Electron Transfer Pathways in a Tetrathiafulvalene-Aluminum(III) Porphyrin-Free-Base Porphyrin Triad Studied Using Electron Spin Polarization

Abstract

The electron transfer pathways 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-Ph-H₂Por (n = 0, 1), free-base porphyrin (H₂Por) is attached covalently to the Al(III) center, while the donor tetrathiafulvalene (TTF) coordinates to Al(III) via an appended pyridine on the opposite face of the porphyrin ring. Excitation of the triad at 532 nm leads to absorption by both AlPor and H₂Por and to two possible charge separation pathways. In the liquid crystalline solvent 5CB, spin polarized transient electron paramagnetic resonance spectra of a weakly coupled radical pair are observed and assigned to the state \({\text{TTF}}^{ \cdot+ } {\text{H}}_{2} {\text{Por}}^{ \cdot- }\). The radical pair spectra are analyzed using a structural model of the complex to determine the dipolar coupling and relative orientation of the radicals. The inertia tensor calculated from the structural model is used to derive the principal values and axes of the order matrix. It is shown that the observed polarization pattern is only consistent with ferromagnetic exchange coupling in the radical pair. The spectra can be reproduced as the sum of two contributions originating in electron transfer from the excited singlet state of AlPor and from the excited the triplet state of H₂Por. The latter is shown to account for the integral emissive net polarization observed in the spectra.