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

## TL;DR

Researchers explain how molecular acidity affects proton transfer in excited states, offering a unified model for designing better photoacids.

## Contribution

The study introduces a unified model for excited-state proton transfer by linking electronic and steric effects with thermodynamics and kinetics.

## Key findings

- Electronic and steric effects govern the thermodynamic driving force of ESPT in water.
- Nonfluorescent photoacids show fast ESPT due to Franck-Condon vibrational energy.
- Thermodynamics-kinetics relationships follow the Bell-Evans-Polanyi principle.

## Abstract

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.

Conventional excited‐state proton transfer (ESPT) theories explain the kinetics of fluorescent photoacids, but face challenges for weakly or nonfluorescent ones. This missing link is bridged by making and dissecting series of green fluorescent protein chromophore derivatives with varied photoacidities via strategic substitutions. Structure‐photoacidity and thermodynamics‐kinetics relationships are revealed with a uniform treatment of ESPT driving force.