Helicity Control in the Aggregation of Achiral Squaraine Dyes in Solution and Thin Films

Abstract Squaraine dyes are well known for their strong absorption in the visible regime. Reports on chiral squaraine dyes are, however, scarce. To address this gap, we here report two novel chiral squaraine dyes and their achiral counterparts. The presented dyes are aggregated in solution and in thin films. A detailed chiroptical study shows that thin films formed by co‐assembling the chiral dye with its achiral counterpart exhibit exceptional photophysical properties. The circular dichroism (CD) of the co‐assembled structures reaches a maximum when just 25 % of the chiral dye are present in the mixture. The solid structures with the highest relative CD effect are achieved when the chiral dye is used solely as a director, rather than the structural component. The chiroptical data are further supported by selected spin‐filtering measurements using mc‐AFM. These findings provide a promising platform for investigating the relationship between the dissymmetry of a supramolecular structure and emerging material properties rather than a comparison between a chiral molecular structure and an achiral counterpart.


Introduction
The high absorption of light in the visible and near-infrared regime displayed by squaraine dyes has intrigued many research groups in the last 60 years. [1][2][3] The unique photophysical properties arise from the intramolecular charge-transfer between the electron-poor quadratic core and the fused electron-rich arene substituents. Previous studies have shown that squaraine dyes can be used in various applications such as (bio-)labelling, [4][5][6] photodynamic therapy, [7] dye-sensitised solar cells [8][9][10][11] and organic bulk heterojunction solar cells. [12] Despite numerous studies investigating the photophysical properties of squaraine dyes, the number of reports dealing with chiral analogues is limited. [13][14][15][16][17][18][19] Nonetheless, their potentialinchiroptical applicationss uch as detection [20,21] and emission [22] of circularly polarised light or organic spintronics [23] is high. The paucityo ft hese reports is likely due to the fact that these fields of application are of recent date.
The emerging research on organic spintronics focusses on improved fundamentalu nderstanding of the role of chirality and the electron spin or electromagnetic fields in both chemical [24,25] and biological systems. [26][27][28] It has been shown that electrons with initially random electron spin-orientation can be spin-polarised by transmission through ah ighlyo rdered chiral layer of doublestranded DNA. [29] Manifestation of spin-polarisation in organic materials is described by the so-called chiral-induced spin selectivity (CISS) effect. [23] Even thought he measured spin polarisation achieved in organic materials is never absolutei nt he systems described until now, [30] various applications such as water-splitting, [24,[31][32][33] enantio-separation [34,35] and enantioselective reactions [36] have been realised.
Inspired by reports on the giant intrinsic circular dichroism of prolinol-derived squarainesi nt hin film, [19] we designed two novel types of symmetrical dyes as depicted in Scheme 1, with the aim to explore the unique photophysical properties of squaraine dyes with applicationi no rganic electronics and spintronics. Previous studies on squaraine dyes have focused often on nitrogen-based heterocycles [4,16,[37][38][39] or N,N-dialkylaminoaryl substituted derivatives [6,12,40] since the formation of the squaraine unit succeeds only when the nucleophilicity of the reagent used for the condensation reaction with squaric acid is sufficiently high. [41] Squaraine dyes that comprise parahydroxy substitution have received less attention. [42][43][44] Replacement of the tertiary amine group in the squaraine precursor by ah ydroxyl group renders the resulting squaraine dye an acid, which readily reacts with traces of water present in organic solvent. [41] Both presented dyes combine squaraine motifs with amide functionalised side chains.T he combination of amide functionalised side chains and planar motifs such as the benzene-1,3,5tricarboxamide, [45] porphyrin [46] or naphthalene diimide( NDI) [47] is well-described in the literature to induce the formation of supramolecular aggregates.T he successful attachment of amide substituents to the squaraine motif in order to induce supramolecular aggregation has so far been reportedo nly once for an achiral squarained ye derivative. [48] By introducing chiral, non-racemic substituents, we anticipate the formation of structuresw ith ap referred helicity upon aggregation in selected solvents or in the bulk. [49,50] Since the dyes contain either N,N-dialkylaminoarylo rh ydroxy aryl substituents, they will be sensitivet od ifferences in pH and solvent polarity.S timuli-responsiveness is considered valuablef or the design of future smartm aterials. [51] The combination of the intriguing photophysical properties of squaraine dyes in the visible regime with the successful preparation of aggregates, which exhibit ordering with preferred helicity,isregarded as ak ey element towards novel materials for application in organic spintronics.

Results and Discussion
Synthesis of chiral (S-) and achiral( a-) SQ-1 and SQ-2 N,N-Dialkylamino-aryl and hydroxyl-aryl substituted squaraine dyes S-SQ-1 and a-SQ-1 as wella sS-SQ-2 and a-SQ-2,w ere prepared according to Scheme2.A fter obtaining an amide substituted precursor (see Scheme S1 in the SupportingI nformation for detailed synthetic procedures), condensation with squaric acid wasperformed using amodification of the original procedure by Sprenger and Ziegenbein. [39] As described by a more recent study, n-butanol was replaced by the lower-boiling n-propanol toa void the formation of the undesired 1,2-condensation product. [17] The successful dye formation was indicatedb yt he development of an intense green (S-SQ-1 and a-SQ-1)o rb lue (S-SQ-2 and a-SQ-2)c olour. S-SQ-1 and a-SQ-1 were obtained in excellent (89-94%)a nd S-SQ-2 and a-SQ-2 in reasonable (14-60%)y ields. The compounds were fully characterised with NMR, MALDI-TOF-MS, elemental analysisa nd IR spectroscopy (spectra are shown in the Supporting Information). These resultss how that the final compounds were obtained in high purity,with aperfect regioselectivity.

Photophysical properties in solution
We first focus on N,N-dialkylaminoaryl based squaraine dyes S-SQ-1 and a-SQ-1.F igure 1s hows the molare xtinction coefficients (e)o fS-SQ-1 and a-SQ-1 as af unctiono ft he wavelength in tetrahydrofuran( THF) solution.B oth dyes show a sharp maximum of e around 670 nm and aw eaks houldera t 620 nm. The recorded spectra were found to be consistent with reports on squaraine dyes in the molecularly dissolved state. [2,11,16] S-SQ-1 and a-SQ-1 are highly fluorescent in the molecularly dissolved state. The emission spectra represent mirror images of the absorptions pectra as clearly noticed in Figure 1. The corresponding Stokes shifts are 15 nm. The high e of 3.2 10 5 and 3.3 10 5 Lmol À1 cm À1 for S-SQ-1 and a-SQ-1, respectively,a nd the narrow absorption bandwidths are the result of the weakv ibronic coupling for the electronic S 0 !S 1 transition along the long molecular axis of the squaraine backbone. [52] Weak vibronic coupling is commonly causedb yalow configurational displacementb etween initial and final state of an electronic transition and accompanied by weak manifestation of vibronic bands in the absorption spectrum.
The small shoulder in the absorption spectrum of S-SQ-1 and a-SQ-1 is assigned to the weakv ibronic progression of the absorption band. Aq uantitative theory on how strong vibrational progressionsa re observed in the absorption spectrum was developed by Huang and Rhys. [53] After fittingt he recorded spectral data as shown in Figure S1, the Huang and Rhys parameterS was estimated to be 0.16, which corroborates the weak vibronic coupling. [54] In contrast to S-SQ-1 and a-SQ-1, S-SQ-2 and a-SQ-2 exhibited shorter wavelengths of maximum absorbance of l = 606 nm (for spectra see FiguresS2-S5) and ad ecreased molar absorptivity (e(S-SQ-2) 606 n m = 1.5 10 5 Lmol À1 cm À1 and e(a-SQ-2) 606 n m = 1.6 10 5 Lmol À1 cm À1 ). The shifts in values suggest al ess pronounced intramolecular charge transfer for S-SQ-2 and a-SQ-2 comparedt oS-SQ-1 and a-SQ-1.
Subsequently,a bsorption and emissions pectra of all dyes were measured in ar ange of solvents (acetonitrile, chloroform, heptane, methanol and water) that differ in polarity. The recorded spectra are shown in Figures S2-S5. For all dyes, the absorption band of the molecularly dissolved speciesd epends on the solvent polarity and responds to the addition of an acid or base. Ad etailed characterisation of the manipulation of the photophysical properties upon the addition of acids or bases is given in the Supporting Information (See FiguresS6-S8). During the study,wefound that S-SQ-2 and a-SQ-2 have alimited chemical stabilityw hen molecularly dissolved in dimethyl sulfoxide. An NMR study on the stability is presented in the Supporting Information ( Figure S9). As ar esult,w ed ecided to focus our further investigationsonS-SQ-1 and a-SQ-1.

Formation of CD active aggregates of S-SQ-1a nd a-SQ-1 in solution
The aggregation of S-SQ-1 and a-SQ-1 and mixtures thereof is induced by adding ac oncentrated solution of dye in THF to water,w hichi sapoor solvent. Upon the addition, an immediate colourc hange fromg reen to blue is observed for both S-SQ-1 and a-SQ-1,w hich suggestst hat the aqueous solution containing 1vol %T HF induces the formationo fa ggregates. The absorption spectra of the aggregated dyes are depicted in Figure 2a.I no rder to ensure complete dissolution and equilibrationd uring the mixing experiments,t he samples were heated and stirred at 90 8C, followed by slow cooling to room temperature beforee ach measurement. Thisp rocedure was followed throughout this study.T he absorption spectrumo f aggregated a-SQ-1 (black line in Figure 2a)s hows one absorption maximum around 590 nm with ab road shoulder towards longer wavelengths.T he wavelength of maximum absorbance is blue-shifted (H-band) with respectt ot he absorption band of the molecularly dissolved species (670 nm). Upon addition of S-SQ-1 to the feed and equilibration followingt he protocol, the rise of an absorption band with maximum around8 00 nm is observed. The band is most pronounced for pure S-SQ-1 (red line in Figure2a) and red-shifted (J-band) with respectt o the absorption band of the molecularly dissolved species. The graduali ncrease of the J-band indicates changes in aggregate structure when tuning the dye feed from a-SQ-1 to S-SQ-1.I n the spectra of all aqueous solutions, the absorption band of the molecularly dissolved speciesisn oticeda sasmall shoulder only,w hichi ndicates the pronouncedc onversion from the molecularlyd issolved state to aggregates. Circular dichroism (CD) spectra were measured for solutions of S-SQ-1, a-SQ-1 and mixtures thereof.T he resultsa re depicted in Figure 2b and show that the aggregates formed by  S-SQ-1 exhibit circular dichroism whereas aggregates of a-SQ-1 do not, as expected.
Since no CD effect was observedf or molecularly dissolved neutralo rc harged forms, it is concluded that the CD effect stemmed from supramolecular interactions. The helical structure of the aggregates is indicated by the absorption band around8 20 nm which displays as trong bisignate Cotton effect. When the content of S-SQ-1 in am ix of S-SQ-1 and a-SQ-1 is increased, the ellipticityt hat is determineda t8 68 nm increases predominantly in al inear fashion as shown in Figure 2c.T his correlation suggests that the aggregates formed by S-SQ-1 and a-SQ-1 in water are either barely mixing or the ability of S-SQ-1 to bias one helical sense is low.
Formation of CD active aggregates of S-SQ-1a nd a-SQ-1 in the solid state The chiroptical properties of S-SQ-1 and a-SQ-1 were further investigated in bulk. Thin films (thickness = 40 nm) were prepared by spin-coating as olution of S-SQ-1 and a-SQ-1 in chloroform onto microscope glass slides. The films were characterised by UV/Vis spectroscopy (Figure 3a). The UV/Vis spectra obtained from the thin films show noticeable differences from those recorded for the aqueous solutions. In the thin films, both dyes showabroad absorption band with am aximum that is red shifted with respecttot he absorption band of the molecularly dissolved species. For a-SQ-1,t his maximumi s found at 790 nm and for S-SQ-1 at 834 nm. Additionally,t he absorption spectra of both compoundse xhibit as houlder 620 nm which is in turn blue-shifted with respect to the absorptionb and of the molecularly dissolved species. The coinciding presence of ar ed-and blue-shifted component in the spectraa re tentatively attributed to the presence of several molecules with different orientations in one structural repeating unit. Since the shapes of the absorption spectra of S-SQ-1 and a-SQ-1 are very similar,astructurally similarpacking of the chromophores in the aggregates,w hich are formed in the thin films, is likely. Having investigated the absorption properties of thin films consisting of the pure compounds, we investigated thin films consisting of spin-coated mixtures of S-SQ-1 and a-SQ-1.A ss hown in Figure3a, the absorbance spectra recorded for all thin films, where 5-50 % S-SQ-1 were present in the mixture showed am aximum of absorbance at 790 nm. The spectra look very similar to the spectrum recordedf or pure a-SQ-1.I ncreasing the content of S-SQ-1 to 75 %i nt he mix results in ar ed-shift of the maximum of absorbance to 799 nm. For pure S-SQ-1,t he red-shift is even more pronounced and the wavelengtho fm aximum absorbance is determined with 834 nm. These resultsi ndicate that thin films, whichc omprise up to 50 % S-SQ-1 in am ix of S-SQ-1 and a-SQ-1,s how an aggregation mode that is structurally very similart ot he aggregation of pure a-SQ-1.T he mixtures containing 75 % S-SQ-1 in contrasts hows an aggregation mode that is approachingt he aggregate structure formed by pure S-SQ-1.
Next, CD spectroscopy was used to investigate the thin films ( Figure 3b). Gratifyingly,t hin films consisting of pure a-SQ-1, do not show any CD effect. In contrast, thin films containing a mixture of S-SQ-1 and a-SQ-1 or pure S-SQ-1 show significant Cotton effects in the CD spectrum.A sn oticedi nF igure 3c,t he CD effect is mosti ntense for a1 :3 mix of S-SQ-1 and a-SQ-1, and not for pure S-SQ-1.T his observation is in sharp contrast to the results obtainedi nw ater,w here the most intenseC Deffect was measured for the sample containing pure S-SQ-1. Additional spectroscopic datao btained from the Muller matrix is discussed in the Supporting Information.
The spectroscopic data suggestst hat a-SQ-1 forms helical aggregates with P-and M-helices as ar acemic mixture, as the addition of as mall amount of S-SQ-1 to the feed, induces a CD effect but does not impact the absorbance spectrum of the material. The absence of changes in the absorbance spectrai ndicate that the aggregate's structure is not changed as well. S-SQ-1 is structurally incorporated into the aggregates formed by a-SQ-1.O wing to the chiral information of the stereogenic centres,t his incorporation non-proportionally biases the ratio of the populations of present P-and M-helical aggregates. In helical supramolecular systems, this effect is commonly referred to as the "sergeants-and-soldiers"e ffect. [55] Interestingly, the "sergeants-and-soldiers"e ffect is not operativeo ver the whole range of mixtures.
Once S-SQ-1 becomes the major fraction in the feed, the recorded absorbance spectrum starts to change,i ndicative of a changed structure of the formed aggregates. At the same time, the recorded CD effect is decreased. This observed trend is fascinatings incei ts hows that the maximisation of the amount of stereogenic centresi nt he aggregates-by increasing the amount of S-SQ-1 in the feed-does not yield the thin film with the highestC De ffect. The highestC De ffect is achieved when S-SQ-1 is solely used as ad irector for a-SQ-1, rather than astructuralcomponent.
In order to elucidate whether the measured ellipticity is caused by purely excitonic couplingo ra lso stems from other effects such as cholesteric ordering, thin films with different thicknesses were preparedb ys pin coating. As shown in Figure S10, the ratio of measurede llipticity and film thickness was relativelyc onstant for rather thin films (thicknesses between1 1 and 24 nm). When the film thickness was increased further, an increaseo ft he ratio was noticed.I nv ery thin films, the CD effect can only originate from the mechanism operative at the nm scale, which suggests the presence of excitonicc oupling. The increase of the ratio with increasing film thickness suggests that the recorded CD effect stemsp artly from cholesteric ordering in bulk. In all thin films, the aggregate size is very small and the films do not show birefringence when investigated by polarised optical microscopy (FigureS19), while only a negligible linear dichroism (LD) effect was recorded (Figure S10d).
The absorbance and CD spectra recorded for the thin films differ from the spectra recorded for the aqueous samples. Since the aqueous samples were equilibrated by ah eatinga nd cooling cycle, the impact of annealing on the aggregated structures in thin film was probed. All thin films were annealed for 10 mina t2 10 8Cu nder nitrogen atmosphere which is well below the molecules' decomposition temperature. Subsequently, the optical properties wereinvestigated again. The absorbance spectra recorded after annealing are depicted in Figure 4a.T he thin films with low S-SQ-1 content (0-30 %) exhibit ad ecrease of the former band of maximum absorbance around7 90 nm and the rise of two new absorption bands around6 00 nm and 700 nm, respectively.F or thin films with a higher S-SQ-1 content (50-100 %), the absorbance of the band around6 00 nm intensifies slightly.T he position of the maximum of absorbance is retained around800 nm after annealing. Besidesc hanges in the UV/Vis spectra, the CD spectra recorded for all samples show differences after annealing, too. As depicted in Figure 4b,s amples with low S-SQ-1 content (0-30 %) showed as hift of the wavelength of maximum CD effect to shorter wavelengths. After annealing, the maximum CD effect was noticed around 690 nm (formerly7 90 nm). The CD effect is decreased for the 50 % S-SQ-1 sample and increased for the 75 % S-SQ-1 and the pure S-SQ-1 sample. The changes in both UV/Vis and CD spectrau pon annealing suggest therefore that the aggregates formed during spin coating were the result of kinetic effects. The recorded LD spectraa re depicted in Figure S11a nd indicate that after annealing, LD has negligible contribution to the absorption properties, still.

Spin-filtering properties of thin films by mc-AFM
In order to confirm the exceptional non-linear behaviour found for the spin-coated films, we performed spin-filteringe xperiments.M agnetic-conductive atomic force microscopy (mc-AFM) is an efficient method for studying the spin-selectivity in the electron transport through chiral structuresi ncluding the effect of the interface between the probedm ateriala nd the substrate. [56] The electrical measurement recordst he current transported through at hin film of organic matter deposited on top of ag old-coated nickel surface.D uring the measurement, the sample is magnetised with its magnetisation perpendicular to the surface. Owing to the magnetisation of the Ni/Au substrate, the electrons that are injected into the organic material have ab ias in their populations of electron spin alignments( spin-up and spin-down). It is important to note that the values are determined in the nonlinearr egime which make the measurement sensitive to detect the spin polarisation for the electron conduction. The results obtained from recording 100 I-V curves for both orientations of the magnet and the three samples containing pure a-SQ-1,p ure S-SQ-1 and the 3:1m ixture thereof are depicted in Figure 5. The orientation of the magnet does not have an influence on the I-V curves recorded for a-SQ-1.F or the sample containing purely S-SQ-1,o nly as mall impact is found. The most pronounced dependency of the I-V curves on the orientation of the magnet is found for the 3:1m ixture of both a-SQ-1 and S-SQ-1.
From the recorded measurements, the spin polarisation was determined using SP ¼ I up ÀI down I up þI down hi 100. The spin polarisation at 3V was determined with 0% for a-SQ-1,1 7 AE 5% for S-SQ-1 and 56 AE 8% for the 3:1m ixture. As noticedi nt he Figure 5d, similar values are found for lower potentials, too. The upper limit of 60 %c orresponds to ar atio of about four to one between the two spin states. Various systems, that are based on ahomochiral compound only,report asimilarspin polarisation, too. [29,[57][58][59][60][61][62][63] Studies on comparingc hirala nd achiral materials are reportedl ess often. [30,31] Only one very recently reported study investigated the spin polarisation of mixtures of chiral molecules and their achiral counterparts. [30] Thec ompounds were coassembled into supramolecular nanofibers. The spin selective measurements indicated ac orrelation of the molarc ircular dichroismo ft he nanofibersi nsolution and the spin polarisationw hichw as measured after drop casting and drying. However,b oth maximao fm olar circulard ichroism and spin polarisation were found for the sample containing purely chiral material. In the system presented by us, we find the mosts elective electron transportf or materials with the highest optical activity.S ince this materiald oes however not contain the highest number of stereogenic centres in the series, our findings indicate as trong correlation between spin polarisation during the electron transport and the opticala ctivity of the organic material. As imilar correlation has been found for deoxyribonucleic acid( DNA) based spin filters whichs howed ad rastically reduced spin-selectivee lectron transport when used as singlestrandsi nstead of double-strands. [29] Although the number of stereogenic centresi nb oth types of DNA is equal, the formation of ah ighly-orderedc hiral structure is impairedf or the single-stranded version. With the loss of chiral order,s pin selective properties are lost. The system presentedb yu su nderlines the cruciality of chiral (supramolecular) order.A lthough the number of stereogenic centres is increased when comparing the S-SQ-1 sample with the 3:1m ixture of a-SQ-1 and S-SQ-1,apronounced decrease in spin polarisation is noticed which coincides with the decreaseo fo pticalactivity.

Conclusions
The formation of squaraine dyes is commonly achieved by the reaction of squaric acid with aniline-based precursors. The presenteds tudy shows that anotherc lass of substrates that is suitable for this type of reactioni sg iven by amide substituted hydroxyl-benzenes. Both aniline-a nd hydroxyl-benzene based dyes were prepared in chiral and achiral forms.The dyes exhibit intense colours that can be tuned by the addition of acids or bases.A nilino substituted S-SQ-1 and a-SQ-1 change their colourf rom green to violet upon protonation. Blue hydroxylbenzene substituted S-SQ-2 and a-SQ-2 change their colourt o red when singly deprotonated. S-SQ-1 and a-SQ-1 were aggregatedi na queous solution and in thin films. The aggregates were studied by UV/Vis and CD spectroscopy. When mixtures of S-SQ-1 and a-SQ-1 were aggregated in solution, the CD effect increased predominantly linearly when increasingt he amount of chiral S-SQ-1 in the mix. This linear increasei ndicatest hat the aggregateso f S-SQ-1 and a-SQ-1 are either barely mixingo rt he ability of S-SQ-1 to bias one helicals ense in the mix is low.I nc ontrast, spin-coated thin films of S-SQ-1 and a-SQ-1 resulted in co-assembled structures that displayed an on-linear increaseo ft he CD effect. Surprisingly,t he highest dissymmetry was not measured for at hin film that contained pure S-SQ-1 but a1:3 mix- ture of S-SQ-1 and a-SQ-1 molecules.T he similarity of the absorptions pectrumo ft he 1:3m ixture of S-SQ-1 and a-SQ-1 with respectt ot he absorption spectrum of pure a-SQ-1 indicates av ery similar aggregations tructure for these two samples. Opposed to pure a-SQ-1,t he 1:3m ixture is highly CD active, which in turn suggests that the population between the present P-or M-helices has been altered. Additionally,t his sample exhibits ah igherC De ffect than pure S-SQ-1.I nc ontrast to previouss tudies, the preparation of solid dye structures with the highest CD effect is achievedb yu sing ac hiral compound (S-SQ-1)s olely as ad irector for an achiral compound (a-SQ-1), rather than as tructural component. This nonlinear behaviour was further confirmed by mc-AFM measurements,a st he highest spin selectivity was found for the 1:3 mixture as well. Annealing of the spin-coated thin films resulted in changes of absorption andC Dspectra which suggests the coassembly to be ak inetically controlled process. We note that the degree of circular polarizationi na bsorption displayed by the chiral amide derivates under study is much smaller than observedf or the chiral prolinol derivatives of the squaraine chromophore described by Zablocki et al. [19] Furthermore, we note that for crystals of squaraine dyes, the interaction between light and molecules can be exceptionally strongw ith coupling constantsi nt he order of one eV. [64] This strongc oupling, most likely contributes to unconventional optical properties whichremain yet to be fully explored.
Considering the emerging work on spintronic applications for example, photoelectrochemical water splitting, where devices functionalised with ac hiral, non-racemic active layer yielded higher currents than for achiral counterparts, [31,33] the presented system provides an promising basis to furtherp robe the impact of supramolecular rather than molecular chirality on materialp roperties. Utilisation in spintronic applicationsw ill be the topicoffuture investigations.

Materials and methods
All chemicals were purchased from commercial sources and used without further purification. Dry solvents were obtained with an MBRAUN Solvent Purification System (MB-SPS). Reactions were followed by thin-layer chromatography (TLC) using 60-F254 silica gel plates from Merck and visualised by UV light at 254 nm. Silica column purifications were performed on aG race Reveleris X2 automated column machine with Reveleris Silica Flash Cartridges and on aB iotage Isolera 1w ith Biotage SNAP KP-SIL columns. 1 H-and 13 C-NMR spectra were recorded with aV arian Mercury Vx 400 MHz. Deuterated solvents were purchased from Cambridge Isotope Laboratories and are indicated for each measurement. Chemical shifts (d)a re expressed in ppm and are referred to the residual peak of the solvent. Peak multiplicity is abbreviated as s: singlet;d :d oublet, q: quartet;p :p entet;m :m ultiplet;d d: double doublet;d t: double triplet;d q: double quartet. MALDI-TOF spectra were recorded with Bruker Autoflex Speed using a-cyano-4-hydroxycinnamic acid (CHCA) or trans-2-[3-(4-tert-butylphenyl)-2-methyl-2propenyl-idene]-malononitrile (DCBT) as matrix. The stock solutions used for the optical measurements and spin coating were prepared by weighing the necessary amount of compound for the given concentration and transferring into as crew-capped vial. The desired concentration was adjusted by the addition of solvent using GilsonTM MICROMANTM Positive-Displacement Pipets. In order to ensure complete dissolution and equilibration during experiments performed in aqueous solution, the samples were heated and stirred at 90 8Cf or 20 min, followed by slow cooling to room temperature before each measurement. Spectroscopic measurements were performed in high precision cells made of quartz suprasil (Hellma analytics). The optical path length was 10 mm. For spin coating, 50 mLo ft he solution containing the dye were deposited on microscope glass slides and subsequently spun (Headway Research Inc.,c onc. 5mm,1 0mm or 20 mm,s pinning speed 1000 rpm or 2000 rpm, 45 s). The microscope glass slides had previously been cleaned by sonication in acetone, demineralised water and isopropanol for 5min each. For the determination of optical properties after annealing, the spin coated samples were annealed at 210 8Cf or 10 min under nitrogen atmosphere. UV/Vis, CD and LD spectroscopy were performed on aJ ASCO J-815 CD spectrometer either aJ ASCO Peltier MPTC-490S temperature controller with ar ange of 278-373 Ko raJ ASCO Peltier PFD-425S/15 with ar ange of 263-383 K. Separate UV/Vis spectra were recorded on aJ ASCO V-750 UV/Vis spectrometer.F luorescence spectroscopy was performed on aP erkinElmer LS 50 Bl uminescence spectrometer.O ptical microscopy using crossed polarisers was performed on an optical microscope from Jenaval. Images were recorded using an Infinity1 camera provided by Lumenera. AFM imaging was performed using an Asylum Research MFP-3D mounted on an anti-vibration stage. Silicon tips were supplied by Nanoworld (SIN 89 229F10L1333). The dimensions of the tips were:l ength of 150 mm, width of 27 mma nd thickness of 2.8 mm. The spring constant was 7.4 Nm À1 and soft tapping-mode was used to obtain an image. Ar esolution of 512 points and lines was used for the images. The images were processed using Gwyddion (v.2 .53) software. mc-AFM measurements were performed on 10 nm thick thin films deposited by spin coating from ac hloroform solution on top of ag old-coated nickel surface (Ni/Au 120/8 nm thicknesses). During the AFM measurement (contact mode), the substrate was magnetised with the magnetic pole up or down by external magnetic field of 0.5 T. The AFM set-up was ac ustom-designed RHK machine capable of reaching am agnetic field of 1T .T he AFM cantilevers were platinum-coated silicon tips supplied by Micromasch (HQ:DPE-XSC11, spring constant:2 .7 Nm À1 ). During each measurement, an 8-10 nn force was exerted with the tip onto the probed surface. The AFM tip is grounded while the potential at the Au/Ni substrate was varied from À3t o+ 3V .R eflectivity and Muller matrix spectroscopy was performed on aW -VASE ellipsometer from J.A. Wollam.

Syntheses
We synthesised the substrates for S-SQ-1 and a-SQ-1 as well as S-SQ-2 and a-SQ-2 according to literature procedures. [39,[50][51][52] Ad etailed summary is given in the Supporting Information. The squaraine dye formations were performed using am odification of the original procedure by Sprenger and Ziegenbein. [16,35] a-SQ-1: Af lask filled with 80 mL of 50/50 n-propanol/toluene was fitted with aD ean-Stark trap with cooler and heated to 110 8C. After the trap was fully filled with solvent and manually emptied once, squaric acid (218 mg, 1.9 mmol, 1equiv.) was added. Heating and stirring were continued for 15 minutes before N-(3-(dimethylamino)phenyl)octanamide (1.0 g, 3.8 mmol, 2equiv.) was added. The solution turned dark green and heating with stirring was continued. After 20 hours of reaction, the mixture had ad eep turquoise colour.T he reaction mixture was cooled down and the solvent was