Stimuli-responsive phenothiazine-based donor-acceptor isomers: AIE, mechanochromism and polymorphism

Article abstract of DOI:10.1039/c9tc05192a

The development of solid-state stimuli responsive materials has escalated following their intriguing colour switching properties and versatile material and optoelectronic applications. In this article, we have designed and synthesized donor-acceptor (D-A)

Full text of DOI:10.1039/c9tc05192a

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DOI: 10.1039/C9TC05192A
Stimuli-responsive phenothiazine-based donor-acceptor isomers:
AIE, mechanochromism and polymorphism
a
a
a*
Anupama Ekbote , Shaikh M. Mobin and Rajneesh Misra
aDepartment of Chemistry, Indian Institute of Technology Indore,
Indore- 453552, India
*
Corresponding author
Email: rajneeshmisra@iiti.ac.in

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Abstract
DOI: 10.1039/C9TC05192A
The development of solid-state stimuli responsive materials has escalated following their
intriguing colour switching properties and versatile materials and optoelectronic applications.
In this article, we have designed and synthesized donor-acceptor (D-A) isomers namely p-PTZ,
m-PTZ and o-PTZ by functionalizing the para, meta and ortho positions of benzothiazole
(BT) ring with phenothiazine (PTZ) moiety. The Suzuki cross-coupling reaction of mono
boronate ester of phenothiazine PTZ with corresponding bromo derivatives of phenyl
benzothiazole BT generated the isomers p-PTZ, m-PTZ and o-PTZ. The attachment of donor
PTZ at different positions of acceptor phenyl BT could exert influence on the donor-acceptor
character and the molecular packing modes in the isomers resulting in discrete photophysical
and electronic properties. The isomers manifest contrasting emission properties in different
solvents as a consequence of twisted intramolecular charge transfer (TICT) state. The non-
planar twisted structures of the isomers bring about aggregation induced emission (AIE)
characteristics and reversible mechanofluorochromism (MFC) behaviour. The isomers display
self-reversible colour switching by virtue of the different accessible conformers of PTZ
moiety. The p-PTZ isomer occurs as polymorphs owing to the conformational flexibility of
PTZ moiety. The structural and morphological differences in polymorphs of p-PTZ were
studied in detail using single crystal X-ray analysis, scanning electron microscopy (SEM) and
powder X-ray diffraction (PXRD) studies. The density functional theory (DFT) and time-
dependent density function theory (TDDFT) calculations were used to explore the electronic
properties of the isomers in ground and excited state. The isomers were capable of sensing
trifluoroacetic acid in solution. A detailed comparison of the photophysical, electronic and
structural properties of p-PTZ, m-PTZ and o-PTZ has been done with a view to study the
outcome of positional change. The adopted design opens up different possibilities for study of
novel stimuli responsive materials.
1

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Introduction
DOI: 10.1039/C9TC05192A
The development of emissive organic molecules responsive to external stimuli (pressure,
1
temperature, heat, acid etc.) has advanced the production of organic light emitting diodes
(OLEDs), memory and data storage devices, sensors, security inks, fluorescent probes and
2
switches, pressure sensors and other optoelectronic devices. Mechanofluorochromic (MFC)
have emerged as smart stimuli-responsive materials, which have their solid-state emission
3
profiles reversibly responsive to mechanical grinding. A considerable emission in solid state
4
is essential for construction of MFC materials. The typical organic fluorophores like pyrene,
anthracene have substantial amount of π- π stacking in their aggregated state owing to which
they dissipate their excited state energy non-radiatively which is well known as aggregation
5
caused quenching (ACQ). This drawback has been successfully addressed by Tang et al. by
establishing the idea of aggregation induced emission (AIE). The non-planar framework of
AIEgens minimize the π-π stacking and as a consequence of restriction of intramolecular
6
rotation (RIR) result in enhancement of emission in aggregated state. The mechanically
induced switchable emission leads to phase transformation and can be regulated easily by a
7
variety of factors. The MFC behaviour can be tuned by subtle changes in the molecular
packing which are dependent on twisting or planarity in the structures, steric hindrance,
inclusion of alkyl chains or heavy atoms, intermolecular interactions due to heteroatom or
8
halogen effect, positional isomerization etc. Polymorphism is a phenomenon in which organic
molecules that can adapt different confirmations exist in more than one crystalline form for the
9
same molecular skeleton. The generation of polymorphs can result in distinct molecular
packing modes which is also an effective strategy for realizing MFC behaviour in solid state.10
The D-A, D-A-D or D-π-A organic frameworks have been utilized to create emissive
1
1
solid-state materials displaying MFC behaviour. The incorporation of non -planar twisted
AIEgens like tetraphenylethylene (TPE), hexaphenylsillole (HPS), cyanostilbene etc. in
2

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organic donor-acceptor architectures assist in achieving high emission in aggregated state by
DOI: 10.1039/C9TC05192A
reducing stacking and can easily get converted in different metastable states by forming more
1
2
planar structures. Phenothiazine (PTZ) is a familiar core with good utility in devices like
OLEDs, solar cells, chemosensors and thermally activated delayed fluorescence (TADF)
materials. The PTZ moiety has also surfaced as a desirable component for MFC materials
following its strong electron donating nature due to “S” and “N” atoms and an unusual butterfly
1
3
shape of the central heterocyclic ring. Benzothiazole (BT) is a popular electron acceptor
which has many pharmacological as well as material applications with easy functionalization
and facile synthesis.14 The integration of BT moiety with PTZ moiety may give rise to
interesting emissive properties in solid state. In our previous work, we have coupled the BT
1
5
core with TPE and investigated its AIE and MFC properties. In the present contribution, we
have coupled the PTZ moiety to the phenyl ring of the BT moiety at its para, meta and ortho
position to yield D-A positional isomers p-PTZ, m-PTZ and o-PTZ respectively. The
combination of BT and PTZ could produce highly emissive solid-state materials with
fascinating AIE and MFC properties. The free heteroatoms in the two moieties have the ability
to display supramolecular interactions and weak hydrogen bonding that can easily affect the
solid-state properties. The functionalization of the donor PTZ at para, meta and ortho positions
of acceptor phenyl BT was exploited to modulate the donor-acceptor character and molecular
packing in the isomers. The N-atom of PTZ moiety was alkylated using propyl chain for
increasing its solubility. The twisting induced by positional isomerization results in
enhancement in intensity in aggregated state with reversible MFC and effectively sense
trifluoroacetic acid (TFA). Additionally, the conformational flexibility of N-alkylated PTZ
moiety give rise to self-reversible colour switching and polymorphism. The isomers display
contrast charge transfer characteristics due to formation of twisted intramolecular charge
transfer (TICT) state. The present study shows a detailed comparison of the AIE,
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mechanochromic, solvatochromic and photophysical of the D-A positional isomers p-PTZ, m-
DOI: 10.1039/C9TC05192A
PTZ and o-PTZ.
Results and Discussions
Synthesis
The isomers p-PTZ, m-PTZ and o-PTZ involving a positional change of the phenothiazine
(PTZ) moiety at the para, meta and ortho positions of the phenyl benzothiazole BT ring were
synthesized as discussed in Scheme 1. The condensation reaction of 2-aminobenzenethiol 1
with 4-bromobenzaldehyde a, 3-bromobenzaldehyde b and 2-bromobenzaldehyde c using
ammonium chloride in methanol/water produced the respective benzothiazole intermediates
BT-1, BT-2 and BT-3 (Scheme S1).16a The PTZ moiety was converted into N-propyl
phenothiazine boronate PTZ-3 ester by a three-step approach (Scheme S1).16b The alkylation
of PTZ using N-propyl iodide resulted in 10-propyl-10H-phenothiazine PTZ-1 which on
selective bromination with bromine in acetic acid yielded 3-bromo-10-propyl-10H-
phenothiazine PTZ-2. The Pd(dppf)Cl
pinnacolatodiboron in 1,4-dioxane afforded N-propyl phenothiazine boronate ester PTZ-3.
The PTZ-3 was coupled in presence of Pd(PPh with BT-1, BT-2 and BT-3 via Suzuki-cross
coupling reaction to successfully form the desired isomers p-PTZ, m-PTZ and o-PTZ in 73%,
1% and 79% yields, respectively. All the isomers have good solubility in common organic
2
catalysed borylation reaction of PTZ-2 and bis-
)
3 4
7
solvents such as tetrahydrofuran (THF), dichloromethane (DCM), chloroform, toluene etc. and
were completely characterized by H NMR, 13C NMR, HRMS and single crystal X-ray
analysis.
1
4

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S
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Br
DOI: 10.1039/C9TC05192A
N
N
S
BT-1
N
S
S
N
p-PTZ
73%
N
S
N
4,
Br
K CO , Pd(PPh
2
3
3
)
BT-2
O
THF : water
: 1
80 °C, 12h
S
B
4
O
PTZ-3
S
N
m
-
PTZ
S
7
1%
N
Br
N
S
BT-3
S
N
o
-PTZ
79%
Scheme 1 Synthesis of isomers p-PTZ, m-PTZ and o-PTZ.
Photophysical Properties
The photophysical properties of the isomers p-PTZ, m-PTZ and o-PTZ were examined in
chloroform solution by electronic absorption and emission spectroscopy (Fig. 1). The
absorption spectra (Fig. 1(A)) of the isomers show bands at shorter wavelength region (250–
*
3
30 nm) which could be assigned to the π-π transitions. The p-PTZ isomer shows a very strong
absorption band at 375 nm possibly due to the intramolecular charge transfer (ICT) occurring
from the donor PTZ to the acceptor BT moiety. A significant change was observed in the
optical properties on positional change. The charge transfer band at 372 nm in m-PTZ and o-
PTZ are weakened as compared to p-PTZ. This could possibly be explained by the formation
of a twisted intramolecular charge transfer state due to the higher amount of twisting in meta
and ortho isomers as compared to para. The emission spectra of the isomers also confirm the
different optical properties of p-PTZ (Fig. 1(B)). All the isomers show emission between 400–
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5
00 nm which could be ascribed to the localized excited (LE) states in the molecules. Other
DOI: 10.1039/C9TC05192A
than the LE emission, the p-PTZ exhibits a strong emission at 549 nm which could be ascribed
to ICT transition. On the other hand, the m-PTZ and o-PTZ have significant LE emissions and
low intensity ICT emissions at longer wavelength (above 500 nm). The photophysical data are
summarized in Table 1 and the effect of positional change on the ICT transition have been
explored in detail using solvatochromic studies.
5
4
3
2
1
0
(B)
(A)
4
31 nm
549 nm
p-PTZ
m-PTZ
o-PTZ
1
.0
407 nm
p-PTZ
m-PTZ
o-PTZ
0.8
4
58 nm
0.6
0.4
543 nm
0.2
531 nm
ICT
0.0
400
450
500
550
600
650
700
250
300
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Fig. 1 Electronic absorption spectra (A) and normalized emission spectra (B) of isomers p-
-5
PTZ, m-PTZ and o-PTZ measured in chloroform (2×10 M).
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DOI: 10.1039/C9TC05192A
Table 1 Photophysical properties of isomers p-PTZ, m-PTZ and o-PTZ.
b
-1
–1 a
λ
em (nm)a
ɸ
f
Compounds
λ
ab[nm] (ε[Lmol cm ])
p-BT
m-BT
o-BT
272(18620), 324(18200), 375(11280)
272(29675), 316(18760), 372(4920)
265(36730), 314(19265), 372(5280)
407,431,549
407,431,458,531
407,431,458,543
0.63
0.11
0.16
a
-5
b
Measured in chloroform (2×10 M), Fluorescence quantum yields recorded using quinine
sulphate as a standard in 0.5 M H
2
SO solution.
4
Solvatochromism
The ICT transition arising in D-A molecules is easily affected by changes in solvent polarity
and could be further influenced by the positional change in p-PTZ, m-PTZ and o-PTZ.17a The
isomers were dissolved in solvents of different polarity from non-polar to polar (cyclohexane,
toluene,
THF,
chloroform,
DCM,
1,4-dioxane,
dimethylformamide (DMF),
dimethylsulphoxide (DMSO)) and the effect of solvent polarity and positional change on ICT
transition was explored in detail by emission (Fig. 2) and absorption spectroscopy (Fig. S1).
The p-PTZ isomer emits at 471 nm due to the localized-excited (LE) state with a shoulder at
5
00 nm corresponding to the ICT transition cyclohexane. The emission intensity of the ICT
increases in toluene while the LE emission appears as a low intensity peak. The ICT peak is
further stabilized and red shifted on increasing the polarity and emits at 637 nm in DMSO. The
ICT transition creates a polarized excited state stabilized by reorganization of solvent
molecules around it thereby lowering its energy resulting in a red shifted weakened emission
in polar solvents.17a In cyclohexane, the m-PTZ isomer similarly exhibits LE emission at 448
nm and emission at 470 nm as a shoulder due to charge transfer (CT) state. On increasing the
polarity from toluene to DCM, the CT emission is significant, whereas the LE emission appears
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as a low intensity peak. The CT emission is stabilized in moderately polar solvent like DCM
DOI: 10.1039/C9TC05192A
with red shifted emission at 590 nm. The close vicinity of the LE and CT states rationalizes the
dual emission in solvents of various polarity.18a Although, the intensity of CT emission
substantially decreases in highly polar solvents such as DMF and DMSO with the LE emission
remaining intact. The drastic reduction in emission intensity in highly polar solvents could be
associated to TICT state.18b The free rotation around the single bond in the meta isomer makes
it twisted which is accompanied with an instantaneous charge transfer from donor to acceptor
forming a relaxed perpendicular TICT state. In non-polar solvents, a high intensity LE emission
was observed at lower wavelength due to D-A coplanar conformation stabilized by mesomeric
interactions, whereas in highly polar solvents the D-A relaxed perpendicular TICT state creates
a non-radiative channel resulting in little or no emission.18c,d The o-PTZ isomer emits strongly
at 473 nm in cyclohexane from CT state along with LE emissions below 450 nm. The increase
in polarity results in stabilization of the CT emission accompanied by bathochromic shift. In
DMSO, the isomer exhibits dual emission at lower wavelength due to LE states and higher
wavelength due to CT state. The ortho isomer shows significant LE emission in highly polar
solvents and the emission from CT state is slightly quenched owing to the TICT state. The
ortho isomer has higher twisting as compared to meta isomer, however the electronic
communication at the ortho position is more pronounced as compared to meta position. The
synergistic effect of the TICT state and the electronic communication at ortho position results
in intermediate solvatochromic behaviour as compared to the para and meta isomer. The
absorption spectra (Fig. S1) of the isomers are not much affected by changing the solvent
polarity which establishes existence of more polarized excited state in contrast to ground
electronic state.17b The quantum yield for the isomers in different solvents has been given in
Table S1.
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DOI: 10.1039/C9TC05192A
Fig. 2 Emission spectra of (A) p-PTZ, (B) m-PTZ and (C) o-PTZ in solvents of varying
polarity. (λexc = 370nm) (Photographs taken under 365 nm UV illumination).
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DOI: 10.1039/C9TC05192A
Aggregation induced emission
5
0
(A)
Water Vol %
(B)
Water Vol %
0
%
14
12
10
8
0%
1
0%
0%
30%
0%
0%
0%
0%
0%
0%
5%
10%
20%
30%
40%
50%
60%
70%
80%
90%
95%
2
40
4
5
6
7
30
8
9
9
20
6
4
10
2
0
0
400
450
500
550
600
650
700
750
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
14
(D) 50
(C)
Water Vol %
0
%
CT
1
0%
0%
0%
0%
0%
0%
0%
80%
0%
5%
p-PTZ
m-PTZ
o-PTZ
12
Different
aggregates
at 40% and 70%
3
4
0
2
4
1
0
8
6
4
2
0
5
6
7
30
9
9
2
0
10
0
0% 10% 20%30% 40% 50%60% 70%80% 90%95%
400
450
500
550
600
650
Water vol %
Wavelength (nm)
Fig. 3 Emission spectra of (A) p-PTZ, (B) m-PTZ and (C) o-PTZ in DMSO-water mixtures
-5
with different water fractions. (Luminogen conc. 2×10 M) (D) Plot of intensity vs water
fraction; intensity calculated at λmax (λmax with highest intensity considered for calculation).
The butterfly type structure of PTZ and the twisting in the isomers provided non-planar
molecules with aggregation induced emission characteristics which were investigated using
emission spectroscopy (Fig. 3). The isomers p-PTZ, m-PTZ and o-PTZ are readily soluble in
DMSO but insoluble in water; hence consistent increase of water in DMSO initiated the nano-
aggregate formation. The p-PTZ has an emission maximum at 639 nm in pure DMSO contrast
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to the standard AIEgens as a result of the exclusive ICT transition present in the isomer. The
DOI: 10.1039/C9TC05192A
emission intensity of ICT peak decreases on increasing the water fraction upto 30% and shifts
bathochromically to 658 nm, which shows the stabilization of intramolecular charge transfer
peak in polar medium. At 40% water fraction, the aggregation occurs which enhances the
intensity accompanied by blue shifted emission at 485 nm. The aggregation causes intensity
enhancement upto 50% water fraction, although a slight drop in intensity was observed at 60%
water fraction which could be caused by the larger sized aggregates which are less exposed to
radiation. The 70% DMSO-water fraction experiences a substantial enhancement in emission
intensity with appearance of new peak at 511 nm. This could be attributed to the formation of
two different types of aggregates at 40% and 70% water fraction. 19a The higher water
percentage (i.e. 80% and above) again experience minor drop in intensity due to formation of
larger sized aggregates.19b,c Thus, the emission at low water fractions in p-PTZ isomer is
mainly due to the ICT which later at higher water fractions is majorly governed by
aggregation.19d In p-PTZ isomer, the AIE behaviour was observed at different wavelengths for
two water fraction 40% and 70%, which may be due to formation of two different aggregates.
The m-PTZ and o-PTZ isomers display contrast AIE behaviour as a consequence of the TICT
state. Both the isomers have weakened emissions in pure DMSO solutions. The emission
intensifies at 50% DMSO-water and 60% DMSO-water for m-PTZ and o-PTZ respectively
owing to aggregate formation. The emission reduces at higher water percentage similar to p-
PTZ. The meta and ortho isomer behave as typical AIEgens unlike the para isomer. The unique
AIE properties in the isomers are an outcome of the TICT state formed due to positional
change. The pictures of AIE behaviour are given in Fig. S2.
1
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DOI: 10.1039/C9TC05192A
Single crystal X-ray analysis
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Fig. 4 (A) Crystal structures of polymorphs p-PTZ GC and p-PTZ YC in its different
DOI: 10.1039/C9TC05192A
confirmations, Crystal packing diagram and the intermolecular interactions of (B) p-PTZ GC
and (C) p-PTZ YC.
Fig. 5 (A) Crystal structure of m-PTZ and (B) crystal packing diagram of m-PTZ depicting
the intermolecular interactions.
1
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DOI: 10.1039/C9TC05192A
Fig. 6 (A) Crystal structure of o-PTZ and (B) crystal packing diagram of o-PTZ depicting the
intermolecular interactions.
The single crystal X-ray analysis is useful for structure elucidation and inference of properties
in aggregated state. The slow evaporation of p-PTZ isomer in DCM serendipitously resulted
in two different types of crystals. The polymorphs were denoted as p-PTZ GC (P2
system) and p-PTZ YC (Pna2 orthorhombic system) according to their physical appearances;
p-PTZ GC being green coloured rod shaped and p-PTZ YC being yellow plate shaped crystal
Fig. 7(A)). In p-PTZ GC, the PTZ moiety attaches at para position with a dihedral angle of
3.36º in a way that the N and S of PTZ are on the same side as compared to N and S of BT
1
monoclinic
1
(
1
moiety (Fig. 4(A)). In p-PTZ YC, the dihedral angle between BT and PTZ moiety is 27.15º
which is higher compared to p-PTZ GC and the N and S of PTZ moiety are anti to the N and
1
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S of BT moiety (Fig. (4(A)). The major difference in the polymorphs can be visualized in the
DOI: 10.1039/C9TC05192A
terms of orientation of the propyl chain attached to the N atom of PTZ. The central six
membered ring of PTZ moiety prefers a boat confirmation due to the butterfly shape of PTZ
moiety. In p-PTZ GC, the propyl chain is quasi axial whereas in p-PTZ YC it is quasi
2
0
equatorial (Fig. 4(A)) in context to the boat confirmation of PTZ moiety. In p-PTZ GC, two
molecules lie perpendicular in a head to tail arrangement and each molecule stacks parallelly
in respective direction forming a tightly packed 2D stacked structure (Fig. S3 (A)). In the
aggregated state two molecules of p-PTZ GC exhibit two C–H·····π interactions C(20)–
H(10)·····π(red centroid C20 C21 C22 C23 C24 C25, 3.340 Å), C(13)–H(24)·····π(blue centroid
C13 C14 C15 C16 C17 C18, 2.880 Å) and one lone pair·····π interaction i.e. S(2)·····π(blue
centroid C13 C14 C15 C16 C17 C18, 3.822Å) (Fig. 4(B)) . In p-PTZ YC, each molecule is
surrounded by one head to head stack molecule and another molecule remaining in an
antiparallel arrangement which forms 2D stacked ladder type structure (Fig. S3 (B)). The
interactions observed in p-PTZ YC are two C–H·····π interactions C(7)–H(24)·····π(blue
centroid C7 C8 C9 C10 C11 C12, 3.121 Å), C(18)–H(21)·····π(red centroid C13 C14 C15 C16
C17 C18, 2.880 Å) and one lone pair·····π interaction i.e. S(2)·····π (blue centroid C7 C8 C9 C10
C11 C12, 3.922Å) (Fig. 4(C)). The m-PTZ isomer crystallizes as P2
slow evaporation of compound in hexane. The crystals for o-PTZ isomer were generated by
evaporation of ethyl-acetate/hexane mixture as P2 /c monoclinic system. The dihedral angle
between the donor (PTZ) and acceptor (BT) in m-PTZ is 46.83º (Fig. 5(A)) and in o-PTZ is
0.90º (Fig. 6(A)). There is a clear increment in the dihedral angle between the donor
1
/c monoclinic system by
1
7
benzothiazole moiety and the acceptor phenothiazine in the following order o-PTZ>m-
PTZ>p-PTZ, thereby altering the degree of twisting in the isomers, which could affect its
solid-state emission properties. In the solid state of m-PTZ isomer, each molecule interacts
with four other molecules by three C–H·····π interactions i.e. C(14)–H(4)·····π(centroid C14 C15
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C16 C17 C18 C19, 3.025 Å), C(20)–H(26A)·····π(blue centroid C20 C21 C22 C23 C24 C25,
DOI: 10.1039/C9TC05192A
3
.306 Å), C(25)–H(23)·····π(blue centroid C20 C21 C22 C23 C24 C25, 3.216 Å) and one lone
pair·····π interaction i.e. S(2)·····π(red centroid C8 C9 C10 C11 C12 C13, 3.898Å) (Fig. 5(B)).
The multiple interactions give rise to a tight 2D sheet type packing in meta isomer (Fig. S4
(A)). On the other hand, the o-PTZ isomer has no C–H·····π interactions, however exhibits one
lone pair·····π interaction S(2)·····π(red centroid C20 C21 C22 C23 C24 C25, 4.314 Å) and two
different kind of weak hydrogen bonding interactions from N and S of BT moiety i.e. C–
H(23)·····N(1) (2.641 Å) and C–H(19)·····S(1) (2.908 Å) (Fig. 6(B)). The absence of multiple
interactions in o-PTZ isomer results in a loosely packed zig-zag 2D arrangement (Fig. S4 (B)).
The variable packing and twisting in the isomers could initiate intriguing properties in solid
state. The p-PTZ GC exhibits sheet like morphology, whereas the morphology of p-PTZ YC
has small aggregates. The crystal information for the isomers is compiled in Table S2. The
polymorphs of p-PTZ isomer were also studied using scanning electron microscopy (SEM) as
shown in Fig. S5.
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DOI: 10.1039/C9TC05192A
Mechanochromism
(B)
(A)
486 nm 524 nm
484 nm
5
36 nm
1
.0
1.0
0.8
0.6
0.4
0.2
0.0
p-PTZ Pristine powder
p-PTZ GC
p-PTZ YC
Pristine powder
Grinded
Intermediate State
Fumed
0.8
0.6
0.4
0.2
0.0
4
00
450
500
550
600
650
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
(D)
(C)
451 nm
458 nm
447 nm
4
38 nm
486 nm
504 nm
460 nm
1
.0
440 nm
1.0
0.8
0.6
0.4
0.2
0.0
Pristine
Grinded
Fumed
Pristine
Grinded
Fumed
Self reversible
0.8
Self reversible
0.6
0.4
0.2
0.0
400
450
500
550
600
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
1
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Fig. 7 Solid state emission spectra of (A) p-PTZ and its polymorphs (p-PTZ GC and p-PTZ
DOI: 10.1039/C9TC05192A
YC), (B) p-PTZ, (C) m-PTZ and (D) o-PTZ as their pristine, grinded, fumed and self-
reversible solids and photographs taken under 365 nm UV illumination.
Fig. 8 Repeated switching of the solid-state fluorescence of (A) p-PTZ in its intermediate and
strongly grinded state and (B) p-PTZ, (C) m-PTZ and (D) o-PTZ by repeated grinding and
fuming cycles.
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DOI: 10.1039/C9TC05192A
Table 2 Peak emission wavelengths (λ, in nm) of p-BT, m-BT and o-BT as pristine, grinded
and fumed solids.
Emission
Compounds
a
λpristine (nm)
λground (nm)
λfumed (nm)
Δλ (nm)
p-PTZ
484
524 (intermediate state)
36 (strong grinding)
486
52
5
m-PTZ
o-PTZ
440
447
486
504
438
451
47
58
a
Grinding-induced spectral shift, Δλ = λground – λfumed/pristine
The non-planarity and twisting in the isomers lead to significant emission in the solid state
which is responsive to mechanical force. Additionally, the inclusion of PTZ moiety resulted in
polymorphs for the para isomer owing to the formation of different conformers due to
flexibility of the butterfly confirmation. Taking in consideration the above structural features,
the mechanochromic behaviour of the isomers and polymorphism of p-PTZ was investigated
in detail using solid state emission spectroscopy (Fig. 7). The p-PTZ pristine powder exhibits
green emission at 484 nm, whereas its polymorphs p-PTZ GC and p-PTZ YC emit at 490 nm
and 494 nm respectively. The grinding of p-PTZ pristine powder in mortar pestle resulted in
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bathochromically shifted bright green emission at 524 nm. The green emission red shifted to a
DOI: 10.1039/C9TC05192A
yellow emission at 536 nm on further strong grinding, which dynamically reversed to an
intermediate emission at 524 nm in about one minute. The self-reversible change from yellow
emission (536 nm) to green emission (524 nm) has been recorded in a video clip attached in
the ESI and is reversibility was studied upto 7 cycles (Fig. 8(A)). The intermediate green
emission at 524 nm was stable although restored to the pristine state with bluish green emission
at 486 nm on fuming with dichloromethane for 4–5 minutes. The p-PTZ isomer exhibited a
three-fold mechanochromic behaviour from bluish green to yellow via green emission with a
spectral shift of 52 nm. The three-fold stepwise colour switching in p-PTZ could be ascribed
to the flexible conversion of different conformers of PTZ moiety. The m-PTZ and o-PTZ
isomers display a blue emission in its pristine state at 440 nm and 447 nm respectively. A bright
green emission is generated on grinding at 486 nm and 504 nm for m-PTZ and o-PTZ
respectively. The grinded solids of m-PTZ and o-PTZ restored their original blue emission at
4
38 nm and 451 nm respectively on fumigation with hexane for 5–7 minutes. The grinding
induced spectral shift for m-PTZ and o-PTZ is 47 nm and 58 nm respectively. The grinded
solid of m-PTZ when left at room temperature for over a week displayed self-reversible
switching of its emission upto 460 nm whereas the grinded solid of o-PTZ switched its
emission merely in 2-3 days self reversibly upto 458 nm which is close to its pristine state. The
self-reversible switching of emission observed in all the isomers could be due to the flexible
2
1
interconversion between different conformers of the PTZ moiety. The highest spectral shift
initiated by grinding was observed in o-PTZ which is associated to a loosely packed structure.
The o-PTZ isomer has absence of multiple C–H·····π interactions and shows weak H-bonds
which could be easily perturbed on grinding assuming a more planar structure resulting in
higher bathochromic shift. The tightly packed crystal structure of m-PTZ driven by extensive
C–H·····π interactions makes it less susceptible to mechanical stimuli. The reversible
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mechanochromism by fumigation could be repeated upto many cycles for all the isomers (Fig.
DOI: 10.1039/C9TC05192A
8
). The wavelength for pristine, grinded, fumed and self-reversible solids are given in Table-2.
The PLQY of p-PTZ pristine powder and its polymorphs p-PTZ GC and p-PTZ YC are 0.22,
.26 and 0.21 respectively. The PLQY for p-PTZ at 524 nm is 0.19 and at 536 nm is 0.20. The
0
fumed solid of p-PTZ has PLQY 0.24. The PLQY for pristine, grinded and fumed solids of m-
PTZ is 0.02, 0.04 and 0.03 respectively. The PLQY for pristine, grinded and fumed solids of
o-PTZ is 0.02, 0.01 and 0.01 respectively.
The powder X-ray diffraction studies were carried out for the pristine, grinded and fumed solids
of the isomers to find out the underlying mechanism for mechanochromic behaviour (Fig. S6).
The p-PTZ GC and p-PTZ YC possess different diffraction patterns confirming its
polymorphic nature. The sharp and intense diffraction patterns for pristine solids of all the
isomers establish their crystalline nature. The intense peaks are lost on grinding and a wide
diffused peak is observed for the grinded solids. The crystallinity is regained on fumigation
which can be seen from the sharp patterns obtained for the fumed solids. The PXRD results
prove that the crystalline pristine solids form an amorphous metastable state on grinding and
the fumigation regenerates the crystallinity. The stimuli reactive colour change can therefore
be related to a phase transition from crystalline to amorphous caused by the disturbance in
molecular packing on applying mechanical force.
Acidochromism
The free N atom present in the BT moiety of the isomers are capable of protonation owing to
which the response of isomers on treating with trifluoroacetic acid (TFA) was studied using
2
2
fluorescence spectroscopy (Fig. 9). The experiment was carried out by titrating solution of
-5
the isomers in CHCl
3
(2×10 M) against known amounts of TFA. The intensity emission
maxima of p-PTZ at 549 nm due to ICT quenches on addition of TFA (10μL-100μL). On
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excess addition of TFA a low intensity blue shifted broad emission was observed at 524 nm.
DOI: 10.1039/C9TC05192A
The addition of TFA (10μL-100μL) to the solution of m-PTZ isomer results in quenching of
its localized emissions (407 nm, 431 nm and 458 nm) and a broad red shifted low intensity
emission peak was seen at 475 nm on excess addition of TFA. The o-PTZ isomer in CHCl
3
displays LE emission (407 nm, 432 nm and 458 nm) and CT emission at 539 nm. The addition
of TFA (10μL-100μL) quenches the LE emissions and causes the CT emission to broaden and
disappear. In excess TFA a broad emission is observed at 475 nm. The quenching efficiency
of the emission maxima for p-PTZ, m-PTZ and o-PTZ is 87%, 61% and 58% respectively.
The effective quenching of the emission of the isomers confirms that they are responsive to
TFA and can act as potential TFA sensors.
Fig. 9 Emission spectra of (A) p-PTZ, (B) m-PTZ and (C) o-PTZ in CHCl on titration with
3
-5
known amounts of TFA (10μL-100μL), (Conc. 2×10 M). (D) Emission photographs of p-
PTZ, m-PTZ and o-PTZ in response to TFA.
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DOI: 10.1039/C9TC05192A
Density functional theory calculations.
The density functional theory calculations were done to analyse the geometries and the
electronic distribution present in ground state for the isomers by utilizing the basis set
2
3
B3LYP/631-G+ (d,p). The optimized structures of the isomers are in accordance with the
single crystal structure. The o-PTZ isomer has an extremely twisted structure as compared to
the m-PTZ and p-PTZ isomer. Fig. 9 depicts the optimized structures and calculated frontier
orbitals whose values are listed in Table 3. The highest occupied molecular orbital (HOMO) is
mainly concentrated on the donor PTZ moiety extending towards the phenyl ring while the
lowest unoccupied molecular orbital (LUMO) is concentrated on the acceptor BT moiety. The
calculated HOMO-LUMO gap for p-PTZ, m-PTZ and o-PTZ are 3.37 eV, 3.42 eV and 3.29
eV respectively. The time-dependent density functional theory (TD-DFT) calculations were
performed at B3LYP/6-31G +(d,p) level to study the electronic properties in excited state and
were found to be in a good agreement with the absorption studies. Table S3 contains the
computed vertical transitions, oscillator strengths, configurations and corresponding calculated
frontier orbitals.
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DOI: 10.1039/C9TC05192A
Table 2 DFT calculations of p-PTZ, m-PTZ and o-PTZ.
DFT calculations
a
a
Compounds
HOMO eV
LUMO eV
HOMO-LUMO gap eV
p-PTZ
m-PTZ
o-PTZ
-5.35
-5.33
-5.03
-1.98
-1.91
-1.74
3.37
3.42
3.29
a
Theoretical values at B3LYP/6-31G +(d,p) level.
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DOI: 10.1039/C9TC05192A
-5.03 eV
Fig. 9 Energy level diagram showing the HOMO and LUMO energy levels of p-PTZ, m-PTZ
and o-PTZ as determined at the B3LYP/6-31G +(d,p) level and their optimized structures.
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DOI: 10.1039/C9TC05192A
Conclusion
In conclusion, a series of D-A positional isomers p-PTZ, m-PTZ and o-PTZ were designed
and synthesized using Suzuki cross-coupling reaction in order to regulate the donor-acceptor
character as well as the intermolecular interactions in solid state. The functionalization of
phenothiazine (PTZ) at para, meta and ortho positions of phenyl BT resulted in contrasting
photophysical and electronic properties. The p-PTZ isomer displayed strong ICT
characteristics owing to enhanced conjugation due to lower degree of twisting. In contrast, the
m-PTZ and o-PTZ displayed quenching of emission in polar solvents due to formation of
TICT state. The non-planar twisted structures of the isomers assist in intensity enhancement in
aggregated state and endow them with reversible MFC characteristics. The MFC behaviour is
associated with generation of metastable states and is reflected from the crystalline to
amorphous phase transformation studied by PXRD. The isomers exhibit self-reversible colour
switching owing to the conformational flexibility of PTZ moiety. The unique butterfly shape
of PTZ moiety allows it to exist in different conformations namely quasi axial and quasi
equatorial in p-PTZ isomer resulting in polymorphism which was also explored using SEM,
PXRD and SCXRD. The single crystal X-ray analysis of the m-PTZ reveals multiple C–H·····
π
interactions driven tightly packed structure, which makes it less susceptible to grinding as
compared to o-PTZ which has weak H-bonds and absence of C–H·····π interactions thereby
exhibiting highest grinding induced spectral shift. The DFT optimized structures are in
accordance with the single crystal structures. The isomers due to the presence of free N atom
display sensing of trifluoroacetic acid. The work is of great importance for designing MFC
materials with self-recovering mechanism. The adaptable confirmations of PTZ moiety can
regulate the intermolecular interactions resulting in molecules with distinct photophysical
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properties. The chosen strategy opens new avenues for producing smart stimuli responsive
DOI: 10.1039/C9TC05192A
materials with modulate solid state properties.
Experimental details
General methods
Chemicals were used as received unless otherwise indicated. All oxygen or moisture sensitive
1
13
reactions were performed under nitrogen/argon atmosphere. H NMR (400 MHz) and C NMR
. 1H NMR
(100 MHz) spectra were recorded on a Bruker Avance III instrument by using CDCl
3
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) and m
1
3
(
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). UV−visible absorption
spectra were recorded on a Perkin Elmer Lamba 35 UV−visible spectrophotometer. Emission
spectra were taken in a The Fluoromax-4C, S/n.1579D-1417-FM Fluorescence software Ver
3
.8.0.60. The relative quantum yields in solution were calculated using Quinine sulphate in 0.5
M H SO solution as standard. The quantum yields in solid state were measured using K-sphere
2
4
Integrating sphere. The excitation and emission slits were 2/2 nm for the emission
measurements and 3/3 for solid quantum yield measurements. All the measurements were done
at 25 °C. HRMS were recorded on a Bruker-Daltonics micrOTOF-Q II mass spectrometer.
Single-crystal X-ray structural studies of p-PTZ GC and p-PTZ YC were performed on a CCD
equipped SUPERNOVA diffractometer from Agilent Technologies with Oxford Instruments
low-temperature attachment under argon/nitrogen using standard Schlenk and vacuum-line
techniques. Single-crystal X-Ray structural studies for m-PTZ was collected using (BRUKER
KAPPA APEX II CCD Duo) with graphite monochromatic Mo Kα radiation (0.71073 Å), on
an Xcalibur, Eos, Gemini diffractometer. Single crystal X-ray structural studies for compound
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o-PTZ were collected on a Bruker D8 VENTURE diffractometer equipped with CMOS Photon
DOI: 10.1039/C9TC05192A
1
00 detector and MoKα (λ = 0.71073 Å) radiation was used. The acidochromism was
performed using 0.1 M stock solution of TFA in CHCl . The PXRD studies were done using
3
powdered samples on Rigaku SmartLab, Automated Multipurpose x-ray Diffractometer,
equipped with a high-accuracy theta-theta goniometer featuring a horizontal sample mount.
Scanning Electron Microscope (FE-SEM), was performed on Supra55 Zeiss, GEMINI
Technology.
Synthesis and characterization of p-PTZ, m-PTZ and o-PTZ.
General procedure for the synthesis of p-PTZ, m-PTZ and o-PTZ. BT-1 (1.03 mmol), PTZ-
3
(1.03 mmol) and K
2
CO
3
(5.16 mmol) were dissolved in a mixture of tetrahydrofuran (20.0
(0.10 mmol) was added to the
mL)/ H O (4.0 mL) and the solution was degassed. Pd(PPh
2
)
3 4
resulting mixture and was stirred at 80 ºC 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 70:30 in vol) to yield the desired product p-PTZ as a
green powder. The same procedure was employed using intermediates PTZ-3 and BT-2 to
obtain product m-PTZ as white powder. The product o-PTZ was also obtained as white powder
by applying the above procedure using intermediates PTZ-3 and BT-3.
1
p-PTZ Yield: 73%; H NMR (400 MHz, CDCl
3
, 25 ºC): δ 8.13 (d, J = 8.0 Hz, 2H), 8.08 (d, J
=
8.0 Hz, 1H), 7.92 (d, J = 8.0 Hz, 1H), 7.66 (d, J = 8.0 Hz, 2H), 7.50 (t, J = 8.0 Hz, 1H), 7.37–
7
2
1
1
.46 (m, 3H), 7.15 (d, J = 8.0 Hz, 2H), 6.87–6.94 (m, 3H), 3.86 (t, J =8.0 Hz, 2H), 1.83–1.91(m,
1
3
H), 1.04 (t, J =8.0 Hz, 3H); C NMR (100 MHz, CDCl , 25 ºC): δ 167.7, 154.2, 145.1, 144.8,
3
42.4, 135.3, 135.0, 134.0, 132.1, 128.9, 128.0, 127.5, 127.3, 126.8, 126.3, 125.9, 125.7, 125.4,
25.1, 124.3, 123.1, 122.5, 121.6, 115.5, 115.4, 49.2, 20.1, 11.3, 0.0 ppm; HRMS (ESI): calcd.
+
for C28
H
22
N
2
S
2
[M + H] : 451.1297. Found: 451.1296.
2
8