Triphenylene Synthesis Essay

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  • 1. Introduction

    Supramolecular gels are unique colloidal soft materials built up by the self-assembly of small molecules (low molecular weight gelators, LMWG) in a given solvent through the formation of a 3D network. Due to the reversible nature of the non-covalent supramolecular interactions that held the system together (hydrogen bond, ion-ion and dipole-dipole interactions, π-π stacking, etc.) supramolecular gels are thermosensitive and can be transformed reversibly to a fluid (sol) by heating above the gel-sol transition temperature (Tgel), and vice versa. Moreover, supramolecular gels have the chemical and physical functional properties inherent to the gelator molecules and, if the functionality can be modulated externally, the gels become sensitive to chemical [1] or physical stimuli [2], such as pH, ions, light, etc. This unique characteristic makes of every supramolecular gel system a potential functional soft material with application in a wide variety of technological areas: from bio-medicine to electronic devices and/or media for chemical reactions [3], sensing [4], self-healing materials [5], molecular template for preparation of inorganic nanoparticles [6], and many others [7]. Among the wide variety of structurally diverse families of LMWG (e.g., peptides, steroids, saccharides, nucleobases, dendrimers, etc.) gelators based on π-systems with more than one fused or conjugated aromatic ring, so called π-gelators, are of special interest due to their intrinsic electronic properties derived from the delocalized π-electron system such as luminescence, charge carrier mobility, and electronic conductivity [8]. Therefore, π-gels are promising soft materials with potential application in imaging, sensing and in high-tech organic electronic devices such as LEDs [9], FETs and PVDs [10,11]. 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 [12] leading to anisotropic carrier transport materials with higher mobility than conventional semiconductors [13,14]. 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 [15,16]. 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) [17]. In this area Shinkai and co-workers [18] 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 [19] and alcohol [20] moieties linked through spacers to the triphenylene core.

    In this work we report on the supramolecular organogelling properties of six asymmetrical hexaether derivatives of triphenylene mono-functionalized with carboxylic and primary amine groups (Figure 1). Although compounds 26 have already been synthesized and characterized as discotic liquid crystals [21,22,23], and extensively used as intermediates in the synthesis of more complex triphenylene derivatives, their gelling ability has not been reported so far. We demonstrate below that the presence of a carboxylic acid (COOH) or amine (NH2) group attached to the triphenylene core generates stable, pH-sensitive supramolecular π-organogels. In order to understand the gelation process we studied the effect of the spacer length finding a different gelation scope for the acid and basic derivatives accounting for a different self-assembly process. Under self-catalyzed conditions, the fibrillar superstructure of the gel of the amine derivative 5 was successfully used as template for the in situ sol-gel polymerization of tetraethoxysilane (TEOS) and the further preparation of silica nanotubes.

    Figure 1. Structures of triphenylene derivatives 16. The triphenylene core is shown in blue.

    Figure 1. Structures of triphenylene derivatives 16. The triphenylene core is shown in blue.

    2. Results and Discussion

    2.1. Gelation Scope and Thermal Stability

    Our study started with triphenylene derivative 2, which was synthesized following reported literature procedures [21]. This acid was able to gel methanol and other small alcohols at 5 °C, generating stable, turbid gels. This simple mono-acid was reported and characterized in several works as an intermediate in the synthesis of other more complex triphenylene derivatives but, to the best of our knowledge, the gelling property has not been reported so far. There is only one example in the literature of a mono-substituted triphenylene π-gelator that bears a primary alcohol with similar gelation ability [20]. In these triphenylene alcohols the self-assembly was proposed to be driven by the π-π interactions between the triphenylene cores with solvation of the OH groups in the periphery. To study the structure/gelling property relationship of the carboxylic derivatives we synthesized the series of triphenylene acids 13 with spacers of different length [21,22], we chose alkyl chains of 3, 5, and 10 carbon atoms in order to have the COOH functionality inside, on the edge and outside the hydrophobic corona generated by the other five hexyloxy substituents. We also included in our study the triphenylene derivatives 46 with a primary amine functionality. These triphenylene amines were reported to form columnar mesophases when cooled from the isotropic liquid phase with a tendency to lead to glassy structures [23]. This behavior caught our attention on these amines as good supramolecular π-gelators candidates. The gelation scope of compounds 16 was studied with the simple test tube method over a wide range of organic solvents (Table 1). When no gelation was observed after resting the tube at room temperature it was cooled in the fridge at 5 °C. In general, the three carboxylic acids were insoluble in extreme polarity solvents such n-hexane and water (entries 1 and 7) and soluble in middle polar solvents such as ether, ethyl acetate and dichloromethane. Although none of the acids gelled methanol at room temperature, in all cases cooling at 5 °C induced a fast gelation. They also gelled other small primary alcohols such as ethanol and n-butanol. To evaluate the effect of the alkyl group of the alcohol on the thermal stability, the gel-sol transition temperature (Tgel) for gels of compound 2 in methanol, ethanol and n-butanol were determined by the inverted tube method at the same concentration of gelator. Analysis of the Tgel and the dielectric constants (ε) of the alcohols clearly showed that as the ε increases so does the Tgel accounting for higher thermal stability (Table 2). This trend suggests that the stabilization effect is related, among other factors, to the dielectric constant of the alcohols. Surprisingly, even though the gelation was only observed at 5 °C with the three alcohols, the Tgel were always higher and in the case of the methanolic gel it was stable even at room temperature.

    Table 1. Gelation tests of triphenylene derivatives 16.

    EntrySolvent123456
    1n-HexaneIIIIIG (0.25)
    2TolueneSSSSSG (0.35)
    3DichloromethaneSSSSSS
    4EthyletherSSSSSS
    5EthylacetateSSSSSS
    6MethanolTG * (0.1)TG * (0.2)TG * (0.2)ITG * (0.5)TG * (1.0)
    7WaterIIIIII

    Gelation test: * at 5 °C; G: Gel; TG: Turbid gel; S: Soluble; I: Insoluble; in brackets: Critical concentration for gelation (wt %).

    Table 2. Gel-sol transition temperatures for gels of acid 2 (1 wt %) in three different alcohols and their dielectric constants.

    EntrySolventTgel (°C)ε *
    1Methanol3133.6
    2Ethanol1725.2
    3n-Butanol818.2

    * Dielectric constants at 20 °C [24].

    A totally different behavior was found for the triphenylene amines 46; while compound 4 with the shortest spacer did not gel any solvent, amine 6 with the longer spacer was able to gel hydrocarbons such as n-hexane and toluene even at room temperature to give transparent, stable gels. Triphenylene amine 5 with the spacer length that leads the functionality on the edge of the hydrophobic corona only gelled small alcohols at 5 °C, in a similar way as acids 13.

    To estimate the thermal stability of the gels we studied the variation of the gel-sol transition temperature (Tgel) with the concentration of gelator for acids 13 in methanol (Figure 2a) and in n-hexane and toluene for amine 6 (Figure 2b). Tube inversion experiments were performed to measure Tgel, this method was selected because of its simplicity and widespread use in the field of gel-phase materials. In all cases as the concentration of gelator increased, Tgel also increased until a plateau region was reached at about the same concentration for the three acids (1.5 wt %). Near the critical concentration for gelation (CCG) the methanolic gels of 13 were almost transparent but, as the concentration increased, they became turbid, accounting for partial precipitation and low solubility. At low concentration of gelator the Tgelvs. concentration profile for the three acids is almost the same. For concentrations above 1.5 wt % the acid with the shortest linker (1) shows a slightly higher stability with a Tgel of 40 °C. The n-hexane gel of amine 6 has the higher thermo stability with a maximum Tgel of 70 °C reached at a low concentration near the CCG (0.5 mg/100 μL, Figure 2b). In aromatic hydrocarbons, such as toluene, of higher polarity compared to n-hexane, the maximum Tgel decreases to 28 °C. From these experiments we can conclude that the amine derivatives 46 have a very different gelation scope than the carboxylic acid derivatives since in these solvents (n-hexane and toluene) hydrogen bond interaction with the polar amine heads is not possible.

    Figure 2. Tgelvs. concentration plots as a measure of thermal stability. (a) Methanol gels based on triphenylenes 1, 2, and 3; and (b) hydrocarbon gels based on triphenylene amine 6.

    Figure 2. Tgelvs. concentration plots as a measure of thermal stability. (a) Methanol gels based on triphenylenes 1, 2, and 3; and (b) hydrocarbon gels based on triphenylene amine 6.

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