A solid approach has been developed to ascertain the correlation of electron transfer with molecular vibration in a quantitative manner. Specifically, the reaction coordinate is identified by maximizing the linear Pearson’s correlation coefficient between atomic displacement and diabatic energy gap. In the limit of fast molecular vibration, the rates of electron transfer driven by multiple vibrational modes have been derived respectively under the strong and weak vibronic coupling conditions. As shown by the back electron transfer of a betaine-30 molecule from its first excited state to its ground state after being photoexcited, only 7 out of 210 vibrational modes are essential by cumulatively accounting for more than 60% of the total reorganization energy. Since all essential vibrational modes are significantly faster than thermal fluctuation, the back electron transfer is primarily driven by intramolecular quantum tunneling. Interestingly, the calculated reaction driving force of 1.95 eV is substantially greater than the reorganization energy of 0.58 eV, placing the reaction in the inverted Marcus region. Nevertheless, a sizable Franck-Condon factor of 1.58×10-3 eV-1 is still achieved due to the large vibronically weighted zero-point energy of the essential vibrational modes. After determining the electronic coupling strength as 0.14 eV by the constrained density functional theory, the overall electron transfer rate at 300 K is found to be 0.30 ps-1, which agrees nearly perfectly with experimental values. In this talk, we will also discuss our recent application of the functional mode electron transfer theory on singlet fission, an interesting photochemical process that holds great promise to circumvent the long-standing Shockley-Queisser thermodynamic limit for solar energy conversion.

Functional mode electron transfer theory

249th National Meeting (2015)

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