α-Cyanostilbene and fluorene based bolaamphiphiles: Synthesis, self-assembly, and AIEE properties with potential as white-light emissive materials and light-emitting liquid crystal displays

Article abstract of DOI:10.1039/d0tc04571c

Novel bolaamphiphiles containing two α-cyanostilbene units interconnected with 2,7-substituted-9,9-dialkylated fluorene as the central core and a diol unit at each terminal have been synthesized efficiently by a palladium catalyzed Suzuki coupling reaction and the Knoevenagel condensation reaction as key steps. POM, DSC and XRD investigation reveals that these compounds can organize into single wall triangular honeycomb columnar phases. A unique packing model was established in which the twisted π-conjugated backbones stacked alternatively into the walls and were connected through the end diol groups to form a triangular honeycomb, and the almost perpendicularly distributed alkyl side chains filled the cells. Such packing was additionally stabilized by the local anti-parallel dipole interactions of α-cyanostilbene and the H-bonding between CN?H. These compounds showed the AIEE effect and emitted yellow luminescence which could be further utilized to generate white light emission by coating on a commercially available blue LED lamp. Additionally, light-emitting liquid crystal display (LE-LCD) device could be obtained by dissolution of such AIE active distinctive luminogens in commercially available nematic LCs.

Full text of DOI:10.1039/d0tc04571c

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Journal of Materials Chemistry C
α-Cyanostilbene and fluorene based bolaamphiphiles: synthesis, self-assembly, View Article Online
DOI: 10.1039/D0TC04571C
AIEE properties with potentials as white-light emissive materials and
light-emitting liquid crystal displays
Deling Zhang, Yuantao Liu, Hongfei Gao, Qing Chang, Xiaohong Cheng*
Key Laboratory of Medicinal Chemistry for Natural Resource, Ministry of Education; Yunnan
Research & Development Center for Natural Products; School of Chemical Science and
Technology, Yunnan University, Kunming, 650091, P. R. China. E-mail:
xhcheng@ynu.edu.cn
Abstract: Novel bolaamphiphiles containing two α-cyanostilbene units interconnected with
2,7-substituted-9,9-dialkylated fluorene as the central core and diol unit at each terminal have
been synthesized efficiently by palladium catalyzed Suzuki coupling reaction and Knoevenagel
condensation reaction as key steps. POM, DSC and XRD investigation reveals that these
compounds can organize into single wall triangular honeycomb columnar phases. A unique
packing model was established in which the twist π-conjugated backbones stacked alternatively
into the walls and were connected through the end diol groups to form triangular honeycomb, the
almost perpendicular distributed side alkylchains filled the cells. Such packing was additionally
stabilized by the local anti-parallel dipole interactions of α-cyanostilbene and the H-bonding
between CN…H. These compounds showed AIEE effect and emitted yellow luminescence which
could be further utilized to generate white light emission by coating on the commercially blue
LED lamp. Most interesting light-emitting liquid crystal display (LE-LCD) device could be
obtained by dissolution such AIE active distinctive luminogen to commercially available nematic
LCs.
1 Introduction
Control of the self-assembly of π-conjugated molecules into predictable ordered arrangements is
crucial for their application as optoelectrical devices.1 This can be realized by imprinting
information in the molecular components to influence their assembly leading finally to defined
complex nanostructures with expected properties.2 In this sense, study liquid crystal phases
formed by π-conjugated rod like molecules are attractive because of their importance for study the
basic mechanism of molecular self-assembly, their self-healing properties with formation of defect
free and well defined nanostructures as well as their wide applications.3
Bolaamphiphiles consisting of a π-conjugated rodlike core, two terminal diol groups, and
lipophilic side chains, are able to form various complex nanostructures including a number of
polygonal honeycomblike liquid-crystal phases, and various novel lamellar and 3D-ordered
mesophases.4,5,6,7,8,9,10 In the honeycomb structures, the rodlike rigid cores organize into the
cylinder frameworks held together by H-bond between the terminal diol groups, the formed cells
inside the honeycomb are filled with the side lipophilic chains.4 Till now, functional groups such
as oligophenyl, oligothiophene11 azobenzene,12 2,1,3-benzothiadiazole (BTD)13 and triazole14
etc have been introduced into the π-conjugated rodlike core of bolaamphiphiles. The obtained
self-assembly principle of such bolaamphiphiles would guide their assembly in a predicted way15

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DOI: 10.1039/D0TC04571C
and should be very important for their applications as optoelectrical devices.16,17
The fluorene derivatives are famous because of their strongly blue fluorescence emission and
highly thermal stability.18 9,9-Dialkylfluorene has been used as the π-conjugated link to avoid the
dye aggregation and suppress the charge recombination.19,20,21 Due to the SP3 hybrid of C-9, the
9,9-dialkyl chains are almost perpendicular to the conjugated backbone, and the central fluorene
unit usually twists out of the molecular π-conjugated coplane.22
In another side, compounds containing α-cyanostilbene (CS) motif [-Ar-CHC(CN)-Ar-] belong
to one of the π-conjugated substrates that exhibit aggregation-induced emission (AIE) due to
restricted molecular rotations and the special supramoleuclar aggregation accompanying with the
distinctive electric and geometrical features. Different secondary bonding interactions (different
modes of local dipole coupling, C-H…π interaction, C-H…N hydrogen bond, π-π stacking) are
essential for the molecular packing.23 Because of their superior emissive characteristics in
aggregation state, such CS compounds have found wide applications as optical materials and for
sensing biological materials.24 It should be very interesting to introduce fluorene (F) or
cyanostilbene (CS) units into the rigid core of bolaamphiphiles. The question arose, can such CS
and F contained bolaamphiphiles self-assemble into novel honeycomb phase? Can the
characteristics encoded by CS and F in the molecules be transferred into the self-assembly
system? Which photophysical properties would display by such designed bolaamphiphiles? So far
to our acknowledgement there is no effort to introduce either fluorene (F) or cyanostilbene (CS)
units into the rigid core of bolaamphiphiles. Thus herein novel series of bolaamphiphiles (FT/n)
(Scheme 1) containing two CS units interconnected with 4,7-substituted 9,9-dialkylated fluorene
as the central core and diol unit at each terminal have been efficiently synthesized by palladium
catalyzed Suzuki coupling reaction and Knoevenagel condensation reaction as key steps as shown
in Scheme 1. The relationship among molecular structures, the mesogenic properties,
photophysical properties and aggregation properties, as well as their potentials as optoelectronic
devices were studied.
2 Results and discussion
2.1 Synthesis
Target CS-F bolaamphiphiles FT/n were synthesized by using C-C coupling and Knoevenagel
reactions as key steps. Fluorene was converted to 2,7-diiodofluorene 1 by KIO3-I2, under acid
condition,25 followed by alkylation with 1-bromoalkane afforded 9,9-dialkylated diiodefluorenes
2/n, which were further coupled with (5-formyl thiophene-2-yl) boronic acid under standard
Suzuki conditions, leading to the aromatic aldehydes 3/n. The 2-(4-hydroxyphenyl)acetonitrile 4
was etherified with allylbromide to afford 5, the double bond of 5 was then dihydroxylated by
co-oxidants OsO4 and NMMNO. Then the diol groups of
6 was protected with
2,2-dimethoxypropane to produce acetonides 7. Knoevenagel reactions between aromatic
aldehydes 3/n with acetonitrile 7, followed by deprotection of the 1,2-O-isopropylideneglycerol
units produced target compounds FT/n. FT/n were purified through column chromatography.
Details for synthesis procedures are shown in Supporting Information.

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DOI: 10.1039/D0TC04571C
NC
OH
4
iv
i
NC
I
I
I
I
1
5
O
ii
v
H2n+1Cn CnH2n+1
OH
O
NC
NC
2/n
6
O
O
OH
iii
vi
H2n+1Cn CnH2n+1
7
O
OHC
CHO
S
S
3/n
vii
H2n+1Cn CnH2n+1
CN
NC
NC
8/n
S
S
O
O
O
O
viii
O
O
H2n+1Cn CnH2n+1
CN
S
S
HO
O
O
OH
FT/n:
n = 12, 14, 16
HO
OH
Scheme 1 Synthesis of FT/n. Reagents and conditions: (i) I2, H2SO4, CH3COOH, KIO3, H2O, 80 °C, 12 h; (ii)
CnH2n+1Br, NaOH, toluene, H2O, TBAB, RT, 12 h; (iii) (5-formylthiophen-2-yl)boronic acid, Pd(PPh3)4, K2CO3,
H2O, THF, 78 °C, 15 h; (iv) K2CO3, CH3CN, allylbromide, reflux, 6 h; (v) OsO4, N-Methylmorpholine N-oxide
(NMMNO, 60% aqueous solution Aldrich), H2O, acetone, 50 ºC, 5 h; (vi) 2,2-dimethoxypropane, pyridinium
p-toluene sulfonate, RT, 5 h; (vii) ethanol, sodium methoxide, reflux 12 h; (viii) 10% HCl, methanol, reflux, 12 h.
2.2 Mesomorphic properties
The liquid crystalline characteristics of bolaamphiphiles FT/n were studied by POM, DSC and
XRD and the results are summarized in Table 1.
Table 1 Transition temperatures, corresponding enthalpy values and lattice parameters of bolaamphiphiles
FT/n a
Comp.
FT/12
n
T (°C) ∆H(kJ·mol-1)
a (nm)
ncell
nwall
12
Tg 49 ColhexΔ/p6mm 143b Iso
(Iso140b ColhexΔ/p6mm 47 Tg)
Tg 40 ColhexΔ/p6mm 137 [0.8] Iso
(Iso 131 [0.9] ColhexΔ/p6mm 36 Tg)
Tg 39 ColhexΔ/p6mm 113 [0.2] Iso
(Iso 110b ColhexΔ/p6mm 35 Tg)
4.03
3.56
1.19
FT/14
FT/16
14
16
3.92
3.98
3.17
3.08
1.06
1.03
a
measured by DSC (second heating/cooling at 10 K min-1); Tg: glass transition temperature; ColhexΔ/p6mm:
triangular honeycomb hexagonal columnar phase; ncell: number of molecules in hypothetical 3D unit cell; nwall
:
b
average intercellular wall thickness; calculation means of ncell and nwall are displayed in Table S4; temperature
measured by POM.

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All compounds FT/n display columnar phase (Table 1) as demonstrated by the typical sDpOhIe:r1u0.l1i0ti3Vc9ie/wD0ATrtCicl0e4O5n7l1inCe
fanlike textures with some dark regions as observed under POM (see Fig. 1). The main
π-conjugation route should be vertical to the column long axis because all the LC phases are
optically negative. The wide angle scatterings of X-ray diffractions showed diffuse maxima
around d = 0.45-0.46 nm (Figs. S2b, S3b and S4 in the Supporting Information), confirming the
LC properties of all mesophases.
(a)
(b)
(c)
Fig. 1 The textures of FT/n observed under POM: (a) Colhex/p6mm phase of FT/12 cooling at 120 °C; (b)
Colhex/p6mm phase of FT/14 cooling at 110 °C; (c) Colhex/p6mm phase of FT/16 cooling at 100 °C. The bottom
insets on each texture show the same texture with λ-retarder plate, the indicatrix orientation in the compensator is
shown on the top of (a), the columnar phase is optically negative as indicated by the orientation of the blue shifted
and yellow shifted fans; this indicates that in the columns the orientation of the intramolecular π-conjugation route,
coinciding with the long axis of aromatic cores is generally vertical to the column long axis.
1/2
The ratio of the positions of the small angle X-ray spacings is 1 : 3 : 2 as typical for a
two-dimensional p6mm lattice (Figs. 2a, S2a and S3a). The lattice parameters (ahex) around 3.9-4.0
nm (see Table 1 and Tables S1-S3) are within the scope of the molecular length (L = 3.9-4.4 nm),
this is expected for a triangular honeycomb LC.11b For the suggested triangular honeycomb
structure, there is about ncell = 3.1-3.6 molecules in each hypothetical 3D unit cell (calculated
process see Table S4). Because each unit cell is surrounded by three walls, the thickness of the
walls (nwall = ncell/3) is only about one rodlike core (nwall = 1.0). According to the SAXS pattern,
the electron density map of triangular cylinder phase has been reconstructed (Fig. 2b), in which
purple and blue regions refer to the high electron density areas, green regions are the medium
electron density shells, and red areas are the lowest electron density regions, representing H-bond
network of terminal glycerol groups, aromatic rigid cores and flexible alkyl chains, respectively.
Fig. 2 shows molecular dynamics (MD) annealed model which further confirmed that tight space
filling and right lattice parameters are obtained from this arrangement. In order to gain insight into
the molecular alignment in honeycomb, we calculated the optimized molecular structure of FT/12
according to density functional theory (DFT) (Gaussian 09W, B3LYP/(6-31G, d)), the optimized
molecular structure shows a little twist of the π-conjugated backbone with two cyano groups
located at the same side and the sulfurs of the two thiophenes point to another side of the
backbone (see Fig. S8). Simultaneously two alkyl chains are arranged almost vertical to the
molecular backbones. It should be mentioned that the dihedral angle between the fluorene and CS
units could be changeable during the molecular assembly.22 Taking this conformation in mind, and
considering the intermolecular local dipoles as well as C-H…N hydrogen bond interactions, the
following packing model was established (Fig. 2c). The honeycomb frames were constructed by
walls of antiparallel aligned twisted rodlike aromatic units arranged vertical to the resulting

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channel as revealed by the UV investigation (H-aggregation, see section 2.3), the planes of thVeiew Article Online
DOI: 10.1039/D0TC04571C
fluorenes should parallel to each other and in the plane of the hexagonal lattice, and both chains of
one molecule went into the same central lipophilic columns and those from the adjacent molecule
filled the space in the other adjacent lipophilic columns. Due to the SP3 hybrid of C-9 of fluorene,
the 9,9-dialkyl chains are almost perpendicular to the conjugated backbone and they are not in the
plane of the fluorene. The driving force for this unique organization is the microsegregation of the
incompatible components, and the SP3 hybride of C-9 of fluorene and the antiparallel of the rigid
core along the columns as well as the antiparallel coupling between the local dipoles in adjacent
molecular sheets assisted by multiple C-H…N hydrogen bonds. All these helped the formation of
single wall cylinder phase as shown in Figure 2c. Such packing showed the good space filling
without crowding and void, and was in accordance with the dense packing principle meaning the
minimum energy and maximum space filling of the packing system. Meanwhile such packing
ensures the intramolecular π-conjugation way to be vertical to the column long axis as revealed by
POM investigation.
Additionally, a weak and diffuse peak centered at 0.85 nm (Figs S2b, S3b and S4) is considered
as the assumed distance between the sulfur atoms of every other thiophene unit in packings of
alternating antiparallel organization of the molecules.26
The C-H…N hydrogen bonds could be confirmed by the FT-IR experiments (Fig. 3). In solid
state, the FT-IR spectrum of the representative compound FT/12 showed three characteristic
peaks at 2208cm-1, 3200 cm-1 and 3444 cm-1, which could be assigned to C≡N stretching vibration
of cyano unit, the C-H…N hydrogen bond vibration and O-H stretching vibration of diol
respectively. Whereas in LC state, the peaks showed obvious blue-shift (from 2208 to 2164 cm-1)
and the peak of 3200 cm-1 became stronger, which indicated that H-bond existed between N atoms
of cyano unit and hydrogen of H-C=, and the hydrogen bond between them will be strengthened in
the LC state.
(a)
(b)
(c)

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Fig. 2 (a) SAXS pattern of FT/16 at T = 100 ºC; (b) Reconstructed electron density from the SAXS pattern of (a);
(c) The possible self-assembly model of triangular honeycomb phase; the enlarged cylinder wall area is illustrated
by a molecular stick model, where the lateral chains are replaced by two methyl groups and the terminal glycerol
groups are omitted. The possible intermolecular hydrogen bonding between the N atom of cyano group and the H
atom of vinylene (black dotted lines) as well as the local dipoles (purple arrows) have been marked.
Fig. 3 FT-IR spectra of FT/12 in solid and LC states.
2.3 Photophysical and electrochemical properties
Fig. 4 Normalized UV-vis absorption and PL spectra of FT/12 in THF (1.0 ×10-6 mol L-1) and in thin film state.
The excitation wavelength of FT/12 in THF is 442nm and FT/12 in thin film state is 434nm.
The UV-vis and fluorescence spectra of representative compound FT/12 were studied (Fig. 4).
The UV-vis spectra of FT/12 in dilute solution of THF displayed one maximum peak at 442 nm.
The UV-vis spectra of FT/12 in the thin film displayed a little blue-shift at 424 nm. This
suggested that an H-type parallel π-π stacking mode in the solid state was formed.27 The emission
spectra in dilute solution exhibited two maximum emission peaks at 498 nm and 530 nm,
respectively and in solid state exhibited maximum peak at 578 nm (Fig. 4), indicating a much
larger Stokes shift in the solid state (154 nm) comparing with that in THF solution (88 nm).
T
he HOMO and LUMO energy levels of FT/12 in thin films were investigated by cyclic
voltammetry (Fig. 5).28 FT/12 exhibited one reversible reduction process giving an energy gap
el
(Eg ) of 2.22 eV, which was near to the value induced from absorption in thin film (2.41 eV) (Fig.
S7).29 The electronic features of FT/12 from DFT calculations (B3LYP/6-31G(d)) (Fig. S8)
suggested that the electron densities of HOMO are homogenously distributed over the whole

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conjugated core, whereas those of LUMO are also delocalized over the whole conjugDaOteI:d10c.1o0r3Ve9ie/wD0ATrtCicl0e4O5n7l1inCe
except the two terminal phenyl groups connected with diols. The low energy gap (2.22 eV) and
LUMO value (-3.96 eV) indicating electron injection property of this molecule.30
Fig. 5 Cyclic voltammogram of FT/12 on glassy carbon electrode in 0.1 mol/L Bu4NPF6 in acetonitrile solution
with a scan rate of 50 mV.s-1. Reference electrode: SCE. EHOMO = -(Eox + 4.74 eV), ELUMO = -(Ered + 4.74 eV), Eg =
ELUMO - EHOMO
.
2.4 Aggregation-induced enhanced emission (AIEE)
AIEE characteristics of these compounds in solid states were investigated in a mixture solution of
THF/water. As shown in Fig. 6 and Fig. S9, the fluorescence intensity displayed a weakening
tendency as the water fraction of the THF/water mixture increased from 0% to 70%. After the
water fraction reached 80%, the fluorescence intensity increased with increasing the water fraction
up to 90%, which was about 1.4 fold higher than that in the pure THF.
(a)
(b)
Fig. 6 (a) The fluorescence spectra of FT/12 in THF/H2O mixtures with different fractions of H2O (fw), excited at
λ = 330 nm (1 × 10-5 M); (b) The fluorescence intensity vs fw.
2.5 White light emission
Bolaamphiphiles FT/n exhibited yellow emission under 365 nm UV light, which indicated that it
was very possible to construct a white light emission device by coating FT/n film on the
commercial blue LED chip.31 Therefore the yellow fluorophor FT/12 was coated on the surface
of a commercial blue LED buld. When the LED was switched on, white light can be obtained (see

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inset of Fig. 7a). Two bands with maximum at 460 and 540 nm repectively were detected (Fig. 7a),
the CIE coodinates were defined to be (0.31, 0.33), which belongs to the white-light domain(Fig.
7b). Thus just by dip-coating bolaamphiphiles FT/n onto the commercial available blue lamp pith,
facil and low-cost WLEDs can be achieved.
(b)
(a)
Fig. 7 (a) PL spectrum of UV-LED lamp with the film containing FT/12, inserts show the lamp off and on states
respectively; (b) CIE chromaticity coordinates of the coated UV-LED lamp (0.31, 0.33).
2.6 Polarized emission spectra, the dichroic ratio and LE-LCD device
(a)
(b)
Fig. 8 Polarized fluorescence spectra of light-emitting LC cell in electric field-off status: a) SLC9023 + 0.5 wt%
FT/16; b) TEB300 + 0.5 wt% FT/16. The dichroic ratio (NF) was calculated from the formula: NF = F///F┴, where
F// and F┴ are the rubbing direction of the LC cell perpendicular and parallel to the polarization direction of the
detector, respectively.

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(a)
DOI: 10.1039/D0TC04571C
Voltage off
Voltage on
(b)
(c)
Voltage off
Voltage on
Fig. 9. Photograph of the LE-LCD devices with patterned alignment in the electric field-off and field-on status
using light-emitting LC mixture. The LC mixture used for a) is SLC9023 + 0.5 wt% FT/16 and for b) is TEB300 +
0.5 wt% FT/16; c) Schematic representation of the structures of the LE-LCD device with patterned alignment in
the electric field-off and field-on states using the LC mixture.
Recently it was demonstrated that it is possible to apply AIE-active mesogens to manufacture
LE-LCD,32 which was considered as an excellent alternative to conventional LCDs for enhanced
performance. In order to assess the potentials of FT/n in LE-LCD, FT/16 was selected as a
representative to blend with commercially available nematic liquid crystals SLC9023 and TEB300
respectively at a concentration of 0.5% w/w (details for sample preparation is shown in SI). The
dichroic ratio of polarized emission spectra of the mixture were investigated.33 The measured
dichroic ratios were respectively (Figure 8a and 8b). Such results indicated that the LC mixture of
FT/16 and the nematic LCs could be applied for fabrication of the LE-LCDs. Therefore, the LC
cell was prepared firstly with patterned ITO glass substrates. Then the monodirectional alignment
of LC mixtures were obtained by rubbing the polyimide (PI) aligned layer (details for device
fabrication is shown in SI). A UV lamp was applied to irradiate the LC mixture, and a polarizer
with the transmission direction parallel to the LC aligned direction was applied to detect the
change of PL efficiency. In the electric field-off state, there would be a bright image in the LC-cell
under UV illumination. After application of an electric field (1 KHz, 8 v) to the LC cell, only the
central region without ITO is still light emissive. The LE-LCD devices manufacture with a
mixture of SLC9023 + 0.5 wt% FT/16 and TEB300 + 0.5 wt% FT/16 can display the switching
from almost bright to dark yellow/green status observed by naked eyes as shown in Figure 9.

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3 Conclusion
Therefore, we have successfully developed a novel series of bolaamphiphilic liquid crystals with
triangular honeycomb phase by incorporating 9,9-dialkylated fluorene and cyanostilbene in the
aromatic rodlike core, the planes of the fluorenes should parallel to each other and in the plane of
the hexagonal lattice, and both chains of one molecule went into the same central lipophilic
columns and those of the adjacent molecule filled the space in the other adjacent lipophilic
columns. Besides the driving forces usually described for previously reported bolaamphiphiles,567
the additional interplay among the twist π-conjugated system as well as the antiparallel coupling
between the local dipoles in adjacent molecular sheets assisted by multiple C-H…N hydrogen
bonds also promoted this specific arrangement in triangular honeycomb LC organization. This
approach led to a new type of Bolaamphiphiles consisting of LC molecule with twist π-conjugated
core and nearly perpendicularly distributed lateral alkyl chains which should be interesting for
both the fundamental research and practical applications. Finally such bolaamphiphiles exhibited
AIEE property, and the potential application of such bolaamphiphiles as both WLED and LE-LCD
devices have been initially demonstrated.
Acknowledgements
This work was supported by National Natural Science Foundation of China (Nos. 21664015 and
21865038) and program for Innovative Research Team (in Science and Technology) in University
of Yunnan Province for financial supports (2018FY001(-012)). We thank beamline 1W2A at
Beijing Accelerator Laboratory and beamline BL16B1 at Shanghai Synchrotron Radiation Facility
(SSRF), China.
Conflicts of interest
There are no conflicts to declare.
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Table of contents
Fluorene and α-cyanostilbene based bolaamphiphiles could self-assemble into triangular
honeycomb phases with unique molecular arrangements, and showed AIEE effect. Their potential
application as both WLED and LE-LCD devices have been initially demonstrated.