Microwave-assisted crystallization inclusion of spiropyran molecules in indium trimesate films with antidromic reversible photochromism

Article abstract of DOI:10.1039/c2jm34618d

An indium trimesate metal-organic framework (JUC-120) which possesses a cubic zeolitic MTN topology, as a new analogue to MIL-100(Al, Fe or Cr) compounds, has been successfully synthesized. Its structure exhibits a mesoporous cage (26 A) and high thermal stability. The assembly of nitrobenzospiropyran derivatives (BSP) into JUC-120 nanocrystals (BSP/JUC-120) is achieved by a microwave-assisted crystallization inclusion approach. The BSP/JUC-120 films have been prepared on quartz wafers by a spin-coating method. The successful encapsulation of BSP molecules into the mesopores of the JUC-120 structure has been verified by N₂ adsorption and TGA measurements. The photochromic properties of BSP/JUC-120 films are studied by the UV-Vis and fluorescence spectroscopies. More interestingly, metastable open merocyanine (OMC) species are directly generated from the closed spiropyran form (CSP) without photoirradiation and stabilized for a long period in the BSP/JUC-120 film. The open merocyanine isomer bleaches to the closed spiropyran form by ultraviolet or visible light, and the coloration is regained upon standing in the dark, exhibiting antidromic photochromism. Moreover, the BSP/JUC-120 film shows high reversibility and thermal stability of photochromism. This highly efficient MOF film is expected to be promising in the applications of optical devices.

Full text of DOI:10.1039/c2jm34618d

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PAPER
Microwave-assisted crystallization inclusion of spiropyran molecules in
indium trimesate films with antidromic reversible photochromism†
Feng Zhang,a Xiaoqin Zou,a Wei Feng,b Xiaojun Zhao,a Xiaofei Jing,a Fuxing Sun,a Hao Rena
ac
*
and Guangshan Zhu
Received 14th July 2012, Accepted 21st September 2012
DOI: 10.1039/c2jm34618d
An indium trimesate metal–organic framework (JUC-120) which possesses a cubic zeolitic MTN
topology, as a new analogue to MIL-100(Al, Fe or Cr) compounds, has been successfully synthesized.
ꢀ
Its structure exhibits a mesoporous cage (26 A) and high thermal stability. The assembly of
nitrobenzospiropyran derivatives (BSP) into JUC-120 nanocrystals (BSP/JUC-120) is achieved by a
microwave-assisted crystallization inclusion approach. The BSP/JUC-120 films have been prepared on
quartz wafers by a spin-coating method. The successful encapsulation of BSP molecules into the
mesopores of the JUC-120 structure has been verified by N2 adsorption and TGA measurements. The
photochromic properties of BSP/JUC-120 films are studied by the UV-Vis and fluorescence
spectroscopies. More interestingly, metastable open merocyanine (OMC) species are directly generated
from the closed spiropyran form (CSP) without photoirradiation and stabilized for a long period in the
BSP/JUC-120 film. The open merocyanine isomer bleaches to the closed spiropyran form by ultraviolet
or visible light, and the coloration is regained upon standing in the dark, exhibiting antidromic
photochromism. Moreover, the BSP/JUC-120 film shows high reversibility and thermal stability of
photochromism. This highly efficient MOF film is expected to be promising in the applications of
optical devices.
MOFs offers new perspectives with expected properties and
Introduction
broadened applications. In addition to the modifications by
selecting specific ligands or grafting functional groups via post
synthesis,8 the host–guest chemistry of MOFs allows an imple-
mentation of desirable properties by embedding guest molecules
or clusters in the cavities. Recently, the utilization of MOFs for
supporting metal nanoparticles has been attempted for hetero-
geneous catalysis9 and hydrogen storage.10 Therefore, the
investigation of MOFs for host materials is attracting increasing
attention although it is still in an early stage and thus remains a
big challenge.
Metal–organic frameworks (MOFs) are emerging as an extensive
class of novel nanoporous crystalline materials, the structure of
which is composed of metal ions or clusters joined by a variety of
organic linkers through strong coordination bonds.1 The versa-
tile organic ligands and inorganic components enable molecular
engineering of MOFs with topologically diverse and pleasing
structures.2 A library of possible structures with different prop-
3
ꢀ
erties of large pore size range (3.0–48.3 A), very high surface
areas, and adjustable internal surface properties make MOFs of
interest in gas storage,4 catalysis,5 magnetism,6 and other
potential applications.7 Designing a target structure with specific
properties and functionalities provides endless exploration in
materials science. Besides the extraordinary degree of variability
in MOF structures themselves, the tailored functionalization of
In addition to the aforementioned issues of internal properties,
the external surface is another key factor for tailoring the
properties of MOF materials for advanced applications. The
possibility of controlling particle dimensions, creating more
active sites and engineering crystal surfaces has been initiated for
predetermined purposes. While great effort is continuing on the
synthesis side, the parallel exploitation of MOF materials for
industrial applications is currently under-developed. In partic-
ular, the processing of MOFs as thin films is a nascent domain
and has attracted considerable interest for many applications,11
such as membranes for separations,12 and chemical sensors.13
Furthermore, applying a thin film to a material allows us to make
better tools, and leads to a smaller, flexible design and more
efficient electronic or optical devices. Thus, many benefits are
aState Key Laboratory of Inorganic Synthesis and Preparative Chemistry,
Jilin University, Changchun, China. E-mail: zhugs@jlu.edu.cn; Fax: +86
431-8516-8331
bKey Lab of Groundwater Resources and Environment, Ministry of
Education, Jilin University, Jiefang Road 2519, Changchun 130021,
People’s Republic of China
cQueensland Micro and Nanotechnology Centre, Nathan campus, Griffith
University, QLD 4111, Australia
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm34618d
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J. Mater. Chem., 2012, 22, 25019–25026 | 25019

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expected by the processing of MOF crystals into thin films with
superior performances. Meanwhile, the supramolecular organi-
zation of guests into the uniform pores of MOF films provides a
general methodology to gain control over molecular properties
of guests and host–guest interactions. Further, MOFs supply
additional stability and high dispersion of the incorporated
active species due to their large range in pore sizes, well-defined
pore systems, high adsorption ability and high thermal stability.
From this point of view, MOF films are considered to be good
candidates as the host materials.
applications. The feasibility of this concept has been exemplified
by assembly of BSP compounds into the big mesopores of
indium trimesate MOF via a microwave synthesis approach and
then formed films by spin-coating method.25 The structure of
indium trimesate, designated as JUC-120, possesses the cubic
zeolitic MTN topology based on indium cluster and trimesic acid
(H3BTC) ligand, which is a new analogue to MIL-100(Al, Fe or
Cr). Additionally, the titled spiropyran compound, BSP
(Scheme 1), is successfully incorporated inside the supercages of
JUC-120 crystals by crystallization, assisted by microwave
heating, with the resultant product named BSP/JUC-120. The
crystallization inclusion process involves the direct synthesis of
host materials from the precursor, and the co-inclusion of dye
molecules in the mixture solution can occur simultaneously
during hydrothermal/solvothermal crystallization. Microwave-
assisted crystallization inclusion25 is employed in this study based
on the following considerations. Microwave heating enables the
preservation of the structure of the chromophore in the accom-
modated molecules which are sensitive towards hydrolysis and
thermal heating during the conventional process. Also, micro-
wave synthesis is an energy-saving process, involving a short
reaction time. Moreover, when the size of the guest molecules
exceeds the diameter of the cavity window of the host material,
crystallization inclusion will show its superior advantages in
comparison to the impregnation method. Furthermore, the
photochromatic behaviors of the BSP/JUC-120 film are studied
by UV-Vis and fluorescence spectroscopy. Upon photo-
irradiation of the BSP/JUC-120 film, the stabilization of the open
merocyanine isomer bleaches to the closed spiropyran form, and
the coloration is regained upon standing in the dark, exhibiting
antidromic reversible photochromism.
Following the successful examples of zeolites for the incor-
poration of luminescent dye molecules or other organic
compounds,14 an exploration of MOF films as host materials for
photochromic molecules is underway. Photochromic materials
can be defined as a class of compounds that change their color
reversibly upon irradiation with light or by electromagnetic
radiation.15 Among numerous photochromic compounds,16 spi-
ropyran derivatives represent as a main family member of
photochromic molecules.17 The principle of photochromism for
spiropyrans is based on the interconversion between the col-
ourless ‘‘closed’’ spiropyran form (CSP) and a highly colored
‘‘open’’ merocyanine form (OMC), such as 1-(2-hydroxyethyl)-
3,3-dimethylindolino-60-nitrobenzopyrylospiran (BSP, Scheme
1). Upon UV irradiation of the stable colorless CSP form, the
cleavage of the C–O bond takes place through a pericyclic
reaction, followed by unfolding of the molecule to a colored
planar OMC form which reverts back to the CSP form by
spontaneously fading in the dark (direct photochromism).18
Meanwhile, in less common cases, the photoinduced decolou-
ration of the OMC form bleaches to the colorless CSP form, and
regains colouration upon standing in the dark (reverse photo-
chromism).19 Photochromism has been extensively studied, and
photochromic materials have attracted much attention for a wide
range of practical and advanced applications, including optical
data storage,20 digital electronics,21 lasing effect22 and biocom-
patible materials.23 However, most spiropyran molecules in the
crystalline form generally do not undergo photochromic inter-
conversion, which limits their applications. Additionally, the
efficiency of these organic photochromic compounds is quite low
due to photodegradation processes and fast relaxation of the
energy-rich photoinduced state. Thus, the immobilization of
photochromic molecules into rigid cavities of nanoporous
materials has been attempted because of enhanced stability
against thermal relaxation and improved photoswitching
reversibility.24
Experimental
Materials
In(NO3)3$xH2O (aladdin reagent, Co., Ltd, Shanghai, China,
99%), trimesic acid (also called 1,3,5-benzenetricarboxylic acid,
H3BTC, Sigma Aldrich, 99%), 1-(2-hydroxyethyl)-3,3-dimethy-
lindolino-60-nitrobenzopyrylospiran (TCI, >93.0%, BSP), anhy-
drous ethanol, methanol and N,N-dimethylformamide (DMF) as
solvents were used as received.
Preparation of BSP/JUC-120 suspension
The precursor solution of JUC-120 was prepared by mixing
In(NO3)3$xH2O (0.08 g, 0.215 mmol) and trimesic acid (0.068 g,
0.324 mmol) in a mixed solvent of H2O–DMF (1 : 1, 10 mL).
Then, the solution above was transferred into a Teflon-lined
microwave reactor (XT-9900 Intelligent Microwave Digestion
System, Xintuo Analytical Instruments Co., LTD, Shanghai,
The objective of the present study is to assemble the photo-
chromic molecules in the nanoporous films and investigate the
photochromic properties for advanced photonics or optics
ꢀ
China) and heated at 120 C for 120 seconds at 1000 W with a
constant pressure of 1 MPa. The suspension obtained was
purified by centrifugation in three subsequent cycles (5000 rpm)
using DMF and H2O, respectively, and dried at 80 ꢀC. BSP/JUC-
120 was also prepared by the same procedure as stated
above, except that the organic photochromic compound BSP
(0.057 mmol, 0.02 g) was introduced into the precursor solution
under constant stirring for about 60 min in the dark. Further, the
excess BSP on the external surface of JUC-120 was removed with
Scheme 1 Molecular structure and photoisomerization of 1-(2-hydroxy-
ethyl)-3,3-dimethylindoline-60-nitrobenzospiropyran (BSP).
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methanol in a Soxhlet extractor until no BSP molecules could be
detected in the supernatant solution, and then the BSP/JUC-120
powders were re-dispersed in 10 mL methanol for further use.
(l > 420 nm) generated by xenon lamp (500 W) with UV-cut filter
were used to irradiate the sample. UV-Vis spectra were recorded
before and after irradiation with an external UV-light (Hayashi
LA-410, 25 mW cmꢂ2) for different time durations at room
temperature using a Perkin-Elmer Lambda 900 spectrophotom-
eter. Emission spectra were collected by fluorescence spectros-
copy (Perkin-Elmer LS55 luminescence spectrometer) before and
after exposure to external visible light (500 W xenon lamp with
an interference filter, l > 420 nm) at different irradiation times.
The time dependence of fluorescence intensity upon visible light
irradiation was described by fitting the rate constant k (sꢂ1) of
the monoexponential function using the following eqn (1):
Preparation of BSP/JUC-120 film
Prior to the preparation of MOF films, quartz wafers used as the
substrates were pre-treated according to the reported literature.26
Thin films of BSP/JUC-120 were prepared on quartz plates
(1.5 ꢁ 1.5 cm) by spin-coating the suspension of 5 g Lꢂ1 with a
speed of 5000 rpm for 30 s.27
Characterizations
At ¼ A0exp(ꢂkt) + Ath,
(1)
The crystalline structure of JUC-120 crystals and BSP/JUC-120
films were determined by X-ray diffraction (XRD) measurements
using a Riguku D/MAX2550 diffractometer with Cu-Ka radia-
where At is the peak intensity at lmax with test time and Ath
reflects the thermal equilibrium between the closed and opened
forms.24f
ꢀ
tion (l ¼ 1.5418 A) running at a voltage of 50 kV and a current of
200 mA. A solid-state 13C CP/MAS NMR measurement was
carried out on a Bruker Avance III model 400 MHz NMR
spectrometer at a MAS rate of 5 kHz. Raman spectra were
recorded on a LabRam Aramis Raman microscope system
(Horiba-JobinYvon) equipped with a multichannel air cooled
charge-coupled device (CCD) detector. Spectra were excited
using the 633 nm line of a HeNe narrow bandwidth laser (Melles
Griot). FTIR spectra were collected on a Nicolet Impact 410
FTIR spectrometer at room temperature in the range of 400–
4000 cmꢂ1, with potassium bromide pellets. Dynamic light
scattering (DLS) measurements of the suspension were per-
formed with a Malvern Zetasizer-Nano instrument. The films
were also characterized by field emission scanning electron
microscopy (FE-SEM: JEOS JSM6700F). Nitrogen adsorption–
desorption measurements on the JUC-120 and BSP/JUC-120
powders were carried out using an Autosorb iQ2 adsorptometer,
Quantachrome Instruments. Prior to the tests, the as-synthesized
JUC-120 was refluxed in methanol for 24 h. Then, the JUC-120
and BSP/JUC-120 powders were degassed under vacuum over-
night at 150 and 100 ꢀC, respectively, in order to remove the
methanol solvent trapped within the pores. The measurements
were carried out at 77 K. Specific surface areas were determined
using the Brunauer–Emmett–Teller (BET) equation over the
range of relative pressures P/P0 between 0.01 and 0.3. The pore
size distribution was calculated by applying the nonlocal density
functional theory (NLDFT) method. TGA analysis was per-
Results and discussion
General characterization of materials
The X-ray diffraction patterns collected from JUC-120 powder
and the BSP/JUC-120 film deposited on the quartz wafer are
shown in Fig. 1. The XRD pattern of JUC-120 (Fig. 1b) is similar
to the simulated MIL-100(Cr)28a (Fig. 1a) by judging all apparent
peak positions. Namely, as-synthesized JUC-120 is considered to
possess MIL-100 structural type28 based on the perfectly
matching XRD data, and also supplemented as a new analogue
to MIL-100(Al, Fe or Cr). Fig. 1c shows the XRD pattern of the
BSP/JUC-120 film. No substantial differences are observed
between the initial JUC-120 powder (Fig. 1b) and the BSP/JUC-
120 film (Fig. 1c). As can be seen, the crystallinity is retained after
BSP loading in the BSP/JUC-120 film.
To further elucidate the structure of the JUC-120 MOF
material, NMR and Raman spectroscopy were employed to
study the organic and inorganic components of JUC-120.The
13C MAS NMR spectra of JUC-120 and the activated MIL-
100(Al) are shown in Fig. 2, the spectra of which were recorded
with a contact time of 5 ms. The spectrum of JUC-120 contains
two main groups of peaks located at d ¼ 134 and 142 ppm, which
are attributed to the C atoms of the phenyl ring, and the signal
ꢀ
formed on the BSP/JUC-120 and JUC-120 samples in air (5 C
minꢂ1) (Netzch Sta 449c thermal analyzer). For measuring BSP
content in JUC-120 crystals using an analytical protocol, the
typical procedure involves the digestion of BSP/JUC-120
powders (10 mg) in 25 mL EtOH containing 40 mL of HCl (37%
aqueous solution) with sonication. Subsequently, the solution
was transferred into a volumetric flask. The concentration of
BSP was determined by UV spectroscopy using a standard plot,
as shown in Fig. S1.†
Photochromism tests
Before the photochromism experiments, all samples were sealed
to avoid any contamination, wrapped with tin foil, and stored at
room temperature in the dark. External UV source (Hayashi LA-
410, 25 mW cmꢂ2, 320 nm < l < 400 nm) and visible light source
Fig. 1 XRD patterns of (a) simulated MIL-100(Cr), (b) as-synthesized
JUC-120 powder and (c) the BSP/JUC-120 film.
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Fig. 4 Structure of JUC-120 (a) viewed along the [101] direction, and (b)
MTN topology including two types of mesoporous cages.
(1.49 ꢁ 0.85 ꢁ 0.7 nm) would presumably be located in meso-
porous cages of the MOF structure (JUC-120) by taking into
account the molecular size and pore size.
Fig. 2 13C MAS NMR spectra of (a) the activated MIL-100(Al) and (b)
The thermal stability of JUC-120 crystals was also followed by
temperature-dependent XRD measurements. Fig. S3† shows the
evolution of the XRD pattern for the as-synthesized sample after
calcination in air at different temperatures. As seen in Fig. S3,†
it caꢀn be found that JUC-120 retains the same sꢀtructure below
370 C, but its structure collapses at about 400 C. This result
shows that JUC-120 is a quite stable MOF material (stable up to
370 ꢀC in air), which has a higher thermal stability than its
JUC-120 samples.
coming from the carboxylic groups at d ¼ 172 ppm also
appears.29 Almost all the 13C MAS NMR signals of JUC-120 are
similar to its analogue, MIL-100(Al). The NMR result combined
with supplementary IR spectra (Fig. S2†) sheds light on the
structure similarity of JUC-120 and MIL-100(Al).
counterparts of MIL-100(Cr, Fe) (275 C in air).28a,b This high
ꢀ
Additionally, Raman scattering measurements were carried
out to give some information on the microenvironments of the
inorganic components of the activated MIL-100(Al) (Fig. 3a)
and JUC-120 (Fig. 3b) at room temperature. In the range of
100–600 cmꢂ1, the four Raman peaks of the JUC-120 sample
located at 137 and 177 cmꢂ1 are assigned to the Iꢂn1–In vibra-
tions,30b and the other two peaks at 362 and 473 cm are asso-
ciated with the stretching vibrations of In–O–In.30 The slight shift
between JUC-120 and MIL-100(Al) is interpreted as resulting
from the different atom radii of indium and aluminum.
thermal stability provides a basis for the efficient evacuation of
the solvent molecules in the pores.
The surface morphologies of JUC-120 and BSP/JUC-120 films
were monitored by scanning electron microscopy (SEM). The
SEM images of JUC-120 and BSP/JUC-120 films are shown in
Fig. 5. As can be seen in Fig. 5a, JUC-120 nanoparticles around
750–850 nm in size with octahedral shape homogenously cover
the quartz substrate. Meanwhile, the top view of the BSP/JUC-
120 film is also shown in Fig. 5b. The crystals in the BSP/JUC-
120 film exhibit a similar size and shape as JUC-120. The particle
size of both samples in ethanol is determined by DLS measure-
ments (Fig. 5c and d). The particle size distribution curves
contain peaks around 794 and 657 nm for JUC-120 and BSP/
JUC-120 respectively, and this result is consistent with the SEM
Based on the observations above, JUC-120, with a similar
structure to MIL-100(Al), is constructed of indium(III) trimers
linked by tricarboxylate groups that form supertetrahedra as
building blocks, resulting in a cubic zeolitic MTN topology
(Fig. 4). After the removal of the solvent molecules occluded in
the pores, the cages with the free internal diameters of about 2.0
and 2.6 nm are accessible through small windows of 0.46
and 0.82 nm. Therefore, spiropyran derivative (BSP) molecules
Fig. 5 SEM images of (a) JUC-120 and (b) BSP/JUC-120 films depos-
ited on quartz wafers. DLS data of (c) JUC-120 and (d) BSP/JUC-120
suspensions in ethanol.
Fig. 3 Raman spectra of (a) the activated MIL-100(Al) and (b) JUC-120
samples.
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analysis. The optical picture of BSP/JUC-120 suspension in
ethanol is shown in Fig. S4.†
Thermogravimetric analysis (TGA) is a fundamental and
effective method to determine the content of organic moieties in
porous materials.31 TGA curves of the activated JUC-120 and
BSP/JUC-120 samples are presented in Fig. 6. Compared with
the reference curve of the activated JUC-120 sample, there is a
distinct weight loss for the BSP/JUC-120 sample between 235
and 330 ꢀC, which can be attributed to the partial decomposition
or combustion of the BSP species occupied in the pores. By
analyzing the TGA data, it is calculated that there is approxi-
mately 8–9 wt% BSP loaded in the BSP/JUC-120 crystals, the
amount of which is also verified by an analytical protocol with
UV-Vis spectroscopy using a standard curve of BSP solutions
(Fig. S1†).
Fig. 7 N2 adsorption–desorption isotherms of JUC-120 and BSP/JUC-
The pore structure and related textural properties were fol-
lowed by nitrogen sorption measurements. Nitrogen adsorption–
desorption isotherms for JUC-120 and BSP/JUC-120 are shown
in Fig. 7, and the physical property list of these two samples is
given in Table 1. For both samples, a steep increase in the
adsorbed volume at low relative pressure is observed followed by
a slight secondary uptake. There is much smaller uptake of
nitrogen for the BSP/JUC-120 sample if taking its counterpart
(JUC-120) as a comparison. Additionally, the surface area
120 samples.
prepared sample before irradiation and the other one is after
irradiation with an external UV source. The color change of the
BSP/JUC-120 film from pink to colorless is induced by UV light
(320 nm < l < 400 nm). This means that photochromism is taking
place during UV light irradiation. Conversely, the color of this
film would revert back to pink in the dark, which sheds light on
its reverse photochromism. In order to study in detail, UV-Vis
spectroscopy is used to study the optical properties of the BSP/
JUC-120 film. UV-Vis spectra of the BSP/JUC-120 film during
the irradiation by an external UV light are shown in Fig. 8b, and
its corresponding absorbances versus irradiation time is depicted
in Fig. S5.† Before photoirradiation, the film shows an absorp-
tion maximum at 556 nm together with one weaker absorption at
523 nm, originating from BSP molecules in an OMC form.24c,i
These results indicate that the BSP molecules in the OMC form
are stabilized in the assembled JUC-120 film, which is rarely
observed. When exposed to the UV light, this absorption band at
556 nm suffers a continuous decrease with increasing irradiation
time, indicating a conversion of OMC to CSP. It is well known
that the OMC form of BSP molecules possesses a metastable
configuration, which is found in the polar and hydrophilic
microenvironments. Meanwhile, those BSP molecules in the
stable CSP form are present in non-polar and hydrophobic
systems. This antidromic photochromism behavior for BSP
molecules incorporated in the JUC-120 structure can be
explained by the relatively high polarity of JUC-120 nanocrystals
due to the polar In–OH and In–H2O groups in the structure.27
These polar (In–OH and In–H2O) groups allow the stabilization
of the molecules with an open merocyanine structure, and thus
decelerate the rate of thermal OMC-CSP isomerizations, which
coincides with the observation of spiropyran molecules trapped
in other porous solid matrices.24cꢂi
(SBET) and pore volume are reduced from 1456 to 267 m2 gꢂ1
,
and 0.636 to 0.123 cm3 gꢂ1, respectively, after BSP is loaded into
the JUC-120 crystals (Table 1). These results are expected due to
the large amount of BSP encapsulated in the pores. Estimation
of the pore size distribution adopted by the nonlocal density
functional theory (NLDFT) method shows three maxima at 14.3,
ꢀ
20.3 and 26 A for JUC-120, attributed to small and large cages. It
is noteworthy that the created large pores would serve as good
hosts for the encapsulation of BSP molecules.
Photochromic properties of BSP/JUC-120 film
Inspired by the successful encapsulation of BSP molecules inside
the mesopores, this type of material is further employed to
investigate the optical properties. Based on the aforementioned
advantages of thin film construction, BSP/JUC-120 films were
prepared on quartz substrates by a spin-coating method. Two
optical images of the film are shown in Fig. 8a. One is the freshly
Table 1 Textural data of surface areas, pore size and pore volume
measured by N2 sorption
Pore volume
(cm3 gꢂ1
Pore diameter
ꢀ
(A)
SBET (m2 gꢂ1
)
)
JUC-120
BSP/JUC-120
1456
267
0.636
0.123
14–26
—
Fig. 6 TGA curves of the activated JUC-120 (black) and BSP/JUC-120
samples (gray).
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the visible region at 578 and 616 nm upon the irradiation of the
film (Fig. 9a). The photorelaxation from OMC (pink) to CSP
(colorless) proceeds slowly, as observed from the decrease of the
fluorescence intensity with irradiation time. Additionally, in
order to measure the kinetics for photo-induced decoloration of
the OMC species, the time-resolved emission is followed by the
peak intensity at 578 nm in the fluorescence spectra for 143 min,
yielding an exponential decoloration curve (Fig. 9b). The curve is
fitted using curve-fitting software. As can be seen, the first-order
exponential model fits accurately the kinetics of photobleaching
of the BSP/JUC-120 film (eqn (1)). The fitting parameters for the
spectroscopic data are displayed in Table 2. This slow decay rate
in the kinetics measurement may be due to strong host–guest
interactions between the BSP molecules and JUC-120 crystals.
The BSP molecules are confined in the cages of JUC-120 crystals
during crystallization assisted with microwave heating, avoiding
the formation of In–BSP precipitates (Fig. S6†). The pore
confinement would isolate the BSP molecules each other in the
different cages, which is indicated by the presence of 0.84 mole-
cules occupied in one pore (the calculation method is described in
the ESI†). This confinement effect would also increase the steric
hindrance of the ring-closing process from the OMC to CSP
form, and hence leads to lengthen the weighted-average half-life
of the excited species (bleaching by visible light) as observed in
Fig. 8 (a) Photographic images of the BSP/JUC-120 film with and
without external UV irradiation, (b) UV-Vis spectra of the BSP/JUC-120
film before (1) and after the UV irradiation at different times ((2): 2 min,
(3): 5 min, (4): 10 min, (5): 20 min).
this study (kvis ¼ 6.89 ꢁ 10ꢂ4
s
ꢂ1, t1/2 ¼ 1006 s, Table 2). This
half-life value gives an indication for strong lightfastness, far
longer than that in DMF solution (kvis ¼ 1.64 ꢁ 10ꢂ2 sꢂ1, t1/2
¼
¼
Control over the fading speed of photochromic materials is
interesting for applications requiring a particularly fast or slow
fading speed, such as in ophthalmic lenses, photochromic coat-
ings and security printing ink. Recently, BSPs have been shown
to exhibit fluorescence in the visible region for their excited/
energy-rich OMC form, not the CSP form. Therefore, the fluo-
rescence properties of the BSP/JUC-120 film are monitored by
supplementary fluorescence spectroscopy to study the kinetics of
the decoloration of the excited species in the fading process.
Thus, the decoloration process is exemplified by the consecutive
irradiation of the BSP/JUC-120 film from an external and strong
visible light source (500 W xenon lamp with an interference filter,
l > 420 nm). Subsequently, the fluorescence spectra are recorded
with an excitation wavelength of lex ¼ 500 nm. Fig. 9a shows the
evolving spectra with irradiation time. As shown in Fig. 9a, two
distinguished peaks at 578 and 616 nm appear instantly before
the excitation, which are assigned to the OMC species.24i The
fading process is followed by variations of the peak intensities in
42 s) or a polymer (poly-n-butylmethacrylate) matrix (kvis
1.15 ꢁ 10ꢂ2
s
ꢂ1, t1/2 ¼ 60 s).24j Furthermore, compared with
literature data on the half-lives of spiropyrans incorporated into
ordered porous materials, BSP molecules in JUC-120 crystals
exhibit longer half-lives (t1/2) than those of mesoporous materials
(Si-MCM-41, t1/2 ¼ 108–162 s), but a bit shorter than some
faujasite-type microporous materials (HY, t1/2 ¼ 2.3 ꢁ 103 s)
(Table S1†).24e
The resistant ability towards thermal relaxation is another
important factor for the long-term stability of photochromic
materials in order to realize the potential applications. The
fluorescence spectra of the BSP/JUC-120 film in the dark were
recorded at various times at room temperature (Fig. 10 inset).
Negligible differences are observed between the spectra. This
points to a good preservation of the BSP molecules in the OMC
form. In more detail, the variations of fluorescence intensities
(lem ¼ 578 nm) as a function of time are plotted in Fig. 10. No big
loss in the intensity is observed even when placed in the dark after
one week at room temperature. According to the calculation
using eqn (1), the fluorescence decay constant for the thermal
decoloration (kthermal) and half-life (t1/2) of the OMC form in the
JUC-120 film are estimated to be 3.78 ꢁ 10ꢂ9 sꢂ1 and 2.65 ꢁ 108
Table 2 Fitting parameters of the spectroscopic data for the photoin-
duced decoloration of the BSP/JUC-120 film
a
A0
kvis (sꢂ1
)
Ath
R2
0.0989
6.89 ꢁ 10ꢂ4
0.063
0.995
Fig. 9 (a) The fluorescence emission spectra of the BSP/JUC-120 film
over time upon the external visible light irradiation at room temperature;
a
The kinetic constant calculated from the fluorescence intensity decay
curve for photoinduced decoloration.
(b) the decay curve of the corresponding fluorescence intensity (lem
578 nm) versus time.
¼
25024 | J. Mater. Chem., 2012, 22, 25019–25026
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View Article Online
dark for 120 min, judging from the increasing intensities. The
intensity changes of the film within one full cycle reveal a
reversible configuration change of BSP molecules during the
coloration and decoloration processes. The time-resolved fluo-
rescence emission intensities upon the testing cycles by reversibly
switching from visible light irradiation to dark is shown in
Fig. 11b. No significant loss is observed in the intensities after ten
consecutive applications of the film. It is suggested that this film
possesses high reversibility, which provides a basis for the
fabrication of photochromic devices.
Conclusions
A new hybrid MOF (JUC-120) material, with indium as the
metal centers and trimesate as carboxylate ligands, has been
synthesized. The XRD, NMR and Raman characterizations
indicate that JUC-120 possesses a cubic zeolitic MTN topology
as a new analogue to MIL-100(Al, Fe or Cr). The results of
nitrogen sorption reveal that this MOF material possesses large
Fig. 10 Time dependence of the fluorescence intensity (lem ¼ 578 nm)
for the OMC-form of BSP/JUC-120 in the dark at room temperature.
The inset shows the fluorescence emission spectra of the BSP/JUC-120
film for various times (0 h, 3 d, and 7 d).
pore size (26 A) with high surface area (SBET ¼ 1456 m2 gꢂ1) and
s, respectively, which is comparable to the best results in previous
reports (Table S1†).24g This observation shows that the BSP
molecules entrapped in the cages of the JUC-120 crystals can
hardly undergo rotational movements, thus suppressing CSP
isomerization. The long retardation to the thermal-stable CSP
form validates the pore confinement effect, which is consistent
with the results of light-fading experiments. It can be concluded
that the thermal stability of the open merocyanine form of the
accommodated BSP molecules in the porous MOFs is improved
significantly, which provides further proof that JUC-120 MOF is
a good candidate as a host material. Based on the spectroscopy
studies above, there are two decisive parameters in controlling
the photochromic properties of the BSP/JUC-120 film, namely
the polar groups (In–OH and In–H2O) in the structure and pore
confinement.
ꢀ
high thermal stability (up to 370 ꢀC in air). Besides, the assembly
of photochromic compounds into JUC-120 nanocrystals is
exemplified by nitrobenzospiropyran derivative (BSP) using a
microwave-assisted crystallization inclusion approach. About
8–9 wt% BSP compounds are encapsulated into the JUC-120
crystals during the direct synthesis process, which is determined
by TGA analysis and an analytical measurement. Further, BSP
loaded JUC-120 films (BSP/JUC-120) are prepared by spin-
coating of their suspension on quartz wafers. The photochromic
properties of the as-prepared films are investigated by UV-Vis
spectroscopy. It is found that metastable open merocyanine
(OMC) species are entrapped in the BSP/JUC-120 film, which is
attributed to the large pore size and unique host–guest interac-
tions. Additionally, the photochromic properties of BSP mole-
cules in the BSP/JUC-120 film were studied in detail by
fluorescence spectroscopy. The long-term stability of the OMC
configuration rather than the CSP form against light/photo and
thermal relaxation is observed in the JUC-120 crystals due to the
pore confinement effect. Furthermore, the BSP/JUC-120 film
displays high reversibility of photochromism. The BSP/JUC-120
films are a novel type of host–guest material and are of potential
interest for advanced applications in photonics or optical
devices.
The reversibility of photochromism is another required index
in the design of photochromic materials for optical devices.24c
The reversibility of the photochromic properties of the BSP/
JUC-120 film is evaluated by a coloration–decoloration process.
Fig. 11a shows the evolution of the fluorescence intensities for
one coloration–decoloration cycle. It can be seen that photo-
irradiation results in a decrease of BSP molecules in the OMC
form irradiated by the visible light for 20 min, and thereafter, the
portion of BSP molecules in their open form is increased in the
Acknowledgements
We are grateful for the financial support of the National Basic
Research Program of China (973 Program, grant no.
2012CB821700), the Major International (Regional) Joint
Research Project of the NSFC (grant no. 21120102034) and the
NSFC (grant no. 20831002).
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