Flavylium Salts: A Blooming Core for Bioinspired Ionic Liquid Crystals

Article abstract of DOI:10.1002/chem.201901975

Thermotropic ionic liquid crystals based on the flavylium scaffold have been synthesized and studied for their structure-properties relationship for the first time. The mesogens were probed by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD). Low numbers of alkoxy side chains resulted in smectic (SmA) and lamello-columnar (Lam_Col) phases, whereas higher substituted flavylium salts showed Col_ro as well as ordered and disordered columnar (Col_ho, Col_hd) mesophases. Mesophase width ranged from 13 K to 220 K, giving access to room temperature liquid crystals. The optical properties of the synthesized compounds were probed towards absorption and emission properties. Strong absorption with maxima between 444 and 507 nm was observed, and some chromophores were highly emissive with quantum yields up to 99 %. Ultimately, mesogenic and dye properties were examined by temperature-dependent emissive experiments in the solid state.

Full text of DOI:10.1002/chem.201901975

A Journal of
Accepted Article
Title: Flavylium salts: A blooming core for bioinspired ionic liquid
crystals
Authors: Robert Forschner, Jakob Knelles, Korinna Bader, Carsten
Müller, Wolfgang Frey, Andreas Köhn, Yann Molard, Frank
Giesselmann, and Sabine Laschat
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10.1002/chem.201901975
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Flavylium salts: A blooming core for bioinspired ionic liquid crystals
1
1
1
2
1
Robert Forschner , Jakob Knelles , Korinna Bader , Carsten Müller , Wolfgang Frey ,
Andreas Köhn 3, Yann Molard 4, Frank Giesselmann 2, Sabine Laschat 1,*
1 Institut für Organische Chemie, Universität Stuttgart
Pfaffenwalding 55, 70569, Stuttgart, Germany
E-mail: sabine.laschat@oc.uni-stuttgart.de
2 Institut für Physikalische Chemie, Universität Stuttgart
Pfaffenwalding 55, 70569, Stuttgart, Germany
E-mail: frank.giesselmann@ipc.uni-stuttgart.de
3 Institut für Theoretische Chemie, Universität Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart, Germany
E-mail: koehn@theochem.uni-stuttgart.de
4 University Rennes, CNRS, ISCR—UMR6226, ScanMAT—UMS 2001Krista
35000 Rennes, France
E-mail: yann.molard@univ-rennes1.fr
Keywords: ionic liquid crystals; self-assembly; X-ray diffraction; UV/Vis spectroscopy;
fluorescence.
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Graphical Abstract
Flower power: In the search for new mesogenic cores for ionic liquid crystals, we were
inspired by the anthocyanidins, responsible for the bright red to blue colours in many plants.
The novel flavylium triflates were examined for their liquid crystalline and dye properties.
Abstract
Thermotropic ionic liquid crystals based on the flavylium scaffold have been synthesised and
studied for their structure-properties relationship for the first time. The mesogens were probed
by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X‐ray
diffraction (XRD). Low numbers of alkoxy side chains resulted in smectic (SmA) and lamello-
columnar (LamCol) phases, while higher substituted flavylium salts showed Colro as well as
ordered and disordered columnar (Colho, Colhd) mesophases. Mesophase width ranged from
13 K to 220 K, giving access to room temperature liquid crystals. The optical properties of the
synthesised compounds were probed towards absorption and emission properties. Strong
absorption with maxima between 444 nm and 507 nm was observed, and some chromophores
were highly emissive with quantum yields up to 99 %. Ultimately, mesogenic and dye
properties were examined by temperature dependent emissive experiments in the solid state.
Introduction
Ionic liquid crystals (ILCs) are an emerging class of soft matter materials which combine the
best of two worlds, i.e. the fluidity and adjustable polarity of ionic liquids with the anisotropic
properties of liquid crystals.[1–6] The vast majority of ILCs consist of nitrogen-containing
cations such as ammonium, pyridinium, imidazolium or guanidinium salts and analogues
thereof, while cations with other heteroatoms (O, S, P, …) are less commonly employed.
While oxonium ions usually only occur as short-lived intermediates in reactions, their high
reactive character can be tamed by embedding the positively charged oxygen in an aromatic
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system. Such pyrylium derivatives, like e.g. 2,4,6-triphenylpyrylium salts show strong
fluorescence and anion-π interactions.[7,8] They have been successfully utilized for electron
transfer reactions,[9] (photo)organocatalysis,[10–12] white light fluorophores[13] and laser dyes.[14]
Scheme 1
Liquid crystalline oxonium salts reported in the literature (Scheme 1) are solely based on di- or
triphenyl pyrylium cations 1,[15] 1,4-disubstituted benzenes functionalized with two pyrylium
units 2[16] and condensed xanthylium derivative BNAX 3.[17] We found it quite surprising, that
ILCs based on the probably most prominent organic oxonium salt, the flavylium cation A-Fla-
B never has been reported to the best of our knowledge. Those salts are an important scaffold
in natural and synthetic dyes. For example, anthocyanins, i.e. hydroxylated and O-glycosylated
flavylium salts provide the largest family of water-soluble plant dyes, which protect plants
against photooxidation, serve as food colourants[18] and attractant for insects.[19] Key features
of these natural dyes are their pH-dependent coloured species ranging from red to blue, their
ability for complexation of metals and formation of different types of aggregates.[20,21] Synthetic
flavylium derivatives have been successfully utilized for organic/inorganic hybrid pigments,[22]
or photosensitizer for dye-sensitized solar cells.[23–26]
Because of their nearly planar structure and the strong tendency for aggregation into dimers and
higher aggregates[27] they are particularly attractive candidates for ILCs. Additionally,
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flavylium salts possess unique structural features, i.e. the ionic moiety is located in the centre
of the mesogenic core rather than as peripheral headgroup and the molecular shape is
unsymmetrical. We anticipated that the substitution pattern of the rigid A- and flexible B-ring
should enable tailoring of both liquid crystalline self-assembly as well as linear optical
properties of flavylium salts. Here we report the first flavylium ILCs showing a rich
polymorphism and promising absorption and emission characteristics. The results are discussed
below.
Results and Discussion
Synthesis
Scheme 2
i) C12H25Br, K2CO3, DMF, 80 °C, 3 h; ii) 1. C12H25Br, K2CO3, NaI, CH3CN,
105 °C, 2 d; 2. LiAlH4, Et2O, rt, 1 h; 3. DDQ, dioxane, rt, 1 h; iii) H2O2, H2SO4,
CHCl3, CH3OH, rt, 18 h; iv) ethynylmagnesium bromide, THF, rt, 3 h; v) IBX,
EtOAc, 80 °C, 18 h.
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For the syntheses of the desired flavylium salts a protocol by Chassaing[28] was applied were
the A-ring of the flavylium salt is derived from a phenol and the B- and C-rings are generated
from an ethynyl ketone. As shown in Scheme 2, a series of hydroxy substituted
arylaldehydes 5a-e was converted into the corresponding alkoxy-substituted
arylaldehydes 6a-e via Williamson etherification in 84 - 97 %, except for the 3,4,5-
trisdodecyloxyphenylcarbaldehyde 6f, which was obtained in three steps from ethyl gallate 1
in 83 % overall yield.[29] Aldehydes 6a-f and benzaldehyde 6g were then treated with ethynyl
magnesium bromide to give the propargylic alcohols 8a-g in almost quantitative yield after
aqueous workup, followed by IBX oxidation to the corresponding ethynyl ketones 9a-g. Dakin
oxidation with H2O2 and H2SO4 of aldehydes 6a-f yielded in the phenol derivatives 7a-f in
61 – 96 %.[30]
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Scheme 3
Treatment of an equimolar solution of a phenol 7a-f and an ethynyl ketone 9a-g in EtOAc with
an excess of trifluoromethanesulfonic acid resulted in the formation of the desired flavylium
salt A-Fla-B (Scheme 3). The variation in the yields of the flavylium salt A-Fla-B was mostly
due to the differences in solubility. Some derivatives, e.g. V-Fla-1 and 2-Fla-2, show low
solubility in EtOAc and precipitated directly from the reaction solution at room temperature.
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Others, like 3-Fla-1 and 3'-Fal-3' display good solubility at room temperature and therefore
were recrystalized at low temperatures. The crude products were purified by recrystallization
from the reaction solution. With 7a no formation of the product was observed, therefore the
alkoxy substitution in meta-position is crucial for the reaction. This is the reason phenols 7b
and 7c with an additional methoxy substituent were used to obtain the derivatives with one
alkoxy sidechain attached to the A-ring. The flavyliums salt 3-Fla-0 could not be isolated and
3'-Fla-0 could not be obtained in satisfactory purity. It should be emphasized, that the solid
flavylium salts were bench-stable and solutions did not show any colour change or loss of
colour upon storage over more than 6 months.
Solid state properties
Figure 1 Single crystal X-ray structure representations of V-Fla-1 in the solid state[31]
(H = light blue, C = white, O = red, S = yellow, F = green, in b) and c) hydrogens
are omitted for clarity). a) Hydrogen bond interactions of the triflate anion given by
dashed lines. b) Stacking interaction of the flavylium cation. c) bc view along the
a-axis showing the interdigitated layer structure.
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Recrystallisation of V-Fla-1 from EtOAc provided suitable crystals for single crystal analysis.
The compound crystallizes with one ion pair in the asymmetric unit of the centrosymmetric
space group P-1. The flavylium cation is almost planar with a torsion angle of 5° between the
chromenylium and the phenyl moiety. The oxygen atoms of the triflate anion works as acceptors
for a couple of hydrogen bond interactions (Figure 1a). First, there are π(C-H) donors of the
chromenylium and the phenyl moieties. The (H…O) interval of the relevant distances is 2.34
to 2.39 Å. Second, a weaker interaction is evident with the methyl C-H function of the methoxy
group. The (H…O) distance range is 2.54 to 2.70 Å. And finally, there is a weak interaction
between a C-H donor of the alkyl chain and the O6 of the triflate anion with a (C29-H29…O6)
distance of 2.48 Å. The cation built up a layer type stacking interaction with a pairwise 180°
rotated orientation of the molecules forced by a slight π-π stacking interaction of the
chromenylium cores (Figure 1b). In detail, the pyrylium core interacts with the benzene part of
the chromenylium and vice versa. The distance of the centroids is in both cases 3.66 Å.
Additional each pair is also stabilized by a slight stronger π-π stacking generated only between
the pyrylium cores with a distance of 3.56 Å. Remarkably the phenyl groups of the flavylium
moieties are not involved in π-π stacking interactions. This is most likely due to the strong ionic
and dipolar interaction of the benzopyrylium moiety, surpassing the possible contribution of
the π-π interaction from the phenyl group. This packing motive seems to be universal for
flavylium cations and will be important for the following discussion of the liquid crystalline
properties. In the bc view of the packing diagram there is a layer-type orientation of the
molecules along the c-axis evident (Figure 1c). The centre part of the cation and the triflate
anions form a polar layer which alternates with the nonpolar layer consisting of the aliphatic
interdigitated chains.
Liquid crystalline properties
The thermotropic behaviour of the synthesized flavylium salts A-Fla-B were examined by
polarizing optical microscopy (POM), differential scanning calorimetry (DSC) as well as wide-
and small-angle X-ray scattering (WAXS and SAXS). The results of the DSC experiments are
summarized in Table S1 and an overview of the observed phases is presented in Figure 2.
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Figure 2 Overview of the observed mesophase of the flavylium salts A-Fla-B (a detailed
version of this diagram with the mesophase width can be found in Figure S1).
Some compounds show decomposition of the material in the DSC. However, the TGA
measurements of the series iV-Fla-B showed that the compounds are stable to 200 °C, except
iV-Fla-3, which decomposes at about 150 °C (Figure S7). Flavylium salts with no side chain
on the B-ring, i.e. V-Fla-0, iV-Fla-0, 2-Fla-0 and 3'-Fla-0 were non-mesomorphic irrespective
of the number and position of the side chain on the A-ring and showed only Cr-Cr transitions.
In addition, V-Fla-2 with one side chain at the A-ring and two side chains at the B-ring as well
as V-Fla-3' and iV-Fla-3' with sterically crowded B-ring were non-mesomorphic. For clarity,
the following discussion is organized according to the type of mesophase (lamellar, columnar
rectangular and/or hexagonal) formed by the flavylium salt.
Flavylium salts with lamellar mesophases
Flavylium salts with one or two side chains on the A-ring and one side chain on the B-ring
resulted in lamellar mesophases. For example, vanillin-derived flavylium salt V-Fla-1 with one
dodecyloxy chain on both A- and B-ring displayed two liquid crystalline phases, i.e. a melting
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transition at 129 °C followed by a mesomorphic transition at 201 °C and a clearing point at
214 °C upon first heating in the DSC (Figure S2b). Under the POM the lower-temperature phase
showed uncharacteristic textures, whereas Maltese cross textures and a strong tendency for
homeotropic alignment were observed upon heating into the high temperature mesophase.
Upon cooling from the isotropic liquid, the compound displayed bâtonnets textures (Figure 3a),
indicating a SmA mesophase. Upon further cooling into the low temperature phase fan shaped
textures were visible (Figure 3b) which can be observed in smectic and columnar mesophases,
but also has been reported for lamello-columnar LamCol mesophases.[32]
Figure 3 POM micrographs of a) V-Fla-1 at 205 °C (magnification 200x) and b) at 188 °C
(magnification 100x). All pictures were taken between crossed polarizers upon
cooling from the isotropic phase with a cooling rate of 5 K min-1.
X-ray diffraction of V-Fla-1 showed upon cooling from the isotropic phase the typical
diffraction pattern of an oriented SmA mesophase consisting of a sharp layer reflex (001) and
the higher order reflex (002) in the small angle region (Figure S9a). The wide-angle region
displayed two broad halos at 4.66 Å and 3.64 Å resulting from the molten alkyl chains and
presumably short aggregates of the mesogens, respectively (Figure 4a). Both reflexes are
oriented perpendicular to the layer reflexes. The experimental layer spacing d = 29.7 Å from
the SAXS measurement is significantly smaller than the molecular length obtained from single
crystal structure analysis (L = 41.7 Å). Usually, the layer spacing of a SmA phase is 5 – 10 %
smaller than the molecular length, due to the axial disorder of the mesogens defined by the
order parameter.[33] In the case of V-Fla-1 the observed d/L ratio of 0.71 is unusual and cannot
solely be explained by a low order parameter. However, the difference between flavylium ILCs
and conventional mesogens with smectic phases is that their charge is located in the centre of
the molecule. The close proximity to the counter ion leads to an expansion of the effective cross
section of the core. The resulting free space between the alkyl chains is then filled by
interdigitation of the neighbouring hydrophobic layers, forming an SmA1 phase. This packing
model is further supported by analogy with the crystal structure of V-Fla-1, which reveals
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interdigitation of alkyl chains and a lattice parameter of c = 26 Å similar to the experimentally
determined layer spacing.
Cooling of the sample into the lower-temperature phase at 170 °C resulted in a more
pronounced wide-angle reflex at 3.54 Å as a result of the growing intracolumnar stacks, which
were already weakly present in the SmA phase (Figure 4b). Additionally, a sharp reflex with a
layer spacing of 10.04 Å is observed. The perpendicular orientation towards the layer reflex, as
well as the similarity of the length to the b-axis (10.27 Å) of the single crystal structure, led to
the assumption that this reflex originates from the lateral distance of the short columnar stacks.
The scan over χ of a slowly cooled sample revealed that this reflex is split into two reflexes
with ± 10° with respect to the centre (Figure 4c).
Therefore, we assume that the lower-temperature phase is a lamello-columnar phase LamCol, in
which the layers are build up by short stacks of flavylium cations (Figure 4d). Within these
stacks the molecules are organized in an antiparallel manner as observed in the crystal structure,
in order to enable π-π-interactions and reduce charge repulsion. The splitting of the lateral
intercolumnar distance can be explained by an alternating tilt of these mesogenic stacks with
±10° in respect to the layer normal from one layer to another, comparable to the anticlinic SmC
phase. Due to the low number of observed reflexes and the sliding of the layers, further
differentiation regarding the symmetry was not reliable. Further evidence for this hypothesis
can be given by calculating the volume of the elemental cell using the length of the 001 and the
010 reflex, as well as the intracolumnar distance. With the volume in hand the number of
molecules per elemental cell Z can be calculated according to Lehmann,[29] which in the case
of the lower-temperature phase of V-Fla-1 resulted in Z = 1.07 assuming a density of 1 g ml-1.
The occurrence of the LamCol phase can be rationalized by the strong tendency for the formation
of vertical aggregates of the flavylium cation and a weak layer coupling.[34] At high
temperatures the stacking is unfavourable, therefore the calamitic shaped molecules form a
SmA phase as expected. However, the presence of the wide-angle reflex corresponding to the
π-π-reflex indicates, that aggregation already occurs in the SmA phase. Upon decrease of the
temperature the molecular stacks are growing until the phase transition into the LamCol phase
occurs.
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Figure 4 Diffractogram and the diffraction pattern of the oriented sample of V-Fla-1 in the
a) SmA phase at 210 °C and b) LamCol phase at 170 °C after cooling from the
isotropic state (cooling rate: 5 K min-1). c) 2D-SAXS pattern of the LamCol
mesophase and the χ-scan of the diffraction peaks obtained by slow cooling
(0.2 K min-1) from the SmA phase into the LamCol phase. d) Proposed packing of
the molecules in the LamCol mesophase. e) Temperature dependent layer spacing of
the (001) reflex (■), the (002) reflex (●) and the (010) reflex (▲) in the SmA (hollow
symbols) and the LamCol phase (filled symbols) of compound V-Fla-1. The
measurement was performed via the second heating (rate: 2 K min-1).
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Further information of the mesophase was obtained by the temperature-dependent SAXS
measurements (Figure 4e). In the LamCol phase, the layer spacing decreased only very slightly
with increasing temperature. At the LamCol -SmA phase transition the layer spacing dropped
significantly by ~2 Å and upon further increase of temperature a negative thermal expansion of
the (001) reflex was observed, which is typical for the SmA phase.
The DSC of the isovanillin-derived flavylium salt iV-Fla-1 with one dodecyloxy side chain on
both A- and B-ring showed only one mesophase between 47 °C and 131 °C (Figure S3a). Under
the POM an uncharacteristic grainy texture was observed. The XRD pattern showed an intense
(001) reflex and the higher order reflexes (002) and (004) (Figure S10a-b). In addition, one
further reflex at 10.4 Å was observed as well as a diffuse halo centred around 4.84 Å and a π-π
reflex at 3.63 Å. For this compound no oriented sample could be obtained, but due to the
similarity of the diffraction pattern compared to V-Fla-1 we surmised that this phase is also a
LamCol phase. However, it should be noted that the layer spacing of iV-Fla-1 shows a stronger
dependency on the temperature than V-Fla-1 in the LamCol phase (Figure S10c).
Figure 5 Polarized optical micrographs of a) 2-Fla-1 at 135 °C (magnification 100x) upon
heating, b) 2-Fla-1 at 115 °C (magnification 100x) upon cooling from the isotropic
liquid, c) fan-shaped textures at 135 °C and at d) 110 °C. 2-Fla-1 in a rubbed
polyimide cell (homogeneous alignment, cell gap: 3 µm) upon cooling. 2-Fla-1 in
a single side rubbed nylon cell, cell gap: 1.6 µm) at e) 135 °C and f) 110 °C.
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Flavylium salt 2-Fla-1 with two dodecyloxy chains at the A-ring and one chain at the B-ring
showed three transitions upon heating in the DSC. An endothermal melting transition at 46 °C
and 100 °C followed by a first order transition at 126 °C with a small transition (less than
0.7 kJ mol-1) and a clearing transition at 145 °C was detected (Table S1, Figure S4a). Under the
POM 2-Fla-1 showed filament-like textures in the highest mesophase upon heating (Figure 5a).
Cooling from the isotropic liquid resulted in a pronounced homeotropic alignment, interrupted
by occasionally occurring Maltesian crosses. Upon entering the lower-temperature mesophase
fan-shaped textures were observed (Figure 5b). In a polyimide coated cell, already the higher-
temperature phase showed fan-shaped textures, characteristic for the SmA phase (Figure 5c).
The lower-temperature phase showed broken fan-shaped textures characteristic for the SmC
phase (Figure 5d). Additionally, the birefringence increases drastically. However, attempts to
determine optical tilt angles in both polyamide and nylon test cells failed despite good
alignment, since the lower-temperature phase appears to be uniaxial, disproving a SmC phase.
Cooling into the lowest temperature phase resulted in grainy and darkened textures.
To gain further insight into the observed phases, 2-Fla-1 was examined by temperature
dependent XRD measurements. In the small-angle region at 130 °C, a sharp reflex indexed as
(001) of the SmA phase is observed (Figure 6a, Figure S11). The layer spacing of 31.1 Å is
smaller than the molecular length due to interdigitation of the alkoxy side chains, as also seen
in the SmA phase of V-Fla-1. The wide-angle region displayed a diffuse halo at 4.75 Å oriented
perpendicular to the layer reflex (Figure 6b). Therefore, this phase has been identified as a
partially interdigitated SmA phase.
The mesophase at 120 °C showed besides the (001) reflex the higher ordered reflex (002). The
χ scan of the wide-angle region of the oriented sample showed that the reflexes oriented
perpendicular to the layer reflexes and no significant difference towards the SmA phase could
be observed. However, the reflex corresponding the intramolecular distance is more prominent
in this phase indicating a longer correlation length of the lateral intermolecular distance. The
temperature dependent SAXS measurement revealed, that the layer spacing decreased
continuously with increasing temperature from 102 °C to 145 °C (Figure 6c). Considering the
results from the POM and the X-ray analysis, we assume that this phase is a SmA' phase, with
the difference that the layers are built up by flavylium dimers.
The lowest temperature mesophase at 90 °C showed the layer reflexes (001), (002), (003) and
(004) along the meridian, as well as the intercolumnar reflex (010) oriented perpendicular to
the layer reflexes. Thus, the overall appearance of the XRD pattern showed similarities to the
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LamCol phase of V-Fla-1. The layer spacing of 34.0 Å is larger as compared to V-Fla-1,
showing that interdigitation is less pronounced in 2-Fla-1, due to the additional side chain. The
intercolumnar distance of the columns is 10.6 Å and no splitting of this reflex could be detected.
The WAXS showed a diffuse halo at 4.60 Å and the intracolumnar reflex at 3.50 Å along the
equator.
Figure 6 a) SAXS and b) WAXS diffractograms of 2-Fla-1 at 130 °C, 120 °C and 90 °C (from
top to bottom). b) Temperature dependent layer spacing of the 001 and 002 reflex on
cooling from the isotropic state. Transition temperatures are given by the dashed
lines. c) Temperature-dependent layer spacing of the (001) reflex (■) and the (002)
reflex (●) in the LamCol phase (filled symbols), the SmA' phase (gray symbols) and
the SmA phase (hollow symbols).
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Flavylium salts with columnar rectangular phases
Figure 7 Polarized optical micrographs of a) iV-Fla-2 at 196 °C (magnification 200x) and
b) iV-Fla-3 at 120 °C (magnification 200x). All pictures were taken by cooling
from the isotropic phase with a cooling rate of 5 K min-1.
In contrast to the non-mesomorphic vanillin-derived flavylium salt V-Fla-2 (melting
point: 195 °C) the corresponding isovanillin-derived flavylium salt iV-Fla-2 and iV-Fla-3
showed Colro mesophases between 52 °C and 199 °C and between 50 °C and 156 °C in the
DSC, respectively (Figure S3c). Under the POM both compounds displayed spherulite-like
textures and line defects, characteristic for columnar mesophases (Figure 7).
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Figure 8 X-ray diffractograms of a fibre sample of a) iV-Fla-2 at 110 °C and b) iV-Fla-3 at
120 °C with the corresponding 2D diffraction pattern with the azimuthal angles of
the diffuse wide-angle reflexes. The fitting of the wide-angle area is given in grey
with the sum (bold line) of the distinct Gauss peaks (dashed lines). White arrows
indicate the direction of the fibre. c) Proposed packing model of the Colro phase and
the stacking within the column (the representation of the discoid has been
simplified, for a more detailed discussion see Figure 10 and the corresponding text).
The diffraction pattern of an oriented fibre of iV-Fla-2 consisted of sharp reflexes in the small-
angle region (Figure S12a). The reflexes could be assigned as (11), (02), (12), (22), (23) and
(31) of a columnar rectangular ordered mesophase Colro with p2gg symmetry. The lattice
parameters of the elemental cell are a = 35.9 Å, b = 50.2 Å and a Z-value of 4, indicating that
one disk is formed by two molecules (it should be noted that a p2mg symmetry, where on disc
consists of one molecule cannot be completely ruled out). The wide-angle region of a fibre
sample showed a diffuse halo of the molten alkyl chains centred around 4.71 Å. This reflex is
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split into two reflexes with an azimuthal angle of 39° with respect to the meridian (Figure 8a).
Furthermore, a relatively sharp reflex at 3.52 Å as well as an additional diffuse reflex at 3.44 Å
was observed. The diffraction pattern of iV-Fla-3 (Figure S12b) showed fewer and less intense
higher order reflexes compared to iV-Fla-2, but the same phase geometry of p2gg with slightly
larger lattice parameters (a = 36.3 Å, b = 51.0 Å, Z = 4) was observed. The wide-angle region
showed also three reflexes: a diffuse halo at 4.63 Å with a tilt of 39°, a relative sharp reflex at
4.06 Å with no tilt as well as an additional diffuse reflex at 3.39 Å with a tilt of 30° with respect
to the meridian (Figure 8b). Furthermore, a reflex is observed at 7.46 Å.
The occurrence of a relative sharp reflex in the wide-angle region in discotic phases is usually
referred to the periodic arrangement of the aromatic cores, but for the flavylium salts with a
Colro mesophase the situation seems to be different, since the intracolumnar distance of
iV-Fla-3 would be too large for typical π-π-stacking. To rationalize the wide angle reflexes I-
III, and their different azimuthal angles in Figure 8, the mesogens can be divided into three
parts: the alkoxy side chains I, the triflate anion II and the aromatic core III. From the diffraction
pattern of the iV-Fla-2 fibre sample the tilt of the mesogens with respect to the column axis is
mainly governed by the alkyl chains. The observed tilt angle of 39° can be obtained directly
from the azimuthal angle. Presumably the almost spherical anions form the linear, non-tilted
backbone of the columns as depicted in Figure 8c. The small difference in the distances of the
anions (3.52 Å) and cations (3.44 Å) can be explained by the tilt between the aromatic cores of
12° calculated by using the formula α = cos-1 (dIII / dII). This angle is quite small and therefore
could not be determined directly form the diffraction pattern due to overlapping of both
reflexes.
For iV-Fla-3 the tilt of the alkyl chains is similar to iV-Fla-2 but the tilt between the aromatic
cores is much higher. First, this becomes noticeable by the splitting of the corresponding reflex
III with an azimuthal angle of 30° in the diffraction pattern and secondly by the increased anion-
anion distance of 4.06 Å as a result of the aromatic tilt while the distance between the aromatic
cores remains almost identical. Using the above mentioned formula, the calculated tilt of the
flavylium cation is 33° and therefore in good agreement with the experimental value and
supports the assumed packing of the molecules within the column. Further evidence can be
found by comparing the width of the peaks in the wide-angle area. The aromatic reflex in the
less tilted iV-Fla-2 is sharper compared to the more tilted iV-Fla-3. A smaller tilt results in a
longer correlation length and therefore in a sharper reflex. The results provide a useful tool to
estimate the tilt of the columns in the Colr phases in ILCs where no planar aligned sample can
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be obtained. Such detailed information about the intracolumnar stacking can be important e.g.
for ionic conductivity.[35] An comprehensive packing model for the columnar phases of the
flavylium salts will be discussed below using 2-Fla-3 as an example.
Flavylium salts with rectangular and hexagonal phases
Figure 9 Polarized optical micrographs of V-Fla-3 at a) 125 °C (inset: dendritic germ
observed below the clearing point, picture taken with slightly uncrossed polarizers)
and b) 75 °C between crossed polarizers. Textures of 2-Fla-3 at c) 185 °C and d)
75 °C. All pictures were taken by cooling from the isotropic phase with a cooling
rate of 5 K min-1 and a magnification of 200x.
The vanillin-derived flavylium salt V-Fla-3 as well as 2-Fla-2 and 2-Fla-3 showed a lower
temperature Colro mesophase and a higher temperature Colh mesophase. Flavylium salt V-Fla-3
showed upon heating in the DSC a melting transition at 53 °C, a mesomorphic transition at
100 °C and clearing into the isotropic phase at 190 °C (Figure S2d). Under the POM
uncharacteristic grainy textures were observed for the low temperature phase upon heating,
while the high temperature phase showed line defects. Cooling from the isotropic phase into
the high-temperature mesophase resulted in dendritic growth (Figure 9a). The hexagonal shape
of the liquid crystalline germs indicates a columnar hexagonal mesophase in agreement with
Bouligand.[36] Upon cooling into the low temperature phase, the texture became grainy
especially in previously homeotropic aligned areas (Figure 9b).
XRD experiments of V-Fla-3 (Figure S13) in the higher-temperature mesophase at 140 °C
revealed the reflexes (10), (11), (20) and (21) of the Colh mesophase (p6mm symmetry) with a
lattice parameter of a = 25.8 Å (Z = 1). The WAXS region showed a diffuse halo at 4.51 Å and
a reflex at 3.39 Å originating from the intracolumnar order. As compared to the Colr phase, the
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distances between the anions and those between the aromatic cores are identical and therefore
are represented by a single reflex at 3.39 Å. Furthermore, a reflex at 6.71 Å was observed,
which is twice the layer spacing of the intracolumnar distance. For the further discussion this
reflex will be noted as π-π'.
In the lower temperature phase of V-Fla-3 at 75 °C the SAXS region showed the reflexes (01),
(10), (12), (20), (03), (13), (22), (04), (30), (24) and (15) which could be assigned to a Colr
phase with p2mm symmetry and lattice parameters of a = 57.7 Å and b = 41.9 Å with a Z-value
of 4, indicating that one discoid is formed by 4 flavylium salts. Presumably, the decrease of the
alkoxy chain length at 7-position to a methoxy group in V-Fla-3 resulted in an different volume
requirement as compared to iV-Fla-2, iV-Fla-3 and 2-Fla-3 carrying a dodecyloxy chain at this
position. The WAXS region showed a similar behaviour as discussed above for iV-Fla-3. The
diffuse halo centred around 4.52 Å showed a splitting into two reflexes with an angle of 51°
with respect to the meridian corresponding to the average distance and tilt of the alkyl chains.
The sharp reflex at 3.96 Å showed no tilt, as it originates from the non-tilted anions as seen
above. The reflexes corresponding to the distances of the aromatic cores at 6.74 Å and 3.64 Å
showed both a tilt of 23° respectively and are in agreement with the calculated tilt α = cos-1
(3.64 Å / 3.96 Å) = 23°.
The DSC of flavylium salt 2-Fla-2 with two dodecyloxy side chains at the A- and B-ring
showed a transition at 119 °C and 131 °C into the mesophase as well as a mesomorphic
transition at 165 °C with low enthalpy (-0.8 kJ mol-1) followed by clearing into the isotropic
liquid phase at 180 °C (Figure S4b). Under the POM the compound displayed spherulite like
textures and dendritic growth typical for columnar phases. The small angle diffractogram of
compound 2-Fla-2 at 170 °C showed a single (10) reflex of a hexagonal mesophase with a
lattice parameter of a = 35.5 Å (Z = 2, Figure S14). The WAXS pattern showed a diffuse halo
at 4.68 Å and an additional intracolumnar reflex at 3.49 Å. In contrast to the hexagonal phases
of V-Fla-3, 2-Fla-3, 3-Fla-2 and 3-Fla-3 the π-π' reflex could not be observed. As seen for the
previous flavylium compounds iV-Fla-2 and iV-Fla-3, the lower temperature Colr phase shows
p2gg symmetry with a lattice parameter of a = 61.0 Å and b = 34.5 Å, preserving the
pseudohexagonal lattice of the higher temperature hexagonal phase as indicated by the ratio
a/b = 31/2.
For the compound 2-Fla-3 upon heating a glass transition at 59 °C, a mesophase to mesophase
transition at 102 °C and a clearing into the isotropic liquid at 200 °C was observed. In analogy
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to V-Fla-3 the higher-temperature mesophase showed typical columnar textures, which became
grainy upon entrance into the lower-temperature mesophase (Figure 9c-d).
From the XRD result for 2-Fla-3 a slightly larger lattice parameter of a = 27.4 Å in the higher-
temperature Colho phase and a halo at 4.52 Å as well as the intracolumnar reflexes at 3.44 Å
and 6.75 Å were observed (Figure S15a-b). The small angle peaks of the lower-temperature
mesophase could be perfectly indexed as hexagonal phase with a = 35.4 Å (Z = 2), but since
the wide angle region indicates a tilt similar to V-Fla-3, we assume a Colr phase.
Proper indexation could be achieved for the hk sets: 20/02 and 11/31 both with p2gg symmetry
and a lattice parameter of a = 61.3 Å and b = 35.3 Å, as well as the rather unlikely case of 10/01
with p2mm symmetry and a lattice parameter of a = 30.1 Å and b = 17.7 Å. The wide-angle
region displayed a halo with a layer spacing of 4.52 Å and a tilt angle of 51°, resulting from the
alkyl chains. The layer spacing of the anions is with a value of 3.96 Å larger than the π-π-
distance of the aromatic cores (3.55 Å). The tilt of these cores was determined to be of 23° by
fitting of the χ scan (calculated: 25°).
Figure 10 Proposed packing model of the flavylium salts in the a) Colho and the b) Colro
mesophase viewed from the top (left) and in an side view (right).The flavylium
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cations are displayed as blue arrows (pointing towards the oxonium cation) and the
triflate anions as red dots.
To construct general packing models for columnar phases, usually nanophase segregation and
π-π interactions are decisive. However, in the case of flavyliums salt the additional repulsive
charge interaction located in the centre of the aromatic core plays the major role. Therefore,
most likely flavylium salts are stacked into columns in an antiparallel arrangement (Figure 10a)
similar to the crystal structure of V-Fla-1 (Figure 1). Further evidence for this model can be
found in the observed π-π'-reflex, which corresponds to either the distance between the aromatic
core with the same direction or the distance between the anions. This model can be applied to
all compounds with a Colho phase with one molecule per disc (e.g. V-Fla-3, 2-Fla-3, 3-Fla-2,
3-Fla-3).
In the Colro phases the XRD results revealed that two flavylium salts form one discoid. We
assume, that in addition to the intracolumnar interaction in the lower temperature Colro also
intercolumnar anion-H-bonds occur. In other words, the Colro phase can be considered as two
antiparallel packed columns bond together, as depicted in Figure 10b. Within one discoid the
flavylium salts can be arranged either in a face to face or pointing into same direction. In both
cases the flexible B-ring has to rotated out of plane to avoid steric repulsion. We assume, that
additional intercolumnar anion-H interactions stabilize the lower-temperature Colro phase,
while the higher-temperature Colho phase features only intramolecular interactions. 2-Fla-2 can
be as an intermediate case which forms a Colho but contains two molecules per disc.
Flavylium salts with hexagonal phases
Flavylium salt 3-Fla-2 possessing three dodecyloxy chains at the A-ring and two at the B ring
showed a single broad mesophase between -6 °C and 215 °C in the DSC (Figure S5c) and
characteristic columnar textures under the POM. The XRD (Figure S16a-b) experiments
revealed a Colho mesophase with a slightly larger a-value of 28.6 Å (Z = 1) as compared to the
analogue 2-Fla-3 with an inverted substitution pattern. The wide-angle region displayed the
diffuse halo, as well as the π-π and the π-π' reflex. Compound 3-Fla-3 showed also a Colho
phase, ranging from 56 °C and 209 °C with similar textures and XRD result (for details see
Table S2 and Figure S16c-d). No Colro phase was observed for the substitution pattern 3-Fla-B,
presumably due to steric overcrowding in the above mentioned packing models shown in Figure
10. Additionally, the overall number of alkoxy side chains can be sufficient to stabilise the Colho
phase over a wide temperature range.
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In contrast to the normal thermotropic behaviour of 3-Fla-2 and 3-Fla-3, the flavylium salt
3-Fla-1 lacked a stable mesophase. Upon heating from the solid phase, clearing into the
isotropic phase was detected at 54 °C by DSC and under the POM (Figure S5a). Upon further
heating no other peak could be observed. The cooling curve showed a broad endothermic peak
at 20 °C and a sharp peak at 11°C. These two transitions were enantiotropic and are also
observed in the second and third heating. Surprisingly, under the POM (Figure S17) the
formation of columnar textures could be observed upon heating the substance to 78 °C. These
textures were only observable in thin sample areas between glass or in a polyimide-coated test
cell of 4.6 µm thickness, while thick parts of the sample remain isotropic. At 143 °C these
textures cleared into a second isotropic state. Upon cooling again typical columnar textures was
observed clearing into a fluidic phase at about 78 °C, revealing the enantiotropic nature of this
phase. This shows, that under bulk conditions the compound 3-Fla-1 has no stable mesophase,
but under planar anchoring liquid crystalline properties can be observed. Reentrant phases have
already been reported for compounds, where two SmA phases were separated by a nematic
phase, but the sequence Ire-Col-I is quite rare,[37–40] and to the best of our knowledge an Ire phase
has never been reported for thermotropic ionic liquid crystals.
XRD experiments showed a (10) and (11) reflex in the small angle region and a diffuse halo in
the WAXS measurement (Figure 11a). Since no π-π- reflex is observed, the intracolumnar order
is quite low in contrast to the previously described columnar mesophase. Fortunately, a partially
oriented SAXS pattern of 3-Fla-1 could be obtained by slowly heating the sample into the
columnar phase from the isotropic re-entrant phase. The diffraction pattern consisted of a
diffuse part and sharp reflexes orientated in a hexagonal manner (Figure 11b).
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Figure 11 a) WAXS diffractogram of 3-Fla-1 at 85 °C. The inset displays the SAXS pattern
of a planar aligned domain obtained by slow heating into the Colh phase at 80 °C
(the additional diffuse reflex is most likely due passing of the beam through another
domain without mesophase). b) Proposed stacking of two flavylium cations
V-Fla-3 in the liquid crystalline state.
We assume the unusual phase sequence can be explained by a complex relationship of
intermolecular forces, surface interactions and space filling. The high-temperature isotropic
phase probably consists of monomeric flavylium salts, while the surface is covered by
molecules due to polar anchoring. Cooling the sample causes more and more molecules to
assemble on the surface aligned molecules by intramolecular interactions, leading to the
formation of columns which ultimately results in the observed Colh phase. With decreasing
temperature, the attractive π-π and ionic interactions increase. At some point these interactions
become so strong that stable dimers of 3-Fla-1 are formed, which no longer display any liquid
crystalline behaviour, resulting in the phase transition into the lower-temperature Ire phase.
This behaviour can be explained by comparing 3-Fla-1 to V-Fla-3, with the inverted
substitution pattern. In V-Fla-3 the higher number of side chains is attached to the flexible B-
ring. Torsion of this phenyl moiety causes the side chains in m-position to rotate out of the
aromatic plane and extend into the upper and lower mesogen (Figure 11b). This allows the
mesogen to effectively fill the void left by the mono-substituted side. Additionally, the rotation
of the crowded B-ring disfavours the formation of a dimeric species by steric repulsion. In
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contrast, the higher substituted side in 3-Fla-1 is rigid, therefore space filling is only possible
by strong coiling of the alkyl chains, leading to a rather spherical appearance, disfavouring
liquid crystalline properties.
Flavylium salts with 3'-substitution pattern
Since several of the above discussed flavylium salts showed quite high clearing points we
wanted to push the clearing temperature towards lower temperatures. A lot of research has been
done with respect to reduce the clearing point by using thioether side chains,[41] branched[42] or
swallow-tailed[43] side chains rather than linear alkoxy side chains. In contrast, our aim was to
reduce the clearing point by varying the substitution pattern at the flavylium A- and B-rings.
Therefore the 3'-substitution pattern, derived from 2,3,4-dodecylalcoxybenzaldehyde 5e at the
A- and/or B-ring was tested.
The clearing points of the 3'-mesogens, i.e. A-Fla-3' and 3'-Fla-B could indeed be reduced
compared to their A-Fla-3 and 3-Fla-B counterparts. This effect is more pronounced for
derivatives with the 3'-substituent attached on flexible B-ring. For example, 3-Fla-3 shows a
clearing point at 210 °C and strong decomposition in the DSC, 3'-Fla-3 enters the isotropic
phase at 169 °C and 3-Fla-3' already at 93 °C. 3'-Fla-3 showed only minor differences
compared to 3-Fla-3'.
All compounds of this series A-Fla-3' and 3'-Fla-B, except the non-mesomorphic V-Fla-3' and
iV-Fla-3' derivatives exhibited a columnar mesophase as indicated by the typical columnar
textures observed under the POM (Figure S18). In contrast to the A-Fla-3 and 3-Fla-B
flavylium salts the columnar phases shows almost no intracolumnar order. Similar lattice
parameters were found for compound 2-Fla-3' (a = 27.2 Å), 3-Fla-3' (a = 27.7 Å) and
3'-Fla-3' (a = 29.4 Å) and the calculated Z-value of 1 indicating one molecule per discoid
(Figure S19). The intercolumnar reflex of 3'-Fla-3' is broad, almost comparable to the isotropic
liquid. This behaviour can be explained by either a very small correlation length of the
intercolumnar reflex, or that the mesophase is only stable under planar anchoring conditions,
similar to 3-Fla-1. This would also explain the missing clearing point in the DSCs. In contrast,
the derivatives 3'-Fla-1 and 3'-Fla-2 showed larger a-values with 29.7 Å and 31.4 Å
respectively and a Z-value of 2 indicating two molecules per disk. 3'-Fla-3 again shows a Z-
value of 1 and a lattice parameter of 27.7 Å. The packing model shown in Figure 10 cannot
easily be transferred to these compounds, since the columnar phases are disordered and the
alkoxy substituent in 8-position causes a steric hindrance.
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Dye properties
To obtain insight into the optical properties of the flavylium salts A-Fla-B were examined by
UV/Vis absorption and emission spectroscopy. Chloroform was chosen as solvent, since all
compounds showed good solubility in halogenated solvents. The results are summarized in
Table S3. All compounds showed absorption maxima between 444 and 507 nm. The spectra of
the vanillin-derived series V-Fla-B are shown in Figure 12. The substitution pattern on the
A-ring had only a minor influence (< 4 nm) on the absorption maximum, while some variations
of the extinction coefficients were observed. However, the number and position of substituents
at the B-ring resulted in a red-shift of the absorption maximum, which increased in the series
V-Fla-0 < V-Fla-1 < V-Fla-3' < V-Fla-3 < V-Fla-2.
Figure 12 a) Absorption spectra (bold line, represented by its extinction coefficient
determined by linear regression of a concentration series ranging from 0.2⸱10-5 M
to 7⸱10-5 M) and normalized emission spectra (dashed line) of vanillin-derived
flavylium salts V-Fla-B in CHCl3.
Fluorescence spectra were also measured in CHCl3 by exciting the molecules in their absorption
maxima. The flavylium salts showed emission maxima between 498 - 595 nm. The Stokes shift
increased in the series V-Fla-1 (1467 cm-1) < V-Fla-2 (2416 cm-1) < V-Fla-0 (2721 cm-1) and
V-Fla-3 (3298 cm-1). Interestingly the V-Fla-3' derivative showed a comparable small Stokes
shift (1467 cm-1) as observed for V-Fla-1 but the fluorescence intensity was low. The number
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of alkoxy substituents on the flavylium salt had a strong impact on the absolute fluorescence
quantum yields. While V-Fla-0 with no substituent on the B-ring was only weakly emissive
(ΦF = 4 %), the corresponding analogue V-Fla-1 with one substituent on the B-ring displayed
a very strong green emission with a quantum yield of 97 %. Similar high values have already
been reported by Haucke for 7,4'-dimethoxy substituted flavylium salts.[44] Upon further
increasing the number of side chains in the B-ring, the quantum yield decreased considerably
(V-Fla-2: 5 %). Flavylium salts with three alkoxy side chains were only weakly emissive
(V-Fla-3: < 1 %, V-Fla-3': < 1 %). With increasing the number of alkoxy side chains at the A-
ring the quantum yields also decreased. While vanillin-derived flavylium salt V-Fla-1 and
isovanillin-derived flavylium salt iV-Fla-1 showed almost quantitative fluorescence (97 % and
99 %, respectively), already 2-Fla-1 showed a reduced value of 92 %. With three alkoxy side
chains in 3-Fla-1 the quantum yields drastically dropped below 1 % as seen for 3-Fla-1 and
3'-Fla-1. Possible fluorescence quenching resulting from H-aggregates can be clearly ruled out
due to the low concentration and the fact that no blue shift of absorption and emission within
the series V-Fla-1, iV-Fla-1, 2-Fla-1, 3-Fla-1 and 3'-Fla-1 was observed.
The fluorescence quantum yields seem to strongly depend on the number of alkoxy chains at
the B-ring. The high emission of V-Fla-1, iV-Fla-1 and 2-Fla-1 respectively, carrying only one
alkoxy chain at the B-ring as compared to the only weakly fluorescent flavylium salts with
either no substituent on the B-ring (i.e. V-Fla-0, iV-Fla-0, 2-Fla-0) or two alkoxy chains on the
B-ring (i.e. V-Fla-2, iV-Fla-2, 2-Fla-2) might be rationalized by considering the canonical
Lewis structures. As exemplified for flavylium salts V-Fla-0, V-Fla-1 and V-Fla-
2, respectively possible Lewis structures are shown in Figure S24. According to a previous TD-
DFT study by Woodford[45] in the parent flavylium salt V-Fla-0 (M0) carrying an unsubstituted
phenyl B-ring two Lewis structures V-Fla-0 (M1) and V-Fla-0 (M2) with the positive charge
located at C-2 or C-4 are favoured over Lewis structure V-Fla-0 (M0) with the positive charge
at O-1. Thus, the phenyl ring possesses a high degree of rotational freedom resulting in
decreased fluorescence quantum yields. In contrast, the presence of a para-alkoxy group
attached to the B-ring in V-Fla-1 stabilizes the positive charge at C-2 (or C-4) through
conjugation, resulting in Lewis structure V-Fla-1 (M3). This Lewis structure seems to be
preferred due to the extended π-system. Thus, conjugation of the B-ring leads to planarization
and rigidification resulting in strong fluorescence emission. Further evidence for the
rigidification can be found in the asymmetric peak shape of V-Fla-1, as a result of a similar
geometric of the ground and excited state.
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When a second alkoxy group is present in the B-ring as in V-Fla-2, the contribution of the
Lewis structure V-Fla-2 (M3) to the overall electronic structure is diminished as compared to
V-Fla-1 (M3) because the electron-donating +M-effect of the para-alkoxy group is partially
counterbalanced by the electron-withdrawing –I-effect of the meta-alkoxy group. This increases
the single bond character of the C2-C1’ bond, resulting in an increased rotational mobility and
thus decreased fluorescence quantum yield. This effect becomes even more pronounced in
V-Fla-3, which shows a fluorescence quantum yield below 1 %. These hypotheses were further
supported by preliminary DFT calculations (for details see Supp. Information) with simplified
flavylium salts (alkoxy groups were replaced by methoxy), which indicated an elongation of
the C2-C1’ bond by 6 pm for V-Fla-2 upon transition from the ground state to the excited state,
while the C2-C1’ bond lengths remained almost constant for V-Fla-0 and V-Fla-1.
Furthermore, the calculated oscillator strengths of V-Fla-1 was twice as large as compared to
V-Fla-0, V-Fla-2. A previous computational study by Quina showed that calculations on
hydroxylated flavylium salts are challenging.[46] Therefore our results should be treated with
care. Nevertheless, the above proposed model serves as a useful rationale for the experimental
results.
The lifetime of the excited state in solution was examined by TRSPC (time resolved single
photon counting). The mono-substitution on the B-ring, i.e. V-Fla-1, iV-Fla-1 and 2-Fla-1
resulted in monoexponential emission decays with lifetimes around 3.3 ns (Table S3,
Figure S20). For the unsymmetrical substitution with two alkoxy side chains on the B-ring, the
emission decay profiles could be fitted with two components with shorter lifetimes, revealing
that two emitting species are involved in the fluorescence process. A potential reason for the
two decay times might be the formation of hemiketals and chalcones, which is known for
flavylium salts.[20] The two decay times may also be caused by the presence of rotamers with
intact π-system.
In the solid state only compounds V-Fla-1, iV-Fla-1 and 2-Fla-1 showed enough fluorescence
intensity to obtain reliable data. The emission spectrum of V-Fla-1 was broad and ranged from
about 550 – 850 nm at room temperature with the main contribution at 645 nm (Figure 13,
Figure S21). Additionally, the spectrum of V-Fla-1 was the only spectrum which showed a
contribution at ~770 nm, close to the NIR regime. The compound iV-Fla-1 and 2-Fla-1 showed
similar emission spectra but with different contributions of the single bands. In the solid state
the emission spectra were red shifted compared to the emission in solution (λmax = 484 nm).
Temperature-dependent measurements were performed upon cooling the sample from the
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isotropic phase. With decreasing temperature, the emission intensity increased, and a red shift
was detected. For the compound V-Fla-1 the emission intensity is lowest in the isotropic state
and remains low in the SmA phase. Upon entering the LamCol the intensity increased with a
slow slope which further increased in the solid state. For iV-Fla-1 a similar behaviour has been
observed, with the difference that the intensity increased abruptly upon entering the LamCol
mesophase, in which the luminescence intensity remained almost constant over the whole
phase. In the solid state the intensity increased as expected. 2-Fla-1 showed low intensity in the
isotropic state, as well as in the smectic phases. Upon entering the LamCol phase the intensity
increased in the same manner as iV-Fla-1 and iV-Fla-1 in the crystalline state. Hence, emission
intensity is strongly dependent on the emitters supramolecular organization. While the liquid
phase and mesophases with pronounced fluidity, i.e. the SmA and SmA' phase, show only weak
luminescence, the higher ordered phases, i.e. LamCol and solid phase, show higher fluorescence.
Figure 13 a) Emission spectra of V-Fla-1 (black), iV-Fla-1 (red) and 2-Fla-1 (green) in the
solid state at 50 °C under irradiation with UV light (350 – 380 nm, solution spectra
of V-Fla-1 is given in dashed lines for comparison) and b) temperature dependent
emission intensity at 665 nm upon cooling the sample from the isotropic liquid
(cooling rate of 10 K min-1). For the orientation transition temperatures of the DSC
are given as arrows.
When optical micrographs of V-Fla-1 were examined under UV-light, different behaviours
were observed depending on the temperature. In the isotropic phase, the intensity of the emitted
light appears evenly distributed over the sample area (Figure 14) while, already in the SmA
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