Difluoroboron complexes of peripheral β-diketonates in cyclopheophorbides: Their syntheses and optical properties

Cyclopheophorbide-a prepared from intramolecular mixed Claisen condensation of methyl pyropheophorbide-a (one of the chlorophyll-a derivatives) was complexed with a difluoroboron species at the β-diketonate moiety on the exo-five-membered ring fusing the core cyclic tetrapyrrole π-skeleton. Its 3-substituted chlorin analogs and the related bacteriochlorin were also synthesized by difluoroboronation of the corresponding (bacterio)chlorins possessing a peripheral diketonate group. In solution, all the synthetic complexes showed broad Qy and redmost Soret absorption bands and an intense bluemost Soret band. Their maxima were dependent on the 3-substitution and the 7,8-dihydrogenation. The boron complexes were non-fluorescent, which was comparable to the boron-free counterparts.