RESONANCE RAMAN-SPECTROSCOPY OF A CHLOROPHYLL-PORPHYRIN HETERODIMER -EXCITATION PROFILE IN THE 400-450-NM REGION

Photophysical studies carried out previously on a chlorophyll-porphyri n heterodimer consisting of a zinc methyl pyrochlorophyllide a that is directly bonded at its 3-position to the 5-position of a zinc aethyl- 3,7,13,17-tetramethyl-15-(p-tolyl)porphyrin have shown that the chloro phyll and porphyrin interact within the excited singlet manifold. Ster ic hindrance between adjacent substituents rigidly positions the pi sy stems of both macrocycles perpendicular to one another. A similar orie ntation occurs between the primary donor and accessory bacteriochlorop hyll in the photosynthetic reaction center. To better understand the r elationship between the structure and the electronic states of the chl orophyll-porphyrin heterodimer, we have carried out a series of resona nce Raman measurements in the Soret band region of the chlorophyll-por phyrin heterodimer. Our data yield the assignment of the two individua l Soret bands in the 400-450-nm region to the chlorophyll and porphyri n macrocycles and allow us to characterize the vibrational frequencies present in the ground electronic state of the heterodimer. Coupling t he chlorophyll to the porphyrin via the meso position of the porphyrin does not change the vibrational frequencies of the porphyrin from tho se observed in the corresponding molecule in which a phenyl is attache d to the porphyrin meso position. In addition, the vibrational frequen cies 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.