Photoinduced Intramolecular Proton Transfer of Phenol-Containing Ligands and Their Zinc Complexes

N,N‘-Bis(salicylidene)hydrazine (L_I), a bis-2-hydroxybenzene-type ligand H₂L, its tert-butyl derivative (L_II), and the corresponding Zn²⁺ complexes of the type Zn₂(LH)₂L (Zn−I and Zn−II) were synthesized. The molecular structure of Zn−II was determined by X-ray crystallography at −170 °C. The photoreactions of the four compounds in solution were studied by time-resolved UV−vis spectroscopy using nanosecond laser pulses. A weak but strongly Stokes shifted fluorescence signal of the ligands L_I or L_II is suggested to be due to excited-state intramolecular proton transfer (ESIPT) from the phenolic hydroxy group to the nitrogen of the methine bond in analogy to the fast enol → keto tautomerization of other 2-hydroxybenzenes. A transient with the maximum at 480 nm, bleaching at 370 nm, and a lifetime of 0.01−0.3 ms is attributed to the trans-keto tautomer, formed via internal conversion. The decay occurs via trans → cis isomerization and proton back-transfer to the enol form. Quenching by water indicates a proton-catalyzed reaction. To account for similar fluorescence and transient properties in the cases of the Zn²⁺ complexes, a photoinduced tautomerism at one of the two free phenolic hydroxy groups is proposed. The rapid ESIPT followed by a relatively slow relaxation process is reversible.