Synthesis of 32-nitro-chlorophyll-a derivatives and their electronic absorption/emission data

The formyl group at the 3-position of methyl pyropheophorbide-d, one of chlorophyll-a derivatives, was efficiently transformed to the 32-cis-unsubstituted/methyl- or 32-cis-phenyl-32-trans-nitro-vinyl groups by Henry (nitro-aldol) or nitro-Mannich (aza-Henry) reactions with nitromethane/ethane or nitrophenylmethane, respectively. The introduction of a nitro group at the terminal of the 3-ethenyl moieties directly connected with the chlorin π-system red-shifted and broadened their Qy electronic absorption and fluorescence emission bands in a solution. The fluorescence intensities were largely suppressed by the substitution with the nitro group.