Excited-State Intramolecular Proton Transfer Dynamics

Excited-state intramolecular proton transfer (ESIPT) dynamics describe the ultrafast relocation of a proton within a single molecule across an internal hydrogen bond immediately after electronic excitation. This phototautomerisation converts a ‘normal’ form into a tautomeric isomer, typically in tens to hundreds of femtoseconds, and produces characteristic large Stokes shifts and sometimes dual emission bands. The rate and yield of ESIPT are finely tuned by molecular geometry, hydrogen-bond strength, solvent polarity and, in the solid state, crystal packing. ESIPT underpins a range of applications—from highly sensitive fluorescence probes and organic light-emitting devices to mechanochromic materials and molecular lasing media—by offering rapid excited-state decay channels, environmental responsiveness and enhanced photostability. Recent progress in time-resolved spectroscopies and theoretical simulations has elucidated the mechanistic interplay between ESIPT, excimer formation and electron-induced proton motion, providing a unified picture of how structure and environment govern proton-transfer pathways in excited molecules.

Research from Nature Portfolio

Recent studies have illuminated how ESIPT can couple with other excited-state processes in the solid state and under electron impact. In well-ordered molecular crystals, time-resolved spectroscopic measurements combined with structural analysis have revealed that an initial equilibrium between normal and tautomeric excited states is established in femtoseconds, after which tight lattice packing permits the slower formation of excimer species. The resulting triple emission—originating from the normal excited form, the tautomer and the excimer—spans from blue to red wavelengths and underlies reversible mechanochromic effects when mechanical force alters crystal packing. In parallel, investigations of intramolecular electron-induced proton transfer have shown that molecules capable of ESIPT also undergo proton relocation following attachment of a low-energy electron. By integrating anion photoelectron spectroscopy with density functional theory screening, researchers have mapped a family of chromophores whose electronic signatures support both photon- and electron-driven proton transfers, pointing towards multifunctional molecular devices responsive to optical and electronic stimuli.

Excited-State Intramolecular Proton Transfer Dynamics publication trend

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

Technical terms

Excited-State Intramolecular Proton Transfer (ESIPT): A rapid proton relocation across an internal hydrogen bond within a molecule following electronic excitation, leading to distinct emissive tautomeric states.

Excimer: A short-lived excited dimer formed between an excited molecule and a ground-state partner, often emitting broad, red-shifted light.

Anion Photoelectron Spectroscopy: A technique that measures the kinetic energy of electrons ejected from anions upon photon impact, used to probe electronic states involved in electron-induced processes.

Machine-Learning Explainability (e.g. SHAP): Methods that assess the contribution of individual molecular features to predictive models, enhancing interpretability in AI-driven materials discovery.

References

1. Excited-State Intramolecular Proton Transfer: A Short Introductory Review. Molecules (2021).
2. Mechanochromism induced through the interplay between excimer reaction and excited state intramolecular proton transfer. Communications Chemistry (2019).
3. Intramolecular electron-induced proton transfer and its correlation with excited-state intramolecular proton transfer. Nature Communications (2019).
4. AI‐Powered Mining of Highly Customized and Superior ESIPT‐Based Fluorescent Probes. Advanced Science (2024).
5. Modern Theoretical Approaches to Modeling the Excited-State Intramolecular Proton Transfer: An Overview. Molecules (2021).
6. Excited-state intramolecular proton transfer of 2-acetylindan-1,3-dione studied by ultrafast absorption and fluorescence spectroscopy. Structural Dynamics (2015).
7. Natural Born Laser Dyes: Excited-State Intramolecular Proton Transfer (ESIPT) Emitters and Their Use in Random Lasing Studies. Nanomaterials (2019).