Light Triggered Reversible Aggregation/Dispersion of Hydroxy Azo‐benzenes During Photo Switching: Solvent, Ions Assisted Dispersion, and Induced Quenching Emission

Abstract Hydroxy azo‐benzenes are very well known for their rapid trans ⇆ cis photoisomerization under polar solvents. In contrast, we synthesized two hydroxymethylated‐ hydroxyazobenzene derivatives and investigated their light‐triggered photoisomerization and stability under polar solvents. The result showed just opposite behavior and very slow trans ⇆ cis isomerization was observed (minutes‐days). Thus, the UV‐vis spectrum revealed a very low decrease in the absorbance of π–π* transition. Surprisingly, the isomerization process became faster with the addition of ionic species. Here, we attempted to understand the underlying cause of the unusual photo‐switching behavior. The presence of hydroxymethyl and fluorine substituents was found to have a significant effect on the stability of the trans and photo‐isomerized cis products. Here, it was confirmed with NMR and DLS studies that the unusual photostability of trans compounds was caused by polar solvent‐assisted aggregation (hydrodynamic radius, R H 5660–1720 nm) which underwent dispersion (R H, 220–68 nm) and formed a significant stable solvated photoproduct under photoirradiation. Furthermore, this aggregation‐dispersion was found to be very slowly reversible. Further, the fluorescence emission demonstrated a characteristic dispersion‐induced quenching. Regular photoswitching was observed under non‐polar conditions in benzene where an expected blue shift of π–π* transition and an increase in the intensity of n–π* transitions.