Switching to a Reversible Proton Motion in a Charge-Transferred Dye

We report on the steady-state, pico- and femtosecond time-resolved emission studies of 6-amino-2-(2-methoxyphenyl)benzoxazole (6A-MBO) and 6-amino-2-(2-hydroxyphenyl)benzoxazole (6A-HBO) in different solvents. We observed an intramolecular charge transfer (ICT) reaction following by slow (relatively) solvent relaxation, which happened in the same time domain for both molecules. The ultrafast ICT reaction happens in 80-140 fs whereas the solvent relaxation occurs in 0.5-1.1 ps. In 6A-MBO the excited CT species has a lifetime of ∼2.5 ns. However, in 6A-HBO and after the ICT reaction, a reversible excited-state intramolecular proton transfer (ESIPT) reaction takes place in the formed enol charge transfer (ECT*) species producing a keto (K*) type tautomer. Depending on the solvent, the forward ESIPT reaction (ECT* → K*) happens in 40-175 ps while that of the reverse one (ECT* ← K*) occurs in 240-990 ps. Kinetic isotopic effect (OH/OD exchange) study in acetone shows that the reversible ESIPT reaction occurs via tunneling, while we suggest that in acetonitrile solution it evolves along the IHB and solvent coordinates. Our results show a reversible proton motion coupled to charge-transfer reactions opening the way to new explorations of charge- and proton-transfer dynamics and spectroscopy.