Excimer Dynamics in Aromatic Molecular Systems

Excimers—short-lived excited dimers formed when a photon promotes one aromatic monomer into an excited electronic state that overlaps with a neighbouring ground-state partner—play a pivotal role in photophysics and materials science. In aromatic molecular systems, excimer formation and decay are governed by a delicate interplay of π–π stacking, charge-transfer interactions and vibronic coupling. Following photoexcitation, two monomers approach a bound excited-state geometry, characterised by new intermolecular bonding that does not exist in the ground state. The resulting excimer state typically emits broad, red-shifted fluorescence and relaxes on picosecond to nanosecond timescales. Molecular orientation (eclipsed versus slipped), substituent effects and the local environment (solvent polarity, temperature, matrix rigidity) tune both the formation yield and radiative versus non-radiative decay pathways. Advances in ultrafast spectroscopy and time-resolved computational methods have revealed that conical intersections and charge-resonance contributions modulate excimer lifetimes and emission efficiencies. These dynamic processes underpin applications ranging from organic light-emitting diodes and fluorescence sensors to photodynamic therapy and supramolecular assemblies. Understanding how substituents, heteroatom incorporation and aggregate morphology steer excimer behaviour offers routes to design bespoke materials with controllable excited-state lifetimes and emission colours.

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Recent studies have shown that nitrogen substitution in pyrene derivatives can stabilise distinct excimer geometries. Time-dependent density functional theory investigations reveal that charge-transfer contributions and dipole–dipole interactions govern the balance between dark eclipsed and bright twisted excimer structures, thereby affecting emission barriers and quantum yields.

Investigations into perylene di-imide aggregates have demonstrated how charge-resonance excitations influence singlet and triplet exciton character along slip-stacked motifs. Analysis in terms of Frenkel and charge-transfer diabatic states discloses that small longitudinal displacements can trigger H- to J-aggregate switching and alter excimer-like emission pathways in both spin manifolds.

Foundational work on sulfur-bridged naphthalene dimers established that oxidation state of the bridge atom controls non-radiative decay through conical intersections. By tuning the electron lone-pair participation in excited-state relaxation, these studies provide strategies to design strongly photoluminescent dimers with tailored excimer dynamics.

Excimer Dynamics in Aromatic Molecular Systems publication trend

The graph below shows the total number of articles in excimer dynamics in aromatic molecular systems across all publications each year (not limited to Nature Index journals).

Technical terms

Excimer: A transient excited-state dimer formed between an excited chromophore and a ground-state partner, emitting characteristic red-shifted fluorescence.

π–π* transition: An electronic excitation from a bonding π orbital to an antibonding π* orbital within or between aromatic systems.

Charge-transfer (CT) excitation: An excited-state in which electronic density shifts from one monomer to another, contributing to excimer binding and emission.

Conical intersection: A point of degeneracy between potential energy surfaces that facilitates ultrafast non-radiative decay.

H- and J-aggregates: Types of molecular stacks where dipole–dipole couplings lead to blue-shifted (H) or red-shifted (J) optical bands, affecting excimer formation.

References

1. Eclipsed and Twisted Excimers of Pyrene and 2-Azapyrene: How Nitrogen Substitution Impacts Excimer Emission. Molecules (2024).
2. Impact of Charge-Resonance Excitations on CT-Mediated J-Type Aggregation in Singlet and Triplet Exciton States of Perylene Di-Imide Aggregates: A TDDFT Investigation. Computation (2022).
3. The photophysics of naphthalene dimers controlled by sulfur bridge oxidation. Chemical Science (2017).