Structural dependence on excitation energy migration processes in artificial light harvesting cyclic zinc(II) porphyrin arrays

A series of covalently linked cyclic porphyrin arrays CNZ that consist of N/2 of meso-meso directly linked zinc(II) porphyrin dimer subunits Z2 bridged by 1,3-phenylene spacers have been prepared by Ag^I-promoted oxidative coupling reaction. We have investigated the excitation energy migration processes of CNZ in toluene by using femtosecond transient absorption anisotropy decay measurements by taking 2Z2 composed of two Z2 units linked by 1,3-phenylene as a reference molecule. On the basis of the excitation energy transfer rate determined for 2Z2, we have revealed the excitation energy hopping rates in the cyclic arrays CNZ by using a regular polygon model. The number of excitation energy hopping sites N_flat calculated by using a regular polygon model is close to the observed N_expt value obtained from the transient absorption anisotropy decays for C12Z-C18Z with circular and well-ordered structures. On the other hand, a large discrepancy between N _flat and N_expt was found for smaller or larger arrays (C10Z, C24Z, and C32Z). In the case of C10Z, m-phenylene linked 2Z2 motif with the interchromophoric angle of 120° is not well suited to make a cyclic pentagonal array C10Z based on planar pentagonal structure. This geometrical factor inevitably causes a structural distortion in C10Z, leading to a discrepancy between N_expt and N_flat values. On the contrary, the presence of highly distorted conformers such as figure-eight structures reduces the number of effective hopping sites N_expt in large cyclic arrays C24Z and C32Z. Thus, our study demonstrates that not only the large number of porphyrin chromophores in the cyclic arrays CNZ but the overall rigidity and three-dimensional orientation in molecular architectures is a key factor to be considered in the preparation of artificial light harvesting porphyrin arrays.