A Transient EPR Study of Electron Transfer in Tetrathiafulvalene-Aluminum(III) Porphyrin-Anthraquinone Supramolecular Triads

The stabilization of light-induced charge separation in two axially bound triads based on aluminum(III) porphyrin (AlPor) are investigated using the electron spin polarization patterns of the final radical pair state. In the triads, TTF-(Ph)_n-py-AlPor-AQ, (n=0, 1) anthraquinone (AQ) is attached covalently to the Al(III) center, while the donor tetrathiafulvalene (TTF) coordinates to Al(III) on the opposite face of the porphyrin ring via the appended pyridine (py). The dyad AlPor-AQ has been studied previously (M. Kanematsu, P. Naumov, T. Kojima, S. Fukuzumi, Chem. Eur. J. 17 (2011) 12372.) and shown to undergo fast light-induced charge separation and triplet recombination. Here, it is shown that by coordinating pyridine-appended TTF to the porphyrin, the charge separation can be stabilized. The spin polarized transient EPR spectra of the state TTF·⁺AQ·^- can be observed in both the glass phase and in liquid solution and show that the state is formed from a singlet precursor on a timescale of less than ~0.5 ns. Using structural models to fix the geometry of the radical pair and the strength of the dipolar coupling, it is possible to determine the sign and approximate magnitude of the exchange coupling between TTF·⁺ and AQ·^-. In contrast, other similar triads, which display relatively large ferromagnetic coupling, the exchange coupling is found to be small and antiferromagnetic. This difference can be rationalized as a result of differences in the structure of the bridge between the porphyrin and the acceptor.