(Invited) Triplet Sensitizer Derived, Nanocarbon Bearing Donor-Acceptor Conjugates for High Potential, Long-Lived Charge Separation

Abstract

Donor-acceptor conjugates capable of producing high potential, long-lived charge separated states are invaluable for artificial photosynthesis of converting light energy into either electricity or environmentally friendly fuels. The majority of the donor-acceptor conjugates designed for this purpose are derived mainly from singlet sensitizers.1,2 In such conjugates, charge separation from the singlet excited donor or acceptor due to rapid charge separation and recombination processes 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.3-5 In the present contribution, we summarize our efforts to derive and witness donor-acceptor conjugates where excited state electron transfer exclusively occurs from the triplet excited state. To achieve the goal, two triplet sensitizers, viz., palladium porphyrin and platinum porphyrin are covalently linked to either fullerene, C60 3-5 or (6,5) or (7,6) enriched single wall carbon nanotubes (SWCNTs).6 Using transient absorption spectral studies covering wide temporal and spatial regions, and time-resolved electron paramagnetic resonance (TR-EPR) studies, photoinduced electron transfer from 1MP* (M = Pd or Pt) to C60 in a series of dyads, leading to the formation of MP•+-C60 •- charge separated state of triplet spin character will be demonstrated. Similarly, the role of triplet sensitizers in promoting charge separation in PdP-SWCNT(6,5) and PdP-SWCNT(7,6) in solvents of different polarity will be highlighted. 1. C. B. KC, F. D’Souza, ‘Design and photochemical study of supramolecular donor-acceptor systems assembled via metal-ligand axial coordiation’ Coord. Chem. Rev. 2016, 322, 104-141.2. F. D’Souza, and O. Ito ‘Photosensitized electron transfer processes of nanocarbons applicable to solar cells’ Chem. Soc. Rev. 2012, 41, 86-96.3. 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, 176-185.4. C. Obondi, G. N. Lim, B. Churchill, P. K. Poddutoori, A. van der Est, and F. D’Souza, ‘Modulating the generation of long-lived charge separated states exclusively from the triplet excited states in palladium porphyrin–fullerene conjugates’, Nanoscale, 2016, 8, 8333-8344.5. P. K. Poddutoori, Y. E. Kandrashkin, C. O. Obondi, F. D’Souza and A. van der Est, Triplet Electron Transfer and Spin Polarization in a Palladium Porphyrin–Fullerene Conjugate, Phys. Chem. Chem. Phys. 2018, 20, 28223-28231.6. L. M. Arellano, H. B. Gozebe, M. J. Gómez-Escalonilla, J. L. G. Fierro, F. D’Souza and F. Langa, Photoinduced charge separation in PdP-SWCNT(6,5) and PdP-SWCNT(7,6) donor-acceptor hybrids, manuscript in preparation.