Ligand-induced transformation to cyclic conformation of trimers of porphyrin dendrimer analogs

Abstract

Three zinc porphyrin dendrimer analogs with rigid conjugated backbones were synthesized and connected via a copper-catalyzed Hüisgen 1,3-dipolar cycloaddition reaction to form a trimer with two flexible linkages. Comparison of the ¹H NMR and UV–vis absorption spectra of the trimer with those of the corresponding monomers indicated that the trimer adopted an extended conformation in chloroform. The addition of a Y-shaped ligand with three pyridyl terminals to a chloroform solution of the trimer resulted in the formation of a 1:1 complex between the trimer and ligand. A binding constant of K = 2.7 × 10⁶ M⁻¹ was estimated by UV–vis titration. The characteristic up-field shift of the signals for pyridine protons was observed in the ¹H NMR spectrum of the 1:1 mixture of the trimer and ligand. Theoretical calculations suggested that coordination of pyridyl terminals to three zinc porphyrin cores led to the trimer adopting a cyclic conformation.