Molecular excitons in arylazopyrazole aggregates: a quantum chemical study.

Aggregation of molecular photoswitches may affect their functionality. Fundamentally, interaction of monomers in the aggregated state results in formation of exciton states, which, in turn, govern energy and charge transfer processes in the materials made of the photoswitches. In this work, we study the exciton states of aggregates of arylazopyrazole - the photoswitch which gained popularity in last decade as an alternative to azobenzene - using quantum chemical calculations. We perform cluster excited-state calculations for aggregates including up to 32 arylazopyrazole monomers as well as periodic calculations for the crystal structure. We obtain and analyze the composition of the exciton states, exciton splittings, and monomer-to-aggregate spectral shifts, thus providing quantitative insight into the electronic states and absorption spectra of realistic arylazopyrazole aggregates.