Frequency and time-domain analysis of excited-state intramolecular proton transfer. Double-proton transfer in 2,5-bis(2-benzoxazolyl)-hydroquinone?

2,5-bis(2-benzoxazolyl)-hydroquinone has two reactive sites for excited-state intramolecular proton transfer (ESIPT) AH…B→A…HB. To carry out a frequency and time-domain analysis of the PT the reactive system is modeled in terms of diabatic educt and product potentials for each proton plus a single heavy atom coordinate per proton. The latter are assumed to modulate the proton motion which is treated in the adiabatic approximation. By dissipative propagation of this four-mode system, the structured absorption spectrum of the isolated molecule is simulated. Comparison with the observed vibronic spectra suggests the existence of a double-proton transferred state in the isolated molecule. This is confirmed by semiempirical quantum-chemical calculations.