Polarization-sensitive CARS spectroscopy on free-base porphyrins:: coproporphyrin I tetramethyl ester

The application of polarization-sensitive (PS) coherent anti-Stokes Raman scattering (CARS) spectroscopy for the investigation of highly luminescent free-base porphyrins under Q, band resonance is discussed. For coproporphyrin I tetramethyl ester (CP-I-TME), PS CARS spectra involving resonances with the electronic Q, absorption band as well as polarized spontaneous Raman spectra involving B band resonance are presented. A quantitative evaluation of the CP-1-TME spectra is performed and the results are compared to our previously presented data on free-base octaethylporphine (OEP) and mesoporphyrin IX dimethyl ester (MP-IX-DME), which were obtained under identical excitation conditions. This comprehensive analysis reveals several spectral differences that can be attributed to the different fi-substitution pattern of the porphyrin macrocycle. Additionally, the strong resonance enhancement of totally symmetric modes under Q., band excitation is identified as a common property for OEP, CP-I-TME, and MP-IX-DME; this enhancement selectivity distinguishes the investigated substances from what is generally observed for metallo porphyrins. Copyright (c) 2008 John Wiley & Sons, Ltd.