Synthesis of oxo-, thioxo- and methylene-substituted bacteriochlorins by modifying chlorophyll-a and their electronic absorption in visible and near-infrared regions

Regioisomerically pure 7/8,131-dioxo-bacteriochlorins prepared by modification of naturally occurring chlorophyll-a were reacted with Lawessonʹs reagent in toluene at room temperature or reflux to give 7/8-oxo-131-thioxo- or 7/8,131-dithioxo-products, respectively. The 131-thioxo moiety of the latter products was selectively transformed to the oxo group to afford 7/8-thioxo-131-oxo-bacteriochlorins. The remaining 7/8-thioxo moiety was treated with diazomethane to be successfully converted to the methylene group. Electronic absorption and circular dichroism spectra of all the synthetic bacteriochlorins were measured in 20% (v/v) tetrahydrofuran and hexane. Comparison of these spectra showed that thiocarbonylation induced red-shifts of all the absorption maxima in visible and near-infrared regions and 7-oxo-derived bacteriochlorins afforded their maxima at a longer wavelength than those of the corresponding 8-substituted regioisomers.