Synthesis of: O-carborane-functionalized metal-organic frameworks through ligand exchanges for aggregation-induced emission in the solid state

Article abstract of DOI:10.1039/c9cc06386b

The carborane (CB)-functionalized ligand was installed in a variety of MOFs through postsynthetic ligand exchange processes. This methodology is a general method for preparing o-CB-functionalized MOFs with known frameworks. Furthermore, the photoluminescence (PL) spectra revealed intriguing aggregation-induced emission (AIE) features following the systematic incorporation of o-CB functionalities into framework-type materials.

Full text of DOI:10.1039/c9cc06386b

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Synthesis of o-Carborane-Functionalized Metal-Organic
Frameworks through Ligand Exchanges for Aggregation-Induced
Emission in the Solid State
Received 00th January 20xx,
Accepted 00th January 20xx
Sangdon Choi,a‡ Ha-Eun Lee,a‡ Chan Hee Ryu,b‡ Jooyeon Lee,a Jihyun Lee,c Minyoung Yoon,d
Youngjo Kim,a Myung Hwan Park,*e Kang Mun Lee,*b and Min Kim*a
DOI: 10.1039/x0xx00000x
The carborane (CB)-functionalized ligand was installed in a variety various applications, such as molecular storage, separation, and
of MOFs through postsynthetic ligand exchange processes. This catalysis.15 The ligand tunability and/or installation of functional
methodology is a general method for preparing o-CB-functionalized groups on MOFs are directly related to the performance of
MOFs
with
known
frameworks.
Furthermore,
the MOFs in the target applications. Since both MOFs and o-CB are
photoluminescence (PL) spectra revealed intriguing aggregation- tunable inorganic-based materials, the combination of these
induced emission (AIE) features following the systematic two materials is receiving considerable attention from the
incorporation of o-CB functionalities into framework-type inorganic and materials chemistry fields. The o-CB unit is
materials.
generally considered a benzene substituent in MOF ligands,
serving as a strut in the framework; however, each ligand is
designed for a specific MOF, and the tunability of the MOFs/o-
CB is very limited in this system (Scheme 1).16–19 In addition,
studies on carborane-MOFs have only focused on their
hydrogen storage properties, stability and structural features,
while their unique photophysical properties have not yet been
investigated.
Icosahedral carboranes are inorganic cage molecules that
consist of two carbon atoms and ten boron atoms and are
utilized in the organometallic and materials fields because of
their hydrophobicity and chemical/physical stability. In
particular, o-carborane (o-CB) cages are considered a benzene
substituent
because
they
have
three-dimensional
aromaticity.1,2 In addition, various organometallic species have
been shown to have unique photophysical properties, and as a
result, they have been extensively studied for use in energy
transfer materials.3–11 Recently, the incorporation of carborane
cages into solid-state materials, such as in polymers and porous
metal-organic frameworks (MOFs) along with optoelectronics,
gas separation devices and hydrophobicity sensors, has been
reported.12–14
MOFs are porous polymeric materials that consist of metal
clusters and multitopic organic ligands. Their controllable
porosity, relatively low density, and broad structural diversity as
well as the chemical tunability of their ligands make these
organic/inorganic hybrid materials of great importance for
a. Department of Chemistry, Chungbuk National University, Cheongju, 28644, Korea.
E-mail: minkim@chungbuk.ac.kr
b. Department of Chemistry, Institute of Molecular Science and Fusion Technology,
Kangwon National University, Chuncheon 24341, Korea. E-mail:
kangmunlee@kangwon.ac.kr
c. Department of Nanochemistry, Gachon University, Sungnam 13120, Korea.
d. Department of Chemistry, Kyungpook National University, Daegu 41566, Korea.
e. Department of Chemistry Education, Chungbuk National University, Cheongju,
28644, Korea. E-mail: mhpark98@chungbuk.ac.kr
† Electronic Supplementary Information (ESI) available: experimental detail and
spectroscopic data for 1H and 13C NMR PDF). See DOI: 10.1039/x0xx00000x
‡
S. Choi, H.-E. Lee, C. H. Ryu are equally contributed to this work.
Scheme 1 Method of installing carborane into metal-organic frameworks.
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Herein, we demonstrate a postsynthetic ligand exchange (PSE) due to its low stability in water. A smaller exchange ratio was
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process for the installation of o-CB functionalities into MOFs achieved relative to that of UiO-66; thus,DthOeI:e10x.c1h03a9n/gCe9CtiCm0e63a8n6dB
and the photoluminescence (PL) of o-CB-functionalized MOFs. temperature were increased to 1 day and 40 °C. The retention
For this protocol, o-CB-functionalized benzene-1,4-dicarboxylic of the bulk crystallinity of IRMOF was confirmed by PXRD, and a
acid (BDC-CB) was prepared as the main ligand, and two total of 4% BDC-CB was incorporated into the IRMOF structures,
representative BDC-based MOFs, IRMOF (IsoReticular MOFs) as evidenced by the IR and 1H NMR spectra of IRMOF after acid
and UiO-66 (UiO = University of Oslo), were investigated digestion (Fig. 1, S3 and S4†). Due to its chemical stability, UiO-
(Scheme 1).20,21 Both IRMOF and UiO-66 were synthesized 66 is much easier to handle, better suited to carborane
under simple solvothermal conditions with the appropriate installation through ligand exchanges, and more practically
metal salt (i.e., Zn(NO3)2 for IRMOF and ZrCl4 for UiO-66) and applicable. Therefore, additional functionalizations were
BDC ligand. IRMOF is a Zn(II)-based, landmark MOF that was focused on UiO-66 with chemical tags.
reported in 1999 by Yaghi and co-workers, and various ligand
functionalization processes involving this MOF have been
studied.22 Since the practical uses of IRMOF are limited due to
its low stability to humidity and water, CB installation into
Zr(IV)-based UiO-66 as well as the practicality of the resulting
material were also studied. UiO-66 shows superior chemical and
physical stability due to the high oxophilicity of the zirconium
cation, and a variety of organic functional groups were
successfully installed in UiO-66 frameworks with functionalized
BDCs. The solid-state PL of carborane-bearing MOFs with
different functionalities in the framework was studied, and
unprecedented aggregation-induced emission (AIE) was clearly
visible from the MOFs.
The o-CB-functionalized BDC (BDC-CB, 1) was prepared through
multiple organic reactions (Scheme S1†). All reactions were
performed with the methyl ester form to facilitate separation
by column chromatography. The key intermediate is an alkyne-
functionalized BDC, which can be synthesized by a Sonogashira
cross-coupling with palladium and copper catalysts. Then, the
o-CB cage was generated using decaborane (B10H14). After
hydrolysis under basic conditions, the target BDC-CB was
obtained.
Fig. 1 PXRD patterns of MOFs before and after carborane installation by ligand exchange.
Functionalized UiO-66-R (R = Br, NO2, and Naph) species were
prepared using the solvothermal reaction between ZrCl4 and
BDC-R (or NDC = 1,4-naphthalene dicarboxylic acid). Amino-
functionalized UiO-66-NH2 and hydroxy-functionalized UiO-66-
OH were not employed for this methodology because they have
unique and relatively weak photoluminescence. The retention
of the crystallinity of the UiO-66-Rs during the PSE process was
confirmed by PXRD (Fig. 1 and S5-S7†). Notably, UiO-66-Naph
has the smallest pore and window sizes among the
functionalized UiO-66-Rs, and this prevented it from
undergoing PSE. Therefore, a ligand mixing strategy between
NDC and BDC (1:3 ratio, see the ESI for details†) was employed
to obtain more empty space for ligand exchange. The exchange
ratio was determined by 1H NMR spectroscopy after acid
digestion of thoroughly washed materials, and IR spectroscopy
was employed to confirm the presence of B-H bonds in the
ortho-carborane cages (Figs. S8-S10†). The remaining porosity
in exchanged UiO-66-(R)(CB) was confirmed by N2 adsorption at
77K, and the BET (Brunauer-Emmett-Teller) surface area was
calculated (Figs. S11-S15†). Generally, the surface area
decreased upon the introduction of carborane functionalities;
however, the main pores of the MOFs were retained after
installation of the bulky carborane cages. Thermogravimetric
analysis (TGA) confirmed the thermal stability of the carborane-
As an initial study, the direct synthesis of UiO-66-CB using ZrCl4
with BDC-CB was attempted; however, the o-CB-functionalized
UiO-66-CB was not observed from the screening of various
solvothermal conditions. Since the carborane cage is a relatively
larger functional group than known functional groups in the
UiO-66 pores, we also tried direct synthesis using a mixed ligand
strategy. Several ratios of non-functionalized BDC and BDC-CB
were tested, and carborane-functionalized UiO-66 was not
obtained; only non-functionalized UiO-66 was synthesized.
Therefore, a PSE strategy with the pre-synthesized UiO-66 was
explored. Using PSE, the functionalized ligand could be
incorporated into the MOF pores by exposing the parent MOF
to the target ligand solution.23,24 Dry, non-functionalized UiO-66
was immersed in an aqueous solution of BDC-CB for 30 min, and
UiO-66-CB was obtained by simple centrifugation. The
crystallinity was completely retained during the PSE process, as
evidenced by PXRD (powder X-ray diffraction, Fig. 1 and S1†).
Infrared (IR) spectra displayed signals characteristic of B-H
1
bonds after PSE, and the H NMR spectra after acid digestion
showed that 11% BDC-CB was incorporated into the UiO-66
framework (Fig. S2†). This ratio did not change after extensive
washing, which confirmed that the BDC-CB is not trapped in the
pore but is exchanged into the framework. At the same time,
PSE for IRMOF was also tested in an organic solvent (e.g., THF)
2 | J. Name., 2012, 00, 1-3
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functionalized UiO-66s. The exchanged MOFs were generally were utilized as a basic framework. Interestingly,Vaielwl AthrtiecleoO-nCliBne-
DOI: 10.1039/C9CC06386B
stable up to 300 °C under air (Fig. S16†).
functionalized UiO-MOFs clearly exhibited broad emission in
the range of 450 nm to 600 nm (Fig. 2(c)-(f)), which is
significantly redshifted compared to the emission of o-CB-
functionalized ligands. Furthermore, the PL spectra of UiO-66-
(H)(CB) gradually intensified with increasing content of the
carborane, verifying that the signals resulted from AIE caused
by the presence of o-CBs (Fig. 2(c)). However, the non-emissive
nature of o-CB-functionalized MOFs in the high-energy regions
was also observed. The changes in the emissive behaviors of o-
CB-appended MOFs could be attributed to the desired
intermolecular interactions between the carborane cages,
which are regularly incorporated into the building blocks of the
framework. In contrast, the physical mixture of BDC-CB and
UiO-66 (10 mol% of BDC-CB to UiO-66) did not show significant
redshift in the PL spectra (Fig. S17†). These findings further
suggest that such framework materials containing carborane
moieties could play an important role in controlling the
aggregation effects favored by various o-carborane-based
molecules.
In conclusion, we have successfully prepared o-CB-
functionalized BDC ligands for the installation of carborane
cages into various BDC-based MOFs. Two representative
frameworks, Zn(II)-based IRMOF and Zr(IV)-based UiO-66, were
subjected to o-CB functionalization with a postsynthetic ligand
exchange strategy, and approximately 10% carborane
functionalities were incorporated into MOF structures. The
good functional group tolerance of the carborane installation
was confirmed for UiO-66 since it has superior chemical stability,
which is an important advantage for practical applications. The
maxima in the PL spectra were drastically shifted upon
installation into MOFs, and these changes mainly originate from
AIE. This methodology provides a general approach for the
formation of systematic carborane-functionalized MOFs, which
show intriguing photophysical properties as a novel class of
carborane-bearing framework materials.
Fig. 2 Photoluminescence spectra of carborane-functionalized ligands and UiO-66s.
Prior to investigating the luminescent properties of the o-CB-
appended MOFs, the PL spectra of the o-CB-functionalized
ligands (BDCE-CB and BDC-CB(1)) were acquired. The ligands
This research was supported by the Basic Science Research
Program (2019R1A2C4070584) and the Science Research
Center (2016R1A5A1009405) through the National Research
Foundation of Korea (NRF) funded by the Ministry of Science
and ICT.
showed virtually no emission in solution (DMSO), which is
typical of o-carborane derivatives;25,26 however, both BDCE-CB
and BDC-CB in the solid state displayed intense emission in the
high-energy region over λem = 400 nm (λem, max = 360 nm for BDC-
CB and 345 nm for BDCE-CB, Fig. 2(a)). This finding strongly
indicates that the intense emission bands could be attributed to
o-CB-based AIE induced by the restricted rotation of the
carborane moiety in the rigid structure.27,28 To further check the
AIE phenomenon observed in the solid state of BDC-CB ligand,
the additional PL experiments were performed in DMSO−water
mixed solvents.10,29 Unfortunately, no aggregation occurred
upon increasing volumetric fractions of water in the
DMSO−water mixed solvents and no emission peak was
observed. Next, the emission spectra of o-CB-functionalized
MOFs in the solid state were also investigated. A similar AIE was
observed in the o-CB-functionalized IRMOFs in the region of
350‒400 nm (Fig. 2(b), blue line). However, the only AIE from o-
CB-bearing MOFs could not be precisely measured due to the
strong emission at 442 nm originating from the IRMOF core (Fig.
2(b), dark blue line).30 To clarify the AIE characteristic of o-CB
within MOF structures, UiO-66 MOFs, which are non-emissive,
Conflicts of interest
There are no conflicts to declare.
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Table of Contents Entry
o-carborane-functionalized MOFs were successfully prepared through ligand exchanges, and the PL
were drastically shifted by AIE in the solid state.