Synthesis and optical properties of bacteriochlorophyll-a derivatives having various C3 substituents on the bacteriochlorin π-system

Methyl bacteriopyropheophorbide-a derivatives having a series of substituents at the C3 position were prepared and their optical properties were compared with the corresponding chlorin analogues. Two kinds of oxidation reaction (C3-vinyl -> formyl -> carboxy group) were found to be applicable with a little alteration of the free-base bacteriochlorin macrocycles. The Q(x) and Q(y) electronic absorption peak positions of synthetic bacteriochlorins in CH2Cl2 were affected by the C3 substituents and found to be more sensitive than those of the chlorins. The observed Q(x)/Q(y) peaks in their monomeric states were shifted to a longer wavelength in the order of 1-hydroxyethyl < hydroxymethyl < acetoxymethyl < vinyl < acetyl < carboxy < formyl < 2,2-dicyanoethynyl group. Zinc complex with the C3-hydroxymethyl group formed self-aggregates in a nonpolar organic solvent, which showed the largest red-shift of the Q(y) band (2380 cm(-1), 726 nm in THF to 878 nm in 1% THF-cyclohexane) among those of the synthetic self-aggregative (bacterio)chlorins examined.