A New Biscarbazole-Based Metal–Organic Framework for Efficient Host–Guest Energy Transfer

Article abstract of DOI:10.1002/chem.201805636

A new metal–organic framework (MOF), [Zn₆L₄(Me₂NH₂⁺)₄?3 H₂O] (1) was constructed based on [9, 9′-biscarbazole]-3, 3′, 6, 6′-tetracarboxylic acid (H₄L) and Zn²⁺ ions. The porous framework and intense blue fluorescence of the MOF based on the biscarbazole moiety of the ligand could facilitate efficient host to guest energy transfer, which makes it an ideal platform for the tuning of luminescence.

Full text of DOI:10.1002/chem.201805636

DOI: 10.1002/chem.201805636
Communication
&
Host–Guest Systems |Hot Paper|
A New Biscarbazole-Based Metal–Organic Framework for Efficient
Host–Guest Energy Transfer
[
a]
[a]
[a]
[a]
[b]
Qianqian Mu, Jingjuan Liu, Weiben Chen, Xiaoyu Song, Xiaoting Liu,
[a]
[b]
[a]
Xiaotao Zhang,* Ze Chang,* and Long Chen*
In consideration of the mechanism of energy transfer, there
are several principles for the construction of energy transfer
systems with MOFs. Firstly, to simplify the engineering of
ligand-based emission and avoid the emission quenching, tran-
sition-metal ions without unpaired electrons is preferred, espe-
Abstract:
A
new metal–organic framework (MOF),
+
[
Zn L (Me NH ) ·3H O] (1) was constructed based on [9,
6 4 2 2 4 2
9
’-biscarbazole]-3, 3’, 6, 6’-tetracarboxylic acid (H L) and
4
2
+
Zn ions. The porous framework and intense blue fluo-
rescence of the MOF based on the biscarbazole moiety of
the ligand could facilitate efficient host to guest energy
transfer, which makes it an ideal platform for the tuning
of luminescence.
10
[8]
cially those having d configurations. Secondly, because the
energy transfer process requires a short distance between the
donor and acceptor, a highly porous framework with proper
pore structure is preferred as the host so that guest molecules
could contact with the host framework more effectively. Fur-
thermore, the porous framework could also promote the load-
ing and dispersion of the guest, which could facilitate the
energy transfer between the host and guest. For the construc-
tion of porous MOF as host, one of the important factors that
should be considered is the structure of organic ligand. In the
perspective of topology, the connection and configuration of
the ligand could determine the structure of the MOF. It has
been proved that multidentate carboxylate ligands could pro-
mote the formation of highly porous MOFs. For example, sev-
Metal–organic frameworks (MOFs) are porous crystalline mate-
rials constructed by metal ions or metal clusters and organic li-
[
1]
gands through coordination bonds. By varying the metal
ions, organic ligands, and/or synthetic conditions, numerous
MOFs with different structures and properties can be obtained.
Because of their attractive properties, including large surface
areas, tunable porosity, and variable structure, MOFs were
[2]
widely used in the realms of gas storage and separation, cat-
[
3]
[4]
[5]
alysis, sensing, drug delivery and bio-imaging, optoelec-
eral tetra-carboxylate ligands with planar backbones have
[
6]
[7]
[11,12]
tronics, energy storage and conversion, and so on. In addi-
been well studied, such as tetraphenylethylene,
porphy-
[
8]
[13]
[14]
[15]
tion to these applications, luminescent MOFs have been de-
veloped as a unique platform for highly efficient energy trans-
fer utilizing their unique pore structure and photophysical
properties. For example, Hupp and co-workers reported a
quantum dots@porphyrin-based MOFs composites for the en-
hancement of light harvesting via energy transfer from the
rin, tetraphenylbenzene, pyrene, etc. In contrast, ligands
[16]
with steric backbones such as 9,9’-spirobifluorene and tetra-
[17]
phenylmethane were rarely reported. Among the various po-
tential backbones, N, N’-biscarbazole has caught our attention
for its steric geometry, which has rarely been utilized for the
construction of MOFs. On the other hand, the carbazole has
been widely used in the construction of organic luminophores
owing to its unique emission properties, which is also desired
for the construction of energy transfer platform.
[
9]
quantum dots to the MOFs. Du et al. have reported a dual-
emitting dye@MOF composite showing apparent energy trans-
[
10]
fer process from MOF to dye. Our group recently reported a
tetraphenylethylene-based luminescent MOF with excellent
light-harvesting properties with a host to guest energy transfer
In this work, we designed and synthesized a multidentate
ligand, [9, 9’-biscarbazole]-3, 3’, 6, 6’-tetracarboxylic acid (H L),
4
[
11]
efficiency up to 96%.
for the construction of MOF as a platform for energy transfer.
II
Based on this ligand, a three dimensional Zn porous MOF was
+
synthesized, namely [Zn L (Me NH ) ·3H O] (1). This MOF ex-
[
a] Q. Mu, J. Liu, W. Chen, X. Song, Prof. X. Zhang, Prof. L. Chen
Department of Chemistry, Tianjin Key Laboratory of Molecular Optoelec-
tronic Science and Institute of Molecular Plus, Tianjin University, Tianjin,
6
4
2
2
4
2
hibits a ligand-originated broad emission around 460 nm and
a relatively high porosity, indicating its potential as an ideal
host for the construction of energy transfer system. On the
basis of these characteristics, we further investigated the
energy transfer behaviors of the MOF by encapsulating differ-
ent dye molecules into 1 (e.g., coumarin 6 or rhodamine 6G,
Scheme S3). The results show that highly efficient energy trans-
fer could occur between the MOF as host and the dyes as
guest, which prove the current MOF to be an ideal platform
for energy transfer.
3
00072 (P. R. China)
E-mail: zhangxt@tju.edu.cn
long.chen@tju.edu.cn
[
b] X. Liu, Prof. Z. Chang
School of Materials Science and Engineering, National Institute for Ad-
vanced Materials, TKL of Metal and Molecular-Based Material Chemistry,
Nankai University, Tianjin 300350 (P. R. China)
E-mail: changze@nankai.edu.cn
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/chem.201805636.
Chem. Eur. J. 2019, 25, 1 – 6
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Colorless block crystals of 1 could be obtained by solvother-
mal reaction of Zn(NO ) ·6H O and H L in the mixture of N,N-
to ca. 3208C. The N sorption isotherm for 1 recorded at 77 K
exhibits a characteristic type-I profile, which reveals the micro-
2
3
2
2
4
dimethylformamide (DMF), 1,4-dioxane, and H O (V:V:V=2:1:1)
porous nature of 1 (Figure S7). The BET surface area of the
2
2
À1
at 908C for four days. Single-crystal X-ray diffraction analysis re-
vealed that MOF 1 crystallizes in monoclinic space group C2/c
sample was determined to be 789 m g , and the pore size
distribution analysis of 1 indicate that the accessible pore di-
mension is about 9 ꢂ (Figure S8). All these results suggest that
1 could serve as a host for the loading of guest molecules.
In consideration of the carbazole backbone that commonly
exhibits intense deep-blue emissions, 1 is expected to show
similar emissions that could facilitate its application as donor
host for energy transfer. Therefore, the luminescence of the
(
Table S1, Supporting Information). The asymmetric unit of the
II
4À
+
MOF contains 1.5Zn ions, one L ligand and one Me NH2
2
counter ion (Figure 1). Other free solvent molecules were diffi-
H L ligand and 1 were investigated in solid state at room tem-
4
perature. Both of them exhibits intense blue luminescence
upon the excitation of UV light. As shown in Figure S10, H4L
ligand exhibits a broad emission band around 440 nm when
excited at 380 nm. Similarly, MOF 1 displays a broad emission
band at 460 nm, which could be assigned to the emission
from the ligand considering the similar wavelength and band
shape. The slight redshift (20 nm) might be due to the change
of the configuration of the ligand upon coordination to the
[19]
metal ions. The intense and broad emission of 1 suggest its
potential as donor for energy transfer, since highly efficient
energy transfer could be achieved with various acceptors with
different excitations.
To further investigate the potential of 1 as platform for
energy transfer applications, energy transfer system was fabri-
cated with 1 as host and fluorescent dye as guest. As one of
the typical red emissive dyes, Rh6G was selected in consider-
ation of its proper dimension and excitation. As shown in Fig-
ure 3a (see below), the distinctive color change upon in situ
incorporation of Rh6G indicates the successful loading of Rh6G
into 1, which is named as Rh6G@1. The PXRD patterns of
Rh6G@1 composites matched well with that of 1, which indi-
cates that the framework structure of 1 is well retained after
introduction of dye molecules (Figure S11). Furthermore, there
was no leakage of Rh6G guest from the Rh6G@1 after the
sample soaked in DMF for 24 h, which was confirmed by the
UV/Vis spectra of the supernatant (Figure S12). This result sug-
gest that the dye molecules could be well encapsulated in the
host framework of 1, which should be ascribed to the well
matched pore size of 1 (ca. 9 ꢂ) to the molecular size of the
Figure 1. The structure of 1: (a) asymmetric unit; (b) coordination mode of
the L ligand; (c) [Zn (CO ) ] trinuclear clusters; (d) the 3D framework struc-
3 2 8
ture of 1.
4
À
cult to be modulated because of their highly disordered state.
4
À
The L ligand reveals m -(h -h -m )-(h -h -m )-(h -h -m )-(h -h -m )
7
1
1
1
1
2
2
1
1
2
1
1
2
II
coordination mode to bridge seven Zn ions. Zn1 reveals a dis-
torted octahedral coordination geometry defined by six car-
boxyl oxygen atoms from four ligands. For Zn2, it is surround-
4À
ed by six oxygen atoms from five L ligands to give another
distorted octahedral coordination geometry. Zn1 and Zn2
metal centrals were bridged through two bidentate and one
4À
tridentate bridging carboxylate groups from three L ligands,
II
and the neighboring three Zn atoms interconnect with each
other to form a [Zn (CO ) ] trinuclear cluster as secondary
3
2 8
3
[10]
building unit (SBU).
Each [Zn (CO ) ] cluster connects with eight L ligands, and
Rh6G (10.89ꢁ15.72ꢁ15.79 ꢂ ).
As shown in Figure 2, the emission spectra of Rh6G in DMF
4À
3
2 8
4
À
À5
À1
each L ligand links to four [Zn (CO ) ] clusters. The connec-
(2ꢁ10 molL ) exhibits a maximal peak at 567 nm with exci-
tation at 513 nm. The overlap between the emission of 1 and
the excitation spectra of Rh6G suggests the potential of effi-
cient energy transfer from 1 to Rh6G. Solid-state fluorescence
spectra of Rh6G@1 further confirmed the energy transfer pro-
cess. As displayed in Figure 3b, the fluorescence spectra of
Rh6G@1 presents two emission bands at 450 and 565 nm
when irradiated at 380 nm, which are originated from the MOF
host and guest Rh6G molecules, respectively. With the increase
loading of Rh6G, the emission intensity of 1 gradually de-
creased, whereas the emission of Rh6G increased. Meanwhile,
the emission color of the corresponding samples changed
from blue to orange (Figure 3a). These results suggest the oc-
currence of energy transfer from 1 to Rh6G guests. Meanwhile,
3
2 8
tion between the ligands and the clusters generate a three-di-
mensional (3D) porous framework with square 1D channels
2
along the a-axis (~13.6ꢁ15.7 ꢂ ), which are occupied by free
+
Me NH2 counter ions and free solvent molecules. After re-
2
moval of solvent molecules, the accessible volume of 1 is 49%
3
3
(
1267.7 ꢂ of the 3720 ꢂ unit cell volume), as estimated using
[
18]
the SQUEEZE routine in PLATON.
The phase purity of 1 was verified by powder X-ray diffrac-
tion (PXRD) measurements. As show in Figure S5, the PXRD
pattern of the as-synthesized 1 matches well with the simulat-
ed patterns based on the crystal structure, indicating the high
phase purity of bulk sample. The thermogravimetric analysis
(TGA) results reveal that the framework is thermally stable up
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of Rh6G@1 were determined. As shown in Figure 3c, the F of
fl
1
decreased from 11.9 to 0.1%, whereas the F of Rh6G in-
fl
creased from 2.6 to 30.1% with increasing Rh6G loading. The
results proved that the light energy transferred from MOF host
to the guest dye molecules, and the energy transfer efficiency
(
(
h ) estimated from the quenching rate of MOF emission
ET
[21]
h =1ÀF (c)/F (0), where c is Rh6G concentration) reach-
ET
fl
fl
es 98.8% (Table S4). Thus, our example represents one of the
most efficient host–guest energy transfer in light-harvesting
MOF systems (Table S9). Furthermore, we measured the life-
time of the emission of host framework to evaluate the rate
constant of the energy transfer. The average lifetime of 1 (t0)
was measured to be 7.63 ns (Figure 4, black curve). For the
Figure 2. Fluorescence excitation (dotted) and emission (solid) spectra of 1
dotted blue curve monitored at 465 nm, solid blue curve upon excitation at
80 nm) and Rh6G (dotted red curve monitored at 567 nm, solid red curve
(
3
upon excitation at 500 nm, in DMF solution).
Figure 4. Fluorescence decay profiles of pristine 1 (black dots) monitored at
460 nm and the host in Rh6G@1 samples (a, red dots monitored at 455 nm
and e, blue dots monitored at 430 nm), the solid lines represent the best-fit
using a global multiexponential function.
Figure 3. (a) Fluorescence photographs of solid samples of 1 and Rh6G@1
with different loadings of Rh6G upon excited at 365 nm; (b) The fluores-
cence emission spectra for Rh6G@1 upon excitation at 380 nm
Rh6G@1, the average lifetime of MOF host (t ) changed from
DA
(a=0.03 wt%, b=0.05 wt%, c=0.09 wt%, d=0.14 wt%, e=0.17 wt%);
c) the absolute fluorescence quantum yield (Ffl) of 1 and Rh6G under exci-
6.18 to 0.16 ns with increasing Rh6G content (Figure 4, red and
(
tation at 380 nm.
blue curve). Accordingly, the rate constant of energy transfer
À1
DA
À1 [22]
(
k =t
Àt0
)
9
from MOF 1 to Rh6G increased from 0.03ꢁ
ET
9
À1
10 to 6.12ꢁ10 s (Table S5). Both the high hET and kET values
we compared the emission spectra of Rh6G@1-e and that of
thoroughly ground mixture of Rh6G and 1 (Rh6G=0.17 wt%)
measured under the same conditions. The emission spectra of
the mechanically ground mixture only exhibits a broad emis-
sion at about 460 nm, which is originated from 1 (Figure S13).
These results clearly indicate the absence of energy transfer in
the mechanical mixed sample, since the particles of MOF and
dye could not contact well to meet the distance required for
energy transfer with this method. Furthermore, the absence of
emission of dye in this sample should be due to the aggrega-
tion induced quenching in solid state, which accord well with
the weak emission of Rh6G in solid state (Figure S14). Then the
enhanced emission of Rh6G in the MOF may benefit from the
highly dispersion of dye molecules in the host framework of 1
suggest the high efficiency of the energy transfer process. This
excellent energy transfer performance should be ascribed to
the highly porous framework of 1. Because the dye molecules
could be encapsulated well in the framework of 1, a close con-
tact could be expected between the host and the guests. In
consideration of the FRET mechanism of energy transfer, the
short distance between the donor and the acceptor would
benefits the energy transfer process.
Furthermore, owing to the broad emission band and consid-
erable porous framework of 1, we expected that dye molecules
with distinct emissions could be introduced aiming at the tar-
geted construction of luminophores. As a representative cou-
marin derivative, coumarin 6 (C6) is a typical green emissive
fluorescent dye. Since the emission spectra of 1 and the ad-
sorption spectra of C6 are overlapped well, the C6@1 could be
expected to be an ideal green luminophore on the basis of the
effective energy transfer in this system (Figure S15b). C6@1
[
20]
through the avoid of aggregation-caused quenching.
Furthermore, for the quantitative proof of the host to dye
energy transfer process, the fluorescence quantum yield (Ffl)
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was obtained through a similar in situ synthesis procedure
with that of Rh6G@1. The emission color of sample changed
from blue to green after encapsulation of C6 molecules (Fig-
ure S15a). This phenomenon initially indicates that the dye
molecules have been successfully loaded into 1. As shown in
Figure S15c, the fluorescence spectra of C6@1 also exhibits
two emission band at about 440 and 500 nm, which are origi-
nated from 1 and C6, respectively. The emission of MOF host
receded while the green emission of C6 enhanced with the in-
crease of the loading content of C6, similar to the result of
Rh6G@1, indicating the occurrence of energy transfer between
the host and the C6 guests. Similarly, we measured the quan-
tum yields of C6@1 with different dye loading (a–e) (Fig-
Conflict of interest
The authors declare no conflict of interest.
Keywords: energy transfer
systems · in situ encapsulation · metal–organic frameworks
·
fluorescence
·
host–guest
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Manuscript received: November 12, 2018
Revised manuscript received: December 10, 2018
Version of record online: && &&, 0000
2009, 121, 4102–4106.
Chem. Eur. J. 2019, 25, 1 – 6
www.chemeurj.org
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ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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&
These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
&
Host–Guest Systems
Q. Mu, J. Liu, W. Chen, X. Song, X. Liu,
X. Zhang,* Z. Chang,* L. Chen*
&& – &&
A New Biscarbazole-Based Metal–
Organic Framework for Efficient Host–
Guest Energy Transfer
When the MOFs go blue: A metal or-
ganic framework (MOF) was constructed
based on a novel biscarbazole tetracar-
boxylic acid ligand. This blue fluorescent
MOF serves as an ideal platform for
host–guest energy transfer.
&
&
Chem. Eur. J. 2019, 25, 1 – 6
www.chemeurj.org
6
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!