Luminescence modulation in liquid crystalline phases containing a dispiro[fluorene-9,11′-indeno[1,2-b]fluorene-12′, 9′′-fluorene] core

Article abstract of DOI:10.1039/c4tc00038b

A luminescent liquid crystalline compound containing a bulky dispiro[fluorene-9,11′-indeno[1,2-b]fluorene-12′, 9′′-fluorene] has been designed and synthesized by di-substitution of a bromo derivative with N-(4-ethynylphenyl)-3,4,5-tris(hexadecyloxy)benzamide fragments. This di-substituted 3π-2spiro derivative forms stable and well-organized mesophases over large temperature ranges. Combination of DSC, POM and SAXS analyses has revealed the formation of a lamellar mesophase between 60 and 150 °C followed by another mesophase with a 2-dimensional lattice of rectangular symmetry that remains up to the isotropization point near 225 °C. In the original molecular packing model deduced from SAXS, the tert-butyl terminal groups fill the centre of hollow columns constituted by both the dihydro(1,2-b)indenofluorene and benzamide fragments and separated from each other by the surrounding aliphatic tails. The merging of the columns yielding the lamellar phase turned out to be governed by the dynamics of both, the micro-phase segregation process and the network of hydrogen bonds. In the various mesomorphic states and in solution, a strong luminescence was observed. The emission spectrum however depends on temperature and drastically changes between both mesophases and the isotropic liquid. In particular, a strong modulation of the emission wavelength occurs at the isotropic to 2D phase transition. This luminescence modulation results from an enhanced contribution of the vibronic peaks at higher energies in the emission profile. The compound was also found to be soluble in 5CB and was integrated in a guest-host LC cell, allowing efficient modulation of the photoluminescence polarization, in the presence or absence of an electrical field.

Full text of DOI:10.1039/c4tc00038b

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Materials Chemistry C
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PAPER
Luminescence modulation in liquid crystalline
phases containing a dispiro[fluorene-9,110-indeno
[1,2-b]fluorene-120,900-fluorene] core†
Cite this: J. Mater. Chem. C, 2014, 2,
4265
a
b
bc
a
*
*
Cyril Poriel
Sebastien Thiery, Benoıt Heinrich, Bertrand Donnio,
´
ˆ
a
*
and Franck Camerel
A
luminescent liquid crystalline compound containing a
bulky dispiro[fluorene-9,110-indeno[1,2-b]
fluorene-120,900-fluorene] has been designed and synthesized by di-substitution of a bromo derivative
with N-(4-ethynylphenyl)-3,4,5-tris(hexadecyloxy)benzamide fragments. This di-substituted 3p–2spiro
derivative forms stable and well-organized mesophases over large temperature ranges. Combination of
DSC, POM and SAXS analyses has revealed the formation of a lamellar mesophase between 60 and
150 ꢀC followed by another mesophase with a 2-dimensional lattice of rectangular symmetry that
remains up to the isotropization point near 225 ꢀC. In the original molecular packing model deduced
from SAXS, the tert-butyl terminal groups fill the centre of hollow columns constituted by both the
dihydro(1,2-b)indenofluorene and benzamide fragments and separated from each other by the
surrounding aliphatic tails. The merging of the columns yielding the lamellar phase turned out to be
governed by the dynamics of both, the micro-phase segregation process and the network of hydrogen
bonds. In the various mesomorphic states and in solution, a strong luminescence was observed. The
emission spectrum however depends on temperature and drastically changes between both
mesophases and the isotropic liquid. In particular, a strong modulation of the emission wavelength
occurs at the isotropic to 2D phase transition. This luminescence modulation results from an enhanced
contribution of the vibronic peaks at higher energies in the emission profile. The compound was also
found to be soluble in 5CB and was integrated in a guest–host LC cell, allowing efficient modulation of
the photoluminescence polarization, in the presence or absence of an electrical field.
Received 7th January 2014
Accepted 18th March 2014
DOI: 10.1039/c4tc00038b
www.rsc.org/MaterialsC
a pentaphenylenyl core with two uorenyl or xanthenyl units via
two spiro bridges allowed obtaining efficient emitters for blue
OLED applications.2,4,6,7 A further modication of the geometry
Introduction
Spiro compounds have been widely used in Organic Light
Emitting Diodes (OLED) since their rst incorporation in 1997
by the Salbeck group.1 Of particular interest in this wide family
of chromophores are the “3p–2spiro” compounds which are
constituted of three p-systems linked by two spiro bridges.2–4
This type of 3p–2spiro molecular structure can lead to uo-
rophores possessing very different properties depending on the
geometry of the molecular scaffold or p-systems.5 As an
example, the association of a dihydroindeno[1,2-b] uorenyl or
of the molecular scaffold has recently opened a new avenue to
produce blue light in OLEDs through the use of intramolecular
excimer emission.8 Although, the incorporation of these mole-
cules in a liquid-crystalline architecture could be a way to
modulate their exceptional luminescence properties, no meso-
morphic materials incorporating the dispiro[uorene-9,110-
indeno[1,2-b]uorene-120,900-uorene] (DSF-IF) segments were
reported until now, to the best of our knowledge.
Yet, the mesomorphism would not only lead to the modu-
lation of the photoluminescent properties, but also allow
switching between several states in response to various external
stimuli, which would be of great interest due to their potential
application for memory devices, sensors and informational
displays.9 Indeed, electric-eld controlled photoluminescence
switching has been achieved in many systems by adding dyes
into liquid crystalline hosts,10 evidencing the potential interest
of neat photoluminescent liquid crystalline materials.11
a
´
Institut des Sciences Chimiques de Rennes, UMR 6226 (CNRS-Universite de Rennes 1),
Campus de Beaulieu, 35042 Rennes, France. E-mail: cyril.poriel@univ-rennes1.fr;
franck.camerel@univ-rennes1.fr
b
´
Institut de Physique et Chimie des Materiaux de Strasbourg (IPCMS), UMR 7504
´
(CNRS-Universite de Strasbourg), 23 rue du Loess, BP 43, 67034 Strasbourg Cedex
2, France
cComplex Assemblies of So Matter Laboratory (COMPASS), UMI 3254 (CNRS-Rhodia/
Solvay-University of Pennsylvania), CRTB, 350 George Patterson Boulevard, Bristol, PA
19007, USA. E-mail: bdonnio@ipcms.unistra.fr
Thus, the present work investigates this concept and
demonstrates that the DSF-IF scaffold can be efficiently
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4tc00038b
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Fig. 1 Chemical structure of the liquid crystalline compound 1 based on a dispiro[fluorene-9,110-indeno[1,2-b]fluorene-120,900-fluorene]
scaffold.
functionalized to generate mesophases with strong lumines- strongly stabilize supramolecular organisations over larger
cence properties. In order to reach this goal, a protomesogenic temperature range.12
fragment, which has been already successfully used with pyrene
The molecular structure and purity of 1 were conrmed
1
derivatives to promote mesogens and molecular gelators,12 was using H, 13C NMR and mass spectroscopy. A set of aromatic
graed on a DSF-IF derivative (Fig. 1). Hydrogen bonding peaks integrating for even numbers of protons conrmed the
amides allow then the emergence of molecular self-assemblies formation of the expected molecular structure and the
stable over large temperature ranges.
symmetrical substitution pattern on the 2,8 positions of the
In addition to the original temperature-dependent bulk dihydroindeno[1,2-b]uorenyl scaffold.
luminescence properties, the potential application of such
anisotropic materials in guest–host LC systems was also evalu-
ated. For this purpose, the material was dissolved in the stan-
dard nematic liquid crystal matrix 5CB (4-cyano-40-n-pentyl-1,10-
biphenyl) and we have demonstrated that marked photo-
luminescence polarization and modulation responses can be
obtained from the DSF-IF derivative in LC cells.
Spectroscopic characterizations in solution
The UV-Vis spectrum of 1 presents two main bands at 379 and
399 nm leading to a wide optical energy gap of ca. 2.98 eV
(Fig. 2). The maximum appears hence to be strongly bath-
ochromically shied by ca. 52 nm compared to that of its
constituting building block DSF-IF(t-Bu)4, leading to an
impressive gap contraction (3.49 eV vs. 2.98 eV).14 This red shi
is assigned to the extension of the p-conjugation induced by the
presence of the N-(4-ethynylphenyl) fragment in the para posi-
tion of the phenyl–phenyl linkage of the dihydroindeno[1,2-b]
Results and discussion
Synthesis
The synthesis of 1 (Scheme 1) starts with the preparation of the uorenyl core. Thus, the optical properties of 1 are almost fully
key precursor, namely 2,200,7,700-tetra-tert-butyldispiro[uorene- governed by the “N-(4-ethynylphenyl)/dihydroindeno[1,2-b]u-
9,60-indeno[1,2-b]uorene-120,900-uorene]
(DSF-IF(t-Bu)4), orene/N-(4-ethynylphenyl)” moiety, the electronic density of
constituted of a dihydroindeno[1,2-b]uorenyl core linked via both Highest Occupied and Lowest Unoccupied Molecular
spiro bridges to two 2,7-tert-butyl uorenyl moieties. As previ- Orbitals (HOMO and LUMO respectively) being distributed on
ously reported, electrophilic bromination of DSF-IF(t-Bu)4 leads this fragment (Fig. S1†). This extension of the p-conjugation
to the dibromination of the dihydroindeno[1,2-b]uorenyl core appears to be relatively efficient through the rigid triple bond
providing DSF-IF(t-Bu)4Br2 in high yield (90%).2,13 The lipophilic linkage. For example, compared to another structurally related
alkyne moiety (EPBA) was synthesized in two steps from 3,4,5- extended DSF-IF derivative substituted on the dihydroindeno
tris(hexadecyloxy)benzoic acid chloride and 4-trimethylsilyle- [1,2-b]-indenouorenyl core by two 3,4,5-trimethoxyphenyl
thynylaniline.12 Then, compound 1 was obtained in good yield moieties (lmax ¼ 362 nm, DE ¼ 3.17 eV),8 the present chromo-
(46%) via a palladium(0)-catalyzed cross-coupling reaction phore 1 appears to possess a smaller energy gap due to its
between DSF-IF(t-Bu)4Br2 and N-(4-ethynylphenyl)-3,4,5-tris(h- extended p-conjugation (Fig. S1†). To the best of our knowl-
exadecyloxy)benzamide in n-propylamine as a base and solvent. edge, this is one of the most contracted energy gap observed in
The EPBA fragment, carrying an amide function, was selected the literature for a DSF-IF derivative.2
preferentially to a half-disk moiety as the terminal group in
Additionally, 1 presents an intense blue uorescence with a
order to introduce hydrogen bonding which is known to well resolved spectrum presenting several maxima at 411, 435
Scheme 1 Synthetic route for the preparation of 1.
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compound is in its isotropic state. On cooling from the isotropic
liquid, large birefringent domains readily develop over the
whole lm (Fig. 3), whilst the so texture is indicative of a
mesomorphic state (referred thereaer to high-temperature,
HT, phase, vide infra) (Fig. S4 and S5†). Despite numerous
efforts, no characteristic textures could have been developed;
although, the texture is uid and homogeneous, suggesting the
formation of a LC phase. Successive transitions crossed on
cooling down to room temperature were not associated with
visible changes in the optical texture, from which no further
information on the structure of the phases could be deduced
(Fig. S5†). Nevertheless, the quite high uidity of the sample
ꢀ
between 60 and 150 C indicates that the intermediate phase
consists of a “low-temperature” (LT) mesophase, whilst the
texture stiffens below the low-temperature transition and leaves
the nature of the room-temperature state unclear.
Fig. 2 Absorption, emission (lex ¼ 358 nm) and excitation (lem
¼
435 nm) spectra of compound 1 in dichloromethane (c ¼ 8.7 ꢂ
10ꢁ7 mol Lꢁ1).
Characterization by small-angle X-ray scattering (SAXS)
The structures of the various phases were investigated, and
and 467 nm (Fig. 2). These maxima are strongly red shied identied by SAXS measurements.
(59 nm) compared to those of its constituting building block
The pristine (initial) state possesses a local “embryonic”
DSF-IF(tBu)4 (l ¼ 352 and 370 nm),14 highlighting again the lamellar organization evidenced by the presence of 6 broadened
interest of the p-conjugation extension to obtain a blue emitter. small-angle reections in the ratio 1 : 3 : 4 : 5 : 6 : 10 (00l) and
The well-resolved emission spectrum of 1 compared to its unresolved broad scattering signals in the wide-angle region
absorption spectrum suggests a more rigid/planar structure of 1
in the excited state. Indeed, for species that comprise torsional
degrees of freedom in their p-conjugated backbones, varying
degrees of deviation from the classical mirror image behaviour
can be observed. Such differences between absorption and
emission band-shapes may be associated with the exible
nature of the molecules (and in the present case especially the
exibility of the chains).15 However, the Stokes shi dened as
labsꢁlem of 1 appeared to be small (12 nm), highlighting small
rearrangements in the excited state. Thus, despite the presence
of the exible pendant N-(4-ethynylphenyl)-3,4,5-tris(hex-
adecyloxy)benzamide, which may lead to important rotational
freedom, we note that the absorption spectrum and more
importantly the uorescence spectrum are well resolved, which
is indicative of a very rigid molecular structure. Finally, the
quantum yield of 1 has been measured at ca 0.32 compared to
quinine sulphate (Fig. S2 and Table S1†). This value appears to
be signicantly decreased compared to that observed for DSF-
IF(tBu)4 (0.52 in CH2Cl2) and may be assigned to the introduc-
tion of the exible N-(4-ethynylphenyl)-3,4,5-tris(hexadecyloxy)
benzamide which may lead to a non-radiative de-activation
process as oen observed.16 The perfect match between the
excitation spectrum and the absorption spectrum points to an
efficient radiative deactivation of the excited electronic state.
Thermal behavior
Thermal and polymorphic behavior of 1 was investigated by
Fig. 3 Compound 1 viewed by optical microscopy under crossed-
differential scanning calorimetry (DSC) and polarized optical
polarisers upon cooling (symbolised by the cross in the corner of the
microscopy (POM). DSC analyses show that the compound
exhibits three reversible main rst order thermal transitions
picture) at (a) 195 ꢀC (co-existence of the isotropic liquid and HT
phase) and (b) HT phase at 180 ꢀC (black spots correspond to air
centred at 60, 150 and 225 ꢀC (Fig. S3†). Above 225 ꢀC, the trapped bubbles).
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(Fig. S6, Table S2†). Aer heating the sample in the isotropic structure, but the data collected do not permit the conrmation
liquid, this poorly-organized state is restored on cooling down of such an assumption. The structure of the lamellar packing is
to room-temperature (diffractogram not shown).
characterized by the molecular area S, ratio of the molecular
Above the rst transition, the broadened reections of the volume and layer spacing, S ¼ Vmol/d. The estimation, that a
room-temperature state are replaced by nine sharp reections density at 80 ꢀC lies approximately in the range 0.9–1.0 g cmꢁ3
,
3
˚
in the spacing ratio 1 : 2 : 3 : 4 : 5 : 6 : 7 : 8 : 10 (00l), according leads to a volume comprised between 4600 and 5150 A , and to
2
˚
˚
to a lamellar periodicity of 54.0 A (see Fig. 4a and Table S3†). area S comprised between 80 and 95 A , i.e. slightly more than
2
˚
The wide-angle region only contains overlapping broad scat- the cross-sections of three molten terminal chains (ꢃ68 A ): the
tering signals, due to distances between segregated molecular diverging chains are therefore disordered and folded in order to
fragments, particularly the characteristic scattering maximum compensate for the large molecular area imposed by the bulky
˚
at about 4.5 A of molten aliphatic chains (hch) that conrms the rigid parts.
assignment to a mesophase. The dominating (001) reection
Finally, in the upper temperature range above 150 ꢀC, a
trivially results from the alternation of sub-layers of mesogens complex diffraction pattern is produced, which remains
and molten chains, but here the number of harmonics is unchanged up to the isotropization (ca. 225 ꢀC). Now, 21 sharp
unusually high, and therefore the interfaces between sub-layers reections (Fig. 4b) could be precisely measured and satisfac-
are unusually sharp. With this high degree of cohesion of the torily indexed (Table S4†) according to a centred rectangular
mesogens' sub-layers, it would not be surprising that a 3D lattice, and thus the mesophase is assigned as Colrec-c2mm.
structure persists in spite of the intercalation of the liquid Consequently, this transition corresponds to a lamellar-to-
aliphatic sub-layers. The unresolved shoulders in the mid- and columnar phase transformation.
wide-angle region might be the residue of such a vanishing
The high number of crossed (hk) reections shows that the
structure is perfectly dened along several directions of the
lattice plane, and the modulation of their intensity suggests an
additional segregation process likely between the lateral tert-
butyl groups and the terminal chains (i.e. the lattice contains
more than one peak of electronic density suggesting alternation
of high- and low-electronic density fragments, vide supra). Four
(hk) reections are visible along two main axes of the columnar
rectangular network, namely (20), S, (40), MW, (60), S, (80), M,
and (11), VS, (22), S, (33), M, (44), W (Table S4†) (VS, S, M, MW,
W, VW stand for very strong, strong, medium, medium-weak,
weak, very weak and correspond to the reection intensities).
The latter series, (hh), dominates largely over the former one,
(h0), an indication of a strong anisotropy within the lattice
plane. Moreover, the intensity of the (hh) reections decreases
regularly with h, in contrast to the (h0) series, for which the
intensity goes through a minimum for the rst harmonics. A
second intensity minimum is found in the perpendicular
direction to the (h0) series, (0 2k) (e.g. (02), S, (04), W, (06),
absent, (08), M, Table S4†), associated with an increase of the
electronic density between the nodes of the network. Finally, in
the other directions of the lattice plane, the rst, high order
reection (31) is absent, and two higher order reections, (13)
and (51), are either absent or too weak, respectively, but the
third one is intense ((42), S, (84), S), conrming the strong
anisotropy in the plane, and the modulation of the electronic
density. This detailed analysis shows that the variation of the
intensity of the various reection sets is compatible with a
model of hollow columns (vide infra).
Supramolecular organization in the mesophases
A model for the mesophase structure consisting of the assembly
of “hollow” columns is a priori not unexpected from the similar
electronic density of tert-butyl groups and terminal chains on
the one hand, and from the contrast with the high electronic
densities of rigid fragments, on the other hand. The presence of
aliphatic moieties at all mesogen ends could then lead to such a
Fig. 4 X-ray diffractograms of compound 1 recorded at 80 ꢀC (LT or
lamellar phase) and at 160 ꢀC (HT or rectangular phase). In (b) the
superposed diffractogram was recorded on a synchrotron line (red),
showing the good agreement between both techniques.
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type of stratied structure. A more detailed inspection of the
packing and supramolecular organization needs additional
information about the molecular conformation and geometry
that could be extracted from a reference crystalline structure
(structure from the Cambridge Structural Database, CSD
number: XIVHEQ). The model compound bears the same tert-
butyl-uorene (TBF) and dihydro(1,2-b)indenouorene (InF)
fragments as compound 1, but without the terminal N-(ethy-
nylphenyl) benzamide (EPBA) fragments and the terminal
alkoxy chains. This molecular structure directly gives access to
both TBF and InF dimensions and conrms the “double cross-
like” molecular conformation.
The crystalline structure resulting from such a special
geometry is very informative and consists of the superposition
of the InF fragments associated in dimers, decorated by the TBF
orientated along the self-assembled columns, that drives the
segmentation of the columns by the tert-butyl groups, them-
selves segregated into layers. Similar processes of segregation as
those occurring within the crystalline structure may explain the
formation of the mesophase with hollow columns. In both cases
(i.e. Colrec mesophase and crystalline structure), the formation
of the columns results from the piling of the InF segments,
though in the mesophase, the columns are oriented parallel to
the b-direction, in order to allow the layers containing the tert-
butyl groups to be oriented along the a-axis, and to be evenly
disrupted by the EPBA groups at the extremities of the InF
fragments. Half of the tert-butyl-containing layers are thus
segregated into ribbons, and surrounded by a continuous shell
of rigid parts. The long aliphatic chains, according to the
segregation process found in polycatenar systems,17 are folded
around the columns, and form a continuum, which here is also
fused with the other half of tert-butyls expelled out of the
columns (Fig. 5).
Steric constraints associated with the molecular geometry
and the multiple segregation process imply that the columnar
cross-sections are dened by a precise integer number of
molecules, in contrast to average statistical numbers in classical
polycatenar systems. This specicity permits us to understand
the well-dened nature of the structure and the sharp interfaces
between rigid parts and chains, at the origin of the large
number of high order terms and crossed reections in the dif-
Fig. 5 Model of the hollow columns and columnar rectangular phase
formed by 1.
fractogram. This number is equal to Z/2 ¼ 4 molecules per from that of the rectangular phase, except that the columns are
columnar repeat units. Estimating the molecular volume (Vmol not closed in the lamellar phase. The modulations in the
3
ꢀ
˚
¼ 4845 A at 160 C), one can now access the thickness of an lamellar planes due to the alternation of the uorene segments
elementary ctitious repeat unit according to h ¼ Z Vmol/ab ¼ and the tert-butyl groups likely pre-exist in the lamellar phase,
˚
ꢃ7.2 A, not too different from the distance between the TBF but are not correlated over long distances, in agreement with
˚
segments (d1 ꢃ 7.65 A) found in the crystalline structure of the the slight peak enlargement observed in the X-ray pattern.
model compound (even comparable, since the long aliphatic
The dynamic of the hydrogen bonding network was probed
chains are not involved here). Organization along the columns by variable-temperature FT-IR absorption spectra for compound
can be assimilated to the alternation of monolayers of InF 1 dispersed into KBr pellets. Measurements obtained upon
segments and of monolayers of intercalated TBF segments.
heating and cooling (Fig. 6 and Fig. S7†) show reproducible and
Superposition of the Colrec and the lamellar phase diffrac- reversible spectral changes of the bands associated with the
tion patterns reveals that the reection series (h0) (h ¼ 2n) amide functions at the phase transitions. A weakening of the
coincide with the reection series (00l), except that the former out-of-plane NH bending band (amide V) localized at 720 cmꢁ1
are more intense, the intensity ratio are different and that all (ref. 18) is observed between the poorly organized room
crossed reections have disappeared. Nevertheless, it shows temperature phase and the lamellar phase and this vibration
that the structure of the lamellar phase is not greatly different mode, associated with hydrogen bonded amide functions,
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maximum of efficiency occurs when the folding occurs towards
a same layer containing the tert-butyl groups, starting in a
random direction rst, thus expelling the other half TB layers
outside the columns to be mixed with the aliphatic continuum.
Then, as in a germ growth crystallization process, the next layer
is automatically selected as a row of column, and is continued
until it reaches another growth domain.
Solid state luminescence properties
Luminescence properties of compound 1 in the various material
states detected by DSC and SAXS were investigated with the help
of a uorescence microscope equipped with a heating stage and
a UV-Vis-NIR spectrophotometer based on CCD detection
technology. Fluorescence microscopy was found to be a
powerful tool to probe the molecular organization in thin lms
at the macroscopic scale.12,20
At temperatures above 225 ꢀC, in the isotropic liquid, the
luminescence appears light blue and completely uniform inside
the uid material in which all the molecules are randomly
distributed. It is interesting to note that the luminescence
remains very strong even at such high temperatures. The uo-
Fig. 6 Solid-state IR absorption spectra of compound 1 (2% in KBr
pellet) at the following temperatures (2nd heating): 20, 30, 40, 50 and
60 ꢀC (room temperature state, black lines), 70, 80, 90, 100, 110, 120,
130, 140 and 150 ꢀC (lamellar mesophase, red lines), 160, 170, 180 and
190 ꢀC (rectangular mesophase, blue lines). Graph up, left: superpo-
sition of spectra; up, right: the amide V region with vibration modes of
N–H/O bridges; down, left: the amide I region with C]O stretching
vibration; down, right: the N–H stretching region.
ꢀ
rescence spectra obtained above 225 C display a broad struc-
tureless emission band centred at 492 nm (Fig. 8). In the bulk,
the emission spectra are, as expected, broader and much less
resolved than in solution (Fig. 2) and the maxima are red-shied
by 82 nm. The difference should come from some remaining
intermolecular interactions in the isotropic state, whilst the
interactions with the solvent dominate in the solutions. Upon
cooling, the birefringent domains of the rectangular mesophase
readily develop, as observed by POM (Fig. 3a). Interestingly,
under irradiation at 340 < lex < 380 nm (Hg lamp), the lumi-
nescence coming from the rectangular mesophase appears
dark-blue and is clearly different from that one of the isotropic
phase (Fig. 7). Evidently, 1 is a thermochromic material, which
exhibits temperature induced luminescence changes.21 Spec-
troscopic measurements show that the colour change is due to
an enhanced contribution of the two vibronic peaks at 443 and
completely disappear in the rectangular phase. The collapse of
the hydrogen bonding network in the rectangular phase is
further conrmed by the frequency and prole changes of the
C]O stretching vibration (amide I) above 150 ꢀC. In the
lamellar phase, the C]O stretching vibration appears as a
single band at 1645 cmꢁ1 and is characteristic of H-bonded
amides.19 Above 150 ꢀC, the shi of this vibration mode to
higher frequencies is indicative of the disruption of the
hydrogen bonds. A similar behaviour is found for the nN–H
stretching vibrations around 3300 cmꢁ1
.
The major pertur-
18,19
bations of the energy and intensity of the nCO, nNH and dNH
stretching and deformation vibrations observed highlights that
the hydrogen bonding network present in the lamellar phase is
disrupted above 150 ꢀC when entering into the rectangular
phase.
The phase transitions appear to be strongly governed by the
dynamic of the hydrogen bonding network. The presence of
persisting hydrogen bonds in the lamellar phase forbids the
folding of the chains at the EPBA extremity, and the transition
to the Colrec phase is connected to the collapse of the H-bonding
network. Similarly, at low temperature, this H-bonded edice
would also perturb the lamellar supramolecular arrangement.
The mechanism that leads to the interruption of half of the
tert-butyl groups is a consequence of the difference between the
sections of InF bearing the TBF moieties, on the one hand, and
the EPBA fragments, on the other hand. The folding of the
mobile extremities of the EPBA fragments permits the lling of
Fig. 7 Co-existing isotropic liquid (light-blue) and HT (dark-blue)
phases at 195 ꢀC observed upon irradiation at 340 < lex < 380 nm
(black areas correspond to trapped air bubbles) (same area as in
the available volume whilst enhancing the segregation, and the Fig. 3a).
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terminal N-(ethynylphenyl) benzamide (EPBA) fragments, the
maxima in the solid state (443, 468 and 492 nm) are shied by
more than 30 nm compared to those in solution (411, 435 and
467 nm). This large red-shi clearly indicates that the EPBA
pendant units allow a more efficient p–p stacking in the solid
state.
To sum up, the luminescence properties of 1 are sensitive to
the organisation inside the liquid crystalline state and the
emission intensity and colour can be modulated with the
temperature.
Modulation and polarization of the photoluminescence
The strong molecular anisotropy of the rod-shaped compound 1
(length ¼ 7.7 nm; diameter ¼ 1.3 nm) led us to explore the
possibility to modulate and to polarize the photoluminescence
intensity using a nematic liquid crystalline host. For this
purpose, compound 1 was dissolved in the nematic liquid
crystal 5CB. It was found that 1 is well soluble in 5CB and the
nematic phase of the LC host and the TN/I transition tempera-
ture (Fig. S10†) are weakly affected up to 1 weight%. Observa-
tions by polarizing microscopy and under UV irradiation
conrmed the homogeneity of the LC mixtures and stability
over several months. In contrast, the parent compound DSF-
IF(t-Bu)4, without the exible pendant units, is not soluble in
5CB. Even the evaporation of dichloromethane solutions con-
taining DSF-IF(t-Bu)4/5CB mixtures also led to the formation of
heterogenous suspensions. A crucial role of the exible pendant
units is to ensure good solubility and stability in 5CB.
The LC mixture was introduced by capillarity in a LC cell with
a 5 mm gap between the ITO coated glass (1 cm ꢂ 1 cm). The
polyimide (PI) layers on both ITO surfaces were rubbed in one-
direction in an antiparallel alignment. The cell was then
observed by optical microscopy without using a polarizer
(Fig. 9a). A voltage of 20 V was requested to switch the orien-
tation of 5CB molecules in the direction perpendicular to the
windows.
Fig. 8 Bulk emission spectra of the compound 1 in a slender piece of
glass at 240 ꢀC in the isotropic liquid, at 182 ꢀC in the rectangular
mesophase, at 80 ꢀC in the lamellar phase and at 25 ꢀC in the poorly-
organized room-temperature phase under excitation at 340–380 nm.
468 nm (Fig. 8). Temperature-dependent solid state absorption
spectroscopy reveals that the absorption band remains
unchanged between the isotropic phase and the rectangular
phase (Fig. S8†). So, the luminescence modulation observed is
not due to a slight bathochromic shi or enlargement of the
absorption band in the isotropic phase causing a re-absorption
of the uorescence at the higher energy side. This modulation
likely explained by an exaltation of these two high-energy
vibronic peaks from the emission prole is likely due to the
organisation of the molecules into an anisotropic medium
leading to a specic orientation of the transition dipole
moments with the excitation beam.22 Upon further cooling
down to room temperature, the intensity of the luminescence at
the higher energy side continuously increases down to room
temperature, but without signicant change of the uniform
dark-blue uorescence texture between both mesophases.
The unsubstituted compound DSF-IF(t-Bu)4 shows an emis-
sion band displaying two maxima at 358 and 368 nm in the
solid state at room temperature (Fig. S9†), shied by 10 nm
compared to the solution spectrum, indicative of weak inter-
actions in the solid state. With compound 1 bearing the
As the voltage is OFF, the molecules of 1 are parallel to the
cell windows and a strong blue luminescence is observed
(Fig. 9b, le). The 1/5CB mixture displays a strong structured
emission band, spanning from 400 to 650 nm, centred at
442 nm (Fig. 10a). The position of the emission band, which is
Fig. 9 (a) Experimental setup for polarized photoluminescence measurements with compound 1/5CB mixtures; (b) schematic representation of
the molecular organisation inside the LC cell without and with applied electric field and effect on the emitted blue light.
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eld-off state while it goes to minimum in the eld-on state.
This electrically switchable and repeatable photoluminescence
modulation observed shows that the luminescent compound 1
can be efficiently used in optical switches.
The ability of the aligned 1/5CB mixture to generate polar-
ized emission was also evaluated. For this purpose, an analyser
was inserted between the CCD spectrometer and the cube lter.
Without bias, the long molecular axis of the molecule is
oriented along the rubbed PI direction with a planar anchoring
parallel to the surface (Fig. 9b, 0 V). The photoluminescence
intensity was evaluated as a function of the angle between the
analyzer and the rubbed direction. At 0ꢀ, the analyzer's axis is
parallel to the long molecular axis of 1 and perpendicular at 90ꢀ.
As can be seen in Fig. 11, the aligned 1/5CB sample shows a
strong angular dependence of photoluminescence intensity
coming from 1. These measurements clearly show that the
intensity of the photoluminescence spectra is much larger in
the parallel conguration than in the perpendicular one. The
degree of polarization can be calculated by r ¼ (I// ꢁ It)/(I// +
It), where I// and It represent the polarization parallel (0ꢀ)
and perpendicular (90ꢀ) to the rubbed PI direction, respectively.
The resulting polarization ratio of 0.3 is comparable with the
values determined for polycatenar oligothiophenes.24
Fig. 10 (a) Emission spectra measured in field-off (0 V) and field-on
(20 V); (b) time-dependent switching of PL intensity in response to
several cycles of field-off (0 V) and field-on (20 V) (lex ¼ 340–380 nm)
([1] ¼ 0.1% w/w in 5CB).
slightly red-shied by 32 nm compared to the emission band in
diluted solutions (Fig. 2), together with a well-resolved emission
prole conrms the good dispersion of the luminescent DSF-IF
derivative in the LC host. It should be noted that with the short
acquisition times used (100–500 ms), the luminescence of 5CB
was not detected at all. As the voltage switches ‘ON’, the reor-
ientation of the 5CB molecules stimulates a macroscopic
orientation of the molecules of 1 with their long axes perpen-
dicular to the cell window, as shown in Fig. 8b, right. As a result
of this reorientation, the observed PL intensity of 1 is reduced by
ꢃ40% (Fig. 10a). This remarkable uorescence modulation
appears to be much more marked than that observed for
example with europium(III) complexes23 and of the same order
of magnitude as the one observed with inorganic CdSe/ZnS
nanorods.10d These results demonstrate that, at the molecular
scale, the excitation is more efficient when the electromagnetic
wave is perpendicular to the long axis of the dihydro(1,2-b)
indenouorene core. The PL intensity instantaneously (<1 s)
increases again when the voltage switches ‘OFF’. The reversible
switching of intensity in response to several cycles of eld-off
(0 V) and eld-on (20 V) is presented in Fig. 10b. The electric
eld was switched manually with an arbitrary period. It can be
Fig. 11 (a) Photoluminescence intensity as a function of the analyzer
angle. (b) Polar graph presentation. The excitation wavelength is 340–
seen that the PL intensity quickly reaches maximum during the 380 nm and the registration wavelength is 442 nm.
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Journal of Materials Chemistry C
These results illustrate for the rst time that aligned samples further purication. The optical gap was calculated from the
of DSF-IF derivatives can emit polarized light and that modu- absorption edge of the UV-vis absorption spectrum using the
lation of the luminescence can be induced by application of formula DEopt (eV) ¼ hc/l, l being the absorption edge (in
electric elds. The large structural anisotropy of 1 coupled to its meter). With h ¼ 6.6 ꢂ 10ꢁ34 J s (1 eV ¼ 1.6 ꢂ 10ꢁ19 J) and c ¼
good solubility in liquid crystalline medium makes this class of 3.0 ꢂ 108 m sꢁ1, this equation may be simplied as: DEopt (eV) ¼
compounds suitable for optical applications in LC devices.
1237.5/l (in nm). Photoluminescence spectra were recorded
with a PTI spectrouorimeter (PTI-814 PDS, MD 5020, LPS 220B)
using a xenon lamp. Quantum yields in solution (fsol) were
calculated relative to quinine sulfate (fref ¼ 0.546 in H2SO4 1 N).
Conclusion
The graing of N-(ethynylphenyl) benzamide (EPBA) fragments fsol was determined according to the following equation,
carrying hexadecyl carbon chains on the bulky DSF-IF core has
allowed the emergence of strongly luminescent liquid crystal-
line phases. Three different supramolecular organizations were
revealed, starting namely with an initial poorly-organized
phase, followed by a rst transformation into a lamellar phase,
and a second transition into a columnar mesophase. Based on
variable-temperature SAXS measurements and on the crystal-
line structure of a parent molecule, models of the various
fsol ¼ fref ꢂ 100 ꢂ [(Ts ꢂ Ar)/(Tr ꢂ As)] ꢂ [(ns/nr)2]
where, subscripts s and r refer respectively to the sample and
reference. The integrated area of the emission peak in arbitrary
units is given as T, n is the refracting index of the solvent (ns ¼
1.421 for dichloromethane) and A is the absorbance. 3 solutions
of different concentrations of the substrate (A < 0.1) and 3
solutions of the reference (quinine sulfate) were prepared. The
molecular organizations were proposed. In the columnar phase,
quinine sulfate concentration was chosen so as the absorption
two supramolecular dimers, created through the piling of
of the reference and the substrate was the same at the excitation
wavelength. 3 quantum yields were then calculated at this
wavelength and the average value was reported.
dihydro(1,2-b)indenouorene fragments, are organized in a
disk-like repeat unit with half of the tert-butyl groups trapped in
the centre and the rest of the tert-butyl fragments mixed with
Variable-temperature FT-IR absorption spectra were per-
the long aliphatic chains arranged around the columns. The
formed on KBr pellets, by using a spectrometer Digilab FTS3000
piling of these self-assembled units leads to the formation of
and a home-built oven with an accuracy of 0.1 ꢀC. UV-visible
spectra were recorded on mm-thick lms between quartz glass
slides, by using a spectrometer Agilent Cary 100 and a home-
ꢀ
built oven with an accuracy of 0.1 C.
“hollow” columns self-organized in a rectangular lattice. The
transition to the lamellar phase at low temperature is explained
by the opening of the columns through the unfolding of the
EPBA extremities. The thermal transitions are driven by steric
Optical microscopy investigations were performed on a
constraints and by the dynamic of the hydrogen bonding
Nikon H600L polarising microscope equipped with a Linkam
networks. Temperature-dependent solid-state luminescence
“liquid crystal pro system” hotstage. The microscope is also
equipped with a UV irradiation source (Hg Lamp, l ¼ 340–380
measurements have also revealed that this compound displays
thermochromic luminescence properties with a marked pho-
nm) and an ocean optic USB 2000+ UV-Vis-NIR spectropho-
toluminescence colour change at the isotropic liquid-to-Colrec
tometer based on CCD detection technology. This set-up allows
phase transition. The functionalized DSF-IF core could also be
the recording of luminescence spectra on solids, liquids, liquid
dissolved in the nematic liquid crystal 5CB and switching of the
photoluminescence emission could be realized in aligned
samples or by applying electrical elds to a LC cell. Such highly
functional materials are very appealing for applications in
optoelectronics, requiring the generation or the control of light
under electrical stimuli. DSF-IF derivatives with different
molecular topologies are currently under investigation.
crystalline materials and gels from ꢁ196 ꢀC up to 420 ꢀC
between 350 and 1100 nm. For electro-optical measurements,
compound 1 was dissolved in the 4-cyano-40-n-pentyl-1,10-
biphenyl (5CB) nematic liquid crystal and the LC mixture was
introduced into a pre-assembled LC cell provided by Linkam (5
mm gap, capillary ll, ITO coated and anti parallel alignment).
5CB (>98%) was purchased from TCI Europe and used without
further purication. The voltage was applied with a standard
potentiostat. Differential scanning calorimetry (DSC) was
Experimental part
General
carried out by using a NETZSCH DSC 200 F3 instrument
equipped with an intracooler. DSC traces were measured at
10 ꢀC minꢁ1 down to ꢁ30 ꢀC. The XRD patterns were obtained
with a transmission Debye–Scherrer-like geometry. A linear
300 (1H) and 75.5 MHz (13C) NMR spectra were recorded on a
Bruker Avance 300 spectrometer at room temperature using
perdeuterated solvents as internal standards. FT-IR spectra
were recorded using a Varian-640 FT-IR spectrometer equipped
with a PIKE ATR apparatus. High Resolution Mass Spectra were
recorded with Varian MAT 311 instrument by the Centre
˚
focalized monochromatic Cu-Ka1 beam (l ¼ 1.5405 A) was
obtained using a sealed-tube generator (600 W) equipped with a
bent quartz monochromator. In all cases, the crude powder was
lled in Lindemann capillaries of 1 mm diameter and 10 mm
wall-thickness. The diffraction patterns were recorded with a
curved Inel CPS120 counter gas-lled detector linked to a data
´
Regional de Mesures Physiques de l'Ouest, Rennes. UV-visible
spectra were recorded using an UV-Visible spectrophotometer
SHIMADZU UV-1605. The UV-visible spectra were recorded
using an UV-Visible spectrophotometer SHIMADZU UV-1605.
Dichloromethane (analytical grade, VWR) was used without
˚
acquisition computer (periodicities up to 70 A) and on image
plates scanned by STORM 820 from Molecular Dynamics with
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˚
50 mm resolution (periodicities up to 120 A). The sample ST (ANR-11-BS07-020-01). CP and ST thank the CINES for
temperature was controlled within ꢄ0.01 ꢀC and exposure times computing time. Nicolas Beyer is warmly acknowledged for
were varied from 1 to 24 h.
his technical assistance in the temperature-dependent
Full geometry optimization was performed with Density measurements.
Functional Theory (DFT)25,26 and Time-Dependent Density
Functional Theory (TD-DFT) calculations were performed with
the hybrid Becke-3 parameter exchange27–29 functional and the
References
Lee-Yang-Parr non-local correlation functional30 (B3LYP)
implemented in the Gaussian 09 (Revision B.01) program
suite31 using the 6-311G+(d,p) basis set and the default
convergence criterion implemented in the program. The gures
were generated with GaussView 5.0.
20,80-Dibromo-2,200,7,700-tetrakis(1,1-dimethylethyl)-dispiro-
[9H-uorene-9,60(120H)-indeno[1,2-b]uorene-120,900-[9H]uo-
rene] (DSF-IF(t-Bu)4Br2) was synthesized as previously
described. The synthesis and the characterization of the
precursor DSF-IF(t-Bu)4 can be found in previous studies.14
1 (a) J. Salbeck, N. Yu, J. Bauer, F. Weissortel and H. Bestgen,
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¨
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Compound 1
A Schlenk ask was charged with DSF-IF(t-Bu)4Br2 (0.023 g, 0.024
mmol) and 2 equiv. N-(4-ethynylphenyl)-3,4,5-tris(hexadecyloxy)
benzamide (0.057 g, 0.060 mmol) and n-propylamine (10 mL).
The mixture was degassed with argon. Pd(PPh3)4 (10 mg) was
then added and the mixture was stirred at 60 ꢀC overnight. The
solvent was evaporated in vacuo, water was added to the residue
and the product was extracted with dichloromethane. The
organic layer was washed with water and ltered over hygroscopic
cotton wool and evaporated. The residue was puried by column
chromatography on silica gel using CH2Cl2 as the eluent. The
residue was precipitated from dichloromethane/methyl alcohol
to give 0.030 g (46%) of a white solid. 1H NMR (CDCl3, 300 MHz,
ppm) d 7.80 (4H, d, J ¼ 7.8, ArH), 7.68 (s, 2H, NH), 7.62–7.51 (m,
6H, ArH + ABsys), 7.50–7.36 (m, 10H, ArH + ABsys), 7.19 (s, 2H,
ArH), 7.00 (s, 4H, ArH), 6.86 (s, 2H, ArH), 6.74 (4H, d, J ¼ 1.8 Hz,
ArH), 4.10–3.95 (m, 12H, OCH2), 1.90–1.64 (m, 12H, CH2), 1.57–
1.39 (m, 12H, CH2), 1.39–1.25 (m, 144H), 1.19 (s, 36H, CH3 (tBu)),
0.90–0.85 (m, 18H, CH3); 13C NMR (CDCl3, 75 MHz, ppm) d 165.6
(C]O), 153.4 (Cq), 151.1 (Cq), 150.2 (Cq), 148.6 (Cq), 141.8 (Cq),
141.75 (Cq), 141.5 (Cq), 139.4 (Cq), 138.0 (Cq), 132.4 (CH), 131.2
(CH), 129.8 (Cq), 127.1 (CH), 125.25 (CH), 120.9 (CH), 120.3 (CH),
119.75 (CH), 119.4 (CH), 116.1 (CH), 105.9 (CH), 90.2 (C]C), 73.7
(OCH2), 69.6 (OCH2), 66.1 (Cspiro), 35.0 (Cq), 32.1 (CH2), 31.6
(CMe), 30.45 (CH2), 29.9 (CH2), 29.816 (CH2), 29.7 (CH2), 29.5
(CH2), 26.2 (CH2), 22.8 (CH2), 14.3 (CH3); HRMS (ESI+, CH2Cl2/
CH3OH: 70/30): Found: 2681.0530 (0 ppm) [M + Na]+, 1040.8409
(0 ppm) [M2 + Na]+, 964.8106 (1 ppm) [M3 + Na]+; required for
C186H268N2O8: 2658.06; IR (ATR, cmꢁ1) n ¼ 3302 (NH), 2955, 2919,
2851, 1646 (C]O), 1581, 1515, 1494, 1467, 1426, 1404, 1362,
1336, 1291, 1260, 1237, 1206, 1180, 1115, 1020, 820. UV-vis
(CH2Cl2): l nm (3, Mꢁ1 cmꢁ1) 341 (79000), 379 (110000), 399
(113000).
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