Two 3D Cd(II) Metal-Organic Frameworks Linked by Benzothiadiazole Dicarboxylates: Fantastic S@Cd6 Cage, Benzothiadiazole Antidimmer, and Dual Emission

Article abstract of DOI:10.1021/acs.inorgchem.6b02863

On the basis of the same benzothiadiazole (BTD) ligand 2,1,3-benzothiadiazole-4,7-dicarboxylic acid (H₂L), two new isomers of three-dimensional (3D) BTD-derived Cd(II) metal-organic frameworks 1-2 {[S@Cd₆L₆]·xH₂O}_n were obtained by the different solvothermal reactions, which were structurally similar. Surprisingly, structural analyses reveal that in 1 or 2, one free sulfur atom was fixed in a Cd(II) cluster cage by strong intermolecular interaction to form the secondary building unit (SBU) S@Cd₆. Each SBU S@Cd₆ is connected by six L^2- ligands and further extended into the 3D porous framework. In this work, the BTD antidimmer was evidenced by structural analysis and photophysical study. Furthermore, either 1 or 2 showed the uncommon dual emission, while only one emission was observed in the solution of ligand H₂L. The dual-emission mechanism was also realized by the structural analysis and photophysical study. Interestingly, although there is slight difference in structure (regular octahedral cage in 1 and slightly distorted octahedral cage in 2), the changes in N₂ adsorption capability and photophysical performance between 1 and 2 are obvious, where 2 shows smaller Brunauer-Emmett-Teller surface area, broader absorption of antidimmer, and longer dual-emission lifetimes. Interestingly, either 1 or 2, the dual emission was clearly red-shifted by increasing the solvent polarity or the acidity of ambience, respectively.

Full text of DOI:10.1021/acs.inorgchem.6b02863

Article
pubs.acs.org/IC
Two 3D Cd(II) Metal−Organic Frameworks Linked by
Benzothiadiazole Dicarboxylates: Fantastic S@Cd6 Cage,
Benzothiadiazole Antidimmer, and Dual Emission
Qing Cheng,†,# Xiao Han,†,# Yue Tong,† Chao Huang,‡ Jie Ding,*,† and Hongwei Hou*,†
†College of Chemistry and Molecular Engineering, Zhengzhou University, Henan 450001, P. R. China
‡Center for Advanced Materials Research, Zhongyuan University of Technology, Zhengzhou 450007, P. R. China
S
* Supporting Information
ABSTRACT: On the basis of the same benzothiadiazole (BTD)
ligand 2,1,3-benzothiadiazole-4,7-dicarboxylic acid (H2L), two
new isomers of three-dimensional (3D) BTD-derived Cd(II)
metal−organic frameworks 1-2 {[S@Cd6L6]·xH2O}n were
obtained by the different solvothermal reactions, which were
structurally similar. Surprisingly, structural analyses reveal that in
1 or 2, one free sulfur atom was fixed in a Cd(II) cluster cage by
strong intermolecular interaction to form the secondary building
unit (SBU) S@Cd6. Each SBU S@Cd6 is connected by six L2−
ligands and further extended into the 3D porous framework. In
this work, the BTD antidimmer was evidenced by structural
analysis and photophysical study. Furthermore, either 1 or 2
showed the uncommon dual emission, while only one emission
was observed in the solution of ligand H2L. The dual-emission
mechanism was also realized by the structural analysis and photophysical study. Interestingly, although there is slight difference in
structure (regular octahedral cage in 1 and slightly distorted octahedral cage in 2), the changes in N2 adsorption capability and
photophysical performance between 1 and 2 are obvious, where 2 shows smaller Brunauer−Emmett−Teller surface area, broader
absorption of antidimmer, and longer dual-emission lifetimes. Interestingly, either 1 or 2, the dual emission was clearly red-
shifted by increasing the solvent polarity or the acidity of ambience, respectively.
carrier or electron acceptor. Especially, the highly polarized
feature of BTD species usually supports it to afford well-
ordered crystal structure by intermolecular interactions (such as
heteroatom contacts and π−π interactions).16f When planar
BTD units are held in a cofacial geometry at a suitable distance
in crystalline state, it is appropriate to observe the photo-
physical behavior of uncommon dimer.20 Until now, despite the
intriguing properties, only Chen group reported few BTD-
derived Nd-MOFs for enhanced CO2 sorption.21 In this
relatively unexplored area, elaborate design and deep photo-
physical research will be beneficial for the fast development of
optical-functional BTD-derived MOF material.
In this paper, we selected the analogue of famous
“terephthalic acid”, 2,1,3-benzothiadiazole-4,7-dicarboxylic acid
(H2L), as the linker to construct new BTD-derived MOF.
Herein, two novel MOF 1 and 2 were successfully obtained
with the same BTD ligand. It is surprising that in the
structurally similar MOFs 1 and 2, one free sulfur atom was
fixed in a Cd(II) cluster cage to form the secondary building
unit (SBU) S@Cd6. Each SBU S@Cd6 is connected by six L2−
INTRODUCTION
■
Undoubtedly, metal−organic framework (MOF), an attrac-
tively versatile platform for achieving long-range organization
and order as well as exploitable property,1,2 has been attracting
the gazes of many chemists and material scientists during the
past decade.3−5 In particular, such optical-functional systems
were gradually used to explore the amazing applications,6,7 such
as light-emitting devices,8 light-harvester,9 nonlinear optics,10
photocatalysis,11,12 and sensor technology.13,14 Since then, we
are very interested in constructing new chromophore-derived
MOF and exploring the relationship between photophysical
properties and the structures to get the excellent optical-
functional materials. In our previous work of BODIPY-derived
MOF, the special coordination modes are capable of forming
the two BODIPY units in coplanar arrangements to display the
uncommon J-dimer absorption band.15
Herein, 2,1,3-benzothiadiazole (BTD), another famous π-
conjugated chromophore,16−19 was selected to construct a new
series of optical-functional MOFs. Actually, BTD derivative is
normally efficient chromophore with high fluorescence
quantum yield. Meanwhile, because of the strong electron-
withdrawing capacity of heterocyclic unit as well as relatively
high reduction potential, such derivative is a possible electron
Received: November 25, 2016
© XXXX American Chemical Society
A
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ligands and further extended into a three-dimensional (3D)
porous framework. In this work, the two adjacent BTD units
are held in an antidimmer mode with the transannular distance
from 3.47 to 3.49 Å (shorter than the value of theoretical
prediction) that was further evidenced by the corresponding
photophysical study. Furthermore, either 1 or 2 showed the
uncommon dual emission, while only one emission was
observed in the solution of ligand H2L. The dual-emission
mechanism was also realized by the structural analysis and
photophysical study. More interestingly, although there is slight
difference in structure, the photophysical performance change
between 1 and 2 is distinct, where broader antidimmer
absorption is observed in 2 as well as longer dual-emission
lifetimes.
Table 1. Crystallographic Data and Structure Refinement
Details for Complexes 1 and 2
complex
1
2
formula
fw
C48H30N12O33S7Cd6
2201.66
C48H24N12O30S7Cd6
2147.61
T/K
293
293
crystal system
space group
a (Å)
trigonal
trigonal
R3
̅
R3
̅
19.676(3)
19.676(3)
19.300(4)
90
19.377(3)
19.377(3)
19.810(4)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
90
90
120
120
6471(2)
3
6442(2)
3
EXPERIMENTAL SECTION
■
Z
Materials. All chemical reagents were purchased from commercial
suppliers without further purification. For 2,1,3-benzothiadiazole-4,7-
dicarboxylic acid (H2L), the synthetic procedure is according to the
literature (Scheme S1).22
Dcalcd (g·cm−3
)
1.695
1.661
abs coeff (mm−1
F (000)
)
1.701
1.704
3198.0
1.080
3108.0
1.116
Physical Measurements. 1H NMR (400 MHz) spectra were
measured on a Bruker Avance-400 nuclear magnetic resonance
(NMR) spectrometer. Thermal stability studies were performed on a
Netzsch STA 449C therma analyzer at a heating rate of 10 °C/min,
from 30 to 900 °C under the air atmosphere. Power X-ray diffraction
(PXRD) patterns were measured on a PANalytical X’Pert PRO
diffractometer with Cu Kα1 radiation. The results from the PXRD
patterns are in close agreement with those calculated from the single-
crystal structure determination. The gas sorption isotherms of
coordination polymers 1 and 2 were collected on a Micromeritics
3Flex surface area and pore size analyzer under ultrahigh vacuum in a
clean system, with a diaphragm and turbo pumping system. Ultrahigh-
purity (>99.999%) N2 gas was applied in all adsorption measurements.
The experimental temperature was maintained by liquid nitrogen (77
K). Prior to measurement, supercritical CO2 (SC−CO2) activation
was used to remove the solvent molecules in samples 1 and 2 by a
custom-built system. Typically, bulk sample of complex 1 was washed
by absolute ethanol (EtOH) three times, dried under vacuum, and
then transferred into stainless steel column. After 20 min of soaking
and venting of supercritical CO2 by a DB-80 simplex pump, the
column pressure regulator was set at 100 bar by soaking SC−CO2, and
the column temperature was raised to 40 °C. SC−CO2 in the column
was gradually vented after 14 h. The activation samples were placed in
sealed containers for nitrogen adsorption. Thermogravimetric analysis
(TGA) experiments and PXRD were performed to make sure that the
solvent molecules were removed completely and the framework
structures retained crystallinity after activation, respectively.
Single-crystal X-ray data for compounds 1 and 2 were collected on a
Rigaku Saturn 724 CCD diffractometer equipped with a graphite
monochromator and Mo Kα (λ = 0.710 73 Å, 293 1 K) radiation.
Data collections were performed using ϕ and ω scan. Non-hydrogen
atoms located from the difference Fourier maps were refined
anisotropically by full-matrix least-squares on F2, using SHELXS-
97.23 All hydrogen atoms were included in the calculated positions and
refined isotropically using a riding model. Determinations of the
crystal system, orientation matrix, and cell dimensions were performed
according to the established procedures. Lorentz polarization and
multiscan absorption correction were applied. All calculations were
performed using SHELXL 97 and PLATON 99. The numbers of guest
molecules for 1 and 2 were calculated by the TGA. The
crystallographic data for 1 and 2 are listed in Table 1. Moreover,
the selected bonds distances and bond angles are summarized in
Supporting Information Table S1.
GOF
data/restraints/parameters
3401/0/165
0.0366
3413/0/165
0.0401
a
R1(I > 2σ(I))
b
wR2(I > 2σ(I))
0.0906
0.1017
a
b
2
2
R1 = ∑∥F0| − |Fc∥/∑|F0|. wR2 = {∑[w(F0 − Fc )2]/
2
∑[w(F0 )2]}1/2
.
were conducted on a FL-4600 fluorescence spectro-photometer at
room temperature. The emission decay lifetime was measured on an
Edinburgh instrument FLS980 fluorescence spectrometer. In this
emission lifetime experiment, two excitation wavelengths were chosen.
Under the irradiation of 330 nm light, the tested emission lifetime
wavelength range is from 350 to 450 nm with 20−30 nm intervals
(those are 350, 370, 390, 410, 430, 450, 480, 500, 520, 540, 560, 580,
600, and 620 nm), while another emission lifetime wavelength range is
500−620 nm with 20 nm interval (those are 500, 520, 540, 560, 580,
600, and 620 nm) excited at 405 nm. Herein, the details of fitting
results were in the Supporting Information (Table S2).
Synthesis of {[S@Cd6L6]·9H2O}n (1). H2L (0.1148 g; 0.5 mmol) and
CdCl2 (0.2284 g, 1 mmol) were dissolved separately in dimethylfor-
mamide (DMF)/H2O (10:10, v/v) solution. Meanwhile, 10 mL of
adenine (0.0405 mol L−1) DMF solution was also prepared, and mixed
with the H2L and CdCl2, DMF/H2O (10:10, v/v) solutions. Each 0.5
mL of the above mixed solution was added to 3.5 mL of DMF/EtOH
(1.5/2, v/v) solution in a 10 mL glass tube. The reaction mixture was
heated at 80 °C for 3 d. After the cooling treatment, red-brown crystals
of 1 were washed several times with mother liquor and dried (yield:
72%, based on H2L ligand). Anal. Calcd for C48H30Cd6N12O33S7 (%):
C, 26.16; H, 1.36; N, 7.63. Found (%): C, 26.31; H, 1.07; N, 7.77. IR
data (KBr, ν/cm−1): 3440 (s), 1584 (s), 1497 (w), 1401 (s), 1313
(m), 1222 (w), 1112 (w), 1040 (w), 905 (w), 838 (m), 828 (m), 818
(m), 779 (m), 665 (w), 623(w), 573 (w), 479 (w).
Synthesis of {[S@Cd6L6]·6H2O}n (2). A mixture containing Cd-
(NO3)2·4H2O (0.0308 g, 0.1 mmol) and H2L (0.0224 g, 0.1 mmol)
was dissolved in 5 mL of DMF/EtOH/H2O (1:2:2, v/v/v). The
mixture was heated at 120 °C in a 20 mL Teflon-lined autoclave for 3
d. After the cooling treatment, reddish-brown crystals of 2 were
separated and washed with mother liquor (yield: 80%, based on H2L
ligand). Anal. Calcd for C48H24Cd6N12O30S7 (%): C, 26.82; H, 1.12;
N, 7.82. Found (%): C, 26.50; H, 1.55; N, 7.71. IR data (KBr, ν/
cm−1): 3316 (s), 1579 (s), 1542 (s), 1401 (s), 1386 (s), 1312 (s),
1221 (w), 1145 (w), 1031 (w), 871 (w), 834 (m), 779 (m), 624 (w),
570 (w), 476 (w).
Photophysical Measurements. The UV−vis absorption spectra
in the solid state were measured by a Cary 5000 spectro-photometer
equipped with a 110 nm diameter integrating sphere. Spectra in the
solution were measured by an equipped V-750 spectro-photometer.
The measurements of steady-state emission spectra in the solid state
RESULTS AND DISCUSSION
As we know, the ligand terephthalic acid is a super linker. In
this platform, many MOFs were constructed, including the
■
B
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famous pioneer MOF-5. However, because of the structural
character of terephthalic acid, the direct application in optical-
functional material was limited. In this work, we selected one
analogue of terephthalic acid, 2,1,3-benzothiadiazole-4,7-
dicarboxylic acid (H2L), as the linker to construct optical-
functional BTD-derived MOF. Herein, two new BTD-derived
MOFs 1 and 2 were successfully obtained with the same
chromophore linker H2L by the different solvothermal
reactions, respectively. In the DMF/EtOH/H2O (2.5:2:0.5,
v/v/v) media, complex 1 crystallizes in a hexagonal system with
ligands and further extended into the 3D porous framework
(Figure 1; the perspective view and topology of 1 was shown in
Figure S1). This pore is regular, the size of which is 13.1 × 13.1
× 13.1 Å3.
o
The dihedral angle is 46.8 between the plane of O4, C1,
and O1 and plane of O2, C5, and O3 (Figure 1). Significantly,
the two adjacent chromophore linkers (BTD unit) are held in a
cofacial geometry (in opposite direction, antimode) with the
transannular distance of 3.47 Å. This transannular distance is
shorter than the distance of antidimer predicted by theoretical
calculation (4.00 Å),20a but it is longer than that in anti-
[2.2](4,7)benzothiadiazolophane (average distance 2.94 Å).20b
Herein, we preliminarily inferred that there was BTD
antidimmer in the complex 1. As shown in Figure S2, the
peak positions of the experimental PXRD spectrum match well
with that of simulated one, indicating the excellent phase purity
of complex 1. In addition, application of the SQUEEZE routine
in PLATON indicates a void volume of 29% in the total cell
volume. These channels are probably occupied by solvents,
which are further confirmed by TGA. From the TGA
experiment (Figure S3), there are two successive weight losses
of 8.83% (calcd 8.81%) before 190 °C. It is ascribed to the
absence of solvents in the cavity and free S atom in the S@Cd6
cage, which was further supported by the result from 1H NMR
spectrum for complex 1 immersed in CD3CN for one week
(the DMF signal was resulted from the DMF molecules
adhered in the surface of crystal, Figure S4). Over 190 °C, the
framework of sample 1 was decomposed gradually, which was
also proved by the PXRD patterns for complex 1 heated under
130 and 180 °C, respectively (Figure S2). Thus, gas sorption
measurement for the as-synthesized sample 1 after SC−CO2
activation was conducted up to a relative pressure (p/p0) of 1.0
on the activated frameworks at STP. N2 adsorption isotherm
demonstrates that MOF 1 exhibited the Brunauer−Emmett−
Teller (BET) surface area of 404.34 m2/g (Figure S5).
Interestingly, MOF 1 showed the good chemical stability
(Figure S2) in intermediate acidic (pH = 3) or basic condition
(pH = 11).
non-centrosymmetric R3 space group. As shown in Figure S1,
̅
there are four separated Cd(II) ions, one deprotonation L2−
ligand in the asymmetric unit of 1, where each of Cd(II) ions
adapts an octahedral geometry. In this Cd-MOF, it is notable
that adenine is not an auxiliary ligand but possibly a template
agent. In addition, both of the carboxylic groups in the ligand
adopt μ2-η2-chelate/bridge coordination fashions. Surprisingly,
one free sulfur atom S2 was fixed in an octahedral Cd(II)
cluster cage, which was formed by six Cd(II) ions. In this
regularly octahedral cage, the distance of Cd−S2 was 2.88 Å,
which was longer than a typical Cd−S bond.24 As we know, the
precursor (BBr2) of ligand H2L was derived by o-phenylenedi-
amine.25 Hence, in the solvothermal condition, it was possible
that a few free sulfur atoms were produced by the dissociation
of the ligand, which further were trapped in Cd(II) cluster cage.
Furthermore, S@Cd6 cage is also the secondary building unit
(SBU; Figure 1). Each SBU S@Cd6 is connected by six L2−
It is interesting that without adenine in similar DMF/EtOH/
H2O (1:2:2, v/v/v) media, when the solvothermal reaction
temperature raised to 120 °C and CdCl2 was replaced by
Cd(NO3)2·4H2O, Cd-MOF 2 crystallized in the same
Figure 1. Schematic representation of the structure of complex 1.
Hydrogen atoms and solvent molecules were omitted for clarity.
hexagonal crystal system with non-centrosymmetric R3 space
̅
Figure 2. Schematic representation of the structures of 2, where hydrogen atoms and solvent molecules were omitted for clarity. (a) Cavity mode A.
(B) Cavity mode B.
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Figure 3. (a) UV−vis absorption spectra of H2L in the DMF/EtOH/H2O (2.5:2:0.5, v/v/v, red) media and DMF/EtOH/H2O (1:2:2, v/v/v, violet)
media at room temperature, and (b) UV−vis absorption spectra of 1 (solid line), 2 (dash line), and H2L (dot line) in solid state at room
temperature.
group. More surprisingly, there are many similarities in the
structures of 1 and 2. For instance, the asymmetric unit of 2
also contains four separated Cd(II) ions, one deprotonation
L2− ligand (Figure S1). As shown in Figure S6, one S2 atom
was also surrounded by six Cd(II) ions to form a cage core S@
Cd6, which topologically serves as an uncommon 6-connecting
node to further form a porous 3D framework. Moreover, the
parallelogram-shaped cage of 2 is of 13.0 × 13.0 × 13.0 Å3.
Notably, although the L2− ligands in 1 and 2 adopt the same
coordination mode (Figures S1 and S6), the Cd(II) ion in 2
possesses a slightly distorted octahedral fashion. Normally, the
S@Cd6 SBU in 2 is a distorted octahedral geometry, in which
the distance of Cd−S2 is in the range of 2.878−2.880 Å. As a
consequence, there were two cavity modes (cavity mode A and
cavity mode B, Figure 2). In cavity mode A, the transannular
distance of two adjacent BTD units is completely equal (3.47 Å,
Figure 2a) with complex 1. In contrast, in cavity mode B, the
dihedral angle between the plane of O4, C3, and O3 and plane
some interesting photophysical properties that are different
than those of monomeric H2L in solution. In this work, the
solution UV−vis absorption spectra (Figure 3a) of H2L and the
solid-state UV−vis absorption spectra (Figure 3b) of 1, 2, and
H2L were measured at room temperature.
As shown in Figure 3a, the ligand H2L actually showed very
similar behaviors in DMF/EtOH/H2O (2.5:2:0.5, v/v/v,
reaction solvent for the preparation of complex 1) and
DMF/EtOH/H2O (1:2:2, v/v/v, reaction solvent for the
preparation of complex 2), because of similar absorption
spectrum of ligand H2L in pure DMF, EtOH, or H2O media
(Figure S9a). There are several intense absorption bands before
370 nm and one intermediate absorption band around 400 nm.
In addition, the intensity of absorption band around 400 nm
can be fitted by the Lambert−Beer Law (in the absorbance
range of 0−1, Figure S9b), indicating that this lowest-energy
excited state is ascribed to the monomeric H2L species
polarized by highly polar solvent. In contrast, in power sample
H2L, one new absorption band in the lower energy level was
observed at 470 nm, while such three bands observed in
solution were visibly blue-shifted. The blue-shifted π→π*
transition of monomeric BTD group could be resulted from the
absence of the solvent polarization interaction. Because power
sample H2L was amorphous, there was no clear structure
information to reveal the feature of the new absorption band at
470 nm. Thus, although it is tempting to ascribe such band to
π−π stacking, we need to do more experiments with caution
before drawing any conclusions.
More interestingly, similar behaviors also were displayed in
both crystalline samples 1 and 2. For complexes 1 and 2, the
mechanism of shifted monomeric BTD absorption band was
complicated, which might mainly be influenced by the changed
electron-withdrawing character of carboxylate groups at the 4,7-
positions by being coordinated with transition metal ion15 as
well as the polarization interaction of remaining solvent in the
cavity. In addition, in comparison with powder sample H2L,
there was a prolonged tail of absorption around 470 nm in
either 1 or 2. Especially in case 2, such absorption band was
broadened to lose some feature of peak. As we know, the BTD
unit is commonly planar that distinctly enhances the odds of
π−π stacking in solid state. As exhibited in Figures 1 and 2,
only the coplanar BTD moieties were observed in complex 1 or
2. The transannular distance was approximately in the range of
strong π−π interaction, which was also shorter than the value of
theoretically predicted antidimmer. Hence, such absorption
o
of O1, C7, and O2 is 45.6 , which results in the transannular
distance of two adjacent BTD units being incompletely equal
(3.47 and 3.49 Å; Figure 2b). Hence, it is foreseeable that
different from that in complex 1, there were probably two kinds
of BTD antidimmer in the complex 2. In addition, the phase
purity was confirmed by the PXRD analyses (Figure S7).
Because of the similarities in the structure, the framework of
sample 2 was decomposed gradually in similar temperature,
which was proved by the TGA and variable-temperature PXRD
results (Figures S3 and S7). There is a void volume of 29% in
the total cell demonstrated by the SQUEEZE routine in
1
PLATON. Additionally, in H NMR spectrum for complex 2
immersed in CD3CN for one week (Figure S4), the water
signal with lower intensity compared to that in 1 also indicated
the presence of crystalline water molecules, while weak DMF
signal resulted from the DMF molecules adhered in the surface
of crystal. Surprisingly, because of the slight difference in
structure, the BET surface area of 102.23 m2/g (Figure S8) was
distinctly smaller than that of 1, which was calculated from N2
adsorption isotherm of 2. Moreover, MOF 2 also showed the
good chemical stability in intermediate acidic or basic condition
(Figure S7).
Photophysical Behavior Study. As described above, in
complex 1 or 2, the two adjacent chromophore linkers (BTD
unit) are held in a cofacial geometry (antimode), where the
transannular distance is shorter than the value of theoretically
predicted antidimmer.20a This structural character may induce
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Figure 4. (a) Emission and (b) excitation spectra of 1 in the solid state at room temperature (λex = 280 nm, red; 300 nm, orange; 320 nm, yellow;
340 nm, green; 360 nm, olive; 380 nm, blue; 400 nm, magenta; 420 nm, violet. λem = 390 nm, red; 420 nm, orange; 460 nm, yellow; 500 nm, green;
520 nm, olive; 540 nm, blue; 580 nm, magenta; 620 nm, violet).
band around 470 nm (Figure 3b) is clearly assigned by the
transition of antidimmer. In comparison with 1, because of two
kinds of BTD antidimmer in the complex 2, the broad
absorption band of antidimmer is actually including more than
one peak.
Commonly, more sensitive emission spectroscopic experi-
ment comparing to absorption test could demonstrate the
further detail for photophysical performance of sample (Figure
4, Table 2, Figures S13). In this work, H2L dilute solution
Surprisingly, when the ligand H2L was coordinated with
Cd(II) ion, either complex 1 or 2 showed the totally different
emission behavior (Figure 4 and Figure S13) in comparison
with that of H2L in solution. Unfortunately, excited at 470 nm,
neither 1 nor 2 was absent of antidimmer emission. As we
knew, large stocks shift is common in BTD-based species;
therefore, such emission was probably in near-infrared (NIR)
range. Moreover, it totally quenched in concentrated solid state.
As shown in Figure 4a, when the excitation light shifted to
longer wavelength, the emission peak was also red-shifted. This
phenomenon is uncommon, indicating that more than one
emissive excited state is present in the system. At the same
time, in the corresponding excitation spectroscopic experi-
ments, the emission at 500 nm is like a “watershed”. The
emission before 500 nm was mainly contributed by the
transition centered at 316 nm (ν = 31 650 cm−1), while the
emission after 500 nm was mainly contributed by the transition
centered at 372 nm (ν = 26 900 cm−1). In comparison with
H2L dilute solution, the energy gap shortens (∼4750 cm−1,
0.59 eV) shown in the excitation spectra of complex 1, and the
overlap is also visible. Hence, from these results, it is reasonable
that the decay way of transition centered at 316 nm was not
only IC but also irradiation decay. This conclusion needs to be
further confirmed by the emission lifetime study (Table 2 and
Table S2).
Table 2. Absorption and Steady-State Fluorescence
Properties of Crystals 1, 2, Power H2L, and H2L Solution at
Room Temperature
sample
λab, nm
λem, nm
τF, ns
b
a
a
1
2
236, 292−333 , 372, 468
418, >520
418, >540
c
2.3, 5.5
b
205, 284−328 , 372, 480
2.5, 7.8
d
b
H2L
209, 236, 312−356 , 470
c
c
c
e
b
H2L
260, 284−348 , 396
467, 486
474, 489
f
b
H2L
254, 284−348 , 394
a
Because of the bad signal resolution under in the irradiation of light
b
(>420 nm), the position of this emission peak was not clear. Broad
signal including multipeaks. Not measured in this paper. The
c
d
e
powder sample. In the DMF/EtOH/H2O (2.5:2:0.5, v/v/v) media.
f
In the DMF/EtOH/H2O (1:2:2, v/v/v) media.
Emission decay curves were measured in solid state at room
temperature with irradiation at 330 nm (30 300 cm−1) or 405
nm (24 700 cm−1) detection at different wavelengths across the
emission spectrum. Excited at 330 nm, the logarithmic plot
shows a typical single-exponential behavior (for 1: 2.3 ns; for 2:
2.5 ns). In contrast, when complex 1 under the irradiation of
405 nm light, double exponential fitting was the best choice,
with a fast component (τ1 ≈ 1.9 ns) and a slower component
(τ2 = 5.5 ns). The full analysis of the decay curves (see Table S2
in the Supporting Information) confirms that more than 60% of
the observed luminescence is due to the slow component at
lower energy level. Similarly, with complex 2 excited at 405 nm,
double exponential fitting was also appropriate with a fast
component (τ1 ≈ 2.1 ns) and a slower component (τ2 = 7.8
ns). In this case, more than 74% of the observed luminescence
is due to the slow component at lower energy level.
showed the emission with one shoulder peak excited in the
range of 260−440 nm, where the valley of intensity was
observed in the irradiation of 320 nm. The peaks of emission
intensity were observed in the irradiation of 280 and 396 nm,
respectively. The corresponding excitation spectra also
indicated that the light in the range of 310−330 nm was
almost no contribution for the sample emission. Furthermore,
the light at 282 nm (ν = 35 450 cm−1) or 390 nm (ν = 25 650
cm−1) was the most powerful supporter for the emission. These
results proved that (a) there was only one emission in the H2L
dilute solution, which was ascribed to the lowest-energy π→π*
transition of monomeric BTD group (ν = 25 650 cm−1); and
(b) because of the relatively big energy gap (∼9800 cm−1, 1.22
eV), the main decay of transition around 35 400 cm−1 was not
irradiation decay but internal conversion (IC) that transferred
the energy to the lowest-energy transition of monomeric BTD
group.
At last, from the structural and spectroscopic information, we
can sketch a model for the understanding this dual-emission
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Figure 5. (a) Emission spectra of 1 in solid state (black bolded) and ground 1 dispersed in different organic solvents (red: in petroleum ether; olive:
in ethanol; and violet: in methanol); (b) emission spectra of 1 in solid state (black bolded) and ground 1 dispersed in different aqueous media at
room temperature (red: in ambience of pH = 11, sodium hydroxide aqueous solution; magenta: in water; olive: in ambience of pH = 4, nitric acid
aqueous solution; and violet: in ambience of pH = 3, nitric acid aqueous solution). λex = 370 nm.
mechanism. Different from the ordinary emission mechanism
model in H2L dilute solution (Figure S14a), complex 1 or 2
after the excitation process showed that the decay of excited
state at higher-energy level (centered at 316 nm, ν = 31 650
cm−1) could be accomplished by IC to lower-energy excited
state of monomeric BTD group or lowest-energy excited state
of BTD antidimmer, while the irradiation decay process
(fluorescence) was also allowed. For the decay of excited
state at lower-energy level (centered at 372 nm, ν = 26 900
cm−1) in complex 1 or 2, there were still two main processes,
namely, IC to lowest-energy excited state of BTD antidimmer
and irradiation decay. In contrast, the lowest-energy excited
state of BTD antidimmer was decayed by a non-irradiation way.
In comparison with 1, the longer dual-emission lifetimes
observed in 2 probably resulted from the enhanced IC process
to suppress non-irradiation decay, since there were two kinds of
BTD antidimmer in the complex 2. More importantly, even
after the SC−CO2 activation procedure, both 1-desolvated and
2-desolvated still displayed the similar dual-emission feature as
well as the similar absorption bands (Figures S15 and S16), in
comparison with either 1 or 2. It is possibly ascribed to only
crystalline water in the cavity of sample MOF before solvent-
removal procedure.
As we know, chromophore molecule commonly shows the
interesting solvatochromic behavior in solution, which is
originated from the feature easily polarized by the ambient
solvent molecules. As follows, the corresponding emission also
is influenced by the ambient solvent molecules. Herein, because
comparing to the relatively free solution environment, the
arrangement of chromophore species was restricted by the
center metal ion in porous MOF; the solvatochromic behavior
research may reveal more information from the connection
between photophysical property and the structure. In this work,
a series of organic solvents (petroleum ether, toluene, ethanol,
ethyl acetate, dioxane, acetone, methanol, and ethylene glycol)
and different aqueous media (pH = 3, pH = 4, pure water, and
pH = 11) were chosen for the solvatochromic behavior research
(Figure 5 and Figures S17−S22). In comparison with the
similar solvatochromic behavior of 2, that of MOF 1 was more
pronounced; thus, the detail was described below. Although
there was a slight shift in the lowest-energy absorption band,
the change of emission behavior in MOF 1 was distinctly
visible. As shown in Figure 5a, under the irradiation of 370 nm
light, pure MOF 1 showed a dual-emission performance (λem =
423, 439 nm), while this dual emission was clearly red-shifted
by increasing the solvent polarity. More interestingly, the large
red-shift of emissive excited state with lower energy (λem = 439
nm) was observed compared to that of emissive excited state
with higher energy (λem = 423 nm). Such change should be
attributed to the emissive excited states with higher energy (λem
= 439 nm) that displayed some charge transfer (CT) character
after the coordination between ligand H2L and center metal
Cd(II) ion. Moreover, this red-shift phenomenon was much
more pronounced in aqueous media, from blue region to green
region, which was observed by increasing the acidity of
ambiance (Figure 5b). In addition, after 8 h immersed in
acidic or basic media, this phenomenon still was the same as the
initial state of MOF 1 (Figure S21), which was consistent with
the result from the PXRD experiment (Figure S2). Before
considering any hypothesis of the solvatochromic mechanism in
aqueous media, the control experiment of ligand H2L was
accomplished in same condition. As shown in Figure S23a,
there was a slight shift in the lowest-energy absorption band in
different aqueous solution of ligand H2L. The red-shifted
emission in ligand H2L was distinctly observed by increasing
the acidity of ambiance (Figure S23b), whereas the change of
excitation spectra was slightly (Figure S23c) that was consistent
to the absorption spectra. As we know, the BTD species is a
typical electron acceptor. When pH-sensitive carboxyl group
and N atoms in BTD species were under the acidic condition,
the electron-donating capability of carboxyl group and the
electron-withdrawing capability of BTD species were amplified,
respectively. It resulted in an enhancing CT character of lowest-
energy excited state in ligand H2L under the acidic condition
with small energy gap comparing to that in pure water system.
In contrast, under the basic condition, the deprotonated
carboxyl group was a weak electron acceptor, while the
electron-withdrawing capability was weakened in BTD species.
This result indicated a large energy gap of lowest-energy excited
state in ligand H2L under the basic condition with negligible
CT character compared to that in pure water system. Similarly,
we surmised that the electron distortion of noncoordinated N
atoms in 1 under the acidic or basic condition was probably an
important factor, which might trigger the whole electron
arrangement change of the coordinated ligand L2−. As follows,
the similar red-shift phenomenon was observed in MOF 1.
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CONCLUSION
■
Herein, two new BTD-derived MOFs 1 and 2 were successfully
obtained, which were structurally similar. It is interesting that
one free sulfur atom was fixed in a Cd(II) cluster cage to form
the SBU S@Cd6, which further extended into a 3D porous
framework. MOF 1 showed a larger BET surface area (404.34
m2/g) than that of 2 (102.23 m2/g). More interestingly,
uncommon BTD antidimmer was evidenced by the corre-
sponding structural and photophysical experiment. Further-
more, because of the structural character, either 1 or 2 showed
the special dual emission, the mechanism of which was realized
by the structural analyses and photophysical study. Researching
the solvatochromic behavior of 1 and 2 revealed that the dual
emission was clearly red-shifted by increasing the solvent
polarity or the acidity of ambiance, respectively. These results
indicate that chromophore-derived MOF can be a powerful
platform to deeply understand the relationship between
photophysical properties and the structures, which is extremely
beneficial to design and construct the excellent optical-
functional materials.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02863.
■
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Selected bond lengths and angles, PXRD, TGA, and
additional figures. CCDC1504823−1504824 crystal data
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X-ray crystallographic information (CIF)
X-ray crystallographic information (CIF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: jieding@zzu.edu.cn. (J.D.)
*E-mail: houhongw@zzu.edu.cn. (H.H.)
ORCID
■
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Hongwei Hou: 0000-0003-4762-0920
Author Contributions
#Q.C. and X.H. contributed equally to the work and should be
considered cofirst authors.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation (Nos. 21371155, 21301157, and
21671174).
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