Spacers for Geometrically Well-Defined Water-Soluble Molecular Rulers and Their Application

The synthesis and application of monodisperse oligo(para-phenyleneethynylene)s (oligoPPEs) with side chains that are adjustable to specific needs, such as water solubility, on a very late stage of the multistep synthesis are described. The adjustable side chains allow for circumventing problems associated with the isolation of highly hydrophilic compounds during early stages of a synthesis. Furthermore, these oligoPPEs can be stocked as adaptable advanced building blocks for rapid assembly of tailor-made spacers and rulers. A rapid growth synthesis provides oligoPPEs with alkyne termini protected with the orthogonal groups trimethylsilyl and 1-hydroxy-1-methylethyl (HOP) and with propargyloxy side chains protected with the triisopropylsilyl group. The three protecting groups allow independent modification of the two termini and the side chains. The HOP group not only acts as a protecting group but also as a polar tag for simple product isolation. We demonstrate one application of these oligoPPEs as spacers for the water-soluble Gd rulers of the type Gd-PyMTA-spacer-Gd-PyMTA with Gd-Gd distances of 2.1-10.9 nm. For this purpose, the terminal alkyne units were used for backbone assembly and attachment of the ligand PyMTA, and the porpargyloxy side chains were used to attach water solubilizing poly(ethylene glycol) chains through a click reaction after spacer assembly.