Highly efficient triplet-triplet intramolecular energy transfer and enhanced intersystem crossing in rigidly linked copper(II) porphyrin-free base porphyrin hybrid dimers

Intramolecular energy transfer as well as excited-state relaxation of gable-type copper(II) porphyrin-free base porphyrin dimers, in which the two halves are linked via a benzene or a naphthalene, was studied by transient absorption and fluorescence spectroscopy. Photoexcitation at 532 nm of the dimer gives rise to transient absorption spectra identical to that of triplet-triplet (T-T) absorption in the free base monomer; however, the absorption intensities in the dimers are more than four times larger than that of the monomer, indicating efficient intramolecular energy transfer from the copper porphyrin to the free base counterpart. Presence of pyridine in the dimer solutions decreases the initial optical densities, while the free base monomer shows no change. The decrease is ascribed to competition between energy transfer via triplet manifolds and quenching by pyridine in the excited trip-quartet and/or trip-doublet states of the copper porphyrin moiety. The analysis of the generated yield of the T-1 free base in the various pyridine/toluene fractions leads to energy transfer rates of sub-nanoseconds. On the basis of the relative absorption intensities as well as the evaluated energy transfer rates, energy transfer efficiency of the dimer in toluene was estimated as almost unity. On the other hand, with the selective excitation of the free base half, the dimers exhibit fluorescence spectra identical to that of the monomer. Fluorescence lifetimes were determined in toluene as 430 +/- 30 and 890 +/- 30 ps for the benzene-bridged dimer (Cu-Bz-H-2) and the naphthalene-bridged dimer (Cu-Np-H-2), respectively. Fluorescence intensities of Cu-Bz-H-2 and Cu-Np-H-2 are 1/28 and 1/13 of that of the monomer, respectively. Intersystem crossing (ISC) in the free base part is remarkably enhanced in the hybrid dimer. We propose that the interaction between an unpaired electron in the copper(II) and the triplet of the free base via exchange coupling gives rise to the partially allowed character of ISC in the free base half of the hybrid dimers.