A luminescent benzothiadiazole-bridging bis(salicylaldiminato)zinc(ii) complex with mechanochromic and organogelation properties

Article abstract of DOI:10.1039/c8dt00665b

Herein, a new bis(salicylaldiminato)Zn(ii) Schiff base complex, BTZn, derived from benzo[c][1,2,5]thiadiazole-5,6-diamine was designed and synthesized. It exhibited unique mechanical force-induced luminescence change characteristic. Upon mechanical grindi

Full text of DOI:10.1039/c8dt00665b

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PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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A luminescent benzothiadiazole-bridging
bis(salicylaldiminato)zinc(II) complex with mechanochromism and
organogelation properties
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaoxian Song, Hanbo Yu, Xianju Yan, Yuewei Zhang, Yang Miao, Kaiqi Ye* and Yue Wang*
DOI: 10.1039/x0xx00000x
A new bis(salicylaldiminato)Zn(II) Schiff-base complex BTZn, derived from benzo[c][1,2,5]thiadiazole-5,6-diamine has been
designed and synthesized. It exhibited unique mechanical force induced luminescence change characteristic. Upon
mechanical grinding, as-prepared BTZn solid crystalized from ethanol/dichloromethane solution displayed a high-contrast
emission-colour variation from yellow (emission maximum λem = 545 nm) to red (λem = 645 nm) and such emission variation
can be erased through solvent vapour treatment.The reversible emission colour alteration between yellow and red can be
repeatedly performed. The thermal annealing of as-prepared BTZn solid resulted in a more ordered orange phase with
emission maximum of 575 nm. The multi-stimuli-responsive luminescence mechanism has been investigated by the SEM,
powder X-ray diffraction (XRD) and thermal analyses. It was demonstrated that the mechanical force can induce the
morphology transformation from crystalline to amorphous phase, which was accompanied by the change of BTZn molecualr
packing modes. The BTZn based solids have molecular packing dependent emission characteristic. XRD experimental results
revealed that for yellow emissive as-prepared BTZn solid molecualr columnar square arrangement was adopted. On the
other hand, BTZn complex exhibited the ability to organize into organic luminescent gels that were constructed by one
dimensional BTZn molecualr nanofibrils. The BTZn xerogels also displayed mechanochromic property. Accordingly, BTZn
based solids may be the potential candidates for the development of new stimuli-responsive materials.
www.rsc.org/
mechanochromic luminescence behaviour.7, 8 In contrast, zinc
complexes with distinct luminescence mechanochromism are
very rare.9 Actually, the zinc complexes with rich
photoluminescence (PL) properties are also particularly
attractive for the inventions of functional materials due to that
zinc element is abundant and less expensive compared with the
noble metals. The study of stimuli-responsive materials based
on zinc complexes is beneficial to the development of low cost
sensor materials.
Introduction
Luminescent materials that respond to external stimuli are of
scientific and technological interest owing to their wide
potential applications in the fields of various sensors,
optoelectronic and medical devices.1 For stimuli-responsive
materials, mechanochromic luminescent materials, which
change their emission colours based on the response to
external mechanical forces (such as grinding, crushing, shearing
and rubbing) and can subsequently revert to their original
emission states by another external stimulus, have attracted
much attention. This is due to that the property can be used to
develop memory-recording devices, motion or damage sensors,
and security inks.2 Although a number of pressure-sensitive
luminescent transition-metal complexes have been reported,3
the amount of metal complexes with mechanochromic
luminescence feature is still limited compared with the organic
compounds that have responsive property to mechanical force
stimuli.4 So far, most of the reported mechanochromic metal
The mechanochromism behaviours were often attributed to
the molecular packing changes within metal complex or organic
compound solids. Generally, the intermolecular interactions
such as π···π stacking, hydrogen bonding, weak coordination
bonding and metal···metal interactions dominate the molecular
packing modes. The intermolecular interactions have dramatic
influence on the emissive properties of molecules based
solids.10 On the other hand, the mechanical force induced phase
transformations are frequently accompanied by the changes of
intermolecular interactions and sequentially lead to the
alteration
of
emission.11
Therefore,
to
achieve
5,
6
complexes are gold and platinum based complexes.
addition, some copper and silver complexes exhibited
In
mechanochromism, developing the strategies that can
efficiently regulate the intermolecular interactions should be
focused on. It is worth to note that most of luminescent
mechanochromism compounds can only show a single phase
transformation process that are accompanied by the
alternation of two emission colours. The metal complex or
State Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun 130012, P. R. China. E-mail:
yekq@jlu.edu.cn; yuewang@jlu.edu.cn
† Electronic Supplementary Information (ESI) available: supplementary figures and
tables. See DOI: 10.1039/x0xx00000x
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Scheme 1. Molecular structure and synthesis procedure of the BTZn complex.
organic molecule based solids with multiple colour and benzo[c][1,2,5]thiadiazole-5,6-diamine were stirred and
luminescence change property is still a challenge.12 Recently, heated at reflux temperature for 3 h. The complex was isolated
bis(salicylaldiminato)zinc(II) Schiff-base complexes have drawn as yellow solid with good yield and confirmed by NMR, mass and
a lot of attention due to their multifunctional characteristics, elemental analysis spectroscopies. The synthetic details of the
such as, non-linear optical materials, supramolecular assembly, intermediates and target product are presented in
fluorescent molecular probes and active materials for organic Experimental Section. In MALDI-TOF mass spectrum of BTZn
optoelectronics.13 However, there is only a few examples of complex the dominant peak was attributed to the parent
Schiff-base zinc(II) complexes exhibiting thermochromic, molecular ion and a peak corresponding to molecular dimer
vapochromism and mechanochromism behaviors.14,9 It is species (Fig. S1) was also recorded, which was often observed
noteworthy
that,
Zn(II)
complexes
often
adopt for Zn(II) Schiff base complexes and generated base on
pentacoordination structures with relatively weak axial binding intermolecular Zn···O interactions.17 Therefore, in the BTZn
contact of Zn···donor interactions. It was demonstrated that the solid the molecular dimers really exist. The presence of lateral
axial intermolecular Zn···O interactions can result in the alkyl chains that attach to the salicylidene rings of the Schiff
formation of dimeric or oligomeric Schiff-base zinc(II) base makes complex BTZn moderately soluble in common
complexes.15 Generally, the weak axial Zn···O interactions are organic solvents. This property is essential for the investigations
sensitive to external stimuli resulting in different aggregation of photophysical and assembly properties in solutions. On the
states, 16 which may be employed to develop smart materials.
other hand, the lateral alkyl chains also have dramatic effect on
In this paper, a new Zn(II) Schiff base complex BTZn (Scheme supramolecular assembly properties.18 In this study the Schiff
1) with mutil-color mechanochromism luminescent property is base ligand was modified by 1,3,4-thiadiazole with the electron
reported. BTZn based solid exhibited luminescence changes withdrawing property, and the benzothiadiazole-containing
upon mechanical, organic solvent and thermal stimuli. To the salicylaldiminato moiety and hydroxybenzyl are electron
best of our knowledge, BTZn represents the largest acceptor (A) and donor (D), respectively. The introduction of
mechanochromic luminescence shift (around 100 nm) for zinc 1,3,4-thiadiazole moiety not only extended the π-conjugated
complexes. To understand the intriguing emission colour area of ligand, but also established a donor–acceptor–donor
alternation behaviour of BTZn solids, comprehensive (D–A–D) type molecule with intramolecular charge transfer (ICT)
investigations on structural, photophysical and morphological feature. Generally, the large planar π-conjugated ligand based
properties were performed. Furthermore, the BTZn complex complexes have abundant emission, assembly and
displays gelation characteristic in some organic solutions and mechanochromism properties.
the xerogel of BTZn also exhibits mechanochromic
luminescence feature.
Photophysical Properties in Solution
The UV-vis absorption and photoluminescent (PL) spectra of
BTZn complex in different solvents are shown in Fig. 1. The
absorption spectra of BTZn in noncoordinating solvents of
Results and discussion
toluene and DCM display identical profile with two intense
bands at around 330 and 450 nm, respectively. However, in
Synthesis
Zinc(II) complex BTZn was synthesized with benzothiadiazole-
containing salicylaldiminato Schiff base ligand through a facile
and straightforward procedure (Scheme 1). In ethanol solution,
zinc acetate dihydrate, 4-(hexyloxy)-2-hydroxybenzaldehyde
coordinating solvents of THF and DMSO, two defined
absorption bands were recorded in the region from 300 to 400
nm and a slight red-shift (around 10 nm) of absorption was
observed. Similar results were obtained for the reported Zn(II)
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Schiff bases complexes. This phenomena was attributed to that mechanochromic phenomenon (Fig. 2b). WhenVietwheArticglreoOunlninde
aggregates are always formed in noncoordinating solvents, powder was exposed to organic DOvIa: 1p0o.1u0r3s9,/C8sDuTc0h0665aBs
while monomeric species exist in coordinating solvents due to dichloromethane (DCM), tetrahydrofuran (THF) and toluene,
the axial coordination of the solvent to the Zn(II) metal center.15 the emission rapidly recovered to the initially yellow colour (Fig.
It was demonstrated that BTZn exhibited remarkable solvent- 2a-iii). However, heating the ground sample at 125 °C for 2 min
dependent emission property (Fig. 1b). Upon increasing solvent resulted in an orange luminescent solid with emission maximum
polarity obvious red-shift of emission was observed suggesting at 575 nm (Fig. 2a-iv). Under grinding-fuming and grinding-
the ICT excited sate characteristic of BTZn complex. Due to the heating stimuli the luminescence alternation processes can be
fully filled d shell of the central Zn(II) ion, the observed repeated
many
circles,
indicating
the
reversible
fluorescence emission of the complex should originate from the mechanochromic behaviour of BTZn solid (Fig. 2c). Interestingly,
ligand-centred excited state.19
some marks can be easily written by using a stick on the as-cast
film prepared from a saturated
Fig. 1 Normalized UV-vis absorption and fluoroscence spectra of BTZn in different
solutions (1.0×10-5 M).
Mechanochromic Property
Fig. 2 (a) Photo images recorded under UV light (λex = 365 nm) for BTZn solids with
different phases: (i) as-prepared solid, (ii) ground powder, (iii) sample fumed with DCM
vapour, and (iv) solid annealed at 125 °C . (b) Normalized PL spectra of BTZn solids with
different phases. (c) Emission switching characteristics of BTZn solids upon repeating
grinding-fuming and grinding-heating processes.
The
as-prepared
BTZn
solid
crystallized
from
ethanol/dichloromethane solution displayed yellow emission
under UV (365 nm) illumination (Fig. 2a-i). Upon simply grinding
with a mortar and pestle, the as-prepared BTZn solid changed
into a red emissive powder (Fig. 2a-ii). The PL maximum of as-
prepared solid is similar to that of toluene solution of BTZn, this
phenomenon may be attributed to that aggregates are often
formed in noncoordinating solvents. In toluene solution BTZn
molecules should adopt aggregated state. The PL spectra
indicated that the grinding treatment has induced a remarkable
spectral red-shift by 100 nm and the emission maximum
underwent a shift from 545 to 645 nm suggesting a typical
dichloromethane solution on the quartz slides (Fig. 3a). Once
the film was treated with a small drop of DCM, the red marks
could be erased. The reversible and sensitive luminescence
switching based on BTZn complex may have potential
applications in sensing, ambient monitoring and security inking.
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We demonstrated that the mechanochromic behaviour of
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To understand the mechanochromism mechanism, powder
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DOI: 10.1039/C8DT00665B
BTZn can be characterized by absorption spectra. The UV-vis X-ray diffraction measurements (Fig. 5) were conducted, which
spectra of the film in different states were compared in Fig. 3b. provided direct information on molecular packing modes of
The absorption maximum of the yellow emissive as-casting film different phases based on BTZn. The as-prepared solid showed
located at 450 nm with a shoulder at around 470 nm. Grinding multiple intense diffraction pattern with several split peaks.
treatment induced a broad absorption spectrum, suggesting Therefore, the as-prepared solid with thin nano-fiber structure
that the molecular packing mode has changed under grinding feature should adopt a polycrystalline state and composed of a
treatment. Upon fuming the ground film with DCM vapour for mixture of BTZn samples with different phase due to that the
2 min, the absorption spectrum exhibited blue-shifted and as-prepared solid was prepared by rapid recrystallization
almost recovered into the initial profile of as-casting film, process. For the as-prepared solid sample, 125 °C heating
indicating that the molecular aggregate restored into the treatment for 5 min has not resulted in phase transformation,
original mode. In contrast, thermal annealing resulted in a while further higher temperature treatment (165 °C and 2 min)
significant red-shift of the absorption maximum. The longer leaded to the generation of orange emissive crystals with highly
wavelength absorption feature implied a J-aggregate
ordered lamellar structure feature and a sharp diffraction peak
at 2θ = 4.27°, and a series of peaks at 2θ =
Fig. 3 (a) Photo images recorded for stimuli-responsive property. (b) Normalized UV-vis
spectra of BTZn thin films with different phases.
state may be adopted in annealed film. Accordingly, in as-
casting film, BTZn molecules may adopt an H-like aggregate.
The morphologies of different phases were recorded by
fluorescence microscopy and scanning electron microscopy
(SEM) images (Fig. 4). The as-prepared solid crystallized from
ethanol/dichloromethane solution showed a one-dimensional
(1D) morphology feature and is composed of green emissive
fibrillar network (Fig. 4a and b). It was demonstrated that some
salphen complexes exhibited similar 1D nano-assembly
property,13b, 13f which are similar to the nature of BTZn in this
study. After mechanical shearing, BTZn solid changed into a red
emissive film (Fig. 4c and d), indicating the amorphous
characteristic. The DCM vapour treated sample showed
Irregular green emissive aggregates on the substrate surface,
which may suggest the tendency of phase transformation from
amorphous to ordered phase (Fig. 4e and f). Heating treatment
leaded to the formation of block-like crystalline aggregates,
demonstrating that more ordered structure was achieved (Fig.
4g and h). Above experiment results provided valuable
information for understanding the different morphologies
induced by various external stimuli.
Fig. 4 Fluorescence microscopy and SEM images for BTZn in different states: (a, b) as-
prepared solid, (c, d) ground film, (e, f) solid fumed with DCM vapour, and (g, h) solid
thermally annealed at 125 °C for 2 min.
8.60, 12.83, and 17.14 were recorded. These peaks
corresponding to d-spacing of 20.68, 10.27, 6.89 and 5.17 Å,
respectively, with a ratio of 1:1/2:1/3:1/4, suggesting a perfect
layered structure. (Fig. 5a).
In contrast, the ground solid sample displayed only
ambiguous diffuse halo suggesting the formation of amorphous
phase. After fuming with DCM vapour for 2 min, the diffraction
pattern showed a set of strong diffraction peaks, which are
different from that of as-prepared solid (Fig. 5b). It is worthy to
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note that the diffraction peaks in the low angle region of the then ground into amorphous red phase, which was finally
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solvent vapour fumed sample displayed 2θ values of 3.24, 4.58, heated at 125 °C for 2 min, the obtained orange crystalline solid
6.48, 7.22, and 9.73°, corresponding to d-spacings of 27.25, displayed identical X-Ray patterns to that of the sample
19.28, 13.63, 12.24, and 9.08 Å, respectively, with a ratio of prepared by grinding the as-prepared solid and then heating at
almost 1:1/
2:1/ 4 :1/ 5:1/ 9. This diffraction pattern is 125 °C for 2 min (Fig. S2). These XRD results revealed that the
√ √ √ √
consistent with
a high-symmetry 2D (two-dimensional) mechanochromic process of BTZn solid was accompanied by the
columnar square arrangement with a lattice constant of 27.25 transformation between crystalline and amorphous phases. The
Å. Based on solvent vapour treatment the amorphous ground crystalline structure of solvent vapour fumed sample is different
powder has changed into crystalline columnar phase with a from that of the thermally treated sample.
higher symmetry. For the ground amorphous sample, 125 and
Thermodynamic properties of the BTZn based samples with
165 °C heating treatment leaded to the formation of crystalline different phases were checked by differential scanning
phase with intensity diffraction peaks (Fig. 5b), in which except calorimetry (DSC) and thermogravimetric analysis (TGA)
the peak at 10.2° all of the other peaks are identical to that measurements to check the phase transformation processes
observed for the sample of as-prepared solid that was heated (Fig. S3). BTZn complex was stable up to 300 °C , and displayed a
at 165 °C for 2 min (Fig. S2).
melting point at around 340 °C with concurrent decomposition.
For the as-prepared sample, there was an endothermic peak at
around 165 °C . While a small exothermic peak at 125 °C and a
weak broad endothermic peak at around 135 °C were observed
for the ground sample. Solvent vapour fuming treatment to the
ground sample resulted in the solid with identical thermal
property to that of as prepared solid. It is proposed that the
phase transitions induced by fuming and heating treatments are
most likely relevant to the solvent molecules adsorption and
release process, since a significant weight loss (around 5.5wt %)
occurs within the temperature range from 130 to 180 °C for the
initial as-prepared and fumed samples. The grinding treatment
resulted in a small amount release of solvent molecules. On the
other hand, the heating treatment (125 °C ) generated the
sample that does not show any discernible thermic peak or
weight loss before decomposition temperature. Therefore, for
the heated sample solvent molecules have been completely
eliminated. In the 1H-NMR spectrum of as-prepared solid
sample an additional peak at δ 3.30 ppm, which is associated
with the existence of water molecules, was observed, and this
signal nearly disappeared after the sample was heated at 165 °C
for 2 min (Fig. S4 and S5). Therefore, the as-prepared solid
should have the position of BTZn·2H2O due to that upon heating
the weight loss of 5.5% is close to the water content (5.3%) of
the BTZn·2H2O complex. For the CH2Cl2 vapour fumed sample,
the 1H-NMR spectrum (Fig. S6) presented a peak at δ 5.76 ppm,
which was assigned to CH2Cl2. For the samples that were heated
no solvent or water signal was observed in the 1H-NMR spectra.
To figure out the mechanochromic mechanism of BTZn solid,
a possible supramolecular structure transformation process is
presented in Fig. 6a. The DFT optimized results demonstrated
that BTZn molecule adopts a equilateral triangle like structure
feature and the molecular dimension is about 20 Å (Fig. 6b). The
as-prepared yellow emissive solid crystallized from
ethanol/dichloromethane solution may be composed of one-
dimensional molecular columns. Although the accurate
molecular stacking mode is unavailable, it is presumed that the
BTZn molecules should aggregate into dimers with antiparallel
mode (Fig. 6c), which then assemble into columns based weak
interdimer π···π stacking interactions. It is well known that in
solid state some Zn(II) Schiff-base complex molecules often
aggregate into solids based on intermolecular Zn···O
interactions in absence of axial coordinating ligand.14 Since the
Fig. 5 (a) X-ray powder diffraction of as-prepared BTZn solid before and after heating at
125 °C and 165 °C . (b) X-ray powder diffraction of the as-prepared solid upon grinding,
and the ground sample upon DCM vapour fuming and heating at 125 °C and 165 °C.
Therefore, for the ground sample the heating treatment
resulted in the formation of layered phase accompanied by
another phase, which showed diffraction peak at 10.2°with d-
spacing of 8.66 Å. For the amorphous BTZn sample, heating
treatment induced the generation of mixture crystalline phases.
When the as-prepared solid was heated at 165 °C for 2 min and
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lattice constant of 27.25 Å is obviously larger than the
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Upon mechanical grinding, the well-organized V1ieDw AcroticlulemOnnlianer
dimension of the BTZn molecule, solvent molecules should exist arrangement should be destroyed and DthOeI: 1m0.1o0l3e9c/uCl8aDrTd0i0m66e5rBs
in the as-prepared sample. The side alkyl chains on the form disordered aggregate resulting in amorphous phase.
salicylidene rings could provide Van der Waals interactions Under solvent vapour atmosphere the BTZn molecular dimers
between molecular columns, leading to the formation of 2D can spontaneously arrange themselves into ordered crystalline
columnar square structure. This class molecular columnar phases with high-symmetry columnar square structure feature.
structures were commonly observed in Zn(II) Schiff-base
complexes.20
Fig. 6 (a) Schematic presentation of the BTZn molecular packing transformation process induced by multi-stimuli. (b) The theoretically estimated BTZn molecular structure. (c) BTZn
molecular dimer based on Zn···O interactions.
Under thermal annealing condition, the complex dimers prefer with small diameter of several tens nanometers. The xerogel
to arrange into a well-defined and more condensed lamellar obtained from toluene showed a rope-type morphology formed
packing structure with layer period distance of 20.68 Å, in which by the assembly of numerous bundled and slender fibers (Fig.
J-aggregates stabilized by π···π stacking interactions between 7b). In o-dichlorobenzene solution BTZn molecules can form 3D
the benzothiadiazole rings may be adopted. The thermal nano-network based on thin fibrils (Fig. S7). The XRD pattern of
treatment induced orange solid of BTZn complex with a close xerogel exhibited two obvious diffraction peaks corresponding
packing lamellar arrangement is the thermodynamically stable to d-spacings of 37.6 and 18.8 Å, respectively, which were in a
phase.
ratio of 1:1/2, indicating that a lamellar-like assembly structure
with an interlayer distance of 37.6 Å was adopt in BTZn xerogel
(Fig. S8). The period length is approximately twice dimension of
Gelation Property
The 1D fibrillar structure feature prompt us to check the BTZn molecule, suggesting that in gel BTZn molecules
gellization property of BTZn complex. The organic gel formation aggregated into different supramolecular structure.
ability of BTZn was tested in various solvents by means of the
The BTZn xerogel also exhibited mechanochromic
‘‘stable to inversion of a test tube’’ method.21 The results are characteristic. As shown in Fig. 7c, upon grinding a part of the
listed in Table S1. Notably, the complex easily forms gels in xerogel by a spatula, sharp emission contrast between yellow
aromatic solvents, such as toluene, p-xylene and o- and red was achieved. Upon solvent (such as toluene or DCM)
dichlorobenzene. For the selected solvents the low critical vapour fuming, the ground sample recovered into original
gelation concentration of complex BTZn was 1 mg/mL, yellow emission. Heating treatment at 125 °C for 2 min only
suggesting the strong gellization characteristic. Time- resulted in the luminescent change of ground xerogel, while the
dependent fluorescence emission spectra showed slightly blue- emitting colour of unground xerogel gradually changed into
shifted and the emission intensity gradually decreased upon orange based on a longer heating time (30 min). We deduced
cooling the hot solutions (2×10-3 M) to room temperature, that the mechanofluorochromic mechanism of the as-prepared
suggesting that H-like aggregates formed in the gel phase (Fig. solid could be applied to the xerogel. The solvent molecules can
7a). The gels still exhibited strong luminescence with emission be fixed in the nanonetwork during the gel
maxima at around 542 nm, which is similar to that of the as-
prepared solid. Transmission electron microscopy (TEM) of the
xerogels revealed that BTZn molecules formed 1D nanofibers
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were recorded on a Thermo Fisher ITQ1100 mass sVpieewctArrtoicmle eOtnelinre.
Matrix-assisted laser desorption/ionizDatOioI:n10.1(0M39A/CLD8DI)T00w6e65rBe
obtained on a Bruker Autoflex speed TOF/TOF (TOF = time-of-
flight) spectrometer, samples were dissolved in chloroform
before analysis. Elemental analyses were performed on a Vario
Micro (Elementar) spectrometer. UV-vis absorption spectra
were recorded by a Shimadzu UV-2550 spectrophotometer and
PL emission spectra were recorded by a Shimadzu RF-5301 PC
spectrometer. TGA and DSC was carried out on TA Q500 and TA
Q20 thermogravimeter at a heating rate of 10 °C min-1 under a
nitrogen atmosphere, respectively. FESEM images were
performed on a JSM 6700F field emission scanning electronic
microscope. TEM (transmission electron microscopy)
characterization was performed with a JEOL JEM-2100F
microscope operated at 200 KV. Power X-ray diffraction data
were recorded by a PANalytical B.V. Empyrean diffractometer
with Cu-Kα radiation (λ = 1.5418 Å), operating at 40 kV and 40
mA.
Fig. 7 (a) Time-dependent photoluminescence spectra upon cooling the hot solutions in
toluene (2×10-3 M). Inset: Photograph of BTZn gel from toluene under normal and UV-
light. (b)TEM images of BTZn xerogel obtained from toluene gel. (c) Photographs of the
xerogel recorded under UV light (λex = 365 nm) and the responsive properties to the
stimuli of grinding, fuming and heating.
formation and grinding could result in part of solvents release.
The ground xerogel is more sensitive to thermal stimuli than the
unground one suggesting that the latter should be more stable
than the former.
Preparation of Materials
CHCl3 and triethylamine (Et3N) used for organic synthesis were
freshly distilled over CaH2 under a nitrogen atmosphere. Thionyl
chloride (SOCl2) in high-performance liquid chromatography
(HPLC)-grade was purchased from Aladdin (Shanghai, China). All
the other chemicals and reagents were used as received from
commercial sources without further purification. Compounds
benzo[c][1,2,5]thiadiazole-5,6-diamine22 and 4-(hexyloxy)-2-
hydroxybenzaldehyde23 were synthesized based on a slightly
modified method reported earlier. All reactions were
performed under nitrogen atmospheres.
Synthesis of 5,6-dinitrobenzo[c][1,2,5]thiadiazole To a 250 mL
flask, 4,5-dinitrobenzene-1,2-diamine (1.1 g, 5.6 mmol), CHCl3
(80 mL) and Et3N 2.3 mL, 22.7 mmol) were added. The solution
was stirred until total dissolution of the diamine. SOCl2 (0.9 mL,
11.4 mmol) was slowly added dropwise under ice-cooled
condition, and the reaction mixture was then heated at reflux
with stirring for 5 h. The solvent was then removed with a
rotatory evaporator and water (150 mL) was added.
Concentrated hydrochloric acid was added to the water
solution and the pH value of water solution was controlled at
around 2. The mixture was extracted with dichloromethane and
the combined organic layer was dried over anhydrous MgSO4
and concentrated. The crude product was purified by column
Conclusions
In summary, a bis(salicylaldiminato Zn(II) Schiff-base complex
BTZn with unique mechanochromic property was prepared and
characterized. BTZn based solid is a new class mechanochromic
materials. Detail photophysical, SEM, X-ray diffraction
characterizations of the different phases provided deep insights
into the mechanochromism phenomena. The mechanochromic
and solvent or thermal annealing processes were accompanied
by the ordered molecular columns disassembly and reassembly
behaviours, respectively. The assembly molecular packing
changes resulted in the emission alterations of BTZn samples
with different phases. The thermal treatment resulted in the
formation of solvent-free and well defined lamellar structure.
Moreover, in some organic solution BTZn complex displayed
excellent gellization ability and assembled into nanofiber
network. The BTZn based xerogel also exhibited
mechanochromic characteristic, which was similar to that
shown by as-prepared BTZn solid. The 1D supramolecular self-
assembly property endows BTZn based solids or xerogel with
mechanochromic feature. The BTZn based 1D nano-structure is
generated based on the weak interdimer interactions that can
be easily destroyed by mechanical forces leading to disorder
aggregation structure and emission change. The multicolour
emission alteration property of BTZn solids or xerogel implies
that this complex may be employed to develop multifunctional
chromic materials.
chromatography
using
a
mixture
of
dichloromethane/petroleum (1:1) as the eluent to afford pale
yellow solids (845 mg, 67 %). 1H NMR (500 MHz, CDCl3): δ 8.67
(s, 2H). GC-MS m/z: 225.98 [M]+ (calcd: 225.84).
Synthesis of benzo[c][1,2,5]thiadiazole-5,6-diamine 5,6-
dinitrobenzo[c][1,2,5]thiadiazole (678 mg, 3.0 mmol) and iron
powder (3.36 g, 60.0 mmol) were suspended in acetic acid (60
ml) and the mixture was stirred at 65 °C for 7 h under N2. After
cooling down to room temperature, the resulting mixture was
poured into 5% NaOH aqueous, and extracted with diethyl ether
(3 × 100 mL). The organic layer was collected, dried over
anhydrous MgSO4 and followed by purification by column
chromatography using a mixture of dichloromethane/ethyl
acetate (10:1) as the eluent, affording a white solid of 224 mg
Experimental Section
General Information
NMR was measured on Bruker Avance 500 MHz spectrometer
at room temperature with tetramethylsilane (TMS) as internal
standard. Gas chromatography-mass spectrometry (GC-MS)
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J. Name., 2013, 00, 1-3 | 7
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(45% yield). H NMR (500 MHz, DMSO): δ 6.74 (s, 2H), 5.81 (s,
4H). GC-MS m/z: 166.03 [M]+ (calcd: 166.14).
Acknowledgements
View Article Online
DOI: 10.1039/C8DT00665B
This work was supported by the National Natural Science
Foundation of China (91333201), the National Basic Research
Program of China (2015CB655003), Program for Chang Jiang
Scholars and Innovative Research Team in University (No.
IRT101713018).
Synthesis of 4-(hexyloxy)-2-hydroxybenzaldehyde 2,4-
dihydroxybenzaldehyde (2 g, 14.5 mmol), 1-bromohexane (2.6
g, 16 mmol), KHCO3 (2.2 g, 21.7 mmol) and catalytic amount of
KI (0.1 g) were mixed in dry acetone (100 mL) and the mixture
was refluxed for 48 h. It was then filtered to remove the
insoluble solid. The warm solution was neutralized with dilute
hydrochloric acid and extracted twice with CH2Cl2 (100 mL). The
combined extracts were concentrated to give a purple solid. The
product was purified by column chromatography using silica gel
(200–300 mesh) eluting with a mixture of dichloromethane and
petroleum (v/v=1:1) followed by evaporation of solvent to yield
colourless oily product (2.5 g, 78%). 1H NMR (500 MHz, CDCl3):
δ 11.47 (s, 1H), 9.70 (s, 1H), 7.41 (d, J = 8.7 Hz, 1H), 6.53 (d, J =
8.7 Hz, 1H), 6.41 (s, 1H), 4.00 (t, J = 6.5 Hz, 2H), 1.86 – 1.73 (m,
2H), 1.51 – 1.30 (m, 6H), 0.91 (t, J = 6.3 Hz, 3H). GC-MS m/z:
222.13 [M]+ (calcd: 222.19).
Synthesis of [N,N-Bis(4-hexyloxy-2-hydroxybenzylidene)-5,6-
benzo[c][1,2,5]thiadiazole-diaminato]ZnII (BTZn) A mixture of
4-(hexyloxy)-2-hydroxybenzaldehyde (444 mg, 2.0 mmol),
benzo[c][1,2,5]thiadiazole-5,6-diamine (166 mg, 1.0 mmol), and
zinc acetate dihydrate (220 mg, 1.0 mmol) in ethanol (40 mL)
was heated at reflux with stirring for 3 h under a nitrogen
atmosphere. After cooling to room temperature, the resulting
precipitates were collected by filtration and then crystallized
from ethanol/dichloromethane (v/v=3:1). The yellow crystalline
powder was dried in vacuum (80 °C, 10−2 Torr) for 5 h and finally
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