STUDIES OF CRYSTALLINE AGGREGATES OF PHEOPHORBIDE AND BACTERIOPHEOPHORBIDE BY ELECTROABSORPTION (STARK-EFFECT) SPECTROSCOPY AND POWDER X-RAY-DIFFRACTION

Absorption and electro-absorption (Stark effect) spectra of polycrysta lline and lyophilized samples of pheophorbides and bacteriopheophorbid es have been measured in order to obtain information on the consequenc es of intermolecular interactions on electronic structure. The results support the general hypothesis that large red shifts in the absorptio n spectra of photosynthetic aggregates are associated with increased d ipolar character in the excited state due to mixing with intermolecula r charge transfer states. In some cases, new bands, which are difficul t to detect or absent in the absorption spectrum, are observed in the electro-absorption spectrum, and these may be charge-transfer transiti ons. The structures of several polycrystalline aggregates, either grou nd single crystals or lyophilized powders, were analyzed by powder X-r ay diffraction. In some cases, the powder diffraction of lyophylized p owders was found to be nearly identical to that predicted from single crystal diffraction data on the same chromophore. High quality electro -absorption spectra were also obtained and analyzed for the monomeric pigments.