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 demonstrations 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), 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 the first funding period, we studied exciton states, exciton dynamics and photoisomerization in azobenzene aggregates using quantum chemical methods and molecular dynamics simulations in conjunction with a transition density matrix analysis. We found that ground-state conformational disorder leads to partial localization of the ππ* excitons and strong localization of the nπ* excitons. Subsequent photodynamics upon ππ* excitation results in ultrafast exciton localization (on a sub-100 fs timescale), with exciton transfer occurring in the ππ* manifold and being inhibited in the nπ* manifold. Upon nπ* excitation, extremely ultrafast (~10 fs) additional localization of initially localized nπ* excitons is observed. Moreover, quantum yields of the trans→cis photoisomerization in the aggregated state were computed using quantum mechanical description of several molecules and they were found to be lowered in comparison to the monomer. In the proposed project, we will complement the performed studies by addressing (i) exciton dynamics and isomerization in aggregates of cis-azobenzene, (ii) the dynamical role of electronic communication with a nonmetallic surface upon chemisorption thus taking into account realistic grafting on a support, and (iii) exciton states and exciton dynamics in another class of molecular photoswitches—arylazopyrazoles. The project will contribute to further understanding of aggregation / complexation effects on photochemistry and photophysics of molecular switches.

• DFG Programme: Research Grants