(Invited) Donor-Acceptor Systems from Triplet Photosensitizers for High Potential, Long-Lived Charge Separation

Abstract

Generation of high potential, long-lived charge separated states in donor-acceptor systems are invaluable for converting light energy into either electricity or fuels. The majority of the donor-acceptor conjugates designed for this purpose are derived mainly from short-lived singlet sensitizers.1,2 In such conjugates, charge separation from the singlet excited donor or acceptor results in a short-lived charge separated state of singlet spin character. Alternatively, by utilizing high-energy triplet sensitizers one could generate long-lived charge separated states. The radical ion-pairs of triplet spin character slowly charge recombine to the ground state due to spin forbidden 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.3-5 Here, we summarize our efforts to synthesize donor-acceptor conjugates bearing triplet sensitizers wherein excited state electron transfer exclusively occurs from the triplet excited state.3-6 To achieve the goal, two types of triplet sensitizers, viz., palladium and platinum porphyrins, and b-iodo functionalized BODIPYs are covalently linked to the well-known electron acceptor, fullerene, C60. Using femtosecond transient absorption studies covering wide temporal and spatial regions, and time-resolved electron paramagnetic studies, photoinduced electron transfer from 3MP* (M = Pd or Pt) to C60 in these dyads, leading to the formation of MP•+-C60 •- charge separated state of triplet spin character is demonstrated. Similarly, the role of triplet sensitizers in promoting charge separation in I2BODIPY-C60 and I2azaBODIPY-C60 dyads6 is also highlighted. 1. B. KC, F. D’Souza, Coord. Chem. Rev. 2016, 322, 104-141.2. D’Souza, and O. Ito ‘Chem. Soc. Rev. 2012, 41, 86-96.3. Obondi, G. N. Lim, F. D’Souza, J. Phys. Chem. C 2015, 119, 176-185.4. Obondi, G. N. Lim, B. Churchill, P. K. Poddutoori, A. van der Est, and F. D’Souza, Nanoscale, 2016, 8, 8333-8344.5. K. Poddutoori,Y. E. Kandrashkin, C. O. Obondi, F. D’Souza and A. van der Est, Phys. Chem. Chem. Phys. 2018, 20, 28223-28231.6. Shao, H. B. Gobeze, V. Bandi, C. Funk, B. Heine, M. J. Duffy, V. Nesterov, P. A. Karr, F. D’Souza, ChemPhotoChem, 2020, in press.