Recent Developments in Synthesis and Applications of Triptycene and Pentiptycene Derivatives

MICROREVIEW

DOI: 10.1002/ejoc.201100684

Recent Developments in Synthesis and Applications of Triptycene andPentiptycene Derivatives

Yi Jiang[a] and Chuan-Feng Chen*[a]

Keywords: Supramolecular chemistry / Synthetic methods / Host–guest systems / Molecular devices / Rotaxanes / Arenes /Triptycenes / Pentiptycenes

Iptycenes are a class of aromatic compounds with arene unitsfused to the [2.2.2]bicyclooctatriene bridgehead system. As aresult of their unique structures, more and more iptycenes,especially triptycene and pentiptycene derivatives, havebeen efficiently synthesized. These compounds have wideapplications in many research areas, including molecularmachines, molecular balances, catalysis, materials chemistry,

Introduction

With the goal of testing its radical activity, Bartlettet al.[1] first synthesized triptycene (1, Figure 1) by a multi-step procedure in 1942. Interest in triptycene, with itsunique structure, increased gradually after Wittig andLudwig[2] reported the easy synthesis of 1 in one step frombenzyne and anthracene, which was eventually also to makethe synthesis of triptycene a standard undergraduate labo-ratory “experiment”. In 1981, Hart[3b] first proposed theconcept “iptycene”, based on triptycene, denoting the

[a] Beijing National Laboratory for Molecular Sciences, CAS KeyLaboratory of Molecular Recognition and Function, Instituteof Chemistry, Chinese Academy of Sciences,Beijing 100190, ChinaFax: +86-10-62554449E-mail: [email protected]

Yi Jiang graduated and received his Ph.D. from the Institute of Chemistry, Chinese Academy of Sciences, Beijing in 2010(supervisor: Professor Chuan-Feng Chen). Since 2010, he has been working as an assistant professor in Qingdao Instituteof Bioenergy & Bioprocess Technology, Chinese Academy of Sciences. His current research interests include supramolec-ular chemistry based on novel synthetic hosts, as well as polymer science.

Chuan-Feng Chen graduated and received his Ph.D. from Nanjing University in 1994. After working as a postdoctoralfellow at ICCAS for two years, he became an associate professor at the same institute in 1996. From 1998 to 2001 heworked as a visiting scientist at University of New Mexico, USA. He was then promoted to become a full professor ofICCAS in 2001. His current research interests include supramolecular chemistry based on novel synthetic hosts, hydrogen-bond-mediated molecular assemblies, and helicene chemistry.

Eur. J. Org. Chem. 2011, 6377–6403 © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 6377

molecular assembly, host–guest chemistry, and so on. Thismicroreview mainly describes the developments in the syn-thesis and applications of triptycene and pentiptycene deriv-atives during the last ten years. Particular attention is paid tothe synthesis and applications of hosts derived from trip-tycene and pentiptycene systems in recent years.

number of arene planes separated by a bridgehead system.Compounds 1 and 2 (Figure 1), containing three and fiveseparated arene units, for example, were called triptyceneand pentiptycene, respectively. Since then, the door ofiptycene chemistry has truly been opened.

Figure 1. Structures of triptycene (1) and pentiptycene (2).

At the beginning of its development, iptycene chemistrywas focused almost entirely on the synthesis of triptyceneand its derivatives. Until the 1980s their potential applica-

Y. Jiang, C.-F. ChenMICROREVIEWtions were developed only gradually. In recent years,though, iptycene derivatives, especially triptycenes[4] andpentiptycenes,[4b,4c,5] have attracted much attention andmore and more applications for them in molecular ma-chines, supramolecular chemistry, materials science, andother research areas have been found. Here we mainly focuson syntheses and applications of triptycene and pentip-tycene derivatives during last ten years. Particular attentionis paid to the synthesis and applications of novel triptycene-and pentiptycene-derived hosts in recent years.

2. Synthesis and Applications of TriptyceneDerivatives

2.1. Triptycene Derivatives in Molecular Machines

Molecular machines,[6] capable of mimicking the behav-ior of macroscale objects, such as shuttles, switches, ratch-ets, brakes, gears, rotors, motors and nanocars, have at-tracted much attention, because of the potential for the useof such devices to manipulate objects on nanometer scalesor to construct future “intelligent” materials as functionalcomponents. In the course of the last 20 years, the field ofmolecular machines has experienced spectacular develop-ment. Various external stimuli,[7] such as chemical, electri-cal, or light irradiation stimuli, have been employed to in-duce the functions described above. Triptycene, which hasthree arene units fused to the [2.2.2]bicyclooctatrienebridgehead system with D3h symmetry, looks very much likea macroscale gearwheel, so triptycenes were utilized fromthe 1980s onwards as potential components for molecularmachines.[8] In recent years they have shown many moreapplications in the construction of new molecular machines.

In 2002, Garcia-Garibay[9] et al. proposed a new molecu-lar architecture with a function analogous to those of mac-roscopic compasses and gyroscopes. To elaborate the newconcept, they developed a simple approach for the synthesisof the molecules 3a–3d (Figure 2, a), each consisting of acentral aromatic group coupled with two axially positionedethynyltriptycene units. Semiempirical calculations by theAM1 method indicated that rotation about the tryptycene–alkyne and aryl–alkyne single bonds in the gas phaseshould be essentially frictionless, and indeed, dynamicallyaveraged 1H and 13C NMR spectra both suggested rapidrotation of 3 in solution. On analysis of the crystal structureof 3a, however, it was found that in the solid state the rota-tion of the phenylene group of 3a is prevented by interdigi-tation of adjacent molecules and by close contacts with thesolvent of crystallization. With the goal of preventing theinterdigitition observed in the packing structure of 3a, Gar-cia-Garibay et al.[10] synthesized the compounds 3e and 3f(Figure 2, b) from 2,3-dimethylbuta-1,3-diene with use ofDiels–Alder cycloadditions and Pd0-catalyzed coupling asthe key reactions. Single-crystal X-ray analyses of 3e and3f showed that the adjacent molecular gyroscopes were, asexpected, separated by the presence of the peripheral methylgroups. Moreover, although it was found that in the solid

www.eurjoc.org © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Org. Chem. 2011, 6377–64036378

state the free volume generated around the central phenyl-ene unit was filled by solvent molecules, preliminary 2HNMR dynamic measurements indicated that gyroscopic ro-tation in crystals of 3e occurred through a 180° site ex-change (twofold flip) with a very low rotational barrier of4.4 kcalmol–1,[11] only 1.4 kcal mol–1 higher than the in-ternal barrier for ethane in the gas phase.

Figure 2. a) Synthesis of compounds 3a–3d. b) Structures of 3e and3f.

In 2004, McGlinchey[12] began a major effort to con-struct triptycene-based molecular rotors in which migratingmetal-based components were used to control the barriersof rotation of the triptycene moieties. They successfully ob-tained the target compound 9-(1H-inden-3-yl)triptycene (4,Figure 3, a) by treatment of 9-(1H-inden-3-yl)anthracenewith benzyne. However, they found that treatment of 4 with

Figure 3. a) Synthesis of compounds 4 and 5. b) Representation ofthe molecular brake.

Triptycene and Pentiptycene Derivatives

Cr(CO)6 resulted in the formation of the metal complex 5,in which the chromium tricarbonyl unit is coordinated toone of the blades of the triptycene, rather than to the six-membered ring of the indenyl substituent to produce themetal complex 6, which might be due to the steric crowding.Furthermore, they found that there was a 2:1 mixture oftwo rotamers at low temperature and that the rotation bar-riers for 4 and 5 were 12 and 13 kcalmol–1, respectively.

In 2007, McGlinchey and Nikitin[13] reported the synthe-sis of 9-(1H-inden-2-yl)triptycene (7, Figure 3, b). They fur-ther determined by 1H NMR and 13C NMR that the rota-tional barrier about the 2-indenyl–triptycenyl single bondin 7 is �9 kcalmol–1 and that this bond undergoes essen-tially free rotation at ambient temperature. Thanks to thereduction of steric crowding in 7, they were thus able tosynthesize the chromium, manganese, and rhenium tricar-bonyl derivatives 8a and 8b–8d, in which the M(CO)3 unitswere attached to the indenyl moiety of 7 in either an η6 oran η5 fashion (Figure 3, b).[14] When the M(CO)3 unit wasattached to the indenyl moiety in an η6 fashion, the sim-plicity of the 1H and 13C NMR spectra of 8a showed thetime-averaged equivalence of the three blades of the pad-dlewheel, and the crystal structure of 8a also revealed thatthere were no steric interactions between the triptycene andthe indenyl moieties, which all indicated that the pad-dlewheel 8a was free to rotate. When, however, the organo-metallic fragment M(CO)3 was shifted from the six-mem-bered ring of the indene to the five-membered ring, the ste-ric crowding increased. The 1H NMR spectra of 8b and ofthe isostructural η5-Mn(CO)3 and η5-Re(CO)3 complexes8c and 8d, respectively, each showed a 2:1 splitting of theblades of the triptycyl moiety, thus breaking its originalthreefold symmetry. Furthermore, the X-ray crystal struc-tures of 8b–8d provide structures of the systems in their“OFF” states, meaning that the bulky tripods had, as ex-pected, blocked the rotation of the three-bladed molecularpaddlewheel.

More recently, McGlinchey and Nikitin[15] have furtherdeveloped the novel triptycene-based molecular machine 9(Figure 4), named a “molecular dial”, in which two ferro-cenyl groups are attached to the 9- and 10-positions of trip-tycene. The 1H and 13C NMR spectra of 9 showed the exis-tence of meso and racemic rotamers (1:2) over the 253–323 K range. Moreover, 2D-EXSY NMR spectroscopicdata recorded with different mixing times also revealed thatthese interconversions between the rotamers proceeded in astepwise manner (rac-meso-rac, for example), thus behavingas a set of molecular dials.

Figure 4. Dynamic behavior of 9,10-diferrocenyltriptycene (9).

Eur. J. Org. Chem. 2011, 6377–6403 © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjoc.org 6379

Since the 1990s, Kelly and his co-workers have devotedmuch attention to triptycene-based molecular rotors, suchas molecular brakes[16] and molecular ratchets.[17] Moreinterestingly, they also developed compound 10 (Figure 5),a rationally designed, chemically driven molecular motorprototype, which achieved 120° unidirectional rotationaround the triptycene/helicene bond.[18] In an attempt toachieve 360° unidirectional rotation they recently[19] synthe-sized the triptycene derivatives 11 and 12. Unfortunately,the rotation of the triptycene moieties in 11 and 12 did notoccur as designed, which might be due to the formation ofhydrogen bonding between the hydroxypropyl group andthe functionality present in 11 but absent in 10, or to Bürgi–Dunitz (or similar) interactions.

Figure 5. Structures of the triptycene derivatives 10–12.

2.2. Triptycene Derivatives in Molecular Balances

Noncovalent interactions play a great role in all chemicaland biological processes. In biological systems, noncovalentinteractions have been found in complexes of medicinaldrugs and targeted enzymes,[20] in addition to the promi-nent role they play in DNA and RNA structures.[21] Mean-while, in chemical systems, noncovalent interactions havebeen applied in templated synthesis,[22] molecular recogni-tion,[23] and the construction of new materials with uniqueproperties.[24] Understanding of the mechanisms involved innoncovalent interactions is necessary for the design of sup-ramolecular systems and biologically active agents. How-ever, the precise geometry of an interaction of interest in aconformationally dynamic biomolecule is hard to deter-mine, and noncovalent interactions in natural complexesare often too complicated to be studied in detail.

Folding molecules – as “molecular balances”[25] – offeran attractive platform for study of noncovalent interactions,because the relative stabilities of the conformational statesare governed by the intramolecular contacts and solvent in-teractions that are present in one conformation but absentin the others. Oki[26] first reported on triptycene-based mo-lecular balances in the 1970s, in an investigation into howthe interactions between substituents in the 1-(peri) and 9-positions (bridgehead) influenced the rotational barriersabout the C–C bonds. Triptycene derivatives have in recentyears shown many more applications as molecular balances,based on Oki’s seminal work.

In 2005, Gung et al. reported the series of triptycene de-rivatives 13–16 (Figure 6), which served as molecular bal-ances for direct measurements of π–π interactions in theparallel-displaced orientation.[27] They determined the ra-

Y. Jiang, C.-F. ChenMICROREVIEWtios of the syn and anti conformers by variable-temperatureNMR spectroscopy and calculated the free energies fromthe syn/anti ratios. Moreover, they further prepared theseries of model compounds 17–21, in which multiple strongdonors and accepters are involved.[28] They found that 17–21 all behaved differently with regard to the aromatic inter-actions in their parallel-stacked configurations. Compounds19–21 showed good correlation between the free energy at-traction and Hammett parameters, but 17 and 18, in whichthe aromatic rings are strongly perturbed by multiple sub-stituents, did not display linear correlation in the Hammettplot. It was proposed that the charge-transfer interactionswere the dominant force.

Figure 6. Structures of compounds 13–25.

Gung et al. have also studied the stacking interactionsbetween aromatic rings and pyridine or pyrimidine rings,with the aid of the series of triptycene-derived scaffolds 22–25 (Figure 6).[29] The 1H NMR spectra of 22–25 showedthat stronger attractive interactions existed in these com-pounds than in those with benzene rings located in the posi-tion of the heterocycles. Consistently with its donor–ac-ceptor character, 22a was found to show the strongest inter-actions. Furthermore, it was also found that the attractiveinteractions in 22–25 are much less sensitive to the substitu-ent effects than in the corresponding carbocycles.[29]

In order to study the interactions between oxygen lonepairs and aromatic rings, Gung et al.[30] also synthesizedcompounds 26 and 27 (Figure 7). They found that attractiveinteractions existed between the oxygen lone pairs and thearomatic rings, even electron-rich aromatic rings. Low-tem-perature 1H NMR spectra gave the free energies of interac-tions and the equilibrium constants. An X-ray crystal struc-ture of 27 showed close proximity between the oxygen andthe center of the aromatic ring. The introduction of α-sub-stituted acetates at the triptycene 1-position in place of the2-methoxyethoxy group afforded the new triptycene deriva-tives 28–29,[31] in which formation of CH–π interactionswas observed. Moreover, the free energies of the interac-


tions and the equilibrium constants were also determinedfrom the ratios of the syn/anti rotamers derived from low-temperature 1H NMR spectra.


2.3. Triptycene-Based Complexes in Catalysis

Starting in the 1990s, Brynda and Geoffroy[32] reportedthe synthesis of a set of stable phosphanes, germanes, ar-sanes, and silanes containing the bulky 9-triptycenyl frame-work as a protective group for stabilization of the unstableP–H and Ge–H bonds, which are also associated with toxic-ity. Gelman et al.[33] have recently developed a new ap-proach to the synthesis of several palladium complexes(compounds 30a–c and 31 in Figure 8, a) bearing 1,8-di-phosphanyl-triptycene components, based on the conceptof “sterically induced bending”. The structures were deter-mined by 31P NMR and X-ray diffraction analysis. As aresult of the introduction of the bulky triptycenyl groups

Figure 8. a) Structures of 30–32. b) Suzuki cross-coupling, c) cyan-ation of aryl halides, and d) selective hydrocyanation of butadienecatalyzed by the triptycenyl complexes.


these palladium complexes were highly stable, which wasimportant for their potential applications in catalysis.

In the light of the above observations, Gelman et al.[34]

devoted their efforts to investigation of the catalytic poten-tial of palladium complexes possessing the new triptycene-based bidentate ligands as catalysts for C–C bond-formingreactions (Figure 8, b). They found that the complexes 30a,30b, and 32a/Pd(AcO)2 could promote Suzuki coupling re-actions between aryl chlorides and arylboronic acids. Bothelectron-deficient and electron-rich aryl chlorides couldbe made to react smoothly with phenylboronic acid toform the desired biphenyls in good to excellent yields. Thetriptycenyl complex 32a/Pd(AcO)2 could also be used as acatalyst in cyanations of aryl bromides (Figure 8, c).[35] Notonly did the complex exhibit high catalytic activities forthese transformation reactions, but the catalytic system isalso perfectly stable to air, and the reactions could be per-formed without rigorous exclusion of oxygen. Moreover,the amines that had served in previous reports as co-ligandsto allow additional stabilization of the palladium catalystswere not needed at all, and the nontoxic cyanide sourceK3Fe(CN)3 could be employed in place of CuCN.

More recently, Vogt[36] reported an improved route forthe synthesis of 32c. Treatment of 32c and Ni(cod)2 af-forded the complex (32c)Ni(cod), which was then appliedto the hydrocyanation of buta-1,3-diene to give pent-3-en-enitrile (3PN, up to 98%) with high selectivity (Figure 8, d).

2.4. Triptycene Derivatives in Materials Chemistry

Swager[37] first introduced the concept “internal free vol-ume” (IFV), which was defined as the free volume sweptout by the aromatic faces of a triptycene. Complementarystructures containing triptycenes produce IFV, with forma-tion of local cavities because of the inability of such struc-tures to pack efficiently. The formation of this IFV provideda unique opportunity to develop new materials.

The IFV can be increased when the triptycene moiety isenlarged by increasing the size of its arene blades. Kinget al.,[38] for example, reported the two new triptycene deriv-atives 33a and 33b (Figure 9). These had triphenylene

Figure 9. Extended triptycene derivatives 33 and 34 with largeIFVs.


blades and were synthesized by biphenylation of hexabro-motriptycene with [Li(THF)4]2·Zr(biphe)3. Jiang andChen[39] also synthesized the 1,10-phenanthroline-based ex-tended triptycene derivatives 34a and 34b. Lee et al.[40] re-cently reported a series of new oxadisilole-fused triptycenesand extended triptycenes, which could function as precur-sors of triptycenes and extended triptycynes.

The attachment of triptycene frameworks to polymerscan also afford polymers with large IFVs. In 2008, Swageret al.[41] reported a novel poly(2,6-triptycene) after homo-polymerization of 2,6-dibromotriptycene and 2,6-diiodo-triptycene in a nickel(0)-mediated Yamamoto-type polycon-densation polymerization. This polymer might be a novelmaterial with good solubility and thermal stability becauseof its high triptycene content.

Triptycenes with these unusual IFVs have attracted muchattention from materials chemists in recent years. In 2001,Swager and his co-workers[37] first found that the IFV be-tween the three aromatic faces of the triptycene derivative35 (Figure 10) allowed for the threading of liquid crystals(LCs) or of a stretched polymer through these spaces, mini-mizing the IFVs. They showed by polarized UV/Vis and IRspectroscopy that, when compound 35 was added to LCsor to polymers, the LCs or polymers could fill the emptyvoids created by 35, while minimally disrupting the align-ments of the LCs or polymers. Inspired by these results,Swager et al. further synthesized the fluorescent blue dyes37–39 and 41 (Figure 10) in which different numbers oftriptycene units were incorporated. When these triptycene-based dyes were dissolved in 4-(trans-4-pentylcyclohexyl)-benzonitrile as a common LC host, the order parameterscalculated by polarized UV/Vis spectroscopy were increasedrelative to those of the dyes 36 or 40, which showed thatthe IFVs of the triptycenes in these dyes enhanced the chro-


Y. Jiang, C.-F. ChenMICROREVIEWmophore alignment in the LC host. Moreover, they alsofound that the switch times of the LC host increased, whichwas due to the incorporation of the triptycene moieties.[42]

The capability to control the orientations and electronicstructures of molecules with extended structures plays amajor role in the construction of materials with optimizedproperties. Swager and his co-workers[43] realized that intro-duction of a triptycene-based polymer to a LC host mightallow the production of a polymer liquid crystal solution inwhich the polymer and the LC host could be mutuallyaligned. They thus first successfully synthesized the fluores-cent poly(phenylene vinylene)s 42 and poly(phenylene eth-ynylene)s 43 (Figure 11), containing triptycene frameworks,through Suzuki and Sonagishira reactions. Because theIFVs of the rigid triptycene groups in these polymers pre-vented the aggregation of the polymers, they were com-pletely soluble in typical organic solvents. Homogeneousalignment of 42 or 43 in 4-(trans-4-hexylcyclohexyl)phenyl-isothiocyanate as a liquid crystal host was achieved by useof surface-rubbed test cells and the order parameters werethen calculated by polarized absorption spectroscopy. Itwas found that the order parameters were high and that thelong axes of the polymers aligned with the directors of thenematic LCs. When a voltage (9 V) was applied across thecell, the polymers showed realignment with the LCs direc-tors, which was further evidence that the polymers in theLC host formed a solution. More importantly, it was foundby absorption spectroscopy that the liquid crystal solutionsabsorbed further towards the red, which showed that theeffective conjugation lengths of the polymers were en-hanced.

Figure 11. Structures of the polymers 42 and 43.

Supramolecular polymers, as combinations of supra-molecular chemistry and polymer science, open anotherdoor for the construction of new materials with enticingproperties. Working on this concept, Swager’s group furtherdesigned and synthesized the poly(phenylene ethynylene)s44 (Figure 12, a), end-capped with hydrogen bonding moie-ties, in high yields.[44] On going from methanol solution togel formation, the NH peaks of the polymers, characterizedby FT-IR, sharpened up remarkably and shifted to lowerwavenumbers, which suggested that strong hydrogen bond-ing (Figure 12, b) was taking place and that an extensivehydrogen bonding network was being formed upon gela-tion. Moreover, polarized absorption of liquid crystal solu-tions of the polymers in parallel aligned test cells showedthat the polymers end-capped with hydrogen-bonding moie-


ties displayed much higher order parameters than their non-functionalized counterparts. Together, these results indi-cated that the hydrogen bonds between the polymer chainsdramatically increased the alignment of the electronic poly-mers 44 in LCs.[44]

Figure 12. a) Structures of compounds 44. b) Mode of dimerizationof the hydrogen-bonding functional group through quadruple hy-drogen bonding.

In 2002, Swager’s group also synthesized the symmetricaltriptycene derivatives 45a–f and the unsymmetricaltriptycene derivatives 46a–d (Figure 13). It was found that45a–f showed monotropic behavior, indicating that the li-quid crystalline phases are all kinetically accessible, but notthermodynamically stable. Increasing the alkyl chainlengths decreased the clearing temperatures, and onlynematic mesophases were displayed in the symmetricaltriptycene liquid crystals. In the case of the unsymmetrical46a–d, highly complex mesophase behavior was observed,with the systems remaining fluid over wide temperatureranges. Moreover, subtle decreases in the clearing tempera-tures were found with respect to the same length chains inthe symmetrical series.[45]

Figure 13. Structures of the triptycene derivatives 45 and 46.


The IFVs of triptycenes also provide opportunities to de-sign low-κ dielectric materials. The polymers 47, 48, and49 (Figure 14), with incorporated triptycene moieties, werereported by Swager in 2003. Dielectric studies of these poly-mers showed that the insertion of ring-fused triptycene moi-eties resulted in lower dielectric constants, with 49 dis-playing even lower dielectric constants than the materials47 and 48. It was found by differential scanning calorimetry(DSC) and thermogravimetric analysis (TGA) that all thesepolymers had good thermal stability, which could be dueboth/either to decreased interactions between the back-bones of neighboring chains and/or to rigidified solid-statestructures. More interestingly, nearly all the polymers

Figure 14. Structures of the polymers 47–50.

Figure 15. a) Synthesis of the polymer 52. b) Structures of 53 and 54.


showed water absorptions of 1.1 wt.-% or less, which wasimportant for maintenance of low dielectric constants afterapplication of the polymers.[46] In addition, Swager and hisco-workers[47] further reported the triptycene polyester 50(Figure 14), with improved mechanical properties. Thepolyester 50 displayed a higher glass transition temperaturethan its non-triptycene counterpart: from 25 to 55 °C. Thepolyester 50 also showed a modulus value and strength sig-nificantly higher than those of the non-triptycene polyesterat room temperature. Moreover, it was found that eventhough the triptycene polyester films lost a great deal ofductility during the tension heat-treatment process, they stillhad ductility comparable to that of the as-cast room tem-perature non-triptycene polyester films. It was suggestedthat the triptycene polyester adopted favorable conforma-tions to minimize the IFVs of the triptycene units and thatthe resulting assembly was responsible for the improvementof the mechanical properties of the polymer.

Because of the inability of triptycene-based polymericstructures to pack efficiently, in 2007 McKeown and Buddstarted to apply triptycene in the construction of polymersof intrinsic microporosity (PIMs). Treatment of 51 (Fig-ure 15, a) with the commercially available 2,3,5,6-tetrafluo-roterephthalonitrile afforded a triptycene network based on9,10-diethyl-2,3,6,7,12,13-hexahydroxytriptycene (52) inquantitative yield. Nitrogen adsorption measurements at77 K indicated that the Brunauer–Emmett–Teller (BET)surface area of 52 was 1064 m2 g–1 and that the H2 uptakeat 1 bar/77 K was 1.65% by mass, both of which werehigher than for any previously prepared PIM.[48] In 2010 itwas further found that the gas adsorption behavior couldbe tuned by variation of the alkyl chains at the 9- and 10-positions. Shorter or branched alkyl chains provided thematerials of greatest microporosity, whereas longer alkylchains appeared to block the microporosity created by therigid organic framework.[49]

Y. Jiang, C.-F. ChenMICROREVIEW


MacLachlan[50] recently reported a series of triptycene-based metal salphens (e.g., 53 and 54, Figure 15, b) withintrinsic molecular porosity. It was found that increasingthe numbers of salphen moieties or triptycene frameworkswas an effective means to afford porosity, and that smallerIFVs resulted in lower surface areas. We therefore deducedthat polymers with multiple triptycene-based metal sal-phens might afford higher porosities in the future.

Triptycene can also act as a rigid platform for the investi-gation of photophysical properties. In 2002, Belser’s groupsynthesized the metal complexes 55–57 (Figure 16), basedon triptycene bridging ligands, and their photophysicalproperties were further investigated. The triptycene bridg-ing ligands were prepared from a rigid triptycene spacerconnected to two bipyridine ligands by means of a Horner–Emmons-type reaction. Metal-to-ligand charge-transferbands are observed for all complexes in the UV region ofthe spectra. It was found that the donor effects of methoxysubstituents on the triptycene resulted in higher emissionthan in the case of the unsubstituted triptycene ligand,whereas the presence of a quinone moiety in the triptycenestrongly quenched the emission. Effective quenching of theluminescence of the ruthenium-centered moiety was ob-served in 57.[51]

Cheng et al.[52] also reported the triptycene derivatives58–60 (Figure 16), which were found to have high glasstransition temperatures (Tg). Electrophosphorescent re-search showed that both 58 and 60 had hole-transportingcharacteristics, whereas 59 was a host material with elec-tron-transporting characteristics and 58 could act as a gene-ral material for blue, green, and red electrophosphorescencedevices, providing high brightness and good current andpower efficiencies.


Covalently linked donor-bridge-acceptor (D-B-A) mole-cules, in which the bridges constitute σ-bond buildingblocks, play great roles in elucidation of important aspectsof intramolecular charge and energy transfer. In 2008,Ratner and Wasielewski synthesized the family of com-pounds 61–64 (Figure 17), in which a 4-(pyrrolidin-1-yl)-phenyl electron donor and a 10-cyanoanthracen-9-yl elec-tron acceptor are attached through alkyne linkages to thebridgehead carbon atoms of bicyclo[2.2.2]octane and all



three benzoannulated bicyclo[2.2.2]octanes. It was observedby transient absorption spectroscopy in toluene that in-creasing the number of bridging π systems produced littledeviation in photoinduced charge transfer rates, suggestingdominant transport through the σ system. More surpris-ingly, it was also found that the significant changes in hy-bridization undergone by the σ system from 61 to 64 alsoseemed to have little effect on the charge transfer rates.[53]

More recently, Lai and his co-workers synthesized com-pounds 65–67. Their absorption spectra showed significantdifferences between 65 and 66, whereas the absorption andemission spectra of 65 were essentially similar to those of67. The above results indicated that the triptycene in 65offered zero homoconjugation effect.[54]

2.5. Triptycene Derivatives in Crystal Engineering

In 2002, Yang and his co-workers[55] synthesized theseries of triptycene-derived secondary dicarboxamides 68–71 (Figure 18). They further obtained single crystals of thecompounds by slow evaporation of the solvent at roomtemperature, which showed that the compounds could allself-assemble into folded and compact structures. By analy-sis of the crystal structures, it was found that intramolecularamide–amide hydrogen bonding, edge-to-face arene–areneinteractions, and the sulfonyl ester turn units were all neces-sary for the folding in the solid state. Furthermore, theyalso found that the presence of hydrogen-bond-active (e.g.,MeOH and DMSO) and/or aromatic-containing solvents(e.g., toluene) during the crystallization did not affect the

Figure 18. Structures of the compounds 68–76.


intramolecular hydrogen-bonding and arene–arene interac-tions, suggesting that the compact folding structures ap-peared to be favorable. 1H NMR spectroscopy gave furtherconfirmation of these results.[55]

Metal-organic frameworks are also promising candidatesfor the development of new porous and electronic materials.Inspired by the unique three-dimensional structure oftriptycene, MacLachlan’s group recently synthesized thetriptycenylquinoxaline ligands 72 and 73 (Figure 18) bycondensation of 2,3-dihydroxy-1,4-dioxane and the corre-sponding polyaminotriptycenes. Treatment of the ligandswith CuI in acetonitrile (MeCN) afforded Cu·72·MeCNand Cu·73·2MeCN as red crystalline solids, free of inter-penetration. The free volume of the shape-persistent ligand72 generated a channel structure with a void space of 44 %in the lattice, whereas the topology of Cu·73 appeared tohave an undulating layered motif with a void space of 39%in the lattice. Thermogravimetric analysis of Cu·72 andCu·73 showed that they were stable to 350 and 380 °C,respectively. Although the channels were inaccessible to N2,the lattices could be expanded by organic vapors and ap-peared to be very hydrophobic.[56] In 2007 the same groupfurther synthesized the triptycenyl quinoxaline ligands 74–76 (Figure 18). They obtained single crystals of the ligands74 and 75 that suggested that the ligands favored packingin layered structures with intermolecular π stacking. Theligand 74 was stable to 341 °C, whereas 75 and 76 showedmass loss beginning at 169 and 163 °C, respectively. Treat-ment of 74 with CuI in benzonitrile gave a dimer with twomolecules of 74 bridged by Cu2I2, which also demonstratedthat these new ligands all showed the potential to coordi-nate to metals.[57]

2.6. Triptycene-Derived Hosts in Host–Guest Chemistry

Because triptycene derivatives are a class of interestingcompounds, not only with unique three-dimensional rigidstructures, but also richly endowed with reactive positions,we deduced they should be utilizable as building blocks forthe synthesis of new hosts, potentially resulting in the devel-opment of new supramolecular systems with specific struc-tures and properties. We have therefore recently synthesizeda series of novel triptycene-derived hosts and investigatedtheir potential applications in molecular recognition andmolecular assemblies.

2.6.1. Synthesis and Structures of Triptycene-Derived Hosts

Inspired by the fact that crown ethers can not only formcomplexes with metal ions, but can also show specific com-plexation capabilities with secondary ammonium ions orparaquats, we designed and synthesized several differentkinds of novel triptycene-derived crown ethers – includingthe triptycene-derived macrotricyclic polyethers 77,[58]

78,[59] and 79,[60] the triptycene-derived tris(crown ether)s80[61] and 81,[62] the triptycene-derived molecular tweezers82,[63] 83,[64] and 84,[65] and the triptycene-derived bis-macrotricyclic host 85[66] (Figure 19) – by treatment of thecorresponding polyhydroxytriptycenes with ditosylate com-

Y. Jiang, C.-F. ChenMICROREVIEWpounds under high-dilution conditions. Their structureswere all confirmed by 1H NMR and 13C NMR spec-troscopy, mass spectrometry, and elemental analysis. More-over, we obtained an X-ray structure of 77, which showedthat this macrocyclic host contains two lateral crown cavi-ties and one rich-electron central cavity with a size of ca.10.2�13.9 Å2 and so can exhibit specific complexationabilities with different guests, especially organic guests.

Figure 19. Structures of triptycene-derived crown ether hosts.

Calixarenes,[67] a class of well defined phenol-derived cy-clic oligomers bridged by methylene groups, have in the pasttwo decades come to be called “the third generation of hostmolecules” after crown ethers and cyclodextrins. However,it is noteworthy that calix[4]arene has too small a cavity tocomplex with ordinary organic molecules except for somesolvents, whereas calix[6]arene and larger calixarenes haveso many conformations that their cavities are also actuallydifficult to utilize. We deduced that replacement of one ormore phenol groups with suitable triptycene moieties in thecalix[4]arene might result in the development of a class ofnovel calixarenes with specific structures and properties. Wefirst tried to synthesize the triptycene-derived calix[6]arenes88 and 89 (Figure 20),[68] and found that these dia-stereomeric compounds could be conveniently synthesizedin one-pot reactions between the triptycene derivative 86


and either p-tert-butylphenol (87a) or p-phenylphenol (87b)in o-dichlorobenzene in the presence of 4-methylben-zenesulfonic acid. The macrocycles 88 and 89 could alsobe obtained by a two-step fragment-coupling approach. Ontreatment of 88 and 89 with BBr3 in dry dichloromethane,we further obtained the demethylated compounds 91 and92, respectively, in high yields (Figure 21). Moreover, wealso found that treatment of 88a and 91a and of 89a and92a with AlCl3 in toluene at room temperature resulted inthe debutylated products 93 and 94, respectively.


Figure 21. Structures of the demethylated calix[6]arenes and debu-tylated calix[6]arenes.

In the 1H NMR spectra of 88 and 89, a pair of doubletsignals with a Δδ value of about 0.90 ppm were observedfor the methylene protons in 88, whereas 89 showed a pairof doublets with a Δδ value of about 0.12 ppm, which sug-gested that 88 has a syn orientation of the two triptycenemoieties and that 89 is an anti orientation isomer. More-over, the 13C NMR spectra of 88 and 89 each showed onlyone signal for the methylene carbons, and no obviouschanges in the methylene proton signals were observed withincreasing temperature in variable-temperature 1H NMR


spectra. These observations are all consistent with highlysymmetrical structures and fixed conformations in solution.For the demethylated products 91a, 91b, 92a, and 92b andfor the debutylated macrocycles 93 and 94 the 1H NMRand 13C NMR spectral features for the bridged methyleneprotons are quite similar to those in the precursors, indicat-ing that they also adopt fixed conformations in solution.

X-ray crystal structures of the macrocyclic compounds88a, 89a, 89b, 91a, and 92 were determined and showedthat the macrocycles all have highly symmetrical structuresand specifically fixed conformations in the solid state, con-sistently with the results obtained in solution. In the case of88a the macrocyclic molecules were able to form a tubularassembly with the aromatic rings as the wall by the bridgesof adjacent molecules. Moreover, the macrocycle 88a couldfurther self-assemble into layers, which then alternatelystacked to form a 3D macroporous structure.

With compounds 90a and 90b to hand, we[69] also syn-thesized the series of triptycene-derived calix[5]arenes 95a–c (Figure 22) through heat-induced fragment coupling reac-tions between 90a and 90b and the p-substituted phenols87a and 87b in xylene at reflux for two days. It was alsofound that treatment of compounds 95 with BBr3 in dryCH2Cl2 gave the demethylated compounds 96a–c in highyields. Treatment of 96a and 96b with AlCl3 in toluene atroom temperature resulted in the debutylated products 97aand 97b in 82 % and 75% yields, respectively. Moreover, wefound that the macrocycles 95a and 95b could not only bedebutylated but could also be demethylated to yield 97a and97b, respectively, in high yields under the same conditions.

Figure 22. Structures of triptycene-derived calix[5]arene derivatives.

The 1H NMR and 13C NMR spectra of 95a–c showedthat these triptycene-derived calix[5]arenes all have Cs-sym-metric structures and adopt fixed cone conformations insolution. Similarly, the 1H NMR spectra of the demethyl-


ated compounds 96 and 97 also indicated that they have thesame fixed cone conformations as their precursors. Vari-able-temperature 1H NMR experiments with 95a, 96a, and97a in [D6]DMSO all showed no obvious changes in themethylene proton signals with increasing temperature evenup to 373 K, which not only is in accordance with fixedconformations, but also indicates that the conformationalinversion barriers of these compounds are very high. It canthus be concluded that the introduction of the triptycenemoiety, with its rigid 3D structure, is the most significantfactor determining the conformational mobilities of thetriptycene-derived calix[5]arenes, in comparison with the in-tramolecular hydrogen bonds and the bulky tert-butylgroups in the para positions of the phenol systems. Further-more, the crystal structures of 95a, 96a, and 95c showedthat they all adopted fixed cone conformations, also con-sistently with the results in solution.

Heterocalixarenes,[67,70] in which the carbon linkages be-tween the aromatic units are replaced by heteroatoms, haveattracted much interest in recent years owing to their readyavailability, tunable cavities, and potential applications insupramolecular chemistry. Oxacalixarenes and azacalixar-enes, two important classes of heterocalixarenes, haveshown rapid growth in calixarene chemistry as a result ofincreasing efforts devoted to their synthesis and conforma-tional investigation. Most researchers, however, have onlyfocused on the synthesis of the macrocycles, and their appli-cations in supramolecular chemistry are still few in number,due to the small cavities or nonfixed conformations of themacrocycles.

We[71] have recently synthesized two novel oxacalixar-enes – 98a and 98b (Figure 23) – through SNAr reactionsbetween 2,7-dihydroxytriptycene and 2,3,5,6-tetrachlo-ropyridine in the presence of cesium carbonate. Structuralstudies showed that the macrocyclic compounds 98a and98b are a diastereomeric pair, in which 98a is a synI isomerwith boat conformation and 98b is a anti isomer in chairconformation. Under the same conditions, a series oftriptycene-derived oxacalixarenes – 99a and 99b, 100a and100b,[71] and 101a and 101b,[72] each as a diastereomericpair – were also conveniently synthesized.

Figure 23. Structures of the triptycene-derived oxacalixarenes 98–101.

The crystal structures of the macrocycles 98a, 98b, and100a showed that they could all assemble into organic tubu-lar structures and further porous architectures in the solidstate, and that multiple chlorine bonding interactions in-

Y. Jiang, C.-F. ChenMICROREVIEWcluding C–Cl···Cl, C–Cl···O, and C–Cl···π interactionsplayed an important role in formation of the tubular as-semblies.[71] In the case of 101a it was observed that therewere two molecules with different orientations in the crystalcell, one molecule adopting a 1,3-alternate conformationwith a highly symmetrical structure and a cavity of13.29� 10.99 Å2 (wider rim) and 8.56� 8.84 Å2 (narrowerrim), and another molecule adopting a distorted 1,3-alter-nate conformation with a cavity of 12.75 �10.81 Å2 (widerrim) and 8.51 �9.45 Å2 (narrower rim) surrounded by twotriptycene subunits and two 1,8-naphthyridine subunits.Moreover, the nitrogen atoms in the 1,8-naphthyridine com-ponents are all positioned inside the cavity.[72]

Treatment of 2,7-diaminotriptycene with cyanuric chlo-ride, 1,5-difluoro-2,4-dinitrobenzene, or 3,5-dicyano-2,6-dichloropyridine (in place of m-phenylenediamine as thenucleophilic reagent for direct nucleophilic aromatic substi-tution reactions), in dry THF in the presence of DIPEA atroom temperature, gave the five azacalixarene pairs 102aand 102b, 103a and 103b, 104a and 104b, 105a and 105b,and 106a and 106b (Figure 24). As well as by one-pot syn-thesis, these macrocycles could also be afforded in totalhigher yields by a two-step fragment-coupling method.Structural studies showed that the syn isomers 102a–105aall have high symmetry and adopt boat conformations,whereas the anti isomers 102b, 103b, and 104b exhibit fixedcurved boat conformations at room temperature, but theanti isomer 105b adopts a normal chair conformation insolution.[73]

Figure 24. Structures of the triptycene-derived azacalixarenes 102–106.

Fortunately, we also obtained single crystals of 102a,102b, 103a, and 104a for X-ray analysis.[73] The resultsshowed that as long as the triptycene subunit is present,neither alternation of another aromatic subunit nor


derivatization on the triptycene moiety can change the boatconformations of these macrocycles, which suggests that therigid triptycene moiety plays a crucial role in the fixed con-formations of the macrocycles.

Interestingly, we also found that azacalixarene 106acould self-assemble into a novel aromatic single-walled or-ganic nanotube in the solid state.[74] As shown in part aof Figure 25, the unit cell contains two molecules of 106a(denoted as 1A and 1B), both of which adopt boat-like con-formations with similar cavities. Symmetrical expansionaround the pyridine rings of the macrocycle shows that twomolecules of 1A are situated in opposite positions and gen-erate a rectangular geometry with the four cyano groupsand four NH sites pointing in the same direction (Fig-ure 25, b and c). Two molecules of 1B are also located faceto face and generate a rectangular geometry rotated withrespect to the former one by 90° owing to the H-bonds(average dCN···HN = 2.23 Å, θCN···HN = 153.291) formed bycyano groups and NH sites between 1A and 1B. An almostsquare-shaped architecture with a pore size of15.2�16.4 Å2 is thus subsequently formed. Because of theadjacent position of a NH group and a cyano group onboth sides of a pyridine ring of 106a, one pyridine ring of1A could interact with two molecules of 1B through twopairs of N–H···N�C hydrogen bonds. Two pyridine rings inone molecule of 106a can contribute four pairs of hydrogenbonds in total and participate in two zigzag H-bond chainswith other molecules for self-assembly. The tetramer de-scribed above can thus also be expanded by the other NHgroups and cyano groups that have not interacted withother molecules. With suitable geometrical constraints ofthe building blocks and the relatively strong noncovalentinteractions, a square-based organic nanotube is formedspontaneously. We believe that the specific structural fea-

Figure 25. a) Single-crystal structure showing the asymmetric unitof 106a. b) Molecular structure representation of a 1D H-bondchain. c) Space-filling representations of a nanotube based on 106a.


tures of such organic nanotubes might find potential appli-cations in fields such as recognition and transportation ofpolycyclic aromatic hydrocarbons.

On treatment of pyridine-2,6-dicarbonyl dichloride with2,7-diaminotriptycene in dry THF in the presence of Et3Nwe also obtained the diastereomeric pair 107 and 108 in26% and 20 % yields, respectively (Figure 26).[75] Their 1HNMR spectra are very different from one another, but eachshow only one signal for the NH protons and two singlesignals for the bridgehead protons of the triptycene moie-ties, suggesting that both macrocycles have highly symmet-rical structures. The crystal structures showed that com-pound 107, as a syn isomer, adopts a cone conformation,and that compound 108 is an anti isomer. As a result of theintramolecular hydrogen bonds between the amide protonsand the adjacent pyridyl nitrogen, the conic cavity sur-rounded by four residues in 107 showed a wider rim of11.97� 16.04 Å2 and a narrower rim of 7.34 �10.95 Å2.Moreover, it was found that the four carbonyl groups at-tached to the wider rim and the four amide NH protonsdefined the narrower rim. Similarly to 107, the intramolecu-lar hydrogen bonds in 108 also played an important role inthe formation of its specific conformation. The pyridinerings in 108 were parallel to each other and inclined by6.59° to the plane formed by four carbonyl oxygen atoms,whereas the two triptycene moieties were anti-connected bythe pyridyl amide subunits (distances between the parallelplanes of the face-to-face benzene rings in triptycene of 6.42and 6.66 Å, respectively), which resulted in an almost pris-matic cavity with a chair conformation.

Figure 26. Structures of the macrocycles 107 and 108.

In order to design and synthesize novel triptycene-de-rived hosts, the new 2,6,14- and 2,7,14-trinitro-substitutedtriptycene derivatives 109a and 109b (Figure 27) were ef-ficiently synthesized in 64 % and 21% yields, respectively,by nitration of triptycene with concentrated HNO3 at 80 °Covernight. Easy reduction with Raney nickel in the presenceof hydrazine hydrate then afforded the corresponding tri-aminotriptycenes 110a and 110b in almost quantitative



yields. From these triaminotriptycenes the series of 2,6,14-and 2,7,14-trisubstituted triptycene derivatives 111–113were prepared.[76]

We[77] have recently conveniently synthesized the novelnanosized molecular cage 114 (Figure 28) in a three-stepprocess starting from 2,7,14-triiodotriptycene. From thecrystal structure of 114 we found that the adjacent 1D sup-ramolecular structures are connected with one anotherthrough three pairs of C–H···π interactions, resulting in a2D-layer structure and a further microporous structure.Interestingly, the mesitylene molecules were found to be lo-cated in the channels.

Figure 28. Structure of the molecular cage 114 (left), and view ofthe microporous structure self-assemblied from 114 (right).

2.6.2. Applications of Triptycene-Derived Hosts

As a result of the introduction of the triptycene moiety,with its unique three-dimensional rigid structure, thetriptycene-derived hosts described above not only have spe-cific structures but also contain large electron-rich cavities,which might open a new door for studies of molecular re-cognition and assembly. Consequently, we have found thatthese novel hosts exhibited powerful complexation abilitiestoward different kinds of organic guests, especially electron-deficient guests, resulting in a series of new supramolecularsystems.

Complexation with Secondary Ammonium Salts

We found that the triptycene-derived macrotricyclic host77, containing two dibenzo-24-crown-8 (DB24C8) subunits,formed a [3]pseudorotaxane-type complex 77·(115a)2 withtwo equivalents of dibenzylammonium salts 115a (Fig-ure 29). Moreover, it was found that the host–guest ex-change is slow on the 1H NMR timescale at room tempera-ture, so the association constants K1 and K2 for the firstand second binding events were calculated to be 1.2� 104

and 2.4�103 m–1, respectively. A single-crystal structure of77·(115a)2 showed that two guest 115a units were threadedsymmetrically through the center of the DB24C8 cavities ofhost 77, resulting in a “gull-wing” structure. The formationof the complex 77·(115a)2 encouraged us further to designand to construct a series of dendritic [3]pseudorotaxane-type complexes based on 77 and 115c–e.[78]

Similarly, we found that the bis-macrotricyclic host 85could form the 1:4 complex 85·(115a)4 with the salt 115a.[66]

A single-crystal structure of 85·(115a)4 showed that 85 con-tains two symmetrical macrotricyclic moieties and that twodibenzylammonium ions are threaded through the centers


Figure 29. Structures of the dibenzylammonium salts 115a–f.

of the DB24C8 cavities of each macrotricycle in 85, re-sulting in the 1:4 complex. Encouraged by this result, wealso synthesized the bis-dibenzylammonium salt 115f, andfound that host 85 and 115f could form the 1:2 handcuff-like complex 85·(115f)2, which was characterized by 1HNMR, 2D NMR, diffusion-ordered NMR, and ESI-MS.

The complexation abilities of the macrocycles 77 and 78with the (9-anthryl)benzylammonium salt 115b were thentested. Compound 77 also formed the 1:2 complex77·(115b)2, similar to the complex 77·(115a)2, and the asso-ciation constants K1 and K2 for the first and second bindingevents were calculated to be 8.0� 103 and 1.6�103 m–1,respectively. 1H NMR experiments and a single-crystalstructure showed that in the complex the two 9-anthracylgroups were selectively positioned outside the cavity of thehost 77 (Figure 30).[79] In the case of the host 78, althoughit also formed a 1:2 complex – 78·(115b)2 – with 115b, 1HNMR experiments and a single-crystal structure showedthat the two anthracyl groups were selectively positionedinside the crown cavities, completely unlike in the case ofthe complex 77·(115b)2. The complexation and disassocia-tion of the complex 78·(115b)2 could be chemically con-trolled by addition of base and acid. Moreover, Ba2+ ionwas able to induce the fluorescence enhancement of com-plex 78·(115b)2 considerably, which might thus be utilizableas a selective supramolecular fluorescence probe for Ba2+

ion.[59]

On the basis of the above results, we further tried to syn-thesize interlocked molecules that might act as molecularmachines and intelligent materials. We thus synthesized the


Figure 30. Single-crystal structures of a) 77·(115b)2, andb) 78·(115b)2.

dibenzylammonium salt 116 (Scheme 1), and found that themacrotricyclic host 77, as expected, formed the 1:2 complex77·(116)2 with 116. Treatment of 77·(116)2 with 3,5-dimeth-ylbenzoic anhydride in the presence of a catalytic amountof tri(n-butyl)phosphane afforded the novel [3]rotaxane 117in 37% yield. Thanks to the two terminal propargyl groupsin the [3]rotaxane, we were further able to synthesize thelinear main-chain poly[3]rotaxane 119 (Scheme 1) by treat-ment of the [3]rotaxane 117 with the diazide 118 (1 equiv.)in DMF in the presence of a stoichiometric amount of CuI.Size exclusion chromatography analysis of the polymershowed the average molecular weight (Mm) to be ca.43 kDa, with a polydispersity index of 1.42. According tothe Mm value, each polymer chain is composed of ca. 14repeating units.[80]

Scheme 1. Synthesis of the linear main-chain poly[3]rotaxane 119.

By the same strategy as above, we further synthesized the[3]rotaxane 120 (Figure 31) by click chemistry and subse-quent regioselective methylation of the triazole group,which bears a dibenzylammonium ion and a N-methyltri-azolium station with different affinities for the DB24C8subunits of host 77. We found that the shuttle process be-tween the macrocyclic host and the two different stationscould be efficiently chemically controlled by the use of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and TFA.[81]


Figure 31. Structure of the [3]rotaxane 120.

Complexation between the host 82 and dibenzylammo-nium salts was also studied. As expected, the host 82 wasable to bind two 115a guests to form the bis[2]pseudorotax-ane-type complex 82·(115a)2 (Figure 32). The complexation

Figure 32. Structures of the complexes 82·(115a)2 and 121–123.

Figure 33. Structures of 124–128.


between 115a and the host 82 in an acetonitrile solutionoccurred in a slow exchange process with the associationconstants K1 = 7.2(�1.8)�102 m–1 and K2 = 77 �22 m–1.[63]

1H NMR spectroscopy and ESI mass spectrometry re-vealed that the triptycene-derived homotritopic host 80could bind three dibenzylammonium salts containing twoterminal vinyl groups to form the 1:3 tri[2]pseudorotaxane121 (Figure 32), similarly to the case of 82·(115a)2. With121 to hand, olefin metathesis was performed. When a solu-tion of 121 in dichloromethane was treated with the second-generation Grubbs’ catalyst, the reaction was found to pro-ceed smoothly, exclusively giving the triple metathesis prod-uct 122 in 82% yield. This was then treated with Adam’scatalyst to afford the [2](3)catenane (also known as a [4]-pseudocatenane) hydrogenation product 123 quantita-tively.[61]

By a strategy similar to that described above we also ob-tained the chiral [2](3)catenane 124 (Figure 33) by startingfrom the chiral triptycene-derived homotritopic host 81.[62]

Although it was difficult to neutralize the ammoniumgroups in 124 with triethylamine, tributylamine, or diiso-propylethylamine, we found that they could be deproton-ated by DBU. We consequently further synthesized theseries of [2](3)catenane derivatives 125–128. The CD spectraof the host 81 and the complexes 126–128 showed that theCotton effect of the (R)-1,1�-binaphthyl chromophore at241 nm was greatly reduced relative to the host, whereas anew positive Cotton effect at 248 nm was found in these[4]pseudocatenanes. This could be attributed to chiralitytransfer from the binaphthyl units to the macrocycles resid-ing in the cavities of the host 81.

More recently, we also found that the macrocycle 80 andthe tri-dibenzylammonium salt 129 could form the 1:1 com-plex 80·129, which then afforded the [2](3)catenane 130(Figure 34) through an olefin metathesis reaction. More-over, we further found that the trisdialkylammonium mono-cyclic ions 131 could form a magic-ring [2](3)catenanethrough two olefin metathesis steps, suggesting that thecombination of multivalency and olefin metathesis providesa powerful tool for the synthesis of complex interlockedmolecules.[82]


Figure 34. Structures of 129–131.

Complexation with Paraquat Derivatives and DiquaternarySalts

We also investigated complexation of the triptycene-de-rived macrotricyclic host 77 with differently functionalizedparaquat derivatives.[58] We first found that the host 77

Figure 36. a) Processes of ion-controlled binding and release of the guest in the complex 77·133. b) Acid/base-controlled selective complex-ation process between the ternary complex containing benzidine and a binary complex.


formed a 1:1 complex with paraquat (132a, Figure 35), andthe association constant for the complex 77·132a was deter-mined to be 4.0�105 m–1. A single-crystal structure of thecomplex showed paraquat (132a) included in the center ofthe host and the two N-methyl groups in the guest posi-tioned in the two DB24C8 cavities, resulting in a pseudorot-axane-like structure. Similarly to the formation of the com-plex 77·132a, we found that the paraquat derivatives 132b–e and 132h–m (Figure 35) were all included in the cavity ofhost 77 to form 1:1 complexes. In the cases of the paraquatderivatives 132f and 132g, containing two β-hydroxyethyland two γ-hydroxypropyl groups, respectively, it was inter-estingly found that the guests formed 2:1 complexes withthe host 77, with two guests simultaneously threading thecentral cavity of the host.

Figure 35. Structures of the paraquat derivatives 132a–m and thediquaternary salts 133 and 134.


It was also found that 77 could bind the diquaternarysalts 133 and 134 to form the 1:1 complexes 77·133 and77·134, respectively, both in solution and in the solidstate.[83] Moreover, when KPF6 was added in excess to thesolutions of 77·133 or 77·134, the proton signals of thecomplexes totally disappeared, indicating the dissociationof the complexes. When 18-crown-6 was added to the abovesystems, the proton signals of the complexes recovered, sug-gesting that the complexes had been formed again. A newion-induced molecular switch had thus been achieved (Fig-ure 36, a).

A single-crystal structure analysis of 77·133 revealed thatthe diquat molecule could thread from the central cavity of77 and occupy half of its cavity, which suggested that therewas free volume available for the inclusion of another guestwithin the cavity of the host 77. Consequently, we foundthat the host 77 could simultaneously bind diquat and elec-tron-rich aromatics (e.g., biphenyl-4,4�-diol) to form stableternary complexes both in solution and in the solid state.Important roles in the formation of these stable complexeswere played not only by charge-transfer interactions be-tween the electron-rich host and the electron-deficientguests, but also by the face-to-face π-stacking interactionsbetween the host and the guests. Moreover, it was alsofound that a selective complexation process between a ter-nary complex containing benzidine and a binary complexcan be effectively controlled through the addition of acidand base (Figure 36, b).[84]

More recently we also tried to synthesize self-assembledinterwoven cages based on the triptycene-derived bis-macrotricyclic host 85 and the branched paraquat deriva-tives.[85] Similarly to the formation of the complex 77·132i,the bis-macrotricyclic host 85 was able, as expected, to bindthe paraquat derivative 132i to form the 1:2 complex85·(132i)2. The average association constant was calculatedto be 9.0� 103 m–1. Furthermore, we also found that thehost 85 could assemble with the branched paraquat deriva-tives 135 and 136 (Figure 37) to form the novel supramolec-ular cages (85)3·(135)2 and (85)4·(136)2, respectively, as veri-fied by 1H NMR and 2D NMR spectroscopy and ESI-MS.Moreover, the hydrodynamic radii (or Stokes radii) of85·(132i)2, 135, 136, (85)3·(135)2, and (85)4·(136)2 were de-termined to be 21.26, 21.61, 24.24, 32.70 and 32.70 Å,respectively, consistently with the calculated results.[85]

As a class of novel molecular tweezers, compounds 82,83 and 84 were also applied in complexation with paraquatderivatives. It was found that the host 82 could form 1:1stable complexes with 132a or 132c both in solution and inthe solid state. A single-crystal structure analysis of 82·132a(Figure 38) showed the paraquat included in the center ofthe host 82 with the two N-methyl groups positioned inthe two DB24C8 cavities, resulting in a pseudorotaxane-likestructure. Moreover, we found that the complexation pro-cesses between 82 and the guests 115a and 132a could bechemically controlled by use of acid and base. We foundthat the molecular tweezer 83, thanks to its electron-richcavity, could form 1:1 stable complexes with paraquat deriv-atives with different functional groups in solution.[63] Elec-


Figure 37. Graphic representation of the syntheses of the self-as-sembled interwoven cages.

trochemical experiments showed that both the cathodic andthe anodic peaks corresponding to the first and second one-electron reduction process of the bipyridinium core moved

Figure 38. a) Single-crystal structure analyses of the complexesbased on tweezer-like hosts. b) Structure of CBPQT (137).


Scheme 2. Syntheses of the oxocalixarene-based [2]rotaxanes 138a and 138b.

to less negative values upon the addition of 83, suggestingthat the formation of the complexes could the result ofcharge-transfer interactions and that the complexes mightdissociate upon two one-electron reductions of the bipyridi-nium salts. A single-crystal structure analysis of 83·132a re-vealed two tweezer-like molecules with different orienta-tions, with each molecule tweezering a bipyridinium guestby different complexation modes.[64] We found that the host84, differently from 83, could bind either the guest 132a orcyclobis(paraquat-p-phenylene) (CBPQT, 137, Figure 38, b)to form 1:1 complexes in different modes. Single-crystalstructure analyses showed that in the complex 84·132a thetwo triptycene moieties in 84 were positioned on differentsides of the crown ether, whereas the paraquat ring was lo-cated approximately at a crystallographic center of sym-metry to form a pseudo-sandwiched structure with the twotriptycene moieties. For the complex 84·137 the two trip-tycene moieties in 84 were positioned on the same side ofthe crown ether to form a tweezer-like cavity and 84 and137 formed a pseudo-ternary complex in which the two mo-lecules acted not only as hosts but also as guests. Electro-chemical experiments showed that both complexes wereformed as a result of charge-transfer interactions, and didnot dissociate upon the first one-electron reduction processof the bipyridinium ring.[65]

As well as the triptycene-derived crown ether hosts, wealso found that the tetraoxocalix[2]triptycene[2]naphthyrid-ine compounds 101a and 101b also showed strong complex-ation capabilities with paraquat derivatives (Scheme 2).[86]

1H NMR and ESI-MS showed that the hosts 101a and 101bwere able to form 1:1 complexes with paraquat derivativescontaining different functional groups in solution. A single-crystal structure analysis of the complex 101a·132a showedthe linear axle guest 132a threaded through the cavity ofthe macrocycle 101a and the bipyridinium unit distortedwith a dihedral angle of 27.4° between the two pyridiniumrings, possibly the result of the nonsymmetrical environ-ment provided by the macrocycle.

Treatment of 101a·132g or 101b·132g with 3,5-di-tert-but-ylbenzoic anhydride in the presence of a catalytic amountof tri(n-butyl)phosphane, based on the formation of


[2]pseudorotaxanes between 101 and 132, afforded the[2]rotaxanes 138a and 138b, respectively (Scheme 2). A sin-gle-crystal structure analysis of 138a showed that the axlewas bent and that one stopper group was positioned closeto the 1,8-naphthyridine unit of the wheel, resulting in ascorpion-like assembly. Moreover, it was found that thedethreading/rethreading processes of the resulting [2]pseu-dorotaxanes could be easily controlled through acid/basestimuli or Hg2+ association/dissociation.

Complexation with Neutral Organic Guests

Recently we also found that the macrotricyclic hosts 77and 79 exhibit complexation abilities towards neutral guestssuch as anthraquinones and pyromellitic diimides in thepresence of metal ions. In the case of the host 77, it wasable to bind the tetra-azide terminal anthraquinone deriva-tive 139 (Scheme 3) in the presence of K+ ions to form a1:1 complex with an association constant of9.1(�0.8) �102 m–1.[87] A single-crystal structure analysis ofthe complex 77·139·(KPF6)2 showed the anthraquinonering threaded through the host cavity resulting in a cascadecomplex structure. In view of this result, the three novelpotassium-ion-templated [2]rotaxanes 140–142 were conve-niently synthesized in high yields by the “threading fol-lowed by stoppering” approach (Scheme 3). Because thepotassium ions act not only as templates during the stop-pering reactions but also as nonslipping chocks to shrinkthe inner diameters of the cavities, we further found thatthe rotaxanes 140 and 141 could be destroyed by removalof the potassium ions with 18-crown-6, but under the sameconditions the dumbbell and ring components of the rotax-ane 142 remained interlocked.

We found that the macrocycle 79, similarly to 77, couldalso form cascade complexes both with anthraquinone(143, Figure 39) in the presence of K+ ions and with thepyromellitic diimide 144 in the presence of Li+ ions,[60]

which was verified by 1H NMR, ESI-MS, and single-crystalstructure analyses. Moreover, the host 79 could bind theparaquat salt 132a to form a 1:1 complex both in solutionand in the solid state. Because both Li+ and K+ ions werekey components in the formation of the two cascade com-


Scheme 3. Synthesis of the potassium-ion-templated [2]rotaxanes 140–142.

plexes, removal of the metal ions by addition of 18-crown-6 or 12-crown-4 could dissociate the previously formedcomplexes. Consequently, we had found that three switch-able processes between the host 79 and the guests 132a, 143,and 144 could be efficiently performed by control with lith-ium or potassium ions.

Figure 39. Structures of compounds 143 and 144.

Thanks to the introduction of rigid triptycene moieties,the triptycene-derived calixarenes 88b and 89b and the ox-acalixarene 101a were all found by a fluorescence methodto exhibit complexation abilities towards C60 and C70. Com-pound 88b was found to be able to form 1:1 complexes withC60 and C70, and the association constants were determinedto be 6.9(�0.2) �104 and 5.2(�0.2)� 104 m–1, respectively.We further found that 89b, like 88b, could also form 1:1complexes with both C60 and C70, with correspondingassociation constants of 8.7(�0.3) �104 and5.9(�0.4)� 104 m–1, respectively.[68b] In the case of the oxo-calixarene 101a, we found that it could also form 1:1 com-plexes with C60 and C70, with association constants deter-mined to be 7.5(�0.3)� 104 and 9.0(�0.3)�104 m–1,respectively.[72]

Complexation with Squaraine Dyes

Squaraines, a family of fluorescent near-IR dyes withspecific photophysical properties for wide potential applica-tions in areas such as imaging, nonlinear optics, photovol-taics, and ion sensing, have attracted increasing interest.[88]

The use of these dyes is limited, however, by their inherentreactivity with nucleophiles and tendency to form non-fluo-rescent aggregates in water. We recently found that thetriptycene-derived hosts 107 and 108 could bind thesquaraine 145b (Figure 40) to form the 1:1 complexes107·145b and 108·145b, with association constants betweenthe hosts and the guest in chloroform of6.8(�0.3) �105 m–1 and 1.3(�0.3)� 106 m–1, respectively.[75]

Interestingly, it was found that the guest protons in the


complex 107·145b all displayed two sets of signals, suggest-ing that half of the guest molecule was positioned at thewide rim of the macrocycle whereas the other half was atthe narrow rim. The guest could therefore display environ-ment-induced-asymmetry upon complexation with the host107 with the cone conformation. A single-crystal structureanalysis of 108·145b showed the squaraine threadedthrough the host 108 to form a [2]pseudorotaxane-typecomplex in the solid state, with the cyclobutene core of 145bpositioned in the centre of 108. When the guest 145b andthe complexes 107·145b and 108·145b were separately dis-solved in THF/water solution (4:1) they displayed similarblue colors, but we found that the guest 145b underwenthydrolytic decomposition to turn colorless after 4 days,whereas the solutions of the complexes retained their bluecolors for several weeks, which suggested that macrocyclichosts 107 and 108 could protect the squaraine dye fromdecomposition in polar solvents.

Figure 40. Structures of the squaraine dyes 145a–g.

Further studies showed the formation of the complexes107·145a and 108·145a between the wheels 107 and 108band the squaraine dye 145a. In the case of 145c it was foundthat the [2]pseudorotaxane-type complex 108·145c formed,whereas no complexation between 107 and 145c occurredat room temperature, although when the mixture of 107 and145c in CDCl3 was heated at 333 K for more than six daysthe [2]rotaxane 107·145c could be obtained through theslippage method. In the cases of the squaraine dyes 145dand 145e, neither could not thread through hosts 107 or108, even after being heated at 333 K for several days, indi-cating that N,N-di-n-butyl and dibenzylamino groups arelarge enough that the squaraines did not form [2]rotaxanesby the slippage method with the triptycene-derived macro-cycles 107 and 108. In the case of the nonsymmetrical


Figure 41. Structures of the isomeric [2]rotaxanes 146a, 146b, 147a, 147b, and 148a–c.

squaraine 145f, 1H NMR studies showed the formation ofthe complex 108·145f. For 107, two isomeric [2]pseudorot-axane complexes based on 107 and 145f were obtained, andthe complexation showed a slight selectivity.[89]

Similarly to the cases of the squaraines 145d and 145e,the squaraine dye 145g with two different bulky stoppersalso could not form complexes with the hosts 107 and 108at room temperature or even at 333 K. However, when wechose the squaraines 145d, 145e, and 145g with bulky stop-per groups as the templates for the synthesis of [2]rotaxanesthrough clipping reactions, the series of isomeric squaraine-based [2]rotaxanes 146–148 (Figure 41) could be obtainedin total yields of 31–37 %.[89] The structural features andphotophysical properties of these inclusion complexes werefurther investigated, and the results showed that the chemi-cal stabilities of the [2]rotaxanes 146–148 were greater thanthose of the free squaraines.

3. Synthesis and Applications of PentiptyceneDerivatives

3.1. Synthesis and Applications of Central-Ring-Functionalized Pentiptycene Derivatives

An important feature of pentiptycene is the presence ofa “sterically shielded” central benzene ring, unlike in thecase of triptycene. Such a feature has also led to the use ofpentiptycene in the construction of fluorescent chemosen-sors, molecular machines, low-dielectric-constant materials,porous solids, and so on.[5] The synthesis of pentiptycene-based functional molecules mainly relies on central-ring-prefunctionalized pentiptycene building blocks, and a use-


ful approach toward the preparation of these buildingblocks is the derivatization of pentiptycene quinone.

Inspired by an example of the synthesis of triptycene qui-nones through reactions between anthracene and quinonesin excess in acetic acid,[90] we have recently developed apractical and efficient method for one-pot syntheses ofiptycene quinones, including a series of pentiptycene quin-ones, heptiptycene quinones, and noniptycene quinones byuse of p-chloranil in place of the excesses of quinones asoxidants.[91] Treatment of the triptycene quinone 149(Scheme 4) with anthracene or 1,4-dimethoxyanthracenegave the pentiptycene quinone 151a and the dimethoxypen-tiptycene quinone 151b in 78 and 82% yields, respectively.Compound 151b was then demethylated by CAN oxidationto give the pentiptycene diquinone 152 in nearly quantita-tive yield.

Scheme 4. Synthesis of the pentiptycene quinones 151–152.


Figure 42. Structures of the iptycene quinones 153–158.

Similarly, the series of iptycene quinones 153–158 (Fig-ure 42) were synthesized by the same strategy.

Yang and his co-workers[92] recently synthesized a seriesof central-ring functionalized pentiptycenes based on thereadily prepared pentiptycene monoquinone 151a. It wasfound that the reaction between 159 and hydroxylamine inthe presence of hydrochloric acid (2 equiv.) afforded themono-oxime 160 in high yield (Scheme 5). However, thecorresponding dioxime was not obtained. Reduction of 160

Scheme 5. Synthesis of the pentiptycene derivatives 159–166.

Scheme 6. Synthesis of the pentiptycene derivatives 168a–e.


in dichloromethane with stannous chloride afforded theaminophenol 161. Furthermore, it was interestingly foundthat by controlling the concentration of reagents and theorder in which they are added to the THF solution of 161,the amino group in 161 can either be removed to form thepentiptycene phenol 162 or converted into a nitro group asin 163 in almost quantitative yield simply by use of tert-butyl nitrite and H3PO2. From the O-alkylated derivative164 of 162, the iodopentiptycene 165 and bromo-

Y. Jiang, C.-F. ChenMICROREVIEWpentiptycene 166 can be readily prepared. From 165 and166, a series of pentiptycene derivatives[92] with extended π-conjugated backbones were further synthesized.

Anzenbacher et al.[93] recently reported a simple methodfor the synthesis of the 1,4-diarylpentiptycenes 168a–e(Scheme 6) in two-step fashion and in moderate overallyields from 1,4-dibromo-2,5-dichlorobenzene (167) in aprocess including double base-promoted dehydrohalogena-tion followed by aryne cycloaddition to anthracene. Thecapabilities of the resulting 1,4-diarylpentiptycenes to act asTNT sensors in solution and in the solid state was alsostudied by Swager’s procedure.[94] It was found that thefluorescence of these pentiptycene derivatives could be con-siderably quenched in the presence of vapors of nitroaro-matic compounds, which suggested their potential applica-tions in sensing of explosives.

Halogenated pentiptycene derivatives are importantbuilding blocks for the synthesis of conjugated pentip-tycene-based macromolecules. Although previous attemptsto prepare halogenated pentiptycenes had been unsuccess-ful, Yang and his co-workers[95] recently reported a newprocedure for the synthesis of halogenated pentiptycenes.Starting from the pentiptycene 161, they prepared the halo-genated pentiptycene phenols 169–171 (Figure 43) either inone-pot fashion or by a two-step method through the di-azonium salt.

Figure 43. Structures of the pentiptycene derivatives 169–171.

Scheme 7 further shows the synthesis of the dihalo-pentiptycenes 175–177. The key step was found to be thePd-catalyzed reduction of the triflate group in 172 with tri-ethylsilane together with utilization of the more reactive Pd

Scheme 7. Synthesis of the dihalopentiptycenes 175–177.


catalyst Pd(PPh3)4. This generates the nitropentiptycene173, and these reactions appear to be more efficient for aryltriflates containing electron-withdrawing substituents. Afterthe pentiptycene dihalides had been prepared, these deriva-tives showed good reactivity, and a variety of pentiptycene-incorporated π-conjugated systems were constructedthrough Heck, Suzuki, and Sonogashira reactions.[95]

Like the triptycene moiety, the rigid pentiptycene moietycan also act as a fourfold rotor in the construction of mo-lecular machines. Yang et al.[96] built the pentiptycene-de-rived light-driven molecular brakes 178 (Figure 44, a), inwhich the trans/cis photoisomerizable dinitrostryl groupsbehave as photoresponsive brake components. Variable-temperature NMR studies showed that thanks to the smallrotational barriers (ΔG� ≈ 4.5 kcal mol–1 at 298 K), the pen-tiptycene rotators in the trans form (trans-178a, the brake-off state) could undergo free rotation (krot ≈ 109 s–1). Suchrotation is almost blocked, however (with krot = 3 s–1, ΔG�

≈ 16.4 kcalmol–1), at 298 K in the cis forms (cis-178a), thebrake-on state as the result of the intercalation of the dini-trophenyl group into the U-shaped cavities of the rotator.The rates of rotation in the brake-on and brake-off statestherefore differ by nearly nine orders of magnitude at298 K. Furthermore, by replacing the nitro groups in thebrake component with smaller (H and methoxy) or larger(isopropyl and tert-butyl) substituents, they synthesized theseries of pentiptycene analogues 178b–e (Figure 44, a).

The substituent effects on the performance of the brakeswere studied further. Consequently, it was found that at298 K, compounds trans-178 rotated with rates at a magni-tude of krot = 108–109 s–1 for all five compounds, but forcompounds cis-178 the rates varied from 2–6 s–1, dependingon the natures of the substituents in the brake component.The substituents exert steric repulsion, electronic induction,and specific attractive electrostatic interactions with thepentiptycene rotor in the ground state as well as the transi-tion state of cis-178 along the rotation coordinate.[97] Asshown in Figure 44 (b), Yang and his co-workers also syn-thesized the new pentiptycene derivative 179, containing an


Figure 44. Schematic representation of a) the light-driven molecular brakes 178, and b) the combined light-driven and electrically drivenmolecular brake 179.

enone group, and further developed a new approach to ef-ficient control of molecular brakes by a combination of di-rectional photochemical trans/cis and electrochemicalcis/trans isomerization. The authors found that because ofthe different extents of steric interaction with the indanoneunit (the brake component) in the two stereoisomers, thetrans/cis switching resulted in a 500-fold change in theBrownian rotation rate of the pentiptycene group (the ro-tor) at room temperature.[98]

3.2. Synthesis and Applications of Pentiptycene-DerivedHosts

The design and synthesis of pentiptycene-based hostsand their applications in host–guest chemistry are still muchless explored than in the triptycene case. On the other hand,the H-shaped scaffold of pentiptycene is inherently differentfrom the Y-shaped scaffold of triptycene, and its derivativescan also be used as a class of useful building blocks forconstruction of new hosts. In the light of previous studies,we[99] thus recently synthesized a series of peripherally o-dimethoxy-substituted pentiptycene quinones and their o-quinone derivatives. We first obtained the pentiptycenequinone 182 (Scheme 8), containing one o-dimethoxyben-zene unit, in 75% yield by treatment of the quinone 149with 2,3-dimethoxyanthracene (180) or by treatment of an-

Scheme 8. Synthesis of the pentiptycene quinone 182.


thracene with the triptycene quinone 181 at reflux in aceticacid in the presence of p-chloranil.

By a similar strategy we also prepared the peripherallyo-dimethoxy-substituted pentiptycene quinones 183–187(Figure 45). Furthermore, a series of o-quinone derivativeswere synthesized by CAN oxidation of 182–187. In particu-lar, we found that if two ortho-dimethoxybenzene moietieswere disposed in syn fashion with respect to the centralquinone unit, only one of them would be oxidized by excessCAN in aqueous acetonitrile.

Single-crystal structure analyses showed that the pentip-tycene quinines, with unique 3D rigid structures, could allself-assemble into 3D microporous structures in the solidstate. For pentiptycene quinones containing both dimethox-ybenzene unit(s) and quinone group(s) simultaneously,interesting intramolecular charge-transfer interactions andelectrochemical properties were also studied by UV/Visspectroscopy and electrochemical analysis.[99]

With the peripherally substituted pentiptycenes to hand,we then tried to synthesize pentiptycene-derived hosts. Ontreatment of the pentiptycene bis(catechol) 188 (Figure 46)with the ditosylate 189 under high-dilution conditions inthe presence of Cs2CO3, the pentiptycene-derived host 190was obtained in 36% yield. 1H NMR and ESI-MS sug-gested that the host 190 could bind CBPQT (137) to formthe 1:1 complex 190·137 with an association constant of


Figure 45. Structures of the peripherally o-dimethoxy-substituted pentiptycene quinones 183–187.

Figure 46. Structures of compounds 188 and 189 and of the hosts 190–194.

2.7 �103m–1. A single-crystal structure analysis revealedthat the CBPQT4+ ring was included in the cavity of host190, whereas one benzene ring of the DB24C8 moieties waspositioned inside the CBPQT4+ cavity, implying that apseudo-ternary complex had been formed in which theCBPQT4+ ring acted both as a guest and as a host (Fig-ure 46). The electrochemical behavior of CBPQT4+ in theabsence and in the presence of the host 190 was alsostudied, and the results showed that the formation of thecomplex was the result of charge-transfer interactions.[100]

We also found that both 191 and 192, like the host 190,could bind CBPQT (137) to form the 1:1 complexes191·137 and 192·137, with association constants of2.52(�0.04)�102m–1 and 1.07(�0.06)�103m–1, respec-tively. Single-crystal structure analyses further showed that


in both complexes 191·137 (Figure 47) and 192·137, notonly is half of the CBPQT4+ ring included in the cavity of191 or 192, but also one benzene ring of the pentiptyceneframework is positioned inside the cavity of the CBPQT4+

ring, making it hard to tell which components in the com-plexes were the guests and which the hosts, because bothare both hosts and guests for one another. Electrochemicalexperiments suggested that both complexes were stabilizedby charge-transfer interactions.[101] Moreover, we alsofound that the host 191 could bind paraquat salts contain-ing a series of functional groups to form 1:1 complexes.It could, for example, bind the paraquat derivative 132l,containing two anthracene groups, to form the 1:1 complex191·132l with an association constant of 285.3 m–1. A sin-gle-crystal structure analysis of 191·132l showed that the


guest 132l was positioned in the cavity of the tweezer-likepentiptycene mono(crown ether) and was adjacent to theDB24C8 moiety. In addition, the bipyridinium unit in thecomplex was distorted, with a –18.76° dihedral angle be-tween the bipyridinium rings. Moreover, the binding andrelease of the paraquat could also easily be controlled byaddition and removal of potassium ion.[102]

Figure 47. Single-crystal structures of complexes 190·137 and191·137.

Similarly to the complexes between the host 191 andparaquat salts, complexation between pentiptycene-derivedbis(crown ethers) and CBPQT4+ salts can also be controlledby the addition and removal of potassium ion. We[103] fur-ther synthesized the pentiptycene-derived bis(crown ethers)193 and 194 (Figure 46). It was found that the host 193 wasalso able to bind CBPQT4+ salts to form a 1:1 complex193·137 with an association constant of 1.4(�0.1) �103m–1,but host 194 could bind CBPQT4+ salts to form the 1:2complex 194·(137)2, with an average association constant of



7.4(�0.2)� 103m–2. It was further found that the complex-ation mode in 194·(137)2 in the solid state is obviously dif-ferent from that between 190 and 137. The triptycene-likecrown ether moiety of one side of molecule 194 and thepentiptycene scaffold of its adjacent molecule formed anopen cavity, in which one CBPQT4+ ring was included.Meanwhile, the terminal benzene ring of each DB24C8moiety of 194 was positioned inside the cavity of theCBPQT4+ ring. Pentiptycene-derived bis(crown ether)s con-taining two 24-crown-8 moieties could form complexes withpotassium ions, and the subsequent complexation of thecations would introduce extra electrostatic repellent force tothe cationic CBPQT4+ ring(s), resulting in dissociation ofthe previously formed complexes between the pentiptycene-derived crown ethers and the CBPQT4+ ring(s). Moreover,18-crown-6 is a very strong sequestering agent for potas-sium ion, so the potassium ion-controlled binding and re-lease of the CBPQT4+ ring(s) in the complexes based onthese pentiptycene-derived bis(crown ethers) were also dem-onstrated.[103]

More recently, we also synthesized a series ofpentiptycene-derived rigid tweezer-like molecules.[104] Bycondensation of o-diaminobenzene (or o-diaminotripty-cene) with the corresponding o-quinones 195–200 in eth-anol at reflux, the tweezer-like molecules 201–209 (Fig-ure 48) were efficiently synthesized. A single-crystal struc-ture analysis of 202 suggested that the molecule showedlittle distortion in the pyrazine rings in the solid state. Moreinterestingly, compound 207 was found to show complex-ation ability towards C60 with an association constant of

Y. Jiang, C.-F. ChenMICROREVIEW3.5 �103m–1, which might be due to the π–π interaction andthe van der Waals forces between the sterically fitted con-cave and convex π surfaces of the pentiptycene andtriptycene moieties and C60.[104]

4. Conclusion

We have detailed recent developments in the synthesis oftriptycene derivatives and their applications in many re-search areas, including molecular machines, molecular bal-ances, catalysis, materials chemistry, and molecular engi-neering. In particular, it was noteworthy that because of theunique three-dimensional rigid structure and electron-richcavity of triptycene, its derivatives have been found to beuseful building blocks for the design and synthesis of dif-ferent kinds of new triptycene-derived hosts. These hostsexhibited powerful complexation abilities toward differentkinds of organic guests, which have resulted in a seriesof new supramolecular systems with specific structuresand properties. On the other hand, in comparison withtriptycene derivatives, studies on the synthesis and applica-tions of pentiptycenes are still limited. However, the uniquestructural feature of pentiptycene has been attracting in-creasing interest. In particular, the synthesis of central-ringfunctionalized pentiptycene derivatives has been well inves-tigated, and more and more applications in areas such asfluorescent sensors and molecular machines have interest-ingly been discovered in recent years. Moreover,pentiptycene derivatives were also found to be useful build-ing blocks for the synthesis of novel hosts, and these hostshave shown efficient complexation abilities toward cyclobis(paraquat-p-phenylene), paraquat derivatives, and C60.We believe that the triptycene and pentiptycene derivativeswith unique structures will find more and more applicationsin wide fields in the near future, and that our recent resultsin the synthesis and applications of triptycene andpentiptycene derived hosts will also open a door foriptycene-based host–guest chemistry.

Acknowledgments

The authors thank the National Natural Science Foundation ofChina (grant numbers 20625206, 20772126, 20972162), the ChineseAcademy of Sciences, and the Ministry of Science and Technologyof China (grant numbers 2007CB808004, 2011CB932501) for fin-ancial support.

[1] P. D. Bartlett, M. J. Ryan, S. G. Cohen, J. Am. Chem. Soc.1942, 64, 2649–2653.

[2] G. Wittig, R. Ludwig, Angew. Chem. 1956, 68, 40–40.[3] H. Hart, S. Shamouilian, Y. Takehira, J. Org. Chem. 1981, 46,

4427–4432.[4] a) C.-F. Chen, T. Han, Y. Jiang, Chin. Sci. Bull. 2007, 52, 1349–

1361; b) T. M. Swager, Acc. Chem. Res. 2008, 41, 1181–1189;c) J. H. Chong, M. J. MacLachlan, Chem. Soc. Rev. 2009, 38,3301–3315; d) L.-W. Zhao, Z. Li, T. Wirth, Chem. Lett. 2010,


39, 658–667; e) C.-F. Chen, Chem. Commun. 2011, 47, 1674–1688.

[5] J.-S. Yang, J.-L. Yan, Chem. Commun. 2008, 1501–1512.[6] a) J.-P. Sauvage, Molecular Machines and Motors, Structure

Bonding, Springer, Berlin, Germany, 2001; b) V. Balzani, A.Credi, M. Venturi, Molecular Devices and Machines – A Jour-ney into the Nano World, Wiley-VCH, Weinheim, Germany,2003.

[7] a) J. D. Badjic, C. M. Ronconi, J. F. Stoddart, V. Balzani, S.Silvi, A. Credi, J. Am. Chem. Soc. 2006, 128, 1489–1499; b)A. M. Brouwer, C. Frochot, F. G. Gatti, D. A. Leigh, L. Mott-ier, F. Paolucci, S. Roffia, G. W. H. Wurpel, Science 2001, 291,2124–2128; c) N. Armaroli, V. Balzani, J.-P. Collin, P. Gaviná,J.-P. Sauvage, B. Ventura, J. Am. Chem. Soc. 1999, 121, 4397–4408; d) C.-J. Chuang, W.-S. Li, C.-C. Lai, Y.-H. Liu, S.-M.Peng, I. Chao, S.-H. Chiu, Org. Lett. 2009, 11, 385–388.

[8] a) F. Cozzi, A. Guenzi, C. A. Johnson, K. Mislow, W. D.Hounshell, J. F. Blount, J. Am. Chem. Soc. 1981, 103, 957–958;b) Y. Kawada, H. Iwamura, J. Am. Chem. Soc. 1981, 103, 958–960.

[9] C. E. Godinez, G. Zepeda, M. A. Garcia-Garibay, J. Am.Chem. Soc. 2002, 124, 4701–4707.

[10] C. E. Godinez, G. Zepeda, C. J. Mortko, H. Dang, M. A. Gar-cia-Garibay, J. Org. Chem. 2004, 69, 1652–1662.

[11] M. A. Garcia-Garibay, C. E. Godinez, Cryst. Growth Des.2009, 9, 3124–13128.

[12] L. E. Harrington, L. S. Cahill, M. J. McGlinchey, Organome-tallics 2004, 23, 2884–2891.

[13] K. Nikitin, H. Müller-Bunz, Y. Ortin, M. J. McGlinchey, Org.Biomol. Chem. 2007, 5, 1952–1960.

[14] K. Nikitin, H. Müller-Bunz, Y. Ortin, M. J. McGlinchey,Chem. Eur. J. 2009, 15, 1836–1843.

[15] K. Nikitin, H. Müller-Bunz, Y. Ortin, J. Muldoon, M. J.McGlinchey, J. Am. Chem. Soc. 2010, 132, 17617–17622.

[16] T. R. Kelly, Acc. Chem. Res. 2001, 34, 514–522.[17] T. R. Kelly, I. Tellitu, J. P. Sestelo, Angew. Chem. 1997, 109,

1969; Angew. Chem. Int. Ed. Engl. 1997, 36, 1866–1868.[18] a) T. R. Kelly, H. De Silva, R. A. Silva, Nature 1999, 401, 150–

152; b) T. R. Kelly, R. A. Silva, H. De Silva, S. Jasmin, Y.Zhao, J. Am. Chem. Soc. 2000, 122, 6935–6949.

[19] T. R. Kelly, X. Cai, F. Damkaci, J. Am. Chem. Soc. 2007, 129,376–386.

[20] G. Kryger, I. Silman, J. L. Sussman, J. Physiol. (Paris) 1998,92, 191–194.

[21] K. M. Guckian, B. A. Schweitzer, R. X. F. Ren, C. J. Sheils,D. C. Tahmassebi, E. T. Kool, J. Am. Chem. Soc. 2000, 122,2213–2222.

[22] D. B. Amabilino, J. F. Stoddart, Chem. Rev. 1995, 95, 2725–2829.

[23] C. A. Hunter, J. K. M. Sanders, J. Am. Chem. Soc. 1990, 112,5525–5534.

[24] S. Deechongkit, H. Nguyen, E. T. Powers, P. E. Dawson, M.Gruebele, J. W. Kelly, Nature 2004, 430, 101–105.

[25] I. K. Mati, S. L. Cockroft, Chem. Soc. Rev. 2010, 39, 4195–4205.

[26] M. Oki, Acc. Chem. Res. 1990, 23, 351–356.[27] B. W. Gung, X. Xue, H. J. Reich, J. Org. Chem. 2005, 70, 3641–

3644.[28] B. W. Gung, M. Patel, X. Xue, J. Org. Chem. 2005, 70, 10532–

10537.[29] B. W. Gung, F. Wekesa, C. L. Barnes, J. Org. Chem. 2008, 73,

1803–1808.[30] B. W. Gung, Y. Zou, Z. Xu, J. C. Amicangelo, D. G. Irwin, S.

Ma, H.-C. Zhou, J. Org. Chem. 2008, 73, 689–693.[31] B. W. Gung, B. U. Emenike, M. Lewis, K. Kirschbaum, Chem.

Eur. J. 2010, 16, 12357–12362.[32] M. Sundermeier, A. Zapf, S. Mutyala, W. Baumann, J. Sans, S.

Weiss, M. Beller, Chem. Eur. J. 2003, 9, 1828–1836.[33] C. Azerraf, S. Cohen, D. Gelman, Inorg. Chem. 2006, 45, 7010–

7017.


[34] O. Grossman, C. Azerraf, D. Gelman, Organometallics 2006,25, 375–381.

[35] O. Grossman, D. Gelman, Org. Lett. 2006, 8, 1189–1191.[36] L. Bini, C. Müller, J. Wilting, L. Chrzanowski, A. L. Spek, D.

Vogt, J. Am. Chem. Soc. 2007, 129, 12622–12623.[37] T. M. Long, T. M. Swager, Adv. Mater. 2001, 13, 601–604.[38] C. L. Hilton, C. R. Jamison, H. K. Zane, B. T. King, J. Org.

Chem. 2009, 74, 705–707.[39] Y. Jiang, C.-F. Chen, Synlett 2010, 1679–1681.[40] B.-J. Pei, W.-H. Chan, A. W. M. Lee, J. Org. Chem. 2010, 75,

7333–7337.[41] Z. Chen, T. M. Swager, Macromolecules 2008, 41, 6880–6885.[42] T. M. Long, T. M. Swager, J. Am. Chem. Soc. 2002, 124, 3826–

3827.[43] Z. Zhu, T. M. Swager, J. Am. Chem. Soc. 2002, 124, 9670–9671.[44] J. Hoogboom, T. M. Swager, J. Am. Chem. Soc. 2006, 128,

15058–15059.[45] T. M. Long, T. M. Swager, J. Mater. Chem. 2002, 12, 3407–

3412.[46] T. M. Long, T. M. Swager, J. Am. Chem. Soc. 2003, 125,

14113–14119.[47] N. T. Tsui, A. J. Paraskos, L. Torun, T. M. Swager, E. L.

Thomas, Macromolecules 2006, 39, 3350–3358.[48] B. S. Ghanem, K. J. Msayib, N. B. McKeown, K. D. M. Harris,

Z. Pan, P. M. Budd, A. Butler, J. Selbie, D. Book, A. Walton,Chem. Commun. 2007, 67–69.

[49] B. S. Ghanem, M. Hashem, K. D. M. Harris, K. J. Msayib, M.Xu, P. M. Budd, N. Chaukura, D. Book, S. Tedds, A. Walton,N. B. McKeown, Macromolecules 2010, 43, 5287–5294.

[50] J. H. Chong, S. J. Ardakani, K. J. Smith, M. J. MacLachlan,Chem. Eur. J. 2009, 15, 11824–11828.

[51] A. Beyeler, P. Belser, Coord. Chem. Rev. 2002, 230, 29–39.[52] H.-H. Chou, H.-H. Shih, Ch.-H. Cheng, J. Mater. Chem. 2010,

20, 798–805.[53] R. H. Goldsmith, J. Vura-Weis, A. M. Scott, S. Borkar, A. Sen,

M. A. Ratner, M. R. Wasielewski, J. Am. Chem. Soc. 2008, 130,7659–7669.

[54] X. Gu, Y.-H. Lai, Org. Lett. 2010, 12, 5200–5203.[55] J.-S. Yang, C.-P. Liu, B.-C. Lin, C.-W. Tu, G.-H. Lee, J. Org.

Chem. 2002, 67, 7343–7354.[56] J. H. Chong, M. J. MacLachlan, Inorg. Chem. 2006, 45, 1442–

1444.[57] J. H. Chong, M. J. MacLachlan, J. Org. Chem. 2007, 72, 8683–

8690.[58] Q.-S. Zong, C.-F. Chen, Org. Lett. 2006, 8, 211–214.[59] J.-M. Zhao, Q.-S. Zong, C.-F. Chen, J. Org. Chem. 2010, 75,

5092–5098.[60] T. Han, C.-F. Chen, J. Org. Chem. 2007, 72, 7287–7293.[61] X.-Z. Zhu, C.-F. Chen, J. Am. Chem. Soc. 2005, 127, 13158–

13159.[62] X.-Z. Zhu, C.-F. Chen, Chem. Eur. J. 2006, 12, 5603–5609.[63] T. Han, Q.-S. Zong, C.-F. Chen, J. Org. Chem. 2007, 72, 3108–

3111.[64] X.-X. Peng, H.-Y. Lu, T. Han, C.-F. Chen, Org. Lett. 2007, 9,

895–898.[65] Y. Jiang, J. Cao, J.-M. Zhao, J.-F. Xiang, C.-F. Chen, J. Org.

Chem. 2010, 75, 1767–1770.[66] J.-B. Guo, J.-F. Xiang, C.-F. Chen, Eur. J. Org. Chem. 2010,

5056–5062.[67] a) Calixarenes 2001 (Eds.: Z. Asfari, V. Böhmer, J. Harrowfield,

J. Vicens), Kluwer, Academic Publishers, 2001; b) C. D. Guts-che, Calixarenes Revisited – Monographs in SupramolecularChemistry (Ed.: J. F. Stoddart), Royal Society of Chemistry,Cambridge, 1998.


[68] a) X.-H. Tian, X. Hao, T.-L. Liang, C.-F. Chen, Chem. Com-mun. 2009, 6771–6773; b) X.-H. Tian, C.-F. Chen, Chem. Eur.J. 2010, 16, 8072–8079.

[69] X.-H. Tian, C.-F. Chen, Org. Lett. 2010, 12, 524–527.[70] a) B. König, M. H. Fonseca, Eur. J. Inorg. Chem. 2000, 2303–

2310; b) M.-X. Wang, Chem. Commun. 2008, 4541–4551; c) W.Maes, W. Dehaen, Chem. Soc. Rev. 2008, 37, 2393–2402; d) H.Tsue, K. Ishibashi, R. Tamura, J. Synth. Org. Chem. Jpn. 2009,67, 898–908.

[71] C. Zhang, C.-F. Chen, J. Org. Chem. 2007, 72, 3880–3888.[72] S.-Z. Hu, C.-F. Chen, Chem. Commun. 2010, 46, 4199–4201.[73] M. Xue, C.-F. Chen, Org. Lett. 2009, 11, 5294–5297.[74] M. Xue, C.-F. Chen, Chem. Commun. 2011, 47, 2318–2320.[75] M. Xue, C.-F. Chen, Chem. Commun. 2008, 6128–6130.[76] C. Zhang, C.-F. Chen, J. Org. Chem. 2006, 71, 6626–6629.[77] C. Zhang, C.-F. Chen, J. Org. Chem. 2007, 72, 9339–9341.[78] Q.-S. Zong, C. Zhang, C.-F. Chen, Org. Lett. 2006, 8, 1859–

1862.[79] J.-M. Zhao, Q.-S. Zong, T. Han, J.-F. Xiang, C.-F. Chen, J.

Org. Chem. 2008, 73, 6800–6806.[80] Y. Jiang, J.-B. Guo, C.-F. Chen, Chem. Commun. 2010, 46,

5536–5538.[81] Y. Jiang, J.-B. Guo, C.-F. Chen, Org. Lett. 2010, 12, 4248–4251.[82] Y. Jiang, X.-Z. Zhu, C.-F. Chen, Chem. Eur. J. 2010, 16, 7140–

7143.[83] T. Han, Q.-S. Zong, C.-F. Chen, J. Org. Chem. 2007, 72, 3108–

3111.[84] T. Han, C.-F. Chen, Org. Lett. 2007, 9, 4207–4210.[85] J.-B. Guo, Y. Jiang, C.-F. Chen, Org. Lett. 2010, 12, 5764–5767.[86] S.-Z. Hu, C.-F. Chen, Chem. Eur. J. 2011, 17, 5424–5431.[87] T. Han, C.-F. Chen, J. Org. Chem. 2008, 73, 7735–7742.[88] S. Sreejith, P. Carol, P. Chithraa, A. Ajayaghosh, J. Mater.

Chem. 2008, 18, 264–274.[89] M. Xue, Y.-S. Su, C.-F. Chen, Chem. Eur. J. 2010, 16, 8537–

8544.[90] A. Wiehe, M. O. Senge, H. Kurreck, Liebigs Ann./Recl. 1997,

1951–1963.[91] X.-Z. Zhu, C.-F. Chen, J. Org. Chem. 2005, 70, 917–924.[92] J.-S. Yang, C.-W. Ko, J. Org. Chem. 2006, 71, 844–847.[93] G. V. Zyryanov, M. A. Palacios, P. Anzenbacher Jr., Org. Lett.

2008, 10, 3681–3684.[94] D. Zhao, T. M. Swager, Macromolecules 2005, 38, 9377–9384.[95] J.-S. Yang, J.-L. Yan, Y.-X. Jin, W.-T. Sun, M.-C. Yang, Org.

Lett. 2009, 11, 1429–1432.[96] J.-S. Yang, Y.-T. Huang, J.-H. Ho, W.-T. Sun, H.-H. Huang,

Y.-C. Lin, S.-J. Huang, S.-L. Huang, H.-F. Lu, I. Chao, Org.Lett. 2008, 10, 2279–2282.

[97] W.-T. Sun, Y.-T. Huang, G.-J. Huang, H.-F. Lu, I. Chao, S.-L.Huang, S.-J. Huang, Y.-C. Lin, J.-H. Ho, J.-S. Yang, Chem. Eur.J. 2010, 16, 11594–11604.

[98] Y.-C. Chen, W.-T. Sun, H.-F. Lu, I. Chao, G.-J. Huang, Y.-C.Lin, S.-L. Huang, H.-H. Huang, Y.-D. Lin, J.-S. Yang, Chem.Eur. J. 2011, 17, 1193–1200.

[99] J. Cao, H.-Y. Lu, C.-F. Chen, Tetrahedron 2009, 65, 8104–8112.[100] J. Cao, Y. Jiang, J.-M. Zhao, C.-F. Chen, Chem. Commun.

2009, 1987–1989.[101] J. Cao, H.-Y. Lu, J.-F. Xiang, C.-F. Chen, Chem. Commun.

2010, 46, 3586–1988.[102] J. Cao, H.-Y. Lu, X.-J. You, Q.-Y. Zheng, C.-F. Chen, Org.

Lett. 2009, 11, 4446–4449.[103] J. Cao, J.-B. Guo, P.-F. Li, C.-F. Chen, J. Org. Chem. 2011,

76, 1644–1652.[104] J. Cao, X.-Z. Zhu, C.-F. Chen, J. Org. Chem. 2010, 75, 7420–

7523.Received: May 15, 2011

Published Online: August 24, 2011

Documents

Recent Developments in Synthesis and Applications of Triptycene and Pentiptycene Derivatives