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[Masaki Ishii](https://orcid.org/0009-0008-6846-9610), Yuto Nakai, [Yu Yamashita](https://orcid.org/0000-0001-7966-3197), Katsuto Onishi, Tan-hao Shi, Nobutaka Shioya, Hideki Sakai, [Takeshi Hasegawa](https://orcid.org/0000-0001-5574-9869), Tomoki Ogoshi, [Katsuhiko Ariga](https://orcid.org/0000-0002-2445-2955), Taizo Mori

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[Pseudorotaxane monolayers of pillar[5]arene and linear fatty acids at the air–water interface](https://mdr.nims.go.jp/datasets/5ac429a5-0a40-4904-b6c2-456d8d6172bf)

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Pseudorotaxane monolayers of pillar[5]arene and linear fatty acids at the air–water interface10138 |  Chem. Commun., 2025, 61, 10138–10141 This journal is © The Royal Society of Chemistry 2025Cite this: Chem. Commun., 2025,61, 10138Pseudorotaxane monolayers of pillar[5]arene andlinear fatty acids at the air–water interface†Masaki Ishii, *ab Yuto Nakai,ab Yu Yamashita, a Katsuto Onishi,c Tan-hao Shi,cNobutaka Shioya,d Hideki Sakai,b Takeshi Hasegawa, d Tomoki Ogoshi,ceKatsuhiko Ariga abf and Taizo MoridPseudorotaxanes, which are formed by macrocyclic host moleculesand linear guest molecules, show potential in molecular devices andsurface applications. In this study, ethoxy-functionalized pillar[5]arene(P5A) and amphiphilic linear fatty acid guests were self-assembled intooriented monolayers at the air–water interface. The fatty acid structuredictates the monolayer formation: [2]pseudorotaxane-based, [3]pseu-dorotaxane-based and phase-separated monolayers. These findingsprovide insights into P5A-based pseudorotaxane monolayers, facilitat-ing their integration into advanced functional materials.Pseudorotaxanes are formed by a macrocyclic host moleculeand a linear guest molecule that exhibits specific non-covalentinteractions with the host.1 The tiling of oriented pseudoro-taxanes particularly in two-dimensional planes is promising forapplications such as molecular shuttles,2,3 surface wettabilitycontrol,4 sensors,5 catalysts,6 and transistors.7 Compared withthe commonly used self-assembled monolayer (SAM) methodfor surface modification, the Langmuir--Blodgett (LB)technique8 imposes significantly fewer material limitationsbecause of the lack of chemical bonds between the pseudor-otaxane terminal functional groups and the substrate surface.Additionally, this method allows for control of the density andorientation of pseudorotaxanes through in-plane mechanicalcompression.Pillararenes, first developed in 2008,9 are a class of macrocyclichost molecules, with cyclopentamer (P5A) and cyclohexamer (P6A)derivatives being the most commonly used. Compared to othermacrocyclic host molecules, such as crown ethers and calixarenes,pillararenes offer advantages in terms of easy synthesis, flexiblefunctionality, and high symmetry.10 In particular, P5A formspseudorotaxanes with saturated alkanes via C–H� � �p inter-actions.11,12 Recent studies have demonstrated that P5A-basedpseudorotaxanes can exhibit bistable states,13,14 offering appli-cations such as molecular machines, similar to those investi-gated in ‘‘blue box’’ systems.3 Accordingly, two-dimensionaltiling of P5A-based pseudorotaxanes can maximize the expres-sion of switchable surface functionalities. Therefore, under-standing P5A-based pseudorotaxane monolayers at the air–waterinterface is expected to open new avenues for various applications.In this study, we found that pseudorotaxanes, which iscomposed of ethoxy-functionalized pillar[5]arene (EtP5A) andlinear fatty acid guests, were oriented and assembled in amonolayer at the air–water interface. The chemical structureof the fatty acid guest dictates the resulting monolayer struc-ture; short-chain linear fatty acids lead to [2]pseudorotaxanemonolayers, long-chain linear fatty acids form [3]pseudorotax-ane monolayers, and a fatty acid guest unsuitable for EtP5Aundergoes phase separation (Fig. 1a).To elucidate the monolayer state of Langmuir films at theair–water interface, we measured the surface pressure–area(p–A) isotherms (Fig. 1b). Pure EtP5A exhibited a lift-off ofthe surface pressure near 1.3 nm2 and monolayer collapse at10.3 mN m�1. Interestingly, given the good agreement with thetheoretical calculations (1.42 nm2), the pure EtP5A with simpleethoxy substitution was found to orient its cavities perpendi-cular to the water surface, as observed in previous studies usingpillararene with longer and more complex substitution.15,16The relatively low collapse pressure (pC) suggested weak inter-molecular interactions within the plane. To obtain a stable andoriented pseudorotaxane monolayer, we mixed a chloroformsolution with amphiphilic G5-18, which has been widelyemployed in insoluble monolayer studies.8,17 Mixing with G5-18 at a molar ratio of 1 : 1 decreased the limiting cross-sectionalarea (AL) per EtP5A molecule from 1.28 nm2 to 0.91 nm2, asa Research Center for Materials Nanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), Tsukuba, Ibaraki, Japanb Graduate School of Science and Technology, Tokyo University of Science, Noda,Chiba, Japan. E-mail: 7221701@ed.tus.ac.jpc Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto, Japand Institute for Chemical Research, Kyoto University, Uji, Kyoto, Japane WPI Nano Life Science Institute, Kanazawa University, Kakuma-machi, Kanazawa,Japanf Graduate School of Frontier Science, The University of Tokyo, Kashiwa, Chiba,Japan† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5cc01040cReceived 26th February 2025,Accepted 5th June 2025DOI: 10.1039/d5cc01040crsc.li/chemcommChemCommCOMMUNICATIONOpen Access Article. Published on 05 June 2025. Downloaded on 7/4/2025 10:46:35 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View Issuehttps://orcid.org/0009-0008-6846-9610https://orcid.org/0000-0001-7966-3197https://orcid.org/0000-0001-5574-9869https://orcid.org/0000-0002-2445-2955http://crossmark.crossref.org/dialog/?doi=10.1039/d5cc01040c&domain=pdf&date_stamp=2025-06-10https://doi.org/10.1039/d5cc01040chttps://doi.org/10.1039/d5cc01040chttps://rsc.li/chemcommhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5cc01040chttps://pubs.rsc.org/en/journals/journal/CChttps://pubs.rsc.org/en/journals/journal/CC?issueid=CC061055This journal is © The Royal Society of Chemistry 2025 Chem. Commun., 2025, 61, 10138–10141 |  10139calculated by extrapolating the p–A isotherms. The reduction inmolecular area by as much as 0.37 nm2 does not occur if EtP5Aand G5-18 form no complexes or 1 : 1 complexes, suggesting theformation of [3]pseudorotaxane structure at the air–water inter-face. Note that the AL per EtP5A molecule of 0.91 nm2 is closeto the sum of one-half of the AL of 1.28 nm2 for a single PAmolecule and the AL of 0.2 nm2 for an alkyl chain, whichsupport [3]pseudorotaxane formation. Furthermore, the [3]pseudo-rotaxane structure might contribute to the formation of a morein-plane rigid film as confirmed by the significant increase in pC of43.2 mN m�1 compared to 10.3 mN m�1 of pure EtP5A. On theother hand, when mixed with G6, a guest molecule incompatiblewith the cavity size of EtP5A, p–A isotherm shifted toward largermolecular areas corresponding to the cross-sectional area of G6,with minimal changes in pC compared to pure EtP5A. This behaviorindicates phase separation within the two-dimensional film.18Similar phase separation was observed when EtP5A and G5-18were sequentially spread on the water surface (Fig. S6, ESI†). Notethat host–guest chemistry strongly influences these phenomena,as demonstrated by the formation of pseudorotaxanes with G6and phase separation with G5 when using EtP6A as the host(Fig. S7, ESI†).To investigate the molecular orientation and pseudoro-taxane formation, we performed multiple-angle incidenceresolution spectroscopy (MAIRS) measurements on LB filmstransferred from the water surface. This method involves tiltingthe thin film relative to the optical axis and analyzing theinfrared absorption spectra acquired at varying polarizationangles.19 The MAIRS method can separate the out-of-plane (OP)and in-plane (IP) absorption components of the film materials.Unlike traditional approaches that combine transmission andreflection methods,17 the MAIRS method offers the advantageof measuring one identical film with fewer substrate con-straints. EtP5A exhibited similar absorption characteristics inboth the powder and LB films (Fig. 1c and d). According todensity functional theory (DFT) calculations, the most intenseabsorption bands at 1502 cm�1 and 1408 cm�1 were mainlyconsisted of aromatic C–H bending, aromatic C–H stretching,CH2 wagging, and CH3 wagging vibrations (Section 1.4, ESI†).The strong transition moments along the cavity direction ofEtP5A can be useful for assessing the molecular orientation.For both LB films of pure EtP5A and the mixture of EtP5A andG5-18, these vibrational bands were more prominent in the OPspectrum than in the IP spectrum, indicating the verticalorientation of the EtP5A cavity, both at the air–water interfaceand after transfer to solid substrates. This vertical orientationof EtP5A corresponds to the fact that the AL obtained by p–Aisotherm is close to the calculated cross-section value ofFig. 1 Pseudorotaxane of EtP5A at the air–water interface. (a) Molecular structures and schematics of monolayer states for employed EtP5A host andguests. (b) p–A isotherms for pure EtP5A (black line), mixture with G5-18 (red line) and with G6 (blue line). (c) DFT-calculated IR spectrum of EtP5A withexperimental one of the powder (Section 1.4, ESI†). MAIRS spectra of LB films for pure EtP5A, mixture with G5-18 and pure G5-18 in the 1550–1300 cm�1(d) and 3000–2800 cm�1 (e) regions. The OP (pink line) and IP (light blue line) indicate the out-of-plane and in-plane directions of the LB films,respectively.Communication ChemCommOpen Access Article. Published on 05 June 2025. Downloaded on 7/4/2025 10:46:35 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5cc01040c10140 |  Chem. Commun., 2025, 61, 10138–10141 This journal is © The Royal Society of Chemistry 20251.42 nm2 for pure EtP5A. Additionally, pure G5-18 exhibitedcharacteristic vibrational peaks due to CH2 trans-zigzag packingowing to its crystallinity, specifically CH2 antisymmetricstretching vibrations (2920 cm�1) and CH2 symmetric stretch-ing vibrations (2850 cm�1) (Fig. 1e).20,21 Upon mixing withEtP5A, these peaks shifted to higher wavenumbers, indicatingthat the alkyl chains adopted a gauche conformation andsupporting the formation of inclusion complexes with EtP5A.Considering that the formation behavior of EtP5A-basedpseudorotaxanes depends on the chain length of the guestmolecules,12 the monolayer state was investigated using guestswith carbon numbers ranging from 14 to 26, which canindependently form insoluble monolayers. In this study, themolecules were mixed at a 1 : 1 molar ratio before spreading tothe air–water interface. The p–A isotherms for each system,guest-dependent AL, and guest-dependent collapse areas and pCare shown in Fig. 2a, b, and c, respectively. All the molecularareas were calculated per EtP5A molecule. Compared to pureEtP5A, the pC for all mixed systems increased, suggesting thatpseudorotaxane formation and concomitant lateral hydropho-bic interactions between alkyl chains improved monolayerrigidity. The constant AL and pC indicate that EtP5A forms anearly identical two-dimensional film when mixed with guestshaving carbon numbers greater than 18, where [3]pseudorotax-ane is stabilized as discussed later. In contrast, the mixedsystems with G5-14 and G5-16 exhibited larger AL and lowerpC than those with G5-18 and other guests. Note that a similarcollapse area of 1.20 nm2 for pure EtP5A and the mixture withG5-14 implies that the mixed system mainly consists of a 1 : 1complex, that is, [2]pseudorotaxane. The gradual shift in the ALwith increasing carbon number indicates that the stoichiometry ofpseudorotaxane formation depends on the carbon number of theguest molecules.12 In other words, it is plausible that the composi-tion of the monolayers—[2]pseudorotaxane, [3]pseudorotaxane,and uncomplexed molecules—varies depending on the type ofguest molecule, given that the p–A isotherms provide averageinformation about the Langmuir films on the water surface, whichextend over an area of several hundred cm2.To assess the effect of the molecular mixing ratio, weanalyzed p–A isotherms for a mixture with G5-20, which has asufficiently long carbon chain to stabilize the monolayer state,as shown in Fig. 2b and c. Here, when nA represents the numberof molecules of A, the host–guest stoichiometric ratio (SR) isdefined as nG5/nEtP5A in the mixing solution. The horizontal axisin Fig. 3a and the AL in Fig. 3b are plotted against the area perEtP5A molecule. Given that the collapse behavior of Langmuirfilms depends on the compressibility of the entire film, thecollapse area is represented as a mean value for all mixedmolecules (Fig. 3c). As expected, p–A plots approached thecharacteristics of pure EtP5A (or pure G5-20) as the stoichio-metric ratio approached zero (or infinity) (Fig. S9, ESI†). Inter-estingly, the AL per EtP5A molecule and the collapse behaviorexhibited a clear bifurcation at a stoichiometric ratio of 3/7.Within each of the two groups, the AL can be approximated by alinear trend with an intersection of approximately 0.33. Thedecrease in the molecular area of binary Langmuir films isoften indicative of specific attractive interactions.8,22 In thissystem, such a trend strongly suggests pseudorotaxane for-mation. Assuming that all G5-20 molecules participate incomplex formation under excess conditions of EtP5A (e.g.,SR = 1/9, 2/8, 3/7), the number of EtP5A molecules involvedin each pseudorotaxane can be estimated from the AL. Theresults are 2.1, 2.3, and 2.0 EtP5A molecules, respectively,strongly indicating that [3]pseudorotaxane is the dominantspecies in the mixed monolayer films. Conversely, under con-ditions where G5-20 is in excess, the decrease in area surpassesthe value expected from molecular interactions associated withpseudorotaxane formation, making it challenging to describethe behavior with a simple additivity rule. Further investiga-tions are necessary to elucidate this phenomenon. Nonetheless,the identical collapse behavior implies that the monolayersinclude [3]pseudorotaxane.In conclusion, we systematically investigated monolayers ofethoxy-substituted P5A and mixtures with fatty acid guests.Linear fatty acid guests lead to pseudorotaxane monolayerswith P5A cavities oriented toward the water surface. The mixingratio and chain length of the guests influenced the stoichiometryof pseudorotaxane formation and monolayer composition. Thisstudy provides insight into P5A-based pseudorotaxane mono-layers for various applications. The simple procedure employedfor monolayer formation also offers the possibility of introdu-cing functional properties by appropriately selecting host andguest molecules. Furthermore, by removing guest moleculesafter the formation of aligned pseudorotaxane films, thesematerials can be explored for applications in sensors andfilters.23–25Fig. 2 Pseudorotaxane monolayers depending on the guest alkyl length. (a) p–A isotherms for mixtures of EtP5A and G5-n. (b) Correlation betweenextrapolated limiting area per EtP5A molecule and guest carbon number. (c) Characteristics of monolayer collapse for mixtures of EtP5A and G5-n.ChemComm CommunicationOpen Access Article. Published on 05 June 2025. Downloaded on 7/4/2025 10:46:35 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5cc01040cThis journal is © The Royal Society of Chemistry 2025 Chem. Commun., 2025, 61, 10138–10141 |  10141M. I. and Y. N. conducted comprehensive experiments andanalyses, with the support of Y. Y. The synthesis of pillarareneswas carried out by K. O. and T. S. under the supervision of T. O.The MAIRS measurements were overseen by T. H. and N. S. Thisstudy was conducted by K. A. and H. S., and T. M. M. I. wassupported by JST, the establishment of university fellowshipstowards the creation of science technology innovation(Grant Number: JPMJFS2144). This work was also supportedby KAKENHI (Grant Numbers: JP25H00898, JP23H05459 andJP23K04703). The calculations in this study were performed onthe Numerical Materials Simulator at NIMS.Data availabilityThe data supporting this article have been included as part ofthe ESI.†Conflicts of interestThere are no conflicts to declare.References1 M. Xue, Y. Yang, X. Chi, X. Yan and F. Huang, Chem. Rev., 2015, 115,7398–7501.2 J. D. Badjic, V. Balzani, A. Credi, S. Silvi and J. F. Stoddart, Science,2004, 303, 1845–1849.3 Y. Liu, A. H. Flood, P. A. Bonvallet, S. A. Vignon, B. H. Northrop, H.-R.Tseng, J. O. Jeppesen, T. J. Huang, B. Brough, M. Baller, S. Magonov,S. D. Solares, W. A. Goddard, C.-M. Ho and J. F. Stoddart, J. Am. Chem.Soc., 2005, 127, 9745–9759.4 E. Katz, O. Lioubashevsky and I. Willner, J. Am. Chem. Soc., 2004,126, 15520–15532.5 M. J. Langton and P. D. Beer, Acc. Chem. Res., 2014, 47, 1935–1949.6 S.-Y. Chou, H. Masai, M. Otani, H. V. Miyagishi, G. Sakamoto,Y. Yamada, Y. Kinoshita, H. Tamiaki, T. Katase and H. Ohta,et al., Appl. Catal., B, 2023, 327, 122373.7 P. Wu, B. 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Mater.,2024, 34, 2312304.Fig. 3 Stoichiometric analysis of pseudorotaxane at the air–water interface. (a) p–A isotherms for mixtures of EtP5A and G5-20. (b) Correlation betweenextrapolated limiting area per EtP5A molecule and stoichiometric ratio. (c) Characteristics of monolayer collapse for mixtures of EtP5A and G5-20.Communication ChemCommOpen Access Article. Published on 05 June 2025. Downloaded on 7/4/2025 10:46:35 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5cc01040c