Fluorescence studies of excited state intramolecular proton transfer (ESIPT) in molecules isolated in solid argon: 3-hydroxyflavone and 2,5-bis(2-benzoxazolyl)hydroquinone

The fluorescence properties of 3-hydroxyflavone and 2,5-bis(2-benzoxazolyl)hydroquinone were investigated in solid Ar matrixes at 15 K. In both cases dominant emission from the tautomeric form of the mols. was obsd. The excitation spectra of this fluorescence and a weak fluorescence at shorter wavelength showed that the latter is due to mols. with a different ground state, presumably dimers or conformers lacking an internal H bond. For both mols. no fluorescence from the normal form was found, indicating the absence of a barrier to ESIPT

On the barrier to excited-state intramolecular proton transfer (ESIPT): the controversial assignment of the spectra of 2,5-bis(2-benzoxazolyl)hydroquinone in solid argon

The spectra of 2,5-bis(2-benzoxazolyl)hydroquinone (I) in solid argon at 15 K reveals two species. The major species (96%) undergoes very efficient excited-state intramol. proton transfer (ESIPT) after excitation to S1. It is assigned to the isolated I mol. (solvated by argon atoms) in which ESIPT is not thermally activated. Either no energy barrier exists or it can be overcome by proton tunneling with a rate const. greater than 8 * 1010 s-1. The minor species produces only fluorescence with a normal Stokes shift, indicating the absence of ESIPT. It is tentatively assigned to a dimer of I

Excited-state intramolecular proton transfer in jet-cooled 2,5-bis(2-benzothiazolyl)hydroquinone

The fluorescence of 2,5-bis(benzothiazolyl)hydroquinone was studied for isolated mols. cooled in a supersonic free neon jet. It consists of a strongly Stokes-shifted fluorescence band, and it is assigned to the mol. which is formed by excited-state intramol. proton transfer. The fluorescence excitation spectrum shows a progression in a 114-cm-1 vibrational mode of the excited state. The obsd. spectrum is more congested than that of the parent oxazole under otherwise identical exptl. conditions. The thiazole has no significant intrinsic barrier to excited-state intramol. proton transfer. Weak vibronic bands were obsd. by fluorescence excitation in the adjoining long-wavelength spectral region which could indicate nonvertical transitions to the proton-transferred excited mol