Synthesis of strapped zinc chlorophyll derivatives and their complexation with a single axial ligand

Zinc chlorins having a bridged moiety between 3- or 8- and 17-positions were designed as protected chlorophylls at one of π-faces, aiming at the investigation of asymmetric coordination ability towards the central metal. These strapped chlorins were synthesized by the cyclization of the dihydroxylated chlorins with dicarboxylic acid dichlorides under highly diluted conditions. The synthetic zinc chlorins complexed with pyridine as an axial ligand in benzene to form 5-coordinated species. The 1:1 binding constants determined by UV–vis titration method were smaller than those of the corresponding acyclic (unstrapped) compounds.