Triphenylene

Triphenylene is an organic compound with the formula (C₆H₄)₃. It is a flat polycyclic aromatic hydrocarbon (PAH) that has a highly symmetric and planar structure consists of four fused benzene rings. Triphenylene has delocalized 18-π-electron systems based on a planar structure, corresponding to the symmetry group D_3h. It is more resonance stable than its isomers chrysene, [[Benz(a)anthracene|benz[a]anthracene]], [[Benzo(c)phenanthrene|benzo[c]phenanthrene]], and tetracene, hence resists hydrogenation. It is a light yellow powder, insoluble in water.

Triphenylene serves as a fundamental building block in discotic liquid crystals, where its planar, disc-like structure facilitates the formation of columnar mesophases, enabling applications in organic electronics. It's also being used as the base of covalent and metal organic frameworks.

Discovery and first synthesis

Triphenylene was first separated by German chemists H. Schmidt and in 1880 from the pyrotic product of the thermal decomposition of benzene vapor. Though triphenylene is previously referred to as chrysene, Schmidt and Schultz realized that it is an isomer of chrysene, and successfully identified and named it as triphenylene.

Later in 1907, Carl Mannich first synthesized triphenylene through a two-step reaction from cyclohexanone and confirmed its planar structure.

center|thumb|369x369px|Mannich's hypothesis of potential condensation pathways

Mannich then dehydrogenated dodecahydrotriphenylene into triphenylene with two methods: zinc dust distillation and copper-catalyzed dehydrogenation. He confirmed the product was identical to the pyrolysis product from benzene by reproducing Schmidt and Schultz's experiment and comparing the samples. Mannich also characterized triphenylene's properties, derivatives, and oxidation reactions, and confirmed it as a fully aromatic polycyclic hydrocarbon. This pathway first diazotizes and iodinates o-bromoaniline through HCl, NaNO₂, and KI to produce o-bromoiodobenzene with a yield of 72-83%. Then form o-bromophenyl lithium using Li and ether. Add benzene to the organolithium intermediate to get triphenylene with a yield of 53-59%. Impurities of biphenyl are then removed with steam distillation.

center|thumb|578x578px|Synthesis of triphenylene through trimerization of benzyne

Another method involves trapping benzyne with a biphenyl derivative. This method started with removing the trimethylsilyl group from 2-(trimethylsilyl)phenyl trifluoromethanesulfonate using cesium fluoride, generating benzyne. Benzyne then reacts with 2-bromobiphenyl in the presence of Pd(dba)₂ and tri(o-tolyl)phosphine as catalysts and produces triphenylene with a yield of 76%.

center|thumb|675x675px|Synthesis of triphenylene via the palladium-catalyzed annulation of benzyne

Application

Discotic liquid crystal and organic electronics

Triphenylene and its derivatives have been widely used in discotic liquid crystal and organic electronics as the core moiety due to its robust discotic molecular architecture.

thumb|Stacking of triphenylene derivatives creates conducting channels

Due to its planar structure and π-conjugated system, triphenylene has a rigid discotic structure. This enables it to self-assemble and form highly ordered, long, cylindrical columns. Tripenylene derivatives, with flexible aliphatic side chains, can modulate intermolecular interactions. This maintains molecular mobility under a wide temperature range and avoids excessive crystallization, and corresponding bad processability and solubility.

Recent studies synthesized new polymer structures incorporating triphenylene units and found that these materials exhibit high photoluminescence and electroluminescence efficiencies. Their emission spectra are well-suited for blue light applications, demonstrating stability and promising performance for next-generation blue emitters. Similar to the properties mentioned in DLC applications, triphenylene and its derivatives have high conductivity, and further affect the conductivity of MOFs and COFs.