Resonance Raman spectroscopy of a chlorophyll-porphyrin heterodimer: Excitation profile in the 400-450-nm region

Photophysical studies carried out previously on a chlorophyll-porphyrin heterodimer consisting of a zinc methyl pyrochlorophyllide a that is directly bonded at its 3-position to the 5-position of a zinc 2,8,12,18-tetraethyl-3,7,13,17-tetramethyl-15-(p-tolyl)porphyrin have shown that the chlorophyll and porphyrin interact within the excited singlet manifold. Steric hindrance between adjacent substituents rigidly positions the π systems of both macrocycles perpendicular to one another. A similar orientation occurs between the primary donor and accessory bacteriochlorophyll in the photosynthetic reaction center. To better understand the relationship between the structure and the electronic states of the chlorophyll-porphyrin heterodimer, we have carried out a series of resonance Raman measurements in the Soret band region of the chlorophyll-porphyrin heterodimer. Our data yield the assignment of the two individual Soret bands in the 400-450-nm region to the chlorophyll and porphyrin macrocycles and allow us to characterize the vibrational frequencies present in the ground electronic state of the heterodimer. Coupling the chlorophyll to the porphyrin via the meso position of the porphyrin does not change the vibrational frequencies of the porphyrin from those observed in the corresponding molecule in which a phenyl is attached to the porphyrin meso position. In addition, the vibrational frequencies of the chlorophyll within the heterodimer remain unperturbed from those observed in the chlorophyll alone. Our data are consistent with a ground-state structure for the heterodimer that is very similar to that of its component macrocycles.