Strong solid emission and mechanofluorochromism of carbazole-based terephthalate derivatives adjusted by alkyl chains

Article abstract of DOI:10.1039/c5tc00267b

Three 2,5-dialkylcarbazole-substituted terephthalate derivatives, in which carbazole and ethoxylcarbonyl groups are used as electron-donating and -accepting moieties, respectively, were synthesized. Owing to the presence of steric hindrance between ethoxylcarbonyl and carbazole groups, three compounds show intense blue fluorescence in both solution and the solid state. The fluorescence quantum yields of compounds with octyl and hexadecyl groups in the solid state exceed 95%. Single-crystal structures of three compounds were obtained and used to interpret the strong emission in the solid state. More interestingly, three compounds exhibited alkyl length-dependent mechanofluorochromism. The compound with ethyl groups exhibited the largest spectral shift under force stimuli, but that with a hexadecyl moiety did not change its emission color after grinding. Because of strong fluorescence in solution and the solid state, we believe that they can be used as luminescent materials and sensors. This journal is

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ARTICLE TYPE
Strong Solid Emission and Mechanofluorochromism of Carbazole-based
Terephthalate Derivatives Adjusted by Alkyl Chains
a
a
b
a
a
a
a
Pengchong Xue,* Jiabao Sun, Peng Chen, Peng Gong, Boqi Yao, Zhenqi Zhang, Chong Qian, Ran
Lu,*
a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Three 2,5-dialkylcarbazole-substituted terephthalate derivatives, in which carbazole and ethoxylcarbonyl
groups are used as electron-donating and –accepting moieties, respectively, were synthesized. Owing to
the presence of steric hindrance between ethoxylcarbonyl and carbazole groups, three compounds show
intense blue fluorescence in both solution and solid state. The fluorescence quantum yields of compounds
with octyl and hexadecyl groups in solid state exceed 95%. Single-crystal structures of three compounds
were obtained and used to interpret the strong emission in solid state. More interestingly, three
compounds exhibited alkyl length-dependent mechanofluorochromism. The compound with ethyl groups
exhibited largest spectral shift under force stimuli, but that with hexadecyl moiety did not change its
emission color after grinding. Because of strong fluorescence in solution and solid, we believe that they
can be used as luminescent materials and sensors.
1
0
1
5
Introduction
An organic compound emitting strong fluorescence in solution
and solid state is very important for its real applications as
2
2
3
3
4
4
0
5
0
5
0
5
optoelectronic materials, sensors for metal ions, anions, neutral
1
molecules.
However, the traditional aggregation-caused
quenching (ACQ) effect in solid state, owing to internal
conversion, intersystem crossing, intramolecular charge transfer,
intermolecular electron transfer, excimer or exciplex formation,
or isomerisation, results in drastically negative effects on the
Scheme 1. Molecular structures and photos under natural and 365 nm
light.
2
50 free rotation of single bond is restricted. For example, Chen and
co-authors designed several tetrahydro[5]helicene derivatives
with rigid twisted structure, and found that they emitted strong
device performance and sensitivity of the sensors. Tang et al.
firstly introduced an effective methodology to overcome ACQ,
3
which is called aggregation-induced emission (AIE). The AIE
1
0
fluorescence in solution and solid state. Another kind of strong
emissive fluorophores is 2,5- disubstituted 1,4-diaminobenzene
derivatives, developed by Shimizu, have been synthesized and
active molecules are highly fluorescent in solid state, but no or
4
very weak emission is observed in solution state, which limited
5
5
their wide applications. So, organic fluorophores with strong
emission in both solution and solid state should been developed.
To realize the intense luminescence in solid state, several
1
1
used in organic light-emitting diodes. Herein, we designed and
synthesized three carbazole-based terephthalate derivatives
(BECT, BOCT and BHCT, Scheme 1), in which carbazole and
ethoxylcarbonyl groups are used as electron-donating and –
5
strategies including J-aggregation formation, the introduction of
6
7
bulky substituent, the formation of excimer, and restriction of
8
60 accepting moieties, respectively. Owing to the presence of steric
hindrance between ethoxylcarbonyl and carbazole groups, three
compounds show intense fluorescence in both solution and solid
state. Fortunately, single-crystal structures of three compounds
were obtained and used to interpret the strong emission in solid
intramolecular rotation (RIR) or planarization were developed.
Generally, AIE molecules have twisted configurations and freely
rotated single bonds, such as silole, tetraphenylethene, 9,10-
9
distryrylanthracene, and cyano-diphenylethene derivatives. In
solution, such molecules emit weak fluorescence because of RIR.
Intramolecular rotation in solid or crystal state is suppressed, and
the twisted configuration leads to loose stacking, which
eliminates strong π-π interaction between fluorophores and then
promotes strong emission. How do molecules possess intense
luminescence in both solution and solid state? One novel strategy
is to construct molecules with twisted conformation, in which
65
state. More interestingly, three compounds exhibited alkyl length-
dependent mechanofluorochromism.
Experimental section
Measurements
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1
13
H and C NMR spectra were recorded on a Bruker- Avance III
Synthetic routes of the compounds BECT, BOCT and BHCT
were shown in Scheme 2. The intermediate of 4 , 9, 10 and 11
1
2
13
(400MHz) spectrometer with CDCl3 as solvent and
tetramethylsilane (TMS) as the internal standard. Mass spectra 45 were synthesized by the reported methods. The detailed
were obtained with Agilent 1100 MS series and AXIMA CFR
MALDI-TOF (Compact) mass spectrometers. Elemental analysis
was performed on a Perkin-Elmer 2400. The UV-vis absorption
experimental procedures were as follows:
5
0
5
0
5
0
5
Diethyl 2,5-bis(9-ethyl-9H-carbazol-3-yl)terephthala-te (BECT).
Toluene (10 ml) and water (10 ml) were added into a mixture of 9
(0.71g, 2.2 mmol), 4 (0.38 g, 1 mmol), K CO (1.5g, 11mmol)
spectra were obtained using
a
Mapada UV-1800pc
2
3
spectrophotometer. Photoluminescence measurements were taken 50 and Pd(PPh ) (15 mg). The reaction mixture was stirred at 90° C
3
4
on
a
Cary Eclipse fluorescence spectrophotometer. The
under N for 24 hours. After cooled to room temperature, the
2
1
1
2
2
3
3
fluorescence quantum yields of BECT, BOCT and BHCT in
solutions were measured by comparing to a standard (quinine
sulfate in 1N H SO aqueous solution, Φ = 0.65). The excitation
solution was extracted with methylene chloride. The organic
extracts were combined, and dried over anhydrous Na SO .
Upon evaporating off the solvent, the crude product was purified
2
4
2
4
F
wavelength was 365 nm. The absolute fluorescence quantum 55 by silica column with petroleum/methylene chloride (V/V = 1/2)
yields were measured on an Edinburgh FLS920 steady state
spectrometer using an integrating sphere. Fluorescence decay
experiment was measured on an Edinburgh FLS920 spectrometer
equipped with an nF900 nanosecond flash lamp. The XRD
patterns were obtained on an Empyrean X-ray diffraction 60 2H), 7.98 (s, 2H), 7.43-7.53 (m, 8H), 7.26 (m, 2H), 4.45 (q, J =
instrument equipped with graphite-mono-chromatized CuKα
as the eluent to yield BECT (0.52 g, 85%) as a white powder.
Elemental Analysis (%): calculated for C H N O : C, 78.92; H,
5.96; N, 4.60; found: C, 78.90; H, 5.99; N, 4.57. H NMR (400
4
0
36
2
4
1
MHz, CDCl , TMS): δ 8.18 (d, J = 1.2Hz, 2H), 8.15(d, J = 7.7Hz,
3
7.2Hz, 4H), 4.14 (q, J = 7.1Hz, 4H), 1.48 (t, J = 7.2 Hz, 3H), 0.95
1
3
radiation (λ = 1.5418 Å), by employing a scanning rate of 0.026°
(t, J = 7.1 Hz, 3H). C NMR (101 MHz, CDCl , TMS): δ168.81,
3
-
1
s
in the 2θ range from 5 to 30°. The samples were prepared by
141.12, 140.41, 139.50, 133.87, 132.12, 130.96, 126.51, 125.85,
123.04, 120.58, 120.40, 119.03, 108.62, 108.12, 61.23, 37.70,
casting crystal powders, ground solid and fuming samples on
glass slides at room temperature. The ground solids for steady 65 13.85, 13.82. MALDI-TOF MS: m/z: calcd. for C H N O :
4
0
36
2
4
+
and time-resolved fluorescence and XRD measurements were
obtained by simply grinding the crystals with a mortar and pestle.
Single crystals were obtained in the mixture of CH Cl and n-
608.3; found: 609.6 [M+H] .
Diethyl 2,5-bis(9-octyl-9H-carbazol-3yl)terephthalate (BOCT).
The synthesis of BOCT is the same as that of BECT. The crude
product was purified by silica column with petroleum/methylene
2
2
hexane by a slow solvent diffusion method. The molecular
optimal configurations were used to obtain the frontier orbitals of 70 chloride (V/V = 1/2) as the eluent to yield BOCT (90%) as a
BECT and BOCT by density functional theory (DFT)
calculations at B3LYP/6-31G(d) level with the Gaussian 09W
program package.
white powder. Elemental Analysis (%): calculated for
C H N O : C, 80.38; H, 7.78; N, 3.61; found: C, 80.39; H, 7.83;
5
2
60
2
1
4
N, 3.58. H NMR (400 MHz, CDCl , TMS): δ 8.17 (s, 2H), 8.15
3
Single crystals of BECT, BOCT and BHCT were selected for X-
ray diffraction studies in a Rigaku RAXIS-RAPID diffractometer 75 (m, 2H), 4.37 (t, J = 7.2 Hz, 4H), 4.14 (q, J = 7.1 Hz, 4H), 2.00 –
(d, J = 7.7 Hz, 2H), 7.99 (s, 2H), 7.57 – 7.44 (m, 8H), 7.32 – 7.24
using graphite-monochromated MoKα radiation (λ = 0.71073°A).
The crystals were kept at room temperature during the
datacollection. The structures were solved by direct methods and
1.86 (m, 4H), 1.50 – 1.18 (m, 20H), 0.96 (t, J = 7.1 Hz, 6H), 0.90
1
3
(t, J = 6.6 Hz, 6H). C NMR (101 MHz, CDCl , TMS) δ168.87,
3
141.10, 140.89, 140.01, 133.88, 132.10, 130.91, 126.47, 125.81,
122.92, 120.50, 120.31, 118.97, 108.86, 108.35, 61.21, 43.26,
2
refined on F by full-matrix least-square using the SHELXTL-97
Program. The C, N, O and H atoms were easily placed from the 80 31.82, 29.43, 29.22, 29.04, 27.37, 22.63, 14.09, 13.80. MALDI-
subsequent Fourier-difference maps and refined anisotropically.
TOF MS: m/z: calcd. for C H N O : 776.5; found: 777.7
52 60 2 4
+
[
M+H] .
Diethyl
BHCT). The synthesis of BHCT is the same as that of BECT.
2,5-bis(9-hexadecyl-9H-carbazol-3yl)terephthalate
(
8
9
9
5
0
5
The crude product was purified by silica column with
petroleum/methylene chloride (V/V = 1/2) as the eluent to yield
BHCT (88%) as a white powder. Elemental Analysis (%):
calculated for C H N O : C, 81.55; H, 9.26; N, 2.80; found: C,
6
8
92
2
4
1
8
1.51; H, 9.31; N, 2.81. H NMR (400 MHz, CDCl , TMS): δ
3
8.17 (d, J = 1.6 Hz, 2H), 8.15 (d, J = 7.7 Hz, 2H), 7.98 (s, 2H),
7
4
5
.55 – 7.43 (m, 8H), 7.31 – 7.25 (m, 2H), 4.37 (t, J = 7.2 Hz, 4H),
.13 (q, J = 7.1 Hz, 4H), 2.00 – 1.84 (m, 4H), 1.48 – 1.19 (m,
2H), 0.96 (t, J = 7.1 Hz, 6H), 0.90 (t, J = 6.8 Hz, 6H). C NMR
1
3
(
101 MHz, CDCl ) δ 168.87, 141.10, 140.88, 140.00, 133.87,
3
132.11, 130.90, 126.47, 125.81, 122.91, 122.89, 120.50, 120.30,
1
2
18.97, 108.85, 108.35, 61.21, 43.26, 31.95, 29.72, 29.68, 29.65,
9.63, 29.57, 29.49, 29.39, 29.05, 27.39, 22.72, 14.15, 13.81.
MALDI-TOF MS: m/z: calcd. for C H N O : 1000.7; found:
1001.9 [M+H] .
4
0
68 92
2
4
+
Scheme 2. Synthesis route of BECT, BOCT and BHCT.
Synthesis of BECT, BOCT and BHCT
2
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Result and Discussion
35 central benzene ring, and then increase the radiative transition
probability. In addition to the steady-state spectral measurements,
time-resolved fluorescence spectra can provide more information
on the excited state of fluorophores. The results suggest that the
emission decay of three compounds is single exponential and
Photophysical properties in solution
Three compounds were synthesized by a simple Suzuki reaction
1
(
Scheme 2) in high yields. They were all characterized by H
1
3
5
NMR, C NMR, elemental analysis and mass spectroscopy. It 40 they have the similar fluorescence lifetimes (from 3.73 to 3.80 ns,
was found that as-synthesized solids could emit intense
fluorescence under 365 nm light irradiation (Scheme 1).
Fig. S12, Table S1). Radiative (k ) and nonradiative (k ) rate
r nr
constants of BECT in cyclohexane are 0.10 and 0.16 ns . Larger
knr should be responsible for moderate Φ in solution.
–
1
When we increased the solvent polarity, three compounds exhibit
polarity-dependent solvatochromism of fluorescence (Fig. 1 and
S10). In cyclohexane, BECT emitted blue fluorescence. Its
solution had an emission peak at 463 nm. The emission maxima
further shifted to 475, 484 and 521 nm in ethyl acetate, CHCl3
and DMF, respectively. BOCT and BHCT have the similar
spectral changes. It was also found that Φs in different solvents
except in DMF are large (from 0.35 to 0.61). Such change in
fluorescence spectra indicates that the excited state has a larger
4
5
5
6
6
5
0
5
0
5
1
6
dipole moment than that in ground state. The quantum chemical
calculation of BECT indicates that the HOMO state density of
BECT is mainly distributed at the carbazole and central benzene
ring. In contrast, the electron density of the LUMO mainly
locates at central benzene ring and ester groups. The stimulated
transition at ca. 394 nm ascribes to the HOMO LUMO
transition. Thus, an excited state with large dipole moment will
emerge after light irradiation. It is in accordance with the electron
push-pull molecular structure and suggests also that polarity-
dependent solvatochromism of fluorescence should be ascribed to
an intramolecular charge transfer transition, be not originated
from twisted intramolecular charge transfer one because that
there was not shift of fluorescence spectra when DMSO solution
1
7
of BECT was cooled to 0 ˚C to from glass solid (Fig. S13).
Fig. 1 (a) Fluorescence of BECT in different solvents; (b) photos of
BECT solutions under 365 light irradiation. λex = 365 nm; (c) HOMO and
LUMO distribution of BECT obtained after conformation optimization
by quantum chemical calculation.
1
0
Firstly, the absorption and fluorescence spectra of three
compounds in solutions were measured and compared. As a result,
they possess the similar absorption and fluorescence bands in
different solvents (Fig. S10, Table S1). For example, two
absorption peaks at 337 and 352 nm for BECT in toluene were
observed. The absorption peaks of BOCT and BHCT located at
1
2
2
3
5
0
5
0
3
40 and 352 nm. Similar absorption peaks show that their
aromatic moieties have similar molecular conformations. They
have also the similar emission spectra with maxima around 465
nm in toluene. Their fluorescence quantum yields (Φ) are
moderate (from 0.42 to 0.49). Such high Φs relative to general
AIE molecules suggests that the free rotation of single bonds in
1
4
compounds in our case may been suppressed. In order to verify
this hypothesis, the optimal conformation was obtained by
density functional theory (DFT) calculation at B3LYP/6-31G(d)
level. As shown in Fig. S11, two dihedral angles between two
carbazole rings and benzene ring are large and are 52.8º and 61.2º.
The ester groups also stray from the plane of benzene ring, and
twist angles are 22.3 º and 33.6 º. Non-coplanar conformation
should be ascribed to the steric hindrance between carbazole and
Fig. 2 (a) Fluorescence spectra of BECT in THF-water mixtures. λex
=
70
365 nm.
As discussed above, the as-synthesized solids can emit strong
fluorescence, which implies that molecular aggregation dose not
induce fluorescence quenching. To study the emission property of
aggregation, their emission spectra were measured in THF–water
1
5
ester groups. Such enhanced steric hindrance will induce
restriction of free rotation of single bond, linked carbazole and 75 mixtures (three compounds are soluble in THF and water is a
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1
8
poor solvent). As shown in Fig. 2b, the THF solution of BECT
emits strong blue fluorescence. The emission gradually quenched
with addition of water and the emissive peak redshifted, which
mixture was turbid and a redshifted absorption band with a level-
off tail was obviously observed in the 80 vol% mixture (Fig. S14).
BOCT and BHCT exhibited similar spectral change in THF-
arising from the increased solvent polarity, liking in DMF. When 10 water mixture (Fig, S15). These spectral changes confirm that
5
the water fraction increased to 80%, the emissive intensity swiftly
increased, which induced by molecular aggregation because the
strong emission could be also observed in small aggregates.
Table 1. Twist angles and photophysical data of three compounds in crystals.
θ1 (º) θ2 (º) θ3 (º) θ4 (º) θ +θ (º) θ +θ (º) λem (nm)
–
1
–1
τ (ns)
k (ns )
k (ns )
1
4
2
3
r
nr
1
4
.96 (37%)
.31 (63%)
BECT 57.2 45.9 57.2 46.8
104.0
103.1
455
0.15
0.14
BOCT 44.9 49.3 46.4 50.1
BHCT 30.4 52.5 30.4 52.1
95.0
82.5
95.7
82.9
459
469
4.20 (100%)
5.78 (100%)
0.23
0.16
0.0047
0.0087
4
4
5
5
6
6
0
5
0
5
0
5
overlaps between aromatic rings for three compounds crystals are
observed, as shown in Fig. 3. The nearest distances between two
carbons of aromatic units in BOCT and BHCT crystals are 3.61
and 3.89 Å, respectively, suggesting no π-π interaction. It may
explain why their excited states decay via single relaxation
pathway. In addition, in their crystal structures, there are many
kinds of intermolecular weak interactions, including CH···π,
CH···O=C and CH···O–C (Fig. S17). These weak interactions
may restrict the intramolecular rotation and then block the
nonradiative channel, thus making enhanced emission of BOCT
2
1
and BHCT. However, a short distance of 3.53 Å between two
parallel carbazole units (Fig. 2a) may allow BECT in excited
state transfers energy to adjacent molecule and then its
2
2
fluorescence is quenched, which may interpret a moderate Φ
and the existence of a short fluorescence lifetime (1.96 ns) for
BECT crystals. Therefore, the non-coplanar molecular
conformation should be responsible for the strong emission in
solid state because coplanar carbazol-3-yl-vinyl based trimer and
pentamer emitted weak fluorescence in solid film and gel
2
3
phases. Moreover, twist angles between ester groups and
benzene ring (θ1 and θ3) or carbazole and benzene ring (θ2 and
θ4), are alkyl chain length-dependent (Fig. S18 and Table 1).
BECT have the largest sums of θ1 and θ4 or θ2 and θ3. The
smallest sums for BHCT are observed. It is believed that BHCT
and BECT have best and poorest π-conjugated structures,
respectively. Therefore, longer emissive wavelengths for
molecules with longer alkyl chains in crystal states were observed.
MFC properties in solid state
1
5
Fig. 3 Molecular structures and interactions between two molecules for
BECT (a), BOCT (b) and BHCT (c) in their crystals. Insets are photos of
crystals under 365 nm light irradiation.
To further understand the relationship between molecular
structure and strong emissions in solid state, their photophysical
properties in crystal states and crystal structures were studied.
BECT emitted strong blue fluorescence with a maximum at 455
nm (Fig. S16). Solid-state fluorescence quantum yield for BECT
is 0.51. BOCT and BHCT have red-shifted emissive maxima at
2
2
3
3
0
5
0
5
4
59 and 469 nm, respectively. Their Φs in crystal state are very
high, and reach 0.98, and 0.95, respectively, which are larger than
those in solutions and illustrate the characteristic aggregation-
1
9
induced emission enhancement (AIEE). The fluorescence decay
of BECT crystals is double exponential and average lifetime is
3
.44 ns (Fig. S17, Table 1). Thus, similar k and k to those in
r nr
solution are obtained. Single exponential decay for BOCT and
BHCT are observed and their τs increase to 4.20 and 5.78 ns. The
similar k s to that BECT are evaluated, but k s of BOCT and
r
nr
BHCT rapidly decrease, suggesting that larger Φs of BOCT and
BHCT are mainly ascribe to the suppression of non-radiative
2
0
transition. Crystal structures will give more information about
solid state luminescence. As shown in Fig. 3, three compounds
have non-coplanar conformation in crystals. Such non-planarity
caused by the steric repulsion between ester groups and carbazole
moieties reduces the intermolecular π-π interactions. No obvious
Fig. 4 Normalized fluorescence spectra of BECT (a), BOCT (b) and
7
0
BHCT (c) in pristine crystals and after grinding and then fuming. λex
60 nm. (d) Schematics of color changes under mechanical force and
solvent fuming stimuli.
=
3
4
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Journal of Materials Chemistry C
Table 2. Emissive peaks of solids, spectral shifts and fluorescence
lifetimes of ground powders.
wavelength from 455 to 480 nm (Fig. 4a), meaning a spectral
shift of 25 nm. After grinding, ground BOCT powder has an
emission peak at 474 nm, indicating a redshift of 15 nm.
25 However, an almost same emission peak as that of crystal sample
was found after applying mechanical grinding to the BHCT
crystals. Based on these spectral changes, we can conclude that
the length alkyl chain can regulate the MFC properties of
b
λ
pristine
λ
grind
ΔλPFC
τ (ns)
a
(
nm)
25
15
1
BECT
BOCT
BHCT
455
459
469
480
474
470
3.07 (46%); 5.68 (54%)
3.81 (87%); 6.78 (13%)
5.77 (100%)
a
b
ΔλPFC = λgrinding-λpristine; lifetimes of ground powders.
2
6
materials.
3
0
To determine different MFC behaviors, X-ray diffraction (XRD)
patterns and time-resolved spectra were investigated. The crystals
of BECT and BOCT show many sharp and strong peaks. The
ground powders have very weak diffraction peaks (Fig. 5),
suggesting the solids after grinding is amorphous. After fuming
by CH Cl vapor, sharp and strong diffraction peaks, similar to
35
2
2
those of the crystals, appeared again. Such spectral changes
suggest the reversibility of MFC behaviors for BECT and BOCT
is based on the reversible phase transformation between crystal
2
7
and amorphous states.
5
Fig. 5 XRD patterns of BECT (a), BOCT (b) and BHCT (c) in crystals
and after grinding.
As discussed above, three compounds are non-coplanar and
electronic push-pull structure, and then mechanofluorochromic
2
4
(
MFC) behaviours are expected. By simply grinding the BECT
1
1
2
0
5
0
crystals with a mortar and pestle, a white powder with blue-green
emission was obtained. When the ground powder was exposed to
CH Cl vapor for several seconds, the fluorescence of ground
2
2
powder rapidly recovered to blue (Fig. 4d). This process of
fluorescence color change can be repeated many times,
4
0
Fig. 6 Time-resolved fluorescence spectra of (a) BECT, (b) BOCT and (a)
BHCT in crystal and ground states. λex = 365 nm.
2
5
suggesting an obviously reversible MFC behaviour. After
grinding, the fluorescence color of BOCT is also converted from
blue to blue-green. However, grinding did not lead to any change
in fluorescence color of BHCT. Fluorescence spectra were used
to monitor such a color transformation under grinding and fuming
stimuli. The data were listed in Table 2. Applying mechanical
grinding to the BECT crystals resulted in a red-shift of emissive
The fluorescence lifetimes of BECT and BOCT became longer
after grinding (Table 1 and 2). For instance, ground BECT solid
still has two fluorescence lifetimes, but they prolong to 3.07 ns,
and 5.68 ns, respectively (Fig. 6a). BOCT after grinding
possesses two lifetimes too, and a longer τ of 6.78 ns appears (Fig.
45
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Journal of Materials Chemistry C
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6
b). As discussed above, longer wavelengths for BOCT and
† Electronic Supplementary Information (ESI) available: [NMR, MS,
UV-vis, fluorescence, and time-resolved fluorescence spectra,
photophysical data in solution and solid state, molecular packing in
crystals. CCDC 1031692, 1031667, and 1031666 for BECT, BOCT and
BHCT, respectively]. See DOI: 10.1039/b000000x/
‡ Footnotes should appear here. These might include comments relevant
to but not central to the matter under discussion, limited experimental and
spectral data, and crystallographic data.
BHCT crystals were observed because of smaller twist angle
sums. So, we suppose that planarity in molecular structure under
mechanical force stimulus should be responsible for MFC
behaviors of BECT and BOCT. Because the sums of twist
angle θ1 and θ4 or θ2 and θ3 in BECT crystal are largest relative
to those in BOCT crystal, meaning a poorest π-conjugation, a
better π-conjugation for BECT is anticipated if the twist angles
decrease after grinding. Therefore, ground BECT solids emit
fluorescence with longer wavelength and have a large spectral
shift under force stimulus. On the other hand, we found that the
60
2
8
5
0
5
0
1
(a) Q. Wang, D. Ma, Chem. Soc. Rev., 2010, 39, 2387; (b) L. Yao, S.
Zhang, R. Wang, W. Li, F. Shen, B. Yang and Y. Ma, Angew. Chem.
Int. Ed., 2014, 53, 2119; (c) H. Shi, J. Dai, L. Shi, M. Wang, L. Fang,
S. Shuang and C. Dong, Chem. Commun., 2012, 48, 8586; (d) Y. Cai,
L. Li, Z. Wang, J. Z. Sun, A. Qin and B. Z. Tang, Chem. Commun.,
2014, 50, 8892; (e) H. Zhou, Q. Ye, W. T. Neo, J. Song, H. Yan, Y.
Zong, B. Z. Tang, T. S. A. Hor and J. Xu, Chem. Commun., 2014, 50,
1
1
2
(002) and (003) diffraction peaks of single crystals are stronger
than those in stimulated pattern (Fig. S20), meaning oriented
growth along with 001 direction. After grinding diffraction peaks
still exist and are similar to those of stimulated and fuming
pattern (Fig. 5c), which illuminates that mechanical grinding only
destroyed oriented stacking, and the molecular packing in crystal
cell did not change, so MFC behaviour for BHCT was not
observed. Time-resolved fluorescence spectra also prove it.
Fluorescence decay of ground BHCT powders is also single
exponential and the lifetime is the same as crystal one (Fig. 6c).
These spectral observations reveal that the length alkyl chains
affect the molecular packing in crystal and then determine the
responsive properties to external mechanical force stimuli.
1
3785.
(a) J. B. Birks, Photophysics of Aromatic Molecules, Wiley, London,
970; (b) A. Qin and B. Z. Tang, Aggregation-induced emission:
fundamentals, Wiley, United Kingdom, 2013.
J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu, H. S. Kwok,
X. Zhan, Y. Liu, D. Zhu and B. Z. Tang, Chem. Commun., 2001,
2
3
4
1
1
740.
Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev., 2011, 40,
361.
5
5 (a) T. E. Kaiser, H. Wang, V. Stepanenko and F. Würthner, Angew.
Chem., Int. Ed., 2007, 46, 5541; (b) P. Xue, R. Lu, G. Chen, Y.
Zhang, H. Nomoto, M. Takafuji and H. Ihara, Chem. – Eur. J., 2007,
1
3, 8231; (c) An, S. K. Kwon, S. D. Jung and S. Y. Park, J. Am.
Chem. Soc., 2002, 124, 14410.
2
5
Conclusions
6 (a) Y. E, X. Ma, Y. Zhang, Y. Zhang, R. Duan, H. Ji, J. Li, Y. Che and J.
Zhao, Chem. Commun., 2014, 50, 13596; (b) T. Qin, G. Zhou, H.
Scheiber, R. E. Bauer, M. Baumgarten, C. E. Anson, E. J. W. List
and K. Müllen, Angew. Chem., Int. Ed., 2008, 47, 8292; (c) B. Li, J.
Li, Y. Fu, and Z. Bo, J. Am. Chem. Soc., 2004, 126, 3430; (d) P. Xue,
B. Yao, J. Sun, Z. Zhang and R. Lu, Chem. Commun., 2014, 50,
Three carbazole-based terephthalate derivatives were designed
and synthesized. Due to steric hindrance, molecules adopt non-
coplanar conformation and emit strong fluorescence in both
solution and solid state. The fluorescence quantum yield of
compounds with two octyl and hexadecyl groups reaches 0.98
and 0.95, respectively. Moreover, compounds with ethyl and
octyl groups exhibited mechanofluorochromic behaviour, but the
presence of hexadecyl groups did not lead to the fluorescence
change under mechanical force stimuli. The crystal structures and
time-resolved fluorescence spectra suggest that the difference in
the length of alkyl chain affect molecular packing and then
determines their response to external stimuli. Because of strong
fluorescence in solution and solid, we believe that they can be
used as luminescent materials and sensors.
1
0284.
3
0
7
8
9
(a) P. Xue, B. Yao, J. Sun, Z. Zhang, K. Li, B. Liu, R. Lu, Dyes
Pigment, 2015, 112, 255; (b) J. Gierschner and S. Y. Park, J. Mater.
Chem. C, 2013, 1, 5818; (c) D. Yan, A. Delori, G. O. Lloyd, T.
Friscic, G. M. Day, W. Jones, J. Lu, M. Wei, D. G. Evans and X.
Duan, Angew. Chem., Int. Ed., 2011, 50, 12483.
(a) X. Du and Z. Y. Wang, Chem. Commun., 2011, 47, 4276; (b) J. Shi,
N. Chang, C. Li, J. Mei, C. Deng, X. Luo, Z. Liu, Z. Bo, Y. Q. Dong
and B. Z. Tang, Chem. Commun., 2012, 48, 10675; (c) B. K. An, S.
K. Kwon, S. D. Jung and S. Y. Park, J. Am. Chem. Soc., 2002, 124,
3
5
1
4410.
(a) M. Zheng, M. Sun, Y. Li, J. Wang, L. Bu, S. Xue, W. Yang, Dyes
Pigment, 2014, 102, 29; (b) Z. Zhao, J. W. Y. Lam and B. Zhong
Tang, Soft Matter, 2013, 9, 4564; (c) P. Xue, B. Yao, Y. Zhang, P.
Chen, K. Li, B. Liu, and Ran Lu, Org. Biomol. Chem., 2014, 12,
4
0
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21103067, and 21374041), the
Youth Science Foundation of Jilin Province (20130522134JH),
the Open Project of the State Key Laboratory of Supramolecular
Structure and Materials (SKLSSM201407), the Open Project of
State Laboratory of Theoretical and Computational Chemistry
7
110; (c) J. He, B. Xu, F. Chen, H. Xia, K. Li, L. Ye and W. Tian, J.
Phys. Chem. C, 2009, 113, 9892; (d) T. Han, Y, Zhang, X. Feng, Z.
Lin, B. Tong, J. Shi, J. Zhi and Y. Dong, Chem. Commun., 2013, 49,
7
049.
4
5
10 M. Li, Y. Niu, X. Zhu, Q. Peng, H. Lu, A. Xia and C. Chen, Chem.
Commun., 2014, 50, 2993.
1 (a) M. Shimizu, R. Kaki, Y. Takeda, T. Hiyama, N. Nagai, H.
1
(K2013-02).
Yamagishi, and H. Furutani, Angew. Chem. Int. Ed., 2012, 51, 4095;
(b) M. Shimizu, Y. Takeda, M. Higashi, T. Hiyama, Angew. Chem.
Int. Ed., 2009, 48, 3653.
Notes and references
12 N. Cocherel, C. Poriel, J. Rault-Berthelot, N. Audebrand, A. M. Z.
Slawin and L. Vignau, Chem. Eur. J., 2008, 14, 11328–11342.
a
5
5
0
5
State Key Laboratory of Supramolecular Structure and Materials,
1
1
3 G. Liu, F. Wang, S. Chaunchaiyakul, Yukie Saito, A. K. Bauri, ΙΙ
T.Kimura, Yuji Kuwahara, and N. Komatsu, J. Am. Chem. Soc.,
College of Chemistry, Jilin University, Changchun, P. R. China.
E-mail: xuepengchong@jlu.edu.cn, luran@mail.jlu.edu.cn
2
013, 135, 4805–4814.
b
Key Laboratory of Functional Inorganic Material Chemistry (MOE),
4 D. Zhao, G. Li, D. Wu, X. Qin, P. Neuhaus, Y. Cheng, S. Yang, Z. Lu,
School of Chemistry and Materials Science, Heilongjiang University, No.
74, Xuefu Road, Nangang District, Harbin, P. R. China
X. Pu, C. Long, and J. You, Angew. Chem. Int. Ed., 2013, 52, 13676.
6
|
Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

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Page 7 of 8
Journal of Materials Chemistry C
1
1
5 X. Zhao, P. Xue, K. Wang, P. Chen, P. Zhang and R. Lu, New J.
Chem., 2014, 38, 1045.
6 (a) Y. Zhang, D. Li, Y. Li and J. Yu, Chem. Sci., 2014, 5, 2710; (b) X.
Shen, Y. Wang, E. zhao, W. Yuan, Y. Liu, P. Lu, A. Qin, Y. Ma, J.
Sun and B. Z. Tang, J. Phys. Chem. C, 2013, 117, 7334.
1
7 (a) R. Hu, E. Lager, A. Aguilar-Aguilar, J. Liu, J. W. Y. Lam, H. H. Y.
Sung, I. D. Williams, Y. Zhong, K. S. Wong, E. Peña-Cabrera and B.
Z. Tang, J. Phys. Chem. C, 2009, 113, 15845; (b) Y. Gong, Y. Zhang,
W. Z. Yuan , J. Z. Sun, and Y. Zhang, J. Phys. Chem. C, 2014, 118,
1
0998; (c) P. Galer, R. C. Korošec, M. Vidmar, and B. Šket, J. Am.
Chem. Soc., 2014, 136, 7383.
1
1
8 M. Koenig, T. Torres, V. Barone, G. Brancato, D. M. Guldi and G.
Bottari, Chem. Commun., 2014, 50, 12955.
9 (a) X. Y. Shen, Y. J. Wang, H. Zhang, A. Qin, J. Z. Sun and B. Z.
Tang, Chem. Commun., 2014, 50, 8747; (b) S. Kaur, A.Gupta, V.
Bhalla and M. Kumar, J. Mater. Chem. C, 2014, 2, 7356.
0 T. Harada and H. Yoshimura, Phys. Chem. Chem. Phys., 2014, 16,
2
2
2
1
4947.
1 J. Chen, C. C. W. Law, J. W. Y. Lam, Y. Dong, S. M. F. Lo, I. D.
Williams, D. Zhu, and B. Z. Tang, Chem. Mater., 2003, 15, 1535.
2 Y. Dong, B. Xu, J. Zhang, X. Tan, L. Wang, J. Chen, H. Lv, S. Wen,
B. Li, L. Ye, B. Zou and W. Tian, Angew. Chem., Int. Ed., 2012, 51,
1
0782.
2
2
2
3 G. Hong, C. Qian, P. Xue, X. Liu, Q. Wang, M. Liu, P. Gong and R.
Lu, Eur. J. of Org. Chem. 2014, 28, 6155.
4 P. Xue, P. Chen, J. Jia, Q. Xu, J. Sun, B. Yao, Z. Zhang and R. Lu,
Chem. Commun., 2014, 50, 2569.
5 (a) Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu and J.
Xu, Chem. Soc. Rev., 2012, 41, 3878; (b) X. Zhang, Z. Chi, Y.
Zhang, S. Liu and J. Xu, J. Mater. Chem. C, 2013, 1, 3376; (c) Y.
Sagara and T. Kato, Nat. Chem., 2009, 1, 605; K. Ariga, T. Mori and
J. P. Hill, Adv. Mater., 2012, 24, 158; (d) Y. Sagara, T. Mutai, I.
Yoshikawa, and K. Araki, J. Am. Chem. Soc., 2007, 129, 1520; (e) S.
Yamaguchi, I. Yoshikawa, T. Mutaia and K. Araki, J. Mater. Chem.,
2
012, 22, 20065; (f) S. Yagai, S. Okamura, Y. Nakano, M. Yamauchi,
K. Kishikawa, T. Karatsu, A. Kitamura, A. Ueno, D. Kuzuhara, H.
Yamada, T. Seki and H. Ito, Nat. Commun. 2014, 5, 4013.
2
2
6 (a) X. Luo, W. Zhao, J. Shi, C. Li, Z. Liu, Z. Bo, Y. Q. Dong, and B. Z.
Tang, J. Phys. Chem. C, 2012, 116, 21967; (b) M. Zheng, D. T.
Zhang, M. X. Sun, Y. P. Li, T. L. Liu, S. F. Xue and W. J. Yang, J.
Mater. Chem. C, 2014, 2, 1913; (c) L. Bu, M. Sun, D. Zhang, W. Liu,
Y. Wang, M. Zheng, S. Xue and W. Yang, J. Mater. Chem. C, 2013,
1
2
, 2028; (d) Y. Sagara and Takashi Kato, Angew. Chem. Int. Ed.,
011, 50, 9128; (e) M Sase, S. Yamaguchi, Y. Sagara, I. Yoshikawa,
T. Mutai and K. Araki, J. Mater. Chem., 2011, 21, 8347.
7 (a) Z. Zhang, P. Xue, P. Gong, G. Zhang, J. Peng and R. Lu, J. Mater.
Chem. C, 2014, 2, 9543; (b) P. Xue, B. Yao, J. Sun, Q. Xu, P. Chen,
Z. Zhang, and R. Lu, J. Mater. Chem. C, 2014, 2, 3942; (c) W. Liu, Y.
Wang, M. Sun, D. Zhang, M. Zheng and W. Yang, Chem. Commun.,
2
013, 49, 6042; (d) H. Li, Z. Chi, X. Zhang, B. Xu, S. Liu, Y. Zhang
and J. Xu, Chem. Commun., 2011, 47, 11273; (e) R. Misra, T. Jadhav,
B. Dhokale and S. M. Mobin, Chem. Commun., 2014, 50, 9076; (f) Q.
Qi, X. Fang, Y. Liu, P. Zhou, Y. Zhang, B. Yang, W. Tian and S. X.
Zhang, Rsc. Adv., 2013, 3, 16986.
2
8 (a) X. Zhang, Z. Chi, H. Li, B. Xu, X. Li, W. Zhou, S. Liu, Y. Zhang
and J. R. Xu, Chem.–Asian J., 2011, 6, 808.B. Xu, Z. Chi, J. Zhang,
X. Zhang, H. Li, X. Li, S. Liu, Y. Zhang and J. Xu, Chem.–Asian J.,
2
011, 6, 1470.
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DOI: 10.1039/C5TC00267B
Graphic Abstract
Carbazole-based terephthalate derivatives could emit strong fluorescence in both solution and solid
state, and the emission response to force stimuli in solid state could be controlled by the length of alkyl
chain.