# Signatures in Vibrational and Vibronic Spectra of Benzene Molecular Clusters

## TL;DR

This paper explores how molecular aggregation affects the vibrational and electronic spectra of benzene clusters.

## Contribution

The study reveals how aggregation and vibronic coupling influence forbidden electronic transitions and IR intensities in benzene systems.

## Key findings

- Vibronic coupling is essential for inducing forbidden transitions in benzene monomers.
- Molecular aggregation alone can induce forbidden transitions in most benzene dimers.
- Aggregation and inducing modes jointly affect the 1A1g → 1B1u transition in clusters.

## Abstract

The photoabsorption
and infrared spectra (IR) of molecular
systems
are heavily influenced by aggregation. In the electronic spectra,
the vibronic coupling effect is of utmost importance. Although treating
both effects simultaneously can be challenging, it is often the only
way to explain the experimental spectrum of molecular clusters. In
this work, we study IR spectra and the vibronic coupling effect in
the electronic photoabsorption spectra in molecular systems composed
of benzene (monomer, dimers, and crystal). Photoabsorption spectra
were generated using the direct vibronic coupling method at the density
functional theory (DFT) level. We also simulated the spectra with
the Liouville-Lanczos approach by calculating the electronic transitions
along the main inducing modes for two forbidden transitions (1A1g → 1B2u and 1A1g → 1B1u). DFT was
also applied to simulate IR spectra. For the monomer, vibronic coupling
was crucial to induce the first and second forbidden transitions.
On the other hand, molecular aggregation was sufficient to induce
the first and second forbidden transitions in almost all dimers. However,
when the vibronic coupling is evaluated for the clusters, the band
in the energy range of the 1A1g → 1B1u transition is affected both by the aggregation
itself and the inducing modes. Moreover, some inducing modes drastically
change the allowed 1A1g → 1E1u transition, depending on the dimer under study due
to symmetry breaking. In terms of IR spectra, clear signatures are
present. For instance, the intensities of the C–H stretching
modes decrease as aggregation increases. This work shows that aggregation
impacts the band shapes differently in relation to the benzene aggregate
structure and the excitation under analysis.