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 Frechet-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 (1a, b) were isolated at the third-generation level. By repetitive sequences of hydrosilylation (with the Karstadt 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 3). 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 pi -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 Frechet-type dendritic wedge. Electrochemical investigations by steady-state voltammetry and cyclic voltammetry showed that the first reduction potential of the PTA oligomer with Frechet-type dendrons is shifted to more negative values as thr dendritic coverage increases. With compounds 5a-c. the first oligomers with a poly(pentaacetylene) backbone were obtained by oxidative Hav 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.