Vibronic exciton

• Comment: There are too many basic errors of science here. As the simplest case, the definition of an excition in the second paragraph is incorrect. Significantly more background reading is needed, I strongly suggest getting help from experts at WT:Physics or WT:Chemistry. Ldm1954 (talk) 22:20, 31 October 2026 (UTC)

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A vibronic exciton is an excited state of a molecular system in which electronic excitation is coupled to molecular vibrations. These can occur due to interactions between the electronic and vibrational motions, creating quantum states that can no longer be considered purely electronic or vibrational. Vibronic excitons have been studied in molecular aggregates and photosynthetic light harvesting systems. This is due to the role they play in excitation energy transfer and optical spectroscopy.

Background

An exciton is a correlated electron–hole excitation in a material. In molecular systems, an electronic excitation can be localised on an individual molecule or become delocalised over several interacting molecules. Such delocalised excitations in molecular aggregates are commonly described using the Frenkel exciton model.

Molecular vibrations can interact with electronic excitations through vibronic coupling. This interaction can produce excited states with mixed electronic and vibrational character. In molecular aggregates, the resulting states can involve both intermolecular electronic interactions and molecular vibrational degrees of freedom.

Physical Description

The behaviour of a vibronic exciton can be described using a quantum-mechanical model that considers electronic states, vibrational modes and their interaction.

A simplified Hamiltonian for such a system can be written as:

$H=H_{\mathrm{e}\mathrm{l}}+H_{\mathrm{v}\mathrm{i}\mathrm{b}}+H_{\mathrm{e}\mathrm{l}\mathrm{-}\mathrm{v}\mathrm{i}\mathrm{b}}$

(where $H_{\mathrm{e}\mathrm{l}}$ represents the electronic part of the system, $H_{\mathrm{v}\mathrm{i}\mathrm{b}}$ describes molecular vibrations, and $H_{\mathrm{e}\mathrm{l}\mathrm{-}\mathrm{v}\mathrm{i}\mathrm{b}}$ represents the coupling between them.)

When this occurs interactions between the three will cause the states to contain electronic and vibrational components. Interactions between electronic states in molecular aggregates will also affect the distribution of the states between adjacent molecules.

The properties of the states will depend on the strength of the electronic coupling between the chromophores, vibrational frequency, molecular geometry and strength of the vibronic coupling.

Vibronic excitons in molecular aggregates

Due to interactions between adjacent molecules, vibronic excitons can be studied in molecular aggregates.

There are many different types of molecular aggregates but two common ones are H and J aggregates. The optical properties will depend on the molecular orientation within the aggregate. H-aggregates tend to have a blue-shifted absorption band whereas J-aggregates will typically have a red-shifted absorption band from the molecular transition.

Vibronic coupling will cause the absorption and emission spectra to change. The molecular packing, electronic coupling, vibrational motion and intermolecular charge transfer have been heavily studied in theoretical work and spectroscopy.

Experimental investigation

Vibronic excitons can be studied using spectroscopy.

Transient absorption spectroscopy can be used to study the changes in optical absorption due to the excitation with short laser pulses. This provides information on the population of the excited states.

Additionally, two dimensional electronic spectroscopy can be used to look at interactions and coherence in excited states. These are very useful for looking at ultrafast processes within molecular aggregates and photosynthetic complexes.

A theoretical model of a femtosecond coherence spectroscopy was presented by Alexander J. Rode, Paul C. Arpin and Daniel B. Turner in 2024. This looked at vibronic excitons in molecular aggregates using molecular dimer and displaced harmonic oscillator models. This looked at the spectral features that could be used to identify vibronic exciton coherences from vibrational coherences within individual molecules.

See also

• Exciton
• Vibronic coupling
• Photosynthesis

References

This article "Vibronic exciton" is from Wikipedia. The list of its authors can be seen in its historical and/or the page Edithistory:Vibronic exciton. Articles copied from Draft Namespace on Wikipedia could be seen on the Draft Namespace of Wikipedia and not main one.

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