Synthesis of chlorophyll derivatives possessing an S-substituted thiomethyl group at the 3-position and their optical properties

Methyl pyropheophorbides possessing an S-alkylated or arylated sulfanylmethyl group at the 3-position were prepared by the reaction of a 3-hydroxymethyl-chlorin with the corresponding thiols in the presence of zinc iodide. The sulfides were readily oxidized to the corresponding sulfoxides and sulfones and the chlorin was smoothly transformed to its metal complexes. Their Qy absorption maxima in dichloromethane were dependent on the S-functional groups at the 3(1)-position and bathochromically shifted by an increase of their electronegativities. The observation was due to their electron-withdrawing effect through homoconjugation of the 3(1)-substituents with the chlorin sr-system via a methylene moiety. Their fluorescence emission quantum yields were similarly affected by the 31-substituents and increased with suppression of the electron-donating ability of the S-atom. The partial fluorescence quenching in the sulfides would be ascribable to photoinduced electron transfer from the 3(1)-S to the singlet excited state of the chlorin moiety in a molecule. (C) 2015 Elsevier Ltd. All rights reserved.