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

Article abstract of DOI:10.1002/chem.202002695

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.

Full text of DOI:10.1002/chem.202002695

Accepted Article
Accepted Article
Title: Helicity control in the aggregation of achiral squaraine dyes in
solution and thin films
Authors: Bert Meijer, Andreas T. Rösch, Anja Palmans, Stefan
Meskers, Jorn Robben, Qirong Zhu, Francesco Tassinari, and
Ron Naaman
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202002695
Link to VoR: https://doi.org/10.1002/chem.202002695
01/2020

10.1002/chem.202002695
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Helicity control in the aggregation of achiral squaraine dyes in
solution and thin films
Andreas T. Rösch,[a] Qirong Zhu,[b] Jorn Robben,[a] Francesco Tassinari,[b] Stefan C.J. Meskers,[c] Ron
Naaman,[b] Anja R.A. Palmans,[a] and E.W. Meijer[a]*
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.
heterojunction solar cells.[12] Despite numerous studies
investigating the photophysical properties of squaraine dyes, the
number of reports dealing with chiral analogues is limited.[13–19]
Nonetheless, their potential in chiroptical applications such as
detection[20,21] and emission[22] of circularly polarised light or
organic spintronics[23] is high. The paucity of these 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 fundamental understanding of the role of chirality and
the electron spin or electromagnetic fields in both chemical[24,25]
and biological systems.[26–28] It has been shown that electrons with
intially random electron spin-orientation can be spin-polarised by
transmission through a highly ordered chiral layer of double
stranded DNA.[29] Manifestation of spin-polarisation in organic
materials is described by the so-called chiral-induced spin
selectivity (CISS) effect.[23] Even though the measured spin
polarisation achieved in organic materials is never absolute in the
systems described until now,[30] various applications such as
water-splitting,[24,31–33]
enantio-separation[34,35]
and
enantioselective reactions[36] have been realised.
Inspired by reports on the giant intrinsic circular dichroism
of prolinol-derived squaraines in thin 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 application in organic electronics and
spintronics. Previous studies on squaraine dyes have focused
often on nitrogen-based heterocycles[4,16,37–39] 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 para-hydroxy
substitution have received less attention.[42–44] Replacement of the
tertiary amine group in the squaraine precursor by a hydroxyl
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 sidechains. The combination of amide
functionalised sidechains and planar motifs such as the benzene-
1,3,5-tricarboxamide,[45] porphyrin[46] or naphthalene diimide
(NDI)[47] is well-described in the literature to induce the formation
of supramolecular aggregates. The successful attachment of
amide substituents to the squaraine motif in order to induce
supramolecular aggregation has so far been reported only once
for an achiral squaraine dye derivative.[48] By introducing chiral,
non-racemic substituents, we anticipate the formation of
structures with a preferred helicity upon aggregation in selected
solvents or in the bulk.[49,50]
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.[1–3] 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,[4–6] photodynamic
therapy,[7] dye-sensitised solar cells[8–11] and organic bulk
[a]
A. T. Rösch, J. Robben, Prof. Dr. A. R. A. Palmans and Prof. Dr.
E.W. Meijer
Laboratory of Macromolecular and Organic Chemistry, and
Institute for Complex Molecular Systems, Department of Chemical
Engineering and Chemistry
Eindhoven University of Technology
P.O. Box 513, 5600 MB Eindhoven (The Netherlands)
E-mail: E.W.Meijer@tue.nl
[b]
[c]
Dr. Qirong Zhu, Dr. Francesco Tassinari and Prof. Dr. Ron
Naaman
Department of Chemical and Biological Physics
Weizmann Institute of Science
Rehovot 76100, Israel
Dr. S. C. J. Meskers
Department of Applied Physics
Eindhoven University of Technology
P.O. Box 513, 5600 MB Eindhoven (The Netherlands)
Supporting information for this article is given via a link at the end
of the document.
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Since the dyes contain either N,N-dialkylaminoaryl or
hydroxy aryl substituents, they will be sensitive to differences in
pH and solvent polarity. Stimuli-responsiveness is considered
valuable for the design of future smart materials.[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, is regarded as a key element towards novel materials for
application in organic spintronics.
Scheme 2. Synthesis route starting from amide functionalised arenes that are
reacted with squaric acid to yield S-SQ-1 and a-SQ-1 as green, and S-SQ-2 and
a-SQ-2 as blue solids.
Scheme 1. Molecular structures of amide functionalised squaraine dyes.
Results and Discussion
Photophysical properties in solution. We first focus on N,N-
dialkylaminoaryl based squaraine dyes S-SQ-1 and a-SQ-1.
Figure 1 shows the molar extinction coefficients (ε) of S-SQ-1 and
a-SQ-1 as a function of the wavelength in tetrahydrofuran (THF)
solution. Both dyes show a sharp maximum of ε around 670 nm
and a weak shoulder at 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 absorption spectra as clearly
noticed in Figure 1. The corresponding Stokes shifts are 15 nm.
The high ε of 3.2•105 and 3.3•105 L mol-1 cm-1 for S-SQ-1 and a-
SQ-1, respectively, and the narrow absorption bandwidths are the
result of the weak vibronic coupling for the electronic S0 → S1
transition along the long molecular axis of the squaraine
backbone.[52] Weak vibronic coupling is commonly caused by a
low configurational displacement between initial and final state of
an electronic transition and accompanied by weak manifestation
of vibronic bands in the absorption spectrum.
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 well as S-SQ-2 and a-SQ-2, were prepared
according to Scheme 2. After obtaining an amide substituted
precursor (see Scheme S1 in the ESI for detailed synthetic
procedures), condensation with squaric acid was performed using
a modification 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 to avoid the
formation of the undesired 1,2-condensation product.[17] The
successful dye formation was indicated by the development of an
intense green (S-SQ-1 and a-SQ-1) or blue (S-SQ-2 and a-SQ-2)
colour. S-SQ-1 and a-SQ-1 were obtained in excellent (89-94%)
and S-SQ-2 and a-SQ-2 in reasonable (14-60%) yields. The
compounds were fully characterised with NMR, Maldi-Tof-MS,
elemental analysis and IR spectroscopy (spectra are shown in the
ESI). These results show that the final compounds were obtained
in high purity, with a perfect regioselectivity.
The small shoulder in the absorption spectrum of S-SQ-1
and a-SQ-1 is assigned to the weak vibronic progression of the
absorption band. A quantitative theory on how strong vibrational
progressions are observed in the absorption spectrum was
developed by Huang and Rhys.[53] After fitting the recorded
spectral data as shown in Figure S1, the Huang and Rhys
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parameter S was estimated to be 0.16, which corroborates the
weak vibronic coupling.[54]
Formation of CD active aggregates of S-SQ-1 and a-SQ-1 in
solution. The aggregation of S-SQ-1 and a-SQ-1 and mixtures
thereof is induced by adding a concentrated solution of dye in THF
to water, which is a poor solvent. Upon the addition, an immediate
colour change from green to blue is observed for both S-SQ-1 and
a-SQ-1, which suggests that the aqueous solution containing 1
vol% THF induces the formation of aggregates. The absorption
spectra of the aggregated dyes are depicted in Figure 2a. In order
to ensure complete dissolution and equilibration during the mixing
experiments, the samples were heated and stirred at 90 °C,
followed by slow cooling to room temperature before each
measurement. This procedure was followed throughout this study.
The absorption spectrum of aggregated a-SQ-1 (black line in
Figure 2a) shows one absorption maximum around 590 nm with
a broad shoulder towards longer wavelengths. The wavelength of
maximum absorbance is blue-shifted (H-band) with respect to the
absorption band of the molecularly dissolved species (670 nm).
Upon addition of S-SQ-1 to the feed and equilibration following
the protocol, the rise of an absorption band with maximum around
800 nm is observed. The band is most pronounced for pure S-
SQ-1 (red line in Figure 2a) and red-shifted (J-band) with respect
to the absorption band of the molecularly dissolved species. The
gradual increase of the J-band indicates changes in aggregate
structure when tuning the dye feed from a-SQ-1 to S-SQ-1. In the
spectra of all aqueous solutions, the absorption band of the
molecularly dissolved species is noticed as a small shoulder only,
which indicates the pronounced conversion from the molecularly
dissolved state to aggregates.
In contrast to S-SQ-1 and a-SQ-1, S-SQ-2 and a-SQ-2
exhibited shorter wavelengths of maximum absorbance of λ = 606
nm (for spectra see Figures S2-5) and a decreased molar
absorptivity (ε(S-SQ-2)606 nm = 1.5 •105 L mol-1 cm-1 and ε(a-SQ-
2)606 nm = 1.6 •105 L mol-1 cm-1). The shifts in values suggest a less
pronounced intramolecular charge transfer for S-SQ-2 and a-SQ-
2 compared to S-SQ-1 and a-SQ-1.
Figure 1. Molar extinction coefficients ε (solid lines) and fluorescence intensity
(dashed lines, λex = 640 nm) of S-SQ-1 and a-SQ-1 recorded in tetrahydrofuran.
The concentrations are 2.6•10-6 mol L-1.
Subsequently, absorption and emission spectra of all dyes
were measured in a range of solvents (acetonitrile, chloroform,
heptane, methanol and water) that differ in polarity. The recorded
spectra are shown in Figures S2-5. For all dyes, the absorption
band of the molecularly dissolved species dependents on the
solvent polarity and respond to the addition of an acid or base. A
detailed characterisation of the manipulation of the photophysical
properties upon the addition of acids or bases is given in the ESI
(See Figures S6-8). During the study, we found that S-SQ-2 and
a-SQ-2 have a limited chemical stability when molecularly
dissolved in dimethyl sulfoxide. An NMR study on the stability is
presented in the ESI (Figure S9). As a result, we decided to focus
our further investigations on S-SQ-1 and a-SQ-1.
Circular dichroism (CD) spectra were measured for
solutions of S-SQ-1, a-SQ-1 and mixtures thereof. The results are
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.
Figure 2. Photophysical properties of aggregates of S-SQ-1 and a-SQ-1 and mixtures thereof, recorded at a total concentration of 1.3•10-5 mol L-1 in H2O containing
1 vol% tetrahydrofuran. (a) UV/Vis spectra; (b) CD spectra; (c) Ellipticity measured at 868nm.
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Since no CD effect was observed for molecularly dissolved
neutral or charged forms, it is concluded that the CD effect
stemmed from supramolecular interactions. The helical structure
of the aggregates is indicated by the absorption band around 820
nm which displays a strong bisignate Cotton effect. When the
content of S-SQ-1 in a mix of S-SQ-1 and a-SQ-1 is increased,
the ellipticity that is determined at 868 nm increases
predominantly in a linear fashion as shown in Figure 2c. This
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.
absorbance at 790 nm. The spectra look very similar to the
spectrum recorded for pure a-SQ-1. Increasing the content of S-
SQ-1 to 75% in the mix results in a red-shift of the maximum of
absorbance to 799 nm. For pure S-SQ-1, the red-shift is even
more pronounced and the wavelength of maximum absorbance is
determined with 834 nm. These results indicate that thin films,
which comprise up to 50% S-SQ-1 in a mix of S-SQ-1 and a-SQ-
1, show an aggregation mode that is structurally very similar to
the aggregation of pure a-SQ-1. The mixtures containing 75% S-
SQ-1 in contrast shows an aggregation mode that is approaching
the aggregate structure formed by pure S-SQ-1.
Next, CD spectroscopy was used to investigate the thin films
(Figure 3b). Gratifyingly, thin 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. As noticed in Figure 3c, the CD effect
is most intense for a 1:3 mix of S-SQ-1 and a-SQ-1, and not for
pure S-SQ-1. This observation is in sharp contrast to the results
obtained in water, where the most intense CD-effect was
measured for the sample containing pure S-SQ-1. Additional
spectroscopic data obtained from the Muller matrix is discussed
in the ESI.
The spectroscopic data suggests that a-SQ-1 forms helical
aggregates with P- and M-helices as a racemic mixture, as the
addition of a small 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 spectra
indicate that the aggregate’s structure is not changed as well. S-
SQ-1 is structurally incorporated into the aggregates formed by a-
SQ-1. Owing to the chiral information of the stereogenic centres,
this 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” effect.[55] Interestingly, the
“sergeants-and-soldiers” effect is not operative over the whole
range of mixtures.
Formation of CD active aggregates of S-SQ-1 and 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 a solution 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 show a broad absorption band with a maximum that is
red shifted with respect to the absorption band of the molecularly
dissolved species. For a-SQ-1, this maximum is found at 790 nm
and for S-SQ-1 at 834 nm. Additionally, the absorption spectra of
both compounds exhibit a shoulder 620 nm which is in turn blue-
shifted with respect to the absorption band of the molecularly
dissolved species. The coinciding presence of a red- and blue-
shifted component in the spectra are 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, a structurally
similar packing of the chromophores in the aggregates, which 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. As shown in Figure 3a, the
absorbance spectra recorded for all thin films, where 5-50% S-
SQ-1 were present in the mixture showed a maximum of
Figure 3. Photophysical properties of thin films prepared from S-SQ-1 and a-SQ-1 and mixtures thereof. (a) UV/Vis spectra; (b) CD spectra; (c) Variation of the
ellipticity with increasing content of S-SQ-1 determined at 790 nm. All thin films have a thickness of approximately 40 nm.
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Figure 4. Photophysical properties of annealed (10 min, 210°C under nitrogen atmosphere) thin films prepared from S-SQ-1 and a-SQ-1 and mixtures thereof. (a)
UV/Vis spectra; (b) CD spectra.
Once S-SQ-1 becomes the major fraction in the feed, the
recorded absorbance spectrum starts to change, indicative of a
changed structure of the formed aggregates. At the same time,
the recorded CD effect is decreased. This observed trend is
fascinating since it shows that the maximisation of the amount of
stereogenic centres in the aggregates – by increasing the amount
of S-SQ-1 in the feed – does not yield the thin film with the CD
effect. The highest CD effect is achieved when S-SQ-1 is solely
used as a director for a-SQ-1, rather than a structural component.
In order to elucidate whether the measured ellipticity is
caused by purely excitonic coupling or also stems from other
effects such as cholesteric ordering, thin films with different
thicknesses were prepared by spin coating. As shown in Figure
S10, the ratio of measured ellipticity and film thickness was
relatively constant for rather thin films (thicknesses between 11
and 24 nm). When the film thickness was increased further, an
increase of the ratio was noticed. In very thin films, the CD effect
can only originate from the mechanism operative at the nm scale,
which suggests the presence of excitonic coupling. The increase
of the ratio with increasing film thickness suggests that the
recorded CD effect stems partly 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 polarized optical
microscopy (Figure S12), 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 a heating and cooling
cycle, the impact of annealing on the aggregated structures in thin
film was probed. All thin films were annealed for 10 min at 210 °C
under nitrogen atmosphere which is well below the molecules’
decomposition temperature. Subsequently, the optical properties
were investigated again. The absorbance spectra recorded after
annealing are depicted in Figure 4a. The thin films with low S-SQ-
1 content (0-30%) exhibit a decrease of the former band of
maximum absorbance around 790 nm and the rise of two new
absorption bands around 600 nm and 700 nm, respectively. For
nm after annealing. Besides changes in the UV/Vis spectra, the
CD spectra recorded for all samples show differences after
annealing, too. As depicted in Figure 4b, samples with low S-SQ-
1 content (0-30%) showed a shift of the wavelength of maximum
CD effect to shorter wavelengths. After annealing, the maximum
CD effect was noticed around 690 nm (formerly 790 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 spectra upon annealing suggest therefore
that the aggregates formed during spin coating were the result of
kinetic effects. The recorded LD spectra are depicted in Figure
S11 and 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-filtering experiments. Magnetic-
conductive atomic force microscopy (mc-AFM) is an efficient
method for studying the spin-selectivity in the electron transport
through chiral structures including the effect of the interface
between the probed material and the substrate.[56] The electrical
measurement records the current transported through a thin film
of organic matter deposited on top of a gold-coated nickel surface.
During 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 a bias in their populations
of electron spin alignments (spin-up and spin-down). It is
important to note that the values are determined in the nonlinear
regime 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, pure S-SQ-1 and the
3:1 mixture 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. For the sample containing purely S-SQ-1, only a small
impact is found. The most pronounced dependency of the I-V
curves on the orientation of the magnet is found for the 3:1 mixture
of both a-SQ-1 and S-SQ-1.
thin films with
a higher S-SQ-1 content (50-100%), the
absorbance of the band around 600 nm intensifies slightly. The
position of the maximum of absorbance is retained around 800
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Figure 5. Spin-dependent conduction through thin films consisting of a-SQ-1 (a), S-SQ-1 (b) and 3:1 mixture thereof (c). The I-V plots show determined averages
and standard deviations over 100 individual measurements. For each sample measurements were performed with the Ni film magnetised with the north pole pointing
up (orange) or down (blue). (d) Spin polarisation calculated from the mc-AFM measurements.
From the recorded measurements, the spin polarisation was
formation of a highly-ordered chiral structure is impaired for the
single-stranded version. With the loss of chiral order, spin
selective properties are lost. The system presented by us
underlines the cruciality of chiral (supramolecular) order. Although
the number of stereogenic centres is increased when comparing
the S-SQ-1 sample with the 3:1 mixture of a-SQ-1 and S-SQ-1, a
pronounced decrease in spin polarisation is noticed which
coincides with the decrease of optical activity.
ꢇꢄ
determined using ꢀꢁ ꢂ ꢃꢄꢄꢅꢆ ꢈꢉꢊꢋꢍ ∙ 100. The spin polarisation at
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3V was determined with 0% for a-SQ-1, 17 5% for S-SQ-1 and
56 8% for the 3:1 mixture. As noticed in the Figure 5d, similar
values are found for lower potentials, too. The upper limit of 60%
corresponds to a ratio of about four to one between the two spin
states. Various systems, that are based on a homochiral
compound only, report a similar spin polarisation, too.[29,57–63]
Studies on comparing chiral and achiral materials are reported
less often.[30,31] Only one very recently reported study investigated
the spin polarisation of mixtures of chiral molecules and their
achiral counterparts.[30] The compounds were coassembled into
supramolecular nanofibres. The spin selective measurements
indicated a correlation of the molar circular dichroism of the
nanofibres in solution and the spin polarisation which was
measured after drop casting and drying. However, both maxima
of molar circular dichroism and spin polarisation were found for
the sample containing purely chiral material. In the system
presented by us, we find the most selective electron transport for
materials with the highest optical activity. Since this material does
however not contain the highest number of stereogenic centres in
the series, our findings indicate a strong correlation between spin
polarisation during the electron transport and the optical activity
of the organic material. A similar correlation has been found for
deoxyribonucleic acid (DNA) based spin filters which showed a
drastically reduced spin-selective electron transport when used
as single-strands instead of double-strands.[29] Although the
number of stereogenic centres in both types of DNA is equal, the
Conclusions
The formation of squaraine dyes is commonly achieved by the
reaction of squaric acid with aniline-benzenes. The presented
study shows that another class of substrates that is suitable for
this type of reaction is given by amide substituted hydroxyl-
benzenes. Both aniline- and 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. Anilino
substituted S-SQ-1 and a-SQ-1 change their colour from green to
violet upon protonation. Blue hydroxyl-benzene substituted S-SQ-
2 and a-SQ-2 change their colour to red when singly deprotonated.
S-SQ-1 and a-SQ-1 were aggregated in aqueous 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 increasing the amount of chiral S-SQ-1 in the mix.
This linear increase indicates that the aggregates of S-SQ-1 and
a-SQ-1 are either barely mixing or the ability of S-SQ-1 to bias
one helical sense in the mix is low. In contrast, spin-coated thin
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coating were prepared by weighing the necessary amount of compound
for the given concentration and transferring into a screw-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 °C for 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 μL of the solution containing the
dye were deposited on microscope glass slides and subsequently spun
(Headway Research Inc., conc. 5 mM, 10 mM or 20 mM, spinning speed
1000 rpm or 2000 rpm, 45 sec). The microscope glass slides had
previously been cleaned by sonication in acetone, demineralized water
and isopropanol for 5 min each. For the determination of optical properties
after annealing, the spin coated samples were annealed at 210 °C for 10
min under nitrogen atmosphere. UV/Vis, CD and LD spectroscopy were
performed on a JASCO J-815 CD spectrometer either a JASCO Peltier
MPTC-490S temperature controller with a range of 278 – 373 K or a
JASCO Peltier PFD-425S/15 with a range of 263 – 383 K. Separate UV/Vis
films of S-SQ-1 and a-SQ-1 resulted in co-assembled structures
that displayed a non-linear increase of the CD effect. Surprisingly,
the highest dissymmetry was not measured for a thin film that
contained pure S-SQ-1 but a 1:3 mixture of S-SQ-1 and a-SQ-1
molecules. The similarity of the absorption spectrum of the 1:3
mixture of S-SQ-1 and a-SQ-1 with respect to the absorption
spectrum of pure a-SQ-1 indicates a very similar aggregation
structure for these two samples. Opposed to pure a-SQ-1, the 1:3
mixture is highly CD active, which in turn suggests that the
population between the present P- or M-helices has been altered.
Additionally, this sample exhibits a higher CD effect than pure S-
SQ-1. In contrast to previous studies, the preparation of solid dye
structures with the highest CD effect is achieved by using a chiral
compound (S-SQ-1) solely as a director for an achiral compound
(a-SQ-1), rather than a structural component. This nonlinear
behaviour was further confirmed by mc-AFM measurements, as
the highest spin selectivity was found for the 1:3 mixture as well.
Annealing of the spin-coated films resulted in changes of
absorption and CD spectra which suggests the coassembly to be
a kinetically controlled process. We note that the degree of
circular polarization in absorption displayed by the chiral amide
derivates under study is much smaller than observed for 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 strong with coupling constants in the order of one
eV.[64] This strong coupling, most likely contributes to
unconventional optical properties which remain yet to be fully
explored.
spectra were recorded on
a JASCO V-750 UV/Vis spectrometer.
Fluorescence spectroscopy was performed on a Perkin Elmer LS 50 B
luminescence spectrometer. Optical 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
89229F10L1333). The dimensions of the tips were: length of 150 μm, width
of 27 µm and thickness of 2.8 µm. The spring constant was 7.4 N/m and
soft tapping-mode was used to obtain an image. A resolution 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 a chloroform solution
on top of a gold-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 a custom-designed RHK machine capable
of reaching a magnetic field of 1 T. The AFM cantilevers were platinum-
coated silicon tips supplied by Micromasch (HQ:DPE-XSC11, spring
constant: 2.7 N/m). 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 -3 to +3 V. Reflectivity
and Muller matrix spectroscopy was performed on a W-VASE ellipsometer
from J.A. Wollam.
Considering the emerging work on spintronic applications
e.g. photoelectrochemical water splitting, where devices
functionalised with a chiral, non-racemic active layer yielded
higher currents than for achiral counterparts,[31,33] the presented
system provides an promising basis to further probe the impact of
supramolecular rather than molecular chirality on material
properties. Utilisation in spintronic applications will be the topic of
future investigations.
Experimental Section
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.[39,50–52] A detailed summary
is given in the ESI. The squaraine dye formations were performed using a
modification of the original procedure by Sprenger and Ziegenbein.[16,35]
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 a Grace Reveleris X2 automated column machine with
Reveleris Silica Flash Cartridges and on a Biotage Isolera 1 with Biotage
a-SQ-1: A flask filled with 80 mL of 50/50 n-propanol/toluene was fitted
with a Dean-Stark trap with cooler and heated to 110 °C. After the trap was
fully filled with solvent and manually emptied once, squaric acid (218 mg,
1.9 mmol, 1 eq.) was added. Heating and stirring were continued for 15
minutes before N-(3-(dimethylamino)phenyl)octanamide (1.0 g, 3.8 mmol,
2 eq.) was added. The solution turned dark green and heating with stirring
was continued. After 20 hours of reaction, the mixture had a deep
turquoise colour. The reaction mixture was cooled down and the solvent
was removed by rotary evaporation to afford a green crude product (1.08
g, 1.79 mmol, 89%). The crude product was recrystallised from acetonitrile
to afford pure a-SQ-1 in high yield (0.97 g, 1.61 mmol, 85%). 1H NMR (400
MHz, Chloroform-d): δ [ppm] = 12.04 and 11.94 (s, 2H), 8.49 and 8.43 (d,
J = 9.2 Hz, 2H), 8.25 and 8.22 (d, J = 2.4 Hz, 2H), 6.45 – 6.42 (m, 2H),
SNAP KP-SIL columns. H- and 13C-NMR spectra were recorded with a
1
Varian Mercury Vx 400 MHz. Deuterated solvents were purchased from
Cambridge Isotope Laboratories and are indicated for each measurement.
Chemical shifts (δ) are expressed in ppm and are referred to the residual
peak of the solvent. Peak multiplicity is abbreviated as s: singlet; d: doublet,
q: quartet; p: pentet; m: multiplet; dd: double doublet; dt: double triplet; dq:
double quartet. MALDI-TOF spectra were recorded with Bruker Autoflex
Speed using α-cyano-4-hydroxycinnamic acid (CHCA) or trans-2-[3-(4-
tert-butylphenyl)-2-methyl-2-propenyl-idene]-malononitrile (DCBT) as
matrix. The stock solutions used for the optical measurements and spin
This article is protected by copyright. All rights reserved.

10.1002/chem.202002695
Chemistry - A European Journal
FULL PAPER
3.18 (s, 12H), 2.62 and 2.56 (t, J = 7.2 Hz, 4H), 1.76 (quin, J = 7.2 Hz, 4H),
1.46 – 1.29 (m, 16H), 0.876 (t, J = 6.0 Hz, 6H); 13C NMR (101 MHz,
Chloroform-d): δ [ppm] = 182.74, 182.53 and 181.52 (2C), 175.86 and
175.33 (2C), 174.19 and 173.87 (2C), 156.88 (2C), 144.09 (2C), 133.95
and 133.30 (2C), 112.76 and 112.55 (2C), 108.64 (2C), 102.48 (2C), 40.63
(4C), 38.18 and 38.01 (2C), 31.93 and 31.89 (2C), 29.62, 29.40, 29.36 and
29.33 (4C), 25.42 and 25.42 (2C), 22.87 (2C), 14.53 (2C); MALDI/TOF
found 602.42 m/z (calculated 602.38); FT-IR (cm-1): 3135 (w), 3096 (w),
2953 (w), 2918 (m), 2872 (w), 2852 (m), 2808 (w), 2676 (w), 1706 (m),
1609 (s), 1578 (s), 1535 (m), 1483 (m), 1458 (m), 1389 (s), 1352 (s), 1278
(s), 1242 (s), 1207 (s), 1179 (s), 1154 (s), 1125 (s), 1090 (s), 1063 (s), 958
(m), 944 (m), 891 (s), 853 (s), 821 (s) 804 (m), 781 (s), 725 (m), 707 (m),
684 (m), 663 (m), 641 (m), 593 (w), 565 (w), 524 (s) 500 (s), 461 (s).
mg, 0.17 mmol, 1 eq.) was added and heating was continued for three
hours. (S)-N-(3,5-dihydroxyphenyl)-3,7-dimethyloctanamide (100 mg,
0.36 mmol, 2.1 eq.) was added. The mixture was heated and stirred for
20 h. The reaction mixture formed a deep blue colour, was cooled down
and concentrated until dryness. The dark blue residue was dispersed in 40
mL ethyl acetate by sonication. After extraction with 20 mL 0.5 M aqueous
hydrochloric acid solution a dark blue solid was filtered off the organic layer.
The obtained product was dried in vacuum and characterised without
further purification. S-SQ-2 was obtained in 11% yield (12 mg, 0.02 mmol).
The low yield was caused by the small scale of the reaction. 1H NMR (400
MHz, DMSO-d6): δ [ppm] = 12.41 (br, 2H), 11.26 (s, 2H), 7.58 (s, 2H), 5.97
(s, 2H), 2.41 (dd, J = 14.2, 6.4 Hz, 2H), 2.23 (dd, J = 14.3, 7.6 Hz, 2H),
1.98 (br, 2H), 1.54 – 1.44 (m, 2H), 1.35 – 1.09 (m, 12H), 0.92 (d, J = 6.7
Hz, 6H), 0.83 (d, J = 6.6 Hz, 12H). MALDI/TOF found 659.34 m/z
(calculated [M]+Na+ 659.33). FT-IR (cm-1): 2955 (m), 2928 (m), 2868 (m),
2620 (br), 1682 (w), 1610 (m), 1583 (s), 1517 (m), 1458 (w), 1426 (m),
1383 (m), 1325 (w), 1229 (s), 1196 (s), 1174 (s), 1125 (s), 1024 (s), 953
(m), 866 (m), 806 (m), 770 (m), 723 (s), 659 (w), 616 (m), 598 (m), 535
(m), 486 (m). Elemental analysis: found 66.91% C, 7.47% H, 4.29% N,
19.84% O (calculated: 67.90% C, 7.60% H, 4.40% N, 20.10% O).
S-SQ-1: A flask filled with 100 mL of 50/50 n-propanol/toluene was fitted
with a Dean-Stark trap with cooler and heated to 110 °C. After the trap was
fully filled and manually emptied twice, squaric acid (99.7 mg, 0.86 mmol,
1 eq.) was added. After 30 minutes, (S)-N-(3-(dimethylamino)phenyl)-3,7-
dimethyloctanamide (0.51 g, 1.7 mmol, 2 eq.) was added to the solution
which turned dark green upon addition. After 20 hours of reaction, the
mixture had a deep turquoise colour. The reaction mixture was cooled
down and the solvent was removed by rotary evaporation to afford a green
crude product (507 mg, 0.77 mmol, 45%). The crude product was
recrystallised from acetonitrile to afford pure S-SQ-1 in high yield (456 mg,
0.69 mmol, 81%). 1H NMR (400 MHz, Chloroform-d): δ [ppm] = 12.08 and
11.94 (s, 2H), 8.55 and 8.49 (d, 9.2 Hz, 2H), 8.31 and 8.30 (d, 2.4 Hz, 2H),
6.48 (dd, J = 9.2 Hz, J = 2.4 Hz, 2H), 3.22 (s, 12H), 2.66 – 2.53 (m, 2H),
2.45 – 2.37 (m, 2H), 2.14 (br, 2H), 1.59 – 1.47 (m, 2H), 1.45 – 1.14 (m,
12H), 1.03 (d, J = 6.6 Hz, 6H), 0.88 – 0.84 (m, 12H); 13C NMR (101 MHz,
Chloroform-d): δ [ppm] = 182.74 and 181.68 (2C), 176.03 and 175.57 (2C),
173.85 and 173.47 (2C), 156.95 (2C), 144.09 and 143.98 (2C), 134.00 and
133.38 (2C), 112.80 and 112.59 (2C), 108.73 (2C), 102.60 (2C), 45.91 and
45.65 (2C), 40.67 (4C), 39.23, 37.37 and 37.23 (4C), 30.86 and 30.83 (2C),
28.15 (2C), 25.08 and 25.01 (2C), 22.87, 22.85, 22.75 and 22.71 (4C),
20.07 and 19.96 (2C); MALDI/TOF found 658.47 m/z (calculated 658.45);
FT-IR (cm-1): 3133 (w), 2953 (m), 2925 (m), 2868 (m), 2681 (w), 1710 (w),
1607 (s), 1574 (s), 1532 (m), 1484 (w), 1455 (m), 1412 (w), 1382 (m), 1354
(s), 1325 (s), 1277 (s), 1241 (s), 1195 (s), 1139 (s), 1124 (s), 1063 (s), 902
(s), 879 (s), 862 (s), 812 (s), 781 (s), 725 (s), 681 (m), 664 (m), 649 (m),
556 (w), 520 (s), 498 (s), 463 (m).
Acknowledgements
The authors acknowledge Chidambar Kulkarni for fruitful discussions. Ralf
Bovee is thanked for conducting MALDI-TOF measurements. We
acknowledge funding from the Dutch Ministry of Education, Culture and
Science (Gravity program 024.001.035).
Conflict of interest
The authors declare no conflict of interest.
Keywords: circular dichroism • helical structures • self-assembly
• squaraine dyes • supramolecular chemistry
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This article is protected by copyright. All rights reserved.

10.1002/chem.202002695
Chemistry - A European Journal
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10.1002/chem.202002695
Chemistry - A European Journal
FULL PAPER
Andreas T. Rösch,[a] Qirong Zhu,[b] Jorn
Robben,[a] Francesco Tassinari,[b] Stefan
C.J. Meskers,[c] Ron Naaman,[b] Anja R.A.
Palmans,[a] and E.W. Meijer[a]*
Helicity control: An unexpected outcome of the co-assembly of achiral squaraine
dye a-SQ-1 with its chiral counterpart S-SQ-1 is reported. A maximum in optical
activity is observed in the supramolecular aggregates of a 3:1 mixture with a-SQ-1
as the major component. Intriguingly, the highest spin-selective electron transport is
observed in these mixtures as well, illustrating the direct relationship between optical
activity and spin-selective electron transport.
Page No. – Page No.
Helicity control in the aggregation of
achiral squaraine dyes in solution and
thin films
This article is protected by copyright. All rights reserved.