Diethynylbenzenes

Surprisingly, the simplest graphdiyne model, PDM 1, had eluded synthesis. Even though substituted derivatives (e. g. 75,85) have been prepared by the standard cyclooligomerization reaction of o-diethynylbenzenes, isolation of the parent molecule failed via this method [8,51]. Utilization of an intramolecular... 

Another limitation of the traditional Cu-mediated cyclooligomerization reaction is generation of differentially substituted PDMs. In the above case, the substitution pattern in the starting o-diethynylbenzene must be maintained on each and every benzene moiety in the oligomeric mixture of PDMs that is produced. Thus, it is impossible to prepare less symmetric systems like 100 via this route. With the intramolecular synthetic approach, however, it should be possi-... 

Beside the polymerization routes of 1,3-cyclohexadiene derivatives repetitive Diels-Alder polyadditions were widely used to prepare arylated PPPs. Stille et al. developed a set of suitable monomers (1,4-diethynylbenzene and 1,4-phenyl-ene-bis(triphenylcyclopentadienone) derivatives) to generate phenylated PPPs (e.g. 17) with molecular weights of 20,000-100,000 [31]. Unfortunately, the repetitive polyadditon does not proceed regioselectively polymers containing para-as well as mefa-phenylene units within the main chain skeleton are formed. 

Ando and co-workers have reported the synthesis of a silyl-carborane hybrid diethynylbenzene-silylene polymer (108) (Fig. 66) possessing high thermal stability.136 The polymer contained Si and —C=C— group in the main chain and m-carborane and vinyl groups in the side chain. The 5% weight-loss temperature of the cured polymer in air was over 1000°C as determined by thermogravimetric analysis. 

Figure 66 The thermooxidatively stable silyl-carborane hybrid diethynylbenzene-silylene polymer 108 containing various carbon-carbon unsaturations. (Adapted from ref. 136.)...

Tetra-dehydrobromination of the compound in benzene with ethanolic potassium hydroxide for the commercial production of 1,4-diethynylbenzene is highly exothermic and needs careful control to avoid hazard. 

Figure 4. The various mechanisms for structural "shrink-wrapping" in bilayer GS inclusion compounds can be gleaned from these illustrative examples, here viewed down the channel direction, (a) quasihexagonal G2IH 2(CH3CN), (b) quasihexagonal G2V-1,4-diethynylbenzene, (c) shifted...

Tour s work showed that polymerization is still a preferred process even though large excesses ofradical traps (1,4-cyclohexadiene) were employed. They obtained poly( 1,4-naphthalene) only as an insoluble brown powder by polymerization of 1,2-diethynylbenzene in benzene solution. This route appears to be an attractive approach to a new dielectric medium ... 

The diethynylbenzene analogues, depending on their individual properties, can be evaporated or sublimed into the reaction zone. The lower limit of the deposition temperature for poly(naphthalene) (PNT-N) was explored, and it was observed that deposition began at a substrate temperature somewhere in range of 150-200°C. A summary of the poly(naphthalene) and poly(tetrafluoronaphtha-lene) deposition conditions described here is given in Table 18.6. 

In THF at 20°C, dendritic monomers 3a-3c show absorption bands at 335 and 278 nm, due to the focal 1,4-diethynylbenzene unit and the dendritic wedges, respectively. On the other hand, dendritic macromolecular rods 2a-2c display a strong absorption band in the visible region (400-460 nm), characteristic of an extended electronic conjugation in the backbone. Upon excitation of the conjugated backbone at 425 nm in THF (abs425 nm = 0.01) at 20°C, 2a-2c show a strong blue fluorescence at 454 nm, where the quantum yield ( FL) has been evaluated to be virtually 100%. Of much interest is the fact that the fl>FL value of 2c stays at nearly 100%, even when the solution is concentrated until the... 

Intramolecular cyclization of o-diethynylbenzene gives us an opportunity to compare results of the thermal and cation-radical variants of the reaction. There are three possible modes of cyclization shown in Scheme 7.25. 

Although the 1,6 cyclization takes place in the thermal process, the cation-radical initiation leads to the 1,5 cyclization (Ramkumar et al. 1996). Chemical oxidation of o-diethynylbenzene bearing two terminal phenyl groups by tris(p-bromophenyl)ammoniumyl hexachloroantimonate as the catalytic oxidizing agent in the presence of oxygen yields 3-benzoyl-2-phenylindenone in 70% yield (Scheme 7.26). 

Scheme 7.26 reflects the final result of the reaction. The initial step of this reaction consists of one-electron oxidation of the substrate. The resulting cation-radical of o-diethynylbenzene transforms into a fulvenyl intermediate, which further reacts with the neutral substrate to yield fulvenyl... 

According to the authors, the 1,5-cyclization mode of o-diethynylbenzene (see Scheme 7.25) is determined by electron state symmetry, which is different from that of the neutral molecule of o-diethynylbenzene (Ramkumar et al. 1996). 

Other exploits have focused on the electronic manipulation of the PPEs via side chains (Table 4, entries 7-16) [36]. If a conjugated side chain is utilized, then the electronic properties of the resulting PPE are influenced. The synthesis of such cross-conjugated PPE-PPV derivatives involves the Pd-catalyzed coupling of a diethynylbenzene derivative with a l,4-distyryl-2,5-diiodoben-zene. The electronic properties of such PPEs are variable by changing the styryl side chain (Table 4, entries 7-12). The polymers are attractive as active layers in light-emitting diodes (LEDs) and thin-film transistors. 

Water-soluble PPEs have been made by the suitable choice of ionic and highly hydrophilic side chains. These PPEs will play a role in bio-type sensing applications (vide infra). A clever example is Schanze s synthesis of a sulfonato-substituted PPE [40, 41]. The NaOH-promoted reaction of [1, 2]oxathiolane-2,2-dioxide with 2,5-diiodohydroquinone gives a bis-sulfonate that can be coupled (Pd-catalyzed) to 1,4-diethynylbenzene to form a water-soluble PPE (Scheme 4) Table 6, entry 1). In a similar approach, Schanze et al. [42] made a... 

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