Insulated Molecular Wires: Dendritic Encapsulation of Poly(triacetylene) Oligomers, Attempted Dendritic Stabilization of Novel Poly(pentaacetylene) Oligomers, and an Organometallic Approach to Dendritic Rods

Multinanometer‐long end‐capped poly(triacetylene) (PTA) and poly(pentaacetylene) (PPA) oligomers with dendritic side chains were synthesized as insulated molecular wires. PTA Oligomers with laterally appended Fréchet ‐type dendrons of first to third generation were prepared by attaching the dendrons ( 8 , 13 , and 17 , respectively, Scheme 1 ) to ( E )‐enediyne 18 by a Mitsunobu reaction and subsequent Glaser‐Hay oligomerization under end‐capping conditions ( Scheme 2 ). Whereas first‐generation oligomers up to the pentamer were isolated ( 1a  –  e ), for reasons of steric overcrowding, only oligomers up to the trimer ( 2a  –  c ) were formed at the second‐generation level, and only the end‐capped monomer and dimer ( 3a ,  b ) were isolated at the third‐generation level. By repetitive sequences of hydrosilylation (with the Karstedt catalyst), followed by allylation or vinylation, a series of carbosilane dendrons were also prepared ( Schemes 3 and 4 ). Attachment of the second‐generation wedge 40 to ( E )‐enediyne 18 , followed by deprotection and subsequent end‐capping Hay oligomerization, provided PTA oligomers 4a  –  d with lateral carbosilane dendrons ( Scheme 5 ). UV/VIS Studies ( Figs. 5 – 10 ) demonstrated that the insulating dendritic layers did not alter the electronic characteristics of the PTA backbone, even at the higher‐generation levels. Despite distortion from planarity due to the bulky dendritic wedges, no loss of π ‐electron conjugation along the PTA backbone was detected. A surprising ( E )→( Z ) isomerization of the diethynylethene (DEE) core in the third generation derivative 3a was observed, possibly photosensitized by the bulky Fréchet ‐type dendritic wedge. Electrochemical investigations by steady‐state voltammetry and cyclic voltammetry showed that the first reduction potential of the PTA oligomer with Fréchet ‐type dendrons is shifted to more negative values as the dendritic coverage increases. With compounds 5a  –  c , the first oligomers with a poly(pentaacetylene) backbone were obtained by oxidative Hay oligomerization under end‐capping conditions ( Scheme 6 ). The synthesis of dendritic PPA oligomers by oxidative coupling of ( E )‐enetetrayne 60 under end‐capping conditions provided oligomers 61a  –  d , which were formed as mixtures of stereoisomers due to unexpected thermal ( E )→( Z ) isomerization ( Scheme 8 ). In another novel approach towards dendritic encapsulation of molecular wires with a Pt‐bridged tetraethynylethene (TEE) oligomeric backbone, the trans ‐dichloroplatinum(II) complex trans‐ 67 with dendritic phosphane ligands ( Fig. 14 ) was coupled under Hagihara conditions to mono‐deprotected 69 under formation of the extended monomer 65 ( Scheme 12 ). Again, an unexpected thermal ( E )→( Z ) isomerization, possibly induced by steric strain between TEE moieties and dendritic phosphane ligands in the unstable complex, led to the isolation of 65 as an isomeric mixture only.