Visible absorption and fluorescence emission of synthetic dimers of chlorophyll-a derivatives linked with a π-conjugated oligoene spacer at the peripheral 3-position

Abstract

Naturally occurring chlorophyll-a was chemically modified to give homo-dimers of methyl pyropheophorbide-a linked with or without a vinylidene group at the 3-vinyl terminal. Access to the hexatriene- and butadiene-linked dyads was achieved through the olefin metathesis of the vinyl group in the peripheral substituent on the chlorin π-system and successive dehydration in the resulting spacer between the chlorin cores. The conjugation of the two chlorin moieties via the oligoene linkers moved the visible absorption and fluorescence emission maxima in dichloromethane to longer wavelengths. Especially, the red-shifted and broadened Soret absorption bands reached to a green-light region around 500 nm, and the Qy absorption maxima were located at a far-red region around 700 nm. The dimerization enhanced the fluorescence emission efficiency with quantum yields of approximately 30%. The intramolecular through-bond interaction between the two chlorin π-systems in the dimers was revealed by comparison with the optical properties of their corresponding monomeric counterparts.