Theoretical study of intramolecular long-range electron transfer reactions between porphyrin and benzoquinone: Ab initio calculations of electronic coupling element

The electronic coupling elements (ECEs) in the long-range intramolecular electron transfer systems with porphyrin-benzoquinone donor/acceptor groups linked by organic spacers an investigated theoretically, A method for calculating the electronic coupling element is developed based on ab initio molecular orbital theory. The ECEs are expressed in terms of the Hamiltonian matrix elements of singly excited configuration interaction wave functions with the localized donor, acceptor, and spacer orbitals. Compared to the hole transfer mechanism, the electron transfer through the unoccupied spacer orbitals is found to be the responsible mechanism in the present systems. The ECEs are examined by the decomposition and pathway analysis methods. The results were that (a) the ECEs strongly depend on the geometries of spacer molecules, (b) the through bond type interaction involving the pi* orbitals of benzene parts in the spacers gives significant contributions to the ECEs, and (c) the interference between the electron pathways plays an important role in determining the ECEs.