| Abstract
| - Gable-type copper(II) porphyrin−free base porphyrin dimers undergo triplet−triplet intramolecular energy transfer from the copper(II) half to the free base with an almost 100% efficiency. On the other hand, in the free base part, intersystem crossing from S1 to T1 is remarkably enhanced because of weak exchange coupling between an unpaired electron in the copper(II) ion and π-electrons in the free base porphyrin.
- Intramolecular energy transfer as well as excited-state relaxation of gable-type copper(II) porphyrin-free baseporphyrin dimers, in which the two halves are linked via a benzene or a naphthalene, was studied by transientabsorption and fluorescence spectroscopy. Photoexcitation at 532 nm of the dimer gives rise to transient absorptionspectra identical to that of triplet−triplet (T−T) absorption in the free base monomer; however, the absorptionintensities in the dimers are more than four times larger than that of the monomer, indicating efficient intramolecularenergy transfer from the copper porphyrin to the free base counterpart. Presence of pyridine in the dimer solutionsdecreases the initial optical densities, while the free base monomer shows no change. The decrease is ascribed tocompetition between energy transfer via triplet manifolds and quenching by pyridine in the excited trip−quartetand/or trip−doublet states of the copper porphyrin moiety. The analysis of the generated yield of the T1 free basein the various pyridine/toluene fractions leads to energy transfer rates of sub-nanoseconds. On the basis of therelative absorption intensities as well as the evaluated energy transfer rates, energy transfer efficiency of thedimer in toluene was estimated as almost unity. On the other hand, with the selective excitation of the free basehalf, the dimers exhibit fluorescence spectra identical to that of the monomer. Fluorescence lifetimes weredetermined in toluene as 430 ± 30 and 890 ± 30 ps for the benzene-bridged dimer (Cu−Bz−H2) and thenaphthalene-bridged dimer (Cu−Np−H2), respectively. Fluorescence intensities of Cu−Bz−H2 and Cu−Np−H2are 1/28 and 1/13 of that of the monomer, respectively. Intersystem crossing (ISC) in the free base part is remarkablyenhanced in the hybrid dimer. We propose that the interaction between an unpaired electron in the copper(II) andthe triplet of the free base via exchange coupling gives rise to the partially allowed character of ISC in the freebase half of the hybrid dimers.
|
