Resonance Raman spectra and excited-state structure of aggregated tetrakis(4-sulfonatophenyl)porphyrin diacid

Resonance Raman (RR) spectra of aggregates of tetrakis(4-sulfonatophenyl)po rphyrin diacid (H4TSPP2-) excited near exciton absorption bands of 489 nm ( J(B) band) and 421 nm (H-B band) were recorded and analyzed, and RR intensi ties of the aggregates and segregated monomers were calculated with time-de pendent resonant Raman formulas for the Franck-Condon mechanism of enhancem ent. Their UV-visible absorption spectra were also calculated spontaneously . On the basis of the RR spectra and computed results, the ground and excit ed-state structures as well as molecular packing of aggregated H4TSPP2- hav e been studied. It was deduced from the analyses that there are large dimen sionless displacements on the excited state along the nu (8), gamma (16), a nd nu (2) coordinates for both aggregated and segregated H4TSPP2-. These no rmal modes involve mainly the CalphaCm, CbetaCbeta bond stretching and Calp haCmCalpha bond angle bending motions. The low-frequency RR bands (< 500 cm (-1)) were greatly enhanced when the excitation wavelength approaches the J (B) absorption band of H4TSPP2- aggregates, which was attributed to narrowi ng of the J(B) band due to the exciton interaction. It was revealed that th e 489 and 421 nm absorption bands of H4TSPP2- aggregates can be attributed to two different exciton manifolds originating from the degenerated B state (S-2 state) of H4TSPP2-. Davydov splitting of the aggregates evaluated fro m molecular exciton theory is well-coincident with the measured values, whi ch supports a one-dimensional ribbon model for H4TSPP2- aggregates.