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