Synthesis and properties of bisphosphole-bridged ladder oligophenylenes

Ladder-type oligophenylenes (LOPP) with bridging heteroatoms are interesting systems as they offer novel electronic and photophysical properties on account of the rigid structural features, more efficient electron delocalization on the coplanar aromatic framework, and strong intermolecular interactions. LOPPs incorporating multiple phosphorous centers combine the excellent electronic properties of phospholes and rigidified conjugated framework of LOPPs, thus positioning themselves as an attractive class of organic semiconductors. To date, there still lacks an effective synthetic methodology towards LOPPs with multiple phosphorous bridges. Herein, we describe the synthesis and properties of a new class of bisphosphole-bridged ladder oligo(p-phenylene)s and the related phosphoxides. The synthesis of phospholes was achieved by a four-fold free-radical phosphanylation reaction of a tetrabromo p-terphenylene or biphenyl-thiophene. Sequential trapping of four highly reactive aryl radicals occurred effectively to give the desired phosphorous-containing ladder compound. The oxides of the phospholes are shown to be strong fluorophores that can be used as potential n-type building blocks for organic semiconducting materials.