Photoexcitations of covalently bridged zinc porphyrin oligomers: Frenkel versus Wannier-Mott type excitons

The effect of increasing chain length on the excited-state properties of two series of covalently linked zinc porphyrin oligomers has been investigated. One series has a 1,4-phenylene bridge, (PB), with n = 1, 2, 3, and 5, and the other series has a 1,4-butadiynylene bridge, (yPy)(n) with n = 1-6. The two series differ dramatically in the nature of the porphyrin-porphyrin interaction. Flash-photolysis time-resolved microwave conductivity measurements (FP-TRMC) show that the excited singlet state (S-1) of the (PB), oligomers has a very small excess polarizability (<20 (3)) which is characteristic of a tightly bound. Frenkel type exciton. In agreement with this, the optical absorption and emission spectra indicate that only electrostatic interactions occur between the transition dipoles of the porphyrin moieties. In contrast, very large excess polarizabilities are found for the S1 state of the (yPy),, series, which indicates that a high degree of electron exchange occurs between the porphyrin moieties. This is confirmed by the marked changes also observed in the optical absorption and emission spectra. The polarizability increases with increasing length of the oligomers up to a maximum of 960 Angstrom (3) indicative of a loosely bound, Wannier-Mott type exciton. In contrast, the triplet excited state of the (yPy)(n) series has an excess polarizability of <50 (3), indicating that it is much more localized than the S-1 state.