Motoko Asano-Someda

Orientation effect of enhanced intersystem crossing in copper(II) porphyrin-free base porphyrin dimers

Abstract Intersystem crossing (ISC) enhanced by a remote unpaired electron through long-range interaction was studied in linear-type copper(II) porphyrin-free base porphyrin dimers in comparison with the gable-type dimers. The excited singlet state of the free base moiety is deactivated to its triplet manifolds quickly due to exchange interaction between the free base π-electrons and copper unpaired d electron. While the ISC rates in the gable dimers show a simple relation with number of bonds of the linkage, two linear-type dimers behave differently. In the case of a p -phenyl bridged dimer, where the two porphyrin halves are almost parallel, the ISC rate is much larger than that expected from the simple through-bond coupling, which is applicable to the gable-type dimers. On the other hand, p -biphenyl bridged dimer, in which two halves are not parallel, does not show such an effect. Parallel orientation of the two terminals can play an important role in the long-range exchange coupling.

Red-shifted emission spectra of several meso-substituted copper porphyrins in fluid solution

Abstract Medium dependence of the emission spectra of several meso-substituted copper porphyrins was studied at 300-77 K. Copper porphyrins emit from the lowest excited (porphyrin triplet) states in fluid solution as well as in rigid media. Emission spectra of several copper porphyrins in toluene liquid solution were red-shifted from those in rigid media such as PMMA (poly(methylmethacrylate)) film. The copper porphyrins, which give red-shifted emission in toluene solution, have 2,4b13(a2ueg)] configuration in the lowest excited states and the amount of the shift depends on meso-substituents ranging up to 1300 cm−1. As the emission spectra in toluene rigid glass at 77 K coincide with those in PMMA film, the observed shift in 2,4[b13(a2ueg)]-type porphyrins is attributable to distortion of the excited molecules in fluid solution. T(2,4,6-(MeO)3)PPCu, in which bulky meso-substituents are likely to suppress torsion of the phenyl rings and distortion of the porphyrin plane, was found to show no shift of emission spectra, in spite of the 2,4[b13(a2ueg)] configuration. In the case of TPrPCu, which has no phenyl group, red shift occurs as a result of the medium being changed to become non-rigid. An out-of-plane distorted structure is proposed. Lifetimes of the emission in toluene solution are remarkably shorter in the copper porphyrins that show a larger emission red-shift from the emission in PMMA film. This relation suggests that the distortion modes are connected with enhancement of the radiationless decay.

Time-resolved EPR spectra of a photoexcited phenanthrene-linked copper(II)-free-base porphyrin dimer: An intermediate-coupling case in a triplet-doublet spin system

Time-resolved electron paramagnetic resonance (TREPR) spectra are presented for a phenanthrene-linked copper(II)-free-base porphyrin dimer. In the lowest electronic excited state, the free-base half is in the (π, π*) triplet state and the copper-porphyrin half is in the doublet ground state due to the copper(II) spin. Because of the interaction between the triplet and doublet spins, the excited state of the dimer is described as a coupled triplet-doublet pair state. By choosing the excitation wavelength, this coupled state is produced via either the intersystem crossing within the free base or the energy transfer from the excited state of the copper porphyrin. The observed TREPR spectra show very large spectral widths compared to that of the triplet state in the free-base monomer. In addition, there is a prominent absorptive band in the center of the spectra regardless of the generation pathway. These features can be interpreted as characteristic properties in an intermediate-coupling case, where the degree of the triplet and doublet mixing largely depends on the molecular orientation relative to the magnetic field.

EPR spectra of gable-type copper(II) porphyrin dimers in fluid solution: extraction of exchange interaction in weakly coupled doublet pairs

A comparison between EPR spectra of rigidly linked dicopper porphyrin dimers and those of the corresponding monocopper dimers (copper porphyrin-free base porphyrin dimers) in fluid solution reveals a very weak exchange interaction between the two copper spins. In these dimers, two porphyrin moieties are linked via an aromatic spacer such as benzene, naphthalene or phenanthrene in a gable-type geometry, with a distance of 10–13 Å. Although essentially all the spectra from the monocopper dimers are the same, exhibiting hyperfine (hf) structure due to the copper and nitrogen nuclei, the EPR spectral patterns of the dicopper dimers depend on the spacer molecule. Differences in hf patterns among the dicopper porphyrin dimers are ascribed to isotropic spin—spin coupling, i.e., exchange coupling between the two copper spins. This is because the anisotropic dipole—dipole interaction is averaged out due to random tumbling of the solute molecules in fluid solution. From the line shape analysis, the absolute value of the exchange interaction (|J|) is found to be 4 × 10−4 cm−1 ≦|J| < 3 × 10−3 cm−1 for the benzene linked dicopper dimer (Cu—Bz—Cu) whereas |J| ∼ 1 × 10−4cm−1 for the other two dimers (Cu—Np—Cu and Cu—Pn—Cu). These values are comparable with or much smaller than the dipole—dipole coupling, which is estimated as about 1–3 × 10−3 cm−1 from the centre-to-centre distance. Since Cu—Bz—Cu shows a significantly larger |J| than Cu—Pn—Cu, despite a slightly longer centre-to-centre distance, and since no correlation could be obtained between |J| and the separation of the two copper atoms, it is likely that the interaction via spacer molecules is dominant between the two halves.

A time-resolved ESR study on triplet-triplet energy transfer processes in a copper(II)-free-base porphyrin dimer

Abstract Energy transfer from a copper porphyrin moiety to its free-base partner is observed in a copper(II)-free-base hybrid porphyrin dimer. By the measurements of T-T transient absorption spectra in toluene, the triplet yield of the free-base porphyrin was found to be increased in the hybrid dimer compared with that in a monomeric free-base porphyrin. Time-resolved ESR study revealed that this intramolecular energy transfer occurs between the triplets. From the simulation of the TR-ESR spectrum, the mean value of the dihedral angle between the donor and the acceptor is estimated as 30 degrees.