Electronic Excitation Properties in Molecular Systems

Electronic excitation in molecules encompasses the transitions whereby electrons absorb energy and move from occupied to unoccupied molecular orbitals, giving rise to new electronic states. These excited states underpin a wide range of phenomena, from photosynthesis and vision to organic electronics and photocatalysis. The energy, character and dynamics of these excitations depend on molecular structure, conjugation length, substituent effects and intermolecular interactions. In particular, the interplay between local excitations, charge‐transfer states and excitonic couplings dictates spectroscopic signatures and energy‐transfer pathways. Advances in quantum chemistry and time‐resolved spectroscopy have deepened our understanding of excitation energies, oscillator strengths and non‐radiative decay mechanisms. At the same time, the design of materials for light‐emitting diodes, solar cells and photocatalysts relies on the precise control of singlet and triplet excited‐state properties as well as the spatial localisation of excitons. Recent progress has therefore focused on resolving complex excited‐state character, quantifying many‐body contributions to excitation energies and mapping exciton delocalisation in molecular aggregates, providing strategies to tailor photophysical responses for technological applications.

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Technical terms

Electronic excitation: The process by which an electron absorbs energy and transitions from an occupied to an unoccupied molecular orbital, creating an excited state.

Exciton: A bound state of an excited electron and the residual hole, which can migrate through a molecular system or aggregate.

Charge‐transfer state: An excited state characterised by significant movement of electron density from one molecular fragment (donor) to another (acceptor).

HOMO/LUMO: Acronyms for the highest occupied and lowest unoccupied molecular orbitals, whose energy separation often approximates the optical gap.

Doubly excited state: An excited state in which two electrons are simultaneously promoted to higher orbitals, exhibiting distinct energy and correlation signatures from singly excited states.

Singlet/Triplet state: Excited states distinguished by the total spin of the electron pair, with singlet having antiparallel spins (total spin zero) and triplet having parallel spins (total spin one), affecting radiative lifetimes and intersystem crossing.

References

1. Classification of doubly excited molecular electronic states. Chemical Science (2023).
2. Energy Component Analysis for Electronically Excited States of Molecules: Why the Lowest Excited State Is Not Always the HOMO/LUMO Transition. Journal of Chemical Theory and Computation (2023).
3. Exciton States of Azobenzene Aggregates: A First‐Principles Study. Advanced Theory and Simulations (2023).