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, optical spectra and exciton delocalization of the exciton states in model clusters of azobenzene molecules are studied. Specifically, 1D linear chains and 2D SAM-like arrangements are considered, and the exciton states are computed 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. The nature of the exciton states is analyzed using transition density matrix analysis. In addition, a connection to periodic systems is made applying the Bethe-Salpeter equation (BSE)/Green's function many-body perturbation theory (GW) approach to a selected system. It is found 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 neighbors. Furthermore, it is analized 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

“…This effect is opposite to what is seen for the strong ππ* band in the UV region (Figure , right). We note that in the simple exciton model (considering an H-aggregate composed of identical monomer geometries with the same nonzero transition dipole moment), a larger enhancement should occur for a smaller separation distance owing to a larger exciton splitting . As discussed previously for the dimeric models, 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 2 h geometry.…”

“…We note that in the simple exciton model (considering an H-aggregate composed of identical monomer geometries with the same nonzero 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 2 h geometry. To quantify this deviation, we computed the NNCC dihedral angles of the monomers for the studied models for the snapshots selected from the ground-state trajectories ( Table S1 ).…”

“…, for SAMs and micellar solutions . Moreover, several studies have reported first-principles calculations of exciton states of azobenzene aggregates and SAMs. − 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

“…Two effects, steric and excitonic, are intrinsically connected, since both become stronger with decreasing intermolecular distances and thus are enhanced at large packing densities. And both effects were proposed to be responsible for hindering azobenzene isomerization in SAMs. , Furthermore, quantum chemical calculations on azobenzene dimers and larger aggregates yielded sizable exciton splittings (of several hundred meV) for the bright, ππ* states, suggesting rapid excitation energy transfer between monomers. − …”