Fluorine-containing bistolanes as light-emitting liquid crystalline molecules

Article abstract of DOI:10.1039/c7ob01369h

We synthesised a series of dissymmetric bistolane derivatives and evaluated their liquid-crystalline (LC) and photoluminescence properties in detail. In measuring LC behaviours, rational structural design based on the dissymmetric molecular structure and electron-density distribution facilitated the production of the LC phase with a wide temperature range (up to 97 °C). In addition, dissymmetric bistolane derivatives were shown to strongly emit blue-photoluminescence in dilute solution and in crystalline states. It was found that dissymmetric bistolanes possess emissive features in even the LC phase and photoluminescence behaviours such as emission intensity and colour were sensitively switched depending on the molecular aggregate structure caused by applying a thermal stimulus.

Full text of DOI:10.1039/c7ob01369h

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ISSN 1477-0520
COMMUNICATION
Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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Fluorine-containing bistolanes as light-emitting liquid crystalline
molecules
Shigeyuki Yamada,*a Kazuya Miyano,a Tsutomu Konno,a Tomohiro Agou,b Toshio Kubotab and
Takuya Hosokaic
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We synthesised
a series of dissymmetric bistolane derivatives and evaluated their liquid-crystallline (LC) and
www.rsc.org/
photoluminescence properties in detail. In measuring LC behaviours, rational structural design based on dissymmetric
molecular structure and electron-density distribution facilitated production of LC phase with wide temperature range (up
to 97 °C). In addition, dissymmetric bistolane derivatives were shown to strongly emit blue-photoluminescence in dilute
solution and in crystalline states. It was found that dissymmetric bistolanes possess emissive features in even the LC phase
and photoluminescence behaviours such as emission intensity and colour were sensitively switched depending on
molecular
aggregate
structure
caused
by
applying
a
thermal
stimulus.
new molecular systems with luminescent characteristics in
mesophases.
Introduction
To identify a new class of luminescent molecules in
mesophases, we designed a research strategy to provide LC
properties to potent luminophores. Among a number of
luminophores, in this study, we selected the bistolane
framework as a luminous core structure because of the linear
In recent years, enormous attention has been paid to the
development of new light-emitting liquid crystalline (LELC)
1
materials that possess both light-emitting and liquid-
crystalline (LC) properties as next-generation display devices
because such LELC materials are among the most promising
candidates to overcome potent disadvantages of conventional
LC display devices such as limited brightness and low energy
efficiency.2 Nevertheless, the discovery of luminous molecules
in the LC phase is one of the most strenuous tasks since
conventional luminophores smoothly quench in the condensed
phase, e.g., crystal and LC phases, due to intermolecular
energy transfer 3 (called the aggregation-caused quenching
(ACQ) effect).4 Therefore, extensive efforts are devoted to
developing a new class of luminophores that are strongly
emissive in condensed phases.
molecular shape and expanded
π
-conjugation structure.7,8
Although there are several reports on bistolane compounds
possessing either LC behaviour7 or photoluminescence in
solution,8 little attention has been paid to bistolane derivatives
equipped with both luminous and LC properties. In addition to
the bistolane framework, we focussed on fluorine-containing
organic molecules with unique character9 as the fluorine atom
has the highest electronegativity, small atomic radius, and
forms C–F bonds with large bond energy. The fact that rod-
shaped organic molecules with fluorine or fluorine-containing
substituents are used in LC display devices as guest LC content
encouraged us that the present molecular design should be
effective.10
Several intriguing phenomena reported by Tang and co-
workers, called aggregation-induced emission enhancement
5
(AIEE) or crystallisation-induced emission enhancement
(CIEE)6 can show enhanced emission in the solid state by
restricting intramolecular motion due to aggregation.
Although these discoveries open avenues to new luminous
molecules in crystalline phases, it is still desirable to develop
Fig. 1 Structural motif for bistolane derivatives.
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Therefore, in this study, we synthesised a series of bistolane
derivatives without and with fluorine-substituents and
evaluated the LC and photophysical properties in detail. In
addition, we tried to elucidate the relationship between
molecular structure and physical properties.
Experimental
General
1H- and 13C-NMR spectra were measured with a Bruker
AVANCE III 400 NMR spectrometer (1H: 400 MHz and 13C: 100
MHz) in chloroform-d (CDCl3) solution and the chemical shifts
are reported in parts per million (ppm) using the residual
proton in the NMR solvent. 19F-NMR (376 MHz) spectra were
measured with a Bruker AVANCE III 400 NMR spectrometer in
CDCl3 solution with CFCl3 (δF = 0 ppm) as internal standard.
Infrared spectra (IR) were determined in a liquid film on an
NaCl plate with a JASCO FT/IR-4100 type A spectrometer; all
spectra were reported in wavenumber (cm–1). High resolution
mass spectra (HRMS) were taken on a JEOL JMS-700MS
spectrometer using fast atom bombardment (FAB) methods.
Elemental analyses were conducted with a Yanaco CHN
CORDER MT-5 instrument. All reactions were carried out using
dried glassware with a magnetic stirrer bar. All chemicals were
of reagent grade and where necessary were purified in the
usual manner prior to use. Column chromatography was
Scheme 1. Synthetic route for bistolane derivatives 2. Reagents and conditions: (i) 3
(1.5 equiv.), 4 (1.0 equiv.), Cl2Pd(PPh3)2 (5 mol%), PPh3 (5 mol%), CuI (10 mol%), Et3N,
reflux, 15 h; (ii) 5 (1.0 equiv.), K2CO3 (1.5 equiv.), MeOH, rt, 15 h; (iii) 6 (1.0 equiv.), ArBr
(1.5 equiv.), Cl2Pd(PPh3)2 (5 mol%), PPh3 (5 mol%), CuI (10 mol%), Et3N, reflux, 15 h.
Typical procedure for the synthesis of 1-(2-phenylethynyl)-4-[(4-
methoxyphenylethynyl]benzene (2aA)
To a mixture of Cl2Pd(PPh3)2 (0.053 g, 0.075 mmol), PPh3 (22
mg, 0.075 mmol), 4-methoxyphenylehynylphenyl acetylene
(6aA) (0.23 g, 1.0 mmol), bromobenzene (0.24 g, 1.5 mmol),
and CuI (28 mg, 0.15 mmol) was added Et3N (20 mL) at 100 °C
(bath temp.) under argon. The reaction mixture was stirred at
that temperature for 15 h. After the solvent was removed
using a rotary evaporator, the crude product was extracted
with AcOEt and washed with saturated aqueous NH4Cl solution
(three times) and brine (once). The organic layer was dried
over anhydrous Na2SO4, filtered, and concentrated in vacuo.
The resultant was subjected to silica-gel column
chromatography (eluent: hexane/CH2Cl2 = 2/1) to provide the
title compound (0.12 g, 0.40 mmol) in 40% yield as a white
solid, which was further purified by recrystallisation from a
mixed solvent of CH2Cl2 and methanol.
carried out on silica gel (Wakogel® 60N, 38–100
ꢀm) and TLC
analysis was performed on silica gel TLC plates (Merck, Silica
gel 60F254).
Synthesis
Prototypical bistolane
Chemical Industries, Ltd. and the bistolane derivatives
synthesised from (4-alkoxyphenyl)acetylene with easy three-
step manipulations based on (i) Pd(0)-catalyzed Sonogashira
cross-coupling reaction with 1-bromo-4-[2-
(trimethylsilyl)ethynyl]benzene ( ), (ii) deprotection of the
1
was purchased from Wako Pure
2
were
3
4
TMS group, and (iii) Sonogashira cross-croupling reaction with
the corresponding aromatic bromide (Scheme 1). A typical
1-(2-Phenylethynyl)-4-[2-(4-methoxyphenyl)ethynyl]benzene
synthetic procedure for the final step to prepare
2 is described
(2aA)
below. The molecular structures of bistolanes 2 obtained were
Yield: 40% (colourless crystal); m.p.: 171 °C determined by
1
DSC; H-NMR (CDCl3):
characterised by multiple spectroscopic analyses, e.g. NMR, IR,
and HRMS, and the purity was proved by elemental analysis to
be sufficient to evaluate their phase transition and
photoluminescence behaviours.
δ 3.84 (s, 3H), 6.89 (d, J = 8.9 Hz, 2H),
7.33–7.38 (m, 3H), 7.45–7.47 (m, 1H), 7.48–7.50 (m, 5H), 7.51–
7.53 (m, 2H); 13C-NMR (CDCl3):
55.3, 87.9, 89.2, 91.0, 91.3,
114.0, 115.1, 122.7, 123.1, 123.4, 128.3, 128.4, 131.3, 131.5,
131.6, 133.1, 159.8; IR (KBr): 3020, 2941, 2842, 2200, 1605,
δ
ν
1518, 1285, 1247, 1180, 1030, 838 cm–1; HRMS (FAB+) m/z
[M+] Calcd for C23H16O: 308.1201; Found: 308.1202; Anal. Calcd
for C23H16O: C, 89.58; H, 5.23. Found: C, 89.22; H, 5.36.
1-[2-[4-(Trifluoromethyl)phenyl]ethynyl]-4-[2-(4-methoxyphenyl)-
ethynyl]benzene (2aB)
Yield: 39% (colourless crystal); m.p. 235 °C determined by DSC;
1H-NMR (CDCl3):
δ
3.84 (s, 3H), 6.89 (d, J = 8.8 Hz, 2H), 7.48 (d,
J = 8.8 Hz, 2H), 7.51 (s, 4H), 7.60–7.63 (m, 4H); 13C-NMR
(CDCl3):
δ
55.3, 87.7, 89.5, 91.5, 91.7, 114.1, 115.0, 121.9,
2 | J. Name., 2012, 00, 1-3
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124.1, 125.3 (q, J = 3.9 Hz), 126.9, 130.0 (q, J = 32.4 Hz), 131.4, = 8.4 Hz, 2H), 7.46 (d, J = 8.4 Hz, 2H), 7.50–7.56 (m, 4H); 13C-
1
131.6, 131.8, 133.1, 159.9, one carbon of JC-F could not be NMR (CDCl3):
δ
13.8, 19.2, 31.2, 67.8, 74.5, 87.5, 92.4, 100.2 (t,
J = 17.5 Hz), 101.4, 114.58, 114.6, 120.7, 125.1, 131.4, 131.8,
2963, 2843, 2210, 1926, 1608, 1520, 1407, 133.1, 136.2–139.0 (dm, J = 248.1 Hz, 1C), 140.2–143.0 (dm, J =
1328, 1251, 1125, 1066, 836 cm–1; HRMS (FAB+) m/z [M+] 262.0 Hz, 1C), 145.6–148.4 (dm, J = 256.9 Hz, 1C), 159.6; 19F-
Calcd for C24H15F3O: 376.1075; Found: 376.1075; Anal. Calcd NMR (CDCl3): –136.3 to –136.5 (m, 2F), –152.99 (t, J = 20.3
Hz, 1F), –162.1 to –162.3 (m, 2F); IR (KBr): 3016, 2964, 2863,
detected due to low solubility in CDCl3; 19F NMR (CDCl3):
63.30; IR (KBr):
δ –
ν
δ
for C24H15F3O: C, 76.59; H, 4.02. Found: C, 76.60; H, 4.27.
ν
2204, 1595, 1523, 1498, 1246, 1137, 992, 960, 831 cm–1; HRMS
(FAB+) m/z [M+] Calcd for C26H17F5O: 440.1200; Found:
440.1199; Anal. Calcd for C26H17F5O: C, 70.91; H, 3.89. Found:
C, 70.59; H, 4.30.
1-[2-(2,3,4,5,6-Pentafluorophenyl)ethynyl]-4-[2-(4-methoxyphen-
yl)ethynyl]benzene (2aC)
Yield: 62% (colourless crystal); m.p.: 149 °C determined by
1
DSC; H-NMR (CDCl3):
δ 3.83 (s, 3H), 6.89 (d, J = 8.8 Hz, 2H),
1-[2-(2,3,4,5,6-Pentafluorophenyl)ethynyl]-4-[2-(4-pentyloxyphen-
7.49 (d, J = 8.8 Hz, 2H), 7.51 (d, J = 8.6 Hz, 2H), 7.55 (d, J = 8.6
Hz, 2H); 19F-NMR (CDCl3):
–136.4 to –136.5 (m, 2F), –152.97
(t, J = 21.2 Hz, 1F), –162.1 to –162.3 (m, 2F); IR (KBr):
2940, 2843, 2211, 1922, 1597, 1496, 1252, 1172, 1028, 985, DSC; H-NMR (CDCl3):
yl)ethynyl]benzene (2eC)
δ
ν
3019, Yield: 20% (pale yellow crystal); m.p.: 128 °C determined by
1
δ
0.94 (t, J = 6.8 Hz, 3H), 1.38–1.48 (m,
836 cm–1; HRMS (FAB+) m/z [M+] Calcd for C23H11F5O: 4H), 1.76–1.83 (m, 2H), 3.96 (t, J = 6.6 Hz, 2H), 6.88 (d, J = 8.6
376.1075; Found: 376.1075; Anal. Calcd for C23H11F5O: C, Hz, 2H), 7.46 (d, J = 8.6 Hz, 2H), 7.51 (d, J = 8.0 Hz, 2H), 7.54 (d,
69.35; H, 2.78. Found: C, 69.57; H, 2.86. The 13C spectra cannot J = 8.0 Hz, 2H); 13C-NMR (CDCl3):
δ 14.0, 22.4, 28.2, 28.9, 68.1,
be obtained due to extremely low solubility in deuterated 74.5, 87.5, 92.4, 100.2 (t, J = 18.3 Hz), 101.3, 114.5, 114.6,
solvent.
1-[2-(4-Ethoxyphenyl)ethynyl]-4-[2-(2,3,4,5,6-pentafluorophenyl)-
120.7, 125.1, 131.4, 131.8, 133.1, 136.2–139.1 (dm, J = 250.1
Hz, 1C), 140.0–142.9 (dm, J = 256.7 Hz, 1C), 145.6–148.5 (dm, J
= 249.4 Hz, 1C), 159.6; 19F-NMR (CDCl3):
δ
–136.4 to –136.5 (m,
2F), –153.00 (t, J = 20.6 Hz, 1F), –162.2 to –162.3 (m, 2F); IR
(KBr): 3019, 2933, 2862, 2204, 1596, 1497, 1286, 1248, 992,
ethynyl]benzene (2bC)
Yield: 61% (colourless crystal); m.p.: 143 °C determined by
1
DSC; H-NMR (CDCl3):
ν
δ 1.43 (t, J = 7.0 Hz, 3H), 4.06 (q, J = 7.0
833 cm–1; HRMS (FAB+) m/z [M+] Calcd for C27H19F5O:
454.1356; Found: 454.1363; Anal. Calcd for C27H19F5O: C,
71.36; H, 4.21. Found: C, 71.25; H, 4.65.
Hz, 2H), 6.88 (d, J = 8.9 Hz, 2H), 7.46 (d, J = 8.9 Hz, 2H), 7.48–
7.52 (m, 4H); 13C-NMR (CDCl3):
14.7, 63.7, 74.4, 87.5, 92.3,
δ
100.2 (t, J = 16.9 Hz), 101.2, 114.6, 114.7, 120.7, 125.1, 131.4,
1-[2-(2,3,4,5,6-Pentafluorophenyl)ethynyl]-4-[2-(4-hexyloxyphen-
131.8, 133.2, 137.7 (dm, J = 253.9 Hz), 141.5 (dm, J = 256.8 Hz),
147.1 (dm, J = 252.5 Hz), 159.3; 19F-NMR (CDCl3):
yl)ethynyl]benzene (2fC)
δ –136.4 to –
136.5 (m, 2F), –152.99 (t, J = 20.7 Hz, 1F), –162.1 to –162.4 (m, Yield: 34% (white solid); m.p.: 120 °C determined by DSC; H-
1
2F); IR (KBr):
1247, 1046, 992, 837 cm–1; HRMS (FAB+) m/z [M+] Calcd for 1.48 (m, 2H), 1.75–1.83 (m, 2H), 3.96 (t, J = 6.6 Hz, 2H), 6.88 (d,
C24H13F5O: 412.0887; Found: 412.0890; Anal. Calcd for J = 8.9 Hz, 2H), 7.46 (d, J = 8.9 Hz, 2H), 7.50–7.56 (m, 4H); 13C-
ν 3018, 2982, 2203, 1595, 1523, 1497, 1285, NMR (CDCl3): δ 0.90–0.94 (m, 3H), 1.32–1.37 (m, 4H), 1.43–
C24H13F5O: C, 69.91; H, 3.18. Found: C, 69.54; H, 3.55.
1-[2-(2,3,4,5,6-Pentafluorophenyl)ethynyl]-4-[2-(4-n-propylphen-
NMR (CDCl3): δ 14.0, 22.6, 25.7, 29.1, 31.5, 68.1, 74.5 (q, J =
3.7 Hz), 87.5, 92.4, 100.2 (td, J = 18.4, 3.7 Hz), 101.2 (q, J = 2.9
Hz), 114.5, 114.6, 120.7, 125.1, 131.4, 131.8, 133.1, 136.2–
139.0 (dm, J = 245.7 Hz, 1C), 140.0–143.0 (dm, J = 256.8 Hz,
1C), 145.6–148.5 (dm, J = 253.0 Hz, 1C), 159.5; 19F-NMR
yl)ethynyl]benzene (2cC)
Yield: 25% (colourless crystal); m.p.: 144 °C determined by
DSC; 1H-NMR (CDCl3):
δ
1.05 (t, J = 7.2 Hz, 3H), 1.82 (qt, J = 7.2,
6.8 Hz, 2H), 3.94 (t, J = 6.8 Hz, 2H), 6.88 (d, J = 8.8 Hz, 2H), 7.46
(d, J = 8.8 Hz, 2H), 7.49–7.56 (m, 4H); 13C NMR (CDCl3):
10.5,
(CDCl3):
δ
–136.4 to –136.5 (m, 2F), –153.00 (t, J = 20.7 Hz, 1F),
3019, 2960, 2848, 2205,
–162.3 to –162.4 (m, 2F); IR (KBr):
ν
δ
1523, 1496, 1245, 992, 832 cm–1; HRMS (FAB+) m/z [M+] Calcd
for C28H21F5O: 468.1513; Found: 468.1513; Anal. Clacd for
C28H21F5O: C, 71.79; H, 4.52. Found: C, 72.03; H, 4.73.
22.5, 69.6, 74.5, 87.5, 92.4, 100.2 (t, J = 16.7 Hz), 101.2, 114.6,
120.7, 125.1, 131.4, 131.8, 133.1, 136.3–139.0 (dm, J = 251.8
Hz, 1C), 140.0–142.8 (dm, J = 256.1 Hz, 1C), 145.8–148.4 (dm, J
= 248.9 Hz, 1C), 159.5, one sp2-carbon signal attached to sp-
carbon in C6H4 moiety is overlapped with a signal at 114.6
X-ray crystallography
ppm; 19F-NMR (CDCl3):
= 20.7 Hz, 1F), –162.1 to –162.4 (m, 2F); IR (KBr):
2210, 1597, 1526, 1496, 1251, 1113, 984, 838 cm–1; HRMS
(FAB+) m/z [M+] Calcd for C25H15F5O: 426.1043; Found:
426.1035; Anal. Calcd for C25H15F5O: C, 70.42; H, 3.55. Found:
C, 70.01; H, 3.95.
δ
–136.4 to –136.5 (m, 2F), –152.99 (t, J
3019, 2966,
Single crystals of the bistolanes 2aC and 2eC were obtained by
ν
slow evaporation from
(dichloromethane/methanol = 1/1) and mounted on a glass
fibre. All measurements for 2aC were made on
diffractometer with filtered Mo-K radition ( = 0.71075 Å)
and rotating anode generator using VariMax with
a
mixed solvent system
a
α
λ
a
a
1-[2-(4-n-Butoxyphenyl)ethynyl]-4-[2-(2,3,4,5,6-pentafluorophen-
PILATUS/DW (Rigaku); all calculations were performed using
the CrystalStructure crystallographic software package. The
structure was solved by direct methods and expanded using
Fourier techniques. On the other hand, the omega scanning
technique was used to collect the reflection data of 2eC using
yl)ethynyl]benzene (2dC)
Yield: 16% (pale yellow crystal); m.p.: 122 °C determined by
1
DSC; H NMR (CDCl3):
δ 0.98 (t, J = 7.2 Hz, 3H), 1.50 (sext., J =
7.2 Hz, 2H), 1.74–1.82 (m, 2H), 3.99 (t, J = 6.4 Hz, 2H), 6.88 (d, J
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Bruker D8 goniometer with monochromatised Mo-Kα
Journal Name
Crystal structure of 2aC (R=CH3, Ar=C6F5)
a
radiation; the initial structure of the unit cell was determined
by a direct method using APEX2. The structural model was
refined by a full-matrix least-squares method using SHELXL-
2014/6.11 All calculations were performed using the SHELXL
program. The crystal data for bistolanes 2aC and 2eC are
summarised in the electronic supplementary information (ESI)
and the indexed data have been deposited in the Cambridge
Crystallographic Data Centre (CCDC) database (CCDC 1552781
for 2aC and 1552782 for 2eC).12
(a) Top view
(b) Side view
(c) Packing structure
Crystal structure of 2eC (R=C5H11, Ar=C6F5)
Phase transition properties
(d)
(e)
The phase transition properties of bistolanes were observed by
polarizing optical microscopy (POM) using an Olympus BX51
microscope equipped with a temperature-controlled stage
(Instec HCS302 microscope, hot and cold stages, and mK1000
temperature controller). The thermodynamic parameters
were determined using a DSC (SII X-DSC7000) at heating and
cooling rates of 5.0 °C min–1. At least three scans were
conducted to confirm reproducibility.
(f)
Photophysical properties
UV-Vis absorption spectra were recorded using a JASCO V-500
absorption spectrometer. Steady-state photoluminescence
spectra were obtained using a Hitachi F-7000 or JASCO FP-
8500 fluorescence spectrometer. Photoluminescence quantum
yields were estimated using a calibrated integrating sphere
system (Hitachi or JASCO). Transient photoluminescence
Fig. 2 Crystal structures of 2aC and 2eC. (a), (d) Top view; (b), (e) side view; (c), (f)
packing structures. Colour legend: C, grey; H, white; O, red; F, light green.
measurements were measured using
fluorescence lifetime system (HORIBA).
a
Fluorocube
Viewing the crystal structures of 2aC and 2eC from the top (Fig.
2a and 2d) and side (Fig. 2b and 2e), the three aromatic rings
formed a coplanar structure in the crystal lattice, though the
three aromatic rings generally may be in rapid equilibrium
between coplanar and twisted conformations through free
rotation of alkyne–aryl single bonds.8a In sharp contrast, the
two bistolanes provided significantly different packing
Computation
All computations were performed with density functional
theory (DFT) using the Gaussian 09 (rev. D.01) package.13
Geometry optimisations were executed with the B3LYP hybrid
structures, as shown in Fig. 2c. 2aC (with
substituent) had an anti-parallel dimer structure located in the
centre that was -stacked ( ··· distance: 343 pm on average)
between electron-deficient pentafluorophenyl and relatively
electron-abundant neighbouring aromatic rings. 2ac
underwent further stabilisation to form crystals through rigid
CH/ interactions (CH··· distance: 295 pm on average) with
two other molecules. The shortest contacts of both and
CH/ interactions were almost equal to the sum of van der
a methoxy
functional and Dunning's correlation consistent basis set cc-
14
pVDZ.
The stationary points were characterised by
π
π π
frequency calculations to confirm that the minimum energy
structures had no imaginary frequencies.
Results and discussion
π
π
π/π
Crystal structures of bistolane derivatives 2aC and 2eC
π
Waals (vdW) radii (carbon: 170 pm and hydrogen: 120 pm),15
indicating that both interactions are attributable to crystal
formation. On the other hand, as shown in Fig. 2f, 2eC (with a
Among the bistolane derivatives
2 synthesised in this
study, fluorinated bistolane 2aC (with a methoxy group) and
2eC (carrying a pentoxy chain) furnished single crystals suitable
for X-ray crystallographic analysis after dual purification by
silica-gel column chromatography and recrystallisation from
the dichloromethane/methanol system.
pentoxy chain) preferentially formed
a parallel dimer
structure, which was distinctly different structure from that of
2aC. Carefully analysing the packing structures of 2eC, a large
Bistolane 2aC was found to crystallise in the P 21/n
monoclinic space group and 2eC afforded crystals in the C1c1
monoclinic space group. The unit cells of 2aC and 2eC
contained four molecules each.
stabilisation effect was observed involving
π
/
π
interaction
: 291 pm
(
π
on average), and van der Waals interaction between the C5H11
··· : 322 pm on average), CH/ interaction (CH···π
π
π
chains (Halkyl···Halkyl: 313 pm on average).
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Phase transition property
effectively decrease the phase transition temperature (Tm) of
Cr→LC due to the atomic radius 1.2 times larger than that of
hydrogen.17 The phase transition behaviour of 2aC revealed
that Tm and Tc for 2aC were 149 °C and 225 °C, respectively (LC
The phase transition properties of bistolanes 2aA–2fC were
evaluated by polarising optical microscopy (POM) and
differential scanning calorimetry (DSC). The phase sequences
and transition temperatures observed are summarised in Table
temperature range (ꢁTLC): 76 °C). Consequently, the electron-
deficient characteristic and steric effect of lateral fluorine
1. The phase transition property of a prototypical
shown for comparison.
1 is also
substituents of the pentafluorophenyl moiety both proved to
successfully effect significant expansion of
As summarised in Table 1, other dissymmetric bistolanes
2bC 2fC with various lengths of alkoxy chain also exhibited LC
ꢁTLC.
Prototypical bistolane
exhibited a direct phase transition from crystal (Cr) to the
isotropic (Iso) phase, indicating that did not show an LC
1 (without any flexible moieties)
–
1
phase.16 In sharp contrast, bistolane 2aA with a dissymmetric
structure displayed a bright-field image with fluidity under
POM observation at 171–181 °C, which means that the 2aA
possesses LC characteristics. Judging from the four-brush
schlieren textured image in the LC phase (Fig. S2 in ESI), it was
concluded that the LC phase of 2aA was a nematic (N) phase.
It is reasonably explained that the distinct phase transition
phases, but different LC behaviours depending on the alkoxy
chain length. Comparing the phase sequences observed in
2aC
–
2fC (1 ≤ n ≤ 6), only N LC phase was seen in 2aC
–2cC (with
shorter alkoxy chain; n ≤ 3), whereas 2dC
–
2fC (carrying longer
alkoxy chain; n ≥ 4) was found to provide two types of LC
phases: we observed the phase sequence Cr→LC1→LC2→Iso.
The POM measurements supported the determination of the
LC1 and LC2 phases; a fan-shaped texture image was observed
in the LC1 phase and a four-brushed schlieren texture image
was obtained in the LC2 phase. Judging from the optical
textures, LC1 and LC2 can be characterised as smectic (Sm) and
N phases, respectively (Fig. S3). Additionally, it was found that
shear stress to the fan-shaped texture in Sm phase caused
significant changes to the POM images from bright-field to
dark-field image. This phenomenon can be usually observed
when the molecular alignment switches from homogenous to
behaviour between 2aA and
enhancement of the flexibility and a dissymmetric molecular
structure by introducing methoxy substituent at the
1 may be attributed to
a
molecular edge. Further substitution of a CF3 group into
another molecular edge, viz. 2aB, also showed the N LC phase
at 235–253 °C, and the N LC region was found to shift to higher
temperature.
Significant increases in phase-transition
temperatures of Cr→N (Tm, melting temperature) and N→Iso
(Tc, clearing temperature) can be attributed to increase of
homeotropic; texture switching behaviour is
observation in the SmA phase.18 Considering a value of
transition enthalpy ( H) (Table S2 in ESI), the H in SmA/N
phase transitions of 2dC
2fC were 0.30, 0.69 and 0.86 kJ mol–1,
respectively. It was reported that the H in SmA N was 0.05–
1.10 kJ mol–1, whereas ca. 2.1 kJ mol–1 of
H was required in
the phase transition of SmB N.19 Judging from
H in Sm/N
a typical
molecular rigidity triggered by the extension of
through the electron-density distribution.
π-conjugation
ꢁ
ꢁ
Subsequently, we demonstrated phase transition
properties of a series of bistolane derivatives (2aC 2fC) with a
–
–
ꢁ
pentafluorophenyl ring as the electron-deficient aromatic ring
since it was reported that laterally-substituted fluorine atoms
ꢁ
ꢁ
phase transition, the obtained Sm phase in this study can be
safely characterized as SmA phase, not SmB phase as well. As
Table 1. Thermodynamic behaviours of the bistolane derivatives.
a consequence, the phase transition property of 2dC
–2fC (with
Bistolane
Phase transition behaviour [°C]a,b
Cr 183 Iso
a longer alkoxy chain) showed distinct properties from that of
1
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
2aC
–
2cC (with a short flexible chain).
To understand the effect of alkoxy chain length among
2fC on the phase transition behaviours, the phase
Cr 173 Iso
2aA
2aB
2aC
2bC
2cC
2dC
2eC
2fC
Cr 171 N 181 Iso
2aC
–
Cr 154 N 181 Iso
transition temperatures were plotted as shown in Fig. 3.
Cr 235 N 253 Iso
Cr 228 N 253 Iso
Cr 149 N 225 Iso
Cr 140 N 225 Iso
Cr 143 N 238 Iso
Cr 140 N 239 Iso
Cr 144 N 220 Iso
Cr 123 N 220 Iso
Cr 122 SmA 134 N 215 Iso
Cr 117 SmA 136 N 216 Iso
Cr 128 SmA 143 N 201 Iso
Cr 96 SmA 144 N 202 Iso
Cr 120 SmA 152 N 197 Iso
Cr 90 SmA 153 N 197 Iso
a
Abbreviations: Cr, crystalline; SmA, smectic A; N, nematic; Iso, isotropic. b Data
obtained from DSC scans at a rate of 5.0 °C min–1. Phase transition temperatures
were determined from the second cycles as the onset of the endothermic signals.
Fig. 3 Phase transition behaviours of 2aC–2fC depending on the chain length (CnH2n+1
)
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Comparing Tc (N→Iso), the highest Tc (238 °C) was observed in deficient aromatic ring. Hence, the HOMO→LUMO electronic
2bC and the Tc values tended to gradually reduce with transition in is associated with * and/or intramolecular
extension of alkoxy chain length. It can be rationally explained charge transfer (ICT). Calculated HOMO–LUMO energy gaps
by thermal activation of molecular motion of the alkoxy chain Eg) were 3.66 eV for , 3.57 eV for 2aA, 3.44 eV for 2aB and
2
π–π
(
ꢁ
1
to induce smooth destruction of the molecular aggregate. 2aC, and 3.42 eV for 2eC; the sequence was in good
Similarly, Tm (Cr→LC) was observed to reduce with longer agreement with the sequence of the absorption edge,
alkoxy chain length. The opposite tendency was observed in concluding that wavelength shift of absorption spectra can be
SmA→N phase transitions: increasing alkoxy chain length led attributed to a change of electronic structure caused by
to higher phase transition temperature due to increased electronic features of substituents.
stability of layer structures through larger van der Waals
forces.
Subsequently, when measuring the photoluminescence of
and in dilute solution, prototype bistolane showed a
1
2
1
sharp emission signal at 348 nm with a slightly broad emission
band at 368 nm, which stemmed from the vibrational
Photophysical behaviour
structure, whereas other bistolane derivatives
2 exhibited a
As mentioned above, bistolane derivatives are known to single photoluminescence band with an emission maximum at
possess intriguing photoluminescence behaviours due to their 377–411 nm.20 In fact,
in dilute solution was visible to purple
extended ex = 365 nm) and
-conjugation.8 Therefore, our interest was directed photoluminescence under UV irradiation (
toward the investigation of photophysical properties of was observed by the naked eyes to emit blue
dissymmetric bistolane derivatives photoluminescence ( em = 0.69–0.88) (Fig. S4).
Initially, we measured absorption and photoluminescence
spectra for in dilute dichloromethane solution. Fig. 4a and
1
π
λ
2
2
.
Φ
2
4b show absorption and emission spectra, respectively, and
the photophysical data obtained are summarised in Table 2.
It was found that all molecules,
1 and 2, showed
absorption bands only in the UV region (200–400 nm),
indicating that the bistolanes possessed transparency over the
entire visible-light region, which is an important feature for
practical application to light-emitting devices. Prototype
1
showed absorption maxima at 320 nm and 340 nm, while the
absorption bands and absorption edges at the lowest energies
for dissymmetric bistolanes (e.g. 2aA, 2aB and 2aC) gradually
shifted to longer wavelengths (Fig. 4a) due to the change in
electron-distribution caused by electronic characteristics of
the substituents.
To understand the long-wavelength shift in 2, calculations
were carried out using density functional theory (DFT) with the
B3LYP functional and the cc-pVDZ basis set. Fig. 4c shows
frontier molecular orbitals and orbital energies for selected
examples. In dissymmetric bistolanes
2 with electron-
abundant and -deficient aromatic rings, e.g. 2aB and 2aC, the
HOMO is mainly localised over the alkoxy-substituted benzene
ring and the LUMO is mainly delocalised over the electron-
Fig. 4 (a) Absorption spectra in dilute solution (CH2Cl2, 10–5 mol L–1), (b) corrected
photoluminescence spectra in CH2Cl2 solution (10–6 mol L–1), and (c) HOMO/LUMO and
the corresponding energies.
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Table 2. Molecular orbital energies obtained from DFT calculation and the photophysical data.
Compound
HOMO/LUMO
Eg) [eV]a
λ
abs [nm]b
x10–3 [L mol–1 cm–1
]
Emission in solution statec
Emission in crystalline state
(ꢁ
(ε
λ
em [nm]
Φem
0.86
0.85
0.77
0.79
0.81
0.69
0.88
0.70
0.76
λ
em [nm]
444
449
420
419
458
420
455
450
450
Φem
–
1
–5.62/–1.96 (3.66)
–5.39/–1.82 (3.57)
–5.56/–2.12 (3.44)
–5.62/–2.18 (3.44)
–5.60/–2.17 (3.43)
–5.60/–2.17 (3.43)
–5.59/–2.17 (3.43)
–5.59/–2.17 (3.42)
–5.59/–2.17 (3.42)
320 (55.6), 340 (34.7)
325 (51.5), 345 (34.9)
330 (55.1), 350 (38.8)
332 (54.2)
348, 368
377
2aA
2aB
2aC
2bC
2cC
2dC
2eC
2fC
0.88
0.72
0.66
0.47
0.68
0.53
0.28
0.47
394
403
331 (50.3)
411
332 (53.6)
409
332 (53.0)
409
333 (50.1)
405
333 (51.7)
407
a
b
Estimated from DFT calculation using the Gaussian 09 program package and the computations were performed at B3LYP/cc-pVDZ level of theory. Measured in
CH2Cl2 solution in 10–5 mol L–1 concentration. c Measured in 10–6 mol L–1 solution in CH2Cl2.
The spectral shift to long wavelength was also observed with a aromatic ring) exhibited photoluminescence with an emission
similar tendency as the absorption behaviours. Comparing band around 420 nm. As described before, in dilute solution,
spectral shapes, the dissymmetric bistolane
emission band unlike prototype , which is because the
photoemission in stemmed from multiple transition
processes like * and ICT due to the large electron-
2 formed a single
1
2
π–π
distribution. In addition, by demonstrating transient
photoluminescence measurements, we measured the time
constants (τ) for the selected examples, viz. 2aA, 2aB, and 2aC;
their were in the range 1.1–1.3 ns, concluding that the strong
τ
photoluminescence observed in dilute solution is fluorescence
(Fig. S6, Table S3). Owing to the similar molecular and
electronic structures among 2aC
the photoluminescence from other dissymmetric bistolanes
2bC 2fC) in dilute solution might also be fluorescence.
Our next interest was the photoluminescence properties of
dissymmetric bistolane derivatives in condensed phases,
–2fC, it can be suggested that
Fig. 5 (a) Corrected photoluminescence spectra of the bistolane derivatives 1 and 2 in
(
–
crystal after recrystallisation; inset: CIE colour diagram obtained from the
corresponding spectrum. (b) Photographs of dissymmetric bistolanes
irradiated by a UV lamp ( ex = 365 nm).
2 in crystal
λ
2
since the development of new luminescent materials in
condensed phases is still challenging, as mentioned before.
Therefore, we examined the photoluminescence properties of
2aB–C with low ꢁEg emitted photoluminescence at long
wavelength, which was the opposite tendency to the
luminescence behaviour in crystal. Photoluminescence in
dilute solution (10–6 mol L–1) is generally considered a radiative
S1→S0 transition involving a sole molecule, not a molecular
aggregate, while crystals can be considered as molecular
aggregates orderly fixed by plural intermolecular interactions.
To understand the opposite luminescence behaviours in
solution and in crystal phases, it is crucial to consider the
molecular aggregates in the crystalline state. Therefore, taking
bistolanes
emission spectra of dissymmetric bistolanes
prepared from recrystallisation process, as well as
; the photophysical data are also listed in Table
2
in crystalline and LC phases. Fig. 5a shows the
2, which were
a
prototypical
2.
1
All molecules were observed to emit photoluminescence
with a single emission maximum at 420 nm or 450 nm, whose
emission colour was visible in the deep-blue range when
the crystal structures of
reported to form herringbone-type packing structures via CH/
interactions (Fig. S1), 21 while 2aC constructed molecular
1 and 2aC into consideration, 1 was
irradiated with UV light (Fig. 5b).
photoluminescence spectra of 2aA, and 2aB , those
without any fluorine-substituent and 2aA) emitted
Comparing the
π
1,
–
C
(
1
aggregates via both
π
Face-to-face packing, e.g.
/
π
and CH/
π
intermolecular interactions.
photoluminescence with emission maximum around 450 nm,
whereas 2aB (with fluorine-containing electron-deficient
π/π interactions, can promote
intermolecular charge transfer to induce smooth S1→S0
–
C
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transition, therefore, suggesting that 2aC results in blue similar situations to 2aC, which may form anti-parallel dimer
photoluminescence at shorter wavelength. structures with weak intermolecular interactions.
In the bistolanes 2aC 2fC with a pentafluorophenyl moiety, Carrying out the transient emission measurements of 2aA
preliminary DFT calculations led us to anticipate that all 2aB, and 2aC in the crystalline phase, were found to be 2.47–
π/π
–
,
τ
moieties may show the same photoluminescence properties 3.19 ns (Fig. S6, Table S3), which indicates that the blue
including spectral shape and emission wavelength. However, photoluminescence observed in crystal was fluorescence,
we observed intriguing photoluminescence characteristics that similar to the photoluminsecence in solution. The values of kr
showed different emission bands depending on the alkoxy and knr calculated from the
τ value observed in crystal were
chain length. Thus, 2aC (methoxy group) and 2cC (propoxy 108 s–1 and 107–108 s–1 (Table S3), respectively, from which
group) emitted deep-blue photoluminescence with a sharp molecular aggregates in crystal contributed to suppress non-
emission band at ca. 420 nm, whereas the other bistolanes radiative quenching processes, leading to strong
ethoxy-substituted 2bC, and butoxy-, pentoxy-, and hexoxy- photoluminescence in the crystalline phase.
substituted 2dC
photoluminescence with a broad emission band at 450–458 photoluminescence behaviour of the dissymmetric bistolanes
nm. As discussed above, the photoluminescence property in in LC phases. To evaluate the temperature dependence of
–
2fC
,
were found to emit light-blue
Finally, our interests were directed toward the
2
crystal is strongly affected by the molecular aggregates photoluminescence, we measured photoluminescence of
involving intermolecular interactions. As described in Fig. 2c bistolane 2aC and 2eC as selected examples with varying
and 2f, both 2aC (λem = 420 nm) and 2eC (λem = 450 nm) measurement temperature. The measurement was conducted
formed molecular aggregates in their crystalline states, but the in two heating and cooling cycles (30↔160 °C)23; the emission
packing structures obtained were distinctly different from each spectra obtained during the first cooling and the second
other: the former constructed a dimer structure stabilised by heating processes are shown in Fig. 6a (2aC) and Fig. 6b (2eC).
π
the latter formed dimers involving
/
π
and CH/
π
interactions aligned in an anti-parallel direction;
2aC was shown to emit deep-blue photoluminescence
π
/π
, CH/π and van der even at 160 °C (N phase, blue-dashed line in Fig. 6a). When
Waals interactions in a parallel direction. From the crystal reducing the measurement temperature, it was found that the
structures of 2aC and 2eC, a slight difference in the closest photoluminescence intensity gradually increased ~2.7 times
interatomic distances between two molecules and higher while maintaining the spectral shape as at 30 °C (Cr
intermolecular interactions was observed. In fact, the closest phase after heating/cooling process, red-solid line).
interatomic distances appearing for the
343 pm for 2aC and 322 pm for 2eC, and the distances of the the Cr to N phase (blue-solid line) resulted in restoring the
CH/ interaction were 295 and 291 pm for 2aC and 2eC emission intensity to the initial intensity, i.e., 2aC was proved
π/π interaction were Continuous heating to 160 °C to induce phase-transition from
π
,
respectively. From the distances of the shortest interatomic to possess reversible photoluminescence characteristic during
contact, it was obvious that 2eC (with a long pentoxy chain) Cr↔N phase transition (Fig. 6a (inset)). The depression of
formed stronger π/π interaction between molecules than 2aC emission intensity by heating is a usual phenomenon because
(with a short methoxy group), leading to a longer wavelength
shift of the emission band. 22 In addition, comparing a
difference of emission maxima in solution and in crystalline
states (ꢁλem(solution–crystal)) for 2aC and 2eC, as shown in
Table 2, the former 2aC was only 16 nm, whereas, in the latter,
the larger long-wavelength shift (45 nm) was observed in
accordance with a variation in a molecular circumstance.
These results imply us that the strengths of intermolecular
interactions worked in crystal are different in both molecules:
in 2aC
,
crystal lattice was constructed with weak
interactions triggered by CH/ interactions,
intermolecular π
resulting in a similar emission in short-wavelength region to
π
/π
the luminescence in solution, whereas 2eC formed crystal with
stronger
disturbance. In our hypothesis based on the experimental
facts, we consider the strength of intermolecular
interaction worked in crystal is a potent factor to induce a
slight longer-wavelength shift of emission maxima in 2eC
π
/π
intermolecular interactions without any
π/π
,
rather than 2aC, though it is still unclear due to the complexity
of the photochemistry in the excited states. Table 2 also
revealed that 2cC emitted photoluminescence at shorter
wavelength at 420 nm, the ꢁλem(solution–crystal) was only 11
nm, which was the same tendency as 2aC. Based on the above
arguments, emission of 2cC in crystal is likely to appear from
Fig. 6 (a) Temperature dependence of the photoluminescence behaviour of 2aC (λex
=
365 nm). Spectrum in N phase (blue-dashed thick line) at 160 °C after 1st heating,
spectrum in Cr phase (red-solid line) at 30 °C after 1st cooling, spectra in Cr phase (red-
dashed thin line) in 2nd heating, spectra in N phase (blue-dashed thin line) in 2nd
heating and spectrum in N phase (blue-solid line) at 160 °C in 2nd heating process;
inset: plots of emission intensity at λmax during 1st cooling (blue square) and 2nd
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heating (red circle) processes. (b) Temperature-dependence of the photoluminescence
behaviour of 2eC ( ex = 340 nm). Spectrum in N phase (blue-dashed thick line) at 160 °C
photoluminescence emitted at 420 nm, whereas packing
λ
structures with strong intermolecular π/π interactions induced
after 1st heating, spectrum in Cr phase (red-solid line) at 30 °C after 1st cooling, spectra
in Cr phase (red-dashed thin line) in 2nd heating, spectrum in SmA phase (green-solid
line) at 140 °C in 2nd heating, spectra in SmA phpase (green-dashed thin line), spectra
in N phase (blue-dashed thin line) and spectrum in N phase (blue-solid line) at 160 °C in
2nd heating process; inset: emission spectra of 2eC in Cr at 30 °C (red-solid line) after
1st cooling, SmA phase at 140 °C (green-solid line) in 2nd heating, spectrum of 2aC in Cr
(red-dashed line) and spectrum of 2eC in Cr (green-dashed line) prepared by
recrystallisation. (c) CIE colour diagram of 2eC at 30 °C (Cr), 140 °C (SmA) and 160 °C (N)
during 2nd heating process.
by the formation of parallel dimer structures, like 2eC (shown
in Fig. 2f), may lead to light-blue photoluminescence emitted
at 450 nm, though the molecular mechanism is still unclear.
As a consequence, dissymmetric liquid-crystalline bistolane
derivatives
2
can precisely switch photoluminescence
behaviours, e.g. emission intensity and emission colour, by
changing the aggregated structure through heat-induced
phase transitions, which would be
a powerful tool in
the thermal energy activates molecular motion to facilitate photoluminescence sensing materials.
non-radiative transition.
Similarly, we measured the photoluminescence properties
Conclusions
of 2eC with varying temperature (30↔160 °C). As shown in
Fig. 6b, emission intensity, spectral shape, and emission
maxima were found to switch depending on the measurement
temperature. In fact, 2eC in the N phase at 160 °C showed a
broad emission band with an emission maximum at 430 nm
(blue-dashed thick line), whereas the emission band in the Cr
phase at 30 °C after 1st cooling caused enhancement of
emission intensity by a factor of four and short wavelength
shift of the emission maximum by 30 nm (red-solid line).
Continuously, raising the temperature to 140 °C caused phase
transition from Cr to SmA, at which point the emission band
became slightly broadened and shifted to 450 nm (green-solid
line). By heating up to 160 °C again, the emission intensity
smoothly diminished to revert to the initial intensity and the
emission maximum shifted back to 430 nm (blue-solid line).
These observations clearly indicate that the emission
behaviour in bistolane 2eC can be switched depending on
changes in the condensed phases caused by thermal stimulus
and the emission colour is controllable by changing the
aggregated structure (Fig. 6c).
In this study, we synthesised dissymmetric bistolane
compounds with alkoxy-substituted electron-abundant
aromatic and/or fluorine-containing electron-deficient
aromatic rings in the same molecule and evaluated phase
transitions and photoluminescence properties.
All
dissymmetric bistolanes were found to appear in liquid-
crystalline phases and underwent significant stabilisation of
the LC phase by incorporating fluorine-containing electron-
deficient aromatic rings to induce an electron-density
distribution. In the photoluminescence measurements, the
dissymmetric bistolane derivatives were shown to intensively
emit blue-photoluminescence in dilute solution and crystalline
phases. It is noteworthy that the luminescence property can
be controlled depending on changes in the electronic structure
and molecular aggregate structure. In addition, we observed
that
2
exhibited remarkable photoluminescence properties
Thus, with
that emitted light even in LC phases.
photoluminescence measurement under heating/cooling, it
was proved that emission intensity and colour changed
depending on phase, based on molecular aggregated
structures. These observations indicate that the luminous
bistolanes would be promising photoluminescence switching
materials, e.g. luminescence sensor, to easily control the
molecular alignment through thermal stimulus. These would
contribute to developing novel luminous materials in LC
phases. Further studies to determine the mechanisms for LC
and photoluminescence behaviours and to develop new
molecules with multiple functionalities such as LC,
photoluminescence, etc., are currently underway in our
laboratory.
As shown in Fig. 6b (inset), moreover, it was observed
unique phenomena, as follows:
(i) Comparing spectra of 2eC in crystals after recrystallisation
(green-dashed line) and after heating/cooling process (red-
solid line), slight wavelength-shift by 30 nm was observed,
meaning that 2eC forms polymorphs depending on the
crystallisation process.
(ii) Spectrum of 2eC in Cr phase after recrystallisation (green-
dashed line) was closely in agreement with that in SmA
phase in 2nd heating (green-solid line), which indicates
that one polymorph obtained by recrystallisation forms
similar molecular aggregation to that in SmA phase.
(iii) Spectrum of 2eC in Cr phase after heating/cooling process
(red-solid line) was almost consistent with a spectrum of
2aC in the Cr phase after recrystallisation (red-dashed line),
indicating that crystal 2eC re-constructs the crystal
structure similar to 2aA after thermal transitions.
Acknowledgements
This work was partially supported by The Kyoto Technoscience
Center and the Hitachi Metals · Materials Science Foundation.
We thank Prof. Osamu Tsutsumi at Ritsumeikan Univ. and
Profs. Kensuke Naka and Hiroaki Imoto at the Kyoto Institute
of Technology for kindly performing physical property
measurements.
Resulting
from
the
temperature-dependent
photoluminescence spectra in 2eC, we hypothesised that the
photoluminescence behaviour of 2eC switched sensitively
depending on the molecular aggregates caused by applying
thermal stimulus: molecular aggregates with anti-parallel
dimer structures caused by a weak intermolecular π/π
interactions, like 2aC (shown in Fig 2c), display deep-blue
Notes and references
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Journal Name
Chanteau, L. Cheng, M. P. Stewart and J. M. Tour, Chem.
Mater. 2005, 17, 1331.
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7
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Leung, R. T. K. Kwok, T. Han, B. Tong, J. Shi, Y. Dong and B. Z.
Tang, J. Mater. Chem. C, 2016, , 10430.
4
23 Phase transition sequences of 2aC and 2eC in the 1st heating
process are as follows: Cr 149 N 226 Iso for 2aC and Cr 128
SmA 143 N 201 Iso for 2eC
.
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Table of contents entry
F
F
F
F
C5H11
O
F
Fluorine-containing dissymmetric bistolanes were found to be promising light-emitting
liquid crystals, which could switch luminescence depends on molecular aggregations.