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 p‐p* 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, RH 5660‐1720 nm) which underwent dispersion (RH, 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 p‐p* transition and an increase in the intensity of n‐p* transitions.

Studies on the effect of fluorine on the interaction of different metal ions with a fluorinated azobenzene-Schiff base: intramolecular C–F activation under polar solvent

“…13,14 These groups of compounds have very characteristic physical and chemical features, which allow them to be used in organic and conducting materials 15,16 and these complexes are also found to be good building blocks for molecular magnets, self-assembly applications, etc. 17–20 Many Schiff bases appear to have very good biological properties like antibacterial, antifungal, anticancer, diuretic activities, etc. 21,22 The azobenzene Schiff base is an extension of the above concept, with the added benefit of extended conjugation to the Schiff base, and it is also widely utilized in different applications.…”