Photophysical properties of long rodlike meso-meso-linked zinc(II) porphyrins investigated by time-resolved laser spectroscopic methods

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. F or this purpose, zinc(II) 5,15-bis(3,5-bis(octyloxy)phenyl)porphyrin (Z1) a nd its directly meso-meso-linked porphyrin arrays up to Z128 (Zn, n represe nts the number of porphyrins) were synthesized. The absorption spectra of t hese porphyrin arrays change in a systematic manner with an increase in the number of porphyrins; the high-energy Soret bands remain at nearly the sam e wavelength (413-414 nm), while the low-energy exciton split Soret bands a re gradually red-shifted, resulting in a progressive increase in the excito n splitting energy. The exciton splitting is nicely correlated with the val ues of cos[pi/(N + 1)] according to Kasha's exciton coupling theory, provid ing a value of 4250 cm(-1) for the exciton coupling energy in the St state. The increasing red-shifts for the Q-bands are rather modest. The fluoresce nce excitation anisotropy spectra of the porphyrin arrays show that the pho toexcitation of the high-energy Soret bands exhibits a large angle differen ce between absorption and emission dipoles in contrast with the photoexcita tion of the low-energy exciton split Soret and Q-bands. This result indicat es that the high-energy Soret bands are characteristic of the summation of the individual monomeric transitions with its overall dipole moment deviate d from the array chain direction, while the low-energy Soret bands result f rom the exciton splitting between the monameric transition dipoles in line with the array chain direction. From the fluorescence quantum yields and fl uorescence lifetime measurements, the radiative coherent length was estimat ed to be 6-8 porphyrin units in the porphyrin arrays. Ultrafast fluorescenc e decay measurements shaw that the S-2 --> S-1 internal conversion process occurs in less than 1 ps in the porphyrin arrays due to the existence of ex citon split band as a ladder-type deactivation channel, while this process is relatively slow in Z1 (similar to1.6 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.