Exciton states of azobenzene aggregates: A first-principles study

Abstract

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.

Visible-Light-Induced Exciton Dynamics and Trans-to-Cis Isomerization in Azobenzene Aggregates: Insights from Surface Hopping / Semiempirical Configuration Interaction Molecular Dynamics Simulations

“…We note that in the simple exciton model (considering an H-aggregate composed of identical monomer geometries with the same non-zero transition dipole moment), a larger enhancement should occur for a smaller separation distance owing to a larger exciton splitting. 25 As discussed previously for the dimeric models, 57 the trend in absorbance observed for the nπ * band upon aggregation is determined by the extent of deviation of the monomers forming an aggregate from the equilibrium, planar C 2h geometry.…”

“…14 Moreover, several studies have reported first-principles calculations of exciton states of azobenzene aggregates and SAMs. [20][21][22][23][24][25] However, these studies address exciton states for fixed nuclear configurations of the aggregates. To go beyond this single-geometry picture, it is necessary to account for ground-state conformational (geometrical) disorder (induced by thermal fluctuations) and excited-state dynamical effects.…”

Exciton localization and transfer in azobenzene aggregates: A surface hopping molecular dynamics study

“…7,22 Furthermore, quantum chemical calculations on azobenzene dimers and larger aggregates yielded sizable exciton splittings (of several hundreds meV) for the bright, ππ * states, suggesting rapid excitation energy transfer between monomers. [23][24][25][26] The simplest theoretical modelling of the steric effects on photodynamics of a photoswitch requires a quantum mechanical (QM) description of a single chromophore, whereas an environment can be described with classical molecular mechanics (MM). This approach has been applied to reveal the role of steric hindrance for azobenzene isomerization in various systems, such as azobenzene (or its derivatives) on a surface, 27,28 in a SAM, 29 attached to RNA, 30 in a micelle, 13 and in a lipid membrane.…”