Excited-State Molecular Sensors: ESIPT and Aromaticity

Research Overview

Excited-state aromaticity provides a powerful framework for understanding and controlling photophysical and photochemical processes.

Its practical exploitation is often complicated by the relationship between favourable aromaticity in an excited state and destabilising antiaromaticity in the electronic ground state.

A particularly promising strategy is to generate aromatic stabilisation only after photoexcitation through excited-state intramolecular proton transfer (ESIPT) .

Recent computational work has demonstrated that proton-transfer-induced excited-state aromaticity can produce pronounced structural and electronic relaxation, including calculated Stokes shifts exceeding 2 eV (Chem. Sci. 2024, 15, 17918–17926) .

Coupling Proton Transfer and Excited-State Aromaticity

My research will investigate how proton transfer, electronic excitation, structural relaxation, and changes in aromaticity can be coupled and controlled through molecular design.

The objective will be to establish quantitative relationships between molecular structure, proton-transfer energetics, excited-state aromaticity, and experimentally observable photophysical properties.

Particular emphasis will be placed on understanding how chemical substitution, heteroatom incorporation, conjugation, molecular rigidity, and intramolecular hydrogen-bonding motifs tune the balance between the normal and proton-transferred excited states.

Potential-Energy Surfaces and Competing Pathways

Ground- and excited-state potential-energy surfaces will be characterised using density-functional, correlated wavefunction, and multireference methods where required.

These calculations will determine the relative energies of normal and proton-transferred structures, proton-transfer barriers, state crossings, conical intersections, and competing non-radiative pathways.

Electronic-structure analysis and excited-state aromaticity descriptors will be combined with non-adiabatic molecular dynamics to examine competition between:

• Excited-state proton transfer
• Fluorescence from normal and tautomeric structures
• Internal conversion
• Intersystem crossing
• Conical-intersection-mediated relaxation
• Environment-induced trapping or deactivation

This dynamical treatment will move beyond static assignments of aromaticity and determine how aromatic character evolves in real time following photoexcitation.

Chromophores with Large and Tunable Stokes Shifts

A major direction will be the computational discovery of new chromophores in which proton-transfer-induced aromatic stabilisation produces large and controllable Stokes shifts.

Large separation between absorption and emission can reduce spectral overlap, self-absorption, and background interference, making such molecules attractive for fluorescence sensing, imaging, and optical materials.

High-throughput calculations will be used to explore families of ESIPT-active molecules and identify the structural features controlling:

• Absorption and emission wavelengths
• Proton-transfer barriers
• Excited-state aromaticity gain
• Normal-to-tautomer emission ratios
• Radiative and non-radiative rates
• Photochemical and thermal stability

Anti-Kasha Emission and Higher Excited States

Most molecular fluorescence follows Kasha’s rule, according to which emission occurs from the lowest excited state of a given multiplicity after rapid relaxation from higher states.

Some chromophores instead display anti-Kasha emission, in which fluorescence originates partly or predominantly from a higher-lying excited state.

Work from Daniel Escudero and co-workers has demonstrated that predicting such behaviour requires a state-resolved kinetic model containing the relevant radiative and non-radiative processes rather than relying only on excited-state energy gaps (J. Phys. Chem. A 2020, 124, 7228–7237) .

Their broader analysis emphasised the central role of electron–vibrational coupling in determining whether higher-state fluorescence can compete successfully with internal conversion (Acc. Chem. Res. 2022, 55, 2698–2707) .

Non-Canonical Spin Dynamics and El-Sayed-Type Behaviour

Anti-Kasha emission and El-Sayed’s rule describe different aspects of molecular photophysics.

Kasha’s rule concerns the electronic state from which emission occurs, whereas El-Sayed’s rule provides a qualitative expectation for when intersystem crossing between singlet and triplet states may be enhanced by a change in orbital character.

Neither should be treated as an absolute law when several closely spaced states, strong vibronic coupling, proton transfer, conformational motion, or state mixing are present.

I will therefore investigate systems in which conventional Kasha and El-Sayed expectations become insufficient. Rather than assigning a pathway solely from state labels, the research will calculate and compare:

• State-specific radiative emission rates
• Internal-conversion rates
• Spin–orbit couplings
• Intersystem-crossing rates
• Electron–vibrational coupling
• Proton-transfer and structural-relaxation timescales

This will allow higher-state emission, dual fluorescence, delayed pathways, and non-canonical intersystem crossing to be distinguished on a quantitative kinetic basis.

An important caution will be to distinguish genuine higher-state emission from alternative explanations such as different protonation states, tautomers, conformers, or molecular aggregates. This issue has been highlighted in work from the Escudero group examining whether apparent excitation-dependent fluorescence is truly anti-Kasha (ChemPhotoChem 2023, DOI: 10.1002/cptc.202200262) .

Connecting Aromaticity with Anti-Kasha Behaviour

Excited-state aromaticity may provide a molecular-level mechanism for engineering the relative lifetimes of different electronic states.

A higher excited state stabilised by aromaticity may retain its energy and remain emissive, while a lower antiaromatic state may undergo strong geometric distortion and rapid non-radiative relaxation.

This connection has been demonstrated for azulene, where contrasting aromatic and antiaromatic character in the S₂ and S₁ states helps explain its characteristic anti-Kasha fluorescence (J. Am. Chem. Soc. 2023, 145, 21569–21575) .

My research will test whether proton transfer can be used to create similar state-selective aromatic stabilisation in new chromophores, thereby tuning both the emitting state and the competition between radiative and non-radiative pathways.

Environment-Responsive Fluorophores and Molecular Sensors

In the longer term, these design principles will be translated towards environment-responsive fluorophores and molecular sensing .

ESIPT is strongly influenced by hydrogen bonding, protonation, polarity, viscosity, conformational restriction, and local molecular environment.

Perturbing the proton-transfer pathway can therefore provide a sensitive optical readout.

I will investigate molecular systems in which analyte binding or environmental change modifies:

• Proton-transfer probability
• Excited-state aromaticity
• Emission wavelength
• Fluorescence intensity
• Normal-to-tautomer emission ratio
• Higher-state versus lowest-state emission

Data-Driven Chromophore Discovery

High-throughput quantum-chemical calculations and machine-learning models will be used to explore extended families of ESIPT-active and aromaticity-switching chromophores.

Initial datasets will contain molecular structures and properties such as:

• Vertical absorption energies
• Relaxed emission energies
• Proton-transfer energetics and barriers
• Aromaticity descriptors
• Oscillator strengths and radiative rates
• Internal-conversion and intersystem-crossing descriptors
• Hydrogen-bonding and environmental sensitivity
• Photochemical stability indicators

Active learning will identify regions of chemical space where additional high-level calculations provide the greatest improvement to predictive models.

Multi-objective optimisation will then search for chromophores combining large Stokes shifts, strong emission, environmental sensitivity, low non-radiative loss, and molecular stability.

Computational Framework

• Ground- and excited-state density-functional theory
• Correlated wavefunction methods
• Multireference electronic-structure calculations
• Excited-state aromaticity descriptors
• Proton-transfer reaction-pathway calculations
• Conical-intersection optimisation
• Non-adiabatic molecular dynamics
• Radiative and non-radiative rate calculations
• Spin–orbit and vibronic-coupling analysis
• High-throughput and machine-learning workflows

Principal Research Objectives

Long-Term Research Vision

This programme will connect fundamental questions in excited-state aromaticity, proton-transfer dynamics, electron–vibrational coupling, and non-canonical emission with predictive chromophore and sensor design.