(Invited) Long-Lived Charge Separated States Exclusively from the Triplet Excited States in Palladium Porphyrin-Fullerene Conjugates

Abstract

Molecular engineering of donor-acceptor conjugates to control their photoinduced processes to yield the much desired long-lived charge separated states has been one of the major goals in artificial photosynthesis. The charge separated states of sufficient lifetime and stored energy are key in ultimately driving the photocatalytic processes of either light-to-electricity conversion or light-to-fuel production. In simple donor-acceptor dyads, charge separation from the singlet excited donor or acceptor due to rapid charge recombination process results in a short-lived charge separated state of singlet spin character. Utilization of high energy triplet sensitizers instead of the traditionally employed singlet sensitizers could be an elegant approach to generate long-lived charge separated states. The radical ion-pairs of triplet spin character charge recombine to the ground state slowly due to spin forbidden nature of the process. However, witnessing such a process purely from the triplet excited state without the interference of the singlet excited state and other competing photochemical events such as excitation energy transfer has been challenging to establish. Here, we report our strategy to observe electron transfer exclusively from the triplet excited state of the donor in PdP-C60 dyads (see figure below). Using transient absorption spectral studies operating at different timescales and time-resolved electron paramagnetic resonance (TR-EPR) studies, photoinduced electron transfer from 1PdP* to C60 leading to the formation of PdP•+-C60 •- charge separated state of singlet spin character has been witnessed in 2 (instead of triplet-triplet energy transfer observed in 1 1) due to the close proximity of the donor and acceptor entities. In contrast, by lowering the energy of the [(TPA)3PdP]•+-C60 •- state owing to the presence of electron rich TPA entities (facile oxidation) compared to that of 3C60*, electron transfer from 3(TPA)3PdP* to C60 leading to [(TPA)3PdP]•+-C60 •- charge separated state of triplet spin character with lifetime in the microsecond range has been observed in 3 in polar solvents. Results of these findings and its significance will be highlighted. C. Obondi, G. N. Lim, F. D’Souza, Triplet-Triplet Excitation Transfer in Palladium Porphyrin – Fullerene and Platinum Porphyrin – Fullerene Dyads’ J. Phys. Chem. C 2015, 119, 176-185. Figure 1