Excited State Intramolecular Proton Transfer Dynamics of Oxadiazole-based Dyes

Excited state intramolecular proton transfer (ESIPT) dynamics of two oxadiazole-based model compounds, 2,5-bis-[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-phenol (SOX) and 2,5-bis-[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-benzene-1,4-diol (DOX) have been investigated by time-resolved fluorescence. SOX and DOX have one and two intramolecular hydrogen bond moieties, respectively, where ESIPT may occur. Time-resolved fluorescence fully resolves the ESIPT dynamics and establishes the two-state conversion between the enol and keto isomers in the S₁ potential energy surface. The apparent inconsistency between the ESIPT rate and the steady-state spectrum is settled by invoking the conformational inhomogeneity including the intermolecular hydrogen bonding with protic solvents. The ESIPT rates of SOX and DOX are not as fast as those of typical ESIPT molecules, which indicates a finite barrier for the reaction. For DOX, ESIPT is enabled for only one hydroxyl moiety, and the ESIIPT rate is twice as fast as that of SOX. We propose a symmetric potential energy surface along the two proton transfer coordinates, and bifurcation of the initial probability density of the enol isomer into the two potential minima representing the keto isomer.