EPR Probes with Well-Defined, Long Distances between Two or Three Unpaired Electrons

Abstract
The synthesis of rod- and star-shaped compounds carrying two or three spin labels as end groups is described. The unpaired electrons are 2.8−5.1 nm apart from each other. The shape-persistent scaffolds were obtained through Pd−Cu-catalyzed alkynyl−aryl coupling and Pd−Cu-catalyzed alkyne dimerization in the presence of oxygen using p-phenyleneethynylene as the basic shape-persistent building block. The spin label 1-oxyl-2,2,5,5-tetramethylpyrroline-3-carboxylic acid (4) was attached through esterification of the terminal phenolic OH groups of the scaffold.