Excited-State Dynamics in Organic Photochemistry

Organic photochemistry centres on the behaviour of molecules after absorption of photons, which promotes electrons into higher electronic states. In these excited states, molecules undergo ultrafast relaxation processes such as internal conversion and intersystem crossing, or they engage in productive photochemical reactions including bond cleavage, proton-coupled electron transfer and isomerisation. The balance between non-radiative decay and photoreactivity is governed by molecular structure, solvent interactions and excited-state potential-energy surfaces. Recent advances in ultrafast spectroscopy and computational dynamics have revealed mechanistic pathways on timescales from femtoseconds to milliseconds, clarifying how transient species give rise to selective chemical transformations. Phenomena such as excited-state aromaticity switching and non-Kasha emission have emerged as design principles for novel photofunctional materials. Understanding these dynamics underpins developments in solar energy conversion, photoredox catalysis, organic light-emitting diodes and bioimaging, by enabling control over excitation pathways, energy dissipation and bond activation in complex organic systems.

Research from Nature Portfolio

Recent studies have introduced a method to achieve net heterolysis of symmetric σ-bonds through a two-stage sequence of thermal homolysis followed by stimulated doublet–doublet electron transfer. In this mechanism, photoexcitation selectively activates one radical fragment, breaking symmetry and permitting heterolytic bond fission even in homopolar systems. The approach exploits wavelength-tunable excitation of distinct radical intermediates to achieve highly chemospecific bond cleavage within multifunctional organic frameworks, offering a new strategy for photochemical bond activation.

Excited-State Dynamics in Organic Photochemistry publication trend

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

Technical terms

Excited state: A higher electronic energy level populated after absorption of a photon.

Internal conversion: A non-radiative process in which an excited molecule relaxes to a lower electronic state of the same spin multiplicity.

Intersystem crossing: A spin-forbidden transition between excited states of different spin multiplicities (e.g. singlet to triplet).

σ-Bond heterolysis: Cleavage of a sigma bond yielding two ionic fragments by unequal electron redistribution.

Kasha’s rule: The empirical observation that fluorescence typically arises from the lowest excited state of a given multiplicity.

Anti-Kasha emission: Radiative decay occurring from an excited state higher than the lowest within the same spin manifold.

References

1. Unimolecular net heterolysis of symmetric and homopolar σ-bonds. Nature (2024).
2. Excited-State (Anti)Aromaticity Explains Why Azulene Disobeys Kasha’s Rule. Journal of the American Chemical Society (2023).
3. Non-Kasha fluorescence of pyrene emerges from a dynamic equilibrium between excited states. The Journal of Chemical Physics (2022).
4. Excitation Wavelength-Dependent Photochemistry. Photochem (2024).