Synthesis of monovinyl- and divinyl-chlorophyll analogs and their physical properties

Chlorophyll-a was chemically modified to methyl pyropheophorbides-a possessing 3,8-diethyl, 3-vinyl-8-ethyl, 3-ethyl-8-vinyl, and 3,8-divinyl groups. Analogous 3-ethyl-7-vinyl- and 3,7-divinyl-chlorins were prepared by derivatization of chlorophyll-b. The synthetic free bases as well as zinc 3-vinyl-chlorins were dissolved in THF and the monomeric diluted solutions were characterized by optical spectroscopy including visible absorption, circular dichroism, and fluorescence emission spectra. The optical data indicated that the 3-vinylation bathochromically shifted the visible (Soret/Qx/Qy) absorption and fluorescence emission maxima, the 7-vinylation moved the Soret/Qx and Qy/emission bands to longer and slightly shorter wavelengths, respectively, and the 8-vinylation induced red shifts of the Soret/Qx maxima and no (or faint red) shifts of the Qy/emission maxima. Zinc complexes of 3,7- and 3,8-divinyl-chlorins showed almost the same optical properties including fluorescence emission quantum yields and lifetimes as well as the same first oxidation potentials, thus, 3,7-divinyl-chlorophyll-a could be considered an alternative pigment to the naturally occurring 3,8-divinyl analog.

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