Recent advances in structurally elaborate triptycenes, triptycene-containing polymers and assemblies: structures, functions and applications

Abstract

Triptycene, a rigid propeller-shaped molecule, was first synthesized in the early 1940s. More recently, many triptycene-containing polymers and molecular assemblies have been developed for a wide range of applications, including guest recognition, material transport, separation, catalysis, and as device components. The advantages of triptycenes lie in their ability to introduce a variety of functional groups on their three-dimensional backbone, with changes in substitution patterns as well as the type of substituents present having a significant impact on the material properties. In this review, we describe the synthesis of triptycene derivatives and polymers, detailing selected examples of triptycene-containing functional polymers. We also focus on the construction of triptycene-based two-dimensional assemblies and polymers, where space-filling designs based on rigid propeller-shaped skeletons are essential. Through a thorough literature survey, future directions and possibilities for the development of triptycene-containing functional materials are discussed.

Introduction

Triptycene is a rigid molecule with a three-dimensional (3D) skeleton consisting of three phenylene rings arranged with interblade angles of 120° (Fig. 1a) [1,2,3,4,5,6,7,8,9,10]. Triptycene derivatives have been utilized in many fields, including supramolecular chemistry and polymer and materials science, and researchers have taken advantage of the structural features of triptycenes in the development of functional molecules, polymers, and assemblies. For example, a number of triptycene-containing polymers (trip-polymers) have been reported [2, 10], many of which exhibit microporosity, can serve as low dielectric materials, and show specific assembly properties arising from the large free volumes present around the rigid triptycene skeleton. Importantly, the properties of trip-polymers largely depend on their substitution patterns, the symmetry of the constituent triptycene units, and the type of substituents present. To date, synthetic methods for accessing triptycene derivatives have been intensely developed, allowing the introduction of various substituents at different positions (Fig. 1b). These structurally elaborate triptycene derivatives are useful for the design of various functional polymers (Fig. 1c–f). Our group has also been engaged in the development of trip-polymers that exhibit unique assembly properties, for which triptycene units with particular substitution patterns [11], different from the majority of previously reported trip-polymers, are essential (vide infra).

Fig. 1

a Molecular structure of triptycene with carbon numbering. b Synthesis and (c–f) schematic drawings of trip-polymers with (c) network, (d) linear, (e) linear ladder, and (f) hyperbranched structures

In this review, based on a thorough literature survey, we describe the synthesis and structures of triptycene derivatives and trip-polymers with various substitution patterns while summarizing recent research trends. To the best of our ability, we attempted to avoid including examples of trip-polymers that have already been described in the book and review by C-F. Chen et al. [1, 8]. We also describe the formation of two-dimensional (2D) assemblies and related polymers using particular types of triptycenes. Triptycene derivatives, when appropriately functionalized, self-assemble into either porous or dense (nonporous) 2D structures through intermolecular π-stacking and/or nested packing of their propeller-blade moieties. These triptycene-based 2D assemblies are useful for the construction of polymeric materials featuring long-range structural order. We also describe various applications of triptycene-based 2D assemblies, including the development of polymer materials with enhanced properties for high-performance organic electronic devices.

Literature survey

Our manual survey of academic papers (other than patents) using Google Scholar^® in March 2024 with the keywords “triptycene + polymer” and “iptycene + polymer” revealed that 364 research publications reported a total of 923 triptycene- and iptycene-containing polymers from 1968 to the present. Note that (i) when polymers with the same structure were reported in different publications, we included them multiple times in the count, (ii) copolymers were counted separately, and (iii) metal-organic frameworks were excluded. Figure 2 shows histograms of the numbers of published papers (Fig. 2a) and trip-polymers (Fig. 2b) reported in these papers, where the colors in Fig. 2b corresponds to the substitution patterns of the constituent triptycene units. Figure 2c shows the percentage of polymers with each substitution pattern of triptycene relative to all trip-polymers; linear polymers linked at the 1,4-positions (338 examples) are the most common, followed by network polymers linked at the 2,7,14(15)-positions (189 examples), linear ladder polymers linked at the 2,3,6,7-positions (118 examples), linear polymers linked at the 2,6(7)-positions (92 examples) and network polymers linked at the 2,3,6,7,14,15-positions, including 2D/3D covalent organic frameworks (COFs) (107 examples) [12]. These polymers account for ca. 90% of the total number of trip-polymers. Both the reported number of papers and the number of trip-polymers rapidly increased around 2010. In the following, we describe the research trends related to triptycene-based polymers and assemblies in chronological order.

Fig. 2

a Histogram of the number of annual publications on trip-polymers. b Histogram and distribution of the number and substitution patterns of trip-polymers reported per year. c Summary of the distribution of substitution patterns in reported trip-polymers

Early-stage examples including engineering polymers, fluorescent sensors, and low-dielectric materials

The first trip-polymer was reported in 1968 [13]. In that period, colorless, heat-resistant engineering plastics were the focus of attention, and structurally rigid triptycene derivatives with a small π-conjugated system were predicted to be potent building blocks in this area [14]. In 1998, Swager et al. reported a seminal paper describing the detection of explosives (TNT; trinitrotoluene) using π-conjugated polymers containing pentiptycene in the main chain [15]. Since then, a series of related papers have been published. Swager et al. also developed low-k polymers that serve as insulating interlayer materials for high-density electronic devices [16]. Trip-polymers with large free volumes around their polymer backbones tend to have low relative permittivity.

PIMs, COFs and 2D polymers (2000s~)

In 2004, McKeown et al. reported ladder polymers with rigid and contorted main chains that yielded microporous freestanding membranes featuring very high specific surface areas [17] and named them “polymers of intrinsic microporosity (PIMs)” [9, 18, 19]. Since then, PIMs have been studied extensively as membranes for gas separation and storage. Structurally rigid triptycenes, to which a variety of substituents can be introduced, are excellent building blocks for PIMs. Yaghi et al. and others introduced the concept of covalent organic frameworks (COFs) using similar porous solid materials in 2005 [12, 20]. Propeller-shaped triptycenes also serve as useful building blocks for 2D and 3D COFs, and an increasing number of triptycene-based COFs have been reported since 2013. The synthesis of exfoliable triptycene-based 2D polymers with single-crystal-like structural order has been achieved using triptycene derivatives consisting of extended propeller blades [21, 22].

Ion-conducting membranes (2010s~)

In 2011, a trip-polymer that serves as an excellent proton exchange membrane for fuel cells was first reported [23]. Then, triptycene-based anion exchange membranes for use as alkaline fuel cells were reported in 2017 [24]. While the initial focus in the development of these ion-exchange membranes was on introducing dense ionic functional groups on the triptycene moieties, later works shifted toward designs that take advantage of the microporous nature of trip-polymers.

Self-assembled materials (2010s~)

The abovementioned examples of trip-polymers all exhibit porosity and utilize micro- and nanopores, e.g., material transport and separation. In 2015, our research group reported that triptycene derivatives with alkoxy groups at the 1,8,13-positions self-assemble to fill the free volume between neighboring triptycene molecules, forming a 2D nested hexagonal packing arrangement, which has been applied in the development of polymers with new functionalities [11].

Chiral polymers (2010s~)

Triptycenes substituted at two or more positions, such as 2,6- or 1,5-substituted derivatives, are inherently chiral [7]. However, until recently, such chiral triptycenes have not been actively explored. Recently, chiral polymers utilizing this triptycene chirality have been reported, and their functions have attracted increased amounts of attention.

In the following sections, we will discuss general synthetic methods for trip-polymers, and their structures and applications. This review builds upon the examples found in the 2013 book of C-F. Chen et al., in which they describe a chapter on trip-polymers [1]. As such, we will focus on results published after this date.

Synthetic routes for accessing triptycene derivatives and triptycene-containing polymers

As shown in Fig. 1b, there are two major ways to synthesize trip-polymers: direct polymerization of unsubstituted triptycene or polymerization using a triptycene monomer (trip-monomer) substituted with a reactive functional group. Unsubstituted triptycene (1) easily reacts at its 2,7,14(,15)-positions under Friedel-Crafts-type conditions. Although monomer (1) can be easily prepared, this polymerization method does not yield network polymers containing triptycenes linked at the 2,7,14(,15)-positions. Trip-polymers linked at other substitution positions require the synthesis of trip-monomers, in which reactive substituents are introduced at the desired positions in advance. One method is to introduce substituents at the 2(,3)(,6),7,14(,15)-positions by Friedel–Crafts-type reactions with 1. The main trip-monomers synthesized starting with this method are shown in Fig. 3. The Friedel–Crafts conditions used include nitration with nitric acid [25, 26], acylation using Lewis acid catalysts such as aluminum chloride or tin chloride, and formylation [27]. For nitration, 2-monosubstituted (2), 2,6- (3) or 2,7-disubstituted (4) and 2,6,14- (5) or 2,7,14-trisubstituted (6) nitro triptycene derivatives have been reported. The ratios of formation can be controlled somewhat selectively by varying the amounts of reagents used, and importantly, all isomers can be separated using silica gel column chromatography (except for optical isomers). This has allowed for the synthesis of trip-monomers with the desired number and patterns of nitro substituents. Amino-substituted triptycenes (7–11), obtained by reducing nitro moieties, are frequently used as monomers in the synthesis of polyamides, polyimides, and Tröger’s base-containing PIMs [9, 19]. Sandmeyer-type reactions via diazotization of the amino forms have led to the insertion of halo substituents, yielding bromo (12–16) and iodo (17–21) triptycenes [26, 28] along with an azide-substituted trip-monomer (22) [29, 30], which have been used in various transition-metal-catalyzed couplings and click polymerizations. In addition, 2,6-diaminotriptycene (8) [31] and its derivatives [32] are relatively easy to separate optically using chiral HPLC, and chiral polymers with interesting optically active functions have been obtained, which will be described in detail later. Although most of the trip-monomers shown here are chiral, many optically active monomers and polymers are expected to be developed in the future.

Fig. 3

Synthesis of trip-monomers from unsubstituted triptycene 1

Baeyer–Villiger oxidation of acylated trip-monomers (23–32) with mCPBA and subsequent hydrolysis have led to hydroxy trip-monomers for use in polyester and polyether synthesis (33–37) [27, 33]. These compounds have also been synthesized by other methods [34] and will be described later. Nitration and reduction of 2,6(,7)-dihydroxytriptycene (34,35) yields a trip-monomer with two hydroxy and two amino groups (38) [33]. This type of monomer is important in the preparation of polybenzimidazoles [35] through thermal rearrangement to the corresponding hydroxy group-containing polyimides, which have been shown to act as excellent gas-separation membranes [36]. Another monomer related to gas separation membranes, the di-ortho-brominated 2,6(,7)-diaminomonomer (39), which is useful in the synthesis of polyimides with reduced free rotation, has recently been reported [37, 38]. A route to synthesize triaminotriptycenes via Beckmann-rearrangement (40) from acetylated triptycenes (23–32) is also known [27].

Amino- and/or bromo-substituted monomers can be synthesized by reacting with unsubstituted triptycene 1 to yield, for example, 2,3,6,7,14,15-hexasubstituted trip-monomers. For the di-amination (42), tetra-amination (43) and hexa-amination (44) of triptycenes, a multistep synthesis via protection, nitration (41), deprotection, and reduction of triaminotriptycene (10,11) was originally reported [39]. Later, Mastalerz et al. reported that amino-substituted derivatives can be synthesized as the corresponding hydrochloride salts in two steps from 1 using fuming nitric acid [40]; however, the yield from 1 is only 18%. On the other hand, for the synthesis of hexabromotriptycene (45), bromination of 1 using Br₂ in the presence of iron proceeded in relatively high yield [41]. Very recently, the addition of I₂ was shown to increase the yield to 90% [42]. It has been reported that hexaaminotriptycene (44) can be synthesized efficiently by palladium-catalyzed Buchwald–Hartwig amination of hexabromide with benzophenone imine, followed by deprotection with hydrochloric acid [43]. This hexa-aminated trip-monomer is widely used as a building block for COFs and network polymers. Sonogashira couplings with di-, tri-, and hexa-brominated monomers have led to ethynyl-substituted trip-monomers (46–48) that can be used in further Sonogashira couplings and click polymerizations. 2,3,6,7,14,15-Hexaethynyltriptycene (48) can be used for Masamune–Bergman polymerizations, resulting in interesting porous network polymers [44, 45], which will be discussed in detail later. Hexa-hydroxylated trip-monomers (50) can also be synthesized by treating the hexa-brominated trip-monomers with sodium methoxide to introduce methoxy groups (49) and then removing the methyl group using BBr₃ [46]. This approach has been used in the preparation of COFs and PIMs. In addition to the synthetic methods described thus far, it is also possible to prepare amino substituents from acyl groups by Beckmann rearrangement and hydrolysis [27] and hydroxyl substituents from amino groups by Sandmeyer-type reactions [25], but as of yet, no polymers have been obtained using monomers in these ways. Moreover, direct borylation of 1 using an iridium catalyst and bis(pinacolato)diboron has been reported to yield 2,6,14(15)-tris[(pinacolato)boryl]triptycene as a mixture of regioisomers [47]. The propeller blades of these triboryltriptycenes can be extended using Suzuki coupling and dehydrogenative cyclization. Thus, triboryltriptycenes could be useful as building blocks for network polymers.

Another method to synthesize trip-monomers is to introduce functional groups into anthracenes, benzynes, or quinones, which are common precursors in the synthesis of triptycenes. In addition to simple modification reactions, anthracene derivatives with various functional groups can be derived from commercially available anthraquinone. Anthracenes can also be synthesized using aromatic electrophilic substitution reactions between relatively electron-rich benzenes and reagents such as aldehydes or dichloromethane. A number of commercially available substituted benzyne precursors, including anthranilic acids and derivatives with triflate and trimethylsilyl groups, have been used.

The synthesis of the most common 1,4-substituted triptycene derivatives is shown in Fig. 4. para-Benzoquinone (51) has been used as a dienophile with anthracenes (52, 53) in Diels-Alder reactions, followed by aromatization to the 1,4-dihydroxy trip-monomer (54, 55) [16, 17]. From this, a perfluorosulfonated intermediate (56) can be formed and converted into a 1,4-diethynyl trip-monomer (57) by Sonogashira coupling [48]. From the 1,4-dihydroxy trip-monomer (54), a para-benzoquinone derivative (59) can be obtained by oxidation, and from the imine derivative (60), it can be prepared through condensation with hydroxylamine, which can be subsequently reduced to yield the 1,4-diamino trip-monomer (61) [49, 50]. Compound 61 can undergo a Sandmeyer-type reaction to yield the 1,4-diiodo trip-monomer (62) [48], which has been used in a further Sonogashira reaction to synthesize the 1,4-diethynyl trip-monomer (57) [51, 52]. The para-benzoquinone form of pentiptycene (64) can be synthesized using an excessive amount of anthracene in a Diels–Alder reaction with para-benzoquinone or by allowing the para-benzoquinone form of triptycene to undergo a second Diels–Alder reaction with anthracene in acetic acid [16, 48]. A reduction of 64 yields the 1,4-dihydroxypentiptycene monomer (63) [52], whereas a nucleophilic attack of 64 with a TMS-acetylide (65) and subsequent oxidation (66) and deprotection yields the 1,4-diethynylpentiptycene monomer (58) [16, 48]. However, it has been noted that the equivalent 1,4-diaminopentiptycene cannot be synthesized by a similar route [4, 48, 53].

Fig. 4

Synthesis of 1,4-substituted trip-monomers

It has been reported that functionalized triptycenes and pentiptycenes can be obtained by reacting substituted anthracenes with benzoquinone 51. The reaction of 1,8-dimethoxyanthracene (53) with 51 yields 1,4-dihydroxytriptycene (55), which has two hydroxy substituents protected by methyl groups [54]. In 2021, a highly functionalized pentiptycene monomer was synthesized by Guo et al. [55]. They synthesized 2,3-dimethoxyanthracene (71) from phthalic anhydride (67) and dimethoxybenzene (68) by stepwise Friedel–Crafts reactions (69) and subsequent reduction/aromatization (70,71). The reaction of 71 with para-benzoquinone 51 gave a pentiptycene (72) bearing upper and lower methoxy groups as a mixture of syn- and anti-isomers. These isomers can be separated by means of silica gel column chromatography. The central para-benzoquinone moiety of 72 can be reduced (73) and etherified (74). The use of ceric ammonium nitrate (CAN) for the reaction with 74 resulted in the selective oxidation of the dimethoxybenzene moieties to yield the ortho-quinone pentiptycene (75). This was reduced via a hydrogenation reaction to yield a tetrahydroxy derivative (76) [55], which has been used in the preparation of PIMs [9].

Figure 5 shows the synthetic pathway for accessing triptycenes with hydroxyl, carboxyl, carboxylic acid chloride, isocyanate, and amino groups as polymerizable substituents at the 9,10-positions. The 9,10-positions of unsubstituted anthracene (77) were chloromethylated (78) and converted to acetoxy groups via nucleophilic substitution (79), followed by a Diels–Alder reaction with benzyne generated from anthranilic acid (80) and amyl nitrite to yield 9,10-diacetoxymethyltriptycene (81). The acetyl group can then be deprotected to the 9,10-dihydroxymethyl trip-monomer (82) [14, 15]. It is also possible to synthesize a 9,10-dihydroxyethyl trip-monomer (86) through the introduction of a cyano group (83) via nucleophilic substitution of the chloromethyl group of 78 and subsequent functional group transformation followed by a Diels–Alder reaction (84, 85) [15]. Oxidation of the 9,10-dihydroxymethyl trip-monomer (82) with chromium trioxide yields the 9,10-dicarboxyl trip-monomer (87), and further treatment with thionyl chloride yields the 9,10-dichlorocarbonyl trip-monomer (88). This synthetic method, reported in 1968 and 1969 [13, 14], is still used today. For example, compound 87 has been utilized for the synthesis of MOFs [56]. 9,10-Diamino Trip-monomer (91) [14, 15] can be synthesized from 88 through a reaction with sodium azide, a thermal Curtius rearrangement of the resulting benzoyl azide (89), and the subsequent hydrolysis of a diisocyanate derivative (90). 9,10-Functionalized triptycene derivatives such as 87 and 91 have been used as building blocks for polyesters and polyamides. Moreover, an interesting synthetic method has been developed for 9,10-dihydroxylated derivatives (94, 95). For example, anthracene substituted with hydroxyl and ethynyl groups at the 9- and 10-positions (92) reacts with norbornadienes or terminal alkynes in the presence of a rhodium catalyst in a [2+2+2] cycloaddition to form 9,10-dihydroxytriptycene 94 [57, 58]. Cycloaddition with terminal alkynes or norbornadienes affords derivatives with or without a substituent at the 14-position, respectively. This synthetic protocol can be applied to the synthesis of a highly substituted, 1,4-dibrominated derivative (95) from a corresponding precursor (93) [57]. Sonogashira coupling polymerization using 95 yields polyphenylene ethynylenes, which can be transformed to polyphenylene vinylenes through intramolecular hydrooxygenation of alkyne moieties.

Fig. 5

Synthesis of 9,10-functionalized trip-monomers

Figure 6 summarizes the trip-monomers that can be synthesized from functionalized anthraquinones. The chloro groups of 1,5-dichloroanthraquinone (96) can be converted to cyano groups (97) using copper cyanide and then to carboxylic acids (98) by hydrolysis [59]. The anthraquinone skeleton is reduced to anthracene by treatment with zinc under basic conditions (99), which is followed by methylation of the carboxylic acid moieties to yield the methyl ester derivative (100). Subsequently, 100 reacts with benzyne generated from anthranilic acid (80) and isoamyl nitrite to yield a triptycene with ester groups at the 1,5-positions (101). Hydrolysis of 101 yields the 1,5-dicarboxytriptycene monomer (102). Here, the 1,5-diaminotriptycene monomer (106) can be synthesized through acid chloride (103), benzoyl azide (104), and isocyanate (105) intermediates, similar to the case of 91 in Fig. 5 [59]. 1,5-Dicarboxytriptycene (102) is chiral, and its enantiomeric separation has been achieved by the formation of diastereomeric salts with the naturally occurring optically active alkaloids synconidine or synconine. Thus, (R,R)- and (S,S)-1,5-dicarboxytriptycene have been obtained using synconidine and synconine, respectively [59]. The use of these 1,5-dicarboxytriptycenes results in the formation of the corresponding optically active 1,5-diamino trip-monomers (106), which can be further transformed into optically active 1,5-dihydroxy trip-monomers (107) by reaction with water via a diazonium salt [59].

Fig. 6

Synthesis of trip-monomers from functionalized anthraquinones

The synthesis of 1,5-dihydroxy trip-monomer (107) has also been reported using the scheme shown in the lower part of Fig. 6 [60]. A sodium salt of anthracene-1,5-disulfonic acid (109), synthesized from the corresponding anthraquinone (108), is subjected to alkali fusion, affording 1,5-dihydoxytriptycene (110). After reacting with acetic anhydride, the resulting 1,5-diacetoxyanthracene (111) reacts with benzyne, followed by hydrolysis to yield the 1,5-dihydroxy trip-monomer (107) [60]. For 2,6-dihydroxytriptycene (34) [34], 2,6-dihydroxyanthraquinone (113) has been used as a starting material. Compound 113 is converted to 2,6-dimethoxyanthracene (116) via methylation and reduction (in no particular order) [61, 62], and then 116 reacts with benzyne followed by demethylation using boron tribromide to yield 2,6-dihydroxytriptycene 34. Therefore, 1,5-dihydroxyanthraquinone (117) may provide an alternative synthetic route to 1,5-dihydroxytriptycene 107, which does not involve alkali fusion. It has been reported that 1,5-dimethoxyanthracene (119) derived from 117, upon reaction with para-benzoquinone (51), yields 1,5-dimethoxytriptycene monobenzoquinone (120) [63]. para-Benzoquinone is useful in the synthesis of benzoquione derivatives of triptycene [64]. 2,6-Diaminoanthracene (122), obtained by the reduction of 2,6-diaminoanthraquinone (121) [65], has been reacted with 1,4-epoxy-1,4-dihydronaphthalene (123) under Diels–Alder conditions to form 124, followed by dehydration using perchloric acid to yield 2,6-diaminobenzotriptycene (125) [66]. The Diels–Alder reaction using 123, which can be carried out even in the presence of amino functionalities, is useful for the synthesis of benzotriptycene derivatives. Compound 125 has been used for the synthesis of microporous polyimides and PIMs consisting of Tröger’s base moieties [66].

Figure 7 summarizes the synthesis of highly functionalized trip-monomers using multisubstituted anthracene derivatives. These triptycene derivatives have attracted much attention in recent years as monomers for use in the preparation of ladder polymer-based PIMs and microporous polyimides. The Friedel–Crafts reaction of 1,2-dimethoxybenzene (68) with various aldehydes (126) in the presence of sulfuric acid yields the corresponding hexa-substituted 9,10-dialkyl-2,3,6,7-tetramethoxyanthracenes (127). By subjecting these anthracenes to a Diels–Alder reaction with benzyne generated from unsubstituted or dimethoxy anthranilic acid (80 or 128), triptycene derivatives methoxy-substituted at the 2,3,6,7- (129) or 2,3,6,7,14,15-positions (130) are obtained [46, 67,68,69]. Deprotection of these methoxy groups using boron tribromide yields the corresponding triptycene 2,3,6,7-tetrahydroxy (131) or 2,3,6,7,14,15-hexahydroxy (132) derivatives [46, 67,68,69,70]. Related hexahydroxytriptycene (50) devoid of alkyl substituents at the 9- and 10-positions (i.e., R¹ = H for 132) was prepared from 2,3,6,7,14,15-hexabromotriptycene 45 (Fig. 3) [46]. The reaction of furan (136) with benzyne derived from the lithiation of substituted benzenes (133–135) yields 1,4-epoxy-1,4-dihydronaphthalene derivatives (137). These compounds undergo Diels–Alder reactions with hexa-substituted anthracenes (127) to yield epoxy products (138), which, through dehydration and demethylation by acid treatment, are transformed into highly substituted 2,3,6,7-tetrahydroxybenzotriptycene (139) [69, 70]. 2,3,6,7-Tetracyanotriptycene derivatives (141) were obtained by palladium-catalyzed cyanation of the corresponding triflated compound (140) derived from 131 [71]. Hydrolysis of the cyano groups of 141 yields 2,3,6,7-tetracarboxyltriptycene (142). The corresponding acid anhydride derivatives (143) were obtained by reacting with acetic anhydride [71]. An acid anhydride derivative devoid of alkyl substituents at the 9- and 10-positions (R¹ = H for 142) can be synthesized by oxidation of 2,3,6,7-tetramethyltriptycene (147) using potassium permanganate [72]. 2,3,6,7-Tetramethylanthracene (145), the precursor of 147, is obtained by a Friedel–Crafts reaction using ortho-xylene (144), dichloromethane, and aluminum chloride. The Diels–Alder reaction between 145 and benzyne yields 147. Similarly, 2,3,6,7,14,15-hexamethyltriptycene (148) can be obtained using anthranilic acid with two methyl groups (146) instead of anthranilic acid (80) as a benzyne precursor. Compound 148 can be transformed to tri-acid anhydride 150 through successive oxidation (149) and intramolecular condensation [73]. 1,3,6,8-Tetramethylanthracene (152) can be prepared through a Friedel–Crafts reaction of meta-xylene (151) in dichloromethane [74]. 1,3,6,8-Tetramethyltriptycene (153) [74] and its monobenzoquinone derivative (154) [75] are obtained by the reaction of 152 with benzyne generated from anthranilic acid (80) and para-benzoquinone (51), respectively. Both 153 and 154 can be selectively nitrated at the 2,7-positions flanked by methyl groups (155 [74] and 156 [75]) using a mixture of potassium nitrate and trifluoroacetic acid. Reduction of these nitrated products using hydrazine and metal catalysts yields 1,3,6,8-tetramethyl-2,7-diaminotriptycene (157) and 13,16-dihydroxy-1,3,6,8-tetramethyl-2,7-diaminotriptycene (158). The latter has been used as a building block for redox-active PIMs [75].

Fig. 7

Synthesis of trip-monomers from multifunctionalized anthracenes

Figure 8 summarizes the synthesis of the 1,8,13-substituted triptycene derivatives. Our group reported that, through the Diels–Alder reaction of 1,8-dimethoxyanthracene (53) and benzyne generated from a methoxy-substituted precursor (162), 1,8,13-trimethoxytriptycene (165) can be prepared along with its 1,8,16-isomer as a minor product (1,8,13/1,8,16 = 2/1) [11]. Compounds 53 and 162 are synthesized in two steps from 1,8-dihydroxyanthraquinone (163) and in four steps from guaiacol (159), respectively. Similarly, 1,8-dimethoxytriptycene (164) can be obtained using 53 and benzyne [76]. By reacting these methoxy-substituted triptycenes with boron tribromide, we have shown that 1,8-dihydroxy and 1,8,13-trihydroxytriptycenes (166, 167) can be synthesized [11, 77]. A wide variety of substituents can be introduced to these hydroxylated derivatives via ether linkages, and the resulting alkoxytriptycenes have been found to exhibit particular 2D assembly properties [11, 77,78,79,80,81], which will be described in detail later. The various 1,8(,13)-substituted derivatives developed thus far include those with terminal olefin moieties (168, 171, 172) [77, 80, 81] and acryloyl groups (169) [78]. Compounds 171 and 172 are used as end modifiers for polydimethylsiloxane (PDMS) [80, 81]. We also prepared a 1,8,13-triptycene initiator with hydroxy termini (170) [79]. Mastalerz et al. also reported a synthetic method for 1,8,13-trihydroxytriptycene 167 at approximately the same time and showed that the ortho-positions of its hydroxy groups can be formylated (173) by the reaction using hexamethylene tetraamine in the presence of trifluoroacetic acid [82]. From hexa-substituted monomer 173, a triptycene-containing 3D COF was synthesized through the formation of a salphen complex [83].

Fig. 8

Synthesis of 1,8,13-substituted trip-monomers

The synthesis of bridgehead-substituted 1,8,13-triptycenes, which involves selective functionalization of the 10-position of 1,8-substituted anthracenes, requires somewhat laborious, multiple protection/deprotection steps [84]. Converting the methoxy groups of 53 to acetoxy groups (174) allows selective bromination at the 10-position using N-bromosuccinimide (NBS). The acetoxy groups of the resulting 175 were then converted back to methoxy groups. The synthesized 10-bromo-1,8-dimethoxyanthracene (176) is lithiated at the 10-position using n-butyllithium and subsequently treated with N,N-dimethylformamide (DMF), yielding 10-formyl-1,8-dimethoxyanthracene (177). A bridgehead-substituted 1,8,13-triptycene skeleton (179) can be synthesized by protecting the formyl group of 177 with ethylene glycol (178) and then treating it with in situ generated methoxybenzyne. Successive deprotection of 179 using hydrochloric acid and boron tribromide yields 10-formyl-1,8,13-trihydroxytriptycene (181) [84]. The formyl group of 181 can be converted to an ethynyl group (182) at this stage using the Ohira–Bestmann reagent [84]. Moreover, terminal ethynyl groups at this position are available for click reactions using copper(I) iodide and triethylamine [85]. The hydroxyl groups of 182 can be functionalized via etherification, and the remaining ethynyl group can be functionalized by nucleophilic substitution through acetylide or Sonogashira coupling. This protocol allows for the synthesis of bifunctional derivatives (183) carrying azide and terminal ethynyl groups [86]. This AB₃-type monomer was found to undergo polymerization in the assembled state, which will be discussed later. 1,8,13-Substituted triptycenes are relatively recently developed derivatives, and their synthesis has been thoroughly reviewed by Shindo et al. [87].

Recent progress in triptycene-containing polymers

Linear polymers

Here, we present some recent examples of triptycene-containing linear polymers with unique structures and interesting properties. Swager et al. reported the synthesis of polyethersulfone (poly-1) by polycondensation using 13,16-dihydroxy-1,8-dimethoxytriptycene (55), bisphenol A (184) and difluorodiphenylsulfone (185) (Fig. 9) [54]. The methoxy groups of poly-1 were deprotected with boron tribromide to form poly-2 with hydroxyl substituents, and bulky pyrazolium chloride (186 and 187) was attached to the hydroxyl groups to form a cross-linked network polymer (poly-3) with a structure in which the pyrazolium ions are densely aggregated (referred to as an ionic highway). Films of poly-3 are reported to show both anion-conducting properties and high stability resulting from suppressed swelling due to its cross-linked nature.

Fig. 9

Highly conductive and stable triptycene-containing anion-exchange polymer with an “ionic highway”

Swager et al. synthesized a simple triptycene-containing polyethersulfone (poly-4) from 1,4-dihydroxytriptycene (54) and 185, chloromethylated its triptycene moieties, and then treated the resulting poly-5 with N-methylimidazole (188) to obtain poly-6 with N-methylimidazolium chloride pendants (Fig. 10) [24]. This polymer not only forms anion exchange membranes for alkaline fuel cells but also provides a scaffold for metal nanoparticles [24, 88, 89]. For example, poly-6 was composited with poly(4-vinylpyridine) and single-walled carbon nanotubes (SWCNTs), and the counter anion (Cl^–) was exchanged with tetrachloroaurate (AuCl₄^–). Upon treatment with sodium borohydride, small (<5 nm) gold nanoparticles (Au NPs) were formed in the composite film (Fig. 10). A field-effect transistor (FET) incorporating a Au NP composite layer was reported to serve as a chemo FET sensor for the detection of carbon monoxide and other gaseous molecules [88]. Moreover, bimetallic Pd/Pt nanoparticles (PdPt NPs) with a size of 1 nm can be synthesized within the poly-6 film (Fig. 10). PdPt NPs react with hydrogen gas with high sensitivity in an oxygen atmosphere, and it has been reported that these materials can be applied as very sensitive hydrogen gas sensors [89]. In these systems, the use of triptycene units allows for the dense accumulation of functional groups as well as the formation of microporous structures.

Fig. 10

a Synthesis of triptycene-containing anion exchange polymers and their application in (b) chemo-FET sensors (e.g., CO gas) and (c) H₂ gas sensors. b Adapted with permission [88] Copyright 2019, American Chemical Society. c Adapted under terms of the CC-BY 4.0 license [89] Copyright 2020, Elsevier

Conjugated polymers derived from the 9,10-diethynylpentiptycene monomer 58 have been used as components of highly responsive chemosensors due to their porous nature [15]. Pentiptycene-containing poly-7 with a benzothiadiazole unit composited with SWCNTs has been reported to serve as a sensor for the chemiresistive detection of solvent vapors such as benzene, toluene and xylene (i.e., BTX) (Fig. 11) [90]. Pentiptycene-containing poly-8 with semiperfluoroalkyl side chains has been reported to function as a fluorescent polymer sensor for the detection of poly(fluoroalkyl) substances (PFAS) (Fig. 11) [91].

Fig. 11

Pentiptycene-containing π-conjugated polymers applied for chemosensing

Swager et al. reported that a palladium-catalyzed reaction between phenolic hydroxyl groups and aryl halides can be used in condensation polymerization to obtain a variety of polyaryl ethers that cannot be synthesized through standard aromatic nucleophilic substitution methods (Fig. 12) [92]. Typically, 1,4-dihydroxy-6,14-di-tert-butyl-triptycene (189), 2,7-dibromospirobifluorene (190) and aromatic dibromides with various photocatalytically active sites (191–196) were reacted in the presence of a palladium catalyst (197) and a monophosphine ligand (198) to afford the corresponding polyethers (poly-9–poly-16). The obtained polymers are solution-processable and form microporous films that exhibit photocatalytic abilities, as shown in Fig. 12 [93]. A ruthenium catalyst can also be used to introduce perfluoroalkyl groups into the spirobifluorene moiety. The resulting polymer carrying fluorophilic side chains (poly-9-C₁₇F₃₅) can be used for the modification of the surface inside perfluoroalkoxy alkane (PFA) tubes. Using these modified PFA tubes, photoreactions can be conducted allowing the reagent to flow while irradiating with light [93].

Fig. 12

Triptycene-containing solution-processable porous photocatalysts obtained by palladium-catalyzed carbon–oxygen bond formation

Triptycene is widely used as a constituent in PIMs due to its rigidity and high intramolecular free volume (Fig. 13) [9, 18]. McKeown et al. reported dioxane-forming ladder polymerization using tetrafluoroterephthalonitrile (199) and 2,3,6,7-tetrahydroxybenzotriptycene with very bulky substituents (139) [69, 70]. The resulting poly-17 was reported to exhibit highly selective gas permeability, significantly outperforming the 2008 Robeson upper bounds for O₂/N₂, H₂/N₂, CO₂/N₂, H₂/CH₄ and CO₂/CH₄ selectivities. Using a similar polymerization method, Guo et al. reported the synthesis of a ladder-type polymer (poly-18) with pentiptycene in the main chain from a tetrahydroxylated pentiptycene (76), a tetramethyl spirobindan tetraol (200) and 201 [55]. High CO₂/CH₄ permselectivity was reported for poly-18. McKeown et al. also reported ladder polymerizations by the reaction of dimethoxymethane with 2,6-diaminotriptycene (8) and 2,6-diaminobenzotriptycene (125) to yield Tröger’s base-containing PIMs poly-19 [94] and poly-20 [66], respectively. Condensation of a trip-monomer with two hydroxy groups and two amino groups (38) and 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (201) in the presence of isoquinoline yielded polyimide poly-21, which, after thermal treatment, was transformed into thermally rearranged polybenzoxazole poly-22 [36]. Li, Yi, Ma et al. reported that a polyimide membrane (poly-23) obtained from 2,6-diaminomonomer (39) with ortho-bromo groups and pyromellitic anhydride (202) showed ca. 8 times greater gas permeability than the corresponding polyimide devoid of bromo groups [38]. Moreover, when the poly-23 membrane was heated to 550 °C, debromination occurred yielding a carbon molecular-sieve membrane, which displayed an almost 9-fold increase in gas permeability while maintaining permselectivity, with CO₂/N₂ and CO₂/CH₄ selectivities of 29.2 and 30.9, respectively, and an unprecedentedly high CO₂ permeability coefficient of 20639 barrer [38].

Fig. 13

Triptycene-containing PIMs useful in gas-separation membranes

Gong, McKeown et al. developed a triptycene-containing polyimide poly-24 with a hydroquinone moiety by reacting 13,16-dihydroxy-1,3,6,8-tetramethyl-2,7-diaminotriptycene (158) with pyromellitic dianhydride (202) in the presence of isoquinoline (Fig. 14) [75]. This polymer has a rigid and randomly contorted main-chain structure in which the rotation of the diimide unit is inhibited by the methyl groups on the triptycene moiety. Accordingly, poly-24 shows excellent solubility in aprotic polar solvents and can be easily cast into freestanding films. Upon treatment with CAN, poly-24 is oxidized to form the para-benzoquinone derivative poly-25, which undergoes a four-step redox process with excellent reversibility (Fig. 14). Lithium-ion battery cells using easily solution-processable poly-24 as a component of the cathode material have been reported to exhibit stable cycling performance [75].

Fig. 14

Triptycene-containing redox-active PIMs useful as electrodes for lithium-ion batteries

Despite the fact that many trip-monomers have chirality, there are only a few examples of optically active triptycene polymers [7, 31, 32]. Examples of the synthesis of such systems are shown in Fig. 15. The polycondensation of optically active 2,6-diaminotriptycene (8), which can be obtained by means of chiral HPLC, with acid anhydride 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (201) in the presence of isoquinoline yields triptycene-containing polyimide poly-27 [31]. This polymer serves as an optically active PIM, and its freestanding film can be used for enantioselective membrane separation. Chen et al. synthesized a chiral triptycene with dihydroacridine blades (203) from optically active 8 [95]. Compound 203 was then reacted with bis(4-bromophenyl)sulfone (204) in the presence of a palladium catalyst to produce optically active poly-28. This polymer shows thermally activated delayed fluorescence (TADF) properties, where the triptycene and sulfone moieties serve as electron donors and acceptors, respectively, and emits circularly polarized luminescence (CPL) with a dissymmetry factor (g_lum) on the order of 10^–3 [95]. Using poly-28, circularly polarized organic light-emitting diodes (OLEDs) were successfully fabricated by means of solution processing. Ikai et al. synthesized optically pure 2,6-diiodotriptycene (18) from the corresponding optical isomer of 8 [96]. This was then used in a Suzuki coupling polymerization with ethynyl-substituted diboronic ester (205) to yield optically active poly-29. Upon treatment with trifluoroacetic acid, poly-29 undergoes a geometrically selective Friedel–Crafts reaction, yielding the ladder polymer poly-30, whose conformation is fixed in a one-handed helical structure [96]. It has been reported that poly-30 exhibits CPL and can be used as a stationary phase for chiral column chromatography.

Fig. 15

Optically active triptycene-containing polymers

Network polymers

Triptycene can be easily tri- and hexa-functionalized by Friedel–Crafts-type reactions (Fig. 3). This approach is also useful for the synthesis of triptycene-containing network polymers; however, the resulting polymers are usually insoluble and devoid of regular structures. Many examples of this type of polymer have been described in a recent review by C-F. Chen et al. [8], so only selected examples are presented here (Fig. 16). McKeown et al. reported that porous 3D network polymers (poly-31) with a Brunauer–Emmett–Teller (BET) specific surface area (S_BET) of 1750 m²/g can be obtained by the reaction of unsubstituted triptycene (1) with dichloromethane in the presence of aluminum trichloride (Fig. 16a, top) [97]. Furthermore, poly-31 can be easily functionalized with nitro (poly-32), amino (poly-33), or sulfo (poly-34) groups [98]. McKeown et al. synthesized a Tröger’s base-containing network polymer (poly-35) by the reaction of 2,6,14- or 2,7,14-triaminotriptycene (10, 11) with dimethoxymethane in the presence of trifluoroacetic acid (Fig. 16a, middle). They found that poly-35 serves as a base catalyst for the Knoevenagel condensation of benzaldehyde and malonitrile [99]. Moreover, the use of various diamines for copolymerization with triaminotoriptycene allows for tuning of the pore size and catalytic activity of the resulting network polymers [100]. Baek et al. reported a very unique solid-state polymerization of triptycene derivatives [45]. They found that 2,3,6,7,14,15-hexaethynyltriptycene (48) forms stable crystalline materials that incorporate acetone and water molecules, but rapid heating causes desorption of the solvent. This triggers Masamune–Bergman cyclization of the ortho-diethynyl moiety to generate highly reactive radicals, leading to explosive reactions and the formation of poly-36 with a porous structure (Fig. 16b, c).

Fig. 16

a Selected examples of triptycene-containing nonregular network polymers. b, c Explosive reactions of 48 forming poly-36 triggered by solvent desorption. b, c Adapted under terms of the CC-BY 4.0 license [45] Copyright 2017, Springer Nature

Porous 2D polymers

Triptycene derivatives that undergo porous 2D hexagonal packing typically have structures with blades laterally extended through the 2,3-, 6,7- and 14,15-positions (Fig. 1). In 2014, King et al. reported a triptycene derivative (206) with photoreactive tetrafluoroanthraceno blades (Fig. 17a, left and center) [101]. This extended triptycene derivative forms porous hexagonal packing in the crystalline state, with a configuration in which the tetrafluoroanthraceno blades are intermolecularly π-stacked (Fig. 17b, left). Then, light irradiation was used to induce [4+4] cycloadditions between the blades, with photoirradiation (460 nm) of a single-crystal sample at 223 K leading to dimerization (207) (Fig. 17b, center) and further photoirradiation at 400 nm forming a 2D polymer (poly-37) (Fig. 17b, right). This two-step photochemical reaction proceeds in a single-crystal-to-single-crystal manner. It has been reported that crystalline samples of this 2D polymer can be exfoliated to monolayer sheets by heating to 50 °C in N-methyl-2-pyrrolidone (NMP). Before this study, the same group reported in 2013 that light irradiation of single crystals of a derivative with nonfluorinated anthraceno blades (208) produced a 2D polymer (Fig. 17a, right) [102]. However, the photopolymerization of 208 was reported to be “not single-crystal to single-crystal transformation”. In 2021, Lackinger et al. reported that vacuum deposition of 206 on a hexacosane-passivated graphite substrate followed by thermal annealing produced a crystalline monolayer film (with a domain size of up to 500 nm) consisting of a porous hexagonal lattice similar to that of a single crystal [103]. When this monolayer sample is irradiated with light using a high-power LED, on-surface photocycloadditions proceed, yielding a 2D polymer.

Fig. 17

a Molecular structures of 206 and 208, which feature photoreactive tetrafluoroanthraceno and anthraceno blades, respectively. b Schematic illustrations of the two-step single-crystal-to-single-crystal transformations of 206 into its 2D polymer poly-37

Based on a similar molecular design, King et al. reported the formation of Langmuir–Blodgett (LB) films at the air/water interface using triptycene derivative 209, which contains a hydrophilic diethylene glycol moiety at the bridgehead position that serves as an anchor for the water layer (Fig. 18a) [104]. A chloroform/hexane solution of 209 was spread on the air/water interface and compressed at 1 °C using an LB trough, and the mean molecular area (MMA) was calculated from the compression isotherm. Phase changes were observed at approximately 155–135 and 80 Ų, and the former MMA was assigned to porous hexagonal p6 packing. STM measurements of a LB film of 209 transferred onto a HOPG substrate show a porous structure that is consistent with the simulated pattern (Fig. 18b). Upon irradiation with 365 nm light, the LB film of 209 underwent photopolymerization, and the obtained thin-film polymer was found to be mechanically hard enough to deform a paper Wilhelmy plate during the Langmuir experiment.

Fig. 18

a Formation of a LB film composed of amphiphilic propeller-shaped 209, which carries a diethylene glycol chain that anchors the molecule to the water surface. b STM image (left) and simulated p6 lattice of 209 (right). c Structures of a series of amphiphilic propeller-shaped molecules (210, 211, and 212) designed for the formation of structurally well-defined LB films and their crosslinked 2D polymers. d Schematic illustration of plasmon-induced [4+4]-cycloaddition/2D polymerization of LB films 210 and 211. b Adapted with permission [104] Copyright 2015, American Chemical Society. Adapted under terms of the CC-BY 4.0 license [107] Copyright 2021, Springer Nature

Similar to 209, several amphiphilic extended triptycene derivatives have been reported to form highly ordered LB films with porous hexagonal structures (210 [105], 211 [106] and 212 [106], Fig. 18c). Schlüter et al. reported the synthesis of 2D polymers using 211, 212 and a 1:1 mixture of 211 and 212 using the LB technique and light irradiation [106]. After being transferred onto a Au(111) substrate, the obtained 2D polymers were characterized by tip-enhanced Raman spectroscopy (TERS), which allowed for the evaluation of the conversion number (X) of the crosslinked structures (i.e., anthracene dimer moieties). The results showed high conversion numbers, with averages of X = 87.7 ± 1.8 (211), 92.0 ± 1.7 (212), and 94.1 ± 2.1% (copolymer). Based on these values, along with the random-growth model, the calculated crystallinities of the 2D polymers were 95.8 ± 1.2, 98.2 ± 1.1 and 99.0 ± 1.4% for 211, 212 and 211/212 (1/1), respectively. In 2021, Shao, Lan, Zenobi, and coworkers performed TERS measurements on LB films of 210 and 211 transferred onto Au(111) substrates using silver chips and found that visible laser irradiation (633 nm) caused intermolecular [4+4] cycloadditions between the blades in the 2D assembly as plasmon-induced chemical reactions (PICRs) (Fig. 18d) [107].

Nonporous 2D assemblies and polymers

Triptycene derivatives that form dense and nonporous 2D hexagonal packing are characterized by structures in which substituents are introduced at either the 1,8,13- or 4,5,16-positions (Fig. 1). In derivatives with these substitution patterns, the free volume around the triptycene backbone can be retained, which is favorable for the formation of 2D nested packing. In fact, most triptycene derivatives reported to form nonporous 2D hexagonal structures in crystals or in the liquid-crystalline state have these substitution patterns [87, 108]. It has been found that 1,8,13-substituted triptycene derivatives can exhibit 2D assembly ability even when another substituent is introduced at the bridgehead position (vide infra). Note that unsubstituted triptycene (1) does not form nested packing in the crystal or on the substrate surface [109, 110]. Therefore, it is essential to choose appropriate substituents and substitution patterns to realize dense 2D hexagonal assemblies.

With the aim of constructing large-area, highly ordered organic thin films through molecular self-assembly, our group developed a tripodal triptycene (213) with long-chain alkoxy groups introduced at the 1,8,13-positions (Fig. 19a) [11]. In the bulk state, this compound forms a “2D nested hexagonal packing + 1D layer stacking” structure (Fig. 19b). The assembly of 213 on a solid substrate yields oriented thin films with 2D hexagonal sheets stacked parallel to the substrate (Fig. 19b) on a variety of inorganic and polymer substrates simply by, e.g., spin-coating or vacuum deposition [11, 111, 112]. Importantly, in thin films of 213, the ordered structure formed on nanometer-length scales can propagate to macroscopic length scales. For example, in through-view XRD measurements of a vacuum-deposited film of 213 (50 nm in thickness) formed on a sapphire substrate (2.0 cm in diameter), the in-plane azimuthal angle dependence of the peak intensity originating from the 2D hexagonal lattice does not change when the measurement position on the film is changed [11]. This observation indicates that the orientation of the 2D hexagonal lattice is aligned across the entire film.

Fig. 19

a Molecular structures of 1,8(,13)-substituted triptycene derivatives 213–215 having alkoxy side chains. b Schematic illustrations of a “2D nested hexagonal packing + 1D layer stacking” structure formed by the self-assembly of 213–215

Derivatives with different alkoxy chain lengths and those with only two or one long-chain alkoxy group also form 2D+1D structures similar to that of 213, whereas their thermal stability largely depends on the triptycene substitution pattern (Fig. 19a) [11, 113]. For example, the 2D+1D assembly of 1,8-bis(dodecyloxy)tripycene 214 (m.p. 134 °C) shows significantly lower thermal stability than that of 213 (m.p. 211 °C). However, the assembly of 1,8-bis(dodecyloxy)-13-methoxytriptycene 215, in which one of the long-chain alkoxy groups in 213 is replaced by a methoxy group, exhibits greater relative thermal stability (m.p. 231 °C). These differences are directly reflected in the properties of the soft materials incorporating the triptycene units (vide infra).

Tripodal triptycenes are also useful as components in organic electronic devices [113,114,115,116,117]. We have found that the performance of flexible organic thin-film transistor devices can be significantly improved by forming several layers of 215 on the surface of a parylene-based polymer dielectric layer [114,115,116,117]. This triptycene-based surface modifier covers structural defects on the surface of the polymer substrate and lowers and homogenizes the surface energy. This leads to improved crystallinity of the organic semiconductor on the dielectric layer and overall performance of the transistors. Although SAMs can be used for the surface coating of inorganic substrates, they cannot be applied to polymer substrate surfaces. Tripodal triptycenes, which can form highly ordered and perfectly oriented films regardless of substrate type, provide a powerful tool for improving the performance of organic electronic devices.

Importantly, tripodal triptycenes exhibit excellent 2D assembly ability even when various functional groups are incorporated [84, 86]. For example, compound 216, in which a sterically bulky spherical C₆₀ is introduced via an ethynyl group at the bridgehead position, as well as its precursor 217 with a terminal ethynyl group (Fig. 20a), also forms 2D+1D structures in the bulk state [84]. The spin-coated thin films of 216 are oriented similarly to those of 215, with a 2D+1D structure consisting of 2D layers of densely arranged C₆₀ units (Fig. 20b). Time-resolved microwave conductivity (TRMC) measurements of these thin films revealed the anisotropic conduction of photocarriers in the in-plane direction. Thus, tripodal triptycenes are useful supramolecular scaffolds [118] that promote the 2D assembly of various functional groups and molecular units, making it possible to improve their anisotropic properties. Face-selective introduction of an azide group and three terminal ethynyl groups into a tripodal triptycene (183) also leads to assembly into a 2D+1D structure both in the bulk state and on solid substrates (Fig. 20c) [86]. When this assembly is heated, intermolecular uncatalyzed Huisgen cycloadditions maintain the assembly structure. The resulting polymer is insoluble and possibly composed of a highly entangled polycatenane-like cyclic structure (Fig. 20c). The solid-phase polymerization of the oriented film of 183 proceeded while retaining the surface nanoterraced structure.

Fig. 20

a Molecular structure of C₆₀-appended tripodal triptycene 216 and its precursor 217. b Schematic illustrations of the 2D+1D structure formed by the self-assembly of 216. c Molecular structure (left) and schematic assembly structures of 183 before and after catalyst-free thermal Huisgen cycloaddition

The incorporation of 1,8,(13)-substituted triptycenes into polymers induces long-range ordered structures through 2D assembly of the triptycene units, greatly enhancing the mechanical properties of the polymers [77,78,79,80]. Such specific 2D assembly can be achieved by incorporating the triptycene units at various sites of polymers, including at both ends [80, 81], in the main chain [77], side chains [78] and at various branch points [79]. Figure 21a shows the chemical structures of poly(dimethylsiloxane) (PDMS) derivatives (poly-38–poly-40) with 1,8(,13)-triptycene units at both ends (poly-38 and poly-39) along with a 1,4-substituted derivative (poly-40). For example, hydrosilylation of olefin-appended 1,8-substituted triptycene 171 (Fig. 8) with hydride-terminated PDMS (PDMS-H, M_n = 18 or 24 kDa) affords poly-38 [80]. In sharp contrast to liquid PDMS-H, the telechelic polymer poly-38 forms a highly viscous solid that exhibits birefringence (Fig. 21b). Rheological measurements revealed that the complex viscosity of poly-38 (approximately 10⁵ Pa·s) is 10⁴ times greater than that of PDMS-H (approximately 10¹ Pa·s). Small- and wide-angle XRD measurements revealed that poly-38 forms a 2D+1D structure with long layer spacings of 18–20 nm, in which 2D sheets of terminal triptycene units are stacked one-dimensionally via PDMS domains (Fig. 21c). Telechelic poly-39 synthesized from 1,8,13-substituted triptycene 172 (Fig. 8) also forms a 2D+1D structure but has an even greater mechanical strength than poly-38 and behaves as a thermoplastic, resulting in a freestanding film without any covalent cross-linking (Fig. 21b) [81]. It has also been shown that poly-39 films exhibit self-healing properties. The only structural difference between poly-39 and poly-38 is the presence or absence of a methoxy group at the 13-position of the terminal triptycene units, and it is thus surprising that such a tiny change in substituent relative to the entire polymer has a significant impact on the mechanical and thermal properties. Moreover, the structural and physical properties of PDMS are largely unchanged in poly-40 (Fig. 21a, b, right), where 1,4-substituted triptycene units without 2D assembly ability are introduced at both ends of the PDMS.

Fig. 21

a 1,8(,13)-Substituted triptycene-containing telechelic polymers poly-38 and poly-39 and 1,4-substituted triptycene-containing poly-40. b Photographs of bulk samples of PDMS-H and the telechelic polymers. c Schematic illustrations of the assembly structures of the telechelic polymers

Ring-closing metathesis of 1,8-olefin-appended 168 (Fig. 8) yields macrocyclic olefin monomer 218, which has been used for the synthesis of main chain-type triptycene-containing polymers (Fig. 22) [77]. We synthesized the homopolymer poly-41 by ring-opening metathesis polymerization of 218 and the copolymer poly-42 by copolymerization of 218 with cyclooctene [77]. Both poly-41 and poly-42 form 2D hexagonal sheet structures with triptycene units and lamellar structures filled with polymer chain domains, which leads to a marked improvement in the mechanical properties of the polymers. In poly-43 without 2D assembly ability, no improvement in physical properties was observed [77]. While the triptycene polymers poly-44 and poly-45, which have oligosiloxane- and ester-based main chains, respectively, also form 2D+1D structures, the urethane-containing poly-46 does not (Fig. 22) [77]. Presumably, the hydrogen bonding ability of the urethane group may be superior to the assembling ability of the triptycene, hampering structural ordering of the polymer.

Fig. 22

Polymers incorporating 1,8-substituted triptycene units (poly-41, poly-42, poly-44–poly-46) in the main chains with a 1,4-substituted unit (poly-43)

We synthesized diblock (poly-47), ABA triblock (poly-48) and random (poly-49) copolymers by reversible addition-fragmentation chain-transfer (RAFT) polymerization of 1-acryloyl-8-methoxytriptycene 169 (Fig. 8) and n-butyl acrylate (Fig. 23a) [78]. The thermal stability of these triptycene-containing copolymers is comparable (2% weight loss temperature: ca. 300 °C) to that of the precursor polymer (poly-50) without triptycene units. However, while poly(n-butyl acrylate) derivatives, including poly-50, are liquids, the copolymers are all viscous solids. For diblock poly-47, both the melting (T_m) and crystallization (T_c) temperatures shift toward higher values with increasing triptycene content. While their triptycene contents are comparable, diblock poly-47 has slightly greater T_m and T_c values than triblock poly-48, whereas the enthalpy changes associated with the phase transitions are in the same range. XRD measurements revealed that both diblock and triblock copolymers form 2D+1D structures with long-range ordering. In contrast, the random copolymer poly-49 does not form a 2D+1D structure and shows no clear phase transition. It is worth noting that rheological measurements show that diblock poly-47 and triblock poly-48 exhibit complex viscosities 10⁴–10⁵ times greater than that of random poly-49 over a wide temperature range. Therefore, in the design of side-chain-type triptycene polymers, the presence of block segments is essential for achieving long-range ordering. Accordingly, we designed poly-51 with triptycenes at both ends of a poly(N-isopropylacrylamide) (PNIPAM) segment (Fig. 23b) [78]. This triblock copolymer yields hydrogels (60 wt% water content) without chemical cross-linking due to the formation of a 2D+1D structure with a 2D triptycene sheet and a hydrated PNIPAM domain. The hydrogel became turbid at 60 °C, and the PNIPAM domain exhibited a lower critical solution temperature (LCST) upon increasing the temperature. VT-XRD measurements of the hydrogel revealed that the interlayer distances decreased with increasing temperature while maintaining the 2D structure. These findings are expected to be applied to the future development of soft actuators that exhibit anisotropic motion in response to stimuli.

Fig. 23

a Molecular and schematic structures of diblock poly-47, ABA triblock poly-48, and random poly-49 carrying 1,8-substituted triptycene-based side chains as well as poly-50, a precursor of poly-48. b Molecular structure of ABA triblock polymer poly-51 with a PINIPAM segment and (c) photographs of its hydrogel. c Adapted with permission [78] Copyright 2023, American Chemical Society

Conclusions

As outlined in this review, triptycene is a versatile scaffold that can be exploited by introducing a number of substituents in various patterns at its blades and bridgehead positions. It is also possible to impart characteristic redox properties to the blade sites. One of the main topics in the development of triptycene-based functional materials, diverse polymers, and molecular assemblies is the design of nanospaces (microporosity) in the assembly state, which is being explored for use in guest recognition, material transport, separation, and catalysis. Rigid three-bladed propeller molecules are suitable for forming porous or nonporous 2D lattices, and they provide excellent building blocks for forming 2D sheet structures that are not easily accessible. Their ability to form dense 2D sheets stems from their characteristic space-filling structure, which enables precise control over the structures and properties of surfaces and interfaces. It is expected that in the future, the development of materials for the realization of unique electronic states that reflect the two-dimensional nature and topology of assemblies will accelerate, leading to new functionalities. From various perspectives, triptycenes will continue to drive the development of functional materials that cannot be realized using planar molecules. Organic synthesis technologies will be the driving force to enable the precise design of such triptycene derivatives.