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

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Abstract

Resonance Raman (RR) spectra of aggregates of tetrakis(4-sulfonatophenyl)porphyrin diacid (H4TSPP2-) excited near exciton absorption bands of 489 nm (JB band) and 421 nm (HB 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 H4TSPP2- have been studied. It was deduced from the analyses that there are large dimensionless displacements on the excited state along the ν8, γ16, and ν2 coordinates for both aggregated and segregated H4TSPP2-. These normal modes involve mainly the CαCm, CβCβ bond stretching and CαCmCα bond angle bending motions. The low-frequency RR bands (<500 cm-1) were greatly enhanced when the excitation wavelength approaches the JB absorption band of H4TSPP2- aggregates, which was attributed to narrowing of the JB band due to the exciton interaction. It was revealed that the 489 and 421 nm absorption bands of H4TSPP2- aggregates can be attributed to two different exciton manifolds originating from the degenerated B state (S2 state) of H4TSPP2-. 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 H4TSPP2- aggregates.

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