Preferential Through-Space Charge Separation and Charge Recombination in V-Type Configured Porphyrin-azaBODIPY-Fullerene Supramolecular Triads

Electron transfer is one of the most fundamental and prevalent processes occurring in chemistry, physics, and biology. In donor acceptor systems with one of the partners' a photosensitizer, upon photoexcitation, transfer of an electron between the photoexcited and ground-state molecules occurs. Factors affecting the geometry, energetics, and dynamics of this process have been one of the intensively studied scientific topics, often by building model donor acceptor conjugates or by utilizing natural systems. A wealth of information, applicable to almost all areas of modern science and technology, has been generated from these studies. In the present study, we demonstrate preferential through-space charge separation and charge recombination in supramolecular triads composed of porphyrin (free-base, zinc, or magnesium at the central cavity) as excited-state electron donor, BF2-chealted azadipyrromethene (azaBODIPY), and fullerene (C-60) as electron acceptors. Because of spatial close proximity of the terminal porphyrin and fullerene entities of the triads as a consequence of the V-type configuration, photoinduced charge separation from the singlet excited porphyrin involves fullerene instead of energetically more favorable covalently linked azaBODIPY entity. Interestingly, charge recombination also follows this path of through-space instead of an electron migration from the fullerene anion radical to the covalently linked azaBODIPY entity. The present study highlights the importance of geometric disposition of donor and acceptor entities in governing not only the forward photoinduced electron transfer but also the dark reverse electron transfer in multimodular donor acceptor conjugates, applicable toward light-energy-harvesting and building optoelectronic devices.