EPR probes with well-defined, long distances between two or three unpairedelectrons

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 ap art from each other. The shape-persistent scaffolds were obtained through P d-Cu-catalyzed alkynyl-aryl coupling and Pd-Cu-catalyzed alkyne dimerizatio n in the presence of oxygen using p-phenyleneethynylene as the basic shape- persistent building block. The spin label 1-oxyl-2,2,5,5-tetramethylpyrroli ne-3-carboxylic acid (4) was attached through esterification of the termina l phenolic OH groups of the scaffold.