NEAR-INFRARED-EXCITED FOURIER-TRANSFORM RAMAN-SPECTROSCOPIC STUDY OF COVALENTLY-LINKED PORPHYRIN VIOLOGEN COMPOUNDS

Fourier transform Raman spectra of two covalently-linked tetraphenylporphyrin-viologen samples were obtained using 1064-nm excitation. It has been found that Raman spectra of the compounds can be viewed as the superposition of spectra of tetraphenylporphine(TPP) and methylviologen-close inspection reveals that frequency shifts and relative intensity variations occur mainly for Raman bands that are attributable to the vibrations of the viologen moiety, while only a decrease in relative intensity of the porphinato macrocycle skeletal modes is discerned. The failure to detect any significant frequency shifts for the Raman bands of the porphyrin moiety is attributed to the 'rigidity'' of this group. The frequency shifts of the putative viologen bands are mainly attributable to ground-state electron density redistribution from the porphinato macrocycle to the viologen moiety induced by the intramolecular coupling. This latter inference is in accord with reported transient absorption and fluorescence measurements. It is concluded that near-IR FT-Raman measurements are useful in identifying structural changes accompanying electron density redistribution which might occur for photoinitiated electron transfer in excited electronic states.