Alkyl length dependent reversible mechanofluorochromism of phenothiazine derivatives functionalized with formyl group

Article abstract of DOI:10.1016/j.dyepig.2017.08.049

A series of D-A typed phenothiazine derivatives functionalized by formyl group (PCAn, n = 1, 2, 4 and 6) with different lengths of N-alkyl chains have been designed and synthesized to systematically investigate the effect of chain length on their solid-state fluorescence properties. The results showed that these compounds emitted strong fluorescence in solutions and solid states with 52%, 42%, 49% and 45% solid-state absolute fluorescence quantum yield (Φ_F), respectively. Their emission wavelengths were strongly affected by solvent polarity, indicating intramolecular charge transfer (ICT) transitions. Interestingly, PCAn solids exhibit not only naked-eye visible and reversible mechanochromic behavior, but chain length-dependent emission properties. PCA1 shows smaller fluorescence spectrum shifts (22 nm) under mechanical force stimuli. Homologs with longer alkyl chains exhibit similar mechanochromic behaviors but larger fluorescence contrasts after grinding except for PCA6. Moreover, the fluorescence emission of ground solid PCA1 and PCA4 can recover at room temperature, PCA2 need high temperatures for fluorescence to be restored, and XRD and DSC revealed that the transformation between crystalline and amorphous states upon various external stimuli was responsible for the MFC behavior. This work demonstrates the feasibility of tuning the solid-state optical properties of fluorescent organic compounds by combining the simple alteration of chemical structure and the physical change of aggregate morphology under external stimuli.

Full text of DOI:10.1016/j.dyepig.2017.08.049

Dyes and Pigments 147 (2017) 537e543
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Alkyl length dependent reversible mechanofluorochromism of
phenothiazine derivatives functionalized with formyl group
Junhui Jia*, Yuying Wu
Key Laboratory of Magnetic Molecules and Magnetic Information Material, Ministry of Education, The School of Chemical and Material Science, Shanxi
Normal University, Linfen, 041004, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of D-A typed phenothiazine derivatives functionalized by formyl group (PCAn, n ¼ 1, 2, 4 and 6)
with different lengths of N-alkyl chains have been designed and synthesized to systematically investigate
the effect of chain length on their solid-state fluorescence properties. The results showed that these
compounds emitted strong fluorescence in solutions and solid states with 52%, 42%, 49% and 45% solid-
state absolute fluorescence quantum yield (FF), respectively. Their emission wavelengths were strongly
affected by solvent polarity, indicating intramolecular charge transfer (ICT) transitions. Interestingly,
PCAn solids exhibit not only naked-eye visible and reversible mechanochromic behavior, but chain
length-dependent emission properties. PCA1 shows smaller fluorescence spectrum shifts (22 nm) under
mechanical force stimuli. Homologs with longer alkyl chains exhibit similar mechanochromic behaviors
but larger fluorescence contrasts after grinding except for PCA6. Moreover, the fluorescence emission of
ground solid PCA1 and PCA4 can recover at room temperature, PCA2 need high temperatures for
fluorescence to be restored, and XRD and DSC revealed that the transformation between crystalline and
amorphous states upon various external stimuli was responsible for the MFC behavior. This work
demonstrates the feasibility of tuning the solid-state optical properties of fluorescent organic compounds
by combining the simple alteration of chemical structure and the physical change of aggregate
morphology under external stimuli.
Received 10 May 2017
Received in revised form
3 August 2017
Accepted 27 August 2017
Available online 30 August 2017
Keywords:
Phenothiazine
Mechanofluorochromism
Chain length-dependent
© 2017 Published by Elsevier Ltd.
1. Introduction
These fluorescent organic molecules can give changes in the color
of their fluorescence under mechanical stress and be restored to
Smart conjugated luminescent materials have attracted a great
deal of attention due to their multi-stimuli responsive emission
characteristics and broad applications in the fields of molecular
probes, organic light-emitting devices (OLED), optical data
recording and data security protection. Particularly, dynamic
altering and switching of solid-state luminescence of organic ma-
terials upon light, heat, pressure or other external factors, is
attracting a lot of interest. Especially, some fluorescent organic
molecules can alter their luminescent properties in response to
external mechanical stimuli such as shearing, grinding, or pressing,
which is known as mechanofluorochromism (MFC) [1e4]. As a kind
of “smart material,” MFC compounds have attracted significant
attention because of their promising applications in pressure sen-
sors, optical storage, rewritable media, and security ink [5e8].
their original state by annealing or fuming with solvent vapor
instead of the chemical alteration of molecular structures [9e14].
However, MFC materials have started to draw increasing research
attention only within the past few years and the types of MFC
materials and in-depth understanding of MFC phenomena are
limited. To date, many kinds of organic molecules have been
exploited to show MFC properties [15e19]. Especially, some
nonplanar
p-conjugated fluorescent molecules, such as tetraphe-
nylethene (TPE), 9,10-divinylanthracene, triphenylamine (TPA) de-
rivatives and organoboron compounds, were preferentially
considered to act as MFC materials, and interestingly, some D-p-A
emissive molecules always contribute to the realization of fluo-
rescence change under mechanical stimuli [20e31]. It is worth
mentioning that phenothiazine was usually introduced to construct
organic luminescent materials as an excellent functional blocks
[32,33]. It has a nonplanar, bowl-shaped configuration, and recently
some studies have found that it can be introduced into molecular
structures to obtain MFC-active fluorescent molecules [34,35].
* Corresponding author.
E-mail address: jiajunhui@sxnu.edu.cn (J. Jia).
http://dx.doi.org/10.1016/j.dyepig.2017.08.049
0143-7208/© 2017 Published by Elsevier Ltd.

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J. Jia, Y. Wu / Dyes and Pigments 147 (2017) 537e543
As is well known the alkyl length can influence molecular
calorimetry (DSC) curves were obtained on a DSC 200 F3 at a
heating rate of 10 ꢀC minꢁ1, and the data of all samples (pristine and
ground) were collected when they were heated for the first time.
Pressing experiment: A quantity of PCAn and KBr powder was
simply mixed in a mortar and then pressed with IR pellet press for
1 min at room temperature under the pressure of 1500 psi.
Annealing experiment: The pressed sample was put into an oven
whose temperature was 30 ꢀC over Tc (cold-crystallization tem-
perature of each compound) for 3 min. Solvent-fuming experiment:
The ground sample was placed above the dichloromethane level
and was exposed to the vapor for 1 min at room temperature. The
molecular configuration was used to obtain the frontier orbitals of
PCAn by density functional theory (DFT) calculations at the B3LYP/
6-31G (d, p) level with the Gaussian 09W program package.
conformations, and change intermolecular interactions and even
packing mode in solid states, which result in different MFC prop-
erties. However, the studies on the effects of alkyl length on MFC
materials are limited owing to the scarcity of MFC materials and the
mechanism is still unclear [36,37]. Therefore, there is still a great
challenge for designing and synthesizing new MFC materials, as
well as the accumulation of knowledge of the relationships be-
tween molecular structures and properties. In this context, much
more systematic molecular design and structure-property investi-
gation on MFC materials are becoming very necessary. To further
understand the effect of alkyl chains on the solid-state fluorescence
and mechanochromic luminescence, in the current work, we have
prepared a series of phenothiazine derivatives functionalized with
benzoyl group (PCAn, n ¼ 1, 2, 4 and 6) with different lengths of N-
alkyl chains (Scheme 1), and their photophysical properties and
MFC behaviors were investigated in detail. The results showed that
these molecules exhibited different MFC activities and their fluo-
rescence emission and MFC behaviors are alkyl length-dependent.
2.2. Synthesis, procedures, and characterization
Compounds 1 and target compounds PCAn were synthesized by
the reported methods [32,33], and the synthetic routes were shown
in Scheme 2.
2. Experimental section
2.2.1. 10-Methyl-10H-phenothiazine-3-carbaldehyde (PCA1)
1.3 mL POCl3 (16 mmol) was added into 2 mL DMF in 25 mL flask
under ice-water bath, after being stirred for 10 min at room tem-
perature, 10-methyl-10H-phenothiazine (1.7 g, 8 mmol) dissolved
in 1, 2-dichloroethane (10 mL) was added into the above mixture
drop-wise under ice-water bath, and then the resulting mixture
was heated to reflux for 12 h. The reaction mixture was cooled to
room temperature and poured into ice-water. The organic layer was
extracted with dichloromethane and the combined organic layers
were washed with saturated brine solution and water, and dried
over anhydrous magnesium sulfate. After filtration and solvent
evaporation, the residue was purified by silica-gel column chro-
matography using petroleum ether/dichloromethane (v:v ¼ 1:1) as
eluent, yellow-green solid was obtained after recrystallization from
dichloromethane and diethyl ether. Yield: 0.96 g, 50%. mp:
88e90 ꢀC. IR (KBr, cmꢁ1): 1676, 1600, 1568, 1464, 1396, 1380, 1328,
2.1. General information
All the raw materials were used without further purification. All
the analytical pure solvents were purchased from Beijing Chemical
Works (Beijing, China). 1H NMR and 13C NMR spectra were recorded
with a Mercury Plus instrument at 600 MHz and 151 MHz by using
CDCl3 as the solvent in all cases. High resolution mass spectra were
performed on AB SCIEX Triple TOF™ 5600þ LC-MS/MS series. FT-IR
spectra were recorded with a Varian 660-IR FT-IR spectrometer by
incorporation of samples into KBr disks. The UVevis absorption
spectra were obtained using a VARIAN Cary 5000 spectrophotom-
eter. Photoluminescence measurements were obtained on a Cary
Eclipse fluorescence spectrophotometer. The fluorescence quantum
yields of PCAn (n ¼ 1, 2, 4, and 6) in various solvents were measured
by comparing with a standard (quinine in 0.1 N H2SO4, ФF ¼ 0.546)
and the excitation wavelength was 365 nm. The solid fluorescence
quantum yields were measured using an Edinburgh Instrument
FLS920 with integrating cavity scanning (ICS) method. Cyclic vol-
tammogram was performed using CHI 604B electrochemical
workstation and measurement was carried out in dry CH2Cl2 with a
scan rate at 100 mV/s. Three electrode configurations were used for
the measurement: a platinum button was used as the working
electrode, a platinum wire as the counter electrode, and Ag/AgCl as
the reference electrode. The solution of Bu4NBF4 in DCM (0.1 M)
was used as the supporting electrolyte. C, H, and N elemental an-
alyses were performed with a vario MACRO cube elemental
analyzer. The XRD patterns were obtained on an Empyrean X-ray
diffraction instrument equipped with graphite-monochromatized
1256, 1204, 752. 1H NMR (600 MHz, CDCl3)
d 9.81 (1 H, s), 7.66 (1 H,
dd, J1 ¼ 1.8 Hz, J2 ¼ 1.8 Hz), 7.60 (1 H, d, J ¼ 1.8 Hz), 7.21e7.18 (1 H,
m), 7.14e7.12 (1 H, m), 7.00e6.98 (1 H, m), 6.85 (2 H, t, J ¼ 8.2 Hz),
3.43 (3 H, s) (Fig. S1). 13C NMR (151 MHz, CDCl3)
d 190.09, 151.11,
144.13, 131.22, 130.46, 127.99, 127.78, 127.42, 127.32, 127.15, 124.03,
123.65, 122.59, 122.44, 114.78, 114.06, 113.72, 77.23, 77.02, 76.80,
35.84. (Fig. S2); MS (HR-MS): m/z 242.0537 [MþH]þ (Fig. S3, Calcd
for C14H11NOS: 241.0561); Anal. Calcd. (%) for C14H11NOS: C, 69.68;
H, 4.59; N, 5.80. Found (%): C, 69.72; H, 4.61; N, 5.77.
2.2.2. 10-Ethyl-10H-phenothiazine-3-carbaldehyde (PCA2)
The synthetic method for compound PCA2 was similar to that of
compound PCA1. It was purified by column chromatography (silica
gel, petroleum ether/CH2Cl2 ¼ 1/1) to give yellow solid (49% in
yield). mp: 100e101 ꢀC. IR (KBr, cmꢁ1): 1672,1600,1576,1468,1398,
Cu K
a
radiation (
l
¼ 1.5418 Å) by employing a scanning rate of
0.026ꢀ sꢁ1 in the 2
q
range from 5 to 50. Differential scanning
Scheme 1. The structures of PCA1, PCA2, PCA4 and PCA6.

J. Jia, Y. Wu / Dyes and Pigments 147 (2017) 537e543
539
Scheme 2. Synthetic routes of PCA1, PCA2, PCA4 and PCA6.
1368, 1325, 1256, 1240, 1200, 756. 1H NMR (600 MHz, CDCl3)
d
9.79
gel, petroleum ether/CH2Cl2 ¼ 1/1) to give yellow solid (75% in
yield). mp: 62e64 ꢀC. IR (KBr, cmꢁ1): 1678, 1600, 1576, 1564, 1474,
1450, 1448, 1441, 1369, 1254, 1226, 1200, 752. 1H NMR (600 MHz,
(1 H, s), 7.64e7.62 (1 H, dd, J1 ¼ 8.4 Hz, J2 ¼ 1.9 Hz), 7.57 (1 H, d,
J ¼ 1.9 Hz), 7.18e7.14 (1 H, m), 7.10 (1 H, dd, J1 ¼ 7.6 Hz, J2 ¼ 1.5 Hz),
6.98e6.94 (1 H, m), 6.90 (2 H, t, J ¼ 8.0 Hz), 3.98 (2 H, m),1.45 (3 H, t,
CDCl3)
d
9.79 (1 H, s), 7.65e7.63 (1 H, dd, J1 ¼ 8.4 Hz, J2 ¼ 1.7 Hz),
J ¼ 7.0 Hz) (Fig. S4). 13C NMR (151 MHz, CDCl3)
d
190.01, 150.30,
7.58 (1 H, d, J ¼ 1.7 Hz), 7.19e7.15 (1 H, m), 7.12e7.10 (1 H, dd,
J1 ¼ 7.6 Hz, J2 ¼ 1.2 Hz), 6.97e6.95 (1 H, m), 6.90e6.87 (2 H, t,
J ¼ 9.2 Hz), 3.89e3.87 (2 H, t, J ¼ 7.2 Hz), 1.83e1.78 (2 H, m),
1.45e1.43 (2 H, t, J ¼ 7.2 Hz), 1.31 (4 H, d, J ¼ 3.4 Hz), 0.88e0.86 (3 H,
143.08, 131.01, 130.17, 128.26, 127.59, 127.49, 124.47, 123.55, 123.27,
115.57, 114.39, 77.23, 77.02, 76.81, 42.47, 12.87. (Fig. S5); MS (HR-
MS): m/z 256.0579 [MþH]þ (Fig. S6, Calcd for C15H13NOS:
255.0718); Anal. Calcd. (%) for C15H13NOS: C, 70.56; H, 5.13; N, 5.49.
Found (%): C, 70.62; H, 5.11; N, 5.53.
t, J ¼ 7.2 Hz) (Fig. S10). 13C NMR (151 MHz, CDCl3)
d 190.03, 150.76,
143.46, 131.04, 130.06, 128.42, 127.57, 127.55, 125.03, 123.82, 123.56,
115.95, 114.79, 48.03, 31.38, 26.74, 26.51, 22.56, 13.96. (Fig. S11); MS
(HR-MS): m/z 312.1413 [MþH]þ (Fig. S12, Calcd for C19H21NOS:
311.1344); Anal. Calcd. (%) for C19H21NOS: C, 73.27; H, 6.80; N, 4.50.
Found (%): C, 73.32; H, 6.83; N, 4.46.
2.2.3. 10-Butyl-10H-phenothiazine-3-carbaldehyde (PCA4)
The synthetic method for compound PCA4 was similar to that of
compound PCA1. It was purified by column chromatography (silica
gel, petroleum ether/CH2Cl2 ¼ 1/1) to give yellow solid (48% inyield).
mp: 54e55 ꢀC. IR (KBr, cmꢁ1): 1676, 1600, 1576, 1564, 1472, 1448,
3. Results and discussion
1440, 1368, 1252, 1228, 1200, 752. 1H NMR (600 MHz, CDCl3)
d 9.79
(1 H, s), 7.65e7.63 (1 H, dd, J1 ¼ 8.4 Hz, J2 ¼ 1.9 Hz), 7.58 (1 H, d,
J ¼ 1.9 Hz), 7.18e7.15 (1 H, m), 7.12e7.10 (1 H, dd, J1 ¼ 7.6 Hz,
J2 ¼ 1.5 Hz), 6.98e6.95 (1 H, m), 6.91e6.88 (2 H, dd, J1 ¼ 11.0 Hz,
J2 ¼ 8.0Hz), 3.91e3.88(2H, t, J ¼ 6.0 Hz),1.83e1.78(2H, m),1.50e1.44
(2 H, m), 0.97e0.94 (3 H, t, J ¼ 9.0 Hz) (Fig. S7). 13C NMR (151 MHz,
3.1. Photophysical properties in solutions
The UVevis absorption and fluorescence emission spectra of
phenothiazine derivatives PCA1, PCA2, PCA4 and PCA6 in THF
(c ¼ 1.0 ꢂ 10ꢁ6 M) were shown in Fig. 1. It was found that all of the
phenothiazine derivatives exhibited three obvious absorption
bands, in the region of 280e300 nm was originating from the
CDCl3) d 190.04, 150.78, 143.47, 131.05, 130.06, 128.43, 127.58, 127.55,
125.06,123.85,123.57,115.96,114.80, 77.23, 77.02, 76.81, 47.69, 28.84,
20.07, 13.75 (Fig. S8); MS (HR-MS): m/z 284.1102 [MþH]þ (Fig. S9,
Calcd for C17H17NOS: 283.1031); Anal. Calcd. (%) for C17H17NOS: C,
72.05; H, 6.05; N, 4.94. Found (%): C, 72.02; H, 6.03; N, 4.96.
phenothiazine and localized
p-p* transitions [38], and the band
appearing at the longer wavelength region of 375e390 nm was
mainly ascribed to the charge transfer (CT) from phenothiazine
donor to formyl group acceptor. In addition, the maximal emission
peaks of PCA1, PCA2, PCA4 and PCA6 in THF emerged at ~507 nm,
which was ascribed to CT emission. It could be confirmed by the
solvent dependent fluorescence emission spectra (Fig. S13).
2.2.4. 10-Hexyl-10H-phenothiazine-3-carbaldehyde (PCA6)
The synthetic method for compound PCA6 was similar to that of
compound PCA1. It was purified by column chromatography (silica
Fig. 1. The UVevis absorption and fluorescence emission spectra of PCA1, PCA2, PCA4 and PCA6 in THF (c ¼ 1.0 ꢂ 10ꢁ6 M, lex ¼ 410 nm).

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J. Jia, Y. Wu / Dyes and Pigments 147 (2017) 537e543
Because all the compounds have the same chromophores, PCA2
was used as the sample. It was clear that the maximal absorption
peak was observed at 375 nm in cyclohexane, a red-shifted peak
(380 nm) for toluene solution appeared, and the DMF solution
possessed an absorption band at 381 nm. The red-shift of the ab-
sorption peak in polar solvents suggests that PCA2 is a polar
molecule [39,40], which can be confirmed by quantum chemical
calculations. And quantum chemical calculations were performed
by time-dependent density functional theory (TD-DFT) calculations
at the B3LYP/6-31G (d, p) level and the calculated dipole moment
was 5.5458 D, which indicated PCA2 had large molecular polarity.
Moreover, it was found that the fluorescence of PCA2 also exhibited
solvatochromism. A structured emission band with a maximum
emission peak at 468 nm and a shoulder peak at 490 nm were
observed in cyclohexane. With solvent polarity increasing, the
emission bands became structureless and the emission wavelength
red-shifted, the maximum emission band of PCA2 in DMF shifted to
547 nm. Red shifts of the emission wavelength in polar solvents
indicated a stronger polarity of PCA2 in excited state than in ground
state. For the other three compounds, similar spectral behaviors
were found because of their same chromophores and the detailed
data were listed in Table S1 and Table S2.
3.2. Theoretical calculation and electrochemical properties
Considering the molecular structures, PCAn are typical D-A
molecules, in which formyl groups act as electron-withdrawing
groups, phenothiazine groups are electron donors. Quantum
chemical calculations also confirmed this conclusion. After geom-
etry optimization, the absorption spectrum was simulated by the
time-dependent DFT calculation at the B3LYP/6-31G (d, p) level. A
wide absorption band with a maximum at 411.7 nm was found
(Fig. S14), which consisted of one strong and one weak absorption
bands. The detailed results were listed in Table S3. The results
showed that electron transitions corresponding to absorption
bands involve six frontier orbitals (Fig. S15 and Table S3) and the
first transition (HOMO/LUMO) will induce charge transfer from
phenothiazine to formyl group, which resulted in an excited state
with a larger polarity relative to that of the ground state. Further-
more, the twisted angles of phenothiazine group reached 154.2ꢀ
(Fig. 2a), suggesting a nonplanar molecular conformation. And the
frontier orbital plots of the HOMO and LUMO were shown in Fig. 2b
and Fig. S16. It was found that the LUMO orbitals were almost
Fig. 2. (a) Optimized molecular structure and (b) electron density distributions of the
frontier molecular orbitals of PCA2.

J. Jia, Y. Wu / Dyes and Pigments 147 (2017) 537e543
541
Table 1
Peak wavelengths (l/nm) of PCAn under various external stimuli.
a
Samples
lannealed
lpressed
lfumed
lrepressed
DlMFC
PCA1
PCA2
PCA4
515
535
533
537
592
594
512
534
535
537
594
596
22
57
61
a
Pressing-induced spectral shift, DlMFC
¼
lpressed - lannealed.
distributed at formyl groups and the near benzene rings, while the
HOMO orbitals mainly populated predominately on the phenothi-
azine moieties. Therefore, PCAn are polar molecules and mecha-
nochromic activities were expected for them in the solid states.
To gain insight into the nature of the ground electronic state of
these four phenothiazine derivatives PCAn, we performed theo-
retical calculations on their energy levels by using the density
functional theory (DFT) method at B3LYP/6-31G (d, p) basis in the
Gaussian 09W program package and cyclic voltammetry (CV)
measurements were performed in dry dichloromethane solutions
with Tetrabutylammonium Tetrafluoroborate as the supporting
electrolyte. Cyclic voltammograms of PCA1, PCA2, PCA4 and PCA6
are shown in Fig. S17, and the corresponding data are listed in
Table S2. It was found that all of the phenothiazine derivatives
exhibited two well-defined reversible oxidation processes and the
half-wave potential located at 1.02, 1.00, 1.02 and 1.02 V (vs. Fc/Fcþ),
respectively. The redox potential of Fc/Fcþ was located at 0.58 eV
which possessed an absolute energy level of 4.8 eV relative to the
vacuum level for calibration in our case. Additionally, the HOMO
energy level was estimated according to the equation of EHOMO ¼ -
(Eox þ 4.22), and the LUMO energy level was calculated using the
empirical equation of ELUMO ¼ EHOMO þ Egap, in which Egap was
estimated from the onset of the absorption spectrum (Egap ¼ 1240/
l
onset). So the HOMO energy levels of PCAn were located
at ꢁ5.24, ꢁ5.22, ꢁ5.24 and ꢁ5.24 eV, and their LUMO energy levels
were located at ꢁ2.32, ꢁ2.28, ꢁ2.34 and ꢁ2.34 eV, respectively.
3.3. MFC properties of PCAn
Some non-planar tetraphenylethene, 9,10-divinylanthracene
exhibit excellent MFC behavior with high contrast [41,42]. Espe-
cially, some functionalized phenothiazine derivatives showed good
MFC behaviors which contained a non-planar butterfly conforma-
tion. And we have previously reported that some phenothiazine-
based compounds exhibited MFC properties [43]. So we predicted
that nonplanar D-A typed phenothiazine derivatives PCAn would
exhibit MFC behaviors.
To check whether PCAn are mechanochromic, their fluorescence
responses toward mechanical and solvent-fuming processes were
studied. Fig. S18 and Fig. S19 were the fluorescence images of PCAn
samples (mixed with KBr in order to save fluorophores) upon a
cycle of pressing, heat-annealing, repressing and solvent-fuming. It
was observed that the pressed PCAn samples emit yellowish-green
to orange colors from PCA1 to PCA4 under UV illumination of a
365 nm lamp. However, when the pressed samples were annealed
before the isotropic melt transition or exposed to solvent vapor
(fuming above dichloromethane for 60s), green or yellowish-green
fluorescence colors appeared again. But this phenomenon did not
happen for PCA6 (Fig. S19 and Fig. S20). It was quite clear that, the
PCAn with longer alkyl chains (n ¼ 2, 4) exhibited more remarkable
fluorescence color changes between pressed and annealed (or
fumed) states (from orange to yellowish-green) than that with
shorter alkyl chain (n ¼ 1, from yellowish-green to green).
Furthermore, when the fumed or annealed samples were re-
pressed, the fluorescence colors were again changed as the first
pressing. This process is reproducible, indicating a reversible MFC

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J. Jia, Y. Wu / Dyes and Pigments 147 (2017) 537e543
behavior [44]. When the samples were pressed, the red shifts of
PCAn fluorescence emission wavelength occurred to some extent.
Meanwhile, the emission spectra of above samples under various
stimuli were recorded on a luminescence spectrophotometer
(Fig. 3), the corresponding spectroscopic data were summarized in
Table 1. As expected, the emission spectra recorded are comparably
consistent with the corresponding fluorescence colors observed
(Fig. S18). It was found that the peak emission wavelengths of
pressed PCAn were 537, 592 and 594 nm, respectively, and the
maximum emission of annealed states emerged at 515, 535 and
533 nm. Obviously, based on the emission spectra, the general
trend of DlMFC
(DlMFC
¼
lpressed - lannealed) increases with the
length of alkyl chains. Therefore, PCAn homologs are also alkyl
chain length-dependent MFC materials, and alkyl chains have
played a functional role in tuning the MFC behavior [44,45].
To understand what happens upon pressing PCAn sample,
powder wide-angle X-ray diffraction (XRD) and differential scan-
ning calorimetry (DSC) experiments were conducted on the ground
and as-prepared solids. As shown in Fig. 4, the diffraction curves of
as-prepared PCAn solid showed sharper and more intense re-
flections, indicative of existence of the well-ordered micro-crys-
talline structures. In contrast, the diffraction peaks of the ground
samples displayed broad and weak peaks, indicating notable
amorphous features [8]. The diffraction peaks, and hence the
crystalline structures, could be recovered after fuming or annealing,
which resulted in blue shifts in the fluorescence spectra. The results
indicated that the MFC properties of the PCAn solid samples should
be attributed to the phase transition between crystalline and
amorphous states [46].
The formation of amorphous state upon grinding has been
further confirmed by DSC experiment. As shown in Fig. 5, the
compounds exhibited obvious melting peak located at 87 ꢀC, 101 ꢀC
and 55 ꢀC, for PCA1, PCA2, and PCA4, respectively, and there was no
change upon grinding. DSC curves showed that PCA1 and PCA2
displayed exothermic transition peak at the lower-temperature
region in the ground state at 40 ꢀC and 51 ꢀC. This broad
exothermic peak could be ascribed to the cold-crystallization
(crystallizing from glass state) of ground PCAn solids upon
annealing [47], which confirmed that the amorphous state was a
metastable state. Unluckily, PCA4 gave no phase transition peak in
ground state. The ground PCA2 solids had high exothermic peak
values, which resulted in their stable MFC behavior at room tem-
perature. The ground PCA1 solid could cold-crystallize at room
temperature since its exothermic peak was located at 40 ꢀC. Thus,
the amorphized PCA1 solid induced by grinding rapidly recovered
its fluorescence at lower temperatures and even exhibited spon-
taneous recovering fluorescence properties at room temperature.
4. Conclusions
A series of bowl-shaped phenothiazine derivatives functional-
ized by formyl group (PCAn, n ¼ 1, 2, 4 and 6) with different lengths
of N-alkyl chains have been designed, The spectral results and
quantum chemical calculations indicate that the compounds are D-
A molecules. Interestingly, these simple non-planar compounds
PCA1, PCA2 and PCA4 exhibit pronounced and reproducible
reversible mechanochromism, not only naked-eye visible and
reversible mechanochromic behavior, but alkyl chain length-
dependent emission properties. XRD and DSC reveal that the
transformation between crystalline and amorphous states upon
various external stimuli is responsible for the MFC behavior. This
work has demonstrated that the subtle manipulation of alkyl
groups of phenothiazine functionalized by formyl group could
endow these compounds with unique and tunable solid-state op-
tical properties. And another phenothiazine derivatives are under

J. Jia, Y. Wu / Dyes and Pigments 147 (2017) 537e543
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This work was supported by grants from the Natural Science
Foundation of Science and Technology Agency of Shanxi Province
(201601D021056), the Natural Science Foundation of Education
Committee of Shanxi Province (20161111), and the Doctor Fund of
Shanxi Normal University (0505/02070262).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2017.08.049.
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