Effects of C8-substituents on spectroscopic and self-aggregation properties of synthetic bacteriochlorophyll-d analogues

As a model compound of bacteriochlorophyll(BChl)-d which is known as an antenna pigment of green photosynthetic bacteria, methyl bacteriopheophorbide-d derivatives having a series of substituents at the 8-position and their zinc complexes were synthesized. Unnatural type analogues were prepared by Wittig reaction of the 8-formyl group, and the 31-epimers were separated by reverse-phase HPLC. Absorption spectra of both the synthetic free-base and zinc chlorins in CH2Cl2 showed that the C8-substituents conjugating directly with the chlorin ring shifted the Soret and Qx peaks to longer wavelengths while retaining the position of the Qy band at around 650 nm. Typically, the observed Qx peaks in wavelength were situated in the order of ethyl ≈ phenylethyl < vinyl ≈ cis-styryl < trans-styryl as the C8-substituent reflecting the degree of the conjugation with the chlorin π-system. In 1% (v/v) CH2Cl2–cyclohexane, all the epimeric zinc chlorins showed red-shifts of the Qy bands (≈690–710 nm) and stronger CD peaks in the red-shifted Qy region, compared to those in CH2Cl2. These spectroscopic changes indicated the formation of well-ordered self-aggregates of the synthetic zinc chlorins in non-polar organic solvents similar to BChl-d aggregates in a natural antenna system. It was shown that the effect of π-conjugation or sterical hindrance of the C8-substituents does not strongly affect the self-aggregation.