Reversible mechanochromic luminescence of phenothiazine-based 10,10′-bianthracene derivatives with different lengths of alkyl chains

Article abstract of DOI:10.1039/c4tc02126f

A series of 10,10′-bis(2-(N-alkylphenothiazine-3-yl)vinyl)-9,9′-bianthracene compounds (PVBAn, n = 2, 8, 12 and 16) 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 solution and in the solid state. Their emission wavelengths were also strongly affected by solvent polarity, indicating intramolecular charge transfer (ICT) transitions. Interestingly, the fluorescence emission and grinding-induced spectral shifts of PVBAn solids are alkyl length-dependent. PVBA2 shows the smallest fluorescence and absorption spectrum shifts under mechanical force stimuli. Homologues with longer alkyl chains exhibit similar mechanochromic behaviors and larger fluorescence contrasts after grinding. Moreover, the fluorescence emission of ground solid PVBA16 can recover at room temperature, but other compounds need high temperatures for fluorescence to be restored. Differential scanning calorimetry experiments reveal that the cold-crystallization temperature difference is responsible for thermal restoration behaviors. 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:10.1039/c4tc02126f

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Reversible mechanochromic luminescence of
phenothiazine-based 10,100-bianthracene
Cite this: DOI: 10.1039/c4tc02126f
derivatives with different lengths of alkyl chains†
Pengchong Xue, Boqi Yao,a Xuhui Liu,b Jiabao Sun,a Peng Gong,a Zhenqi Zhang,a
a
*
a
c
a
*
Chong Qian, Yuan Zhang and Ran Lu
A series of 10,100-bis(2-(N-alkylphenothiazine-3-yl)vinyl)-9,90-bianthracene compounds (PVBAn, n ¼ 2, 8,
12 and 16) 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 solution and in the solid state. Their emission
wavelengths were also strongly affected by solvent polarity, indicating intramolecular charge transfer
(ICT) transitions. Interestingly, the fluorescence emission and grinding-induced spectral shifts of PVBAn
solids are alkyl length-dependent. PVBA2 shows the smallest fluorescence and absorption spectrum
shifts under mechanical force stimuli. Homologues with longer alkyl chains exhibit similar
mechanochromic behaviors and larger fluorescence contrasts after grinding. Moreover, the fluorescence
emission of ground solid PVBA16 can recover at room temperature, but other compounds need high
temperatures for fluorescence to be restored. Differential scanning calorimetry experiments reveal that
the cold-crystallization temperature difference is responsible for thermal restoration behaviors. 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 21st September 2014
Accepted 20th November 2014
DOI: 10.1039/c4tc02126f
www.rsc.org/MaterialsC
molecules can show changes in the color of their uorescence
under mechanical stress and be restored to their original state
by annealing or fuming with solvent vapor.4 Generally, no
Introduction
Conjugated organic molecules exhibiting solid-state uores-
cence are promising optical and optoelectronic materials and
are applied in organic light-emitting diodes, lasers, and
sensors. Among them, some uorescent organic molecules
exhibit tunable and switching solid-state luminescence by
external mechanical stimuli, which is known as mechano-
uorochromism (MFC).1 As a kind of “smart material,”2 MFC
compounds have attracted signicant attention because of their
promising applications in optical storage, pressure sensors,
rewritable media, and security ink.3 These uorescent organic
chemical structure damage occurs during the reversible emis-
sive color change process, and only the molecular packing
model in the solid state is changed under a pressure stimulus.5
However, MFC materials are still at the initial stages of inves-
tigation, and the types of MFC materials and in-depth under-
standing of MFC phenomena are limited. Moreover, the
identication of most MFC compound is still an isolated event.
Therefore, there is still a great demand for identifying new MFC
materials, as well as the accumulation of knowledge of the
relationships between molecular structure and properties. To
date, many kinds of organic molecules have been exploited to
produce MFC properties. For example, Araki et al. and Kato et al.
found that some hydrogen-bonded self-assemblies and liquid
crystals could exhibit responses to force stimuli.6 Harima
designed a series of heteropolycyclic uorescent dyes, which
have MFC behaviors, and suggested that D–p–A emissive
molecules always contribute to the realization of uorescence
change under pressure.7 More importantly, nonplanar p-
conjugated emissive molecules, such as tetraphenylethene,
9,10-divinylanthracene, triphenylamine derivatives and orga-
noboron compounds, are preferentially considered to act as
MFC materials.8 Phenothiazine is an excellent functional block
aState Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, No. 2699 Qianjin Street, Changchun, P. R. China.
E-mail: xuepengchong@jlu.edu.cn; luran@mail.jlu.edu.cn
bTaiyuan Center for Disease Control and Prevention, No. 89 Xinjiannan Road,
Taiyuan, P. R. China
cCollege of Chemistry, Beijing Normal University, No. 19, Xinjiekouwai Street, Haidian
District, Beijing, China
† Electronic supplementary information (ESI) available: NMR and MS spectra;
plot of emission maximum energy vs. solvent polarity; absorption and emission
spectra in different solvents; stimulated absorption spectra and electron
transition data; frontier orbitals; IR, UV-vis, uorescence and time-resolved
uorescence spectra of solids; tables about photophysical data and recovery
times at different temperatures. See DOI: 10.1039/c4tc02126f
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with AXIMA CFR MALDI-TOF (Compact) mass spectrometers. C,
H, and N elemental analyses were performed with a Perki-
nElmer 240C elemental analyzer. The XRD patterns were
obtained on an Empyrean X-ray diffraction instrument equip-
ped with graphite-monochromatized Cu Ka radiation (l ¼
ꢁ1
ꢀ
˚
1.5418 A) by employing a scanning rate of 0.026 s in the 2q
range from 5ꢀ to 30ꢀ. The samples were prepared by casting
crystal powders, ground solids and fuming samples on silicon
slides at room temperature. Differential scanning calorimetry
(DSC) curves were obtained on a Netzsch DSC 204F1 at a heating
rate of 10 ꢀC minꢁ1. The data of all samples (pristine and
ground) were collected when they were heated for the rst time.
Cold-crystallization of ground solids was employed using a
temperature-controlled heating board. Ground powders on a
weighted paper were rst placed on a heating board with a
measured temperature, covered by a glass plate, and then timed
for emissive color recovery. The uorescence decay experiment
was measured on an Edinburgh FLS920 steady state uorimeter
equipped with an nF900 nanosecond ash lamp. The molecular
conguration was used to obtain the frontier orbitals of PVBAn
by density functional theory (DFT) calculations at the B3LYP/6-
31G(d,P) level with the Gaussian 09W program package.
Scheme 1 Synthesis route for obtaining PVBAn.
for the construction of organic luminescent materials.9 It has a
nonplanar, bowl-shaped conguration, and recently it has been
introduced into molecular structures to obtain MFC-active
uorescent molecules.10 Moreover, 9,90-bianthracene has a
twisted conformation due to the strong repulsive interaction of
the hydrogen atoms at the 1,10 and 8,80 positions.11 It is expected
that the molecular material will have excellent MFC properties if
nonplanar phenothiazine and 9,90-bianthracene are integrated
in one molecule.
Moreover, slight differences in molecular structure will
induce distinct emissive behaviors. For instance, changing the
length of the n-alkyl chain can effectively adjust the MFC
behavior and thermodynamic properties.12 In this study, we
have designed and facilely synthesized a series of 10,100-bis(2-
(N-alkylphenothiazine-3-yl)vinyl)-9,90-bianthracenes with ethyl,
octyl, dodecyl and hexadecyl as alkyl side chains (PVBAn,
Scheme 1), and their uorescence properties and MFC behavior
were investigated. The results show that these molecules indeed
exhibit MFC activity and their uorescence emission and MFC
behaviors are alkyl length-dependent. PVBA16 showed an
isothermally reversible uorescence switching of an MFC
material because of its spontaneous restoration at room
temperature.
Synthesis, procedures, and characterization
Compounds 1, 2, 3, 4, and 5 were synthesized by the reported
methods.13 PVBAn were easily obtained by a one-step Heck
cross-coupling reaction using 10,100-dibromo-9,90-bianthracene
and the corresponding N-alkyl-3-vinyl-phenothiazine, as shown
in Scheme 1.
10,100-Bis((E)-2-(10-ethyl-10H-phenothiazine-3-yl)vinyl)-9,90-
bianthracene (PVBA2). Anhydrous K2CO3 (0.55 g, 4.0 mmol) and
Pd(OAc)2 (2.5 mg) were added to a solution of 10,100-dibromo-
9,90-bianthracene (0.50 g, 0.98 mmol), 10-ethyl-3-vinyl-10H-
phenothiazine (0.50 g, 1.96 mmol) and Bu4NBr (1.3 g, 4 mmol)
in dry DMF (20 mL). The mixture was stirred at 110 ꢀC under a
N2 atmosphere for 18 h. Aer cooling to room temperature, the
mixture was poured into water (200 mL) and extracted with
CH2Cl2. The combined organic phases were washed with brine,
and dried with anhydrous Na2SO4. Aer the solvent was
removed, the residue was puried by column chromatography
(silica gel, petroleum ether/methylene chloride ¼ 4 : 1, v/v) to
Experimental section
General information
All the raw materials were used without further purication. All give 0.59 g of an orange-colored solid (70% in yield). Element
the solvents were of analytical reagent grade and purchased analysis (%): calculated for C60H44N2S2: C, 84.08; H, 5.17; N,
from Beijing Chemical Works (Beijing, China); moreover, they 3.27; found: C, 84.03; H, 5.13; N, 3.31; 1H NMR (400 MHz,
were used without further purication. The water used in all the CDCl3) d 8.53 (d, J ¼ 8.9 Hz, 4H), 7.99 (d, J ¼ 16.5 Hz, 2H), 7.61
experiments was puried with a Millipore system. The UV-vis (d, J ¼ 1.7 Hz, 2H), 7.52 (dd, J ¼ 8.3, 1.5 Hz, 2H), 7.46 (m, 4H),
absorption spectra were obtained using a VARIAN Cary 50 Probe 7.26–7.14 (m, 12H), 7.08–6.96 (m, 8H), 4.04 (q, J ¼ 6.8 Hz, 4H),
spectrophotometer. The absorption spectra of solids were 1.53 (t, J ¼ 6.9 Hz, 6H) (Fig. S1†). 13C NMR (101 MHz, CDCl3) d
obtained by measuring their lms on the surface of a silica 144.57, 136.46, 133.71, 133.13, 132.01, 131.52, 129.66, 127.48,
plate. Photoluminescence measurements were obtained on a 127.41, 126.39, 126.14, 125.65, 125.36, 124.99, 123.89, 123.43,
Cary Eclipse uorescence spectrophotometer. The uorescence 122.65, 115.31, 115.26, 53.45, 42.17, 13.04 (Fig. S2†). MALDI-
quantum yields of PVBAn (n ¼ 2, 8, 12 and 16) in various TOF MS (m/z): calcd. for C60H44N2S2: 856.3; found: 857.4 [M +
solvents were measured by comparing with a standard (9,10- H]+ (Fig. S3†).
diphenyl anthracene in benzene, FF ¼ 0.85). Note that the
Other
10,100-bis(2-(N-alkylphenothiazine-3-yl)vinyl)-9,90-
excitation wavelength was 410 nm. Mass spectra were obtained bianthracenes were synthesized as per the same procedure that
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was followed for PVBA2, except that different N-alkyl-3-vinyl- nm in nonpolar n-hexane and cyclohexane, which red-shied to
10H-phenothiazines were used. 422 nm in THF and CH2Cl2, to 421 nm in acetone, and to 426
10,100-Bis((E)-2-(10-octyl-10H-phenothiazine-3-yl)vinyl)-9,90- nm in DMF. The red-shi of the absorption peak in polar
bianthracene (PVBA8). This compound was obtained as a yellow solvents suggests that PVBA8 is a polar molecule,15 which can be
solid with 82% yield. Element analysis (%): calculated for conrmed by quantum chemical calculations.
C
72H68N2S2: C, 84.33; H, 6.68; N, 2.73; found: C, 84.29; H, 6.70;
Quantum chemical calculations were performed by density
1
N, 2.69. H NMR (400 MHz, CDCl3) d 8.53 (d, J ¼ 8.8 Hz, 4H), functional theory calculations at the B3LYP/6-31G(d,p) level.
7.98 (d, J ¼ 16.6 Hz, 2H), 7.61 (s, 2H), 7.52 (d, J ¼ 8.3 Hz, 2H), The calculated dipole moment is 3.03 D. Moreover, we found
7.49–7.40 (m, 4H), 7.26–7.13 (m, 12H), 7.07–6.93 (m, 8H), 3.95 that the uorescence of PVBA8 also exhibits solvatochromism.
(s, 4H), 1.91 (m, 4H), 1.52 (m, 4H), 1.35 (m, 16H), 0.92 (t, J ¼ 6.2 As shown in Fig. 1b and c, PVBA8 emits strong green uores-
Hz, 6H) (Fig. S4†). 13C NMR (101 MHz, CDCl3) d 144.90, 136.49, cence (uorescence quantum yield, FF ¼ 0.4) in hexane, and the
133.72, 133.13, 132.04, 131.52, 129.67, 127.54, 127.40, 127.38, maximal emission peak is located at 524 nm. In cyclohexane,
126.39, 126.09, 125.65, 125.36, 125.06, 124.39, 123.43, 122.67, the emission band slightly shis to 530 nm, whereas the
115.65, 115.58, 47.79, 31.81, 29.28, 27.00, 22.69, 14.16 (Fig. S5†). toluene solution has strong yellow emission (FF ¼ 0.33, lem
MALDI-TOF MS (m/z): calcd. for C72H68N2S2: 1024.5; found: 558 nm). Further increase in solvent polarity leads to contin-
1025.3 (M + H)+ (Fig. S6†).
uous red-shi in uorescence spectra. Note that the acetone
¼
10,100-Bis((E)-2-(10-dodecyl-10H-phenothiazine-3-yl)vinyl)- solution has weak red uorescence. The emission maximum
9,90-bianthracene (PVBA12). The compound was obtained as a reaches 658 nm in DMF with larger polarity, but FF is only 0.02.
yellow solid with 79% yield. Element analysis (%): calculated for In addition, Fig. 2b shows that polar solvents induce wide
C
80H84N2S2: C, 84.46; H, 7.44; N, 2.46; found: C, 84.43; H, 7.49; emission bands. The large shi of the emission band, the
1
N, 2.45. H NMR (400 MHz, CDCl3) d 8.52 (d, J ¼ 8.9 Hz, 4H), decrease of FF, and the broadening of the emission bands in
7.98 (d, J ¼ 16.3 Hz, 2H), 7.61 (s, 2H), 7.52 (d, J ¼ 8.3 Hz, 2H), polar solvents indicate an intramolecular charge-transfer (ICT)
7.49–7.42 (m, 4H), 7.26–7.13 (m, 12H), 7.10–6.93 (m, 8H), 3.95 characteristic for the excited state.16 Lippert–Mataga plots also
(s, 4H), 1.97–1.85 (m, 4H), 1.56–1.47 (m, 4H), 1.45–1.24 (m, conrms the ICT transition because a linear relationship
32H), 0.91 (t, J ¼ 6.8 Hz, 6H) (Fig. S7†). 13C NMR (101 MHz, between maximum emission energy and solvent polarity is
CDCl3) d 144.80, 136.47, 133.69, 133.13, 132.00, 131.51, 129.67, observed (Fig. S13†). For the other three compounds, similar
127.49, 127.40, 127.37, 126.38, 126.08, 125.63, 125.35, 125.02, spectral behaviors were found because they have the same
123.92, 123.45, 122.70, 115.67, 115.61, 47.73, 31.95, 29.68,
29.62, 29.59, 29.38, 29.33, 27.00, 22.73, 14.16 (Fig. S8†). MALDI-
TOF MS (m/z): calcd. for C80H84N2S2: 1136.6; found: 1137.5 (M +
H)+ (Fig. S9†).
10,100-Bis((E)-2-(10-hexadecyl-10H-phenothiazine-3-yl)vinyl)-
9,90-bianthracene (PVBA16). The compound was obtained as a
yellow solid with 85% yield. Element analysis (%): calculated for
C
88H100N2S2; C, 84.56; H, 8.06; N, 2.24; found: C, 84.51; H, 8.10;
1
N, 2.22. H NMR (400 MHz, CDCl3) d 8.53 (d, J ¼ 8.9 Hz, 4H),
7.98 (d, J ¼ 16.4 Hz, 2H), 7.61 (s, 2H), 7.52 (d, J ¼ 7.9 Hz, 2H),
7.49–7.42 (m, 4H), 7.27–7.14 (m, 12H), 7.09–6.93 (m, 8H), 3.95
(s, 4H), 1.98–1.85 (m, 4H), 1.51 (m, 4H), 1.45–1.22 (m, 48H), 0.91
(t, J ¼ 6.4 Hz, 6H) (Fig. S10†). 13C NMR (101 MHz, CDCl3) d
144.97, 136.50, 133.73, 133.13, 131.99, 131.53, 129.67, 127.54,
127.40, 127.37, 126.40, 126.09, 125.65, 125.35, 125.07, 124.38,
123.39, 122.64, 115.60, 115.54, 47.75, 31.96, 29.75, 29.71, 29.63,
29.61, 29.40, 29.34, 27.02, 22.73, 14.17 (Fig. S11†). MALDI-TOF
MS (m/z): calcd. for C88H100N2S2: 1248.7; found: 1249.9 (M + H)+
(Fig. S12†).
Results and discussion
Photophysical properties in solution
Recent studies have demonstrated that a D–p–A–p–D uores-
cent dye molecule with a large dipole is a good choice for
developing MFC dyes.14 Thus, we initially studied their spectral
characteristics in solution to understand their donor–acceptor–
donor p-conjugated systems. Because all the compounds have
the same chromophore, PVBA8 is used as a sample. As shown in
Fig. 1 Normalized solvent-dependent (a) UV-vis absorption and (b)
fluorescence spectra of PVBA8. The concentration of the samples was
10ꢁ5 M. (c) Photos of solutions upon 365 nm light irradiation. The
Fig. 1, PVBA8 has an absorption band with a maximum of 416 concentration of all samples was 10ꢁ5 M. lex ¼ 400 nm.
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Fig. 2 (a) Optimized molecular structure and (b) electron density
distributions of the frontier molecular orbitals of PVBA2.
chromophore (Fig. S14†). The detailed data are listed in Table
S1 and S2.†
Considering the molecular structure, PVBAn are typical D–
p–A–p–D molecules, in which bianthracene acts as an electron-
withdrawing group, two phenothiazine groups are electron
donors and vinyl moieties play the role of p-bridge units.
Quantum chemical calculations also conrmed this conclusion.
Aer geometry optimization (Fig. 2a), the absorption spectrum
was simulated by the time-dependent DFT calculation at the
Fig. 3 (a) Absorption and (b) fluorescence of pristine PVBAn solid.
lex ¼ 400 nm.
B3LYP/6-31G(d,p) level.
A wide absorption band with a
maximum at 453.84 nm was found (Fig. S15†), which consists of
one strong and two weak absorption bands. The detailed results
are listed in Table S3.† The results show that electron transi-
tions corresponding to absorption bands involve six frontier
orbitals (Fig. 2b and S16†) and each transition will induce
charge transfer from phenothiazine to anthracene, which
results in an excited state with a larger polarity relative to that of
the ground state. Furthermore, the twisted angles of the two
anthracenes and between anthracene and the vinyl moiety
reach 90.5ꢀ and 53.2ꢀ, suggesting a nonplanar molecular
conformation. Therefore, PVBAn are polar and nonplanar
molecules and mechanochromic activity is expected for them in
the solid state.17
because of a red-shi in absorption.18 The asymmetric and
symmetric CH2 stretching vibrations in the FT-IR spectra of
PVBA8–16 are located at 2917 and 2850 cmꢁ1 (Fig. S17†),
respectively, indicating that long alkyl chains adopt a trans-
conformation in the solid state.19 Thus, the longer alkyl chains
will prevent aromatic units from being close to one another. In
contrast, ethyl groups with short lengths will allow a signicant
p–p stacking; therefore, the length of the alkyl chain is
responsible for the differences in color. Although X-ray analysis
of single crystals could give more accurate information in
molecular packing, high-quality single crystals of PVBAn were
not obtained. Moreover, PVBA2 has a difference in emission
color as compared to the other compounds. As shown in Fig. 3b,
the maximum emission peak of PVBA2 in pristine solid state is
located at 560 nm, allowing it to have yellow uorescence with
light irradiated at 365 nm. Other compounds have similar
MFC properties of PVBAn
First, the spectral properties of the pristine solids were studied.
It was found that the color and uorescence color of the pristine
solids are relative to the length of the alkyl chains. PVBA2
appended with two ethyl groups at the phenothiazine units is an
orange solid, but other compounds with longer alkyl chains
have a yellow color. Their absorption spectra can explain the
difference in color. As shown in Fig. 3a, an absorption peak at
453 nm for PVBA2 appears, and other compounds have similar
absorption maxima around 422 nm (Table 1). The absorption
band at 453 nm for PVBA2 explains its orange color. It should be
noted that the absorption peaks of the solids, except for PVBA2,
are similar to those in n-hexane, which indicates very weak or no
p–p interaction between aromatic moieties of PVBAn with
longer alkyl chains. In contrast, a p–p interaction exists in solid
PVBA2, and the molecules pack together to form J-aggregates
Table 1 The data of absorption and emission peaks and lifetimes
before and after grinding
labs (nm)
lem (nm)
s (ns)
Pristine Ground Pristine Ground Pristine Ground
PVBA2
PVBA8
PVBA12 422.5
PVBA16 422.5
453
418
433
433
433
441
560
513
519
517
580
568
563
559
2.5
1.9
—
3.4
3.0
—
—
—
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emissions with a maxima around 515 nm, indicating green transfer because the energy acceptors with longer emission
uorescence. The red-shied emission for PVBA2 is ascribed to wavelengths achieve extra lifetime from donors. Grinding also
p–p interaction; thus, different MFC behaviors are expected for caused an increase in the lifetimes of the other compounds with
the different PVBAn.
longer alkyl chains. For example, the ground PVBA8 powder
When the pristine solids were ground with a spatula, orange possesses a longer lifetime (hsi ¼ 3.0 ns) relative to that of the
PVBA2 changed into orange-red powder, and other yellow solids pristine one (hsi ¼ 1.9 ns). The increased p–p interaction and
converted into orange powders. The UV-vis spectra before and energy transfer should be responsible for the increase in s.21
aer grinding were compared (Fig. S18†). The absorption peak
When the ground powders were exposed to organic vapors,
of the PVBA2 pristine solid at 453 nm shis to 443 nm aer such as those of CH2Cl2, CHCl3, THF, toluene, hexane, cyclo-
grinding, which is still red-shied relative to that in n-hexane. hexane and petroleum ether, for several seconds, the color
This result shows that PVBA2 still retained J-aggregate proper- rapidly changed into a new one (Fig. S18†). For example, aer
ties in the ground state; however, p–p interaction is weaker fuming, the color of the ground PVBA8 changed from orange to
than that in the pristine solid aer grinding. Grinding induces a yellow; moreover, fuming also caused a recovery in uorescence
red-shi of the absorption band of PVBA8 from 417 nm to 443 (Fig. 4). These color change processes for the four compounds
nm. Other compounds show a spectral shi similar to that of can be repeated many times, indicating reversible MFC
PVBA8, and all of ground powders have almost the same behavior.22 Therefore, PVBAn exhibits piezochromic behavior,
absorption bands (Fig. S18e†). Such spectral changes clearly not only in the uorescence spectra, but also in the absorption
indicate that all the compounds have similar p–p interactions spectra.
and form J-aggregates aer grinding, independent of the length
To gain an insight into the MFC behavior of PVBAn solids,
of alkyl chains. In other words, mechanical force results in the powder wide-angle X-ray diffraction experiments were con-
disappearance of the steric effect of the long alkyl chains and ducted on the pristine, ground and fuming solids. As shown in
promotes similar J-aggregation between aromatic units.
Fig. 5, the pristine solids show sharp and intense reections,
As discussed above, mechanical force stimulus leading to indicative of the well-ordered microcrystalline structures. In
color change, or MFC, is expected for all of the compounds. As contrast, the ground solids display broad and weak peaks,
shown in Fig. 4, the uorescence spectra of the pristine PVBAn indicating that the ground powders were amorphous.23 Aer
powders shi to longer wavelengths. The ground powders, fuming by CH2Cl2 vapor, sharp and strong diffraction peaks
except for PVBA2, had similar emission bands and yellow similar to those of the pristine solids appeared again, suggest-
uorescence (Fig. S19†). PVBA2 emitted orange uorescence ing the regeneration of the microcrystalline structures. There-
with a maximum at 580 nm aer grinding. The reason why fore, the phase transition of PVBAn solids between crystalline
PVBA2 emits orange uorescence is because of the existence of and amorphous states could be responsible for their MFC
aggregates with low energy levels, which act as energy acceptors behavior. Because the crystalline state is more stable than the
that quench uorescence of short emission wavelength.20 Time- amorphous one, an increase in the mobility of molecules
resolved uorescence spectra were measured to verify this. Note induced by solvent fuming will promote molecular rearrange-
that the uorescence average lifetime (hsi) of the pristine PVBA2 ment from a metastable amorphous structure to a more stable
solid is 2.5 ns, which increases to 3.4 ns aer grinding crystalline state.24
(Fig. S20†). The increase in lifetime could be ascribed to energy
Fig. 4 Normalized fluorescence spectra of (a) PVBA2, (b) PVBA8, (c)
PVBA12 and (d) PVBA16 in the solid state under external stimuli. Inset: Fig. 5 X-ray powder diffraction of (a) PVBA2, (b) PVBA8, (c) PVBA12
schematic diagrams of photos under 365 nm light. lex ¼ 400 nm.
and (d) PVBA16 in the pristine state and after different treatments.
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As discussed above, the permeation of solvent vapors into tunable recovery behavior could be useful for various
the ground powders increases molecular mobility and then applications.
induces color recovery. The increase in temperature also leads
The alkyl length-dependent mechanochromism could be
to accelerated molecular thermal motion; therefore, the interpreted by DSC experiments. Fig. 7 shows that there is
thermal restoration of the ground solids was measured. The clearly one exothermic transition peak in the lower-temperature
result is shown in Fig. 6 and listed in Table S4.† It is clear that region for each ground solid. This broad exothermic peak could
the recovery time becomes shorter at higher temperatures and be ascribed to the cold-crystallization (crystallizing from glass
is relative to the length of the alkyl chain at the same temper- state) of ground PVBAn solids upon annealing26 and also
ature. The restoration of the ground compounds with longer conrms that the amorphous state is a metastable state.
alkyl chains needs shorter time at the same temperature.25 At Amorphized PVBA2, PVBA8 and PVBA12 solids have high
110 ꢀC, the ground PVBA16 powder restored to the original state exothermic peak values, which result in their stable MFC
aer 2 s. The recovery time is 3 s for PVBA12 and 360 s for behaviour at room temperature. The ground PVBA16 solid
ꢀ
PVBA8, but PVBA2 cannot be restored even aer 2 h. At 50 C, could cold-crystallize at room temperature since its exothermic
the uorescence of the ground PVBAn (n ¼ 2 and 8) solids was peak is located at 63 ꢀC.25b,27 Thus, the amorphized PVBA16
retained for more than one week, but the orange ground solid induced by grinding rapidly recovered its uorescence at
PVBA16 solid converted into a yellow one aer 400 s. Another lower temperatures and even exhibits spontaneous recovering
interesting phenomenon is the MFC behavior of the PVBA16 uorescence properties at room temperature. The reason for the
solid. Note that only the ground PVBA16 solid emits yellow compounds with longer alkyl chain possessing lower phase
uorescence; however, the uorescence color gradually transition temperatures could be ascribed to the fact that longer
changed to the original green emission aer ve days, when it alkyl chains provide higher molecular mobility because the
was le standing at room temperature (20 ꢀC). Unlike PVBA16, ordered packing between alkyl chains becomes disordered
the uorescence colors and emission spectra of ground PVBA2, more easily relative to aromatic groups with increase in
PVBA8 and PVBA12 solids remain for over two weeks at the temperature. This point is interpreted by the change in melting
same temperature. These ndings suggest that we can endow points with change in length of the alkyl chain. As shown in
ꢀ
bianthracene derivative dyes with stable or self-recovering MFC Fig. 7, PVBA2 has the highest melting point at 347 C, and the
behavior by tuning the alkyl chain length. MFC materials with lowest melting point at 192 ꢀC is observed for PVBA16. In
addition, PVBA12 and PVBA16 in pristine and ground condi-
tions have an endothermic transition before the melting point,
which could be attributed to a crystal phase transition.
However, the reason for PVBA12 and PVBA16 exhibiting this
transition is still unclear, and this phenomenon requires
further investigation.
Conclusions
Bowl-shaped phenothiazine and nonplanar bianthracene were
integrated into one molecular structure to obtain PVBAn (n ¼ 2,
8, 12 and 16) with different alkyl chains. The spectral results
and quantum chemical calculations indicate that the
compounds are D–p–A–p–D molecules. Interestingly, these
simple and easily prepared bianthracene derivatives exhibit
pronounced and reproducible reversible mechanochromism,
which is dependent on the alkyl chain length. XRD and DSC
Fig. 6 Plot of recovery time of ground powders vs. temperature.
reveal that the transformation between crystalline and amor-
phous states upon various external stimuli is responsible for the
MFC behavior. We have also observed an intriguing MFC
behavior of PVBA16, as well as an isothermally reversible MFC
behavior at room temperature, which is ascribed to the low
cold-crystallization temperature. This work has demonstrated
that the subtle manipulation of alkyl groups of phenothiazine-
based bianthracene could endow these compounds with unique
and tunable solid-state optical properties.
Acknowledgements
This work was nancially supported by the National Natural
Fig. 7 DSC heat curves of pristine (black) and ground (red) PVBAn.
Science Foundation of China (21103067, 51403020 and
J. Mater. Chem. C
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21374041), the Youth Science Foundation of Jilin Province
(20130522134JH), the Open Project of the State Key Laboratory
of Supramolecular Structure and Materials (SKLSSM201407),
and the Open Project of State Laboratory of Theoretical and
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