Photoexcited Energy Relaxation in Porphyrin Nanorings

Abstract

Natural photosynthetic systems convert solar energy to chemical energy with an extremely high efficiency. Synthetic chromophore arrays are promising models for understanding and mimicking energy migration in natural light-harvesting systems. To duplicate the structure and function of natural systems, a multitude of cyclic porphyrin arrays have been synthesized over the past several decades. In a recent breakthrough [Nat. Chem. 2022, 14, 1436–1442], synthetic butadiyne-linked porphyrin nanorings have been shown to exhibit excitation energy transfer on the same timescale and length scale as the natural light-harvesting complex LH2 and with comparable efficiency. In the pursuit of understanding the fundamental underpinnings of the exciton dynamics in such systems, we combined time-dependent density functional theory with the hierarchical equation of motion approach and performed a systematic study of population dynamics, coherence length, and the energy transfer in linear and cyclic multiporphyrin nanostructures with and without the butadiyne moiety. In very good agreement with experiments, we showed that the ultrafast delocalization process of the exciton in symmetric structures occurs during the first 300 fs after the excitation. Migration and localization of the excitation in segments with π-conjugation links happen on a timescale of tens of picoseconds. We also found that coherence length dynamics in multiporphyrin systems are very robust to the environmental effects. Another notable feature of butadiyne-linked porphyrin nanorings is that their energy transfer efficiency can exceed 80%.