Photoswitching of Dihydroazulene Derivatives in Liquid‐Crystalline Host Systems

Photoswitches and dyes in the liquid‐crystalline nematic phase have the potential for use in a wide range of applications. A large order parameter is desirable to maximize the change in properties induced by an external stimulus. A set of photochromic and nonphotochromic dyes were investigated for these applications. It was found that a bent‐shaped 7‐substituted dihydroazulene (DHA) photoswitch exhibited liquid‐crystalline properties. Further investigation demonstrated that this material actually followed two distinct reaction pathways on heating, to a deactivated form by a 1,5‐sigmatropic shift and to a linear 6‐substituted DHA. In addition, elimination of hydrogen cyanide from the photoactive DHA gave both bent and linear azulene dyes. In a nematic host that has no absorbance around 350 nm, it was found that only the linear DHA derivative has nematic properties; however, both 6‐ and 7‐substituted DHAs were found to have large order parameters. In the nematic host, ring opening of either DHA to the corresponding vinylheptafulvene resulted in a decrease in dichroic order parameter and an unusually fast back‐reaction to a mixture of both DHAs. Likewise, only the linear azulene derivative showed mesomorphic properties. In the same nematic host, large order parameters were also observed for these dyes. Double action: Derivatives of the dihydroazulene/vinylheptafulvene photo‐/thermoswitch were prepared and studied for their liquid‐crystalline and switching properties in liquid‐crystal host systems (see figure). These properties were found to be strongly dependent on the position of a substituent in the seven‐membered ring of the dihydroazulene.