Synthesis of monodisperse oligo(1,4-phenyleneethynylene-alt-1,4-triptycyleneethynylene)s.

The synthesis of monodisperse oligo(p-phenyleneethynylene)s 8a(n) with alternating 2,5-dihexyl-1,4-phenylene and 6,14-di-tert-butyl-1,4-triptycylene units and orthogonally protected alkyne end groups is reported. Starting from 6,14-di-tert-butyl-1-(2-triisopropylsilylethynyl)-4-(2-trimethylsilylethynyl)triptycene (5a), 1,4-dihexyl-2,5-diiodobenzene (10), and 1,4-dihexyl-2-iodo-5-(3-hydroxyprop-1-ynyl)benzene (9), oligomers with up to four repeating units, i.e., eight phenyleneethynylene units, were prepared through a partially divergent-convergent route with the alkynyl-aryl (Sonogashira-Hagihara) coupling as the key reaction. The starting compound 5a was prepared from triptycenequinone through a sequence of addition of 2-trialkylsilylethynyllithium, reduction and concomitant elimination of water, conversion of the phenol into a triflate, and finally Pd/Cu-catalyzed coupling with trialkylsilylethyne. A similar access to the key compound for a stringent divergent-convergent route, 6,14-di-tert-butyl-1-(3-hydroxybut-1-ynyl)-4-(2-triisopropylsilylethynyl)triptycene (6), is reported. The optical properties of the oligomers 8a(n) and the corresponding oligo(2,5-dihexyl-1,4-phenyleneethynylene)s in dilute solution are almost identical, whereas they differ significantly for the solid, undiluted compounds.