Fluorescence line narrowing studies on isolated chlorophyll molecules

Fluorescence line-narrowing and resonance Raman properties of various chlorophylls have been measured in organic solvents. Resonance Raman spectroscopy is already a well-established method for the study of photochemical reactions in the various pigment-protein complexes involved in photosynthesis, while fluorescence line-narrowing is still an emerging technique for such systems. Interpretation of these vibrational spectra requires accurate comparative data on the pure isolated pigments. By comparing three different chlorophylls, a, b, and d, which have different substituents on the porphyrin ring, the various spectral lines associated with vinyl and formyl groups on the X and Y electronic axes could be distinguished. The difference between five- and six-coordination of the central Mg atom in FT-Raman spectra was determined by varying the organic solvent used. These chlorophylls are important in photosynthesis: all three in light-harvesting and energy transfer and, in the case of a and d, also in electron transfer. The assignment of spectral bands which we provide here, along with the description of their behavior with respect to the conformation and state of interaction of the pigment molecule, constitutes an essential step if these vibrational techniques are to be exploited to their full potential.