Triplet-triplet intramolecular energy transfer in a covalently linked copper(II) porphyrin-free base porphyrin hybrid dimer: A time-resolved ESR study

Triplet-triplet intramolecular energy transfer in a covalently linked copper(II) porphyrin-free base porphyrin hybrid dimer was examined by time-resolved (TR) ESR measurements of the lowest excited triplet (T-1) state of the free base porphyrin component after laser pulse excitation at 532 nm. The TRESR spectra of the free base moiety in the hybrid dimer in toluene glass at 77 K exhibit a spin polarization pattern different from that for the free base porphyrin monomer. The observed pattern for the dimer cannot be explained by any intersystem crossing (ISC) process, and it is ascribed to intramolecular energy transfer that takes place from the copper porphyrin part to the free base counterpart between the triplet manifolds. On the other hand, the TRESR spectrum in 2-methyltetrahydrofuran (2-MTHF) glass, where the energy transfer is prohibited by the fast deactivation of the excited copper porphyrin, shows the same spin polarization pattern as that of the free base porphyrin monomer. Therefore, the copper porphyrin does not affect spin selectivity of ISC in the free base counterpart. The analysis of the TRESR spectrum in toluene suggests that the energy transfer produces the spin population dominantly into T+1 and T-1 high-field spin sublevels of the T-1 state of the free base porphyrin. In addition to the ESR signals of the T-1 state showing the same fine structure as that for the free base monomer, a moderately intense and narrow emissive band with a larger decay rate was observed at around 324 mT in toluene while in 2-MTHF this band is absent. This emission band may arise from a dimer having a different conformation in which the two porphyrin halves are closer to each other. This kind of conformation gives rise to the strong interaction between the electron spins in the ground state of the copper(II) porphyrin and in the T-1 state of the free base porphyrin.