Resonance Raman Spectra and Excited-State Structure of Aggregated Tetrakis(4-sulfonatophenyl)porphyrin Diacid

Resonance Raman (RR) spectra of aggregates of tetrakis(4-sulfonatophenyl)porphyrin diacid (H 4 TSPP 2- ) excited near exciton absorption bands of 489 nm (J B band) and 421 nm (H B band) were recorded and analyzed, and RR intensities of the aggregates and segregated monomers were calculated with time-dependent resonant Raman formulas for the Franck−Condon mechanism of enhancement. Their UV−visible absorption spectra were also calculated spontaneously. On the basis of the RR spectra and computed results, the ground and excited-state structures as well as molecular packing of aggregated H 4 TSPP 2- have been studied. It was deduced from the analyses that there are large dimensionless displacements on the excited state along the ν 8, γ 16, and ν 2 coordinates for both aggregated and segregated H 4 TSPP 2- . These normal modes involve mainly the C α C m, C β C β bond stretching and C α C m C α 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 H 4 TSPP 2- aggregates, which was attributed to narrowing of the J B band due to the exciton interaction. It was revealed that the 489 and 421 nm absorption bands of H 4 TSPP 2- aggregates can be attributed to two different exciton manifolds originating from the degenerated B state (S 2 state) of H 4 TSPP 2- . Davydov splitting of the aggregates evaluated from molecular exciton theory is well-coincident with the measured values, which supports a one-dimensional ribbon model for H 4 TSPP 2- aggregates.