Excited-State Proton Transfer Mechanisms in Hydrogen-Bonded Molecular Systems

Excited-state proton transfer (ESPT) describes the relocation of a proton along hydrogen bonds immediately after a molecule absorbs light and enters an electronically excited state. In hydrogen-bonded molecular systems, photoexcitation often strengthens specific hydrogen bonds, reshaping the potential energy surfaces and lowering energy barriers for proton motion. Mechanistic pathways may be intramolecular or intermolecular, proceeding via single or double proton shifts, and may occur stepwise or through concerted transitions. These proton transfers frequently couple to intramolecular charge transfer (ICT) or twisted ICT (TICT) processes, which together determine fluorescence lifetimes, emission wavelengths, and non‐radiative decay channels. Understanding ESPT is essential for designing ratiometric fluorescent probes, optimising optoelectronic materials and elucidating proton dynamics in biological macromolecules.

Research from Nature Portfolio

Recent studies have leveraged high‐level quantum‐chemical calculations and spectroscopic methods to unravel ESPT pathways in model systems. Work on bis‐salicylidene benzothiazole (BTS) derivatives has shown that dual intramolecular hydrogen bonds exhibit differential strengthening in the first excited state, yet only one bond provides a kinetically favourable single-proton transfer route. Investigations into anthraquinone derivatives combined time‐dependent density functional theory (TDDFT) with atoms‐in‐molecules analyses to distinguish exothermic, barrierless ESPT processes from endothermic, high‐barrier transfers, highlighting solvent‐dependent modulation of proton movement. Foundational theoretical exploration of quercetin established the existence of two tautomeric excited‐state forms and demonstrated that one form is preferentially generated due to enhanced hydrogen bonding and lower activation energy, providing a blueprint for multi‐proton transfer mechanisms.

Excited-State Proton Transfer Mechanisms in Hydrogen-Bonded Molecular Systems publication trend

The graph below shows the total number of articles in excited-state proton transfer mechanisms in hydrogen-bonded molecular systems across all publications each year (not limited to Nature Index journals).

Technical terms

Excited-state proton transfer (ESPT): Proton relocation along a hydrogen bond following electronic excitation, often altering photophysical properties.

Potential energy surface (PES): A representation of system energy as a function of nuclear coordinates, delineating reaction pathways.

Intramolecular charge transfer (ICT): Redistribution of electron density within a molecule upon excitation, frequently coupled to proton motion.

Twisted intramolecular charge transfer (TICT): An ICT process accompanied by significant molecular torsion, which can modulate fluorescence efficiency.

Tautomerisation: Isomerisation between proton‐shifted structural forms, often occurring in excited electronic states.

Hydrogen bond strengthening: Increase in interaction energy of a hydrogen bond upon excitation, lowering barriers to proton transfer.

References

1. Fluorescent Probes Based on Charge and Proton Transfer for Probing Biomolecular Environment. The Chemical Record (2023).
2. Theoretical insights into excited-state hydrogen bonding effects and intramolecular proton transfer (ESIPT) mechanism for BTS system. Scientific Reports (2020).
3. Combined TDDFT and AIM Insights into Photoinduced Excited State Intramolecular Proton Transfer (ESIPT) Mechanism in Hydroxyl- and Amino-Anthraquinone Solution. Scientific Reports (2017).
4. Theoretical Study of the ESIPT Process for a New Natural Product Quercetin. Scientific Reports (2016).
5. Monitoring penetratin interactions with lipid membranes and cell internalization using a new hydration-sensitive fluorescent probe. Organic & Biomolecular Chemistry (2014).
6. Paying Comprehensive Attention to the Temperature-Dependent Dual-Channel Excited-State Intramolecular Proton Transfer Mechanism of Fluorescence Ratio Probe BZ-DAM. International Journal of Molecular Sciences (2023).
7. Computational Insights into Excited State Intramolecular Double Proton Transfer Behavior Associated with Atomic Electronegativity for Bis(2′-benzothiazolyl)hydroquinone. Molecules (2023).