Excited State Proton Transfer of Quinone Cyanine 9: Implications on the Origin of Super-Photoacidity

Abstract

Quinone cyanine 9 (QCy9) is one of the strongest photoacids known to date. The origin of "super"-photoacidity was investigated via time-resolved fluorescence (TF) with high time resolution to accurately resolve the ultrafast dynamics of the excited-state proton transfer (ESPT) to solvent. The ESPT of QCy9 in water is found to proceed fully by two ultrafast time constants of 40 fs (65 %) and 200 fs (35 %). More importantly, the initially created base form of QCy9 in the excited state undergoes a significant dynamic red shift of over 2700 cm(-1) by solvation. We suggest that the entirely ultrafast reaction rate and the massive solvation of the conjugate base could make QCy9 a particularly strong photoacid. Quantum chemical calculations show that structural diversity leads to the nonexponential behavior of the ESPT dynamics.