Assembly of T-Shaped Amphiphilic Thiazoles on the Air-Water Interface: Impact of Polar Chromophore Moieties, as Well as Dipolarity and π-Extension of the Chromophore on the Supramolecular Structure

Article abstract of DOI:10.1021/acs.langmuir.8b04063

The supramolecular structure essentially determines the properties of organic thin films. In this work, we systematically investigate the influence of the chromophore on the supramolecular structure formation at air-water interfaces by means of the Langmu

Full text of DOI:10.1021/acs.langmuir.8b04063

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge
Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage
Assembly of T-Shaped Amphiphilic Thiazoles on the Air-Water Interface:
Impact of Polar Chromophore-Moieties, as well as Dipolarity and #-
Extension of the Chromophore on the Supramolecular Structure
Maximilian Lutz Hupfer, Martin Kaufmann, Julia Preiß, Dieter
Weiss, Rainer Beckert, Benjamin Dietzek, and Martin Presselt
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04063 • Publication Date (Web): 28 Jan 2019
Downloaded from http://pubs.acs.org on February 1, 2019
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Assembly of T-Shaped Amphiphilic Thiazoles on the Air-Water Interface:
Impact of Polar Chromophore-Moieties, as well as Dipolarity and π-Extension
of the Chromophore on the Supramolecular Structure
Maximilian L. Hupfer1,3, Martin Kaufmann1,2, Julia Preiß1,3, Dieter Weiß2, Rainer Beckert2, Benjamin Dietzek1,3,
Martin Presselt1,3,4,5*
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1
Institute of Physical Chemistry, Friedrich Schiller University Jena, Helmholtzweg 4, 07743 Jena, Germany, E-
mail: martin.presselt@leibniz-ipht.de, Phone: +49 3641 206418
2
Institute of Organic and Macromolecular Chemistry, Friedrich Schiller University Jena, Humboldstraße 10,
07743 Jena, Germany
3 Leibniz Institute of Photonic Technology (IPHT), Albert-Einstein-Str. 9, 07745 Jena, Germany
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Center for Energy and Environmental Chemistry Jena (CEEC Jena), Friedrich Schiller University Jena,
Philosophenweg 7a, 07743 Jena, Germany
5 sciclus GmbH & Co. KG, Moritz-von-Rohr-Str. 1a, 07745 Jena, Germany
*Corresponding author: martin.presselt@leibniz-ipht.de
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KEYWORDS
Langmuir-Blodgett; Langmuir isotherm; non-covalent interactions; organic electronics; aggregate; in
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situ fluorescence
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ABSTRACT
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The supramolecular structure essentially determines the properties of organic thin films. In this work
we systematically investigate the influence of the chromophore on the supramolecular structure
formation at air-water interfaces by means of the Langmuir-Blodgett (LB) technique. Therefore, we
focus on the recently introduced class of double-anchor T-shaped amphiphilic dyes, namely 4-
hydroxy-thiazole chromophores that are centrally equipped with an amphiphilicity-inducing hexanoic
acid. The thiazoles contain hydrophilic subphase-anchor groups in the 2-position (4-N,N-
dimethylaminophenyl (Am), 2-pyridyl (Py), and 4-nitrophenyl (Ni)), while the chromophores are
systematically extended in the 5-position with various substituents. The combination of Langmuir
technique with online fluorescence measurements revealed that the ππ-interactions that are
pronounced in case of 4-methoxybiphenyl derivatives yield the most distinct supramolecular
structures: While in case of Py- and Ni-derivatives ordered J-type supramolecular structures in
microdomains are formed, the Am-derivative forms ordered supramolecular structures that are
more homogeneous, which are, however, not stabilized by J-type dipolar interactions. Because of the
synergetic ππ- and dipolar stabilizations the Ni-derivative bearing the 4-methoxybiphenyl unit forms
exceptionally stable quasi-two-dimensional Langmuir monolayers reaching very high surface
pressures beyond 60 mN/m without any sign of disturbance of the Langmuir monolayer.
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INTRODUCTION
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Organic (opto)electronic devices like solar cells (OSCs)1, light emitting diodes (OLEDs)2, and field
effect transistors (OFETs)3 are steadily gaining importance. The active layer and device properties,
especially spectral4, 5, 6, 7, 8 and radiative9, 10 characteristics, external quantum efficiency11, 12, 13, 14, 15
,
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open circuit voltage16, conductivity17, 18, 19, 20, excited state dynamics21, 22, 23, and electrochemical
potentials4, 24, 25 are tuned by engineering the molecular characteristics and the supramolecular
structure. To control supramolecular structures several techniques, like antiparallel supramolecular
arrangement of dipolar donor-acceptor dyes26, 27, co-self-organization 28, layer-by-layer deposition
(LbL) 29, 30, and (self)assembly at heterointerfaces, have been developed. Another valuable technique
that allows for advanced control of the supramolecular structure formation at liquid-gas interfaces is
the Langmuir-Blodgett (LB) technique,31, 32, 33, 34 which enables supramolecular tuning of optical and
optoelectronic properties of functional amphiphiles.4, 35, 36
The supramolecular geometries and their optoelectronic properties inherently depend on the
molecular parameters37, 38, 39, 40, such as the chromophore, chemical nature, number and position of
polar and non-polar moieties in the amphiphile. It was shown that even the lengths and the type of
spacers41 between polar and non-polar as well as the specific chemical nature of the polar moieties41,
42, 43 heavily influence the geometries of supramolecular structures formed at the air-water interface.
Recently, novel T-shaped amphiphilic dyes have been presented in which the amphiphilicity-
introducing hexanoic acid is linked centrally to rather hydrophobic chromophores.41, 44, 45, 46 The
flexibility of the hexanoic acid41, but also the tunable polarity of the chromophore, particularly
introduction of second hydrophilic subphase anchor groups and introduction of dipolarity,45 make
this class of amphiphiles highly versatile and enables formation of different supramolecular structure
at the air-water interface.41, 44, 45
Basically just two different types of chromophores from this type of T-shaped amphiphiles have been
reported in the literature so far.45 Here, we attempt to systematically study the influence of chemical
variations of the chromophore (see Scheme 1) on the formation of supramolecular structures on air-
water interfaces and the resulting optical properties. The large number of supramolecular structures
that can form from the high number of investigated derivatives at varied Langmuir parameters are
characterized via analysis of the Π(A) isotherm and in situ fluorescence measurements47. UV-vis
spectroscopy has been demonstrated to enable determination of interactions of the dye with the
subphase47 and thermodynamics in amphiphile mixtures48, or changes of the supramolecular
geometry43.
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Scheme 1: Central attachment of a hydrophilic head group to a thiazole chromophore, which is systematically
varied on the 2- and 5-position, via an alkylic linker yields a “T-shaped” amphiphile with various intra- and
intermolecular interactions in a Langmuir monolayer
The T-shaped amphiphiles derived from 4-hydroxy-thiazoles are predestined for in situ fluorescence
characterization as they often feature high fluorescence quantum yields (up to 95% in water49).
Because of the corresponding slow non-radiative excited state decay, 4-hydroxy-thiazoles have been
tested for application in dye-sensitized solar cells (DSSCs)50, as blue emitters in polymer backbones51,
52
and as oligomers in organic field effect transistors53. In this work the thiazole chromophore is
varied by introducing three different hydrophilic subphase anchors in the 2-position of the thiazole,
namely 4-N,N-dimethylaminophenyl (Am), 2-pyridyl (Py), and 4-nitrophenyl (Ni).54, 55 Furthermore,
the chromophore gets systematically π-extended in the 5-position for each of the anchor functions
by substituting the methyl group (Me) by a 4-bromophenyl (Br), and 4-methoxybiphenyl (Bi). Finally,
the chromophores are additionally equipped with
a
third hydrophilic anchor,
a
4-methoxyphenyl-3-sulfonylphenyl (BiSulf) moiety. After presenting the synthesis these
systematically varied amphiphilic T-shaped thiazoles, their optical properties in solution and at an air-
water interface are determined and compared with mean molecular areas derived from Langmuir
experiments to understand the film formation and to derive schemes of supramolecular structures
formed at the air-water interface.
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RESULTS AND DISCUSSION
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To investigate a broad range of the effects of the chromophore, three different core units of the
thiazoles were selected as target structures. A N,N-dimethylaminophenyl-derivative was selected to
represent electron rich substituents, a nitrophenyl-derivative as electron deficient substituent and a
2-pyridyl-derivative as a “neutral”, heterocyclic derivative. Based on the three core units, the
electron rich dimethylphenyl thiazole, the electron deficient nitrophenyl thiazole and the “neutral”,
hetrocyclic pyridyl thiazole, three successively expansions (methyl group, bromphenyl group,
methoxybiphenyl group, sulfonation) of the π-system are targeted. The synthetic route is depicted in
Figure 1.
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Starting from the nitriles 1, the 5-methyl thiazoles 3 are accessible by the Erlenmeyer Route, in which
the nitrile is condensed with thiolactic acid 2, as shown in Figure 1. According to the method by
Calderon56 the pyridyl derivative and the nitro derivative Ni 3 were obtained in 76-81% yields.
However, the amino thiazole couldn’t be isolated because of degradation while the work up
procedure. The alkylation with ethyl 6-bromo-hexanoate not only increases the solubility of the dye
but build up the precursor for the amphiphilic anchor group. It was performed by standard
procedure, which is published for 4-hydroxy-thiazoles50, 57, 58. For the second π-expansion step, the
thiazoles core units has to be synthesized with the well-established Hantzsch thiazole synthesis59, 60,
61. Starting from the thioamides (Am/Py/Ni) 5 the condensation with 6 leads to the 5-Bromophenyl-
4-hydroxy thiazoles 7 which were obtained in very good yields from 60 to 87%. The following
alkylation iv was carried out under the same conditions as in reaction step ii and alkoxy thiazoles 8
were obtained in 72-83% yields. A further expansion of the chromophore is possible by cross
coupling of the 5-(4-bromophenyl)-substituent. Several different cross coupling reactions, like
Buchwald-Hartwig Aminations58, 62, Sonogashira Cross-Coupling63 or Stille Cross Couplings are already
described for 5-Bromophenyl thiazoles. Still no Suzuki reactions on these dyes are known up to date.
However, Arcardi64 et al. performed various Suzuki cross couplings on 4-triflyl thiazoles, which are
structural comparable to our target molecules. With a slight variation of this method (changing the
catalyst from Pd(PPh3)4 to Pd(PPh3)PyBil2) the 5-(4-methoxybiphenyl)-thiazoles 9 were obtained in
moderate 31-56% yields. With the successful syntheses of 4, 8 and 9 the ester group of the
hexanoate sidechain needs to be saponificated to yield the amphiphilic anchor group. Recently the
deprotection of such alkyl esters of 4-alkyoxy thiazoles was published, using a anhydrous ethanolic
potassium hydroxide solution41, 42. Using this procedure, the aminothiazoles AmBr and AmBi, the
pyridylthiazoles PyMe, PyBr and PyBi and the nitrothiazoles NiMe, NiBr, NiBi were isolated in very
good yield from 87-95%. Finally the 5-(4-methoxybiphenyl)-thiazoles AmBi, PyBi and NiBi were
sulfonated by the method of Habenicht43. All target molecules except the sulfonated AmBiSulf,
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PyBiSulf and NiBiSulf are good soluble in chloroform or dichloromethane, which are suitable solvents
for the Langmuir technique. The sulfonic acid group increases the hydrophilicity thus the best
solubility of these was observed in methanol. However a mixture of chloroform and methanol can be
used to achieve the necessary minimum solubility for the Langmuir technique.
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Figure 1: Synthetic route to the target compounds; i) pyridin, ΔT, 0-81%; ii) BrC5H10CO2Et, KPyBiO3, acetone , ΔT,
64-73%; iii) NaOAc, EtOH, ΔT, 60-87%; iv) BrC5H10CO2Et, KPyBiO3, acetone , ΔT, 72-83%; v) p-OMePh-B(OH)2,
KPyBiO3, Pd(PPh3)PyBil2, thf, ΔT, 31-56%; vi) KOH, EtOH (abs.) , 50°C; vii) AcOH, H2SO4, RT – 50°C, 23-39%.
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UV-vis Absorption and Fluorescence Spectroscopy
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The influence of the above introduced modifications to the thiazole chromophores are characterized
by means of UV-vis absorption and fluorescence spectroscopy in solution. The variation in solvent
polarity yields reference spectra for the thiazoles in different supramolecular environments studied
in the subsequent Langmuir sections.
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Table 1. Extinction coefficients (ε(λmax)) (determined via linear regression for concentrations ranging from
c = 10-4 M to 10-7 M, see supporting information Figure Si 64-71), spectral positions of absorption and emission
maxima (c < 10-6M) of 2-(4-N,N-dimethylaminophenyl/2-pyridyl/4-nitrophenyl)-4-hydroxy-5-Y-1,3 thiazoles
with different functionalization
TD-DFT
Toluene
CHCl3
CH3OH
S0 -> S1 dielectric constant: 2.4 dielectric constant: 4.8
dielectric constant: 33
R
Y
vac
ε
abs
em
ε
abs
em
ε
abs
em
[nm]
355
403
415
419
353
400
422
408
429
476
519
487
[M-1cm-1] [nm] [nm] [M-1cm-1] [nm] [nm] [M-1cm-1]] [nm] [nm]
Me
Br
40.724
51.712
396 483
400 494
458
39.204
48.833
394 485
400 497
400 469
345 420
375 454
392 482
478
43.611
75.764
391 485
396 496
397 496
342 422
375 455
386 492
386 484
383 450
411 450
426 484
425 485
Am
Bi
BiSulf
Me
Br
12.512
12.274
43.656
347 420
376 452
393 479
9.127
22.696
36.612
11.145
27.882
28.652
Py
Ni
Bi
BiSulf
Me
Br
18.524
19.525
26.037
391 500
419 525
436 562
432 546
27.348
30.201
36.091
393 588
423 615
439 689
430 669
23.944
24.965
44.795
Bi
BiSulf
4-N,N-Dimethylaminophenyl Derivatives
The extinction coefficient (ε(λ)) spectra of Am (Figure 2) show a pronounced single absorption peak
(cf. quantum chemical results shown in the Supporting Information (SI) Figure SI 37) with an increase
of ε and a slightly red-shift upon extension of the π-electron system63 from AmBr to AmBi (Figure 2).
In AmBi, the CH3O-group adds a weak electron donating (D) character65 to the chromophore and
complements its DπD character, what causes an additional red-shift in the absorption. Introduction
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of an additional SO3Hgroup (AmBiSulf) significantly decreases the solubility in non- and slightly polar
solvents and does not notably shift the absorption maximum as compared to AmBi. In agreement
with recent reports on thiazole chromophores, their absorption spectra negligibly blue-shift upon
increased solvent polarity.44
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However, the extension of the π-electron system causes a red-shifted emission for the different Y
functionalizations from AmBr to AmBiSulf. As an exception, the emission spectrum of AmBiSulf
shows a slightly blue-shifted emission in non- and aprotic polar solvents (toluene, CHCl3), as
compared to AmBi, what might be attributed to the presence of ground state aggregates and a
stabilization of excited states. Additionally, dipolar aggregation could cause a protonation of the
(CH3)2N-group by the SO3H moiety.
Figure 2: Extinction coefficient (ε) (left) and normalized emission (right) of the 2-(4-N,N-dimethyl-
aminophenyl)-4-hydroxy-5-Y-1,3 thiazole core (λexc=405 nm) and the methyl (AmMe) (solid), phenyl-4-bromo
(AmBr) (dotted), phenyl-4-methoxyphenyl (AmBi) (dashed) and 3-phenyl-4-methoxyphenyl-3-sulfonylphenyl
(AmBiSulf) (dotted dashed) functionalization in toluene (dark green, upper row), CHCl3 (violet, middle row),
CH3OH (pink, lower row) (c < 10-6M)
2-Pyridyl Derivatives
The extinction coefficient and emission spectra of the Py-derivatives significantly red-shift upon
extension of the π-system, as shown in Figure 3. Hence, while in case of Am the absorption and
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emission properties were largely determined by the electron-pushing amine moiety, the pyridine
moiety in Py enables enlarging the chromophoric unit upon attaching phenylene groups.
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However, all spectra of the Py-derivatives are blue-shifted as compared to their Am-counterparts
(compare Figure 2 with Figure 3; see Table 1), what is in agreement with reports on similar
chromophores.63 This difference in the spectral position increases with shortening of the π-system,
i.e., as most pronounced for the (Am/Py)Me derivatives. As mentioned for AmBiSulf above, an
introduction of a SO3H-group at the 4-methoxybiphenyl moiety (PyBiSulf) significantly decreases
solubility in non-polar solvents and again, the emission shifts slightly blue due to aggregation in
toluene and CHCl3.
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Figure 3: Extinction coefficient (ε) (left) and normalized emission (right) of the 2-(2-pyridyl)-4-hydroxy-5-Y-1,3
thiazole core (405 nm) methyl (PyMe) (solid), phenyl-4-bromo (PyBr) (dotted), phenyl-4-methoxyphenyl (PyBi)
(dashed) and 3-phenyl-4-methoxyphenyl-3-sulfonylphenyl (PyBiSulf) (dotted dashed) functionalization in
toluene (dark green, upper row), CHCl3 (violet, middle row), CH3OH (pink, lower row) (c < 10-6M)
4-Nitrophenyl Derivatives
Similar to the Py-thiazoles before, the Ni-derivatives show red-shifted absorption (Figure 4) upon
extension of the π-system through the different Y-functionalizations. In contrast to Am and Py, the
Ni-derivatives show a pronounced solvent-dependence of their fluorescence spectra, as shown in
Figure 4. While the fluorescence spectra of Ni dissolved in methanol are similar to those of Am and Py
(λmax = 450-500 nm, Am and Py in all solvents), they significantly red-shift to λmax = 500-575 nm in
case the Ni derivatives are dissolved in toluene. A further red-shift to λmax = 575-700 nm is observed if
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the solvent is CHCl3. In case of NiBi an additional peak between 450-500 nm is present in the CHCl3
fluorescence spectrum. This peak, which features vibrational progression similar to the spectra of
NiBi and NiBiSulf dissolved in CH3OH, actually dominates the fluorescence spectra of the polar
NiBiSulf derivatives when dissolved in CHCl3.
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Figure 4: Extinction coefficient (ε) (left) and normalized emission (right) of the 2-(4-nitrophenyl)-4-hydroxy-5-Y-
1,3 thiazole core (λexc=405 nm) methyl (NiMe) (solid), phenyl-4-bromo (NiBr) (dotted), phenyl-
4-methoxyphenyl (NiBi) (dashed) and 3-phenyl-4-methoxyphenyl-3-sulfonylphenyl (NiBiSulf) (dotted dashed)
functionalization in toluene (dark green, upper row), CHCl3 (violet, middle row), CH3OH (pink, lower row)
(c < 10-6M)
To elucidate the origin of the dual emission of NiBi when dissolved in CHCl3 (Figure 4) dilution studies
have been performed to reveal the possible presence of molecular aggregates that might cause
either of the fluorescence peaks. Variation of the concentration of NiBi in both solvents changes
systematically the ratio between the dual emission peaks, as shown in Figure 5a and b. Because the
fluorescence peak between 450 and 500 nm rises upon increasing concentration (Figure 5b), it is
assigned to molecular aggregate fluorescence, while the 690 nm-emission must originate from the
monomer. The corresponding aggregate absorption, which is virtually invisible in the absorption
spectra in Figure 4, peaks at 378 nm, as revealed by photoluminescence excitation spectroscopy in
Figure 5c. Because of the blue-shift and the dipolar chromophore character of the Ni-derivatives, the
aggregates are presumed to be of H-type. Interestingly, when dissolved in methanol the dimer
emission dominates the fluorescence spectrum, while the monomer emission cannot be detected.
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Figure 5: Molar and self-absorption corrected emission spectra of NiBi in CHCl3 (a) and CH3OH (b) (cf.
0.002 µmol/ml Figure 4) at five different concentrations. Excitation (dark blue, solid), emission (dark blue,
dashed, λex=405 nm) and fluorescence excitation spectra at λem=485 nm (green) and λem=700 nm (pink) of NiBi
in CHCl3 (c) and CH3OH (d) (0.002µmol/ml)
Finally, Am and Py show similar monomeric absorption and emission features and the same
dependencies on molecular substitutions, namely the Me, Br, Bi, and BiSulf substituents causing red-
shifts of the absorption and emission spectra with successive π-system extension (Bi always yielding
the largest red-shift). These results are in good agreement with the literature.41, 42, 43, 63 While the Am
and Py derivatives easily dissolve in the broad range of differently polar solvents (except their BiSulf
derivatives), the Ni derivatives show fluorescence peaks assigned to aggregates, even in the dilute
solutions. Because of their spectral shifts as compared to the monomers we deduce dipolar
aggregation in case of the Ni derivatives, which is expected to impact supramolecular structure
formation at the air-water interface45 as discussed in the following.
Π(A)-Isotherms
In general, surface pressure (Π) vs. mean molecular area (A) isotherms indicate the formation of two-
dimensional monolayers featuring possibly different supramolecular structures, transitions between
them, and their breakdown.41, 66 The supramolecular structures of these monomolecular Langmuir
films are characterized by the molecular packing density or molecular orientations, which are
deduced from the extrapolated minimum molecular area (A0: derived from steep sections (cf.
compressibility modulus shown in Figure SI 64) of the Π(A)-isotherm). To assign distinct
supramolecular structures to Langmuir monolayers the experimentally determined A0-areas are
compared to theoretically derived A0-areas of molecular conformations assumed to be possible at
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the air-water interface.41 The molecular orientations in the individual condensed phases are then
identified by the best fit between theoretical and experimental A0-areas.
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As shown in Figure 6, no Π(A)-isotherms rising at physically meaningful areas were detected for Me-
derivatives (Y=CH3), which turn out to be water-soluble and not sufficiently amphiphilic for the use in
the Langmuir technique. The isotherms further reveal that the largest A0-areas observed do not
exceed 50 Ų. This small area shows that condensed phases are not formed from flat aligned
amphiphiles (parallel to the water surface, thus necessitating areas significantly larger than the
measured A0-areas) but instead the chromophores must be partially dissolved in the aqueous
subphase and vertically aligned or tilted when the Langmuir monolayers are formed. The maximum
A0-areas are obtained for the largest non-polar Y-substituents of type Bi for any of the thiazoles, i.e.
AmBi, PyBi, NiBi.
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4-N,N-Dimethylaminophenyl Derivatives
The Π(A)-isotherm of AmBr in Figure 6 (left) shows a stable and compressible monolayer between 44
and 35 Ų (A0 = 44 Ų, Table 2). A similar A0-area A0,theo = 43 Ų, shown in Figure 6 (AmBr ), is also
obtained when multiplying the molecular length between the COOH, which works as the anchor
group to the water subphase, and Am (the (CH3)2N-group, 16.4 Ų) with the sulfur van der Waals
diameter (2.6 Å). As we have shown previously44, in this supramolecular orientation the (CH3)2N
substituent works as a second anchor67 because of its basicity (N,N-dimethylaniline, pKa = 5.168; cf.
heptaonic acid, pKa = 4.469). Therefore, we conclude that the chromophore is slightly vertically tilted
on the water surface and straightens up in the first phase of the stable and compressible monolayer,
thus causing a reduction of the distance between the two anchors. Further compressions yield a
transition phase of high compressibility, without a second rise of Π. Therefore, we conclude that the
collapse point (ΠC) of the monolayer AmBr is reached at a surface pressure of ΠC > 8.5 mN/m.
Extending Y to the still rather non-polar 4-methoxybiphenyl moiety (AmBi) basically yields just a
small shift of +6 Ų (even if now containing a sterically demanding twisted biphenyl moiety) of the
isotherm (AmBi : A0 = 50 Ų), and an additional phase (A0* = 15 Ų) is observed, as shown in Figure 6.
Because this A0*-area is very small, it might be assigned to upright aligned molecules with both
anchors dissolved in the subphase, as shown in Figure 6 (AmBi*), or to formation of three-
dimensional aggregates. Further modification of Y by attaching a SO3H-group (AmBiSulf) significantly
shifts the isotherm’s onset to smaller mean molecular areas. In accordance to the interpretation of
the small A0*-area of AmBi*, the resulting small A0–area (14 Ų) of AmBiSulf is assigned to partially
dissolved or three-dimensionally aggregated chromophores. However, because the SO3H-group is
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more hydrophilic than the (CH3)2N-group (cf. p-toluenesulfonic acid70, pKa = -2.871) the former is
expected to function as second anchor instead of the latter.
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Table 2. Extrapolated minimum molecular area (A0) derived from Π(A) isotherm of different 4-hydroxy-5-1,3
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thiazoles
Am
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A0 (A0,*)
A0 (A0,*)
A0 (A0,*)
theo
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Figure 6: (left) Averaged mean Π(A) isotherms of (Am) 2-4-N,N-dimethylaminophenyl-; (Py) 2-2-pyridyl; (Ni)
2-4-nitrophenyl-4-hydroxy-5-Y-1,3 thiazoles and the methyl (Me, solid), phenyl-4-bromo (Br, dotted), phenyl-
4-methoxyphenyl (Bi, dashed) and 3-phenyl-4-methoxyphenyl-3-sulfonylphenyl (BiSulf, dotted dashed)
functionalization. (right) Molecular orientations of the different 4-hydroxy thiazoles in the Langmuir film
derived from the extrapolated minimum molecular area (A0) of the Π(A) isotherm. Exemplarily, the
representative molecular dimension of NiMe and NiBi are given. Lewis structures are plotted on top of images
of the electrostatic potential (φ) distribution at the van der Waals surface.
2-Pyridyl Derivatives
With the change to a 2-(2-pyridyl) substitution at the thiazole the molecular amphiphilicity is
significantly altered. The Π(A) isotherm of PyBr starts rising at 22 Ų (A0 = 17.6 Ų; see Figure 6). Such
a small A0 area is obtained when multiplying the molecular y-lengths of PyBr (from the sulfur to the
oxygen of the 4-hydroxy-5-Y-1,3 thiazoles y=6.4 Å , see Figure 6) with the vdW diameter of sulfur. This
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A0 area corresponds to a previously reported one for the same chromophore without the bromine.41
Therefore, we expect a supramolecular structure where the chromophore is tilted with the Py-group
dissolved in the water subphase due to its polarity and acidity (pyridine, pKa = 5.1472). Subsequently,
the isotherm-section being characteristic for PyBr is followed by a second rise of the Π(A) isotherm
after passing a pronounced bend.
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Extending Y to a still rather non-polar 4-methoxybiphenyl moiety increases the A0 areas by 20 Ų
(PyBi : A0 = 46.6 Ų) and again no second condensed phase is observed till the minimum trough area
is reached. We assign the large A0 area to a tilted molecule with both anchors dissolved in the
subphase, as shown Figure 6 (PyBi). Similar to AmBiSulf, attaching of SO3H (PyBiSulf) significantly
shifts the A0 area to smaller values and yields a liquid condensed phase. In accordance to the
interpretation of the small A0* area of PyBi*, the resulting very small A0 area (14 Ų) of PyBiSulf is
assigned to upright aligned and partially dissolved or three-dimensionally aggregated chromophores
(Figure 6, PyBiSulf).
4-Nitrophenyl Derivatives
Introduction of nitrophenyl instead of aminophenyl or pyridine replaces the second strong anchor73,
74
by a less polar moiety. The Π(A) isotherm for the molecule NiBr with a Y = bromophenyl
functionalization shows a highly compressible liquid phase with increasing Π at A < 40 Ų that
transforms to a condensed phase of lower compressibility at higher surface pressures and decreased
areas (A0 = 25 Ų), cf. AmBr and PyBr above. A peak in the isotherm at A = 23 Ų (18 mN/m) indicates
a collapse or strong reorganization of this first stable and compressible monolayer. At further
compressions the surface pressure steeply rises till Π > 50 mN/m, which indicates the formation of a
second phase of a condensed NiBr monolayer with rather low compressibility42 (compressibility
modulus reaching 140 mN/m, as shown in Figure SI 64). The latter is, similar to Am- and PyBr,
characterized by A0* = 15 Ų.
Extension of Y to a 4-methoxybiphenyl moiety (NiBi) also adds a third somewhat dipolar group
(OCH3) to the amphiphile, as shown by the electrostatic potential distribution at the van der Waals
surface of NiBi in Figure 6. Instead of the NO2-moiety, in certain cases the OCH3-group might interact
with the water-subphase, thus causing an inverse orientation, as shown in Figure 6 for a dimer of NiBi
with opposing molecular orientations. The structural motive of NiBi causes a single condensed phase
(A0 = 33 Ų) that reaches exceptionally high surface pressures and relatively low compressibilities as
compared to other thiazoles amphiphiles42 (compressibility modulus reaching ≈100 mN/m, as shown
in Figure SI 64). As clearly visible from the sharp maximum in the compressibility modulus at 27 Ų,
the monolayer starts transforming into multilayers at 27 Ų (between 40 and 45 mN/m). Again,
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further modification of Y by attaching SO3H (NiBiSulf) significantly shifts the isotherm’s onset to
smaller areas (A0 = 21 Ų) and induces a liquid phase, as characterized by the small slope between 0
and 10 mN/m. Because of the additional strong anchor, the tendency of monolayer collapse
decreases.
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It was shown, that the methyl (Me) functionalized thiazoles are not sufficient amphiphilic for the
Langmuir technique, whereas the extension of the π-system (Br, Bi) increases the weight of the
nonpolar moiety and stabilizes the film formation. Here, the pyridyl (Py), but first and foremost the
(CH3)2N (Am) substituent works as a second anchor to the water subphase and enables vertically
tilted supramolecular structures. Though appearing to be a predestined second anchor as well, the
NO2 (Ni) changes the supramolecular structure distinctly, as such derivatives form dipolar aggregates,
which have presumably H-type character, as deduced from the absorption and emission spectra
above. Those interactions have been recently shown to significantly stabilize two-dimensional
Langmuir layers.45 Further insights into the supramolecular structure are gained directly during the
Langmuir-process via online fluorescence monitoring, as detailed below.
In situ Steady State Fluorescence Spectroscopy
Intermolecular interactions between the thiazoles will basically involve dipolar interactions,
particularly in case of the push-pull substitutions pattern and ππ-quadrupole or London dispersion
interactions in case of the π-extended chromophores. Because the balance between these
interactions depends on the polarity of the surrounding medium we have chosen toluene,
chloroform, and methanol as prototype non-polar, medium and highly polar solvents, respectively.
These solvents might be suitable model-media to mimic different molecular assemblies at hetero-
interfaces, as used in the Langmuir technique. In the following we employ UV-vis transmission and
emission spectroscopy to dissolved thiazoles to obtain reference spectra to be compared with those
of typical H-, J-, or π-aggregates that possibly form at the air-water interface.
The supramolecular structure in the Langmuir layers of different types of 4-hydroxy-5-Y-1,3 thiazoles
are characterized by their fluorescence spectra detected in situ upon compression. In the following,
representative emission spectra of the quasi-gas (negligible molecular interactions) to condensed
phases (strong molecular interactions) are compared with solution spectra to determine possible
spectral changes arising from intermolecular interactions in the supramolecular assemblies.
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As compared to the fluorescence spectra of dissolved AmBr, the fluorescence spectra of the
corresponding Langmuir layers are red-shifted and actually vibrationally not resolved at all
investigated surface pressures (see Figure 7; for spectra of systematically varied Π see Figure SI 51).
The disappearance of the vibronic progression as compared to solution is assigned to a superposition
of fluorescence spectra from dyes in slightly varied environments in Langmuir layer within the
measurement spot. The red-shift is assigned to the transition from solution to the condensed state.
Upon increasing Π, i.e. transition from the gas- to the liquid-phase, the emission maximum of AmBr
shifts slightly red (λmax = 518 nm, Π = 0.1mN/m), followed by a continuous blue-shift (λmax = 500 nm,
Π = 12mN/m) in the condensed phase without a change in spectral shape after the collapse point
(ΠC > 11 mN/m; for in situ fluorescence isotherms shown in SI). The above discussed possible
presence of acid-base equilibrium is not reflected in the in situ fluorescence spectra. No blue-shifted
emission that can be assigned to a protonated species (Figure SI 49) is observed (see Figure SI 51).
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In contrast to AmBr, Langmuir layers of AmBi can be compressed to higher surface pressures,
assigned above to a second condensed phase, which gives rise to better resolved vibrational
progression see (Figure 7, Figure SI 52). The latter indicates improved supramolecular order favored
through the extended π-electron system in AmBi as compared to AmBr. In contrast, the fluorescence
spectra of AmBiSulf show no dependence on Π (Figure 7), hence pointing to a smaller supramolecular
order than in case of AmBi and possibly a preaggregation in solution or instantly at the air-water
interface.
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Figure 7: Normalized emission spectra of Langmuir layers at air-water interfaces (solid lines; quasi-2D-phases
varied from gas (Π=0 mN/m), to condensed (Π>0 mN/m)) of the 2-(4-N,N-dimethyl-aminophenyl)-4-hydroxy-5-
Y-1,3 thiazole core (λexc = 405 nm) and the phenyl-4-bromo (AmBr), phenyl-4-methoxyphenyl (AmBi) and 3-
phenyl-4-methoxyphenyl-3-sulfonylphenyl (AmBiSulf) functionalization and solvent spectra (dashed line) for
toluene (dark green), CHCl3 (violet), CH3OH (pink)
2-Pyridyl Derivatives
Similar to Am(Br,BiSulf) the fluorescence spectra of the Langmuir layers of Py(Br,BiSulf) are slightly
red-shifted as compared to the corresponding solutions, show no vibrational progression, and are
independent from Π. Again, no spectral sign of protonation (see Figure SI 50) of the basic anchoring
group (now pyridine) by the COOH moiety is observed (see Figure 8; Figure SI 55). However, PyBi
shows significant extension of the fluorescence spectra to the red when reaching the condensed
phase I, as shown in Figure 8.
Difference fluorescence spectra of the PyBi Langmuir layers (see Figure SI 56), where the quasi-gas
phase spectrum (A = 502 Ų) was subtracted from the remaining spectra, reveal an initial
fluorescence decrease for areas shrinking from 800 to 500 Ų, followed by a steady fluorescence gain
upon further compression into the condensed phases. Consequently, aggregation yields the
extension of the fluorescence spectra to the red upon increasing Π reported in Figure 8: As revealed
by the evolution of the fluorescence difference spectra, aggregates have also formed initially at
spreading the chloroform solution of PyBi, which dissociate in the course of the LB-experiment
provided operation in the quasi-gas-phase. Because of the red-shift, aggregates are assigned as J-
type.
As alternative origins of the red-shifted fluorescence peak, one might consider intra- or
intermolecular protonation of the pyridine moiety by the COOH acid (λem=492 nm (neutral),
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λem=622 nm (protonated), see Figure SI 50), or excimer emission43, 75, which both might change with
Π as the supramolecular structures are changing. However, it appears unlikely that such process
would be exclusively observed for PyBi, but not for all other Py-derivatives.
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Similar to PyBr (which also features similar A0 areas), the fluorescence spectra of PyBiSulf are
independent on Π, red-shifted and show no vibronic progression as compared to the solution
spectra. These equal results for distinctly different amphiphiles (two as compared to three anchors
for PyBr and PyBiSulf) and the very small A0 areas imply formation of amorphous rather than well-
defined supramolecularly structured films that possibly extent to the third spatial dimension rather
than being quasi-two-dimensional prototype Langmuir layers.
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Figure 8: Normalized emission spectra of Langmuir layers at air-water interfaces (solid lines; quasi-2D-phases
varied from gas (Π=0 mN/m), to condensed (Π>0 mN/m)) of the 2-(2-pyridyl)-4-hydroxy-5-Y-1,3 thiazole core
(λexc = 405 nm) and the phenyl-4-bromo (Br), phenyl-4-methoxyphenyl (Bi) and 3-phenyl-4-methoxyphenyl-
3-sulfonylphenyl (BiSulf) functionalization with the solvent spectra (dashed line) in toluene (dark green), CHCl3
(violet), CH3OH (pink)
4-Nitrophenyl Derivatives
The fluorescence spectra of Ni-derivatives on the LB trough are significantly broader than the spectra
of the different solutions discussed above and shown for reference as dashed lines in Figure 9.
Furthermore, their shape and spectral position typically depends stronger on Π than in case of the
Am- and Py-derivatives discussed above, as surveyed in Figure 9.
In detail, the Langmuir layer of the CH3-functionalized core (NiMe) shows a single emission feature
(547 nm) located between the emission of maxima obtained for the rather nonpolar toluene and the
polar CHCl3-soultions (see Figure 9). The emission maximum shifts red (28 nm, Π = 9mN/m; 10 nm,
Π = 14mN/m) when probing the compressible monolayer.
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The opposite trend is observed for the Langmuir layer made from Y=4-bromophenyl functionalized
molecule (NiBr). The main emission feature of the Langmuir layer disappears when probing the
compressible monolayer and an additional blue-shifted band rises upon increasing Π. Hence, the
shape of emission switches from one representing the monomeric emission known from the
CHCl3-solution to an H-type aggregate emission (see CH3OH-solution, Figure 9). The latter
(λem=550 nm) features a pronounced vibrionic progression what further supports the assumption of
H-aggregate formation. For this aggregation each second NiBr dye needs to switch orientation if they
were uniquely aligned in the Langmuir gas phase. This reorganization likely causes the dip in the Π(A)
isotherm discussed above. The accompanied complex supramolecular and morphological
rearrangements during lateral compression of the Langmuir layer likely causes the non-monotonous
development of the ratio between monomer and aggregate fluorescence peaks, as seen in Figure 9
and in Figure SI 59. Formation of a layer over time without lateral compression apparently avoids
reorganization in a dense layer and might be understood as crystallization, as the fluorescence
spectrum gets smoothly converted from the monomer spectrum to the aggregate spectrum within
40 min (see Figure SI 60). Such time is actually granted during the LB-online-fluorescence
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measurements. The Π(A) isotherms recorded during these fluorescence measurements (Π(A)fluor
)
therefore differ from those discussed above (compare Figure 6 and Figure SI 59). The Π(A)fluor
isotherm in Figure SI 59 is shifted to an unphysically small A0 area, thus indicating growth of H-
aggregates in three spatial dimensions, i.e. losing the 2D-growth restriction typically enforced by the
Langmuir technique in case of NiBr.
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Figure 9: Normalized emission spectra of Langmuir layers at air-water interfaces (solid lines; quasi-2D-phases
varied from gas (Π=0 mN/m), to condensed (Π>0 mN/m)) of the 2-(4-nitrophenyl)-4-hydroxy-5-Y-1,3 thiazoles
core (λexc = 405 nm) and the methyl (Me), phenyl-4-bromo (Br), phenyl-4-methoxyphenyl (Bi) and 3-phenyl-
4-methoxyphenyl-3-sulfonylphenyl (BiSulf) functionalization with the solvent spectra (dashed line) in toluene
(dark green), CHCl3 (violet), CH3OH (pink)
In contrast to NiBr, NiBi shows just a red-shifted fluorescence (52 nm, 0.175 eV) upon lateral
compression from the gas (0.5 mN/m) to the condensed phase (36 mN/m), without appearance of
new peaks. Hence, we deduce that the extension of the π-system in NiBi, as compared to NiBr,
hampers reorganization because of increased ππ-interactions, which stabilize a unique molecular
orientation within the supramolecular structure. Within such structure the molecules are apparently
slightly tilted to additionally stabilize the supramolecular structure via J-type dipolar assembly of the
D-π-A dyes, as revealed by the successive red-shift upon increasing Π. The avoided reorganization
due to stabilization of the two-dimensional layer upon enhanced ππ- and dipolar interactions is
presumably the major reason for reaching the exceptionally large surface pressures (Π≈45 mN/m)
before reorganization of the monolayer is observed, as discussed above. However, presumably the
Langmuir layer consists of small domains, as no vibrational progression can be observed in the
Langmuir layer fluorescence.
Contrary to NiBi, the fluorescence maximum of the Langmuir layer of NiBiSulf (λmax = 574 nm) in the
liquid phase (2.2 mN/m) shifts continuously blue (38 nm, 0.149 eV, 29.00 mN/m), what is indicative
for the formation of H-aggregates. The reorganization involved in H-aggregate formation (discussed
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above for NiBr), the high compressibility deduced from the Π(A) isotherms in Figure 6, and the
unphysically small A0 areas indicate formation of three-dimensional aggregates upon lateral
compression in the Langmuir experiment.
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CONCLUSION
In the present work we systematically investigate the influence of the chromophore within the
recently introduced class of double-anchor T-shaped amphiphilic dyes41 on the supramolecular
structures they form on the air-water interface in Langmuir experiments. One hydrophilic subphase
anchor is provided by a COOH moiety that is centrally linked via an alkylic flexible chain to the
chromophore’s core, namely a 4-hydroxy-thiazole. The second anchor is chosen to be integral part of
the chromophore to enable tuning of the molecular orientation upon lateral compression of
Langmuir monolayers employed in this work. These R1-anchors in the 2-position of the thiazole have
been chosen as 4-N,N-dimethylaminophenyl (Am), 2-pyridyl (Py), and 4-nitrophenyl (Ni).54, 55 For
each of these derivatives, the chromophores have been systematically extended in the 5-position by
replacing the methyl (Me) moiety with a 4-bromophenyl (Br), 4-methoxybiphenyl (Bi), and a
4-methoxyphenyl-3-sulfonylphenyl (BiSulf) moiety.
As revealed by standard UV-vis transmission and fluorescence spectroscopy on solutions the Am-
moiety dominates the electronic properties of all investigated Am-derivatives. In contrast, the
absorption and emission spectra systematically shift red upon extension of the π-system in case of
the Py- and Ni-derivatives.
For the given set of substituents in the 5-position of the thiazole, i.e. Me, Br, Bi, BiSulf, just the Ni-
derivatives, essentially NiBr and NiBi, feature dipolar chromophores that give rise to the formation of
H- and J-aggregates at the air-water interface, respectively, as revealed by in situ fluorescence
spectroscopy performed online during Langmuir experiments. From the Π(A) isotherms and the in
situ detected fluorescence spectra we deduce reorganization of the NiBr dyes and 3D- instead of 2D-
growth of their supramolecular structures. In contrast, we find that the diphenylene moiety in NiBi
significantly stabilizes the compressible monolayer formed by the assembly at the air-water
interface. Thus, reorientation is essentially hampered and the dyes form J-aggregates upon lateral
compression of the Langmuir layer. The joint stabilization of this NiBi layer by ππ- and dipolar
interactions gives rise to considerably high surface pressures of ≈45 mN/m42, 44, 45, 76, 77, 78, 79, 80 that are
obtained without any signs of disturbance of the monolayer.
In summary, the combination of the Langmuir technique with online fluorescence measurements
revealed that the ππ-interactions, which are pronounced for the Bi-derivatives, yield the most
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distinct supramolecular structures: While in case of PyBi and NiBi ordered J-type supramolecular
structures in microdomains are formed, as revealed by not vibrationally resolved, red-shifted
fluorescence spectra, AmBi forms ordered supramolecular structures that are more homogeneous,
as revealed by the resolved vibrational progression in the fluorescence spectra, which are, however,
not stabilized by J-type dipolar interactions. Hence, the presented systematic study on the influence
of the chromophore on the supramolecular structures that form at the air-water interface enables
distinct choices of the substitution pattern to target distinct supramolecular structures desired in
optoelectronically active materials.
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UV-vis Absorption and Fluorescence Spectroscopy
For the absorption spectroscopic measurements, a UV-vis (Varian: Cary 5000) spectrometer was used
in transmission mode. The fluorescence measurements using excitation wavelengths larger than
400 nm were performed on a custom built setup (as detailed below), for excitation wavelengths
smaller than 400 nm a different spectrometer (Horiba Fluorolog) was used. The Extinction
coefficients (ε(λmax)) of the absorption maxima were calculated via linear regression over the
concenctration range from c = 10-4 M to 10-7 M.
LB-Isotherms and in situ Fluorescence Spectroscopy of Langmuir layers
For the basic Π(A)-isotherm characterizations solutions of the dyes (1 and 0.1 µmol/ml in 80% CHCl3/
20% CH3OH) were carefully spread onto the subphase (ultra-pure water) of the LB-trough (KSV 5000).
For complete evaporation a time of 10 min were granted before moving the barriers. Then, the
barriers were compressed with a speed of 5 mm/min to record the isotherms.
For the in situ fluorescence spectroscopy studies the same Langmuir parameters were applied at a
KSV NIMA Alternate L 105 LB trough equipped with a custom-built fluorescence setup. This setup
consists of an Isoplane 320 spectrograph with a cooled Pixis CCD-samera from Princeton Instruments.
A fiber coupled 5 mW laser with a 405 nm output wavelength was used as excitation source (incident
under 55°) and the emission were detected under 0° to the normal. Long pass filters with low self-
fluorescence (obtained from ITOS) were used to block scattered excitation light. The data were
corrected by a self-written LabView and C++ program. These programs remove remaining scattered
excitation light and cosmic rays. The maxima of the fluorescence spectra were analyzed by a self-
written Mathematica program.
Density functional theory calculations
Quantum chemical structure optimizations and calculations of electrostatic potential and absorption
spectra on 5_5 and 7 monomers were performed using density functional theory (DFT) and its time-
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dependent derivative (TD-DFT) as implemented in Turbomole81 and applying the GGA (generalized
gradient approximation) functional BP86 (preoptimization), followed by the hybrid functionals and
B3LYP82, the def2-SVP (preoptimization) and -TZVP basis sets83, and the MARI-J approximation in case
of BP86, which have been shown to yield reasonable electronic properties for a large variety of
molecular motives, including push-pull systems and extended π-electron systems37, 40, 84, 85, 86
.
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SUPPORTING INFORMATION
The synthesis, used materials, quantum chemically calculated absorption spectra of various
conformers, detailed fluorescence spectral series, protonation studies, and further details are
provided in the supporting information.
ACKNOWLEDGEMENT
The authors would like to thank the German Research Foundation (DFG PR 1415/2) and
Bundesministerium für Bildung und Forschung (BMBF FKZ 03EK3507) for financial support. J.P.
acknowledges funding from the Nagelschneider Stiftung.
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