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

Fourier transform Raman spectra of two covalently-linked tetraphenylpo rphyrin-viologen samples were obtained using 1064-nm excitation. It ha s been found that Raman spectra of the compounds can be viewed as the superposition of spectra of tetraphenylporphine(TPP) and methylviologe n-close inspection reveals that frequency shifts and relative intensit y 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. Th e failure to detect any significant frequency shifts for the Raman ban ds of the porphyrin moiety is attributed to the 'rigidity'' of this gr oup. The frequency shifts of the putative viologen bands are mainly at tributable to ground-state electron density redistribution from the po rphinato macrocycle to the viologen moiety induced by the intramolecul ar coupling. This latter inference is in accord with reported transien t 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 pho toinitiated electron transfer in excited electronic states.