Time-dependent density functional study of the electronic spectra of oligoacenes in the charge states -1, 0, +1, and +2

TL;DR

This study uses density functional theory to analyze the electronic spectra of small oligoacenes in various charge states, revealing how their absorption properties change with charge and size, relevant for understanding their electronic behavior.

Contribution

It provides a comprehensive theoretical analysis of oligoacenes' electronic spectra across multiple charge states using advanced TD-DFT methods, highlighting charge-dependent spectral features.

Findings

Doubly-ionised oligoacenes show strong near-IR to UV transitions.

Absorption cross-section decreases systematically with increasing positive charge.

Plasmon-like absorption at 17-18 eV remains relatively unaffected by charge state.

Abstract

We present a systematic theoretical study of the five smallest oligoacenes (naphthalene, anthracene, tetracene, pentacene, and hexacene) in their anionic,neutral, cationic, and dicationic charge states. We used density functional theory (DFT) to obtain the ground-state optimised geometries, and time-dependent DFT (TD-DFT) to evaluate the electronic absorption spectra. Total-energy differences enabled us to evaluate the electron affinities and first and second ionisation energies, the quasiparticle correction to the HOMO-LUMO energy gap and an estimate of the excitonic effects in the neutral molecules. Electronic absorption spectra have been computed by combining two different implementations of TD-DFT: the frequency-space method to study general trends as a function of charge-state and molecular size for the lowest-lying in-plane long-polarised and short-polarised $\pi\to\pi^\star$ electronic transitions, and the real-time propagation scheme to obtain the whole photo-absorption cross-section up to the far-UV. Doubly-ionised PAHs are found to display strong electronic transitions of $\pi\to\pi^\star$ character in the near-IR, visible, and near-UV spectral ranges, like their singly-charged counterparts. While, as expected, the broad plasmon-like structure with its maximum at about 17-18 eV is relatively insensitive to the charge-state of the molecule, a systematic decrease with increasing positive charge of the absorption cross-section between about 6 and about 12 eV is observed for each member of the class.