Resonance Raman spectroscopic investigation of directly linked zinc(II) porphyrin linear arrays

We have investigated the resonance Raman (RR) spectra of the directly linked porphyrin arrays in order to elucidate the relationship between excitonic interactions and molecular geometry depending on the number of pigments in the arrays. The RR spectra obtained by photoexcitation at the high-energy exciton Soret bands in the arrays are mainly composed of Raman modes localized on the constituent porphyrin monomers. In contrast, the RR spectra of the arrays with photoexcitation at the low-energy exciton split Soret bands reveal some characteristic Raman bands arising from strong excitonic interactions between the adjacent porphyrins in the arrays. Based on the RR measurements of the isotope labeled analogues and the normal-mode analysis of the dimer, it is suggested that the photoexcitation at the high-energy Soret band produces an electronically excited state largely localized within a monomer unit and that at the low-energy exciton split Soret band the excited state is in a way delocalized throughout the array. Normal mode calculation revealed that some of the RR bands of Z2 arise from vibrational splitting by dimeric interactions. Phonon-like behaviors were observed for some C_m-C_m stretching modes of the arrays, which is ascribed to enhanced polarizability induced by phenyl group movement. Collectively, our data from RR spectroscopic measurements as well as the normal-mode analysis provide a picture of the exciton coupling in relation to the molecular structure of the directly linked linear porphyrin arrays.