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

Abstract

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 3 1-epimers were separated by reverse-phase HPLC. Absorption spectra of both the synthetic free-base and zinc chlorins in CH 2Cl 2 showed that the C8-substituents conjugating directly with the chlorin ring shifted the Soret and Q x peaks to longer wavelengths while retaining the position of the Q y band at around 650 nm. Typically, the observed Q x 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) CH 2Cl 2–cyclohexane, all the epimeric zinc chlorins showed red-shifts of the Q y bands (≈690–710 nm) and stronger CD peaks in the red-shifted Q y region, compared to those in CH 2Cl 2. 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.