Excited-State Aromaticity in Organic Compounds

Excited-state aromaticity refers to the transient stabilisation or destabilisation of cyclic π-electron systems upon photoexcitation. In the electronic ground state many organic compounds obey Hückel’s rule, adopting aromatic character when they possess (4n+2) π-electrons and antiaromatic character when they possess 4n π-electrons. Upon absorption of light, however, Baird’s rule predicts a reversal of these tendencies: systems with 4n π-electrons become aromatic in their lowest ππ* excited state, while those with (4n+2) π-electrons become antiaromatic. This switching of electronic character can drive or hinder photochemical processes, govern ring inversion kinetics, modulate fluorescence properties and enable novel device functionalities. Relief of excited-state antiaromaticity often underpins efficient photoreactions, while gain of excited-state aromaticity can stabilise reactive intermediates. Advances in computational methods and spectroscopic techniques have illuminated these effects in a broad variety of molecules, from small benzene derivatives to large macrocycles and stacked architectures. As a result, excited-state aromaticity has emerged as a unifying framework for understanding photoreactivity, guiding the design of light-emitting materials, photochromic switches, and photochemical transformations with applications in organic electronics, sensing and green photochemistry.

Research from Nature Portfolio

Recent studies have achieved direct energetic quantification of excited-state aromaticity by examining photoexcited chiral [4n]annulene derivatives. Computational and kinetic analyses revealed that photoexcitation lowers activation barriers significantly when planar transition states benefit from excited-state aromatic stabilisation. Another contribution demonstrated the smallest red-emitting fluorophore based on a mono-benzene core, where large Stokes shifts arise from relief of excited-state antiaromaticity rather than extended π-conjugation. Systematic modification of the core scaffold produced a full visible spectrum of emitters, illustrating a new strategy for compact light sources. In a separate advance, an antiaromatic cyclophane consisting of two face-to-face macrocycles was shown to exhibit three-dimensional aromatic current channels in both experimental measurements and theoretical calculations, confirming that close stacking of antiaromatic π-systems can generate stabilising delocalisation pathways and distinctive solid-state photophysical properties.

Excited-State Aromaticity in Organic Compounds publication trend

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

Technical terms

Aromaticity: Energetic and electronic stabilisation associated with cyclic, planar π-electron delocalisation in the ground state.

Antiaromaticity: Destabilisation arising from cyclic conjugation of 4n π-electrons, leading to paratropic ring currents and increased reactivity.

Hückel’s rule: A criterion stating that planar cyclic conjugated molecules with (4n+2) π-electrons exhibit aromatic stability in the ground state.

Baird’s rule: An excited-state counterpart to Hückel’s rule, predicting that cyclic conjugated systems with 4n π-electrons are aromatic in their lowest ππ* states.

π-Conjugation: Overlap of adjacent p-orbitals allowing delocalisation of electrons across bonds, fundamental to aromatic and antiaromatic character.

Proton-Coupled Electron Transfer (PCET): A mechanism involving simultaneous or sequential transfer of an electron and a proton, crucial in excited-state deactivation and photoredox reactions.

Excited-State Intramolecular Proton Transfer (ESIPT): A process in which a proton is transferred within a molecule upon photoexcitation, often influencing fluorescence and photochemical pathways.

References

1. Energetics of Baird aromaticity supported by inversion of photoexcited chiral [4n]annulene derivatives. Nature Communications (2017).
2. Relief of excited-state antiaromaticity enables the smallest red emitter. Nature Communications (2021).
3. Three-dimensional aromaticity in an antiaromatic cyclophane. Nature Communications (2019).
4. Excited-state antiaromaticity relief drives facile photoprotonation of carbons in aminobiphenyls. Chemical Science (2024).
5. Photochemistry Driven by Excited‐State Aromaticity Gain or Antiaromaticity Relief. Chemistry - A European Journal (2023).
6. Impact of Excited-State Antiaromaticity Relief in a Fundamental Benzene Photoreaction Leading to Substituted Bicyclo[3.1.0]hexenes. Journal of the American Chemical Society (2020).