The flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to investigate the nature of the relaxed S(1) state of 9,9'-bianthryl (AA), 10-cyano-9,9'-bianthryl (CAA), and 10,10'-dicyano-9,9'-bianthryl (CAAC). Changes in both the real, Deltaepsilon' (dielectric constant), and imaginary, Deltaepsilon' ' (dielectric loss), components of the complex permittivity have been measured. The dielectric loss transients conclusively demonstrate the dipolar nature of S(1) for all three compounds in the pseudopolar solvents benzene and 1,4-dioxane, and even in the nonpolar solvents n-hexane and cyclohexane. The required symmetry breaking is considered to result from density and structural fluctuations in the solvent environment. The dipole relaxation times for AA (CAAC) are ca. 2 ps for the alkanes and 7.9 (5.3) and 14 (14) ps for benzene and dioxane, respectively. The time scale of dipole relaxation for the symmetrical compounds is much shorter than that for rotational diffusion and is attributed to intramolecular, flip-flop dipole reversal via a neutral excitonic state. The dipole moment of the transient dipolar state is estimated to be ca. 8 D, that is much lower than the value of ca. 20 D determined from the solvatochromic shifts in the fluorescence in intermediate to highly polar solvents which corresponds to close to complete charge separation. For the asymmetric compound, CAA, a dipole moment close to 20 D is found in all solvents, including n-hexane. Dipole relaxation in this case occurs on a time scale of several hundred picoseconds and is controlled mainly by diffusional rotation of the molecules. The mechanism and kinetics of formation of the dipolar excited states are discussed in the light of these results.

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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) n 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 (S1) of the (PB) n oligomers has a very small excess polarizability (<20 Å3) 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) n 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 Å3 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 Å3, indicating that it is much more localized than the S1 state.