| 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 forrapid energy transfer. For this purpose, zinc(II) 5,15-bis(3,5-bis(octyloxy)phenyl)porphyrin (Z1) and its directlymeso−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 numberof porphyrins; the high-energy Soret bands remain at nearly the same wavelength (413−414 nm), while thelow-energy exciton split Soret bands are gradually red-shifted, resulting in a progressive increase in the excitonsplitting energy. The exciton splitting is nicely correlated with the values of cos[π/(N + 1)] according toKasha's exciton coupling theory, providing a value of 4250 cm-1 for the exciton coupling energy in the S2state. The increasing red-shifts for the Q-bands are rather modest. The fluorescence excitation anisotropy spectraof the porphyrin arrays show that the photoexcitation of the high-energy Soret bands exhibits a large angledifference between absorption and emission dipoles in contrast with the photoexcitation of the low-energyexciton split Soret and Q-bands. This result indicates that the high-energy Soret bands are characteristic of thesummation of the individual monomeric transitions with its overall dipole moment deviated from the arraychain direction, while the low-energy Soret bands result from the exciton splitting between the monomerictransition dipoles in line with the array chain direction. From the fluorescence quantum yields and fluorescencelifetime measurements, the radiative coherent length was estimated to be 6−8 porphyrin units in the porphyrinarrays. Ultrafast fluorescence decay measurements show that the S2 → S1 internal conversion process occursin less than 1 ps in the porphyrin arrays due to the existence of exciton split band as a ladder-type deactivationchannel, while this process is relatively slow in Z1 (∼1.6 ps). The rate of this process seems to follow theenergy gap law, which is mainly determined by the energy gap between the two Soret bands of the porphyrinarrays.
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