Mechanochromism and aggregation induced emission in benzothiazole substituted tetraphenylethylenes: A structure function correlation

Article abstract of DOI:10.1039/c5ra04881h

The donor-acceptor benzothiazole substituted tetraphenylethylenes (BT-TPEs) 3a-3c were designed and synthesized to examine the effect of the linkage between the BT and the TPE unit on the photophysical, aggregation induced emission (AIE) and mechanochromi

Full text of DOI:10.1039/c5ra04881h

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Mechanochromism and aggregation induced
emission in benzothiazole substituted
tetraphenylethylenes: a structure function
correlation†
Cite this: RSC Adv., 2015, 5, 29878
Thaksen Jadhav, Bhausaheb Dhokale, Shaikh M. Mobin and Rajneesh Misra*
The donor–acceptor benzothiazole substituted tetraphenylethylenes (BT-TPEs) 3a–3c were designed and
synthesized to examine the effect of the linkage between the BT and the TPE unit on the photophysical,
aggregation induced emission (AIE) and mechanochromic properties. The syntheses of BT-TPEs 3a–3c
were achieved by the Pd-catalyzed Suzuki cross-coupling reaction of bromobenzothiazoles 1a–1c with
4
-(1,2,2-triphenylvinyl)phenylboronic acid pinacol ester (2). The study showed that their photophysical,
AIE and mechanochromic properties are dependent on the linkage between the BT and the TPE unit
ortho, meta, and para). The meta isomer 3b shows the highest grinding induced spectral shift (51 nm)
(
whereas the ortho isomer 3a shows the lowest grinding induced spectral shift (9 nm). The single crystal
X-ray structures reveal the highly twisted conformation and tight packing in BT-TPE 3a compared to 3b.
The thermogravimetric analysis of BT-TPEs shows good thermal stability. The computational study
reveals the donor–acceptor nature of the BT-TPEs. The structure–function correlation indicates that the
mechanochromic and aggregation induced emission (AIE) properties were dependent on the linkage
between BT and TPE.
Received 19th March 2015
Accepted 23rd March 2015
DOI: 10.1039/c5ra04881h
www.rsc.org/advances
7
applications. On the other hand the propeller shaped tetra-
Introduction
phenylethylene (TPE) is weak electron donor and exhibits
8
The research on small organic molecules, showing response to aggregation induced emission (AIE). Recently BT substituted
9
environmental stimuli in the solid state has gained momentum TPEs were reported for their mechanochromic behavior. Our
due to their applications in sensors, displays, uorescent group was interested to explore the effect of linkage between
1
probes and information storage devices. Mechanochromism is TPE and BT unit on the donor–acceptor strength, AIE, and
the response of a material toward mechanical strain, which mechanochromic property of BT-TPE isomers. We have
2
causes changes in their absorption or emission properties. The designed and synthesized three donor–acceptor isomers (ortho,
literature reveals there is a lack of systematic study, to under- meta, and para) BT-TPE 3a–3c and explored their photophysical
stand the exact mechanism and principle of mechanochrom- and mechanochromic properties.
ism. The effect of end groups, alkyl chain length, their position,
hydrogen bonding and strength of donor–acceptor interaction
in different uorophores on mechanochromism have been Result and discussion
3–6
explored.
The mode of molecular packing in the crystal
The syntheses of BT-TPEs 3a–3c are shown in Scheme 1. The
bromo-benzothiazoles 1a–1c were synthesized by the conden-
sation reaction of 2-amino thiophenol and (ortho/meta/para)-
bromobenzaldehyde respectively in the presence of ammonium
chloride and acetic acid. The Suzuki cross-coupling reaction of
benzothiazoles 1a–1c with 4-(1,2,2-triphenylvinyl)phenyl-
structure, variation in the planarity, phase transition between
crystalline phase to amorphous phase are the major factors that
govern the mechanochromism.
The benzothiazole (BT) is weak electron acceptor and has
been used in donor–acceptor systems for various optoelectronic
3–6
10
3 4
boronic acid pinacol ester (2) using Pd(PPh ) as a catalyst
Indian Institute of Technology Indore, Discipline of Chemistry, Chemistry, Indore, resulted 3a–3c in 67%, 78% and 66% yields respectively. The
India. E-mail: rajneeshmisra@iiti.ac.in
purication of the BT-TPEs 3a–3c was achieved by column
†
Electronic supplementary information (ESI) available: Experimental procedures,
chromatography. All the BT-TPEs 3a–3c were well characterized
NMR spectra, UV-vis spectra, mechanochromic effect, computational data and
crystal structural data for 3a and 3b. CCDC 1044876 and 1044877. For ESI and
crystallographic data in CIF or other electronic format see DOI:
1
13
by H, C NMR and HRMS techniques. The BT-TPEs 3a, and 3b
were also characterized by single-crystal X-ray diffraction
technique.
1
0.1039/c5ra04881h
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Scheme 1 Synthesis of BT-TPEs 3a–3c.
The thermal stability of the BT-TPEs 3a–3c was investigated weakly uorescent in solution, due to free molecular rotations
ꢀ
by the thermogravimetric analysis (TGA) at heating rate of 10 C of tetraphenylethylene unit in solution resulting in nonradiative
ꢁ
1
12
min , under nitrogen atmosphere (Fig. S3†). The BT-TPEs 3a decay of excited state.
ꢀ
and 3b show the decomposition temperature at 349 C, and 3c
at 361 C for 5% weight loss.
The AIE property of BT-TPEs 3a–3c were studied by absorp-
tion and uorescence spectroscopy in THF–water mixtures with
ꢀ
The electronic absorption spectra of the BT-TPEs 3a–3c are different water fraction. The BT-TPEs 3a–3c are highly soluble in
shown in Fig. S4,† and the data are summarized in Table S2.† tetrahydrofuran (THF) and insoluble in water, by increasing the
The BT-TPEs 3a–3c exhibit absorption band between 250–400 water fraction in the THF–water mixture, it transforms into
ꢁ
1
nm with extinction coefficient between 34 000–55 000 L M
aggregated particles, resulting change in the absorption and
ꢁ
1
cm . The BT-TPEs 3b and 3c exhibits 14 nm, and 46 nm red uorescence behavior. The BT-TPEs 3a–3c show different AIE
shied absorption compared to the TPE unit (299 nm). On the behavior (Fig. 1). The quantitative estimation of the AIE process
11
other hand 3a show similar absorption behavior like TPE. The was obtained by calculating the uorescence quantum yields for
red shi in the absorption spectra follows the order 3c > 3b > 3a, 3a–3c in the mixtures of water and THF in various proportions,
which reveals that 3c exhibits stronger electronic communica- using 9,10-diphenylanthracene as standard. In pure THF solu-
tion compared to 3a and 3b. The trend further suggests that 3a tion the BT-TPEs exhibit poor uorescence quantum yield,
has more twisted structure and large dihedral angle between the which remains constant until the molecules start aggregating
BT and TPE unit, which is evident from the single crystal X-ray signicantly (AIE effect).
structure and computational studies. The BT-TPEs 3a–3c are
Fig. 1 Fluorescence spectra of (a) 3a, (b) 3b and (c) 3c in THF–water mixtures with different water fractions (10 mM). (d) Fluorescence quantum
yields of the BT-TPEs in water–THF mixtures against different volume fractions of water.
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Fig. 2 Emission spectra of (a) 3a, (b) 3b and (c) 3c as pristine, grinded and fumed solids.
Table 1 Emission wavelengths of 3a–3c under various external stimuli aggregate states of the molecules in the THF–water mixture. The
AIE behavior was further explored by studying the absorption
l
Pristine
l
Grinded
l
Fumed
Dl
(nm)
a
spectra in the THF–water mixtures (10 mM) (Fig. S5†). The
absorption spectra of 3a, and 3b at 80% water fraction, whereas
3c at 60% water fraction started to show light scattering or Mie
effect of the nanoaggregate suspension in the THF–water
BT-TPEs
(nm)
(nm)
(nm)
3
3
3
a
b
c
478
432
458
487
483
484
478
439
460
9
51
26
13
mixtures.
a
Grinding-induced spectral shi, Dl ¼ lGrinded ꢁ lPristine
.
The mechanochromic properties of BT-TPEs 3a–3c were
studied by solid state emission spectroscopy. The emission
spectra of 3a–3c are shown in Fig. 2 and the corresponding data
are summarized in Table 1. The study reveals that mechano-
chromism was found to be dependent on the position of TPE
unit with respect to the BT unit. The BT-TPEs 3a–3c in pristine
form emits at 478 nm, 432 nm, and 458 nm, which on grinding
with mortar and pestle show red shied emission at 487 nm,
It was observed that different percentage of water fraction
was required for 3a–3c to exhibits AIE. The BT-TPEs 3a and 3b
requires more than 80% water fraction, whereas 3c requires
more than 50% water fraction (Fig. 1). The 3a in pure THF
solution show uorescence quantum yield 0.004, which was
increased to 0.368 at 95% water fraction and then slightly
decreased to 0.359 at 98% water fraction. The 3b, in pure THF
solution show uorescence quantum yield 0.005, which was
increased to 0.457 at 95% water fraction. In case of 3c, pure THF
solution show uorescence quantum yield 0.003, which was
increased to 0.484 at 70% water fraction and remains almost
constant upto 95% water fraction, and then decreased to 0.295
at 98% water fraction. The images of BT-TPEs 3a–3c in THF–
water mixture with different water fractions under UV illumi-
nation are shown in Fig. S6,† which clearly show different AIE
behavior, and can be attributed to the formation of various
483 nm, and 484 nm respectively. The BT-TPEs 3a–3c show
grinding induced spectral shi (Dl) of 9 nm, 51 nm, and 26 nm
respectively. In pristine form, the isomers 3a–3c show different
emission behavior and follows the trend as 3a > 3c > 3b, which is
same as the trend in spectral shi (Dl). The grinded BT-TPEs
3a–3c emit in the similar region. The spectral changes
induced by grinding can be restored by solvent fuming. This
mechanochromic conversion can be repeated by repeated
grinding and fuming processes as these processes do not cause
any chemical change (Fig. S7–S9, ESI†).
The solid state absorption spectra (Fig. 3) of 3a–3c were
recorded to understand the emission behavior and D–A
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Fig. 3 Solid state absorption spectra of (a) 3a, (b) 3b and (c) 3c in crystalline and its grinded form.
interaction in solid state under various external stimuli, the
The powder X-ray diffractions (PXRD) of pristine, grinded
data are summarized in Table S3.† The BT-TPE 3a show similar and fumed forms of BT-TPEs 3a–3c (Fig. 4) were studied. The
absorption behavior in pristine and grinded form with blue diffraction patterns for pristine samples show sharp peaks,
shi of 4 nm, and broadening of the shoulder at 471 nm. In case which is characteristic property of the crystalline samples. The
of 3b, pristine form shows two peaks at 398, and 444 nm, and pristine samples upon grinding show broad diffuse bands
aer grinding only one band at 455 nm was observed. The indicating amorphous nature. The grinded samples aer
pristine sample of 3c absorbs at 420 nm, which on grinding fuming with dichloromethane restore the sharp peaks, sug-
shied to 436 nm. The grinding induced red shi in solid state gesting regeneration of the crystalline nature. The PXRD study
absorption spectra indicates planarization and enhanced reveals that the morphological change from the crystalline state
conjugation.
Fig. 4 PXRD curves of 3a(a), 3b(b) and 3c(c) in synthesized, grinded and fumed form.
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Fig. 5 Crystal structure and packing diagram of 3a.
Fig. 6 Crystal structure and packing diagram of 3b.
to the amorphous state and vice versa is associated with the depict the molecular structures which show highly twisted
mechanochromism in BT-TPEs 3a–3c. conformations. In the crystal structure of 3a, two molecules
The photophysical behaviors of BT-TPEs 3a–3c were corre- exist in an asymmetric unit. The two molecules are held
ꢀ
lated with solid state packing by analyzing the single crystals of together via two mutual C(15)–H(15)/S(2) (2.942 A) and
ꢀ
3
a and 3b (details are provided in Table S1, ESI†). Fig. 5 and 6 C(58)–H(58)/S(1) (2.956 A) interactions. The torsional angles
Fig. 7 HOMO and LUMO frontier molecular orbitals of 3a–3c at the B3LYP/6-31G(d) level.
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between the benzothiazole unit and phenyl ring are 4 and 53
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ꢀ
ꢀ
Spectrophotometer. Emission spectra were taken in
a
for 3a and 3b respectively. Further, the torsional angles between uoromax-4p uorimeter from HoribaYovin (model: FM-100).
the phenyl attached to benzothiazole unit and TPE phenyl are The excitation and emission slits were 2/2 nm for the emis-
ꢀ
ꢀ
3
2 and 43 for 3a and 3b respectively. This reveals that 3a has sion measurements. All of the measurements were done at
ꢀ
highly twisted conformation than the 3b.
25 C. HRMS was recorded on Brucker-Daltonics, micrOTOF-Q
The packing diagram of 3a show extensive supramolecular II mass spectrometer. The density functional theory (DFT)
interactions. The two molecule in an asymmetric unit forms calculation were carried out at the B3LYP/6-31G(d) level in the
dimer, which extends into the sheet like structural framework Gaussian 09 program. TD-DFT calculations was performed at
along a-axis via interactions C(45)–H(45)/p(C45–C52) (2.779 the B3LYP/6-31G(d) level in dichloromethane solvent using
ꢀ
ꢀ
A), C(72)–H(72)/p(C34–C39) (3.868 A) and C(6)–H(6)/p(C8– IEFPCM formulation for solvent effect.
ꢀ
C13) (2.837 A). This sheet further extends into the complex 3-D
structural framework via interaction C(4)–H(4)/p(C22–C27)
Synthesis and characterization of 3a–3c
ꢀ
(
3.434 A) (Fig. 5). The supramolecular packing of 3b is shown in
the Fig. 6, in which a rod like framework was formed via mutual
3a. Pd(PPh ) (0.004 mmol) was added to a well degassed
3
4
ꢀ
C(6)–H(6)/p(C24) (2.853 A) interactions and weak p/p solution of 2-(2-bromophenyl)-1H-benzo[d]imidazole (2a) (0.4
ꢀ
staking interaction between C(24)/C(24) (3.360 A). The packing mmol), 5 (0.48 mmol), K
2
CO
3
(1.2 mmol) in a mixture of toluene
2
O (1.0 mL). The resulting mixture
diagram reveals tight packing in 3a than 3b.
(12 mL)/ethanol (4.0 mL)/H
ꢀ
The geometry and electronic structure of 3a–3c were inves- was stirred at 80 C for 24 h under argon atmosphere. Aer
tigated by density functional theory (DFT) calculations using the cooling, the mixture was evaporated to dryness and the residue
Gaussian 09W program. The DFT calculations were performed subjected to column chromatography on silica to yield the
at the B3LYP/6-31G(d) level of theory. The optimized struc- desired product 3a as colorless powder. The compound was
tures for 3a–3c show twisted conformation (Fig. 7). The highest recrystalized from DCM : ethanol (8 : 2) mixtures. Yield: 67.0%.
14
1
ꢀ
H NMR (400 MHz, CDCl , 25 C): d 8.01–8.07 (m, 2H), 7.83 (d,
3
occupied molecular orbitals (HOMOs) of 3a–3c are majorly
distributed on the TPE unit, whereas lowest unoccupied 1H, J ¼ 8 Hz), 7.46–7.51 (m, 3H), 7.38–7.45 (m, 2H), 6.99–7.12
1
3
ꢀ
molecular orbitals (LUMOs) are located on the benzothiazole (m, 19H) ppm; C NMR (100 MHz, CDCl
3
, 25 C): d 167.8, 152.8,
unit, indicating the donor–acceptor behavior (Fig. 7).
143.6, 143.5, 143.5, 143.3, 141.5, 141.2, 140.4, 138.1, 136.7,
1
1
32.7, 131.4, 131.3, 131.3, 130.7, 130.4, 129.9, 129.4, 127.6,
26.5, 125.9, 124.9, 123.3, 121.1, 0.00 ppm; HRMS (ESI): calcd
Conclusion
+
for C39
H
27NS: 542.1937 (M + H) , found 542.1947.
In conclusion we have designed, and synthesized three donor–
3b. Pd(PPh ) (0.004 mmol) was added to a well degassed
3 4
acceptor BT-TPEs 3a–3c by the Pd-catalyzed Suzuki coupling solution of 2-(3-bromophenyl)-1H-benzo[d]imidazole (2b) (0.4
reaction. Their structural, photophysical, aggregation induced mmol), 5 (0.48 mmol), K CO (1.2 mmol) in a mixture of toluene
O (1.0 mL). The resulting mixture
2
3
emission (AIE) and mechanochromic properties were found to (12 mL)/ethanol (4.0 mL)/H
2
ꢀ
be the function of the linkage between the TPE and the BT units. was stirred at 80 C for 24 h under argon atmosphere. Aer
The variation in the D–A interaction, molecular structure and cooling, the mixture was evaporated to dryness and the residue
solid-state packing perturb the mechanochromic behavior of subjected to column chromatography on silica to yield the
BT-TPEs 3a–3c. The results represent the structure function desired product 3b as colorless powder. The compound was
correlation on mechanochromism and will be helpful in the recrystalized from DCM : ethanol (8 : 2) mixtures. Yield: 78.0%.
1
ꢀ
development of mechano sensors.
H NMR (400 MHz, CDCl , 25 C): d 8.29 (t, 1H, J ¼ 1.6 Hz), 8.09
3
(
7
d, 1H, J ¼ 8 Hz), 8.00–8.02 (m, 1H), 7.92 (d, 1H, J ¼ 8 Hz), 7.66–
.69 (m, 1H), 7.49–7.54 (m, 2H), 7.38–7.46 (m, 3H), 7.04–7.17
Experimental details
General methods
1
3
ꢀ
(m, 17H) ppm; C NMR (100 MHz, CDCl
3
, 25 C): d 168.4, 154.5,
1
44.03, 143.99, 143.7, 142.0, 141.7, 140.7, 138.2, 135.4, 134.4,
Chemicals were used as received unless otherwise indicated. All 132.2, 131.74, 131.68, 129.8, 129.7, 128.2, 128.1, 128.0, 126.93,
oxygen or moisture sensitive reactions were performed under 126.86, 126.81, 126.73, 126.7, 126.2, 125.6, 123.6, 122.0, 0.00
1
13
+
nitrogen/argon atmosphere. H NMR (400 MHz), and C NMR ppm; HRMS (ESI): calcd for C H NS: 542.1937 (M + H) , found
3
9
27
(
100 MHz) spectra were recorded on the Bruker Avance (III) 400 542.1942.
1
MHz instrument by using CDCl
3
. H NMR chemical shis are
3 4
3c. Pd(PPh ) (0.004 mmol) was added to a well degassed
reported in parts per million (ppm) relative to the solvent solution of 2-(4-bromophenyl)-1H-benzo[d]imidazole (2c) (0.4
residual peak (CDCl
3
, 7.26 ppm). Multiplicities are given as: s mmol), 5 (0.48 mmol), K
2
CO
3
(1.2 mmol) in a mixture of toluene
(
singlet), d (doublet), t (triplet), q (quartet), dd (doublet of (12 mL)/ethanol (4.0 mL)/H
2
O (1.0 mL). The resulting mixture
ꢀ
doublets), dt (doublet of triplets), m (multiplet), and the was stirred at 80 C for 24 h under argon atmosphere. Aer
coupling constants, J, are given in Hz. C NMR chemical shis cooling, the mixture was evaporated to dryness and the residue
1
3
are reported relative to the solvent residual peak (CDCl , 77.36 subjected to column chromatography on silica to yield the
3
ppm). Thermogravimetric analyses were performed on the desired product 3c as colorless powder. The compound was
Metler Toledo Thermal Analysis system. UV-visible absorption recrystalized from DCM : ethanol (8 : 2) mixtures. Yield: 66.0%.
1
ꢀ
spectra were recorded on
a
Carry-100 Bio UV-visible
H NMR (400 MHz, CDCl
3
, 25 C): d 8.12 (d, 2H, J ¼ 12 Hz), 8.08
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(
d, 1H, J ¼ 8 Hz), 7.91 (d, 1H, J ¼ 4 Hz), 7.68 (d, 2H, J ¼ 12 Hz),
C. Suresh, N. Rath and S. Das, J. Phys. Chem. C, 2009, 113,
11927; (d) R. Thomas, S. Varghese and G. Kulkarni, J.
Mater. Chem., 2009, 19, 4401; (e) Q. Liu, W. Liu, B. Yao,
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and R. Misra, RSC Adv., 2014, 4, 52526.
7
.48–7.52 (m, 1H), 7.37–7.43 (m, 3H), 7.03–7.16 (m, 17H) ppm;
1
3
ꢀ
C NMR (100 MHz, CDCl , 25 C): d 168.1, 154.5, 144.0, 143.96,
3
1
1
1
43.95, 143.92, 143.51, 141.74, 140.6, 138.0, 135.4, 132.6, 132.3,
31.7, 131.68, 131.67, 128.2, 128.15, 128.07, 127.99, 127.7,
26.94, 126.89, 126.83, 126.7, 126.5, 125.5, 123.5, 122.0, 0.00
+
ppm; HRMS (ESI): calcd for C39
42.1938.
H
27NS: 542.1937 (M + H) , found
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5
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Acknowledgements
We acknowledge the support by DST, and CSIR Govt. of India,
New Delhi. T J thanks UGC, B D thanks to CSIR New Delhi for
their fellowships. We acknowledge Sophisticated Instrumenta-
tion Centre (SIC) facility, IIT Indore.
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