Photophysical properties of meso, meso-linked porphyrin arrays: steady-state and time-resolved fluorescence polarization

The photophysical properties of a series of directly meso,meso-linked porphyrin arrays (Zn(II) 5,15-di(3,5-di-tert-butylphenyl) porphyrin (Z1), its dimer (Z2), trimer (Z3), and tetramer (Z4), were investigated by using the steady-state and time-resolved fluorescence polarization spectroscopy. While the Soret band of Z1 exhibits the absorption maximum at 415 nm, those of Z2, Z3, and Z4 are split into two bands because of the strong exciton coupling. The fluorescence band is red-shifted and its lifetime decreases as the number of porphyrin moieties increases. In contrast to Z1, the initial anisotropy values (r0) of Z2, Z3 and Z4 exhibit the negative sign with photoexcitation at 410 nm, indicating a large difference between absorption and emission dipole orientations. However, the positive values are obtained in all compounds with the visible excitation. The emission anisotropy decay appeared to be monoexponential at all excitation wavelengths with a decrease of its rate as the number of porphyrin moieities increases. In addition, the rotational decay is observed to depend on excitation wavelength, which is probably due to the local heating of the solvent by excess vibrational energy. The anisotropy values calculated from the time-resolved anisotropy decay are in a good agreement with those obtained from the steady-state polarized excitation spectra.