Competitive Photoinduced Energy and Electron Transfer in a Donor–Acceptor Triad Consisting of Zinc Porphyrin ‐ Bis‐Styryl‐BODIPY Dyad Coordinated to Fullerene

A newly synthesized and assembled supramolecular triad comprised of BODIPY, ZnP, and C₆₀ entities, probed competitive energy and electron transfer events upon photoexcitation using advanced theoretical and experimental methods. Mimicry of antenna-reaction center events of photosynthesis is demonstrated.

Abstract

To mimic the light-induced processes of the photosynthetic antenna-reaction center, a self-assembled multi-chromophore triad containing triphenylamine-functionalized zinc porphyrin (ZnP(TPA)₃), bisstyryl-boron dipyrrin (BODIPY), and phenylimidazole-appended fullerene, ImC_60, has been constructed to observe competitive energy and electron transfer events. Both steady-state and time-resolved fluorescence techniques revealed that BODIPY acts as an energy acceptor from the singlet excited state of ZnP(TPA)₃. The extended conjugation due to two styryl groups on BODIPY makes it an energy-accepting moiety. A significant emission quenching was observed upon adding ImC₆₀, confirming that this entity acted as a primary electron acceptor in the triad. The thermodynamic feasibility of these events was confirmed by electrochemical studies, which revealed that the redox gap was smaller than the locally excited state. Spectroelectrochemical experiments were performed to characterize the radical cation of the triad spectrally. Furthermore, the directionality of electron transfer was confirmed by charge-difference density maps obtained from time-dependent density functional theory (TD-DFT) calculations. Finally, femtosecond transient absorption spectroscopy was utilized to witness photoinduced events in the triad. The ratio of electron transfer to energy transfer (k_CS/k_EnT ) was found to be ∼3, which confirmed the dominance of electron transfer in producing the BODIPY-ZnP(TPA)₃ ^.+:ImC₆₀ ^.- state among all other competitive processes. The lifetime of the final charge-separated state was found to be ∼100 ps in dichlorobenzene, highlighting successful mimicry of photosynthetic antenna-reaction center events in a competitive fashion.

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