Studies of crystalline aggregates of pheophorbide and bacteriopheophorbide by electro-absorption (Stark effect) spectroscopy and powder X-ray diffraction

Absorption and electro-absorption (Stark effect) spectra of polycrystalline and lyophillized samples of pheophorbides and bacteriopheophorbides have been measured in order to obtain information on the consequences of intermolecular interactions on electronic structure. The results support the general hypothesis that large red shifts in the absorption spectra of photosynthetic aggregates are associated with increased dipolar character in the excited state due to mixing with intermolecular charge transfer states. In some cases, new bands, which are difficult to detect or absent in the absorption spectrum, are observed in the electro-absorption spectrum, and these may be charge-transfer transitions. The structures of several polycrystalline aggregates, either ground single crystals or lyophillized powders, were analyzed by powder X-ray diffraction. In some cases, the powder diffraction of lyophyllized powders 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.