Driving Force and Electronic Coupling Effects on Photoinduced Electron Transfer in a Fullerene-based Molecular Triad¶

Abstract

Tuning thermodynamic driving force and electronic coupling through structural modifications of a carotene (C) porphyrin (P) fullerene (C60) molecular triad has permitted control of five electron and energy transfer rate constants and two excited state lifetimes in order to prepare a high‐energy charge‐separated state by photoinduced electron transfer with a quantum yield of essentially unity (≥96%). Excitation of the porphyrin moiety of C–P–C60 is followed by a combination of photoinduced electron transfer to give C–P·+–C60·− and singlet–singlet energy transfer to yield C–P–1C60. The fullerene excited state accepts an electron from the porphyrin to also generate C–P·+–C60·−. Overall, this initial state is formed with a quantum yield of 0.97. Charge shift from the carotenoid to yield C·+–P–C60·− is at least 60 times faster than recombination of C–P·+–C60·−, leading to the overall quantum yield near unity for the final state. Formation of a similar charge‐separated species from the zinc analog of the triad with a yield of 40% is also observed. Charge recombination of C·+–P–C60·− in 2‐methyltetrahydrofuran yields the carotenoid triplet state, rather than the ground state. Comparison of the results for this triad with those for related triads with different structural features provides information concerning the effects of driving force and electronic coupling on each of the electron transfer steps.

Efficient Low Driving Force Charge Separation in an Electron Deficient Zn-Porphyrin−Fullerene Donor–Acceptor Conjugate

“…Typically, the rate constants at such short separations are up to 2 orders of magnitude larger. 5,6,[20][21][22]26,29,30 Our results agree, however, well with the small driving force for charge separation of around 0.2 eV, vide supra. Interestingly, a previous study on low band gap polymer donor−acceptor OPV blends, based on cyclopentadithiophene derivatives as electron donor and phenyl-C 61 -butyric acid methyl ester as electron acceptor did not result in an observable ET when decreasing the ET driving force to a similarly low −0.18 eV.…”

Mixed Quantum Classical Simulations of Charge-Transfer Dynamics in a Model Light-Harvesting Complex. II. Transient Vibrational Analysis

“…From an experimental standpoint, aliphatic substituents like these are included to control the solubility of the molecule. However, as pointed out by Bahr, the longer chains in the β positions sterically interfere with and force the linker aryl groups into orthogonal orientations . The dihedral angle of these linkers to the porphyrin should have a much larger impact on CT over the side aryl groups.…”

Photoinduced Electron and Energy Transfer in a Molecular Triad Featuring a Fullerene Redox Mediator

“…1000 nm region. This region was not examined for 2 , but the fullerene radical anion absorption accompanying the porphyrin radical cation absorption has been reported in many closely related dyads. − The long-lived DAS is very weak and represents a small amount of fullerene and porphyrin triplet states formed via intersystem crossing.…”