The formation of a high-energy, long-lived radical ion-pair by electron transfer exclusively from the triplet excited state, is demonstrated in a newly synthesized platinum porphyrin-fullerene dyad, in which the porphyrin ring is modified with three electron rich triphenylamine entities. The spin selectivity of the electron transfer leading to the formation of the radical ion-pair is demonstrated using time-resolved optical and EPR spectroscopic techniques. Multi-modular donor-acceptor systems capable of undergoing excited state electron transfer (ET) have been widely investigated as photosynthetic reaction centre models and for their applications in photonics devices. The focus of these studies has been mainly to understand the principles governing the rates of ET and competing processes. The main issue is to find ways of obtaining high ET quantum yields while minimizing the energy losses and prolonging the lifetime of the radical ion-pair. A large body of work has shown that the ET rates are described well by the Marcus equation and that the pre-exponential term describing the electronic coupling between the donor and acceptor, which depends on the nature of the bridging groups and distance between the donor and acceptor, is the most important factor. However, the Franck-Condon term, which depends on the reorganization energy (λ) and driving force (ΔG), also plays an important role. A number of studies have shown, for example, that the use of electron acceptors, such as C60 and C70 in which the charge of the radical anion is highly delocalized, results a small value of λ and a large activation barrier to charge recombination, thus prolonging the lifetime of radical ion-pair.

Exclusive triplet electron transfer leading to long-lived radical ion-pair formation in an electron rich platinum porphyrin covalently linked to fullerene dyad

ACS Fall 2021

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