Fate of Photo-Excited Molecular Antennae – Inter-Molecular Energy Transfer versus Photodegradation Assessed by Quantum Dynamics

The present computational study aims to unravel the competitive photo-induced inter-molecular energy transfer and electron transfer phenomena in a light-harvesting antenna with potential applications in dye-sensitized solar cells and photocatalysis. A series of three thiazole dyes with hierarchically overlapping emission and absorption spectra, embedded in a methacrylate-based polymer backbone, is employed to absorb light over the entire visible region. Inter-molecular energy transfer in such antenna proceeds via energy transfer from dye-to-dye and eventually to a photosensitizer. Initially, the ground and excited state properties of the three push-pull-chromophores, e.g., with respect to their absorption and emission spectra as well as their equilibrium structures, are thoroughly evaluated using state-of-the-art multiconfigurational methods and computationally less demanding DFT and TDDFT simulations. Subsequently, the potential energy landscape for the three dyads, formed by the π-stacked dyes as occurring in the polymer environment, is investigated along linear-interpolated internal coordinates to elucidate the photo-induced dynamics associated to inter-molecular energy and electron transfer processes. While energy transfer among the dyes is highly desired in such antenna, electron transfer, or rather alight-induced redox chemistry, leading to the degradation of the chromophores, is disadvantageous. We performed quantum dynamical wavepacket calculations to investigate the excited-state dynamics following initial light-excitation. Our calculations reveal for the two dyads with adjusted optical properties exclusively efficient inter-molecular energy transfer within 200 fs, while in the case of the third dyad inter-molecular electron transfer dynamics can be observed. Thus, this computational study reveals that statistical copolymerization of the individual dyes is disadvantageous with respect to the energy transfer efficiency as well as regarding the photostability of such antenna.