Self-assembled molecular aggregates have been known for a long time to exhibit unique optical and excitation energy transport properties, making them interesting candidates for energy applications. Nature serves as inspiration, with its many examples of efficient light-harvesting systems for photosynthesis.  Our detailed understanding of these materials has increased tremendously over the past decade, owing to a combination of ultrafast and single-object spectroscopies and microscopic modelling. In this presentation, I will address the optics and dynamics of excitons in large molecular aggregates, consisting of many thousands of molecules. In the first part of the talk, I will report on a scaling study of the energy diffusion constant in linear molecular chains with static and dynamic disorder, described by the Haken-Strobl-Reineker model. Within this well-known model, three different energy scales dominate the exciton dynamics and optical response, namely the exciton band width, the dephasing rate, and the static disorder magnitude. The model allows to describe the transition from quantum to classical diffusion for increasing dephasing rate, where the crossover occurs at a dephasing rate that depends on the ratio of the disorder and the bandwidth. Using a systematic numerical study, we find three scaling regimes, corresponding to strong localization, weak localization, and finite size effects, respectively. I will show that all numerically found scaling relations can be understood in terms of simple physical arguments. In particular, the optimal dephasing rate, at which the diffusion constant reaches its maximum, may be understood from the Quantum Goldilocks principle. In the second part of the talk, I will address the optical properties of disordered tubular molecular aggregates. Such aggregates allow for excitons with very large delocalization sizes, which in turn lead to strong exchange narrowing effects. Recent experiments showed remarkable homogeneity between the properties of individual tubular cyanine dye aggregates. Comparison to microscopic calculations of disorder and interactions, which include molecular dynamics and quantum chemical calculations, demonstrates that this homogeneity at the exciton level is a direct result of exchange narrowing of static disorder for excitons shared by many molecules. At the molecular level, our modelling reveals considerable inhomogeneity. Theory and experiment show good quantitative agreement.

Electronic excitation energy transport and optical response in low-dimensional dye aggregates with static and dynamic disorder

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ACS Fall 2022

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