Light initiated E–Z and Z–E isomerization of isatinphenylsemicarbazones: Tautomeric equilibrium effect

The light and thermally initiated mutual E- and Z-isomer transformations of two efficient isatin N-phenylsemicarbazone colorimetric sensors for strongly basic anions differing in sensitivity, selectivity and sensing mechanism were investigated in solvents of different polarity. The reaction mechanism of photochemically and thermally initiated E–Z isomerization and the absence of back Z–E isomerization is discussed in terms of self-aggregate formation, hydrazide-hydrazonol tautomeric equilibrium, the role of intra- and inter-molecular hydrogen bonds, excited-state proton transfer, solvent polarity and concentration effects. The effect of the activation energies related to conformational and geometric changes of the isomers and the possible analytical application were investigated. Theoretically calculated properties are in good agreement with obtained experimental results and support the proposed reaction mechanism.