Synthesis of cationic N-methylated chlorophyll derivatives and C3-substitution effect on their optical properties

Abstract

Methyl N22-methyl-pyropheophorbides possessing various substituents at the C3-position were prepared by modifying natural chlorophyll-a. Their syntheses were based on the regioselective N22-methylation of a chlorin π-system and/or the functionalization of the C3-substituents in the resulting N22-methyl-chlorins. The synthetic N22-methyl-chlorins were obtained as cationic species due to their high basicity. The N22-methylation largely affected the ultraviolet–visible absorption and fluorescence emission spectra and especially moved the Qy absorption and intense emission maxima to shorter wavelengths and the Qx and intense Soret maxima to longer wavelengths. The C3-substitution effect on the redmost absorption and bluemost emission maxima in the N22-methyl-chlorins was smaller than that in the N22-unsubstitutes. In most cases, the N22-methylation increased the Stokes shifts and decreased the fluorescence emission quantum yields and lifetimes, which were ascribable to the flexibility of the distorted N22-methyl-chlorin π-planes.