Charge and spin transfer dynamics in a weakly coupled porphyrin dimer

The dynamics of electron and spin transfer in the radical cation and photogenerated triplet states of a tetramethylbiphenyl-linked zinc-porphyrin dimer were investigated, so as to test the relevant parameters for the design of a single-molecule spin valve, and the creation of a novel platforms for the photogeneration of high-multiplicity spin states. We used a combination of multiple techniques, including variable-temperature continuous wave EPR, pulsed proton electron–nuclear double resonance (ENDOR), transient EPR, and optical spectroscopy. The conclusions are further supported by density functional theory (DFT) calculations, and comparison to reference compounds. The low-temperature cw-EPR and room-temperature near-IR spectra of the dimer monocation demonstrate that the radical cation is spatially localized on one side of the dimer at any point in time, while the EPR spectra at 298 K reveal rapid hopping of the radical spin density between both sites of the dimer via reversible intramolecular electron transfer. The hyperfine interactions are modulated by electron transfers and can be quantified using ENDOR spectroscopy. This allowed simulation of the variable- temperature cw-EPR spectra with a two-site exchange model and provided information on the temperature-dependence of the electron transfer rate. The electron-transfer rates range from about 10.0 MHz at 200 K to about 53.9 MHz at 298 K. The activation enthalpies ∆‡H of the electron transfer were determined as ∆‡H = 9.55 kJ mol−1 and ∆‡H = 5.67 kJ mol−1 in a 1:1:1 solvent mixture of CD2Cl2:toluene-d8:THF-d8 and in 2-methyltetrahydrofuran, respectively, consistent with a Robin–Day class II mixed-valence compound. Investigation of the spin-density distribution of the photogenerated triplet state of the Zn-porphyrin dimer reveals localization of the triplet-spin density on a nanosecond time scale on one half of the dimer at 20 K in 2-methyltetrahydrofuran and at 250 K in a polyvinylcarbazole film. We highlight how these results impact the design of single-molecule spintronic devices, quantifying fundamental parameters for relevant building blocks, and offer the first step towards a novel platforms for photogenerated high-multiplicity spin states.