Phenothiazine and diphenylsulfone-based donor–acceptor π-systems exhibiting remarkable mechanofluorochromism

Article abstract of DOI:10.1016/j.dyepig.2020.108868

Two donor–acceptor luminophores PTZ-DPS and BPTZ-DPS, which are constructed by electron-donor phenothiazine and electron-acceptor diphenylsulfone moiety, are designed and synthesized. Through the effective combination of phenothiazine and diphenylsulfone unit, PTZ-DPS and BPTZ-DPS molecules adopt twisted molecular conformation, unique intramolecular charge transfer (ICT) behavior and intense fluorescence in both solution and solid state. The solid-state luminescence efficiencies of the two luminophores reach 32.84% and 39.69%, respectively. It is particularly noteworthy that both PTZ-DPS and BPTZ-DPS exhibit switchable mechanofluorochromism with high contrast. Upon external force stimulation, the fluorescence colors of the PTZ-DPS and BPTZ-DPS solid powders that change dramatically from initial bright green and yellow to orange, respectively, are observed. Meanwhile, the emission bands red shift from 499 nm to 564 nm–589 nm and 592 nm, respectively, suggesting the large spectral red shifts of 90 nm and 28 nm. Moreover, the mechanofluorochromic (MFC) behavior of PTZ-DPS and BPTZ-DPS also show good reversibility when fume the ground powders by DCM vapor. PXRD and spectral data suggest that the phase transition between crystalline to amorphous states is responsible for MFC behavior of PTZ-DPS and BPTZ-DPS, and the red-shift in the PL spectrum upon grinding originates from the planarization of the molecular conformation and subsequent planar intramolecular charge transfer (PICT).

Full text of DOI:10.1016/j.dyepig.2020.108868

Dyes and Pigments 184 (2021) 108868
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Phenothiazine and diphenylsulfone-based donor–acceptor π-systems
exhibiting remarkable mechanofluorochromism
,
Hongke Zhoua, Qian Huanga, Xingliang Liua, Defang Xub *, Weidong Zhanga, Shengjie Fua,
Xiucun Fenga, Zhan Zhanga
a School of Chemical Engineering, Qinghai University, Xining, 810016, China
b State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University, Xining, 810016, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Two donor–acceptor luminophores PTZ-DPS and BPTZ-DPS, which are constructed by electron-donor pheno-
thiazine and electron-acceptor diphenylsulfone moiety, are designed and synthesized. Through the effective
combination of phenothiazine and diphenylsulfone unit, PTZ-DPS and BPTZ-DPS molecules adopt twisted
molecular conformation, unique intramolecular charge transfer (ICT) behavior and intense fluorescence in both
solution and solid state. The solid-state luminescence efficiencies of the two luminophores reach 32.84% and
39.69%, respectively. It is particularly noteworthy that both PTZ-DPS and BPTZ-DPS exhibit switchable
mechanofluorochromism with high contrast. Upon external force stimulation, the fluorescence colors of the PTZ-
DPS and BPTZ-DPS solid powders that change dramatically from initial bright green and yellow to orange,
respectively, are observed. Meanwhile, the emission bands red shift from 499 nm to 564 nm–589 nm and 592
nm, respectively, suggesting the large spectral red shifts of 90 nm and 28 nm. Moreover, the mechano-
fluorochromic (MFC) behavior of PTZ-DPS and BPTZ-DPS also show good reversibility when fume the ground
powders by DCM vapor. PXRD and spectral data suggest that the phase transition between crystalline to
amorphous states is responsible for MFC behavior of PTZ-DPS and BPTZ-DPS, and the red-shift in the PL
spectrum upon grinding originates from the planarization of the molecular conformation and subsequent planar
intramolecular charge transfer (PICT).
Phenothiazine
Diphenylsulfone
Intramolecular charge transfer
Mechanofluorochromism
1. Introduction
involves the changes in the molecular packing mode, but does not alter
the chemical structure of molecules. And thus control of molecular
Mechanofluorochromic (MFC) material [1] is an important kind of
intelligent fluorescent material which can change the emission colors or
intensities of solid-state fluorescence under the external force stimula-
tion, such as shearing, pressing, grinding, deformation, etc.). Recently,
the design and synthesis of MFC materials have aroused increased in-
terest due to their fundamental importance and potential applications in
information storages [2], mechano-sensors [3], security inks [4], and
anti-counterfeiting materials [5]. In principle, there are two effective
ways, the chemical and physical methods, to achieve the tuning of the
solid-state fluorescence of conjugated organic materials [6]. The
chemical method involves solid-state chemical reactions, which not only
cause changes in the chemical structure of molecules, but also have some
shortcomings including insufficient conversion, irreversibility as well as
loss of luminescence of reactants. In contrast, the physical method shows
good prospects and advantages. That is because this method only
packing becomes a promising strategy to tune the optical properties of
fluorophores due to the strong dependence of solid-state optical prop-
erties on the molecular packing arrangements. Therefore, various types
of MFC molecules, including the derivatives of tetraphenylethene [7],
carbazole [8], 3,6-bis(aryl)-1,4-diketo-pyrrolo[3,4-c]pyrroles (DPPs)
[9], 9,10-divinylanthracene [10] and oligo(p-phenylene vinylene) [11],
as well as some organoboron complexes [12,13], which can change their
molecular stacking modes, have been designed and prepared to date.
Due to the close relationships between the molecular stacking modes
and the molecular conformations as well as the intermolecular in-
teractions, the desired packing modes can be gained by properly
modifying the molecular structures of specific fluorophores. The change
of high contrast fluorescence color before and after stimulation by
external force and good solid-state luminescence efficiency are two
important indexes to evaluate MFC materials. In general, for an organic
* Corresponding author.
E-mail address: xudefang123@163.com (D. Xu).
https://doi.org/10.1016/j.dyepig.2020.108868
Received 14 August 2020; Received in revised form 12 September 2020; Accepted 15 September 2020
Available online 19 September 2020
0143-7208/© 2020 Elsevier Ltd. All rights reserved.

H. Zhou et al.
Dyes and Pigments 184 (2021) 108868
Scheme 1. The molecular structures of PTZ-DPS and BPTZ-DPS.
Scheme 2. Synthetic routes of PTZ-DPS and BPTZ-DPS.
luminophores, the highly twisted molecular conformation can effec-
tively inhibit intermolecular interactions and strong stacking in the
2. Experimental section
π-π
aggregate state to a certain extent, which avoids the appearance of the
infamous aggregation-induced quenching (ACQ) effect [14] that are
commonly seen in organic fluorescent molecules and thus results in
solid-state fluorescence [15]. Not only that, this twisted molecular
conformation can also promote the dye molecules to form loose packing
in the crystalline state, which are conducive to the changes of stacking
modes under the external force. Therefore, much effort has been devoted
to design and prepare organic molecules with twisted molecular con-
formations, and many MFC fluorophores have been reported [16].
However, in fact, there are still some disputes about the mechanism of
MFC behavior at present. The design rules reported in the literatures are
only applicable to specific molecules. So there is no general design
strategy for the design of MFC molecules. It is difficult to determine
whether the designed molecules have MFC properties. Therefore, it is
urgent to design and prepare new MFC molecules to understand the
underlying mechanism of MFC phenomenon in depth, so as to establish
the internal relationship between molecular structure and performance.
Due to fascinating optical properties and potential applications range
from organic light-emitting diodes (OLED) to chemosensors [17], elec-
2.1. Materials and measurements
General. The 1H and13C NMR spectra were obtained with a Bruker-
Avance III (400 MHz and 100 MHz) spectrometer by using CDCl3 as
the solvent and tetramethylsilane (TMS) as the internal standard. The
high-resolution mass spectrometry (HRMS) was carried out using a
matrix-assisted laser desorption ionization time-of-flight mass spec-
trometry (MALDI-TOF MS, Performance (Shimadzu, Japan)). The IR
spectra were measured with a Nicolet-360 FTIR spectrometer by incor-
poration of samples in KBr disks. Elemental analyses for C, H, N were
carried out on a Perkin-Elmer 240C elemental analyzer. The UV–vis
absorption spectra of solution/solid samples were recorded on a Shi-
madzu UV-2550 spectrophotometer and fluorescence spectra on a Cary
Eclipse fluorescence spectrophotometer. Quantum yields and time-
resolved fluorescence decay profiles of the solid samples were recor-
ded on an Edinburgh FLS920 steady state and time-resolved fluores-
cence spectrometer. Cyclic voltammetry was performed at room
temperature in a three electrode cell by using a Metrohm Autolab
PGSTAT204 Electrochemical Workstation with a scan rate at 50 mV sꢀ 1
.
tron donor-acceptor (D–A) structured
π-systems have been the subject of
The working electrode was a platinum button, the counter electrode was
a platinum wire, and Ag/Ag+ was used as reference electrode.
(C4H9)4NBF4 was used as the supporting electrolyte in dry DCM. Powder
X-Ray diffraction (PXRD) patterns were collected by a Bruker D8 Focus
Powder X-ray diffraction instrument. The HOMO/LUMO orbitals of
target compounds PTZ-DPS and BPTZ-DPS were calculated at the
B3LYP/6-31G (d, p) level with the Gaussian 09W program package.
Mechanofluorochromic and stimuli-recovering experiments
intense research. More importantly, the electronic structures and thus
optoelectronic properties of this kind of luminogens can be facilely
modulated by tuning the electron donor and/or acceptor strengths.
Current researches show that D–A molecules bearing obvious ICT
characteristics and twisted molecular conformation exhibit ideal
solid-state luminescence properties and MFC behavior in vividly con-
trasting colors [16,18]. Herein, we designed and synthesized two D–A
structured molecules PTZ-DPS and BPTZ-DPS (Scheme 1), in which
phenothiazine and diphenylsulfone unit are selected as
electron-donating and electron-accepting segments, respectively.
Through this reasonable combination of bowl-shaped phenothiazine and
diphenylsulfone, the two compounds possessing the twisted molecular
conformation are obtained. As expected, PTZ-DPS and BPTZ-DPS show
obvious ICT effect, intense fluorescence in both solution and solid state,
and high contrast mechanochromic luminescence.
Grinding experiment: The as-prepared solid powders of PTZ-DPS and
BPTZ-DPS were put into a mortar and then ground with a pestle at room
temperature. Solvent-fuming experiment: the ground samples were
suspended above the solvent level in a sealed DCM containing beaker
and then were exposed to the vapor for 30 s at room temperature.
Materials. DMF was was dried over CaH and distilled under reduced
pressure before use. THF was dried and purified by distilling it from
sodium and benzophenone in an atmosphere of dry nitrogen. The other
2

H. Zhou et al.
Dyes and Pigments 184 (2021) 108868
Fig. 1. Normalized UV–vis absorption spectra (a: PTZ-DPS; b: BPTZ-DPS) and normalized PL spectra (c: PTZ-DPS; d: BPTZ-DPS) in solvents with different polarities
(1.0 × 10ꢀ 5 mol Lꢀ 1, λex = 410 nm); Plots of Stokes shift (Δ
υ) vs solvent polarity parameter (Δf) (e: PTZ-DPS; f: BPTZ-DPS); Photos in various solvents under 365 nm
UV light (g: PTZ-DPS; h: BPTZ-DPS. From left to right: Hexane, Cyclohexane, Toluene, Chloroform, Ethyl acetate, THF, DCM, DMF).
reagents and solvents were used as received from commercial suppliers
without further purification.
2.2. Synthesis
2.2.1. (E)-3-(4-((4-bromophenyl)sulfonyl)styryl)-10-ethyl-10H-
phenothiazine (PTZ-DPS)
Compound
3
(1.34 g, 5.29 mmol) and 4,4′-sulfonylbis
3

H. Zhou et al.
Dyes and Pigments 184 (2021) 108868
(bromobenzene) (1.99 g, 5.29 mmol) were dissolved in dry DMF, and
then anhydrous potassium carbonate (2.20 g, 15.92 mmol), tetrabuty-
lammonium bromide (2.50 g, 7.76 mmol) and Pd(OAc)2 (30 mg, 0.134
mmol) was added. The mixture was heated to reflux for 6 h under an
atmosphere of nitrogen. After being cooled to room temperature, the
mixture was poured into 400 mL water with stirring and then extracted
three times with CH2Cl2. The combined organic phases were washed
with brine, and then dried over anhydrous Na2SO4. The solvent was
removed and the crude product was purified on silica gel column
chromatography by using petroleum ether/CH2Cl2 (v/v = 1/2) as
eluent, giving a light yellow-green solid (1.90 g). Yield 65%. 1H NMR
(400 MHz, CDCl3) δ 7.88 (d, J = 8.4 Hz, 2H), 7.81 (d, J = 8.8 Hz, 2H),
7.65 (d, J = 8.4 Hz, 2H), 7.57 (d, J = 8.0 Hz, 2H), 7.30-7.26 (m, 2H),
7.18-7.06 (m, 3H), 6.94-6.86 (m, 4H), 3.96-3.94 (m, 2H), 1.44 (t, J =
6.8 Hz, J = 6.8 Hz, 3H) (Figs. S3 and S4); 13C NMR (100 MHz, CDCl3) δ
142.83, 141.01, 138.88, 132.57, 131.41, 131.37, 130.70, 129.07,
128.29, 128.14, 127.38, 126.88, 126.52, 125.18, 125.15, 125.11,
125.09, 124.87, 124.73, 124.66, 124.61, 124.52, 122.72, 122.67,
115.14, 114.96, 41.97, 13.01 (Fig. S5); HRMS (MALDI-TOF) m/z: [M]+
Calcd for C28H22BrNO2S2 547.0275; Found 547.0266 (Fig. S6); IR (KBr,
Table 1
UVvis absorption and fluorescence Properties of PTZ-DPS and BPTZ-DPS in
different solvents.
Compound
a
Solvent
λabs/nm (
ε
/Mꢀ 1
λem
/
Δ
υst
/
Φfb
cmꢀ 1
)
nm
cmꢀ 1
5143
PTZ-DPS
hexane
312 (20200), 392
(14500)
491
493
533
567
568
577
591
617
491
493
532
563
566
574
589
614
0.35
0.26
0.25
0.25
0.21
0.20
0.17
0.05
0.34
0.26
0.20
0.18
0.18
0.17
0.14
cyclohexane
toluene
chloroform
ethyl acetate
THF
310 (21100), 393
(15100)
5161
6176
7363
7711
7731
8017
8730
5014
5032
5955
7052
7584
7454
7775
8405
309 (19800), 401
(15000)
308 (18900), 400
(14000)
307 (22500), 395
(17400)
307 (23900), 399
(18400)
DCM
306 (23200), 401
(17300)
DMF
312 (21200), 401
(17200)
BPTZ-DPS
hexane
306 (51700), 394
(40100)
cmꢀ 1):
538、689、717、767、941、990、1075、1218、1300、
cyclohexane
toluene
chloroform
ethyl acetate
THF
306 (52000), 395
(40400)
1345、1410、1530、1597、1661. Anal. Calcd (%) for C28H22BrNO2S2:
311 (50900), 404
(41600)
C 61.31, H 4.04, N 2.55; Found: C 61.12, H 4.25, N 2.44.
309 (53100), 403
(42600)
2.2.2. 3,3’-((1E,1′E)-(sulfonylbis(4,1-phenylene))bis(ethene-2,1-diyl))bis
(10-ethyl-10H-phenothiazine) (BPTZ-DPS)
309 (52200), 396
(43900)
BPTZ-DPS was synthesized from compound 3 (2.96 g, 11.70 mmol),
4,4′-sulfonylbis(bromobenzene) (2.00 g, 5.32 mmol) anhydrous potas-
sium carbonate (4.40 g, 31.84 mmol), tetrabutylammonium bromide
(5.00 g, 15.51 mmol) and Pd(OAc)2 (40 mg, 0.178 mmol) following the
same procedure as that of PTZ-DPS, affording a bright yellow solid
(3.26 g). Yield 85%. 1H NMR (400 MHz, CDCl3) δ 7.90 (d, J = 8.4 Hz,
4H), 7.56 (d, J = 8.4 Hz, 4H), 7.27 (t, J = 6.8 Hz, J = 6.4 Hz, 4H), 7.18-
7.05 (m, 6H), 6.95-6.84 (m, 8H), 3.94 (s, 4H), 1.43 (t, J = 6.8 Hz, J = 6.8
Hz, 6H) (Figs. S7 and S8); 13C NMR (100 MHz, CDCl3) δ 145.00, 144.17,
142.40, 139.69, 131.10, 130.85, 130.82, 128.03, 127.38, 126.78,
126.47, 125.16, 124.82, 124.74, 124.70, 124.68, 123.60, 122.67,
115.18, 115.02, 53.50, 42.04, 12.92 (Fig. S9); HRMS (MALDI-TOF) m/z:
[M]+ Calcd for C44H36N2O2S3 720.1939; Found 720.1929 (Fig. S10); IR
308 (53700), 402
(45100)
DCM
309 (53600), 404
(43400)
DMF
310 (50000), 405
(44500)
˂
0.01
a
Δ
υst = υabs-υem.
b
The fluorescence quantum yield (Φf) was measured using quinine sulfate as a
standard (Φf = 0.546 in 0.5 mol Lꢀ 1 H2SO4).
3.2. Photophysical properties of PTZ-DPS and BPTZ-DPS in solutions
In the molecular framework of compounds PTZ-DPS and BPTZ-DPS,
there are typical electron donor phenothiazine and electron acceptor
diphenylsulfone units [22]. The combination of these two units is ad-
vantageous to the occurrence of intramolecular charge transfer (ICT)
process. Generally, the photophysical properties of D–A molecules in
solution are strongly dependent on the polarity of solvents. Therefore,
the optical properties of PTZ-DPS and BPTZ-DPS were investigated by
UV–visible absorption (Fig. 1a and b) and photoluminescence (PL)
spectra (Fig. 1c and d) in varying solvents. As depicted in Fig. 1a and b,
both compounds possess very similar absorption spectra showing two
intense major absorption bands in the solvents of different polarities
with shorter/longer-wavelength absorptions of ca. 310/395 and ca.
310/400 nm, respectively. The former band can be attributed to the
(KBr,
cmꢀ 1):
534、652、694、743、913、984、1078、1190、
1358、1402、1565、1602、1659. Anal. Calcd. (%) for C44H36N2O2S3:
C 73.30, H 5.03, N 3.89; Found: C 73.11, H 5.23, N 4.06.
3. Result and discussion
3.1. Synthesis of PTZ-DPS and BPTZ-DPS
The synthesis of PTZ-DPS and BPTZ-DPS is showed in Scheme 2. The
general route to PTZ-DPS and BPTZ-DPS started from phenothiazine.
Firstly, the 3-aldehyde phenothiazine possessing an ethyl was obtained
by Vilsmeier reactions of N-ethyl-substituted phenothiazine [19], which
was synthesized by the alkylation of phenothiazine by using bromo-
ethane and NaH in DMF [19]. Then, the important precursor 3 was
πꢀ π* transitions, while the latter comes from the ICT transitions be-
tween electron donor phenothiazine and electron acceptor diphe-
nylsulfone segments. This conclusion can be confirmed by the
solvent-dependent absorption spectra [23]. In hexane, the
longer-wavelength absorption bands of PTZ-DPS and BPTZ-DPS appear
at 392 and 394 nm, respectively. With the increase of solvent polarity,
they show a slight red shift trend and reach 401 and 405 nm in DMF. In
contrast, the PL spectra of PTZ-DPS and BPTZ-DPS are strongly affected
by the polarities of the solvents. As shown in Fig. 1c and d, PTZ-DPS and
BPTZ-DPS both exhibit a significant red-shift and broad spectra when
the solvent polarity increased from hexane to DMF, respectively. For
example, in hexane, the emission peaks of PTZ-DPS and BPTZ-DPS both
appear at 491 nm, with the increase of solvent polarity, they red shift to
567 and 563 nm in chloroform, and finally reach 617 and 614 nm in
DMF, respectively, indicating an obvious bathochromic effect.
synthesized
via
Wittig
reaction
between
methyl-
[20].
triphenylphosphoniumiodine (Ph3PCH3I) and compound
2
Finally, Heck reaction [21] catalyzed by Pd(OAC)2 was carried out be-
tween the precursor 3 with 4,4′-sulfonylbis(bromobenzene) to give the
target compounds PTZ-DPS (1.0 equiv of 3) and BPTZ-DPS (2.2 equiv of
3) with good yields of 65% and 85%, respectively. All the intermediates
and the target compounds were purified by column chromatography,
and 1H NMR, 13C NMR, the high-resolution mass spectrometry (HRMS),
and elemental analysis (C, H and N) were employed to confirm the
chemical structures of the new compounds. In organic solvents (CHCl3,
CH2Cl2, THF, benzene and toluene), PTZ-DPS and BPTZ-DPS exhibit
good solubility, but they show poor solubility in alcohols (methanol and
ethanol) and aliphatic hydrocarbon solvents (hexane and cyclohexane).
4

H. Zhou et al.
Dyes and Pigments 184 (2021) 108868
Fig. 2. Optimized geometry for PTZ-DPS and BPTZ-DPS using Gaussian 09 programme at the B3LYP/6-31G(d,p) level.
Accordingly, the fluorescence color of PTZ-DPS and BPTZ-DPS solutions
changes from blue-green to yellow, and finally to orange red (Fig. 1g and
h). The above results directly point to the occurrence of ICT between the
donor and the acceptor [23], which would enhance the dipole-dipole
interaction between the dye and the solvent molecules, and thus de-
creases the excited state energy as a result of the solvent relaxation. In
addition, it should be noted that both compounds exhibit well-defined
fluorescence bands in nonpolar solvents, indicating the existence of
two separated close-lying excited states. We believe that the emission of
PTZ-DPS and BPTZ-DPS in hexane and cyclohexane originates from the
locally excited (LE) state [24]. However, in polar solvents, their spectral
bands become structureless and broader. This further demonstrates the
occurrence of the extensive ICT in polar environment. The Stokes shifts
solvent, respectively.
The orientational polarizability Δf that is described in eq (2) is
chosen as the measure of polarity.
ε
ꢀ 1
n2 ꢀ 1
Δf =
ꢀ
(2)
2ε
+ 1 2n2 + 1
where ε and n are the static dielectric constant and the optical refractive
index of the solvent, respectively.
As shown in Fig. e, f, the linear relationship between Δν and Δf and
the large slope of Δ
υ
and Δf curves (PTZ-DPS: 11760 cmꢀ 1; BPTZ-DPS:
17880 cmꢀ 1) suggest that the dipole moment of ICT excited state is
larger than that of ground state because of the substantial charge
redistribution, which may be stemmed from the relaxation of the
initially formed Franckꢀ Condon excited state. At the same time, the
(Δυ) of PTZ-DPS and BPTZ-DPS in varying solvents are calculated and
collected in Table 1. We can see clearly that the Stokes shifts of the two
compounds increase from 5143 to 5014 cmꢀ 1 in hexane to 8730 and
8405 cmꢀ 1 in DMF, respectively. The solvatochromic behavior of the
two compounds was quantitatively described by the Lippertꢀ Mataga
equation [25].
nearly equal slope of Δυ and Δf curves also means that the ICT degree of
the two compounds is similar. Moreover, we measured the fluorescence
quantum yields (Φf) of PTZ-DPS and BPTZ-DPS in different solvents by
using quinine sulfate (Φf = 0.546, 0.5 mol Lꢀ 1 H2SO4) as the standard,
which were collected in Table 1. Both the two compounds exhibit a
positive solvatokinetic effect. When the solvent polarity increases from
hexane to DMF, the Φf of PTZ-DPS and BPTZ-DPS decreases from 0.35
to 0.34 in hexane to 0.25 and 0.18 in chloroform, and in DMF it is finally
reduced to 0.05 and ˂ 0.01, respectively, which shows a significant
downward trend in response to a positive solvatokinetic effect [8a]. It is
believed that the relatively strong fluorescence of PTZ-DPS and
BPTZ-DPS in nonpolar solvents originates from restricted ICT
2Δf
Δ
υ
=
υa
ꢀ
υe
=
(
μE
ꢀ
μG)2 + constant
(1)
hca3
where υa and υe represent the maximum absorbance and emission
wavenumbers, μG and μE are the dipole moments in the ground and
excited states, and h, c, a, and n are the Planck constant, the speed of
light, the Onsager solvent cavity radius, and the refractive index of the
Fig. 3. Calculated spatial distributions of HOMO and LUMO levels for PTZ-DPS and BPTZ-DPS using Gaussian 09 programme at the B3LYP/6-31G(d,p) level.
5

H. Zhou et al.
Dyes and Pigments 184 (2021) 108868
Fig. 4. The normalized PL spectra of different solids (as-prepared powder, ground powder, and fumed powder) for PTZ-DPS (a, λex = 420 nm) and BPTZ-DPS (b, λex
= 450 nm). The fluorescence images of different solids for PTZ-DPS (c) and BPTZ-DPS (d) under 365 nm UV light.
transitions for the LE state, while the extreme weak emission in polar
solvents is attributed to the fast interconversion from the emissive LE
state to the low emissive ICT state.
3.3. Mechanofluorochromic (MFC) properties
As discussed above, PTZ-DPS and BPTZ-DPS show similar emission
performance in solutions. However, it can be seen that the emitting
colors of the two compounds are significantly different from those of the
as-prepared powders or the ground powders under UV illumination.
Moreover, the emission switch could be obtained by repeating the
treatment of mechanical grinding and fuming with DCM vapor. In order
to investigate such solid emission properties that depend on the
morphology and molecular structure, the PL spectra of PTZ-DPS and
BPTZ-DPS in different solid states are measured. As shown in Fig. 4,
upon illumination by 365 nm UV light, the as-prepared crystalline
powders of PTZ-DPS and BPTZ-DPS could emit strong green and yellow
light with the luminescence efficiencies as high as 32.84% and 39.69%,
respectively. Meanwhile, the emission bands of the two as-prepared
samples locate at 499 nm and 564 nm, respectively. After grinding by
using a mortar and pestle, the as-prepared crystalline powders of the two
compounds are transformed into ground powders. At the same time, the
emission bands of as-prepared samples red shift to 589 nm and 592 nm
(Fig. 3a and b), respectively, which implies that the grinding-induced
large spectral red-shifts for the two luminophores are up to 90 nm and
28 nm, and the bright orange emitting ground solids with the slightly
reduced luminescence efficiencies of 21.49% and 25.05% for both
compounds are observed (Fig. 3c and d). The above test results fully
indicate that PTZ-DPS and BPTZ-DPS have significant MFC nature.
Interestingly, after the ground samples of the two compounds are placed
in DCM vapor for 30 s, the orange emitting ground powders of PTZ-DPS
and BPTZ-DPS could be transferred into green and yellow emitting
solids that are similar to the as-prepared powders. Accordingly, their
emission peaks return to positions that are almost the same as the ones of
the initial powders. And when the fumed samples are re-ground, the
same effectiveness as the first grinding are observed again. This illus-
trates that the MFC behavior of PTZ-DPS and BPTZ-DPS have good
reversibility, and the reversible fluorescence color change could be
repeated many times by alternate grinding and fuming cycles (Fig. S2)
(see Fig. 5).
The electrochemical properties of the target componds PTZ-DPS and
BPTZ-DPS were investigated by cyclic voltammetry (CV) (Fig. S1). From
the redox potential onset, we can calculate the HOMO and LUMO levels
(Table S1). Both PTZ-DPS and BPTZ-DPS exhibit two reversible redox
couples in the cathodic region. PTZ-DPS shows two cathodic redox
processes at the potentials of ꢀ 1.08 and ꢀ 1.82 V (vs. Fc/Fc+). For BPTZ-
DPS, the two cathodic redox processes at a potential of ꢀ 1.11 and
ꢀ 1.75 V (vs. Fc/Fc+). By combination with the electron transition
spectra, we can find that the LUMO energy levels of PTZ-DPS and BPTZ-
DPS are ꢀ 3.70 eV and ꢀ 3.64 eV, and the HOMO energy levels are
ꢀ 6.34 eV and ꢀ 6.26 eV, respectively (Table S1). In order to gain deeper
insight into the effect of the geometric and electronic structures of PTZ-
DPS and BPTZ-DPS on their photophysical properties, density func-
tional theory (DFT) calculations were performed at the B3LYP/6-31G(d,
p) level using Gaussian 09. The optimized molecular structures together
with the frontier molecular orbital profiles that exhibit electronic dis-
tributions in their HOMO and LUMO are shown in Fig. 2 and Fig. 3,
respectively. From Fig. 2, we can see clearly the optimized geometries of
PTZ-DPS and BPTZ-DPS both display the V-shaped conformations with
the bond angles of C–S–C as 109.9◦ and 106.1◦, respectively. At the same
time, the bowl-shaped conformation of phenothiazine unit in two
compounds increases the warping degree of the whole molecule. Such
strongly twisted molecular skeletons of PTZ-DPS and BPTZ-DPS can
effectively prevent close packing and π-π interactions of molecules in the
solid state. In other words, this leads to loose packing of molecules in the
crystalline state, which probably endows the materials with obvious
MFC behavior. As shown in Fig. 3, in each case, the HOMO orbital is
concentrated predominantly on the phenothiazine units and the styrene
moieties that connect with them, whereas the LUMO orbital is delo-
calized over diphenylsulfone segments together with the part of
phenothiazine units. The shifts of electron densities of PTZ-DPS and
BPTZ-DPS upon excitation imply the occurrence of ICT from the elec-
tron donor phenothiazine units to the electron acceptor diphenylsulfone
moieties.
According to the above research results, both PTZ-DPS and BPTZ-
DPS possess reversible stimuli-responsive fluorochromic properties, and
6

H. Zhou et al.
Dyes and Pigments 184 (2021) 108868
Fig. 5. Photographs of the drawing/erasing cycle for PTZ-DPS and BPTZ-DPS under 365 nm UV light lamp.
Fig. 6. PXRD patterns of PTZ-DPS (a) and BPTZ-DPS (b): as-prepared sample, ground sample, and fumed powders.
thus they become a kind of smart materials with the potential applica-
tions in optical storage, security papers, Anti-counterfeiting, etc.
Therefore, we develop the simple, facile, and portable ink-free infor-
mation storage paper. As shown in Fig. 5, after coating the solid powders
of PTZ-DPS and BPTZ-DPS on the filter papers using their suspension in
methanol, and the papers showing green and yellow fluorescence are
obtained. When we draw a rabbit and a dolphin on the above papers
with a glass rod directly, respectively, the clear patterns, which are
maintained and can last for a month or more, are observed in orange
fluorescence against the green and yellow backgrounds. Then, the rabbit
and dolphin patterns can be erased quickly by exposing the drawing
papers to the DCM vapor. In the next drawing, a fish and a pig with
orange fluorescence (under UV lamp) are clearly revealed on the papers
again, respectively.
reflections, which indicates that the molecules of the two compounds
exist as well-ordered microcrystals in the solid powders. However, upon
grinding, these sharp peaks disappear. This is because the ordered
stacking of molecules in the crystalline states for the as-prepared sam-
ples is destroyed under the external force, resulting in the amorphous
states. Moreover, when the ground powders of the two compounds are
fumed in the DCM vapor for 1 min, the sharp and intense reflection
patterns corresponding to the as-prepared samples appear again due to
the recrystallization under this condition. Following grinding and
fuming treatments, the luminescence of PTZ-DPS and BPTZ-DPS sam-
ples can transfer between bright orange-emitting color of the ground
solids (amorphous) and strong green- and yellow-emitting colors of the
as-prepared solids (crystalline), respectively, and thus the highly
contrast emissive difference is observed. Therefore, photophysical and
PXRD measurements reveal that the MFC behavior of PTZ-DPS and
BPTZ-DPS originates from the reversible morphological transformation
between the crystalline and amorphous phases.
To gain insights into the mechanism of the MFC behavior of PTZ-DPS
and BPTZ-DPS, powder X-ray diffraction (PXRD) experiments were
conducted on their samples at different solid states. As shown in Fig. 6,
the PXRD patterns of as-prepared solids show sharp and intense
In order to further obtain the fluorescence excited-state decay
Fig. 7. Time-resolved emission-decay curves of PTZ-DPS (a) and BPTZ-DPS (b) solids before and after grinding.
7

H. Zhou et al.
Dyes and Pigments 184 (2021) 108868
4. Conclusion
Table 2
Fluorescence decay parameters of PTZ-DPS and BPTZ-DPS before and after
grinding.
In summary, we have reported the design and synthesis of two D-A
type luminescent switching materials PTZ-DPS and BPTZ-DPS based on
the building units phenothiazine and electron-acceptor diphenylsulfone.
The results show that the push-pull electronic structures and twisted
molecular conformations not only endow PTZ-DPS and BPTZ-DPS with
unique ICT behavior and intense fluorescence in both solution and solid
state, but also enable them to possess good solid-state luminescence and
remarkable and reversible stimulated fluorescence switching properties.
Under stimulation of external force, the solid powders of PTZ-DPS and
BPTZ-DPS, especially those of PTZ-DPS, exhibit the remarkable color
b
b
Sample
τ1 (ns)a
A1
τ2 (ns)a
A2
<
τ
> (ns)c
PTZ-DPS
As-prepared
Ground
3.63
4.40
3.04
3.82
0.62
0.59
0.68
0.65
7.66
8.54
5.63
8.89
0.38
0.41
0.32
0.35
5.16
6.10
3.87
5.59
BPTZ-DPS
As-prepared
Ground
a
Fluorescence lifetime.
Fractional contribution.
Weighted mean lifetime.
b
c
Fig. 8. Normalized solid UV–vis absorption spectra of PTZ-DPS (a) and BPTZ-DPS (b) in different solid states: as-prepared and ground.
information of PTZ-DPS and BPTZ-DPS solids before and after grinding,
the fluorescence lifetimes are measured. The fluorescence decay curves
of the two compounds are illustrated in Fig. 7. From Table 2, we can
observe that there are two relaxation pathways for the as-prepared and
ground solids of PTZ-DPS and BPTZ-DPS in their fluorescence decays,
which suggests that the two compounds possess independent emissions
change that is respectively from bright green and yellow to bright or-
ange. Moreover, the color and emission intensities of PTZ-DPS and
BPTZ-DPS can be switched by DCM vapors. PXRD study discloses that
the force-induced phase transformation of the molecular packing from
crystalline to amorphous states is response for the MFC behavior, and
the obvious red-shifts of PL spectra upon grinding for the two lumino-
phores come from the planarization of the molecular conformation and
subsequent PICT. This study comprehensively reveals the origins of the
stimuli-responsive luminescence properties of PTZ-DPS and BPTZ-DPS,
and points out that they may be the candidates that can be applied to
sensing, display devices with significant color-changing properties and
other fields.
from the segments with different π-conjugation lengths. The different
pathways played different predominant roles for the solids before and
after grinding. For PTZ-DPS and BPTZ-DPS, the excited molecules of the
as-prepared powders mainly decay through the first pathway with the
A1 as 0.62 and 0.68, respectively. However, for their ground sample, the
values of A1 are 0.59 and 0.65, exhibiting an obvious difference. For the
as-prepared solids of PTZ-DPS and BPTZ-DPS, the fluorescence lifetimes
are estimated to be 5.16 ns and 3.87 ns, respectively, while the values
increase to 6.10 ns and 5.59 ns for the ground sample. We believe that
the change of molecular conformation and arrangements in their ag-
gregation states is responsible for the change of fluorescence lifetimes
[26].
Author Statement
Zhou Hongke: Investigation, Formal analysis, Writing-Original draft
preparation, Software. Huang Qian: Formal analysis, Data curation. Liu
Xingliang: Visualization. Xu Defang: Methodology, Conceptualization,
Investigation, Writing-Original draft preparation, Project administra-
tion. Zhang Weidong: Resources, Validation. Fu Shengjie: Writing-
Reviewing and Editing. Feng Xiucun: Software. Zhang Zhan:
Supervision.
It is well known that the release of the twist strain during the
destruction of the crystalline structure will lead to the planarization of
molecular conformation. And thus it will increase the molecular
conjugation, resulting in the red-shift in the PL spectrum. In the case of
PTZ-DPS and BPTZ-DPS, it is believed that the molecules of the two
luminogens may adopt a more twisted conformation in order to fit into
the crystalline lattice in their crystalline structures. When their crys-
talline structures are destroyed by mechanical force, the molecular
conformation tends to be planar. This leads to the extension of the
conjugated length of the molecule and subsequent planar intramolecular
charge transfer (PICT) [10a,15a], and thus resulting in the obvious red
shift of the emission bands. The above conclusion can be confirmed by
comparing the absorption spectra of the powders before and after
grinding. As shown in Fig. 8, upon grinding, the maximum absorption
peaks of PTZ-DPS and BPTZ-DPS samples red shift from 430 nm to 436
nm–437 nm and 442 nm, respectively.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (NNSFC, No. 21662028 and 21965031), the Project
of Qinghai Science & Technology Department (Grant No. 2020-ZJ-923,
2018-HZ-817 and 2016-ZJY01), the Open Project of State Key Labora-
tory of Plateau Ecology and Agriculture, Qinghai University (Grant No.
2018-ZZ-4), and Qinghai Province “High-end Innovative Thousand
8

H. Zhou et al.
Dyes and Pigments 184 (2021) 108868
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.108868.
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