Excited-State Dynamics in Molecular Aggregates

Excited-state dynamics in molecular aggregates encompasses the mechanisms by which electronic excitations are created, transported and quenched within assemblies of chromophores. Aggregation alters photophysical behaviour by introducing intermolecular interactions that give rise to exciton delocalisation, characteristic spectral shifts in J- and H-aggregates, excimer formation and symmetry-breaking charge separation. The interplay of electronic coupling and environmental factors governs non-radiative decay pathways, while coherent exciton transport in π-stacked architectures can occur on femtosecond timescales. Control over exciton-phonon interactions has enabled suppression of energy-gap-induced losses, facilitating efficient near-infrared emission. In multichromophoric arrays, optimal coupling also dictates singlet fission efficiency, splitting one high-energy exciton into two triplet excitons and offering routes to enhanced solar energy conversion. Understanding these processes through combined experimental and theoretical approaches is critical for the design of advanced light-harvesting systems, organic optoelectronic devices and photonic materials with tailored emission and charge-separation properties.

Research from Nature Portfolio

Recent studies have shown that tuning the strength of excitonic coupling in molecular aggregates can minimise non-radiative decay rates even as the excited-state energy gap narrows. Numerical simulations reveal a non-monotonic relationship between coupling strength and decay, identifying an optimal regime that balances energy-gap reduction with suppressed electron–phonon interactions and points to design principles for efficient near-infrared emitters.

Experimental observation of ultrafast coherent exciton dynamics in helical π-stacks has demonstrated that photoexcited energy can delocalise over multiple molecular units and migrate coherently within tens of femtoseconds. This coherent transport precedes excimer formation, underscoring how aggregate geometry and vibronic coupling shape the lifetime and mobility of Frenkel excitons.

Investigations into slip-stacked perylene bisimide trimers have elucidated multiple pathways to multiexciton generation, including direct singlet fission and excimer-mediated mechanisms. The balance between exciton delocalisation and interchromophoric interaction governs triplet pair formation, informing the development of materials for high-performance organic photovoltaics.

Excited-State Dynamics in Molecular Aggregates publication trend

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

Technical terms

Exciton: A bound state of an electron and a hole generated by photon absorption and capable of migrating through a molecular assembly.

Excitonic coupling: The interaction between transition dipoles of neighbouring chromophores that leads to delocalised excited states and spectral shifts.

J-aggregate: An ordered molecular assembly with head-to-tail dipole alignment, characterised by red-shifted and narrowed absorption bands.

H-aggregate: A molecular assembly with face-to-face dipole alignment, identified by blue-shifted absorption and suppressed fluorescence.

Excimer: A transient excited-state dimer formed when an excited molecule associates with a ground-state partner, often leading to broad, red-shifted emission.

Symmetry-breaking charge separation: The process by which an initially symmetric excited state localises positive and negative charges on different molecular units.

Singlet fission: A photophysical process in which one singlet exciton splits into two triplet excitons, potentially doubling charge-carrier yield in photovoltaic materials.

References

1. One‐Pot Synthesis and Excited‐State Dynamics of Null Exciton‐Coupled Diketopyrrolopyrroles Oligo‐Grids. Advanced Materials (2023).
2. Minimizing non-radiative decay in molecular aggregates through control of excitonic coupling. Nature Communications (2023).
3. Organic molecular aggregates: From aggregation structure to emission property. Aggregate (2021).
4. Direct observation of ultrafast coherent exciton dynamics in helical π-stacks of self-assembled perylene bisimides. Nature Communications (2015).
5. Ultrafast Excimer Formation and Solvent Controlled Symmetry Breaking Charge Separation in the Excitonically Coupled Subphthalocyanine Dimer. Angewandte Chemie International Edition (2021).
6. Exciton Delocalization Counteracts the Energy Gap: A New Pathway toward NIR-Emissive Dyes. Journal of the American Chemical Society (2021).
7. Steering the multiexciton generation in slip-stacked perylene dye array via exciton coupling. Nature Communications (2022).