Triphenylene: Synthesis, Supramolecular Assembly, and Electrochemical Applications

Triphenylene is one of over 100 different polycyclic aromatic hydrocarbons (PAHs). PAHs are chemicals that are formed during the incomplete burning organic substances, such as fossil fuels. They are usually found as a mixture containing two or more of these compounds.

Preparation of substituted triphenylenes via nickel-mediated Yamamoto coupling

A key component in the design of organic semiconducting materials is the ability to tune properties such as the frontier molecular orbital energies and molecular packing. Promoting π-stacking favours molecular orbital overlap that is important for charge transport in these systems. One strategy to promote effective π-stacking is to design disk-shaped molecules that form columnar mesophases, where molecules self-organize into π-stacked arrays, making them attractive candidates as organic semiconducting materials. Substituted triphenylenes have been extensively explored and are well-established discotic mesogens. These triphenylenes are usually prepared via oxidative aryl–aryl bond forming reactions such as the Scholl reaction, which works very well for electron-rich systems but is not effective for electron-poor compounds. Other common approaches for the preparation of triphenylenes include cross-coupling followed by oxidative cyclization, as well as palladium-catalyzed aryne cyclotrimerization. Despite these methods, access to electron-deficient triphenylenes remains a challenge.[1]

The design and synthesis of electron deficient triphenylenes would provide access to potential n-type materials with low LUMO energies. Furthermore, several studies have shown that electron-withdrawing groups on discotic mesogens promote the formation of stable columnar liquid crystalline phases, likely through improved π-stacking interactions. These observations suggest that the preparation of electron-deficient triphenylenes may lead to discotic liquid crystals with broad mesophase ranges. Therefore, there is impetus to develop new approaches for the preparation of electron-deficient triphenylene derivatives. Triphenylenes 2a–f were chosen as targets because they have all been prepared previously by different methods, permitting a comparison of the Yamamoto coupling with previously reported approaches. They were also selected because they included a range of electron-donating and electron-withdrawing groups. Compounds a–f were each subjected to reaction with stoichiometric Ni(COD)2 (1.25 eq.), COD (2 eq.), and 2,2′-bipyridine (1.25 eq.) in THF at room temperature overnight to afford the corresponding triphenylenes in modest to good yields. Using the same conditions, hexamethyltriphenylene 2b was prepared in a 58% yield. To evaluate the utility of the Yamamoto, we compared this synthesis with the previously reported synthetic approaches to 2b. There are two previously described synthetic approaches to 2b. The first involves palladium-catalyzed aryne cyclotrimerization using the corresponding trimethylsilyl aryl triflate, While the second approach involves oxidative cyclization of the corresponding terphenyl, which was prepared by Suzuki coupling. The aryne cyclotrimerization reaction is reported to proceed in 34% yield, and also requires several steps to prepare the aryne precursor, requiring cryogenic conditions.

Substituted triphenylenes show promise as organic semiconductors because of their ability to form columnar liquid crystalline phases featuring extended π-stacked arrays. While there are several methods for preparing triphenylenes, including oxidative cyclization reactions such as the Scholl reaction, as well as transition metal-catalyzed aryne cyclotrimerization, these methods are not effective for electron deficient triphenylenes. Here we demonstrate that the nickel-mediated Yamamoto coupling of o-dibromoarenes is a concise and efficient way to prepare substituted triphenylenes, including electron-deficient systems that are otherwise challenging to prepare. We also demonstrate the application of this approach to prepare electron deficient discotic mesogens composed of triphenylenes bearing imide and thioimide groups.

Supramolecular Assembly of pH-Sensitive Triphenylene Derived π-Gelators

Triphenylene derivatives, with a flat four fused ring π-core, are a well-known family of discotic liquid crystals (DLC) that form columnar assemblies with π-stacked aromatic cores surrounded by alkyl chains leading to anisotropic carrier transport materials with higher mobility than conventional semiconductors. In triphenylene based DLC the distance between rings is about 3.5 Å with a column separation of 20–40 Å depending on the length of the side alkyl chains. Some liquid-crystalline supramolecular gels composites derived from triphenylene DLC and LMWGs have been developed in order to obtain functional materials with modulated electro-optical and conductive properties. Furthermore, some triphenylene derivatives were reported to form π-organogels by themselves due to, for example, cooperative intermolecular hydrogen bonding stabilization of the columnar organization generating Discotic Liquid Crystal Gels (DLCG). In this area Shinkai and co-workers reported the first example of a symmetric triphenylene gelator substituted with six amide groups that exhibit unusual emission properties from an excimer formation, but does not form DLC phases in bulk. Recently, there have been only two reports of simple mono-functionalized asymmetric triphenylenes DLCG containing imidazole and alcohol moieties linked through spacers to the core.[2]

In summary, we have studied the supramolecular organogelling properties of six asymmetrical hexaether derivatives of triphenylene mono-functionalized with carboxylic and primary amine groups. The acid derivatives were able to selectively gel small alcohols independently of the length of the linker, suggesting an assembly where the columnar discotic array is stabilized by solvation of the carboxylic groups, in an analog way as proposed for Kotlewski et al.  for related mono-substituted triphenylenes π-gelator bearing a primary alcohol functionality. In case of the amine derivatives the linker length was a determining factor for the gelling ability. Triphenylene with the amine group inside the alkyl corona was not able to gel any of the solvents while amine gelled small alcohols. The template polymerization allowed a direct view of the real, fibrillar structure of the SAFIN on the gel that was not observable from the SEM images of the collapsed xerogel. In conclusion, the presence of a carboxylic and amine group on the triphenylene core generates pH/temperature responsive π-organogels and these results can be used to design new pH-responsive π-gelators with specific electronic properties. In view of the interesting properties of the gels derived from these simple mono-functionalized triphenylenes a further study of symmetric and asymmetric bi-functionalized triphenylenes derivatives is now in progress.

Employing Triphenylene-Based, Layered, Conductive Metal-Organic Framework Materials

This study compares the use of four layered, conductive MOFs based on the 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) ligand (M3(HHTP)2, M = Co, Ni, Cu, Zn) as working electrode materials for the liquid-phase electrochemical detection of nitric oxide. We employ HHTP-based MOFs as electrochemical NO sensors for the first time, and seek to understand the role of the metal node in electrochemical sensing interactions. The triphenylene-based MOFs used in this study were selected based both on synthetic accessibility and previous use as working electrode materials for electrochemical sensing.This paper describes the first use of conductive metal-organic frameworks as the active material in the electrochemical detection of nitric oxide in aqueous solution.[3]

Four hexahydroxytriphenylene (HHTP)-based MOFs linked with first-row transition metal nodes (M = Co, Ni, Cu, Zn) were compared as thin-film working electrodes for promoting oxidation of NO using voltammetric and amperometric techniques. Cu- and Ni-linked MOF analogs provided signal enhancement of 5- to 7-fold over a control glassy carbon electrode (SANO = 6.7 ± 1.2 and 5.7 ± 1.1 for Ni3(HHTP)2 and Cu3(HHTP)2, respectively) for detecting micromolar concentrations of NO. Zinc-based MOF electrodes offered more limited enhancement (SANO = 3.1 ± 0.5), while the cobalt-based MOF analog had intrinsic redox activity at potentials close to NO oxidation, which interfered with sensing. Combining MOFs with a conductive polymer improved electrode stability under repeated electrochemical scanning (14 ± 3% decrease in signal over 10 scans). This study demonstrates that layered, conductive 2D MOFs have promising applicability for NO detection in aqueous environments.

References

[1]Schroeder ZW, LeDrew J, Selmani VM, Maly KE. Preparation of substituted triphenylenes via nickel-mediated Yamamoto coupling. RSC Adv. 2021 Dec 13;11(62):39564-39569.

[2]Muñoz Resta I, Manzano VE, Cecchi F, Spagnuolo CC, Cukiernik FD, Di Chenna PH. Supramolecular Assembly of pH-Sensitive Triphenylene Derived π-Gelators and Their Application as Molecular Template for the Preparation of Silica Nanotubes. Gels. 2016 Feb 1;2(1):7.

[3]Ambrogi EK, Li Y, Chandra P, Mirica KA. Employing Triphenylene-Based, Layered, Conductive Metal-Organic Framework Materials as Electrochemical Sensors for Nitric Oxide in Aqueous Media. ACS Sens. 2025 Jan 24;10(1):553-562.