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

Photoswitching of hydroxy azobenzene derivatives was studied in different solvents. It was observed that in polar solvents, showed unexpectedly very slow trans⇆cis isomerization. Studies showed that aggregation⇆dispersion was the cause of unusual stabilities. The presence of ionic species was found to promote photoisomerization. Regular photoswitching was observed under non-polar conditions.

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.

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