Effect of a bridge unit on the mechanochromism of phenothiazine-based Schiff bases

Article abstract of DOI:10.1039/c9nj03881g

Two phenothiazine derivatives bridged with Schiff base groups were prepared, and investigations were conducted to study their mechanofluorochromic (MFC) behavior. When a mechanical force was applied, the compound PPD (N,N′-bis(10-ethyl-phenothiazin-2-ylmethylene)-p-phenylenediamine) changed its color from yellow to orange, exhibiting a significant MFC behavior. The reversible MFC behavior of PPD could be accomplished via grinding and fuming with CHCl₃. Spectral analysis indicated that PPD molecules were packed in an H-Aggregate form in the as-synthesized crystal, whilst it had transformed to an amorphous state under external grinding. The X-ray analysis revealed that the transition from the crystalline structure to the amorphous state would result in the color change. Moreover, PED (N,N′-bis(10-ethyl-phenothiazin-2-ylmethylene)-ethylenediamine) with two isolated chromophores did not exhibit an MFC behavior, while no phase transition from crystal to the amorphous solid was found under grinding.

Full text of DOI:10.1039/c9nj03881g

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DOI: 10.1039/C9NJ03881G
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Effect of Bridge Unit on Mechanochromism of Phenothiazine-Based
Schiff Bases
Zicheng Yang,a Pengchong Xue,*,b Lulu Zhang,a Peng Chen*,a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Two phenothiazine derivatives bridged with Schiff base groups have been prepared, and investigation has
been taken upon their mechanofluorochromic (MFC) behavior. When the mechanical force is given, the
compound PPD (N,N’-bis(10-ethyl-phenothiazin-2-ylmethylene)-p-phenylenediamine) change its color from
yellow to orange, exhibiting significant MFC. The reversible MFC behavior of PPD could be accomplished
through grinding and fuming with CHCl3. Spectral analysis indicates that PPD molecules are packed in the H-
aggregate form in the as-synthesized crystal, whilst it has transformed to the amorphous state under the
external grinding. The X-ray analysis reveals that the transfer from the crystalline structure to the amorphous
state would respond to the color change. On the other hand, PED (N,N’-bis(10-ethyl-phenothiazin-2-
ylmethylene)-ethylenediamine) with two isolated chromophores does not exhibit MFC behavior, while no
phase transition from crystal to the amorphous solid has been found under grinding.
fluorescence. In most cases, the molecular structure could
generally be maintained, although a few examples have
revealed the ruins of the chemical bonds for the MFC
Introduction
Lives are involved in the internal and external stimuli (such as
pain, vision, sound) in real time, to which responses are
instinctively made for survival. Stimuli-responsive materials,
which are spontaneously sensitive and responsible to the
external stimuli, have drawn considerable attentions so as to
mimic the nature. And the stimuli-responsive organic
materials, which are featured by the easy tailoring and multi-
functionalities, have the potentials in the applications of data
storage, ion recognition, sensing and electronics etc.1
Therefore, mechanofluorochromic (MFC) materials have been
extensively investigated in the recent ten years.2
The emission color and intensity of the MFC materials could
be altered in response to the discrepant intermolecular
interaction under the external stimuli. The external force
would collapse or rearrange the stacking in the crystals,
resulting in the amorphous state or another crystalline phase.3
And the grounded sample would emit distinct shifted
materials.4
Various π-conjugated chromophores and/or functional
groups with variable conformations have introduced to
assemble the MFC materials that examples are known as
triphenylacrylonitrile,5
tetraphenylethene,6
9,10-
divinylanthracene,7 phenothiazine,8 quinoline9 derivatives.
Some difluoroboron complexes have been reported to
demonstrate MFC behavior as well.10 Meanwhile, full
investigations have been drawn on the contribution of length
of the alkyl chain, t-butyl group and halogen atom etc to
address the influence of molecular structure on the MFC
performance.11 As a famous building block for the construction
of optoelectronic devices in respect to the push-pull electron
structure, phenothiazine is also considerably utilized in the
preparation of MFC molecules for its easy functionalization.12
The non-planar structure of phenothiazine favors to rearrange
the molecular packing in the crystal under the external forces,
resulting in the MFC molecules.8,13
a. Key Laboratory of Functional Inorganic Material Chemistry (MOE), School of
Chemistry and Materials Science, Heilongjiang University, Harbin, 150080, P. R.
China. Email: jehugu@gmail.com
It is noted that the preparation of the organic MFC
molecules has often suffered a complex and harsh synthetic
condition, which would lead to a low yield in turn. Meanwhile,
it remains a challenge to predict the MFC performance of the
b. Tianjin Key Laboratory of Structure and Performance for Functional Molecules,
MOE Key Laboratory of Inorganic–Organic Hybrid Functional Material Chemistry target molecules. Therefore, a relative easy preparation is
College of Chemistry, Tianjin Normal University No. 393, Bin Shui West Road,
Tianjin, P. R. China. Email: xuepengchong@126.com
obviously favored for the fabrication of MFC materials. Schiff
base molecules arisen from the condensation of carbonyl and
amine compounds could be easily synthesized under mild
Electronic Supplementary Information (ESI) available: 1H NMR and 13C NMR
spectra, IR spectra, UV-Vis spectra, emission spectra, TG curve and PXRD patterns. condition in a high enough yield.14 Thus, it is believed it would
See DOI: 10.1039/x0xx00000x
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be a useful strategy to design and synthesize new MFC quantum yields of PPD and PED were performeVdiewinArstioclelidOnloinne
materials in an easy and high yield way. It is known that the Edinburgh FLSP920 with the integrating sphere. Geometrical
DOI: 10.1039/C9NJ03881G
bridge moieties would play key role in the electron optimization for PPD and PED were performed by density
distribution. And the introduction of bridge unit in distinct functional theory (DFT) calculations at B3LYP/6-31G(d) level
length and conformation would definitely lead to a variation in with the Gaussian 09W program package. Electronic transition
the structural packing models and the photophysical data obtained by TD-DFT calculation in CAM-B3LYP/6-31G(d)
properties.15 Two phenothiazine-based Schiff-base derivatives level based on the configuration at ground state.
(PED and PPD, Scheme 1) with flexible and rigid bridge units Syntheses and procedures
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have been prepared, respectively. It is found that PPD
10-ethyl-phenothiazine and 10-ethyl-10H-phenothiazine-3-
carbaldehyde were prepared and purified according to the
assembled with rigid p-phenylenediamine as the bridge16,17
exhibits the MFC behavior, while the compound (PED)
reported methods (Scheme S1).11b,18 The synthetic routes of
connected with ethylenediamine fail to emit the shifted
two compounds were showed in Scheme 1.
fluorescence under the grinding. PPD has displayed a force-
induced color change from yellow to orange that could be
reversed by fuming with CHCl3. Further study indicates that
the stacking transformation from H-aggregate to the
amorphous solid under the external stimuli might be
responsible for the MFC performance. The maintenance of
crystal phase of PED after grinding should be responsible for its
absence of MFC.
Experimental section
Materials and instrumentation
Scheme 1 The synthetic routes and the structures of PPD and
PED.
All raw materials were commercially purchased and used
without further purification. The 1H NMR and 13C NMR spectra
were recorded on a Bruker Avance III 400 MHz spectrometer
at 400 MHz and 100 MHz by using CDCl3 and DMSO-d6 as the
solvents. Elemental analyses were performed on an Elementar
Vario EL cube analyzer. FT-IR spectra were recorded on a
Perkin-Elmer Spectrum One spectrophotometer by
incorporating samples into KBr disks in the experimental range
of 4000–370 cm–1. MS detection was performed on an Agilent
6520 Q/TOF mass spectrometer with an ESI source and an
AgilentG1607A coaxial sprayer (all from Agilent). X-ray powder
diffraction data were collected on a Bruker D5005 X-ray
diffractometer with Cu Kα radiation in the range of 5-40o (λ =
1.5406 Å). DSC measurements were taken using a NETZSCH
STA499F3 QMS403D/Bruker V70 at a heating rate of 10 oC/min
N,N’-bis(10-ethyl-phenothiazin-2-ylmethylene)-p-
phenylenediamine (PPD)
Firstly, p-phenylenediamine (0.54 g, 5 mmol) was added into
20 mL ethanol in 100 mL flask at the room temperature. The
above solution was drop-wise added into an ethanol solution
(40 mL) of 10-ethyl-10H-phenothiazine-3-carbaldehyde (2.60 g,
10 mmol) under refluxing condition, and precipitate was
simultaneously formed. The reaction was allowed to further
reflux for 3h, once the titration had been accomplished. And
then, the reaction mixture was naturally cooled to the room
temperature and the precipitate was collected, washed with
ethanol and dried. Yield: 2.66 g, 91 %. mp: 247-249 °C. IR (KBr,
cm−1): 2975, 2853, 1651, 1596, 1574, 1552, 1496, 1443, 1367,
1
o
1325, 1281, 1246, 1192, 1132, 1101, 968, 752. H NMR (400
under N2 atmosphere in the temperature range of 25–400 C.
MHz, CDCl3, 25 °C, TMS): δ = 8.37 (s, 2H), 7.69 (s, 2H), 7.65 (d, J
= 8.29 Hz, 2H), 7.26 (s, 4H), 7.18 (t, J = 7.55 Hz, 2H), 7.13 (d, J =
8.05 Hz, 2H), 6.97-6.89 (m, 6H), 3.99 (q, J = 6.69 Hz, 4H), 1.47 (t,
Thermogravimetric analyses were conducted on an SDT Q600
thermogravimetric analyzer at a heating rate of 20 oC/min
under air atmosphere in the experimental range of 50–800 oC.
J = 6.86 Hz, 6H). 13C NMR (100 MHz, CDCl3, 25 C, TMS): δ =
°
Cyclic voltammetry (CV) was obtained on
a CHI604C
157.98, 149.85, 147.52, 143.87, 130.80, 128.58, 127.43, 127.38,
127.15, 124.58, 123.69, 122.92, 121.81, 115.26, 114.66, 42.21,
12.93. ESI-MS m/z: [M+H]+ 583.1976. Anal. Calcd. (%) for
C36H30N4S2: C, 74.19; H, 5.19; N, 9.62. Found (%): C, 74.20; H,
5.21; N, 9.61.
N,N’-bis(10-ethyl-phenothiazin-2-ylmethylene)-ethylenediamine
(PED)
voltammetric analyzer in dichloromethane solution by using
Bu4NBF4 (0.1 M) as a supporting electrolyte. Platinum plate
and platinum wire were used as the working electrode and the
counter electrode, respectively. All potentials were recorded
versus Ag/AgCl as a reference electrode (using ferrocene as
the internal standard). The scan rate was maintained at 100
mV/s. The UV-Vis absorption spectra were obtained by using a
Mapada UV-1800pc spectrophotometer. Photoluminescence
measurements were obtained on a Cary Eclipse fluorescence
spectrophotometer. The fluorescence quantum yields of PPD
and PED in toluene solutions were conducted using quinine
sulfate in 0.1 M sulfuric acid as a standard. The fluorescence
Similar method had been used to prepare PED, except for
the addition of ethylenediamine. Yield: 1.98 g, 74 %. mp: 217-
o
219 C. IR (KBr, cm−1): 2978, 2924, 2860, 1617, 1595, 1575,
1552, 1500, 1463, 1439, 1405, 1371, 1328, 1305, 1284, 1269,
1
°
1250, 1190, 1133, 1106. H NMR (400 MHz, DMSO-d6, 25 C,
2 | J. Name., 2012, 00, 1-3
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TMS): δ = 8.49 (d, J = 8.66 Hz, 4H), 8.17 (d, J = 7.69 Hz, 2H), phenyl unit and imine groups were quite small iVniewPPArDtic,lewOhnliinlee
7.87 (dd, J1 = 1.33 Hz, J2 = 1.33 Hz, 2H), 7.61 (dd, J1 = 2.58 Hz, J2 larger twisted angles between the central ethyl group and
DOI: 10.1039/C9NJ03881G
= 2.14 Hz, 4H), 7.47 (t, J = 7.20, 2H), 7.21 (t, J = 7.62 Hz, 2H), imine moieties in PED were observed. As it was well-known,
4.43 (q, J = 7.13 Hz, 4H), 3.94 (s, 4H), 1.30 (t, J = 7.08 Hz, 6H). the smaller twisted angle among the phenyl rings and imine
13C NMR (100 MHz, DMSO-d6, 25 °C, TMS): δ = 162.72, 141.41, groups indicated a significant potential for a better π-
140.50, 127.97, 126.59, 125.95, 122.71, 122.64, 121.25, 121.01, conjugation. TD-DFT calculations were used to obtain the
119.77, 109.89, 109.67, 61.80, 37.60, 14.16. ESI-MS was not electron transition, and the results were listed in Table 1. The
obtained because PED was easily decomposed. Anal. Calcd. (%) electron transition of PED gave an absorption peak at 360.95
for C32H30N4S2: C, 71.87; H, 5.66; N, 10.48;. Found (%): C, 71.89; nm, which was mainly originated from the transition from
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H, 5.67; N, 10.47.
HOMO to LUMO. Its HOMO was mainly located at
phenothiazine moieties, while LUMO density was decreased at
the phenothiazine unit and increased at the imine moieties.
The results indicated that the central ethyl group prevented π-
conjugation of two terminal moieties. The calculated
absorption peak of PPD was located at 449.47 nm, and it was
ascribed to the transition from HOMO to LUMO. The density
distribution change of HOMO and LUMO of PPD suggested
that an intramolecular charge transfer excited state in PPD.
These results indicated that PPD indeed had a larger π-
Results and discussion
Photophysical properties in solutions
The UV-Vis absorption spectra of the two compounds in
toluene were recorded. As shown in Fig. 1, a maximum peak at
287 nm was found in PED toluene solution, and few weak
peaks from 330 to 360 nm were found. Two absorption bands
at approximately 290 and 410 nm were observed for PPD. The
former band was contributed to the local π-π* transition, while
the latter band was ascribed to an extensive π-conjugation
structure.
conjugation, which had induced
a longer absorption
wavelength. Electrochemical measurements were performed
as well to further understand the effect of the bridging unit
(Fig. S8). As shown in Table S1, PED had a lower HOMO and
higher LUMO energy levels in comparison with PPD. And the
larger π-conjugation induced PDD to possess a higher HOMO
and a decreased LUMO, which led to a red-shifted absorption
band.
Fig. 2 Optimal configurations and frontier orbitals of PPD (A)
and PED (B).
Figure 1B showed the emission spectra of PPD and PED. A
maximum emission peak of PPD was located at 525 nm in
toluene solution. PED gave two emission maxima at 335 and
384 nm in toluene solution. The shorter emission wavelength
of PED was in agreement with its localized π-conjugation. In
addition, DFT calculation suggested that the emission of PPD at
525 nm might be arisen from the intramolecular charge
transfer (ICT). To confirm it, solvent-dependent fluorescence
spectra of PPD were measured. As shown in Fig. S9, the
cyclohexane solution had a maximum at 502 nm, which shifted
to 525 nm in toluene solution. And the maximum peak was
found at 556 nm in polar CH2Cl2, exhibiting the orange
fluorescence. The result revealed that the molecule PPD had a
larger polarity in the excited state than that in the ground
state in accordance with the ICT emission.8a,8c,19
Fig. 1 UV-Vis absorption (A) and fluorescence (B) spectra of
PED and PPD in toluene (concentration: 1.0  10–5 M, λex= 300
nm for PED, λex= 420 nm for PPD).
To further understand the difference between the two
compounds in the absorption spectra, quantum chemical
calculations were performed. Firstly, their optimal geometries
were obtained by the energy optimization. As shown in Fig. 2,
the phenothiazine and imine groups in PED and PPD were
almost coplanar. The twisted angles between the central
Mechanochromic behavior
The crystals of PPD were collected as the reaction was
naturally cooled to the room temperature, while it was well-
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known that Schiff base molecules were poorly dissolved in obtained crystals, and an orange powder was colVleiecwtAerdtic.leTOhnelinne,
ethanol. The crystals were yellow and gave yellow the fine powder was exposed to CHCl3 vapor for a few seconds,
fluorescence. Mortar and pestle were used to finely grind the
Table 1. Photophysical data of two compounds in toluene and from quantum chemical calculations (λex= 300 nm for PED, λex=
DOI: 10.1039/C9NJ03881G
420 nm for PPD).
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Electron transition
λem (nm)
(ΦF)
Absorption extinction
Oscillator
strength
λabs (nm)
Transition assignment
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coefficient (cm−1 L mol−1)
(eV)
(nm)
335, 384
(0.21)
HOMO-1LUMO+1
(23.93%)
HOMOLUMO (67.07%)
PED
PPD
287, 354
0.10x105
0.38 x105
3.4349
360.95
PED
PPD
525
HOMO-1LUMO+1
(2.13%)
HOMOLUMO (95.17%)
290, 410
2.7584
449.47
(0.63)
1
for many times under the mechanical and gas stimuli. The H
NMR spectrum of the O-form solid was obtained and
compared with that of the un-ground Y-form, indicative of the
consistence. The result confirmed that the mechanical grinding
had not ruined the chemical structure in the grinding-fuming
cycles.
Fig. 3 Photos of PPD solid before (left) and after (right) grinding
under (A, C) natural and (B, D) 365 nm light.
the orange color quickly turned to yellow as the grinding was
not taken (Fig. 3A). The fluorescence quantum yield changed
from 27 % for the pristine to 25 % for the grounded solid.
The fluorescence change could be repeatedly processed that
a reversible MFC performance was obviously detected.8b We
resorted to the fluorescence and UV-Vis spectra to further
study the color change of the ground and fumed samples. As
shown in Fig. 4, the crystals collected from ethanol exhibited a
maximum emission at 543 nm, whose fluorescence color was
yellow (Y-form) as could be seen with naked eyes. The
emission wavelength was altered from 543 nm to 567 nm,
once the mechanical force had been applied. And the
fluorescence color of the ground samples was orange (O-form).
When the ground crystals had been exposed to the vapor of
CHCl3 for a few seconds, the color of the samples was quickly
turned to yellow. And similar emission spectrum could be
found as what it was for the un-ground crystals. Once again,
the color could be orange, when the fumed samples had been
mechanically ground. Such transformation could be reversible
Fig. 4 Normalized fluorescence spectra of PPD (A) in the solid
state at different states and (B) the maximum fluorescence
emission upon grinding and fuming with CHCl3, repeatedly.
Inset: photos of PPD in response to the grinding and fuming.
Mechanofluorochromism had been achieved as it was
discussed in the context. We turned to further study the UV-
Vis spectra of the as-synthesized and ground samples (Fig. 5).
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The as-synthesized crystals of PPD exhibited a wide absorption respect to the transfer between the crystalline and the
View Article Online
DOI: 10.1039/C9NJ03881G
with a maximum of 372 nm. It was blue-shifted compared with amorphous states.20
the maximum peak in cyclohexane solution (400 nm),
Thermogravimetric analysis suggest their better heat
revealing a face-to-face H-aggregate. The O-form solid had a stability (Fig. S10 and S11). In addition, orange fluorescence of
red-shifted absorption band with a maximum of 415 nm. It ground solid could be restored to yellow one, when the
revealed that the grinding upon the crystals had significantly ground solid was heated at 180 °C for 60 seconds. Differential
destroy H-aggregate in pristine solid.
scanning calorimetry (DSC) measurements were performed to
investigate the phase transition induced by the thermal
annealing.10a,21 The DSC curve (Fig. 7) of the pristine PPD
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exhibited only
a single endothermal peak at 248 °C,
corresponding to its melting point. However, the ground
powders of PPD showed a new exothermal transition peak at
173 °C, indicative of a distinct phase to the as-synthesized
crystals. This exothermal peak can be assigned to the cold-
crystallization process, implying that that the ground solid was
in a metastable state. Under heating, the molecular thermal
motion was intensified to prompt the crystals in a more stable
phase, resulting in the fluorescence recovery.
Fig. 5 UV-Vis absorption spectra of PPD in the as-synthesized
and ground states.
Fig. 7 DSC thermograms of the as-synthesized PPD crystals and
the ground PPD powder under nitrogen atmosphere at a
heating rate of 10 °C/min.
Once ethylenediamine was employed instead of p-
phenylenediamine, PED could precipitate as yellowish crystals
from ethanol as well (Fig. S12). The as-synthesized crystals
emitted strong blue fluorescence under 365 nm light, but
fluorescence color change was not observed, implying the
absence of MFC. It was noted that no distinct shifts were
observed in the UV-Vis absorption and emission spectra of the
ground product in comparison with the pristine as-synthesized
crystals (Fig. S13), which further confirmed the absence of
MFC behavior. The fluorescence quantum yield of PED in
toluene solution was 0.21, which was smaller than that of PPD.
XRD patterns were collected to study the potential phase
transition after grinding. However, the diffraction peaks of
ground powder were almost kept unchanged in comparison
with the as-synthesized samples (Fig. S14). The above results
clearly illustrated that the mechanical force stimuli did not
promote a phase transition and the conversion of π-stacking,
which might be responsible for the unchanged fluorescence
after grinding.
Fig. 6 XRD patterns of PPD in different solid states.
To further study the phase transition during the MFC
process, powder X-ray diffraction patterns of PPD were
collected. Crystalline was confirmed as sharp peaks were
obviously seen in the experimental range of 5-40 degree for
the as-synthesized sample of PPD (Fig. 6). The diffraction peaks
of the ground PPD were weakened and some weak peaks in
the as-synthesized solid disappeared. The results revealed that
the ordered stacking was impaired and the ground solid was in
an amorphous state. Moreover, the diffraction intensity can be
recovered through fuming with CHCl3 and the diffraction peaks
were similar to those of the as-synthesized solid, which proved
the regeneration of the Y-form solid. This result also suggested
that the reversible MFC process of PPD could be achieved in
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(b) Z. Ma, Z. Wang, X. Meng, Z. Ma, Z. Xu, Y. Ma and X. Jia,
Angew. Chem. Int. Ed., 2016, 55, 519.
Conclusions
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DOI: 10.1039/C9NJ03881G
Two Schiff base phenothiazine derivatives with distinct
bridge units had been prepared and characterized, and the
effects of bridging unit on the photophysical and MFC behavior
had been investigated. MFC behavior was observed for PPD
with a rigid and conjugated bridge, while no MFC had been
detected for PED with a flexible and poorly conjugated bridge.
The results indicated that a larger conjugated would have
contributed to the MFC performance.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
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This work was financially supported by the Foundation of
Development Program of Future Expert in Tianjin Normal
University (WLQR201804) and the Science &Technology
Development Fund of Tianjin Education Commission for Higher
Education (2018ZD12).
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DOI: 10.1039/C9NJ03881G
Mechanofluorochromism of Schiff-based phenothiazine derivatives
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Zicheng Yanga, Pengchong Xueb,*, Lulu Zhanga, Peng Chena,*
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Schiff-based phenothiazine derivatives bearing ethylenediamine and p-
phenylenediamine have been prepared. Molecule PDD bridged with rigid p-
phenylenediamine unit show reversible mechanofluorochromism, but one with
ethylenediamine did not change its fluorescence after grinding.