Fluorophobic Effect Promoting Lamellar Self-Assembly of Donor Acceptor Dyes

Article abstract of DOI:10.1002/cphc.201800449

To combine liquid crystalline and linear optical properties in the same molecule, the fluorophobic effect was probed for the first time in donor acceptor dyes. Thus, a series of mono-, bi-, and tricyclic donor acceptor dyes with 1H,1H-perfluorinated alkyl chains of different lengths as donor units and nitrile, malononitrile or barbiturate as acceptor units was synthesized in 5 steps and 1.4–6.6 % overall yield. UV/Vis and fluorescence spectroscopy, cyclic voltammetry and DFT calculations revealed that absorption and emission maxima, Stokes shifts and LUMO energies were mainly governed by the chromophore size and acceptor strengths. The perfluorinated chain was electronically almost decoupled from the remaining chromophore and induced only slight changes of the absorption maxima as compared to the alkyl substituted counterparts. However, in contrast to the non-mesomorphic alkyl donor-substituted derivatives, the perfluorinated donors resulted in self-assembly into partially interdigitated SmA bilayers according to differential scanning calorimetry (DSC), polarizing optical microscopy (POM), X-ray diffraction (WAXS, SAXS) studies and electron density profile calculations.

Full text of DOI:10.1002/cphc.201800449

Accepted Article
Title: Fluorophobic Effect Promoting Lamellar Self-Assembly of Donor
Acceptor Dyes
Authors: Johannes Christian Haenle, Yannick Stöckl, Robert
Forschner, Elena Haenle, and Sabine Laschat
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To be cited as: ChemPhysChem 10.1002/cphc.201800449
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10.1002/cphc.201800449
ChemPhysChem
Fluorophobic Effect Promoting Lamellar Self-Assembly of Donor Acceptor Dyes
Johannes Christian Haenle, Yannick Stöckl, Robert Forschner, Elena Haenle, Sabine Laschat*
Institut für Organische Chemie, Universität Stuttgart
Pfaffenwalding 55, 70569, Stuttgart, Germany
E-mail: sabine.laschat@oc.uni-stuttgart.de
Graphical Abstract
Abstract
To combine liquid crystalline and linear optical properties in the same molecule, the
fluorophobic effect was probed for the first time in donor acceptor dyes. Thus, a series of mono-
, bi-, and tricyclic donor acceptor dyes with 1H,1H-perfluorinated alkyl chains of different
lengths as donor units and nitrile, malononitrile or barbiturate as acceptor units was synthesized
in 5 steps and 1.4 – 6.6 % overall yield. UV/Vis and fluorescence spectroscopy, cyclic
voltammetry and DFT calculations revealed that absorption and emission maxima, Stokes shifts
and LUMO energies were mainly governed by the chromophore size and acceptor strengths.
The perfluorinated chain was electronically almost decoupled from the remaining chromophore
and induced only slight changes of the absorption maxima as compared to the alkyl substituted
counterparts. However, in contrast to the non-mesomorphic alkyl donor-substituted derivatives,
the perfluorinated donors resulted in self-assembly into partially interdigitated SmA bilayers
1
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according to differential scanning calorimetry (DSC), polarizing optical microscopy (POM),
X-ray diffraction (WAXS, SAXS) studies and electron density profile calculations.
Introduction
The merging of liquid crystalline with dye properties can be achieved either by mixtures of dye
guest molecules in liquid crystalline hosts,[1] or by compounds, that combine both properties on
a molecular level. Mixtures of dyes in liquid crystals improved the contrast ratio in displays,[2]
the efficiency and stability of organic solar cells,[3] allowed tuning of the emission wavelength
of lasers [4] and were utilized in applications such as windows with an adaptive transmission.[5]
On the other hand liquid crystalline donor acceptor dyes[6] have been employed as light
responsive materials for drug delivery,[7] components for bulk heterojunction solar cells[8] and
other photovoltaic devices,[8] emissive dendrimers,[9] NIR absorbing and fluorescent
materials,[10] photomodulators of dichroism and birefringence,[11] acido- and photochromic
materials,[12] nematic [60]fullerene supermolecules[13] and second- or third order nonlinear
optical materials.[14] A huge variety of chromophoric units has been incorporated in liquid
crystalline dyes, both discotic mesogens, e.g. perylenes,[15] pyrrole,[16] phenazine[17] and
merocyanine derivatives,[18]as well as calamitic mesogens, e.g. tetrazines,[19] anthraquinones[20]
and azo derivatives.[21]
Among these different building blocks for mesomorphic dyes donor acceptor chromophors play
a prominent role due to their diverse applications mentioned above. However, donor acceptor
chromophors carrying perfluoralkyl donor units rather than alkyl or alkyloxy side chains have
only been rarely investigated. This is rather surprising, because the fluorophobic effect and its
beneficial influence on liquid crystalline self-assembly is long known and has been extensively
studied. As has been recently summarized by Tschierske[21] for perfluoroalkyl chains, the high
polarity and low polarizability of the C-F bond and the helical conformation of the
perfluorinated chain lead to reduced intermolecular interactions and increased lipophilicity.
This results in immiscibility of rigid perfluorinated and flexible aliphatic chains, which
preferably adopt a linear all-trans conformation. It must be emphasized that the fluorophobic
effect originates from contributions of different intra- and intermolecular interactions. The
formation of a liquid crystalline mesophase is based upon molecular segments, that exhibit a
sufficient immiscibility with respect to each other and therefore lead to nanophase
segregation.[22] When combining perfluorinated with hydrocarbon segments via covalent bonds,
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the fluorophobic effects leads to immiscibility of these molecular segments due to low
polarizability and conformational rigidity (compared to aliphatic counterparts). This
immiscibility can drive nanophase segregation resulting in self-ordered mesophase formation.
The incompatibility of perfluorinated, aliphatic, aryl and polar segments has been heavily
exploited in liquid crystal research.[23]
Recently, we investigated the absorption and emission properties of mono-, bi- and tricyclic
donor acceptor dyes A-n-DoH (n = 1, 2, 3) carrying dodecyloxy and piperidyl donor units and
keto, malodinitrile and N,N-dimethylbarbiturate acceptor units (Scheme 1).[24] Experimental
and theoretical results revealed the shift of the ground state polarity from the neutral resonance
form N favored by dyes with dodecyloxy donor moiety irrespective of the chromophor size to
the zwitterionic resonance form Z favored by dyes with a combination of strong piperidyl donor
and barbiturate acceptor.[24] Further studies on related systems were devoted to solvatochromic
effects,[25] dichroism and photostability[26] as well as solid state properties.[27] However, despite
the presence of long side chains such as dodecyloxy in A-n-DoH (DoH = OC12H25) no
mesomorphism was detected. Motivated by the promising previous results on mesophase
stabilization through the fluorophobic effect, we anticipated that grafting of perfluorinated side
chains on the rigidified chromophore A-n-DoF should not only promote the formation of liquid
crystalline phases but should also affect the linear optical properties ultimately leading to
unique materials properties. Through systematic variation of donor, acceptor and chromophore
unit insight into structure–property relationships as well as scope and limitations of the
fluorophobic effect on dye molecules should be obtained. The results towards these goals are
reported below.
Scheme 1.
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Results and Discussion
Synthesis
Two series of donor acceptor dyes A-n-DoF were prepared, i.e. malononitriles Mal-n-DoF and
barbiturates Barb-n-DoF. The synthetic route is outlined in Scheme 2 (an overview of all
compounds and yields can be found in Scheme S1). The hydroxy function of known vinylogous
[28]
acids Keto-1-OH and Keto-2-OH
was converted via an Appel reaction to the bromide,
which was then substituted by the respective 1H,1H-perfluorinated donor to give Keto-1-DoF
and Keto-2-DoF. This synthetic approach was not successful for derivative Keto-3-OH with
the largest tricyclic chromophore. Therefore, Keto-3-OH was reacted with a 1H,1H-
perfluorinated triflate yielding Keto-3-DoF in 15 %. After introduction of the donor group, the
synthesis of the acceptor moiety followed our previously established route.[24] Ketones Keto-
n-DoF were submitted to a HWE reaction with diethyl cyanomethylphosphonate to give nitriles
NC-n-DoF in 62–84 % yield. Subsequent reduction with DIBAL-H provided aldehydes OHC-
n-DoF in . These aldehydes OHC-n-DoF were finally treated with either malononitrile or 1,3-
dimethylbarbituric acid to yield the final products Mal-n-DoF and Barb-n-DoF in 25–99 %. It
should be emphasized that poor yields below 25 % are mostly observed for dienones Keto-n-
DoF, enals OHC-n-DoF and some derivatives with exocyclic diene moiety, which are
particularly prone to Michael addition, autoxidation and polymerization.
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Scheme 2. Synthesis of malononitriles Mal-n-DoF and barbiturates Barb-n-DoF with 1H,1H-
perfluorinated donors. i) For n = 1 and 2: 1. PPh3, CBr4, CH2Cl2, 76–87 %, 2. KOH, 1H,1H-
perfluoroalcohol, MeCN, 14–60 %; ii) For n = 3: KOH; TfOCH2C11F23, THF, 15 %; iii) NaH,
(EtO)2POCH2CN, DMF, 62–84 %; iv) DIBAL-H, CH2Cl2, 14–95 %; v) NEt3, malononitrile,
CH2Cl2, 7–96 %; vi) NEt3, 1,3-dimethylbarbituric acid, CH2Cl2, 43–99 %.
Owing to the exocyclic C=C bond, nitriles NC-n-DoF, malononitriles Mal-n-DoF, and
barbiturates Barb-n-DoF were obtained as E/Z mixtures. An exemplary NOESY spectrum, that
was used to assign E and Z isomers is shown in Figure S1 in the supporting information. The
E/Z ratio was determined from the mean integral ratio of the most deshielded proton signals of
the conjugated carbon chain (Table S1). All nitriles NC-n-DoF and malononitriles Mal-n-DoF
showed nearly equimolar E/Z ratios. Barbiturates Barb-n-DoF exhibited an excess of E isomer
due to the larger acceptor group compared to NC-n-DoF and Mal-n-DoF.
Absorption and emission properties
The optical properties of A-n-DoF were investigated via UV/Vis spectroscopy (Table S2).
Absorption spectra of barbiturates Barb-n-OC12F displayed broad and unstructured absorption
spectra with λmax values at 428 nm, 486 nm and 532 nm, respectively (Figure 1).
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Figure 1. UV/Vis absorption spectra of a) barbiturates Barb-n-OC12F and b) malononitriles
Mal-n-OC12F in CHCl3 at room temperature.
In the series of barbiturates Barb-2-DoF with different donor lengths, absorption maxima did
not change in CHCl3 for Barb-2-OC8F (487 nm), Barb-2-OC10F (488 nm), and
Barb-2-OC12F (486 nm).
Absorption maxima of Barb-n-OC12F in toluene, THF, CHCl3 and DMSO were listed in
Table S2 and were compared with their aliphatic counterparts Barb-n-OC12H. In comparison,
absorption maxima of perfluorinated Barb-n-OC12F were hypsochromically shifted by 4–
54 nm compared to Barb-n-OC12H independently on the chromophore size. This
hypsochromic shift might be rationalized by a stronger inductive effect of the F atoms as
compared to the alkoxy unit. An alternative explanation is the less polar ground state with a
larger contribution of the neutral resonance form N for perfluorinated Barb-n-OC12F. Further
3
information on the ground state polarity can be derived from J(H,H) coupling constants
3
(Table S3).[29] The J(H,H) values of Barb-n-OC12F were systematically smaller (0.1–
0.3 Hz) than coupling constants of aliphatic Barb-n-OC12H. The smaller 3J(H,H) constants of
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Barb-n-OC12F indicated a longer C-C bond caused by a weaker donor and therefore confirm an
increased contribution of N to S0 compared to Barb-n-OC12H.
The absorption spectra of malononitriles Mal-n-OC12F are shown in Figure 1. All derivatives
displayed a broad and unstructured absorption. An increasing donor length for compounds
Mal-2-OC8F (441 nm), Mal-2-OC10F (442 nm) and Mal-2-OC12F (441 nm) did not change
the absorption maxima. The absorption maxima of perfluorinated Mal-n-OC12F (Tables S2)
displayed a hypsochromic shift of 16–31 nm in comparison with aliphatic analogues
3
Mal-n-OC12H. In analogy to barbiturates, coupling constants J(H,H) of perfluorinated
Mal-n-OC12F were smaller (0.1–0.2 Hz) as compared to aliphatic Mal-n-OC12H indicating a
less polar ground state for Mal-n-OC12F.[29a] In order to obtain further insight a computational
analysis of the ground state geometry for model compounds Mal-3-DoF with an 1H,1H-
perfluorinated butyloxy donor was performed. Calculated BLA values showed, that the ground
state S0 of 1H,1H-perfluorinated Mal-3-OC4F (6.20 pm) had a larger contribution of the neutral
resonance form N compared to aliphatic Mal-3-OC2H (5.73 pm).
The emission spectra of Mal-n-OC12F and Barb-n-OC12F were recorded in CHCl3.
Malononitriles Mal-n-OC12F displayed emission maxima in the range max,em = 453–598 nm,
while the corresponding max,em values of barbiturates Barb-n-OC12F were between 478 nm
and 640 nm (Table S4). In comparison with aliphatic compounds A-n-DoH,[24] the Stokes shifts
of 1H,1H-perfluorinated dyes A-n-DoF were smaller by 15–38 nm. With increasing
chromophor size the emission maxima were bathochromically shifted for each series.
Electrochemical properties
The electrochemical behaviour of derivatives Mal-n-OC12F and Barb-n-OC12F (n = 1–3) as
well as Mal-2-OC8F and Barb-2-OC8F was investigated by cyclic voltammetry (0.1 M
NBu4PF6 in CH2Cl2). For all compounds, an irreversible oxidation wave hampered the
determination of HOMO energies (Figure S2). In the reduction wave, solely derivatives
Mal-3-OC12F and Barb-3-OC12F with the largest chromophore showed electrochemically
reversible behaviour. The reduction waves of barbiturates Barb-n-OC12F are shown in
Figure 2. Compound Barb-3-OC12F with the largest chromophore displayed a reversible
reduction, while analogues Barb-2-OC12F and Barb-1-OC12F showed an irreversible
reduction.
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Figure 2. Cyclic voltammogram of barbiturates Barb-n-OC12F in CH2Cl2 at room temperature
(concentration: 2 mg mL–1; 0.1 M NBu4PF6; scan rate: 50 mV s–1).
For derivatives Mal-3-OC12F and Barb-3-OC12F, LUMO energies were calculated from the
half wave potential according to Bazan.[30] These values are listed in Table 1 and compared
with LUMO energies of aliphatic analogues Mal-3-OC12H and Barb-3-OC12H from ref.[24]
Upon comparison of the acceptor moieties, barbiturates Barb-3-OC12F and Barb-3-OC12H
showed smaller LUMO energies of 0.15 eV and 0.14 eV in relation to their malononitrile
counterparts Mal-3-OC12F and Mal-3-OC12H. A change of donor moieties from 1H,1H-
perfluorinated to aliphatic side chain influenced LUMO energies only by 0.01–0.02 eV
indicating that the methylene group which connects the ether unit and the perfluoralkyl chain
acts as an electronic stopcock. Therefore, the choice of donor moiety resulted in a negligible
change of LUMO energies.
Table 1. Half wave potentials E1/2 and LUMO energies of A-3-OC12F and A-3-OC12H.[24]
E1/2 / V
–1.55
LUMOCV / eV
–3.55
Mal-3-OC12F
Barb-3-OC12F –1.40
Mal-3-OC12H
–1.53
Barb-3-OC12H –1.39
–3.70
–3.57
–3.71
Liquid crystalline properties
Whereas none of the dodecyloxy-substituted chromophors A-n-C12H studied previously[24]
showed any mesomorphic properties, all newly prepared dyes A-n-DoF with perfluorinated tails
displayed liquid crystalline behaviour. The mesomorphic properties of NC-n-DoF, Mal-n-DoF
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and Barb-n-DoF (n = 1–3) were investigated by differential scanning calorimetry (DSC),
polarizing optical microscopy (POM) and X-ray diffraction (XRD). Phase transition
temperatures were determined from onset values of the first cooling cycle due to decomposition
issues in some second and third cycles.
For details on the phase behaviour of liquid crystalline dyes A-n-DoF, see Table S5 and Figure
S3 – S7. The DSC curves of nitrile NC-2-OC12F and barbiturate Barb-2-OC12F are shown in
Figure 3. Upon first heating, NC-2-OC12F displayed a transition to the isotropic state at 92 °C
(Figure 3a). All cooling cycles showed a mesophase between 101 °C and 91 °C. The second
and third heating revealed a melting transition of 87 °C and clearing temperatures of 97 °C.
Derivative Barb-2-OC12F showed a clearing transition of 193 °C upon first heating
(Figure 3b). In the first cooling cycle, a clearing transition at 157 °C and crystallisation at 68 °C
were observed. In the second heating cycle, an endothermal transition at 67 °C and a cold
crystallisation at 85 °C as well as a transition to the isotropic phase at 172 °C were seen.
Decreasing clearing temperatures of subsequent cooling cycles indicated a thermal
decomposition of Barb-2-OC12F. In contrast to Sworakowski and Lakahal, who rationalized
decreased isotropization temperatures of azobenzenes and cinnamic acid diacrylates
respectively by E-Z photoisomerization,[31] such scenario could be ruled out in our case.
Prolonged isotropization temperatures during XRD experiments resulted in the conversion of
the samples into black sticky tars. Similar observations were made upon analyzing a thermally
annealed sample by NMR. In summary, nitriles NC-n-OC12F exhibited a thermally stable,
enantiotropic mesophases. Most of malononitriles Mal-n-DoF and barbiturates Barb-n-DoF
displayed thermal decomposition, as evidenced from decreasing isotropization temperature
during repetitive heating/cooling cycles.
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Figure 3. DSC curves of a) NC-2-OC12F and b) Barb-2-OC12F of first three heating (H) and
cooling (C) cycles. Rates 5 K min–1.
The mesophase ranges of nitriles NC-n-OC12F, dimethyl barbiturates Barb-n-OC12F and
malononitriles Mal-n-OC12F (n = 1–3) are shown in Figure 4. For nitriles NC-n-OC12F
(n = 1–3), extension of the chromophore size increased mesophase ranges from NC-1-OC12F
(2 K) over NC-2-OC12F (10 K) to NC-2-OC12F (70 K). In an analogous manner, a larger
chromophore resulted in increasing phase widths from 12 K for barbiturate Barb-1-OC12F to
89 K for Barb-2-OC12F and 91 K in case of Barb-3-OC12F. Within the series of
malononitriles Mal-n-OC12F, an enlargement of the chromophore also increased temperature
ranges from 4 K (n = 1) over 44 K (n = 2) to 112 K (n = 3) as well as stability of the mesophase.
Comparison of acceptor moieties revealed smaller temperature ranges for nitriles NC-n-OC12F
(2–70 K) compared to barbiturates Barb-n-OC12F (12–91 K) and malononitriles
Mal-n-OC12F (4–112 K) with the same chromophore size n. For a specific chromophore size,
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clearing temperatures Tcl increased in the series nitriles NC-n-OC12F < barbiturates Barb-n-
OC12F < malononitriles Mal-n-OC12F.
Figure 4. Mesophase ranges of nitriles NC-n-OC12F, dimethyl barbiturates Barb-n-OC12F
and malononitriles Mal-n-OC12F (n = 1–3).
To analyse the clearing points Tcl based on intermolecular interactions, dipole moments μ were
calculated (for isolated molecules of) A-3-OC12F (Figure 5). The calculated dipole moments μ
increased in the series NC-3-OC12F (μ = 6.0 D) < Barb-3-OC12F (μ = 7.7 D) < Mal-3-OC12F
(μ = 11.8 D). Within this series, clearing points Tcl also increased from 155°Cover 190°C to
210°C. Larger dipole moments increase strength and anisotropy of intermolecular interactions.
This increase led to a more stable mesophase with higher clearing temperatures Tcl due to
enhanced dipolar interactions.[32]
In addition, due to the fact that E/Z mixtures of the merocyanines were employed in the current
study, we surmised that an increasing amount of bent Z isomer should decrease the mesophase
stability in a similar fashion as was reported previously by Meier,[33] Lakehal,[34]
[37]
Sworakowski,[35] Bobrovsky,[36] and Saïdi-Besbes
albeit to a smaller extent because the
respective double bond is not located in the centre of the merocyanine in contrast to the
previously reported stilbenes, azobenzenes and the like. The results in Figure 4 suggest that the
amount of E-isomer indeed contributes to the mesophase range and stability. This effect is most
pronounced in the series of bicyclic merocyanines, where the barbiturate Barb-2-OC12F
containing 75 % E has a much broader phase width as compared to the nitrile NC-2-OC12F
(53 % E) and malononitrile Mal-2-OC12F (51 % E).
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Figure 5. Calculated dipole moments μ and experimentally determined clearing points Tcl of
A-3-OC12F (PBE0/def2-TZVPP, for more details see Table S6 and Figure S7).
Besides the effect of acceptor moiety and chromophore, the influence of the donor chain length
on mesophase properties was investigated for compounds A-2-DoF (Figure 5). In case of nitrile
NC-2-OC8F, no crystallisation was observed by DSC and POM upon cooling to – 50 °C. With
increasing donor chain length, mesophase ranges decreased from > 88 K for NC-2-OC8F and
40 K for NC-2-OC10F to 10 K in case of NC-2-OC12F. In contrast to nitriles NC-2-DoF,
barbiturates Barb-2-DoF and malononitriles Mal-2-DoF exhibited larger mesophase ranges for
donor chain elongation. While barbiturate Barb-2-OC8F was non-mesomorphic,
Barb-2-OC10F and Barb-2-OC12F exhibited phase ranges of 24 K and 89 K, respectively. In
the series of malononitriles, phase ranges increased from Mal-2-OC8F (15 K) over
Mal-2-OC10F (21 K) to Mal-2-OC12F (44 K).
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Figure 6. Mesophase ranges of nitriles NC-2-DoF, dimethyl barbiturates Barb-2-DoF and
malononitriles Mal-2-DoF (n = 1–3).
Under the polarizing optical microscope, nitrile NC-3-OC12F displayed homeotropic
alignment and maltese crosses at 150 °C (Figure 7a). The fan-shaped texture with focal conic
defects of malononitrile Mal-2-OC12F (159 °C) is shown in Figure 7b. All other liquid
crystalline derivatives A-n-DoF displayed analogous textures. The observed textures are
characteristic for smectic A phases (SmA).[38]
Figure 7. POM Textures upon cooling; a) maltese crosses of NC-3-OC12F at 150 °C; b) fan-
shaped texture with focal conic defects of Mal-2-OC12F at 159 °C; cooling rates 5 K min–1.
XRD experiments (SAXS and WAXS) were carried out to study the SmA phases of liquid
crystalline dyes A-n-DoF in detail. The WAXS profile and diffraction pattern of NC-2-OC8F
and Mal-3-OC12F are shown in Figure 8 and Figure S8. Both compounds displayed a strong
SmA fundamental diffraction peak (001), a weak (002) reflection and a diffuse halo in the wide-
angle region. These diffraction patterns are typically observed for SmA phases.[39]
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Figure 8. WAXS profile (intensity vs. scattering angle) and diffraction pattern (inlay) of a) NC-
2-OC8F at 29 °C and b) Mal-3-OC12F at 178 °C.
Compounds A-n-DoF were further investigated by temperature-dependant SAXS
measurements in the mesophase. Based on the (001) reflection, layer spacings d001 were
calculated using Bragg's law through Gaussian fitting of the (001) reflection. For both nitriles
NC-n-DoF (Figure 9 a) and malononitriles Mal-n-OC12F as well as barbiturate Barb-3-OC12F
(Figure 9 b), d001 values decreased with increasing temperature. This negative thermal
expansion gives additional proof for the SmA phase.[40]
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Figure 9. Temperature-dependant layer spacings d001 of a) nitriles NC-n-DoF and b)
malononitriles Mal-n-OC12F and barbiturate Barb-3-OC12F.
The layer spacings d001 were extrapolated to the respective clearing temperatures Tcl for a better
comparability due to the temperature-dependence of d001. The resulting d(Tcl) values are listed
in Table S6 along with the molecular lengths Lmol, which were derived from the all-trans
conformation using Chem3D and MM2 geometry optimization.[41] Extrapolated layer spacings
d(Tcl) indicated a smectic A phase consisting of semibilayer aggregates (SmAd) due to
Lmol < d(Tcl) < 2Lmol.[42] This packing model was also reported for other smectic liquid crystals
with a terminal perfluorinated carbon chain segment.[43] In Figure 10, a proposed model of the
SmAd phase with interdigitating perfluorinated chain segments is shown. The calculated layer
thickness Lcalc is in good agreement with experimentally determined d(Tcl) values (for details,
see Table S7‒Table S9).
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Figure 10. Proposed semibilayer SmAd phase of nitriles NC-n-DoF.
Lcalc,E/Z values, which take the E/Z ratio from the 1H NMR into account, were calculated from
layer thickness Lcalc,E and Lcalc,Z of the respective E and Z isomer (Table S8). Experimentally
determined d(Tcl) values are displayed along with Lcalc,E/Z in Figure 11. For nitriles NC-n-DoF,
calculated Lcalc,E/Z were smaller by 0.9–5.3 Å compared to d(Tcl) (Figure 11a). This can be
explained by an incomplete interdigitation of perfluorinated segments. In contrast to nitriles
NC-n-DoF, Lcalc,E/Z values of barbiturate Barb-3-OC12F and malononitriles Mal-n-OC12F
shown in Figure 11b were larger than d(Tcl). The smaller d(Tcl) values of Mal-n-OC12F and
Barb-3-OC12F are probably due to interdigitation of both perfluorinated segments and polar
acceptor moieties (shown in Figure S9). Layer thicknesses Lfi,E/Z with fully interdigitated
perfluorinated and acceptor groups were only by 0.3–1.3 Å larger as compared to experimental
d(Tcl) values. This small deviation may be rationalized by a thermal molecular fluctuation
within the smectic layers close to clearing temperatures Tcl = 155–210 °C.[40] In this case, a
time-dependant, random molecular tilt reduces the smectic layer thickness as observed in
Figure 11b. It should be emphasized that mesophase ranges are also depended on matching
lengths of core and chain.[48] However, in our case no clear trend could be deduced (Table
S7 - S9).
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Figure 11. Experimentally determined layer thickness d(Tcl), calculated layer spacing Lcalc,E/Z
with interdigitating perfluorinated groups and layer spacing Lfi,E/Z with full interdigitation of
both perfluorinated donor moieties and polar acceptor groups.
Electron density profile ρ(z)
The diffraction pattern of an X-ray measurement origins from a Fourier transformation of the
electron density distribution along the layers of a smectic phase.[44] This means, one can extract
the electron density from the diffraction profile. The deviation ρ(z) from the mean electron
density can be calculated by a discrete cosine transformation according to equation (3).[44] In
this calculation, each (00l) reflection contributes with a coefficient al. The square of the absolute
values |al |2 can be obtained after correction of the Lorentz factor.[45]
(3)
For nitriles NC-2-OC8F, NC-2-OC10F, and NC-2-OC14F as well as malononitriles
Mal-2-OC12F and Mal-3-OC12F, first and second order diffraction reflexes were observed in
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10.1002/cphc.201800449
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the SAXS diffraction profile. In this case, the cosine transformation simplifies to equation (4)
with two cosine terms.
(4)
The d001 and d002 values of A-n-DoF as well as corresponding coefficients a1 and a2 are listed
in Table 2. Owing to the square of the absolute values |al |2, all combinations of coefficients ± al
have to be examined with regard to plausibility.[45]
Table 2. Experimentally determined d001 and d002 values of nitriles NC-n-DoF and
malononitriles Mal-n-OC12F at temperatures T and the corresponding coefficients a1 and a2.
T / °C
29
d001 / Å d002 / Å a1
a2
NC-2-OC8
NC-2-OC10
NC-3-OC12
30.7
41.2
44.9
39.8
43.6
15.4
20.5
22.5
20.0
21.9
1.00
1.00
1.00
1.00
1.00
0.35
0.66
0.51
0.53
0.58
45
107
Mal-2-OC12 133
Mal-3-OC12 178
An overview on the electron densities ρ(z) for all compounds A-n-Do is given in Figure S10.
A typical electron profile is shown in Figure 12 along with a model of the SmAd phase with
both interdigitating perfluorinated segments and polar acceptor moieties. The electron density
profile ρ(z) showed good agreement with the proposed SmAd phase. In the region of
interdigitating perfluorinated chains, ρ(z) displayed an intense maximum. Such a high electron
density in perfluorinated segments compared to aliphatic regions has been previously reported
by Marczuk for partially fluorinated alkanes.[46] Along the aliphatic chromophore, ρ(z) showed
a global minimum. For the adjacent acceptor groups, a local maximum was observed due to an
antiparallel overlap of these polar moieties. Such layered packing is in good agreement with
previous work by Ungar, Percec [44 d)] and Pociecha [47] on polyphilic fluorinated smectic liquid
crystals based on 4-alkoxybenzoates.
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10.1002/cphc.201800449
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Figure 12. Electron density profile ρ(z) and proposed structure model of a SmAd phase with
both interdigitating perfluorinated segments and polar acceptors.
Conclusion
To obtain liquid crystalline donor acceptor dyes, the fluorophobic effect was probed for the first
time on this class of chromophores. For this purpose, a series of compounds A-n-DoF was
prepared where a 1H,1H-perfluoroalkyl donor was grafted onto a mono-, bi- or tricyclic
chromophore unit. These chromophores were equipped with nitrile, malononitrile or barbiturate
acceptor, respectively. The synthesis provided A-n-DoF in 4 steps in 2–6 % overall yield,
starting from vinylogous acids Keto-n-OH. Investigation of the UV/Vis spectra in solution
revealed a bathochromic shift of the absorption maxima with increasing chromophore size n
and a hypsochromic shift upon replacement of the barbiturate by the weaker acceptor
malononitrile. Fluorescence spectra showed a similar trend with smaller Stokes shifts as
compared to the aliphatic counterparts. The results revealed that the single methylene unit
between the ether oxygen and the perfluoralkyl chain in the donor moiety is sufficient to achieve
almost complete electronic decoupling of the perfluorinated tail and the chromophore moiety.
This was further supported by the LUMO energies of the perfluorinated dyes A-n-DoF and their
alkyl-substituted counterparts A-n-DoH, which showed very similar values according to the
cyclic voltammetry experiments.
In strong contrast to the minor influence of the perfluorinated chains on the optical and
electronic properties in solution, the fluorophobic effect strongly promoted liquid crystalline
self-assembly of the donor acceptor dyes A-n-DoF. Except for the bicyclic barbiturate with
OCH2C7F15 chain Barb-2-OC8F all other members of the series A-n-DoF displayed
enantiotropic smectic A mesophases. Mesophase widths and stability increased with increasing
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10.1002/cphc.201800449
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dipole moments, chromophore size and chain lengths. From the X-ray diffraction data, the
electron density profile was calculated, which supported the proposed partially interdigitated
bilayer packing in the SmAd phase.
In conclusion, we have shown for rigidified donor acceptor dyes that the fluorophobic effect
strongly promotes lamellar self-assembly without compromising the absorption and emission
properties of these dyes. The linear optical properties are mostly governed by the size of the
chromophore subunit and the strengths of the acceptor moiety, i.e. the overall molecular dipole
moment. Future work must demonstrate, whether grafting of perfluoroalkyl chains to other
chromophore classes also leads to novel self-assembled materials.
Figure 13 Influence of the acceptor-dependant dipole moment, chromophore size and donor
length on mesophase ranges, clearing temperatures and absorption maxima.
Experimental Section
Detailed information on synthetical methods and measurements are given in the electronic
supporting information.
Acknowledgements
Generous financial support by Deutsche Forschungsgemeinschaft (grant #LA907/17-1), the
Ministerium für Wissenschaft, Forschung und Kunst des Landes Baden-Württemberg, the
20
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10.1002/cphc.201800449
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Bundesministerium für Bildung und Forschung (shared instrumentation grant # 01 RI 05177),
and the Carl-Schneider Stiftung is gratefully acknowledged.
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