INFLUENCE OF β-SUBSTITUENTS ON MACROCYCLE SYMMETRY OF FREE- BASE PORPHYRINS: A POLARIZATION-RESOLVED RESONANCE CARS AND RESONANCE RAMAN SPECTROSCOPIC ANALYSIS

detection Abstract: Resonance CARS and resonance Raman spectroscopies are applied to characterize subtle structural differences in four selected β-substituted free-base porphyrins. A quantitative analysis of the polarization-resolved spectra under various excitation conditions is presented. Porphyrins form the active site of numerous biological systems. Extensive spectroscopic studies on these molecules have been carried out [1], among them resonance Raman spectroscopy [2,3]. Absorption spectra of porphyrins are characterized by strong absorption bands in the visible and near UV region, leading to a strong resonance enhancement of Raman scattering intensities. A significant enhancement of totally symmetric modes is generally achieved with excitation wavelengths coinciding with the B (or Soret) absorption band of porphyrins. Under these conditions, the scattering process is governed by Albrecht’s A term. With excitation wavelengths in the Q band region, however, B term scattering becomes the favoured mechanism, leading to dominant contributions from non-totally symmetric modes. As an example, the absorption spectrum of uroporphyrin I octamethyl ester dissolved in dichloromethane is shown in Fig. 1