Temperature-dependent tautomer fluorescence spectra of 3′,4′-benzo-2′-hydroxychalcone: direct evidence for photoenolization followed by Z→E isomerization in the singlet manifold

Direct evidence for the S1→S1′ photoenolization of 3′,4′-benzo-2′-hydroxychalcone was obtained by the detection of the S1′→S0′ tautomer fluorescence spectra (λmax=600–610 nm) at 77–298 K. The enol-tautomer is barely fluorescent at 298 K, and the fluorescence intensity increases with decreasing temperature. The drastic increase in the S1′→S0′ fluorescence intensity with the 180→77 K temperature drop demonstrates the solvent-derived viscosity barrier for the Z→E isomerization of S1′ to generate S0″ (E-enol-tautomer). Such primary photochemical processes are supported by the detection of the respective transient absorption spectra of E- and Z-enol-tautomers (S0″ and S0′) with lifetimes of 190 and 13 μs at 298 K.