Structure-property relationship in multi-stimuli responsive D-A-A′ benzothiazole functionalized isomers

Article abstract of DOI:10.1039/c8tc04310h

Multichromophoric D-A-A′ molecules comprising of benzothiazole (BT) as A′, benzothiadiazole (BTD) as A and tetraphenylethylene (TPE) as D were designed and synthesized as positional isomers p-BT, m-BT and o-BT by attaching the BTD-TPE moiety ortho, meta a

Full text of DOI:10.1039/c8tc04310h

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Page 1 of 38
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DOI: 10.1039/C8TC04310H
Structureꢀproperty relationship in multiꢀstimuli responsive DꢀAꢀA′ benzothiazole
functionalized isomers.
Anupama Ekbote, Shaikh M. Mobin and Rajneesh Misra*
Department of Chemistry, Indian Institute of Technology Indore,
Indoreꢀ 453552, India
* Corresponding author
Email: rajneeshmisra@iiti.ac.in
TOC
1

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Abstract
Multichromophoric DꢀAꢀA′ molecules comprising of benzothiazole (BT) as A′,
benzothiadiazole (BTD) as A and tetraphenylethylene (TPE) as D were designed and
synthesized as positional isomers pꢀBT, mꢀBT and oꢀBT by attaching the BTDꢀTPE moiety
ortho, meta and para to the phenyl BT unit. The positional change exploited in these isomers
can influence the acceptor strength and molecular packing. Hence, a comparative study of the
photophysical and electronic properties has been carried to study the effect of position. The
pꢀBT and mꢀBT isomers were synthesized by the Suzuki crossꢀcoupling reaction of BTDꢀ
TPE with boronate esters of BT. The Stille crossꢀcoupling reaction was employed for oꢀBT.
The structural features of the isomers endowed them with solvatochromism,
mechanochromism, acidochromism and aggregation induced emission properties which were
studied using emission and absorption spectroscopy. The reversible mechanochromic
behaviour was associated with phase transition from crystalline to amorphous and was used
to develop rewritable ink free paper. The single crystal Xꢀray analysis of pꢀBT and oꢀBT
establishes that mechanochromism synergistically depends on the flexibility and twisting in
the donor and acceptor moieties. The isomers can sense trifluoroacetic acid in solution as
well as solid state. The opted strategy allows modulation of fluorescence properties making
them potential stimuli responsive materials with applications in mechanoꢀsensors, security
inks and optoelectronicꢀdevices.
2

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Introduction
The solid state materials which are susceptible to changes by external stimuli (mechanical
forces, solvent and acid vapours, heat and light) are beneficial1; following their utilization in
a range of promising applications such as optical information storage, fluorescent switches,
memory devices, deformation detectors, optical recording, fluorescent bioꢀprobes, security
systems, mechanical sensors and optoelectronic devices.2 The category of solid state
materials, that manifest change in emission as a response to mechanical force such as
grinding or rubbing are known as mechanofluorochromic (MFC) materials.3 A significant
emission in the solid state is crucial for mechanofluorochromism and is primarily determined
by the nature of molecular packing and intermolecular interactions in the solid state.4 The
response to mechanical forces is associated with phase transitions or the disturbance in the
basic molecular stacking interactions, consequently producing metastable states.5 The factors
such as effective conjugation, donorꢀacceptor interaction, and incorporation of alkyl or aryl
substituent, electronegative or heavy atoms and their position play a pivotal role in regulating
the MFC properties.6 The common organic dyes in their aggregated state quench their
emission due to the aggregation caused quenching (ACQ) which restricts their usage as MFC
materials.7 Recently, the concept of aggregation induced emission (AIE) contributed by Tang
et al. has successfully resulted in materials with enhanced emission in aggregated state. The
nonꢀplanar architecture of AIE luminogens diminishes πꢀπ stacking in aggregated state and
by the virtue of restriction of intramolecular rotations (RIR) induces extensive emission in
aggregated state.8 The ACQ fluorophores have been successfully converted into solid state
emitting materials by attaching to well known AIEgens.9 Tetraphenylethylene (TPE) is one of
the widely used AIEgen because of its easy synthesis, bulky size and propeller shape. The
TPE has been used as a weak electron donor in donorꢀacceptor systems resulting in highly
emissive solid materials with a range of applications in material chemistry and electronics. 10
3

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It is familiar that organic molecules constituting of donor and acceptor fragments
allowing fine tuning of the MFC properties. Organic molecules with DꢀA, DꢀAꢀD, DꢀπꢀA etc.
serve as excellent candidates for MFC with applications in optoelectronics.11 Benzothiazole
(BT) and benzothiadiazole (BTD) are well established electron acceptors and have been
successfully explored for the synthesis of AIE active MFC materials.12 Both BT and BTD
based materials have been successfully employed in organic light emitting diodes (OLEDs)
and organic solar cells (OSCs) and can be combined for potential applications in a
multichromophoric assembly.13
Our group is interested in design and synthesis of MFC materials. We have reported BTꢀTPE
and BTDꢀTPE for MFC and successfully tuned their AIE and MFC behaviour and have
shown that MFC behaviour can be tuned by controlling the strength of donor or acceptor. 11(c),
14
Herein, we have designed and synthesized series of multichromophoric isomers pꢀBT, mꢀ
BT and oꢀBT based on BT, BTD and TPE with an aim of achieving high solid state emission
and MFC behaviour. In this design strategy, we aim to control the acceptor strength and
molecular packing by changing the position of BTDꢀTPE unit ortho, meta and para to BT
unit. The variation of the position with respect to BT unit is expected in modulating the
acceptor strength and molecular packing. The BT unit was selected because of moderate
electron accepting behaviour and presence of two hetero atoms “S and N”, as Sꢀatom is well
known for its strong supramolecular interactions. The comparative study of the change in the
photophysical, AIE and MFC properties of isomers with position has been carried out using
crystal analysis and theoretical calculations. The photophysical, thermal, electrochemical,
solvatochromic, mechanochromic, AIE and acidochromic properties of pꢀBT, mꢀBT and oꢀ
BT have been investigated. The isomers exhibit enhanced emission on aggregation with
distinct MFC and acid sensing.
4

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Results and discussions
Synthesis
The pꢀBT, mꢀBT and oꢀBT are DꢀAꢀA′ type molecules and were designed to evaluate the
change in the photophysical and electronic properties as a result of altering the position of
attachment of the BTDꢀTPE unit to the PhꢀBT moiety. The synthetic route to the isomers pꢀ
BT and mꢀBT is given in Scheme 1 and for oꢀBT is as outlined in Scheme 2.
The benzothiadiazoleꢀtetraphenylethylene (BTDꢀTPE) moiety was attached at the 4ꢀposition,
3ꢀposition and 2ꢀposition of the phenyl ring of benzothiazole to furnish the para (pꢀBT), meta
(mꢀBT) and ortho (oꢀBT) isomers respectively. To achieve this, firstly the
tetraphenylethylene
was
selectively
coupled
to
one
side
of
4,7ꢀ
dibromobenzo[c][1,2,5]thiadiazole using the Suzuki crossꢀcoupling reaction to yield
intermediate BTDꢀTPE (Scheme S1).14 Further, the benzothiazole intermediates BT 4 and
BT
5
required for coupling with BTDꢀTPE were synthesized. 2ꢀ(4ꢀ
bromophenyl)benzo[d]thiazole (BrBT1), 2ꢀ(3ꢀbromophenyl)benzo[d]thiazole (BrBT2) and
2ꢀ(2ꢀbromophenyl)benzo[d]thiazole (BrBT3) were obtained by the condensation reaction of
2ꢀaminothiphenol and respective bromoꢀbenzaldehydes using literature procedures.15 The
Miyaura borylation reaction of BrBT1 and BrBT2 with bis(pinacolato)diboron using [1,1'ꢀ
bis(diphenylphosphino)ferrocene]palladium(II) dichloride as catalyst generated the boronate
esters of benzothiazole at 4 position (BT4) and 3 position (BT5) respectively. 16 However, the
5

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Miyaura Borylation reaction of BrBT3 did not proceed successfully to yield the boronate
ester; alternatively the Stille intermediate oꢀTBTꢀBT6 was synthesized. The inꢀsitu anion
formation of BrBT3 using nꢀBuLi followed by addition of tributyltin chloride supplied the
intermediate oꢀTBTꢀBT6 in 71% yield.
The Suzuki cross coupling reaction of BTDꢀTPE with BT4 and BT5 using Pd(PPh3)4 as
catalyst afforded para and meta isomers pꢀBT and mꢀBT with a yield of 73% and 69%
respectively. The Stille crossꢀcoupling reaction of BTDꢀTPE with oꢀTBTꢀBT6 and
Pd(PPh3)4 as a catalyst resulted in the ortho isomer oꢀBT with a yield of 51%. All the target
compounds (pꢀBT, mꢀBT and oꢀBT) and the newly synthesized intermediate oꢀTBTꢀBT6
1
13
were characterized with H, C NMR spectra and HRꢀESI mass spectroscopy. The para (pꢀ
BT) and ortho (oꢀBT) isomers produced good crystals for single crystal Xꢀray analysis. The
synthesized isomers are highly soluble in common organic solvents such as dichloromethane
(DCM), tetrahydrofuran (THF), chloroform etc., although insoluble in ethanol, methanol,
water and acetonitrile.
6

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Scheme 1 Synthetic routes for pꢀBT and mꢀBT.
Scheme 2 Synthetic route for oꢀBT
Thermogravimetric analysis
High thermal stability is essential for molecules to be utilized in solid state applications. The
thermal properties of pꢀBT, mꢀBT and oꢀBT were evaluated by thermogravimetric analysis
(TGA). The TGA curves are shown in Fig. S1 and the decomposition temperature (Td) are
listed in Table 1. The isomer pꢀBT exhibits the highest decomposition temperature of 405 ºC
corresponding to 5% weight loss. The isomer mꢀBT and oꢀBT show thermal decomposition
temperatures of 398 ºC and 363 ºC respectively for 5% weight loss. The lower twisting in pꢀ
BT provides an extended conjugation leading to high thermal stability as compared to mꢀBT
and oꢀBT.
Photophysical properties
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The optical properties of pꢀBT, mꢀBT and oꢀBT were analysed in THF by UVꢀvis absorption
(Fig. 1 (A)) and fluorescence spectroscopy (Fig. 1 (B)) and the results are compiled in Table
1. The isomers possess two major absorption bands arising from πꢀπ* transitions in lower
wavelength region (250–330nm) and intramolecularꢀcharge transfer (ICT) transitions
8
7
6
5
4
3
2
1
0
(A)
(B)
p-BT
m-BT
o-BT
p-BT
m-BT
o-BT
1.0
0.8
0.6
0.4
0.2
0.0
250
300
350
400
450
500
400
450
500
550
600
650
700
750
Wavelength (nm)
Wavelength (nm)
between the donorꢀacceptor strands in higher wavelength region (390–415nm) with a high
extinction coefficient. The emission maxima of the isomers is in the range of 555–575nm
with blue shifted emission for oꢀBT as a result of the decrease in the conjugation induced by
the highly twisted structure.
Fig. 1 (A) UVꢀvis absorption spectra and (B) normalized emission spectra of pꢀBT, mꢀBT
and oꢀBT in THF (2 × 10ꢀ5 M).
Table 1 Photophysical and thermal properties of pꢀBT, mꢀBT and oꢀBT.
Compounds λab[nm] (ε[Lmolꢀ1cm–1])a
λem (nm)b
Td (°C)c
321(45939), 411(25544)
570
405 °C
pꢀBT
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300(75575), 402(27525)
567
559
398 °C
363 °C
mꢀBT
oꢀBT
279(28335)
307(28838), 396(10068)
a,b Recorded in tetrahydrofuran, c Decomposition temperature
Density functional theory calculations:
The density functional theory (DFT) calculations were conducted at the B3LYPꢀ6ꢀ31G(d,p)
level for further accurate examination of the ground state electronic structures and geometries
of the isomers pꢀBT, mꢀBT and oꢀBT.17 The calculated frontier molecular orbitals are
depicted in Fig. 2 and the values are compiled in Table 2. The DFT optimized structures of
the pꢀBT and oꢀBT isomers are in well accordance with the crystal structures. The isomers
have propeller shaped geometry for TPE moiety and twisted structures with highest twisting
in oꢀBT. The highest occupied molecular orbital (HOMO) is localized on the weakly
donating TPE moiety whereas the lowest unoccupied molecular orbital (LUMO) resides on
the benzothiadiazole moiety which confirms its higher electron accepting nature over
benzothiazole. The HOMOꢀLUMO gap for pꢀBT, mꢀBT and oꢀBT are 2.79 eV, 2.89 eV and
2.99 eV respectively. The HOMO and LUMO are well separated allowing a distinct
intramolecular charge transfer (ICT) from the donor to acceptor. In order to achieve
correlation with the absorption studies, the timeꢀdependent density functional theory (TDꢀ
DFT) calculations were done for the isomers at the CAMB3LYPꢀ6ꢀ31G (d,p) level. Fig. S2
contains the calculated orbitals and the related computed vertical transitions, their oscillator
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strengths, and configurations are tabulated in Table S1. The absorption spectra are in good
agreement with the excited state calculations and affirm an ICT transition.
Fig. 2 Energy level diagram showing the HOMO and LUMO energy levels of
and ꢀBT as determined at the B3LYP/6ꢀ31G(d,p) level.
pꢀBT, mꢀBT
o
Electrochemical properties
The energy levels and electrochemical properties were examined by employing cyclic
voltammometry using tetrabutylammonium hexafluorophosphate (TBAPF6) as supporting
electrolyte for a solution of the isomers in DCM. Fig. S3 shows the irreversible nature of the
cyclic voltammograms and Table 2 lists the corresponding data for the electrochemical study
of pꢀBT, mꢀBT and oꢀBT. The HOMO and LUMO values were calculated from the onset
value of oxidation or reduction wave (Eonset) utilizing the equation HOMO/LUMO = ꢀ (Eonset
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+ 4.4) eV.18 The values for HOMO levels of
and ꢀ5.58 eV respectively and the values of LUMO levels are ꢀ3.62 eV, ꢀ3.62 eV and ꢀ3.68
eV respectively. The electrochemical HOMOꢀLUMO gaps for ꢀBT, ꢀBT and ꢀBT are
1.92 eV, 1.94 eV and 1.90 eV respectively. The CV graphs exhibit one oxidation wave and
pꢀBT, mꢀBT and oꢀBT are ꢀ5.54 eV, ꢀ5.56 eV
p
m
o
Electrochemical properties
HOMO a LUMO a
DFT calculations
HOMOꢀLUMO
HOMO b LUMO b HOMOꢀLUMO
Compounds
eV
eV
gap eV
eV
eV
gap eV
ꢀ5.54
ꢀ5.56
ꢀ3.62
ꢀ3.62
1.92
1.94
1.90
ꢀ5.31
ꢀ5.30
ꢀ5.24
ꢀ2.52
ꢀ2.41
ꢀ2.25
2.79
2.89
2.99
p
ꢀBT
ꢀBT
m
ꢀ5.58
ꢀ3.68
oꢀBT
two reduction waves corresponding to a donor TPE and acceptors BTD and BT respectively.
Table 2 Electrochemical properties and DFT calculations of pꢀBT, mꢀBT and oꢀBT.
a
b
Calculated from CV: Reference electrodeꢀ Ag/AgCl Theoretical values at B3LYP/6ꢀ
31g(d,p) level.
Solvatochromism
The intramolecular charge transfer (ICT) transition is greatly influenced by change in solvent
polarity and is a distinct feature in donorꢀacceptor molecule.19 The target isomers are
comprised of a DꢀAꢀA′ structural unit owing to which the result of change in the solvent
polarity was investigated using electronic absorption spectroscopy (Fig. S4) and fluorescence
spectroscopy (Fig. 3) in a range of solvents from nonꢀpolar to polar (cyclohexane, toluene,
diethyl ether, 1,4ꢀdioxane, THF, chloroform, dichloromethane(DCM), acetone and N,Nꢀ
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dimethylformamide(DMF)). In comparison to the fluorescence spectra, the absorption spectra
exhibited minor changes on changing the polarity of solvent which suggests the presence of
more polarized excited states than the ground electronic states. The
pꢀBT and mꢀBT isomers
manifest a bright green fluorescence in cyclohexane at 515 nm and 513 nm respectively
whereas the
The isomers
o
ꢀBT isomer emits at 504 nm in cyclohexane with a bluish green fluorescence.
ꢀBT, ꢀBT and ꢀBT emit a yellow fluorescence in moderate polar solvents
p
m
o
like 1,4ꢀdioxane, THF, CHCl3. In polar solvent like DMF, a bright orange fluorescence was
observed at 626 nm, 625 nm and 616 nm respectively enabling the isomers to achieve a wide
range of colours in the visible region. The isomers pꢀBT, mꢀBT and oꢀBT exhibit similar
changes in emission on gradually increasing the solvent polarity and a bathochromic shift of
111 nm, 112 nm and 112 nm respectively was observed (Fig. 3). The transfer of electron
from donor to acceptor enhances the dipole moment creating a polarized excited state which
can be stabilized by the reorganization of polar solvent molecules resulting in reduced energy
of the excited state leading to noteworthy bathochromic changes in the emission spectra.
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Fig. 3 Emission spectra of the isomers (A)
pꢀBT, (B) mꢀBT and (C) oꢀBT (excitation
wavelength or λex=370 nm) in solvents of different polarities. (Photographs taken under 365
nm illumination.)
Aggregation induced emission
The isomers were designed with tetraphenylethylene core to accomplish solid state emission
induced by aggregation. The formation of nanoꢀaggregates was initiated by progressive
addition of nonꢀsolvent (water) to the solvent system (DMF) and the AIE character of the
isomers was measured using fluorescence (Fig. 4) and absorption spectroscopy (Fig. S5). The
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isomers pꢀBT, mꢀBT and oꢀBT emit at 626 nm, 627 nm and 618 nm respectively with a pink
fluorescence at 0% water fraction or pure DMF solution due to significant ICT transition,
unlike other TPE containing molecules that are nonꢀemissive in solution. The relative
quantum yields (QY) for
respectively. The ꢀBT isomer shows reduction of intensity upto 40% while
isomer exhibit reduction of intensity upto 30% water fraction. This behaviour can be
attributed to the stabilized CT states resulting from increase in solvent polarity. The ꢀBT,
BT and ꢀBT isomers display sharp intensity enhancement and a blue shift in wavelength
(bright green emission) above 40ꢀ50% water fraction due to formation of nanoꢀaggregates.
The relative QY for ꢀBT, ꢀBT and ꢀBT at 90% water fraction are 0.60, 0.34 and 0.26
p
ꢀBT,
m
ꢀBT and
o
ꢀBT at 0% water fraction are 0.37, 0.20 and 0.21
o
m
ꢀBT and ꢀBT
o
p
mꢀ
o
p
m
o
respectively. It can be concluded that the emission at lower water percentage is governed by
ICT; however greatly influenced by aggregate formation at higher water percentage. Above
70% water fraction, a slight drop in emission intensity is observed due to the formation of the
largeꢀsized aggregates, which decreases the effective dye concentration in the solution and
are less exposed to radiation as compared to smaller aggregates.20 The Mie scattering effect
of light can be witnessed in the electronic absorption spectra (Fig. S5) of pꢀBT, mꢀBT and oꢀ
BT as a consequence of nanoꢀaggregate formation at 40% water fraction for the isomers.21
The recorded AIE phenomenon can be visualized from the photographs given in Fig. S6.
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120
180
(A)
(B)
Water Vol %
0%
Water Vol %
0%
110
100
90
80
70
60
50
40
30
20
10
0
160
140
120
100
80
10%
20%
30%
40%
50%
60%
70%
80%
10%
20%
30%
40%
50%
60%
70%
80%
90%
90%
60
40
20
0
450
500
550
600
650
700
750
500
550
600
650
700
750
Wavelength (nm)
Wavelength (nm)
70
(C)
Water vol %
0%
(D)
p-BT
m-BT
o-BT
2.0
1.5
1.0
0.5
0.0
60
50
40
30
20
10
0
10%
20 %
30%
40%
50%
60%
70%
80%
90%
450
500
550
600
650
700
750
0% 10% 20% 30% 40% 50% 60% 70% 80% 90%
Wavelength (nm)
Water Vol %
Fig. 4 Emission spectra of (A)
p
ꢀBT, (B) mꢀBT and (C) oꢀBT in DMFꢀwater mixtures (0%–
90% water), (D) Plot of fluorescence intensity vs. % of water fraction (fw). Luminogen
concentration: 10 µM; intensity calculated at max.
λ
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Single crystal Xꢀray analysis
Fig. 5 Crystal structure of
depicted by red centroid, N···π and S···π depicted by yellow centroid) in the crystal packing
diagram of ꢀBT 1 (1ꢀD ladder), (B) Intermolecular interactions (CꢀH···π depicted by red and
pꢀBT 1 and pꢀBT 2; (A) Intermolecular interactions in (CꢀH···π
p
yellow centroid, N···π and S···π depicted by blue centroid, CꢀH···N weak hydrogen bonding
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interactions depicted by purple lines) in the crystal packing diagram of
framework).
p
ꢀBT 2 (2ꢀD
Fig. 6 (A) Crystal structure of oꢀBT. (B) Crystal packing diagram of oꢀBT (antiꢀparallel
arrangement and 3ꢀD framework) (C) Intermolecular interactions (CꢀH···π depicted by red
centroid and green line, CꢀH···N weak hydrogen bonding interactions depicted by purple
lines) in the crystal packing diagram of oꢀBT.
The successful interpretation of the observed properties can be achieved by deducing the
structural information achieved from single crystal Xꢀray analysis. The slow evaporation of
dichloromethane (DCM)/hexane and chloroform/hexane solvent mixtures was used to grow
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good quality crystals for
the crystallization of ꢀBT isomer in different batches using the same solvent system by slow
evaporation serendipitously resulted in two different crystal forms namely ꢀBT 1 and ꢀBT
2. The ꢀBT isomer belongs to monoclinic crystal system and P21/n space group. The two
crystalline forms of ꢀBT isomer belong to the triclinic crystal system with Pꢀ1 space group
but differ from each other in orientation of the TPE moiety and degree of twisting resulting in
different crystal packing diagrams. The difference in the crystal packing of ꢀBT 1 and ꢀBT
2 results in interesting emission properties. The crystal structures of ꢀBT 1, ꢀBT 2 (Fig. 5)
and ꢀBT (Fig. 6 (A)) revealed twisted geometries for the tetraphenylethylene moiety as
predicted and extensive twisting was observed in ꢀBT as compared to ꢀBT 1 and ꢀBT 2.
The unit cell of ꢀBT crystal (Fig. S7 (C)) consists of four molecules arranged antiꢀparallel
with multiple intermolecular interactions in contrast to the unit cell of ꢀBT 1 and ꢀBT 2
(Fig. S7 (A, B)) having absence of interactions between two antiꢀparallel arranged molecules.
The crystal packing diagram of ꢀBT 1 (Fig. 5 (A)) reveals C(22)–H(22)···π (red centroidꢀ
pꢀBT (fluorescent green) and oꢀBT (yellow) respectively. However,
p
p
p
o
p
p
p
p
p
o
o
p
p
o
p
p
p
C34–C35–C36–C37–C38–C39, 3.302 Å) and loneꢀpair···π i.e. N(2)···π(3.859 Å) and
S(1)···π (3.738 Å) (π depicted by yellow centroidꢀ C40–C41–C42–C43–C44–C45) leading to
1ꢀD ladder type formation. The
intermolecular interactions leading to a densely packed 2ꢀD framework. The interactions
observed in ꢀBT 2 are C(20)–H(31)···π (red centroidꢀC20–C21–C22–C23–C24–C25, 3.211
pꢀBT 2 (Fig. 5 (B)) contrarily displays extensive
p
Å), C(35)–H(24)···π (yellow centroidꢀC34–C35–C36–C37–C38–C39, 3.226 Å), loneꢀ
pair···π i.e. N(1)···π(3.842 Å) and S(1)···π (3.729 Å) (π depicted by blue centroidꢀ C40–
C41–C42–C43–C44–C45) and weak Hꢀbonding with N of the benzothiazole moiety Cꢀ
H(18)···N(3) 2.748 Å. The crystal packing diagram of
oꢀBT (Fig. 6 (C)) confirms Cꢀ
H(38)···π 3.118 Å, CꢀH(32)···π 3.730 Å (πꢀred centroid) and weak Hꢀbonding with N of the
benzothiazole moiety CꢀH(12)···N(1) 2.686 Å. The presence of abundant intermolecular
18

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interactions in
oꢀBT constructs a head to tail driven tightly packed 3ꢀD framework which
could possibly make it resistant to changes caused by mechanical stimuli.19 The top view and
side views of the crystal packing diagrams for
Fig. S9 and Fig. S10 respectively.
pꢀBT 1, pꢀBT 2 and oꢀBT are given in Fig. S8,
Mechanochromism
The
p
ꢀBT,
m
ꢀBT and
o
ꢀBT were predicted to exhibit mechanoꢀresponsive solid state
meta
emission taking in account the nonꢀplanar framework and AIE nature of TPE. The para
,
and ortho attachment of BTDꢀTPE to benzothiazole moiety could vary the mechanochromic
nature owing to the amount of twisting and/or acceptor strength in these isomers. The isomers
were subjected to mechanical stimuli by grinding with mortar and pestle and the behaviour
was explored using solid state emission spectroscopy (Fig. 7) and the data are tabulated in
Table 3. The crystals of
as pristine samples while the
synthesized sample was considered as pristine. The
structure but differ in their orientation in space resulting in different crystal packing. The
pristine crystals ꢀBT 1 and ꢀBT 2 exhibit a bright green emission at same wavelength of
519 nm. Grinding of the ꢀBT 1 and ꢀBT 2 bathochromically shifts the wavelength to 552
nm with a yellow emission. The fumigation of the grinded forms of ꢀBT 1 and ꢀBT 2 using
DCMꢀhexane for 15ꢀ20 minutes reverted the emission to 513 nm and 516 nm respectively.
The reversibility by fumigation for ꢀBT 1 and ꢀBT 2 was repeated up to 7 times and obtain
wavelength of the fumed samples in the range of 513ꢀ518 nm (Fig. S11). The pristine solid of
ꢀBT and ꢀBT exhibit a bright green emission at 518 nm and dark green emission at 519
nm respectively. The grinded forms of ꢀBT and ꢀBT display yellow and greenish yellow
fluorescence at 542 nm and 537 nm respectively. The fumed (DCM and hexane) solids of
BT and ꢀBT emit at 519 nm and 518 nm, respectively. The spectral shift generated due to
19
p
ꢀBT isomer (
ꢀBT isomer did not form any crystal owing to which the asꢀ
ꢀBT 1 and ꢀBT 2 crystals have same
pꢀBT 1 and pꢀBT 2) and oꢀBT isomer were considered
m
p
p
p
p
p
p
p
p
p
p
m
o
m
o
m
ꢀ
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grinding in
p
ꢀBT 1,
p
ꢀBT 2,
ꢀBT>
m
ꢀBT and
oꢀBT is 39 nm, 36 nm, 26 nm and 19 nm respectively
and follows the order
p
m
ꢀBT> ꢀBT.
o
The powder Xꢀray diffraction technique was used for interpretation of the reversible
mechanochromic behaviour. The PXRD patterns (Fig. S12) for the pristine solids of the
isomers reveal sharp diffraction patterns due to the crystalline nature. The grinded solids
exhibit broad diffused PXRD pattern due to its amorphous nature. However, the sharp
diffraction patterns reappeared on solvent fumigation indicating reversion to crystalline
nature. The reversible mechanochromism can be assigned to morphology change from
crystalline to amorphous caused due to modification of molecular arrangement on grinding.
The different crystalline pristine forms
behaviour but difference in their absolute quantum yields. The absolute quantum yields for
the crystalline pristine forms ꢀBT 1 and ꢀBT 2 are 0.33 and 0.51 respectively. The absolute
quantum yields of the grinded and fumed form for ꢀBT 1 are 0.53 and 0.65 respectively and
for ꢀBT 2 are 0.51 and 0.63 respectively. The grinded and fumed forms of ꢀBT 1 and ꢀBT
2 exhibit similar quantum yields but a noticeable difference can be seen in the quantum yields
of the pristine forms. The ꢀBT 1 pristine crystal has very low quantum yield as compared to
the pristine crystal ꢀBT 2 and the grinded and fumed forms of ꢀBT 1 and ꢀBT 2. The
pꢀBT 1 and pꢀBT 2 exhibit similar mechanochromic
p
p
p
p
p
p
p
p
p
p
difference in the quantum yields of the two pristine crystals could be a consequence of their
crystal packing.22 The
p
ꢀBT 2 crystal confirms presence of abundant intermolecular
interactions and weak Hꢀbonding forming a tightly packed 2ꢀD framework. On the other
hand, the crystal packing of ꢀBT 1 possesses a loosely packed 1ꢀD structure which may
p
result in the loss of excited state energy in terms of vibrational relaxation, lowering the
quantum yield substantially. The two crystals exhibit difference in packing due to different
orientation of TPE in space resulting in distinct PXRD patterns and absolute quantum yields.
The fumed forms
p
ꢀBT 1 and
p
ꢀBT 2 are also crystalline in nature and possess similar PXRD
20

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patterns and quantum yields, however exhibit higher quantum yields as compared to pristine
crystals of ꢀBT 1 and ꢀBT 2. The fumigation of the sample is done over a short period of
time which does not allow the molecule to orient in different ways resulting in similar
quantum yields. The fumed forms of ꢀBT 1 and ꢀBT 2 exhibit highest quantum yields and
p
p
p
p
blue shifted emission as compared to the two pristine crystalline states which may possibly
be an outcome of highest twisting of TPE in these crystalline forms resulting in a densely
packed crystal structure. The absolute quantum yield of
mꢀBT in its pristine, grinded and
fumed forms is 0.74, 0.65 and 0.76 respectively. The absolute quantum yield of
pristine, grinded and fumed forms is 0.34, 0.40 and 0.33 respectively.
oꢀBT in its
The difference in the mechanoꢀresponsive nature of the isomers can be inferred from the
crystal analysis. The aromatic fragments in the ꢀBT and ꢀBT isomers were depicted as BT
p
o
(benzothiazole), BT phenyl (phenyl attached to benzothiazole), BTD (benzothiadiazole) and
TPE (tetraphenylethylene) as shown in Fig. S13 for the measurement of dihedral angles. The
dihedral angle between the BT plane and BT phenyl plane for
8.26º, 8.14º and 37.80º respectively and between BT phenyl and BTD for
and ꢀBT is 35.09º, 34.42º and 62.25º respectively. The higher values of dihedral angle for
BT over ꢀBT 1, ꢀBT 2 suggests an extremely twisted structure for ꢀBT reducing its
susceptibility towards higher bathochromic shift on grinding. The highly twisted structure of
ꢀBT makes it difficult in achieving planarity or furnishing metaꢀstable states after grinding
p
ꢀBT 1,
p
ꢀBT 2 and
o
ꢀBT is
p
ꢀBT 1,
p
ꢀBT 2
o
oꢀ
p
p
o
o
thereby leading to poor mechanochromism. The twisting of donor moiety is essential for
mechanochromism but higher twisting between the donorꢀacceptor moieties makes the
structure rigid to move towards a planar structure. The dihedral angle between BTD plane
and TPE plane in
p
ꢀBT 1,
p
ꢀBT 2 is 44.13º, 46.22º and in
oꢀBT is 40.53º. The higher
twisting of the donor moiety in
p
ꢀBT can also be one of the reasons for increased
21

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mechanochromism. The combined effect of the twisting and flexibility of the DꢀA moieties
varies the degree of mechanochromism in the isomers.
Table 3 Peak emission wavelengths (λ, in nm) of pꢀBT, mꢀBT and oꢀBT as pristine, grinded
and fumed solids.
Emission
λa
Compounds
ꢁ
λpristine (nm)
λground (nm)
λfumed (nm)
(nm)
519
519
552
552
513
516
519
518
39
p
p
ꢀBT 1
ꢀBT 2
36
26
m
ꢀBT
516
519
542
537
19
o
ꢀBT
a Grindingꢀinduced spectral shift, ꢁ
λ = λground – λfumed/pristine
22

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Fig. 7 Solid state emission spectra of pristine, grinded and fumed solids of (A) pꢀBT 1 and pꢀ
BT 2, (B)
m
ꢀBT and (C)
o
ꢀBT (D) writing and erasing on a filter paper coated with
p
ꢀBT,
23

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illustrating ink free rewritable system. (Photographs taken under 365 nm UV light) (Inset:
Pristine crystals ꢀBT 1 and ꢀBT 2)
p
p
Acidochromism
The spectral behaviour of the isomers towards acid was analysed taking in account the lone
pair of N atoms which are susceptible to protonation.20 The solutions of the isomers
ꢀBT in CHCl3 (2×10ꢀ5 M) were titrated against known amounts of trifluoroacetic
p
ꢀBT, mꢀ
BT and
o
acid (TFA in CHCl3 stock solution) and studied using emission spectroscopy (Fig. 8). The
emission maxima for the isomers ꢀBT, ꢀBT and ꢀBT at 569 nm, 566 nm and 562 nm
p
m
o
respectively display quenching in emission intensity on gradual addition of TFA from 10ꢂL
to 100ꢂL. The emission intensity at maximum wavelength quenched upto 91%, 90 % and
51% for
shows highest red shift of the emission maxima from 569 nm to 613 nm with reduced
intensity on addition of 100ꢂL TFA. Similarly, ꢀBT exhibits a bathochromic shift for the
emission maxima from 562 nm to 576 nm on addition of 100ꢂL However in case of ꢀBT,
pꢀBT, mꢀBT and oꢀBT respectively on excess addition of TFA. The pꢀBT isomer
o
.
m
the excess addition of acid only resulted in extensive decrease of emission maxima. The peak
between 460ꢀ465 nm was unaffected and appeared prominently on excess addition of TFA
for all the isomers. The reduction in intensity for emission maxima confirms the response of
isomers towards acid. The increased acceptor strength due to protonation of N atom by acid
could result in a higher donorꢀacceptor interaction consequently causing a bathochromic
shift. The isomers were exposed towards TFA vapours to further explore the changes in its
solid state emission. The emission intensity of
pꢀBT, mꢀBT and oꢀBT was extensively
decreased on fumigation with TFA (Fig. 9) for few minutes. The quenching of intensity can
also be visualized by the colour change of the fluorescent green solids to pale yellow. The
24

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absolute quantum yield of pꢀBT, mꢀBT and oꢀBT in its pristine form is 0.33, 0.74 and 0.34
respectively which on fumigation with TFA decreases to 0.01, 0.05 and 0.09 respectively.
The isomers are highly reactive towards TFA in solution as well as solid state allowing its
use as a fluorescent sensor for TFA.
Fig. 8 Emission spectra of (A)
p
ꢀBT, (B)
m
ꢀBT and (C)
o
ꢀBT in response to known amounts
of TFA in CHCl3.( Concetrationꢀ 2×10ꢀ5 M in CHCl3)
25

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Fig. 9 Emission spectra of the pristine and TFA fumed solids of (A)
p
ꢀBT, (B)
m
ꢀBT and (C)
o
ꢀBT.
Conclusion
In conclusion, we have designed and synthesized multichromophoric DꢀAꢀA′ isomers
incorporating benzothiazole (BT) and benzothiadiazole (BTD) as acceptors and
tetraphenylethylene (TPE) as donor. The acceptor strength was varied by changing the
26

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position of BTDꢀTPE with respect to the ortho
comparative study about the structureꢀproperty relationship has been established for the
isomers. The ꢀBT, ꢀBT and ꢀBT isomers were synthesized by coupling BTDꢀTPE with
, meta and para positions of BT. A
p
m
o
BT using the Suzuki and Stille crossꢀcoupling reactions. The isomers exhibit high thermal
stability essential for fabrication in solid state devices. The isomers display distinct
solvatochromic behaviour in various polarity solvents with emission ranging from blue to
orange with high bathochromic shifts. The nonꢀplanar framework of the isomers results in
enhancement of intensity in the aggregated state. The
p
ꢀBT,
m
ꢀBT and
o
ꢀBT exhibit
ꢀBT and
reversible mechanochromism between green to yellow in the order
p
ꢀBT> ꢀBT> o
m
has been exploited for the development of inkꢀfree rewritable papers. The PXRD studies
reveal that a morphological conversion from crystalline to amorphous state is responsible for
mechanochromism and leads to formation of metastable states. The single crystal Xꢀray
analysis shows that the
3ꢀD framework as compared to
mechanical stimuli. The higher twisting and tight packing of
o
ꢀBT isomer has an extensively twisted structure and tightly packed
ꢀBT, which explains the reason of lower response to
ꢀBT reduces the flexibility of
p
o
the donor moiety to achieve planarization on grinding. The DFT studies reveal good
separation of HOMO and LUMO supporting a distinct ICT transition. The isomers display
changes in emission wavelength and intensity in solution and solid state in response to
trifluoroacetic acid and can act as potential sensors for its detection. The combination of the
three chromophores helps in achieving enhanced emission in solid state suitable for
mechanochromic materials. The positional changes allow fine tuning of acceptor strength and
molecular packing inducing changes in the mechanoꢀresponsive behaviour. The opted
strategy provides a new pathway in design of AIE active mechanochromic materials suitable
for application as mechanoꢀsensors, security paper, light emitting devices and other
optoelectronic applications.
27

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Experimental details
General methods
Chemicals were used as received unless otherwise indicated. All oxygen or moisture
1
sensitive reactions were performed under nitrogen/argon atmosphere. H NMR (400 MHz)
13
and C NMR (100 MHz) spectra were recorded on a Bruker Avance III instrument by using
CDCl3. 1H NMR chemical shifts are reported in parts per million (ppm) relative to the solvent
residual peak (CDCl3, 7.26 ppm). Multiplicities are given as s (singlet), d (doublet), t (triplet)
13
and m (multiplet), and the coupling constants, J, are given in hertz. C NMR chemical shifts
are reported relative to the solvent residual peak (CDCl3, 77.0 ppm). Thermogravimetric
analyses were performed on the Mettler Toledo thermal analysis system. UV−visible
absorption spectra were recorded on a Caryꢀ100 Bio UV−visible spectrophotometer.
Emission spectra were taken in a The Fluoromaxꢀ4C, S/n.1579Dꢀ1417ꢀFM Fluorescence
software Ver 3.8.0.60. The relative quantum yields in solution were calculated using
Rhodamine 6G in ethanol as standard. The quantum yields in solid state were measured using
Kꢀsphere Integrating sphere. The excitation and emission slits were 2/2 nm for the emission
measurements and 3/3 for solid quantum yield measurements. All of the measurements were
done at 25 °C. HRMS were recorded on a BrukerꢀDaltonics micrOTOFꢀQ II mass
spectrometer. The voltammograms were recorded on a CHI620D electrochemical analyzer in
dichloromethane solvent and 0.1 M TBAF6 as the supporting electrolyte. The electrodes used
were glassy carbon as a working electrode, Pt wire as a counter electrode and Ag/AgCl as a
reference electrode. Singleꢀcrystal Xꢀray structural studies of 1 and 4 were performed on a
CCD equipped SUPERNOVA diffractometer from Agilent Technologies with Oxford
Instruments lowꢀtemperature attachment under argon/nitrogen using standard Schlenk and
vacuumꢀline techniques. The acidochromism was performed using 0.1 M stock solution of
28

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TFA in CHCl3. The rewritableꢀink free paper was prepared using Whatman filter paper. The
compound ꢀBT was dissolved in DCM, and then the filter paper was coated with ꢀBT.
After drying, an inkꢀfree plastic tip was used to write “IIT” on the ꢀBT loaded paper.
p
p
p
Synthesis and characterization of
p
ꢀBT and
m
m
ꢀBT
ꢀBT. Pd(PPh3)4 (0.04 mmol) was added to a
General procedure for synthesis of
p
ꢀBT and
well degassed solution of BT4 (0.4 mmol), BTDꢀTPE (0.4 mmol) , K2CO3 (2.0 mmol) in a
mixture of toluene (24.0 mL)/ethanol (8.0 mL)/H2O (4.0 mL). The resulting mixture was
stirred at 80 °C for 24 h under an argon atmosphere. After cooling, the mixture was
evaporated to dryness and the residue was subjected to column chromatography on silica
(Hexane–DCM 30:70 in vol.) to yield the desired product pꢀBT as a green crystals. The same
procedure was employed using BT5 and BTDꢀTPE to obtain the desired product
mꢀBT as a
yellowish green powder.
p
ꢀBT: Yield: 73%; 1H NMR (400 MHz, CDCl3, 25 °C): δ 8.27 (d,
J = 8.0 Hz, 2H), 8.11ꢀ8.14
(m, 3H), 7.94 (d,
J
= 8.0 Hz, 1H), 7.85 (d,
J
= 8.0 Hz, 1H), 7.80 (d,
J = 8.0 Hz, 3H), 7.52 (t,
J
= 8.0 Hz, 1H), 7.41 (t,
J = 8.0 Hz, 1H), 7.22 (d, J
= 8.0 Hz, 2H), 7.05ꢀ7.15 (m, 15H); 13C
NMR (100 MHz, CDCl3, 25 °C): δ 167.5, 154.3, 154.0, 144.0, 143.7, 143.6, 141.6, 140.4,
139.9, 135.2, 135.1, 133.5, 133.3, 131.9, 131.6, 131.5, 131.4, 131.3, 129.8, 128.4, 128.3,
127.8, 127.6, 126.6, 126.5, 126.4, 125.3, 123.3, 121.6, 0.0 ppm; HRMS (ESI): Calcd. for
C45H29N3S2 [M + Na]+: 698.1695. Found: 698.1690.
1
m
ꢀBT: Yield: 69%; H NMR (400 MHz, CDCl3, 25 °C): δ 8.69 (s, 1H), 8.17 (d,
J
= 8.0 Hz,
1H), 8.12 (t,
J
= 8.0 Hz, 2H), 7.94 (d,
J
= 8.0 Hz, 1H), 7.89 (d,
J
= 8.0 Hz, 1H), 7.81 (d, J =
8.0 Hz, 3H) 7.68 (t,
J = 8.0 Hz, 1H), 7.52 (t, J = 8.0 Hz, 1H), 7.41 (t, J = 8.0 Hz, 1H), 7.22
(d,
J
= 8.0 Hz, 2H), 7.06ꢀ7.14 (m, 15H); 13C NMR (100 MHz, CDCl3, 25 °C): δ 167.8, 154.2,
154.0, 153.9, 144.0, 143.7, 143.6, 141.5, 140.5, 138.3, 135.1, 134.1, 133.4, 132.0, 131.8,
29