Synthesis and electronic properties of bridged [8]-, [12]- and [16]-cyclo-para-phenylenes

Abstract

Hoop-shaped π-conjugated macrocycles constitute a new family of π-conjugated systems displaying unique properties induced by their radially directed π-orbitals. The electronic properties of these nanohoops are deeply dependent on their building units and their size. We report herein the first size dependence study of [8]-, [12]- and [16] cyclo-para-phenylenes bridged by nitrogen atoms. The properties were elucidated by ¹H NMR, cyclic voltammetry, UV-vis absorption and fluorescence spectroscopy (in solution and in thin film). Thus, this work reports one of the largest bridged nanohoops synthesized, namely [4]cyclo-para-9-ethylhexyl-2,7-diphenyl-carbazole [16]CPP4N, an analog of [16]CPP, which displays an alternation of four unbridged biphenylenes and four nitrogen bridged biphenylenes. Removing either all the unbridged phenylenes ([4]cyclo-para-9-ethylhexyl-carbazole [8]CPP4N, an analog of [8]CPP) or one building unit ([3]cyclo-para-9-ethylhexyl-2,7-diphenyl-carbazole [12]CPP3N, an analog of [12]CPP) allows decreasing the size of the nanohoop with nevertheless drastically different consequences on the properties. This is what we aim at reporting in this work.