A Zirconocene-Coupling Route to Substituted Poly(<i>p</i>-phenylenedienylene)s:  Band Gap Tuning via Conformational Control

A series of substituted poly(p-phenylenediyne)s (1a−f) was synthesized by the palladium-catalyzed cross-coupling condensation of terminal dialkynylalkanes with substituted diiodobenzenes. Polymerizations were conducted with 1,4-diiodo-5-hexoxy-2-methoxybenzene or 1,4-diiodo-5-hexyl-2-methylbenzene and with 1,6-hexadiyne, 1,7-heptadiyne, or 1,8-octadiyne. Thus, six different poly(p-phenylenediyne)s (1a−f, [ArC⋮C(CH2)nC⋮C]m; Ar = 5-hexoxy-2-methoxyphenylene or 5-hexyl-2-methylphenylene, and n = 2, 3, or 4) were obtained. Intramolecular zirconocene couplings of 1a−f afforded zirconacyclopentadiene-containing polymers 2a−f. These metal-containing polymers were then cleanly hydrolyzed to the substituted poly(p-phenylenedieneylene)s = 5-hexoxy-2-methoxyphenylene or 5-hexyl-2-methylphenylene, and n = 2, 3, or 4). Variation of the polymer structure allows for manipulation of the absorption and emission maxima over the ranges of 316−524 nm and 437−619 nm, respectively. The optical properties of model compounds = phenyl or 2,5-dimethoxy-4-methylphenyl, and n = 2, 3, or 4) are very similar to those for polymers 3a−f. The structures of 9c (Ar = 2,5-dimethoxy-4-methylphenyl, n = 2) and 9d (Ar = phenyl, n = 4) were determined by single-crystal X-ray crystallography. Molecules of 9c are almost completely planar, while molecules of 9d are twisted and have poor π-orbital overlap. The conformation of the diene unit in polymers 3a−f and the model compounds 9a−f are largely responsible for the observed variations in optical properties. Finer tuning of the optical properties for poly(p-phenylenedienylene)s may be achieved via synthesis of copolymers from mixtures of 1,6-heptadiyne and 1,7-octadiyne. The optical properties of the resulting copolymers represent weighted averages for the corresponding homopolymers. Investigation of the photophysical properties of polymers retaining some of the diyne units suggest that defects play a major role in defining the emissive properties of the poly(p-phenylenedienylene)s described here.