Total absorption spectrum of benzene aggregates obtained from two different approaches

The study of molecular aggregation effects on the electronic spectrum is essential for the development of optoelectronic devices. However, investigating the entire valence absorption spectrum of aggregates using quantum mechanical methods is a challenging task. In this work, we perform systematic simulations of the absorption spectrum of benzene molecular clusters up to 35 eV applying two approaches based on time-dependent density functional theory. The results show that depending on the dimer packing, different energy shifts occur for the symmetry allowed \(\varvec{\pi } \varvec{\rightarrow } \varvec{\pi }^{\varvec{*}}\) $\mathit{\pi }\mathbf{\to }{\mathit{\pi }}^{\mathbf{\ast }}$ transition, in comparison to the monomer. The transition intensity increases for the band around 6 eV for larger aggregates from the monomer to dimers and tetramer, indicating the occurrence of the symmetry forbidden (in \(\varvec{D}_{\varvec{6h}}\) ${\mathit{D}}_{\mathbf{6}\mathit{h}}$ point group) \(^{\varvec{1}}{\varvec{A}}_{\varvec{1g}} \varvec{\rightarrow }\) ${}^{\mathbf{1}}{\mathit{A}}_{\mathbf{1}\mathit{g}}\mathbf{\to }$ \(^{\varvec{1}}{\varvec{B}}_{\varvec{1u}}\) ${}^{\mathbf{1}}{\mathit{B}}_{\mathbf{1}\mathit{u}}$ transition. The benzene crystal exhibits a large redshift following the experimental spectrum. Also, the continuum regions of all spectra show a good agreement with the experiments both in gas and solid phases.

Geometry optimization of the monomer was carried out with Gaussian 09 software using the PBE0/def2-TZVP level of theory. We used dimers and tetramer molecular geometries extracted from the experimental crystal structure. The absorption spectra were directly obtained by the Liouville-Lanczos TDDFT approach with plane waves basis set or indirectly by TDDFT pseudo-spectra calculated in a \(\varvec{L}^{\varvec{2}}\) ${\mathit{L}}^{\mathbf{2}}$ basis followed by analytic continuation procedure to obtain complex polarizability. The former is available at Quantum ESPRESSO, and the latter was calculated using Gaussian 09 with the post-processing performed with a code previously developed in our group.