Excited-State Proton Transfer Dynamics of Fisetin in Acetonitrile and Methanol

Fisetin attracts intense attention not only due to its antioxidant and anticancer properties but also because of wide applications in fluorescence probes and sensors, which are based on the dual fluorescence induced by excited-state proton transfer (ESPT). However, to date, its ESPT dynamics remains unknown yet. In this study, we give a comprehensive investigation on ESPT dynamics of fisetin in both protic methanol and aprotic acetonitrile by using femtosecond transient absorption spectroscopy combined with time-dependent density functional theory calculations. In acetonitrile, the ESPT time constant of fisetin is 1.2 ps. In methanol, two distinct intermolecular hydrogen bonding configurations contribute to a fast (< 90 fs) and slow ESPT (11.1 ps), respectively. The slow ESPT in methanol explains the higher emission intensity of normal species than in acetonitrile. The excited-state relaxation of fisetin involves two main vibrational modes: rotation between B and C rings and butterfly-like motion of C ring. Our results give insight into the effect of solvent-solute hydrogen bonding interaction on the dual fluorescence, providing a fundamental guideline for the development of fluorescent probes and sensors based on ESPT.