Quantifying exciton localization and isomerization efficiency in aggregates of molecular switches

• Applicant: Dr. Evgenii Titov
• Subject Area: Theoretical Chemistry: Molecules, Materials, Surfaces ; Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 454020933

Molecular aggregates of chromophores give rise to collective electronically excited states, known as molecular excitons. The excitons play a primary role in numerous processes of biological and technological relevance initiated by photoexcitation of molecular assemblies. Examples are photosynthesis and operation of organic solar cells. These processes are remarkable demonstration of aggregation-induced photophysics. Apart from photophysical phenomena (exciton localization and transfer, internal conversion, etc.), molecular response to absorption of photons may involve the photochemical events, manifested in distinct changes of molecular structure, as is the case for widely studied azobenzene-based molecular switches capable of a reversible trans-cis photoisomerization. Aggregation of azobenzenes may impact their isomerization, e.g., hinder it, as observed experimentally for certain azobenzene-containing self-assembled monolayers. Besides steric reasons (lack of free volume needed for isomerization) the ultrafast exciton dynamics have been hypothesized to influence isomerization efficiency as well. Yet, a comprehensive insight into intertwined photophysics and photochemistry of azobenzene aggregates remains to be established. In this project, we aim at first-principles characterization of exciton states and exciton dynamics as well as photoisomerization in aggregates of azobenzene molecular switches. To this end, we will perform extensive computational study addressing stationary exciton states, time-evolving exciton localization/delocalization, and excitation energy transfer dynamics in these aggregates. Various quantum chemical methods will be applied to calculate the stationary electronically excited (exciton) states, and the nature of these states will be thoroughly examined by means of a transition density matrix analysis. Further, we will investigate the nonadiabatic molecular dynamics and corresponding exciton dynamics by means of mixed quantum-classical surface hopping calculations utilizing suitable semiempirical electronic structure methods. These calculations will shed light on the excited-state decay, structural and exciton dynamics, and will allow one to judge on the role of exciton localization and transfer in the photoisomerization. Moreover, we plan to address ultrafast photoinduced dynamics in novel covalently linked multiazobenzenes.

• DFG Programme: Research Grants