Excited State Proton Transfer of Quinone Cyanine 9: Implications on the Origin of Super‐Photoacidity. Issue 3 (23rd November 2020)

• Record Type: Journal Article
• Authors: Lee, Changmin; Chung, Seyoung; Song, Hayoung; Rhee, Young Min; Lee, Eunsung; Joo, Taiha
• Is Part Of: ChemPhotoChem. Volume 5:Issue 3(2021)
• Page Start: 245
• Page End: 252
• Publication Date: 2020-11-23
• Subjects: excited states -- proton transfer -- photoacidity -- time-resolved spectroscopy -- solvation dynamics; Photochemistry -- Periodicals; Periodicals; Electronic journals; 541.35
• DOI: 10.1002/cptc.202000254
• Languages: English
• ISSNs: 2367-0932

Abstract

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. Abstract : To unravel the origin of "super"‐photoacidity, we investigated the excited‐state proton transfer dynamics of a quinone cyanine dye QCy9, a well‐known "super"‐photoacid. Time‐resolved fluorescence measurements with high enough time resolution to attain precise kinetics and quantum chemical calculations have been performed. We established that truly ultrafast (40 fs) and complete proton transfer, and significant (2700 cm −1 ) solvation of the conjugate base in the excited state make QCy9 a "super"‐photoacid.