Comprehensive study of ultrafast excited-state proton transfer in water and D₂O providing the missing RO^-⋯H⁺ ion-pair fingerprint

Steady-state and time-resolved optical techniques were employed to study the photoprotolytic mechanism of a general photoacid. Previously, a general scheme was suggested that includes an intermediate product that, up until now, had not been clearly observed experimentally. For our study, we used quinone cyanine 7 (QCy7) and QCy9, the strongest photoacids synthesized so far, to look for the missing intermediate product of an excited-state proton transfer to the solvent. Low-temperature steady-state emission spectra of both QCy7 and QCy9 clearly show an emission band at T < 165 K in H₂O ice that could be assigned to ion-pair RO^-*⋯H₃O⁺, the missing intermediate. Room-temperature femtosecond pump-probe spectroscopy transient spectra at short times (t < 4 ps) also shows the existence of transient absorption and emission bands that we assigned to the RO ^-*⋯H₃O⁺ ion pair. The intermediate dissociates on a time scale of 1 ps and about 1.5 ps in H₂O and D₂O samples, respectively.