Charge and Spin Distribution in the Triplet State of a Porphyrin Cofacial Heterodimer via Hyperfine Spectroscopy

The charge and electron spin density distribution in the lowest excited triplet state of a cofacial Al(III) porphyrin-P(V) porphyrin heterodimer (AlPor-O-PPor·PF₆) has been studied at low temperature in different glass-forming solvents using electron nuclear double resonance (ENDOR) spectroscopy and density functional theory (DFT) calculations. The ¹H hyperfine couplings (hfcs) at specific positions of the porphyrin rings have been used to differentiate spin delocalization and charge transfer (CT) character in the heterodimer. While both effects contribute similarly to the zero-field splitting tensor, they can be distinguished by ENDOR spectroscopy through comparing the heterodimer triplet state hfcs with those of the corresponding monomers in their triplet state, oxidized, and reduced radical forms. From ENDOR data and DFT calculations, the triplet state of AlPor-O-PPor·PF₆ is estimated to have ~24% and ~35% CT character respectively, in agreement with the Mulliken partial charges. These results indicate that the CT character is modulated by solvent interactions, depending both on its polarity and coordination of the Al(III) center. The estimate of the CT character in the triplet state and its dependence on the molecular structure and the solvent is crucial to achieve efficient long-lived triplet born charge separation in model photosynthetic systems.