Synthesis of chlorophyll-a derivatives possessing the 3-(2-acylethenyl) group by cross-aldol condensation and their optical properties

Chlorophyll-a derivatives possessing a trans-2-acylethenyl group at the C3-position were prepared by cross-aldol (Claisen-Schmidt) condensation of methyl pyropheophorbide-d having the C3-formyl group with ketones bearing α-hydrogen atoms including (un)substituted acetophenones, acetylarenes, and alkanones under basic conditions. The Qy absorption and fluorescence emission bands of the synthetic chalcone analogs in dichloromethane were largely dependent on the C3³-substituents. Especially, the introduction of electron-donating and withdrawing groups to the C3³-phenyl ring resulted in bathochromic and hypsochromic shifts of the maxima, respectively. Furthermore, fluorescence quantum yields and lifetimes were largely suppressed by the substitution with a nitro group on the C3³-phenyl ring.