOI: 10.1021/acs.cgd.9b01018

Ionic liquids present a versatile, highly tunable class of soft functional materials. Aside from being low melting salts, they can be endowed with additional functionalities. In N-alkylimidazolium halides, which are a prominent class of ionic liquids (ILs), the imidazolium cation was linked via an ether-bridge to an azobenzene unit in order to obtain photoresponsive materials through photoinduced trans-cis isomerization. The azobenzene unit, in turn, was modified with electron-donating or -withdrawing groups such as methyl-, tert-butyl-, methoxy-, N,N-dimethylamino, and nitro groups to study their influence on the photoisomerization and phase behavior. Endowing the imidazolium additionally with a long alkyl chain allows the materials to potentially form liquid crystalline (LC) mesophases before melting into the isotropic liquid. All studied compounds qualify as ionic liquids, and all, except for the nitro-compound, show the formation of smectic mesophases melting to the isotropic liquid. The compounds with the bulkiest aliphatic substituent, the tert-butyl, shows the lowest melting point, the largest mesophase window, and an efficient photochemical trans–cis conversion (>90%). In summary, by tuning sterically and electronically the cationic part of ILs, a photoswitchable room temperature liquid crystal could be developed and design guidelines for photoresponsive ionic liquids could be obtained.