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 <i>T</i> < 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 (<i>t</i> < 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