Exciton states of azobenzene aggregates: A first-principles study

Interaction between azobenzene-containing molecules in supramolecular structures or self-assembled monolayers (SAMs) results in the formation of molecular exciton states. These states determine photophysical and photochemical processes in such assemblies. Here, using first-principles quantum chemical calculations, we study optical spectra and exciton delocalization of the exciton states in model clusters of azobenzene molecules. Specifically, we consider one-dimensional linear chains and two-dimensional SAM-like arrangements, and compute the exciton states by means of time-dependent long-range corrected density functional theory (TD-lc-DFT) and ab initio configuration interaction singles (CIS), for clusters including up to 18 azobenzene molecules. We analyze the nature of the exciton states using transition density matrix analysis. In addition, we make a connection to periodic systems applying the Bethe–Salpeter equation (BSE) / Green’s function many-body perturbation theory (GW) approach to a selected system. We find that the brightest excitons are dominated by local excitations. The energetic location of charge transfer states in the electronic spectra of aggregates depends to a large extent on a given method and distance between nearest neighbours. Furthermore, we analyze how an excitonic delocalization pattern changes with varying molecular orientation in the unit cell of SAMs.