Photophysical Properties of Long Rodlike Meso−Meso-Linked Zinc(II) Porphyrins Investigated by Time-Resolved Laser Spectroscopic Methods

Abstract

The molecular design of directly meso−meso-linked porphyrin arrays as a new model of light-harvesting antenna as well as a molecular photonic wire was envisaged to bring the porphyrin units closer for rapid energy transfer. For this purpose, zinc(II) 5,15-bis(3,5-bis(octyloxy)phenyl)porphyrin (Z1) and its directly meso−meso-linked porphyrin arrays up to Z128 (Zn, n represents the number of porphyrins) were synthesized. The absorption spectra of these porphyrin arrays change in a systematic manner with an increase in the number of porphyrins; the high-energy Soret bands remain at nearly the same wavelength (413−414 nm), while the low-energy exciton split Soret bands are gradually red-shifted, resulting in a progressive increase in the exciton splitting energy. The exciton splitting is nicely correlated with the values of cos[π/(N + 1)] according to Kasha's exciton coupling theory, providing a value of 4250 cm^-1 for the exciton coupling energy in the S₂ state. The increasing red-shifts for the Q-bands are rather modest. The fluorescence excitation anisotropy spectra of the porphyrin arrays show that the photoexcitation of the high-energy Soret bands exhibits a large angle difference between absorption and emission dipoles in contrast with the photoexcitation of the low-energy exciton split Soret and Q-bands. This result indicates that the high-energy Soret bands are characteristic of the summation of the individual monomeric transitions with its overall dipole moment deviated from the array chain direction, while the low-energy Soret bands result from the exciton splitting between the monomeric transition dipoles in line with the array chain direction. From the fluorescence quantum yields and fluorescence lifetime measurements, the radiative coherent length was estimated to be 6−8 porphyrin units in the porphyrin arrays. Ultrafast fluorescence decay measurements show that the S₂ → S₁ internal conversion process occurs in less than 1 ps in the porphyrin arrays due to the existence of exciton split band as a ladder-type deactivation channel, while this process is relatively slow in Z1 (∼1.6 ps). The rate of this process seems to follow the energy gap law, which is mainly determined by the energy gap between the two Soret bands of the porphyrin arrays.