Ring-size controlled dimerization of synthetic zinc chlorophyll derivatives possessing a 1-azacycloalkyl group through mutual coordination of amino moiety to central zinc atom

Abstract

Zinc methyl 3-devinyl-pyropheophorbides-a bearing a (1-azacycloalkyl)methyl group at the 3-position were prepared by chemically modifying naturally occurring chlorophyll-a. The azacyclobutylated molecules closely dimerized through double coordination of the 31-nitrogen atom to the central zinc atom in 1%(v/v) dichloromethane and hexane to give red-shifted visible absorption and reverse S-shaped circular dichroism (CD) bands. The azacyclopentyl analog formed similarly π–π interacted and A-ring overlapped dimers with relatively weakened and lengthened Zn–N bonds to show less split excitonic bands, and its cofacial and anti-parallel supramolecule giving S-shaped CD bands was inversely twisted by the insertion of a methylene group in the 1-azacycloalkyl moiety. One more methylene insertion as in the azacyclohexyl derivative induced a least coordination ability and no more dimerization, so the compound with the most sterically crowded 31-N was monomeric in the less polar organic solvent. Steric factor at around the 31-nitrogen atom controlled the dimerization and affected the supramolecular structure with excitonically coupling visible absorption and CD bands.