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

Dr. Changmin Lee Dr. Seyoung Chung Dr. Hayoung Song Prof. Young Min Rhee Prof. Eunsung Lee Prof. Taiha Joo   

 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.   https://doi.org/10.1002/cptc.202000254