Electronic and Steric Tuning of Molecular Acidity toward Unified Models for Excited State Proton Transfer

Photoinduced proton transfer powers a myriad of functional processes from bioimaging to photocatalysis. However, the elusive structure-photoacidity and thermodynamics-kinetics relationships remain the hurdle for developing such useful tools. Herein, these problems are tackled by systematically investigating photoacids with varied strengths via substitutions on the archetypal green fluorescent protein chromophore. This study quantitatively demonstrates that the thermodynamic driving force of excited-state proton transfer (ESPT) in water is governed by electronic and steric effects exerted by the substituent. Importantly, two different treatments are proposed in calculating ESPT driving force for the fluorescent and nonfluorescent photoacids. In the latter case, the unusually fast ESPT kinetics result from the extra driving force due to the Franck-Condon excess vibrational energy besides the free energy difference, thus providing the missing link in current ESPT theory. Furthermore, the thermodynamics-kinetics relationship for ESPT is unveiled to follow the Bell-Evans-Polanyi principle. The work offers the highly desirable predictive power to engineer photoacids with strategic substituents for targeted properties.