The development of a new experimental method for producing laser wavelengths in the Soret Banci region of bacteriochlorophyll (BChl) and its electrogenerated cation radical permits a more detailed study of the effect of excitation wavelength on the resonance Raman (RR) spectrum than was previously possible. Stimulated Raman scattering (SRS) from H2 or D2 under high pressure is driven with the second or third harmonics of a Nd:YAG laser. Wavelengths ranging from 396.7 to 502.9 nm are generated. The highest energy wavelength is on the shoulder of the strongest Soret transition in neutral BChl. Good quality RR spectra are obtained at this wavelength, and no evidence of photodegradation is observed. In the case of BChl cation radical, however, the effect of excitation wavelength is more dramatic because it is possible to excite near resonance with three different intense electronic transitions by using the available laser lines. In addition, the resulting spectra show clearly that one-electron oxidation of BChl causes distinctive changes in its RR spectrum. Two intense RR Bancis seen in the BChl+• spectrum are absent (1414 cm-1) or only weakly observed (1340 cm-1) in the corresponding spectra for BChl. These results indicate that selective RR monitoring of the formation and decay kinetics of the cation radical in vivo should be possible by using 416.0-nm excitation together with detection of the 1414- or 1340-cm-1 Raman peaks. The RR spectra of BChl and BChl+• are examined in terms of recent ab initio configuration interaction calculations regarding the nature of the electronic transitions in the Soret region. The resonance enhanced Raman spectra agree qualitatively with changes in the molecular geometry which might be expected on the basis of the molecular orbital composition of the excited electronic state relative to the ground state.
ASJC Scopus subject areas
- Colloid and Surface Chemistry