Tuning Aqueous Supramolecular Polymerization by an Acid‐Responsive Conformational Switch

Abstract Besides their widespread use in coordination chemistry, 2,2’‐bipyridines are known for their ability to undergo cis–trans conformational changes in response to metal ions and acids, which has been primarily investigated at the molecular level. However, the exploitation of such conformational switching in self‐assembly has remained unexplored. In this work, the use of 2,2’‐bipyridines as acid‐responsive conformational switches to tune supramolecular polymerization processes has been demonstrated. To achieve this goal, we have designed a bipyridine‐based linear bolaamphiphile, 1, that forms ordered supramolecular polymers in aqueous media through cooperative aromatic and hydrophobic interactions. Interestingly, addition of acid (TFA) induces the monoprotonation of the 2,2’‐bipyridine moiety, leading to a switch in the molecular conformation from a linear (trans) to a V‐shaped (cis) state. This increase in molecular distortion along with electrostatic repulsions of the positively charged bipyridine‐H+ units attenuate the aggregation tendency and induce a transformation from long fibers to shorter thinner fibers. Our findings may contribute to opening up new directions in molecular switches and stimuli‐responsive supramolecular materials.


Introduction
Subtle conformational changes of biomacromolecules, triggered by physiological stimuli,r egulate various complex biological events, such as protein folding and membrane transport. [1] Chemists have long sought to replicate these phenomena by synthesizing artificial molecular switches [2] and machines [3] that show switching between two conformational states in response to external stimuli,s uch as light, [2b] redox, [2c] pH, [2d] or cation/anions. [2e] In recent years, different types of molecular switches derived from diarylethenes, [4a] spiropyrans, [4b] spirooxazines, [4c] fulgides, [4d] and flavylium [4e] have been widely explored,w ith some promising applications in bioimaging, [5a] drug delivery, [5b] organic light-emitting diodes, [5c] molecular electronics [5d] andc atalysis. [5e] In this regard, 2,2'-bipyridine represents an archetypale xample of am olecular switch that can changei ts conformationf rom the linear trans-state to the V-shaped cis-state by acid-induced protonation as well as by metal complexation. [6] To date, conformational switching of 2,2'-bipyridines has been mainly investigated at the molecular level, for example to obtainm olecular hinges [7a] and cavitands. [7b] However,t othe best of our knowledge,b ipyridines have not been exploited as conformational switches in supramolecular polymerization. We foresee that this concept would not only broaden the range of applications of bipyridines but also complement the existing arsenal of tools in stimuli-responsives upramolecular materials.
In this regard,e xploiting molecular conformational changes in response to external stimuli is ap romising way to tune selfassembly processes and create smart materials. [8] The most commonly used methodt oa chieve conformational switching is the use of light as external trigger,w hich allows precise control over the dimensionality and properties of supramolecular assemblies. [9] In contrast, the effect of ion-induced conformational changes on supramolecular polymerization has been comparatively less studied, for which 2,2'-bipyridine would be an ideal candidate. To date, research on bipyridine-based supramolecular polymers has been limited to coordination polymers, [10a] host-guest stimuli-responsive supramolecular poly- [a] Dr.C . Rest, Dr.V .Stepanenko Institut fürO rganische Chemie, UniversitätW ürzburg am Hubland 97078 Würzburg (Germany) mers, [10b-d] and cross-linkeds upramolecular polymers.
[10e-f] However,understanding how the ion-induced changes in molecular shape (trans vs. cis)a ffects self-assembly of bipyridine-based systemsremains elusive. Herein,w ed emonstrate the use of 2,2'-bipyridines as acidresponsive conformational switches to tune aqueous self-assembly processes. [11] To this end, we have designed ab olaamphiphilic derivative 1,i nw hich ac entral 2,2'-bipyridine unit is conjugated at the 4,4'-positions with oligophenyleneethynylene (OPE) fragments bearing hydrophilic triethyleneglycol (TEG) chains (Scheme 1, for synthesis andc haracterization, see the Supporting Information) Interestingly,w ef ound that the acid-induced trans-to-cis conformational change of the 2,2'-bipyridine unit, along with the electrostatic repulsion resulting from protonation, immensely impact the molecular packing and attenuate the aggregation tendency of 1,l eading to a transformation from long fibers to shorterthinner fibers.

Aqueoussupramolecular polymerizationo f1
Ligand 1 is readily soluble in moderately polar solvents, such as THF,dichloromethane and chloroform,suggesting amolecularly dissolved state in these media. However, in more polar, protic solvents such as methanol and water,t he solubility is decreased, which is af irst sign of aggregation. Figure 1a shows the solvent-dependent UV/Vis absorption measurements of 1 at c = 1 10 À5 m. As expected, the absorption spectra in most organic solvents exhibit asimilarshape with am aximum centered at % 336 nm, characteristico fam onomeric state. In aqueous solution, however,the maximum is redshifted GustavoF ernµndez received his PhD in 2009 from the UniversidadC omplutense de Madrid (Spain) under the supervisionofProfs.Nazario Martin and Luis Sanchezf or work on donoracceptor systems based on C 60 .H et hen joined the group of Prof.F rank Würthnera tt he Universityo fW ürzburg (Germany) as aH umboldt postdoctoral researcher working on merocyanine dye aggregates. Between December2 010 and September 2015,heh eadeda ni ndependent junior research group in the same institute focusing on the self-assemblya nd selfsortingo fp-systems.S ince September 2015, he is Professor of Organica nd Supramolecular Chemistry at the University of Münster (Germany), where he headsaresearchg roup working on the self-assembly and stimuli-responsive behavioro f functional BODIPY dyes and metal-containing p-systems.
to 345 nm and the spectrum broadens up to 425 nm suggesting strong intermoleculari nteractionso ft he OPE core. The solvent-dependent emission characteristicso f1 were investigated under the same conditions (Figure1band Figure S1). The maximum in chloroform is located at 448 nm, while it shifts to % 467 nm in THF andd ichloromethane. In contrast, the emission intensity is significantly reducedi na cetonitrile with a maximum at 503 nm, while it is almostq uenched in methanol and water.T his indicates ac lear solvatochromism for 1 that is typical in donor-acceptor-donor systems. [6e] With increasing polarity,t here is ab athochromic shift in the emission maximum ( Figure S1). The observed difference in intensity in emission in polar solvents like acetonitrile,m ethanola nd water can be understood from the differences in solubility of the molecule in these media as well as by the use of the same excitation wavelength (335 nm) for all solvents. The relative emission of 1 in different solvents becomes evident when comparing photographs of the respective solutions under UV light (inset of Figure 1b). The quantum yields were determined in different solvents using quinine sulphate as the standard, [12] andt he values are shown in Ta ble S1.
To unravel the aggregation mechanism of 1 in solution, variable-temperature (VT) absorption measurements were performed. Since the spectralf eatures in different solvents clearly suggest ah igh aggregation tendency of amphiphile 1 in aqueous environment, this mediumw as chosen to investigatet he self-assembly.B esides pure water,asolvent mixture containing 1% of THFw as used to facilitate the sample preparation. This preparation protocol allows to create stable aggregate solutions with invariant absorption spectra over the course of several weeks, so that possible kinetic effects can be discarded. The temperature-dependent studies of 1 at different concen-trations( 1.5-2.9 10 À5 m)i nb oth water and water/THF 99:1 resulted in identical spectralc hanges ( Figure S2), indicating that the small amount of THF has no significant influence on the self-assembly process. To understand the aggregation mechanism, at hin film of compound 1 (preparedb ye vaporation of 30 mLs tock solution in THF) was dissolved in water (c = 2.0 10 À5 m)a nd investigated by VT UV/Vism easurements (Figure 1c). The sample was heatedt oi ts molecularly dissolved state (353 K) and afterwards cooled down slowly (0.1Kmin À1 ) to ensure aggregation under thermodynamic control. It is worth noting that we did not observe al ower critical solution temperature (LCST) upon heatingt he sample. Upon cooling to 326 K, the absorption maximum undergoes ag radualr edshift from 334 to 346 nm through an isosbestic point at 325 nm. Additionally,t he overall absorption slightly increases and a new shoulderband at % 385 nm is observed upon cooling (Figure 1c). To analyze the underlying aggregation mechanism, the spectralc hanges at 385 nm werem onitored as af unction of temperature. The resulting cooling curves( plots of fraction of aggregated species( a agg )a gainst temperature) revealanonsigmoidal plot, characteristico facooperative supramolecular polymerization ( Figure 1d). Particularly relevanti st he fact that the cooling curvesd on ot appear to reach ac lear plateau when the aggregation is presumably complete ( Figure S3). A similar observation was reported by Meijer and co-workersf or the self-assembly of hydrogen-bonded OPVs, and was attributed to ag radualc onversion of the already formed one-dimensionals tructures into bundled assemblies. [13] To facilitatet he curve analysis, the non-sigmoidal plots were fitted to the cooperative nucleation-elongation model. [14] The thermodynamic parameters were determined as follows:e longation temperature (T e ) = 345 K; elongation enthalpy (DH e ) = À262.35 kJ mol À1 ; Gibbs free energy (DG 298 ) = À62.69 kJ mol À1 and the degree of cooperativity (s) = 8.4 10 À4 (Table S2). The cooperative nature of the self-assembly of 1 was also supported by UV/Vis denaturation experiments, [15] where the supramolecular polymers of 1 in water were gradually disassembled by addition of increasing aliquots of monomeric 1 in THF ( Figure S4 and Ta ble S3). Additionally,t he initial stages of the aggregation process (nucleation) of 1 werea lso monitored by diffusion ordered spectroscopy (DOSY) NMR experiments at variable temperatures (Figures S5 and S6).

Molecular conformationa nd morphology of 1b ys olid-state NMR analysis
It has been widely established that bipyridine derivatives preferentially exist in the energetically most stable transoid conformation in solution in neutralm edia. [6] In order to determine whether this is also the case for our system in its solid form, we performed solid-state NMR experiments ( Figure 2). The dried sample obtained after purification was used as such withouta ny further treatment for solids tate analysist oe nsure that the system is in thermodynamic equilibrium. Specifically, we have taken advantage of the 13 Ci sotropic chemical shift as ar eporter of conformational differences by 13 C{ 1 H} CP/MAS NMR spectra as shown in Figure 2. [16a] The application of 2D H-1 Hd ouble-quantum single-quantum (DQ-SQ) and 13 C{ 1 H} heteronuclear correlation (HETCOR)e xperiments are then used to probe the aggregation and stacking behavioro f1. [16b] From the overall line shape, width, and characteristic splitting of the 13 Cs ignals in Figure 2, it is evident that 1 contains two different species in the solid-state, probably because we have used the dried sample as such without any furthertreatment. In particular,t he 13 Cr esolution in the region 145-160 ppm allows us to differentiate between the cis and trans isomerso f1.T he 13 C signals that are most affected by ac onformational change from trans to cis-bipyridine are carbon signals 1a nd 5, as highlighted in Figure 2. Both 13 Cs ignals are shifted to higher ppm values for the cis isomer as supported by gas-phase DFTcalculations of 13 Cc hemical shifts fort he cis and trans monomer (see Ta ble S4). Furthermore, the calculations for 1,w here the glycol chains were substituted by methoxy groups( Figure S7), show that the trans-form is energetically favored by 24.7 kJ mol À1 ,w hich is in excellent agreement with the energetic differenceo fn on-substituted 2,2'-bipyridine. [6] Despite this highers tability of the trans form, deconvolution of the 13 C signals for carbon 5r eveals that the sample of 1 exists as a mixture of both cis-a nd trans-conformations,h owever with a substantial preference for the trans-state (cis/trans % 37:63).
The formation of more than one type of packingf or 1 in the solid-state can also be identified in the 2D solid-state NMR experiments (FiguresS9a nd S10). From the 2D 13 C{ 1 H} HETCOR NMR spectrum, the 13 Cs ignals from carbon positions 1a nd 5 of cis-1 and trans-1 give rise to an umber of different 1 Hp ositions ( Figure S9). These can be grouped into sets of 1 Hs ignals for trans-1 that are located at lower ppm valuesc ompared to cis-1.I tc an also be observed that the trans-form undergoes more pronounced p-p stacking interactions, leading to as hift of the 1 Hd ue to aromatic ring current effects of neighboring molecules of 1. [16c,d] Moreover,a na utocorrelation 1 H-1 Hs ignal of the protonb ound to carbon 5( cf. inset in Figure 2) can be observed in the 2D 1 H-1 HD Q-SQ NMR correlation spectrum only for trans-1,while this signalism issing for the cis-1 species (see red circle in Figure S10). These observations suggest the formation of 1D helical stacks for trans-1 with ar otational displacemento ft he monomeru nits along the fiber-growing direction (see cartoon in FigureS14). [16c] On the other hand, the lack of this correlation signal for cis-1 suggestsadifferent, possibly less ordereds tacking pattern. Thus, the resultsf rom solidstate NMR show that cis-1 and trans-1 are characterized by distinct aggregation behavior and molecular packing motifs, with cis-1 exhibiting less efficient aromatic interactions because of its more distortedc onformation.

Aggregate morphologyof1
The aggregate morphologyof1 was examined by transmission electron microscopy (TEM) on ac arbon-coated copperg rid. The TEM studies of concentrated samples (5.7 and 8.0 10 À4 m) of 1 in aqueouss olution (Figure 3a,b,a nd Figures S11a nd S12) reveal flexible, well-defined nanofibers with ah ighly uniform width of 4-5 nm. As evident from the images, the fibers are denselyp acked and have as trong tendency to bundle, as previously suggested by UV/Vis studies. It also becomes apparent that increasing the concentration of the aggregate solution leads to longer fibers, with al ength that can be estimated to be several hundredso fn anometers. Given that the trans-form of 1 is the major speciesf ormedi ns olution, [6] we can assume Figure 2. Solid-state 13 C{ 1 H} CP/MAS NMRs pectrum of 1 recordeda t9 .4 T with aM AS frequency of 11.0 kHz. The inset shows the assignment of the bipyridinecarbona toms. The grey spectrums hows the complete deconvolution. The signals corresponding to cis and trans isomers are showninb lue and green, respectively.Afull signal assignment of all 13 Cs ignals is giveni n Figure S8. that only this more stable and preorganized conformation is able to engage in supramolecular polymerization, whereas the less stable, distorted cis-form is present,i fa ta ll, only in trace amounts. As imilar behaviorh as been recently observed for self-aggregating Pt II complexes undergoing cis-trans coordination isomerism, [17] wheret he dormant cis-isomer is completely removedf rom the equilibrium due to the stabilization of the aggregation-active trans-form by aggregation and solvation effects. As imilar stabilization of the trans-state is expected here.

Quantum chemical calculationsof1
Based on the hypothesis that amphiphile 1 is primarily present in its linear trans conformation, we next aimed at rationalizing how the monomer units would pack into the 1D stacks. For this purpose, quantum chemical calculations at the dispersion-correctedP M6 semiempiricall evel were performed. The optimized geometry of am onomer is shown in Scheme 1. The molecular length calculated for 1 (4.5 nm) is in excellent agreement with the experimental diameter of fibers of 1 (4-5 nm) measuredb yT EM, indicating that the supramolecular fibers are one molecule thick (Figure 3c). Interdigitation of glycol chains would only be possible when two single fibers interact in ap arallel fashion,g iving rise to thicker bundles. The optimized geometry of the dimer and tetramer reveals that aromatic interactionst ake place along the whole p-conjugated system of 1 and that as light shift is observeda long both the directions (X,Y) perpendicular to the long (Z) axis of the stack (see top view of dimer or tetrameri nS cheme 1a nd Figure S13). This slightly shiftedp acking is in accordance with a redshift in absorption upon aggregation. [18a] Thei nterdisc distance between monomers along the Z-axis is ca 3.35 ,w hile the intermolecular center-to-center distance between two neighboring benzene rings in the stack is ca 3.70 .M any intermolecular CÀH···O short contacts (about 2.2 apart) between the glycol chains help stabilizet he aggregates.T he central bipyridine units in as tack are involved in aromatic (p-p) interactions and in weak intermolecularC ÀH···N hydrogen bonding( d C-H···N % 3.52 ). The heats of formation (DH f )o btained from the PM6 calculations were used to estimate DH f (in kJ mol À1 )f or the following aggregationp rocesses:1+ 1!2 (+ 66), 2 + 1!3( À353), and 3 + 1!4( À340). The initial aggregation step seems to be an on-spontaneous seed formation (dimerization), followed by more spontaneous elongation steps, namely trimer and tetramer formation, which would fit with ac ooperative supramolecular growth. For the initial nucleus formation (1 + 1!2), it can be assumed that the monomer activation predominantly constitutes ap reorganization step, which includes ad ecrease in flexibility of the freely rotatable bipyridine center accompaniedb yaplanarization of the OPE cores. For instance, the torsion angle involving the central rotatableC ÀCb ond and both the nitrogen atoms in the same monomer unit slightly increases from 1778 to 1798 upon dimerization. With the more ordered, planar structureo ft he nucleus, initial contact to another monomer (i.e. 2 + 1!3) is facilitated and sufficiently strong interactions provided. These essentialc hanges requirel oss of conformational freedom, turn-ing the activation into an unfavorable step, as indeedp redicted from the PM6 calculations.
From all the above observations, an aggregation model for 1 can be proposed,a ss hown in Scheme 1a nd Figure S14. At high temperatures ( % 353 K), 1 exists in amolecularly dissolved state. Slow cooling initiatest he self-assembly,w hich is mainly driven by aromatic and hydrophobic interactions in aqueous media. Such close intermoleculari nteractions between the aromatic scaffolds explain the increase in absorbance upon cooling. Furthermore, the redshift of the absorption band upon aggregation suggests as lightly twisted molecular arrangement, as predicted by solid-state NMR studies, most likely to minimize the steric bulkiness posed by the glycol chains of adjacent molecules. The relatively flexible nature of the glycol chains allows the creation of ah ydrophilic shell protecting the conjugated OPE and bipyridine units from the aqueous environment.H owever,w eh ypothesize that not all aromatic fragments are efficiently shielded from the surrounding media, which drives the individual fibers to furthero rganize laterally into dense clusters, as suggestedb yTEM and absorption studies. These superstructures might be further stabilized by the interdigitation of the glycol chains of neighboring monomers mediated by water molecules. [19] Impact of acid on the supramolecular polymerization of 1: cis-locked1-H + + 2,2'-bipyridines in am onomeric state are well-known for their sensitivity towardsv ariousa cids, [6a] leading to the formation of the monoprotonated bipyridine species,e ven in the presence of al arge excess of acid. [6] The monoprotonated bipyridine adduct is favored over the bis-protonated species since the former benefits from intramoleculars tabilization by N-H + ···N interactions (Scheme 1). [6] Consequently,t he protonation of 2,2'-bipyridine is accompanied by ag eometricalc hange from the more extended trans-t ot he V-shaped cis-conformation (Scheme 1). Despite that this conformational change anticipates important implications at the supramolecular level, it is surprising that the pH-responsive behavior of bipyridines has been almost exclusively investigatedi namonomeric state in "good"s olvents. Thus, we sought to investigate whether and to what extentt he protonation of 1 to yield 1-H + + would influence the supramolecular properties of the system.
To this end, we initially investigated the responsivenesso f ligand 1 to the addition of trifluoroacetic acid (TFA) in aqueous solution by UV/Vis titrations tudies. For thesee xperiments, equivalent conditions as those used for previoust emperaturedependents tudies of 1 wereu sed (1.5 10 À5 m,2 98 K). The corresponding absorption spectrumo f1 prior to acid addition clearly indicates ah ighly aggregateds tate. Subsequently,i ncreasinga liquots of ad iluted solution of TFA( water/TFA = 3:1) were added gradually until the pH reached approximately 1.1 and the titration wasm onitored by UV/Vis absorption spectroscopy( Figure S15). Surprisingly,o nly am arginal decrease in absorption without any additional spectral changes is observed upon TFAa ddition, even if large amounts of acid are added (ca. 200 equivalents) and high concentration is used ( Figure S15). These results indicate that the aqueous assemblies of 1 negligibly respond towards TFAu nder the investigated experimental conditions. Proof of that is the absence of a clear redshifted band that is characteristicf or the monoprotonated form of bipyridine. [6e] We hypothesize that the dense molecular packing of the p-backbone inducedb ys tronga ggregationmost likely blocks the accesso ft he surrounding protons in acidic media and consequently prevents the protonation of the bipyridine units. Thisp henomenon is most likely reinforcedb yt he strong solvation and shielding effect of the peripheral glycol chains in aqueous media. The minor decrease in absorption upon TFAa ddition can be explained by slight changes in the hydrogen bondings tructure between the outer hydrophilic shell of the fibers and surrounding water molecules at low pH, which slightly reduce the aggregates olubility.N otably,t he absorption band of 1 in aqueous mediumr emains unchanged over time upon TFAa ddition under highly acidic conditions (pH % 1), whiche mphasizes ar emarkable stabilityo f the assemblies ( Figure S15).
Given that the aqueous assemblieso f1 as such do not respond to acid, another experimentals etup was required to allow the protonation of the bipyridine unit. Solvents such as acetonitrile (MeCN)a nd THF werec hosen for protonation studies of 1 considering the good solubility of both the ligand and TFAi nt hese mediaa nd their miscibility with water.A dditionally, 1 exists in am onomeric state in both MeCN and THF,w hich should facilitatet he exposure of the bipyridine moieties to the acid molecules.F or the titratione xperiments, as olution 1 in MeCN (1.4 10 À5 m)a nd ah ighly diluted TFAs olution (TFA/ MeCN = 1:49) were prepared. Addition of TFAi ns teps of approximately 6equivalents caused remarkable changes in the absorption spectrum, as showni nF igure4a. The absorption band at 334 nm showed ag raduald ecrease with the concomitant formationo fa ni ntense shoulder at around4 00 nm up to % 126 equivalents, after which no significant spectralc hanges were observed. This newly formed transition can be assigned to the monoprotonated form of the bipyridine unit of 1.S ubsequently,d eprotonation studies were carriedo ut using an organic base 1.8-diazabicyclo[5.4.0]undec-7-en (DBU) (Figure S16). The addition of the dilutedb ase (DBU/MeCN = 1:24) resultedi nc omplete and reversible recovery of the spectral features of the neutral( uncharged) ligand 1 ( Figure S16). The emergenceo fasingle isosbestic point during protonation and deprotonation studies indicate ac onformational switch between two distinct speciesi ne quilibrium:t he trans-a nd cisstates (1 and 1-H + + )( Scheme 1). The redshifted band in absorption upon protonation can be rationalized by the formation of an intramolecular NÀH···N hydrogen bridge that causes an increasedp lanarizationa nd, consequently, p-conjugation of the aromatics urface of 1. [6] To complement the UV/Viss tudies, the protonation process of 1 was also followed by 1 HNMR titration experimentsi n CD 3 CN at ac oncentration of 1.4 10 À4 m.T om onitort he protonation in small steps, the concentrated TFA( CF 3 COOD) was diluted using CD 3 CN (1:19). Seven samples with varying amount of TFAw ere prepared, ranging from 0t o1 33 equivalents. As no NMR shifts occur beyond 88 equivalents, the spectrum with 133 equivalents TFAi sn ot shown in Figure 4c.T he 1 HNMR spectrum of 1 in CD 3 CN (Figure 4c,b ottoms pectrum)e xhibits sharp signals, suggestive of am olecularly dissolved state. Upon addition of acid, the aromatic proton signals H a-d undergo ad ownfield shift to ad ifferent extent whereas those corresponding to the oligoethylene chains remain unaffected (Figure S17). More specifically,t he signals for H a ,H b and H c are significantly shiftedd ownfield with the incremental addition of acid (Figure 4c), being this effect most pronounced for the proton signals H b .T he effect on the remaining protons ignals of the aromaticc ore( H d and H e )i sr ather insignificant, considering that these protons are located far from the protonated bipyridine unit. Proof of this is the fact that the proton signal H d undergoes am oderate downfield shift, whilet he most distant protons H e of the terminal phenylene ring (black peaks in Figure4c) are completely unaffected. Comparing the TFAt itration experiments of 1 in water and "good"o rganic solvents, we can concludet hat the formation of the monoprotonated species 1-H + + can only be monitored in detail in the latter media, whereas strong aggregation in water prevents the TFA molecules to penetrate the acid-responsive bipyridineunits.
Since the directa ddition of TFAt oa ggregated 1 in water/ THF (99:1) did not entail as ignificant effect, an alternative preparation method shouldb ei ntroduced to investigate the effect of bipyridinep rotonation on the self-assembly.T ot his end, as mallv olume of ap ale yellow-colored solution of 1 at 4.8 10 À3 m in THF was initially prepared, where 1 exists in a monomeric state. To this solution,alarge excess of concentrated TFAw as added to ensure full protonation of the bipyridine unit, causing an immediate intensificationo ft he yellow color. Subsequently,as mall amount of this solution was diluted with al arge excess of water to reach ac oncentration of 7.1 10 À4 m and the sample wask ept at room temperaturei navial with perforated cap for af ew weeks to allow for equilibrationa nd evaporation of the traces of THF.T he resulting aqueous solution was then investigated by UV/Vis absorption measurements.F igure 4b shows the comparison of the absorption spectra of 1 and its corresponding mono-protonated form 1-H + + in am onomeric (inM eCN) and in an aggregateds tate (water). In order to better identify the differences, all spectra have been normalized. As previously observed, neutral 1 undergoes ar edshift and slight spectralb roadeningu pon aggregation when movingf rom MeCN to water.S imilarly,t he UV/Vis spectrumo f1-H + + also shows ar edshift upon aqueous self-assembly compared to the monomer spectrum in MeCN. Simultaneous to this redshift, the absorption band of aggregated 1-H + + furtherb roadens up to 500 nm. In particular,t he monoprotonated form 1-H + + exhibits ac haracteristic shoulder at 375 nm, which is appreciable both in the monomeric (MeCN) and aggregated state (water). This suggests that the new preparation method (creation of 1-H + + in THF,f ollowed by addition of water and removal of THF) leads to an aggregate spectrum that significantly differs -it is further redshifted-from the aggregate spectrum of the neutralspecies 1.
In analogy to 1,w ea lso attempted to gain mechanistic insights into the aqueous self-assembly process of 1-H + + by UV/ Vis studies. However,t emperature-dependent experiments are unfortunately not applicable under such highly acidic conditions due to the precipitation of the sample upon heating (Figure S18). Therefore, the denaturation( disassembly)o fa ggregated 1-H + + was monitored spectroscopically by gradual addition of aliquots of monomeric 1 in an acidic solution of THF/ TFA( 2 10 À5 m,1 m THF solution,0 .1 m aqueous solution, T = 25 8C). Pleasingly,atransition from self-assembled 1-H + + to the characteristic monomeric absorption spectrum of 1-H + + via an isosbestic point could be identified upon optimization of the experimental conditions of water/THF ratio and amount of TFA (Figure S19 a). Plotting the spectralc hanges at different wavelengths vs. the volumefraction of acidic THF leads to adenaturation curve that cannot be described by the isodesmic or cooperative mechanisms (Figure S19 b-d). In particular, the appreciation of at wo-step curve seems to indicatet hat 1-H + + might follow an anti-cooperative mechanism, as recently observed for amphiphilic OPE-based Pt II complexes. [18b] On this basis, ad ifferent aggregate morphologyi se xpected for the aqueous solutionso f1 and 1-H + + .I no rder to find this out, microscopy investigations were conducted for the same aggregate solutionu sed for UV/Vis studies. TEM on carbon-coated copperg rid revealed one-dimensional assemblies of 1-H + + that show as trong tendency to agglomeration (Figure 5a and S20). Remarkably,t he length of the assemblies upon protonation is considerably smaller than that of the neutrall igand 1.T he TEM studies were furtherc omplemented by atomic force microscopy (AFM) on mica surface (Figures 5b and S21). AFM provided ah igher resolution than TEM and therefore allowed the visualization of agglomerated, individual rods with ad iameter of 3.2 AE 0.3 nm and al ength of up to 70 nm. These structures further self-assemble into denselyp acked regionso nt he hydrophilic substrate.

Quantum chemical calculationsof1 -H + +
To gain insights on the packing of 1-H + + ,d ispersion-corrected PM6 calculations of the cis-locked speciesw ere performed using the MOPAC package, where ground-state geometries of monomer and dimer in vacuum were optimized( Scheme1 and Figure S22). The proton attached to the pyridine helps stabilize the cis form via intramolecular H-bonds to the other pyridine nitrogen (d(NÀH···N) = 2.3 ), while the repulsion between the beta-hydrogens of both pyridines (2.3 apart) hampers the planarization of the ligand,t he torsion angle involving N-C-C-N being 23 degrees. The two individualO PE scaffolds themselves adopt ap lanarc onjugated arrangement. Thea verage distance from one nitrogen to the last oxygen of the corresponding glycol chain is about2nm, which, together with the fiber diameter of around3nm obtainedf rom AFM and TEM suggest az igzag, antiparallel packing of the monomer units of 1-H + + inside the supramolecular fiber.I nt his arrangement,s ubsequentm onomer units are rotated relatively to each other by 180 degrees (see dimer of 1-H + + in Scheme 1), which helps minimize unfavorable repulsions between the positively charged bipyridine-H + units.T his arrangement also renders fibers with av anishingly small dipole moment, whichi s generally preferred by molecules inside supramolecules. [20] Classical molecular dynamics (MD) simulations were performed to furtheri nvestigate this zigzag aggregationu sing the Tinker program and MMFF94 force field. The simulationb ox was comprised of an octamer of 1-H + + surrounded by 5142 (explicit) water molecules. After initial geometry optimization and equilibration, the system was simulated using the NPT ensemble at 298 Ka nd 1a tm for 1ns( Figure 6, further details of the simulations are described in the Supporting Information). The zigzag arrangement was indeed stable in water throughout the whole simulation, althoughm onomer units exhibit some degree of mobility inside the fiber due to the well-known dynamic nature of supramolecular systems. The middle OPE rings of neighboringm olecules were in close contact to stabilize the stacks by attractive aromatic interactions, supported by hydro-phobic forces. Simultaneously,t he flexible glycolc hains created ah ydrophilic shell around the zigzag stackst om inimize the unfavorable contact of the hydrophobic scaffold to the surrounding aqueous media. The glycol chains of every third monomer unit were interacting with each other via weak intermolecular CH···O short contacts. Such type of interactions involving glycol chains have been previously observedt og reatly contribute to the stabilization of aqueous assemblies of metalbased amphiphilic p-systems.
[18a] On average,t he distance between oxygens from the end of the glycol chains of two consecutive monomer units within the fiber was about3 .0 nm, which is in good agreement with the AFM/TEM measurements already discussed. The suggested zigzaga ntiparallel packing model is one of the possible arrangements how the monomer units of 1-H + + can assemble into at hin 1D fiber,a ss hown by microscopy studies. In this arrangement, the positivelyc harged bipyridine-H + units would be alternately oriented to opposite sides to minimizee lectrostatic repulsion and at the same time, the polar TEG chains would shield ag reat part of the supra-molecular column. Mostl ikely,t he increasingly higherr epulsion of charged groups upon fiber growth is responsible for the termination of the supramolecular growth explaining why the 1D fibers upon of 1-H + + have al imited length. Therefore, the acid-responsive bipyridine conformational switch can be efficiently exploited to tune the aggregategrowth.

Conclusions
In summary,w eh ave demonstrated the use of 2,2'-bipyridines as acid-responsive conformational switchesi ns upramolecular polymerization. To that end, we have designed ab ipyridinebased bolaamphiphilic derivative 1 where the 2,2'-bipyridine core has been substituteda tt he 4,4'-positionsw ith OPEf ragments decorated with polar TEG chains.U nder standard conditions, the bipyridine unit of 1 primarily exists in its trans-conformation,l eading to al inear arrangement of the molecular aromatic core. This high degree of preorganization enables ac ooperative supramolecular polymerization of 1 into well-ordered bundled fibers when water is used as aggregation-inducing solvent. The robustnesso ft hese supramolecular structuresi n water is reflected in their high stabilityu pon addition of an acid (TFA), which is attributed to strong hydrophobic and aromatic interactions of the aromatic core and the shielding effect of the peripheral TEGc hains. In contrast, monoprotonation of the 2,2'-bipyridine core to generate 1-H + + can occur efficiently if TFAi sa dded to 1 in am olecularly dissolved state in organic solvents. Interestingly,a ddition of water to the monomeric solution of the monoprotonated form 1-H + + inducess upramolecular polymerization, although to am uch lower extent than neutral species 1.T his attenuated growth of 1-H + + into shorter fibers may be possibly attributedt ot he destabilization of larger aggregates due to the positively charged monomer units as well as the decreased tendency of the positively chargedm onomers to associate duet ob etter stabilization in polar media. We envisage that the use of bipyridines as acidsensitiveu nits to tune self-assembly is expected to open up new directions in the field of conformational switches and stimuli-responsive systems.