Repeated <i>trans</i>↔<i>cis</i> Isomerization and Fluorescence Regulation of a Carbazole-Based Azobenzene Derivative by Acid/Base Addition

Repeated trans↔cis isomerization and fluorescence behaviors of a carbazole-based azobenzene derivative (3CbzAzES) were investigated in toluene solutions. UV light-induced trans→cis conversion (~62%) and visible-light-induced cis→trans back isomerization (~100%) were observed through absorption spectroscopic measurements. Complete cis®trans isomerization was accelerated by thermal treatment and acid addition. Adding trifluoroacetic acid (TFA) to the cis-rich 3CbzAzES solution enhanced the fluorescence by about 10-fold and red-shifted the fluorescence wavelength, compared to respective trans- and cis-3CbzAzES solutions. Furthermore, repeated emission switching was induced by alternately adding of TFA and pyridine.