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Masaki Ishii, Yuto Nakai, Shion Kaneko, Kohei Tanaka, [Yu Yamashita](https://orcid.org/0000-0001-7966-3197), Kenichi Sakai, Hideki Sakai, [Katsuhiko Ariga](https://orcid.org/0000-0002-2445-2955), Masaaki Akamatsu

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This document is the Accepted Manuscript version of a Published Work that appeared in final form in Langmuir, copyright © 2024 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.langmuir.4c03957.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Mechanoelectrical Transduction through Anion Recognition with Naphthalenediimide Monolayers at the Air–Water Interface](https://mdr.nims.go.jp/datasets/a2bc8d01-0f87-4fe7-af18-6eeeb0bd799e)

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Mechano–electrical transduction throughanion recognition with naphthalenediimidemonolayers at the air–water interfaceMasaki Ishii,∗,†,‡ Yuto Nakai,†,‡ Shion Kaneko,†,‡ Kohei Tanaka,‡ Yu Yamashita,†Kenichi Sakai,‡ Hideki Sakai,‡ Katsuhiko Ariga,†,‡,¶ and Masaaki Akamatsu∗,‡,§†Research Center for Materials Nanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan‡Graduate School of Science and Technology, Tokyo University of Science, 2641 Yamazaki,Noda, Chiba 278-8510, Japan¶Department of Advanced Material Science, Graduate School of Frontier Science, TheUniversity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan§Department of Chemistry and Biotechnology, Faculty of Engineering, Tottori University,Tottori, Tottori 680-8552, JapanE-mail: ISHII.Masaki@nims.go.jp; makamatsu@tottori-u.ac.jpAbstractIn biological systems, various stimuli and en-ergies are transduced into membrane poten-tials via ion transport or binding. The appli-cation of this concept to artificial devices mayrealize biomimetic signal transmitters and en-ergy harvesters. In this study, we investigatedthe mechanical control of fluoride anion recog-nition with naphthalenediimide (NDI) mono-layers at the air–water interface. Similar tothe mechanosensitive ion channels in biologicalmembranes, mechanical stimuli modulated thepacking manner of the NDI monolayers, whichreproducibly triggered anion binding and con-comitant shifts in the membrane potential. Fur-thermore, mechanical stimuli resulted in anionbinding or release depending on the structureof the alkyl side chains attached to the NDImolecule, which was explained by the differencein the packing manner of the NDI monolayers.These findings provide insights into the devel-opment of novel mechano–electrical transduc-tion systems that mimic biological processes.IntroductionMembrane potential plays a significant role inliving things, for example, in generating theenergy-carrying molecule ATP, sensing environ-mental stimuli, communicating various types ofinformation, and performing physical motion.1Exploiting the membrane potential system froma biomimetic perspective is expected to fabri-cate a variety of high-performance artificial de-vices. In particular, one important proposal isenergy conversion systems of mechanical stim-uli, i.e., kinetic energy, into electrical energy,which have attracted significant attention in re-cent years for energy harvesting to acceleratecarbon-neutral and Internet of Things (IoT) so-cieties.2,3As shown in Fig. 1, there are several en-ergy conversion mechanisms including the well-known piezoelectric effect, where mechanicalstimuli applied to atoms with long-range or-der in a crystal induces polarization throughrelative atomic displacement.4 In 2012, the tri-boelectric nanogenerator was reported, whichgenerates a potential difference by utilizing elec-1ATOMIC DISPLACEMENTELECTRON TRANSFER AT INTERFACESMEMBRANE FLUIDITYPressedReleasedExpandedThis studyCompressed-- -- - -++++++-- ----++++++Tension++ ++++++ ++++++++++Piezoelectric materialsTriboelectric materialsBiological membraneii-- -+++StretchedPressed-- -+++++++++ +++- - --- ----+-++++++++ +++- - --- --InitialFigure 1: An overview of mechano–electrical transduction mechanisms for energy harvesting.tron transfer induced by friction at the interfacebetween different film materials.5 Unlike thesesystems observed in solid materials, mechano–electrical transduction phenomena with a fluid-ity have been observed only in biological mem-branes. For example, Piezo1 and Piezo2 arewell-known mechanosensitive ion channels6,7 in-volved in many physiological functions suchas light touch sensation, neuronal differentia-tion and erythrocyte volume regulation.1 Haircells in the cochlea of the inner ear also de-tect sound stimuli and transmit signals, whichis also driven by mechanosensitive gating of ionchannels.8Molecular recognition and passive diffusion ofions are critical driving forces for precise con-trol of membrane potential in biological sys-tems. Langmuir monolayers with compositionssimilar to those of biological membranes havebeen extensively studied as suitable models forevaluating and controlling molecular recogni-tion at the air–water interface. Many stud-ies have investigated specific interactions be-tween ligands and proteins, including antibod-ies and enzymes.9 We have tried to finely con-trol various molecular recognition processes us-ing mechanical stimuli.10–14 This was achievedby continuously adjusting the physical shape ofbinding sites contributing to molecular recogni-tion, which can tune the binding constants withtarget species. Through such structural opti-mization of the binding sites, highly selectivemolecular recognition phenomena were demon-strated to successfully mimic the natural en-zyme properties represented by the lock-and-key model. One representative research is adistinction between thymine and uracil, whichdiffer by only one methyl group.13 High re-producible mechano-luminescence was also re-alized as the example of information conversionfrom mechanical stimuli into change in molec-ular electric states through molecular recogni-tion.10,11Considering that ion motion effectivelychanges the membrane potential in biologicalsystems, mechanically induced tuning of ionicrecognition should be a key technique to artifi-cially produce mechano–electrical transductionof a fluidic system. We have recently employednaphthalenediimide (NDI) derivatives15,16 toinvestigate the specific interaction with anions,i.e., anion–π interactions, which affect proteinhigher-order structure formation and enzymereaction mechanisms.17–19In this study, anion recognition in two typesof NDI monolayers was controlled in responseto mechanical stimuli at the air–water inter-face, which was quantitatively evaluated by2the charged membrane potential. The pack-ing manner of NDI molecules dominated anionrecognition, analogous to ionic channels in bi-ological systems, indicating that different sidechains functioned as determining factors for me-chanically induced membrane potentials. Thesynthesized n-NDI with a saturated alkyl sidechain has a high tendency to interact with eachother, which facilitates molecular packing witha vertically aligned NDI core even without suffi-cient mechanical stimuli. The packing manneris so advantageous for anion–π interaction toprepare sandwich structure,20,21 where the n-NDI monolayer is charged at a large moleculararea. At the small molecular area, mechanicallyinduced phase transition of n-NDI monolayerled to pushing anions out, i.e., discharging. Onthe other hand, iso-NDI with a branched alkylside chain showed the opposite trend; a lowpacking tendency due to the steric hindranceof the side chain offered no anion recognitionat the large molecular area, although chargingbased on anion recognition was performed bythe formation of a sandwich structure at thesmall molecular area. To the best of our knowl-edge, this is the first study to demonstrate an-ion binding and release with high reproducibil-ity using a fluidic system. Therefore, this studymay provide a novel platform for mechano–electrical transduction utilizing the membranefluidity of artificial monolayers.Results and discussionFundamental Analyses for PureNDI MonolayersThe amphiphilic NDI derivatives employed inthis study possess the hydrophilic part of thetetraethylene glycol group and the hydrophobicpart of the alkyl groups introduced at the imideN-positions of the NDI core. In order to inves-tigate the effects of alkyl chain structures onthe phase behavior and anion recognition prop-erties of NDIs, we synthesized n-NDI, iso-NDI,and cis-NDI with C8 alkyl chains that are satu-rated, branched, and unsaturated, respectively(Fig. 2a and see Supporting Information).Surface pressure–molecular area (π–A)isotherms were measured to analyze fundamen-tal monolayer characteristics, such as the two-dimensional phase and cross-sectional area ofeach molecule (Fig. 2b). A kink was observed atca. 30mN/m only for n-NDI, suggesting phasetransition from phase (ii) to (iii) (the defini-tion is described below). This phase transitionphenomenon upon mechanical compression isconsistent with previous results for saturatedalkyl chains with carbon numbers 6 and 12.16Molecular cross-sectional area of n-NDI was es-timated to be 0.36 nm2 by extrapolating π–Acurve. This value is close to the NDI core cross-sectional area of 0.31 nm2 calculated using den-sity functional theory (Fig. S8), suggesting thatthe NDI core is close-packed and nearly ver-tically oriented adjacent to each other in thephase (iii).Unlike n-NDI with a saturated octyl group,the π–A isotherm of iso-NDI with a branched2-ethylhexyl group showed no phase transi-tion upon mechanical compression, indicatingthe formation of only one aggregated phase(Fig. 2b). The extrapolated value of the π–Acurve was 0.45 nm2, indicating that the sterichindrance of the branched alkyl chain preventsthe dense packing of the NDI core. For cis-NDIwith an unsaturated octenyl group, the surfacepressure increased from a very small molecu-lar area. The extrapolated value of the π–Acurve was 0.22 nm2, which is comparable to thecross-sectional area of the all-trans conforma-tion of the alkyl chain (0.2 nm2),22,23 but muchsmaller than the NDI core cross-sectional areaof 0.31 nm2. This suggests that cis-NDI canpartially dissolve in the aqueous subphase dueto the unsaturated side chain with relativelyhigh polarity (Fig. S9, 10). We concluded thatdetailed monolayer analysis for cis-NDI is dif-ficult and worthless as well as carboxylic acidswith unsaturated side chains.24 Therefore, thisstudy focused on the comparison between n-NDI and iso-NDI.For further investigation, monolayers weretransferred onto quartz substrates at predeter-mined surface pressures using the Langmuir–Blodgett (LB) technique. The UV–vis ab-sorption spectra of the LB films are shown3ac  n-NDIWavelength (nm)Normalized absorbance (a.u.)330 360 390 42010 mN/m30 mN/md  iso-NDIWavelength (nm)Normalized absorbance (a.u.)In solutionLB film (10 mN/m)LB film (40 mN/m)HydrophilicNDIn-NDI iso-NDI cis-NDIHydrophobicb0.1 0.2 0.3 0.4 0.5 0.6 0.7Molecular area (nm2)010203040506070Surface pressure (mN/m) n-NDIiso-NDIcis-NDI330 360 390 420384389383378Figure 2: Assembling behavior of NDI derivatives at the air–water interface. (a) Chemical structuresof NDIs with different hydrophobic moieties. (b) π–A isotherms for NDIs with indications offixed surface pressures for transferring monolayers. (c) UV–vis spectra of n-NDI in acetonitrilesolution (black line), in LB films transferred at 10mN/m (orange line) and 40 mN/m (red line). (d)UV–vis spectra of iso-NDI in LB films transferred at 10mN/m (light blue line) and 30mN/m (deepblue line). Absorbance intensities are normalized against the value for a 0–0 transition around390 nm.in Fig. 2c, d. Similar to other types of NDIderivatives, characteristic peaks around 390 nmand 370 nm are derived from the 0–0 and 0–1 vibronic transitions of the NDI core, re-spectively.21,25 For n-NDI, the 0–0 vibronictransition peak of LB film transferred at10mN/m were observed at longer wavelengthof 383 nm compared to that in acetonitrile so-lution (378 nm), suggesting the formation of J-aggregates in LB films.26,27 Further red-shiftedpeak was observed for n-NDI LB film trans-ferred at 40mN/m, which exceed phase transi-tion point of 30mN/m (Fig. 2b). This can beexplained by the mechanically induced changein the molecular arrangement and orientation,i.e., the packing manner, which modified theexcitonic coupling28,29 in adjacent NDI cores.However, iso-NDI exhibited no peak shift withincreasing surface pressure, which is consistentwith the absence of a kink in π–A curve of iso-NDI. The π–A isotherm and absorption spectraindicate that the monolayer phase and pack-ing manner of iso-NDI did not change undermechanical compression. As previously con-firmed with a bromine-introduced NDI deriva-tive,16 the introduction of bulky substituentssuppresses changes in the phase of NDI mono-layers upon mechanical compression. Thus, thedifferences in the alkyl chains allowed us tocontrol the packing manner of the NDIs uponmechanical compression.Anion Recognition Properties ofNDI MonolayersTo evaluate the anion recognition behaviors ofNDI monolayers with mechanical compression,we employed an electrolyte subphase dissolving4a  n-NDIMolecular area (nm2)∆VNaF-∆VW (V)-0.2-0.100.10.2Surface pressure (mN/m) 0.2 0.4 0.6 0.8 1 1.2 1.4Surface potential ∆V (V) 0.10.20.30.40.50.60.70102030405060700Pure waterNaF solution(i)(ii)(iii)b  iso-NDI-0.2-0.100.10.2∆VNaF-∆VW (V)0.2 0.4 0.6 0.8 1.0 1.2 1.4Molecular area (nm2)Surface pressure (mN/m) Surface potential ∆V (V) 0.10.20.30.40.50.60.70102030405060700Pure waterNaF solution(i)(ii)Figure 3: Surface potentials of NDI monolayers depending on anion recognition. The surfacepressures and surface potentials are plotted as a function of the molecular area for (a) n-NDI and(b) iso-NDI on water with or without NaF (0.1M). Differences in the surface potentials underthe conditions of pure water and NaF solution are also shown. The surface pressure at the samemolecular area differs from that in Fig. 2, which can be attributed to the effect of compressionspeed.30 However, we emphasize that reproducibility under identical conditions was thoroughlyconfirmed.the fluoride anion (F−) at 0.1M, which can beselectively sensed by NDIs rather than by di-rect electron transfer.31,32 As shown in Fig. 3a,the π–A profile of n-NDI shifted to the largermolecular area with the addition of F− whenthe surface pressure is less than phase transi-tion point. This expansion of the n-NDI mono-layer indicates that F− was taken up by themonolayer driven by the anion–π interaction.16The increased amount in measured moleculararea due to anion binding was 0.10 nm2, ap-proximately twice the ion cross-sectional areaof 0.056 nm2 calculated using the effective ionicradius of F− (1.33Å) according to Pauling.33This can be attributed to the efficient binding ofF− into the n-NDI monolayer through anion–πinteraction and the electrostatic repulsion be-tween densely bound anions. Such efficientanion binding could be due to the controlledmolecular arrangement and orientation at theair–water interface, where translational and ro-tational motions of molecules are restricted intwo dimensions, and the spontaneous molecularpacking with a vertically aligned NDI core con-tributes to anion–π interactions to prepare thesandwich structure.20,21 Suppressed phase tran-sition also supported anion recognition whenthe surface pressure was maintained even closeto critical point (Fig. S11). Note that adsorp-tion of OH− to the n-NDI monolayer can benegligible considering that pH of NaF aqueoussolution was 7.89 in this study and the con-centration of F− was significantly higher by ap-proximately five orders of magnitude than OH−(see Supporting Information section 1.6). In-terestingly, the π–A profiles of n-NDI with or5without NaF overlapped in the phase (iii) af-ter the phase transition, indicating mechani-cally induced anion release. Furthermore, π–Aisotherm of iso-NDI showed no change in thepresence of F−.In-situ surface potential measurements wereperformed simultaneously with usual surfacepressure measurement to investigate molecu-lar arrangement and orientation23,34 of NDIsand anion recognition property of NDI mono-layers in more detail. The surface potentialchange (∆V ) of the Langmuir film is repre-sented relative to the potential of pure wateras follows: ∆V = ∆Vp + Φ0, where ∆Vp isthe potential due to the permanent dipoles ofthe film molecules, and Φ0 is the potential fromthe electric double layer. Generally, the sur-face potential of Langmuir films with electron-donating alkyl chains increases positively withincreasing surface pressure,23,34–36 allowing dis-cussion on molecular density, orientation, hy-dration state, and interactions with chemicalspecies in the subphase. For n-NDI on purewater, the surface potential began to rise at ap-proximately 0.69 nm2, where the surface pres-sure was still zero (Fig. 3a). This increase inthe surface potential indicates an orientationchange of n-NDI from a irregular and randommanner to a vertically aligned one, which canbe derived from island formation with no sur-face pressure, similar to conventional insolu-ble film molecules showing strong intermolec-ular interactions.37,38 The slope of ∆V -A curvechanged around 0.55 nm2, where the surfacepressure began to rise as the n-NDI moleculescovered the entire water surface. For the Lang-muir monolayer, the Helmholtz equation de-scribes the relationship between the surface po-tential and the apparent molecular dipole mo-ment µa which is modeled as a linear com-ponent: µa = ε0A∆V , where ε0 is the vac-uum permittivity.34,36 At molecular areas below0.55 nm2, apparent dipole moment of n-NDIcontinuously decreased (Fig. S12), suggesting asignificant influence of mutual polarization ofdipoles and rearrangement of the hydrophilicparts, commonly observed in high compressionmodulus films.34,35 Therefore, the orientation inthe phase (iii) of n-NDI using surface potentialis not discussed here.In addition to the increase in surface pres-sure discussed above, the comparison of sur-face potentials for n-NDI monolayer in corre-spondence with NaF addition into the subphaseconfirmed that n-NDI monolayer can bind F−at a large molecular area, although it releasesF− at a small molecular area. As shown inFig. 3a, with the addition of NaF to the sub-phase, the surface potential of n-NDI increasedfrom molecular area of 1.07 nm2, approximatelythree times the area of the close-packed struc-ture (0.36 nm2). At the air–water interface, thetranslational and rotational motion of n-NDIis surpassed, so that anions can synergisticallypromote the formation of an effective sandwichstructure for anion–π interaction,20,21 leadingto an orientation change of n-NDI in a verticallyaligned manner. Indeed, apparent dipole mo-ment for n-NDI on NaF solution remained con-stant in the range of 0.70-0.95 nm2 (Fig. S12),indicating almost no change in molecular orien-tation, while the monotonic increase in surfacepotential represents an only increase in molec-ular density. Surface potential measurement isa worthwhile way to evaluate anion-binding be-havior even without changes in surface pressure.Applying sufficient mechanical compression tocause the phase transition to the phase (iii) withclose-packed NDI cores led to overlapping ofboth the surface pressure and surface potentialfor the n-NDI monolayer on water with or with-out NaF, indicating the release of anions uponmechanical compression.Interestingly, the iso-NDI monolayer showedan opposite trend to that of n-NDI in termsof the molecular area-dependent anion recogni-tion behavior. As shown in Fig. 3b, the surfacepotential of iso-NDI monolayer was almost in-dependent of existence of F− in the subphaseat large molecular areas above 0.96 nm2. Thesurface potential increased from 0.81 nm2 ow-ing to the molecular orientation change on purewater, while the increase started at a slightlylarger molecular area of 0.96 nm2 in the case ofadding NaF to the subphase. Similar to n-NDI,the contribution of the early sandwich struc-ture formation can promote orientation change.After the apparent dipole began to decrease6(i) Gas phase(iii) NDI close-packed phase (ii) Anion-binding phase     (NDI sparse-packed phase)- -- ----- -- - -- -- ---baiso-NDI/NaFaqiso-NDI/WMolecular area (nm2)0.2 0.4 0.6 0.8 1.0 1.2 1.4(i)(ii)n-NDI/NaFaqn-NDI/WMolecular area (nm2)0.2 0.4 0.6 0.8 1.0 1.2 1.4(i)(ii)(iii)Figure 4: NDI monolayer phases at the air–water interface. (a) Schematic illustration of monolayerphases for NDIs. (i) Gas phase:NDI molecules can be mostly isolated when the surface pressure iszero and molecular area is quite large. (ii) Anion-binding (NDI sparse-packed) phase: Putting NDImolecules close to each other makes the monolayer recognize anions by sandwich structures, result-ing in the more organized molecular orientation. (iii) NDI close-packed phase: a NDI monolayerpushes anions out through crystallization when a sufficient high surface pressure is applied. (b)Phase diagrams for each NDI monolayer with or without NaF at 20°C. The transition from phase(i) to phase (ii) was determined by the increase in surface potential caused by changes in molecularorientation, whereas the transition from phase (ii) to phase (iii) was determined by the kink in theπ–A isotherm.with the formation of phase (ii) at approxi-mately 0.7 nm2 (Fig. S12), i.e., after the changein molecular orientation settled, the surface po-tential difference ∆VNaF - ∆VW became nega-tive, reaching a minimum of ca. -0.2V (Fig. 3b).We highlight that the negative potential differ-ence is predominantly contributed by anion ad-sorption on the NDI core rather than on theethylene glycol chain (see Supporting Informa-tion section 1.7). Furthermore, the potentialdifference was maintained up to the collapsepressure of the monolayer, indicating that iso-NDI monolayer did not release F− upon me-chanical compression. In the phase (ii), iso-NDI was sparsely packed because of the sterichindrance of the bulky 2-ethylhexyl alkyl chain,which created extra space between the NDIcores. Compared with the phase (iii) of n-NDIwith close-packed NDI cores, the space can beroughly estimated as 0.09 nm2, which is suffi-cient to accommodate F− and is comparable tothe increased molecular area for n-NDI by theaddition of NaF to the subphase. Note thatthe similar values of the space between the NDIcores for both n-NDI and iso-NDI reasonablysupport the formation of sandwich structuresfor synergetic anion–π interaction. In addition,this scenario can explain the lack of significantchanges in π–A isotherm with or without NaFin the subphase.The abstracted phase schematics and phasediagrams are presented in Fig. 4 to overview theanion recognition behavior of NDI monolayers.At sufficiently large molecular areas, neither n-NDI nor iso-NDI interacted with anions, form-ing a gas phase (i) with random molecular ori-entation. With the mechanical compression ofthe monolayer, NDIs first form a sparse-packedphase or an anion-binding phase (ii) with aneffective sandwich structure for anion–π inter-action. The molecular area for phase transi-tion depended on the side chains of the NDImolecules and the existence of NaF in the sub-phase, as summarized in Table 1. Further com-7Table 1: Critical molecular areas for the phase transition of NDI monolayers.n-NDI iso-NDIphase transition (iii)–(ii) (ii)–(i) (ii)–(i)Pure water 0.42 nm2 0.69 nm2 0.81 nm2NaFaq 0.46 nm2 1.07 nm2 0.96 nm2pression induced other phase transitions of then-NDI monolayer to close-packed phase (iii),which was due to the low steric hindrance ofthe saturated alkyl chain. In this transition pro-cess, F− bound at the n-NDI monolayer shouldbe pushed out, accompanied by a change in thepacking manner. On the other hand, the iso-NDI monolayer showed no other phase tran-sition because of the high steric hindrance ofthe branched alkyl chain up to monolayer col-lapse. The sparse-packed structure can provideenough space between each molecule for anionaccommodation driven by anion–π interactioneven at small molecular areas. Therefore, theanion-binding capability and behavior of NDImonolayers can be controlled through molec-ular design and mechanical stimuli. Further-more, considering our previous report that theamount of anion binding varies depending onthe species and concentration of anions,16 op-timizing the molecular structure of NDI couldenable the tuning of its selectivity toward spe-cific target anions.Repeated Anion Recognition at theAir–Water InterfaceGiven that molecular recognition and relatedproperties, including mechano-luminescence inLangmuir monolayers, are reproducibly con-trolled by mechanical stimuli,10,11 we demon-strated the repeated anion binding and re-lease phenomena of identical NDI monolayersthrough successive compression–expansion pro-cesses. Almost equivalent profiles were ac-quired for each process for both surface pressureand surface potential (Fig. S13, Table S1). Themoderate hysteresis indicates that NDI mono-layers can bind and release F− upon mechani-cal stimuli, that is, anion recognition is repeat-able. Interestingly, the anion recognition be-havior of each NDI monolayer showed the op-posite trend, especially when compared at suf-ficiently small and large molecular areas. Then-NDI monolayer binds F− at a large molecu-lar area of 1 nm2, whereas the phase transitionforces it to release F− at a small molecular areaof 0.4 nm2, which is close to the cross-sectionalarea of the NDI core. On the other hand, theiso-NDI monolayer requires greater mechanicalcompression for anion binding and can continueto bind F− even at a small molecular area of0.4 nm2, which is due to decreased intermolec-ular interactions due to steric hindrance of thebranched alkyl chain. Therefore, the differencein the alkyl chain structures led to opposite an-ion recognition behaviors through mechanicalcompression and expansion processes.The surface potentials of the compressionand expansion processes are shown in Fig. 5to discuss mechano–electrical transduction phe-nomenon embracing reproducible anion recog-nition. The difference in the surface potentialof the n-NDI monolayer depending on the pres-ence of NaF in the subphase was obvious at1.0 nm2, whereas it was mainly equivalent at0.4 nm2. However, an obvious difference in thesurface potential for the iso-NDI monolayer wasobserved only at 0.4 nm2. As shown in Fig. 3,the amount and sign of the difference in sur-face potential depends on NDIs and mechani-cal stimuli: plus 0.2V for expanded n-NDI, mi-nus 0.2V for compressed iso-NDI. Note thatthe value should be affected by the complexcontribution of the strength and orientation ofthe NDI molecular dipole, the anion recognitionproperties of the monolayers and anion species;however, it can be optimized through moleculardesign and mechanical stimuli.8expansioncompressionexpansioncompressiona  n-NDIb  iso-NDI1 2 3 4 5 6 7 8 9 10 Process Number-0.100.10.20.30.40.50.60.70.8Surface potential (V)Water NaFaq1 2 3 4 5 6 7 8 9 10 Process Number-0.100.10.20.30.40.50.60.70.8Surface potential (V)Water NaFaqExpandedCompressedAnion-binding phaseGas phaseNDI close-packed phaseAnion-binding phaseExpandedCompressed-- -- - -- -- ---- -- ----- -- - -Figure 5: Repeated mechano-electrical transduction through anion recognition. Changes in thesurface potential through successive compression and expansion processes are summarized for (a)n-NDI and (b) iso-NDI monolayer. Odd process numbers indicate a large molecular area of1.0 nm2and even those with a small molecular area of 0.4 nm2. Non-zero offset of surface po-tentials by the existence of NaF in the subphase suggests that n-NDI forms anion-binding phase atlarge molecular area while iso-NDI does at small molecular area.Inspired by the function of mechanosensi-tive ion channels in biological membranes, con-trolled generation of a difference in the sur-face potential of NDI monolayers was demon-strated through anion recognition. To the bestof our knowledge, this is the first example ofa mechano–electrical transduction phenomenonwith adequate repeatability based on mem-brane fluidity. Our system can be applied to anactual device to produce a potential differenceas the next step, similar to recent mechano–optoelectronic molecular switches.39 A sensorfor specific anions can be manufactured by op-timizing the molecular structure and electrondeficiency of NDI molecules. Another applica-tion may be the XOR gate in digital circuits,40because our system involves two types of mono-layers that generate an opposite sign of a dif-ference in surface potential under applied me-chanical stimuli.ConclusionsIn this study, we demonstrated a novel ap-proach to mechano–electrical transduction withan artificial fluidic membrane through con-trolled anion recognition of NDI monolayers atthe air–water interface. NDI derivatives withdifferent side chains exhibited distinct pack-ing manners in response to mechanical stim-uli, which in turn influenced anion recogni-tion through the anion–π interaction. Specif-ically, n-NDI with saturated alkyl side chainformed three distinct two-dimensional phases:phase (i), (ii) and (iii) with increasing mechan-ical stimuli. Anion binding was observed exclu-sively in the phase (ii) formed under moderatecompression; further compression led to anion9release because of the change in the packingmanner. In contrast, iso-NDI with branchedalkyl side chain formed phase (i) and (ii).Achieving anion binding through phase tran-sition required greater compression in the iso-NDI monolayer than in the n-NDI monolayer.These findings reveal the opposite trend of an-ion recognition in each NDI monolayer withmechanical compression, which was quantita-tively evaluated using surface potential mea-surements. Successive compression and expan-sion processes for the NDI monolayers demon-strated that the surface potential can be repeat-edly modulated through precise anion recogni-tion. The mechano–electrical transduction ap-proach can be applied for fabricating actualdevices based on a mechanically induced ionrecognition monolayer.10ExperimentalMaterialsThe solvents and reagents were purchased fromTokyo Chemical Industry Co. Ltd. (Tokyo,Japan), FUJIFILM Wako Pure Chemical (Os-aka, Japan), Sigma-Aldrich Co. LLC (St.Louis, MO, US), Nacalai Tesque, Inc. (Kyoto,Japan), and GL Science Inc. (Tokyo, Japan)and used without further purification. Detailedinformation on the synthesis of NDI derivativesis provided in the Supporting Information.Monolayer formationEach NDI was dissolved in spectroscopic-gradechloroform (Dojindo Laboratories, Japan) at0.5mg/mL. The solution was spread ontodeionized (DI) water or a NaF-containing aque-ous subphase (0.1M) in an LB trough. Af-ter 15min of chloroform evaporation, the NDILangmuir film was compressed to the predeter-mined surface pressures. Surface pressure andcompression speed was calibrated using stearicacid, which is a widely utilized standard inLangmuir film studies. In the calibration step,reproducibility was confirmed under identicalconditions. The temperature of the subphaseswas controlled at 20.0± 0.2°C using a chiller.In-situ monolayer characteriza-tionsMeasurements of surface pressure–area, π–A,for Fig. 3-5 and surface potential–area, ∆V –Awere performed using a Langmuir trough witha surface potential sensor (KSV NIMA). Thetrough contained a 150 mL subphase and hada working area of 318× 73 = 2.32× 104 mm2.The compression speed of monolayers is fixedat 0.27mm/s.Monolayer transfer for absorptionspectroscopyMeasurements of surface pressure–area forFig. 2 were performed and LB films were pre-pared on a USI-3-777C3 Langmuir–Blodgettsystem (USI). The trough contained a 250 mLsubphase and had a working area of 334× 100= 3.34× 104 mm2. The compression speedof monolayers is fixed at 0.20mm/s. DI wa-ter (> 18.2Ω cm) was prepared using Pure-lab Option R7, Flex (ELGA). Compressedmonolayers were transferred onto quartz bypulling the preimmersed substrate at a rate of0.02mm/s. The quartz substrates were treatedby an ultraviolet–ozone process prior to mono-layer transfer. The absorption spectra weremeasured using a JASCO V-670 instrument.Acknowledgement M.I. was supported byJST, the establishment of university fellowshipstowards the creation of science technology in-novation (Grant Number: JPMJFS2144). Thiswork was also supported by KAKENHI (GrantNumbers: JP20H00392 and JP23H05459), theFoundation Oil & Fat Industry Kaikan, andthe Foundation for the Promotion of Ion En-gineering. We are grateful to Prof. TakeshiKondo and Dr. Toshifumi Tojo for supportingthe surface-potential measurements.Supporting Information Avail-ableThe Supporting Information is available free ofcharge at• Experimental details, synthetic proce-dures, characterization data, additionalπ–A isotherms, UV–vis absorption spec-tra, and DFT calculations (PDF)References(1) Wu, J.; Lewis, A. H.; Grandl, J. Touch,tension, and transduction–the functionand regulation of Piezo ion channels.Trends in biochemical sciences 2017, 42,57–71.(2) Vijayakanth, T.; Shankar, S.; Finkelstein-Zuta, G.; Rencus-Lazar, S.; Gilead, S.;Gazit, E. Perspectives on recent advance-ments in energy harvesting, sensing andbio-medical applications of piezoelectric11gels. Chemical Society Reviews 2023, 52,6191–6220.(3) Chen, K.; Ho, D. 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