Bright blue emissions with temperature-dependent quantum yields from microporous metal-organic frameworks

Article abstract of DOI:10.1039/c1cc15138j

Two 2D microporous metal-organic frameworks have been assembled from a fluorophore ligand, whose quantum yields of strong blue emissions could vary from 40.3% to 74.5% and 13.7% to 25.8% with the decreasing temperature.

Full text of DOI:10.1039/c1cc15138j

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COMMUNICATION
Bright blue emissions with temperature-dependent quantum yields from
microporous metal–organic frameworksw
Qilong Zhu,ab Chaojun Shen,ab Chunhong Tan,ab Tianlu Sheng,a Shengmin Hua and
Xintao Wu*a
Received 18th August 2011, Accepted 25th October 2011
DOI: 10.1039/c1cc15138j
Two 2D microporous metal–organic frameworks have been
assembled from a fluorophore ligand, whose quantum yields of
strong blue emissions could vary from 40.3% to 74.5% and
13.7% to 25.8% with the decreasing temperature.
The luminescent properties of metal–organic frameworks (MOFs)
have attracted considerable interest owing to numerous real
and potential applications in chemical and biological detection,
medical imaging, lighting, displays, and so forth.1 The ordered
nature of MOF structures which include both organic ligands
Scheme 1 Coordination mode and the core structure of H3TTAA ligand.
and metal ions provides a powerful platform for the development
and application of solid-state luminescent materials as they
have a degree of structural predictability, in addition to two
system. Triaminobenzene is nearly identical to the C3N6 core,
however, the three nitrogen atoms of triaminobenzene show slight
well-defined binding sites: metal-based binding sites and organic
linkers in crystalline form.2 On the one hand, the combination
deviation from the central benzene ring, due to steric interactions
between the hydrogen atoms of the amino groups and those of the
of lanthanide ions and conjugated organic ligands usually can
central ring.6 The ligands based on the C3N6 core also have been
produce an antenna effect and a pronounced increase in the
luminescence intensity compared with the free lanthanide
proven to be outstanding fluorophores in our previous work.7
ions.3 On the other hand, the MOFs containing d10 metals
Meanwhile, H3TTAA is more extended than other tricarboxylate
ligands, such as 1,3,5-tris(4-carboxyphenyl)benzene (H3BTB),8 which
and highly conjugated organic linking groups can emit intense
is often used to prepare microporous MOFs. Thus, highly emissive
linker-based luminescence, including intraligand emission as
porous framework structures from the self-assembly between the
to-ligand charge transfer (MLCT).4 A combination of stable
fluorophore ligand and suitable metal ions were anticipated.
well as ligand-to-metal charge transfer (LMCT) and metal-
As expected, a 2D porous framework with strong fluorescence,
{(H2NMe2)[Cd(TTAA)]}ꢀ2H2O (1), has been solvothermally
porosity and intense luminescence in these materials is relatively
uncommon.5 Recently, detailed studies on porous luminescent
MOFs have revealed that the network topology may heavily
synthesized. Under the same synthetic procedure except addition
of 4,40-bipy, another 2D fluorescent porous framework with
different topological structures, {(H2NMe2)[Cd(TTAA)]}ꢀH2O
(2), was surprisingly obtained, despite the fact that the
4,40-bipy did not coordinate to the Cd(II) ions. According to
the sizes of 4,40-bipy and the voids in 2, the hypothesis is put
forward that the 4,40-bipy plays a crucial template role in this
synthesis. Although the ligands in the two frameworks are
arranged in different geometry configurations, they possess the
same coordination mode acting as m7-bridges (Scheme 1). Each
TTAA3ꢁ ligand in 1 and 2 connect four mononuclear Cd units
forming chiral enantiomeric SBUs (Fig. S2 and S5, ESIw).
The frameworks 1 and 2 are both based on the mononuclear
seven-coordinated CdO6N units, in which each Cd(II) is bound
to four ligands with three carboxylate groups and a triazine
nitrogen atom. The bond lengths of Cd–O/N in 1 and 2 are in
the ranges of 2.320(3)–2.488(4) and 2.320(6)–2.476(8) A,
respectively.z
affect the color and intensity of luminescence.4b
Herein we designed a novel tricarboxylate ligand, N,N0,N0 0-1,3,-
5-triazine-2,4,6-triyltris(4-aminomethylbenzoic acid) (H3TTAA),-
based on the optically active and strongly fluorescent C3N6 core
as illustrated in Scheme 1 which was functionalized with 4-methyl-
benzote fragments. The C3N6 core was chosen as a linker, whose
all atoms are totally coplanar to form the great p-conjugated
a State Key Laboratory of Structure Chemistry, Fujian Institute of
Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, 350002, China. E-mail: wxt@fjirsm.ac.cn;
Fax: +86-591-83719238; Tel: +86-591-83719238
b Graduate School of the Chinese Academy of Sciences, Beijing,
100049, China
w Electronic supplementary information (ESI) available: Synthesis,
crystallographic information, supplementary figures, IR, TGA and
XRD. CCDC 838864 and 838865. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1cc15138j
c
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Chem. Commun., 2012, 48, 531–533 531

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Fig. 3 Schematic descriptions of the two (4,4)-connected 2D networks
of (a) 1 and (b) 2 (blue balls represent the Cd atoms; red balls represent
the centers of ligands). The two kinds of helices reflected in the
network of 1 are highlighted.
interact with the adjacent layers by hydrogen bonds, contributing
to an extending 3D framework.
Fig. 1 View of 2D layer comprising alternating right- and left-handed
helical chains in 1.
By PLATON/SOLV10 analysis, the solvent accessible volumes
are as high as 35.5 and 41.2% in frameworks 1 and 2,
respectively. Better insight into such elegant frameworks can
be achieved by the employment of a topology method. From
the connectivity considerations, Cd(II) centers and TTAA3ꢁ
ligands in 1 and 2 are both viewed to be four-connected nodes.
Thus, the framework 1 can be symbolized as an unusual 2D
In 1, the three benzoate groups of the ligand are located at one
side of the central C3N6 plane with the angles between the lines
and the plane of 67.71, 57.91 and 43.21, restraining it from
extending in the other direction, which may be the cause of the
2D layer structure. The convex of one layer is immersed into the
concave of the adjacent layer to get a tightly packed structure
through aromatic pꢀ ꢀ ꢀp stacking interactions (Fig. S2, ESIw).
When viewed down the b axis, the structure presents two
different channels alternately with dimensions of ca. 7.0 ꢂ 10.0 A2
and 7.2 ꢂ 17.0 A2 based on the distances of the opposite atoms
(Fig. 1). In the latter channels, with two arms of each ligand
linking two metal ions, helical chains along the b axis are
formed, in which the two-fold right- and left-handed helical
chains are in an alternate array. One full turn of the helical
chain contains two Cd(II) centers with a pitch of 12.33 A.
These double chains are further interlinked by the coordination
interactions between the triazine rings and the metal ions.
The structure of 2 features a 2D rectangular grid-like network
containing large cavities of ca. 10.4 ꢂ 17.5 A2 based on the
distances of the opposite atoms (Fig. 2). Two benzoate groups
of the ligand are arranged above the central C3N6 plane with
the angles of 11.21 and 12.01 between the lines and the plane,
while another one is arranged below the plane with the angle
of 60.61. Intriguingly, because of the nonplanarity of ligands,
the basic rectangular grid is puckered, different from the
reported grid-like structures in which the basic grids are
usually coplanar or quasi-coplanar.9 Accordingly, the layer
based on the grids is wave-like. The crystalline water molecules
and protonated dimethylamine occupy the large cavities and
(4,4)-connected topology with a Schlafli symbol (42ꢀ64) in
¨
which two kinds of helices can be observed easily (Fig. 3a).
The framework 2 also can be interpreted topologically as a
honeycomb-like 2D (4,4)-connected network with a Schlafli
¨
symbol (43ꢀ63) (Fig. 3b).
The most remarkable feature of the two frameworks is their
bright blue emissions at different temperatures under near-UV
excitation. The solid-state emission spectrum of 1 at room
temperature displays an intense emission band centered at 432 nm
with excitation maximum at 318 nm (Fig. 4). The luminescent
quantum yield was determined by means of an integrating
sphere and was found to be 40.3%; and the lifetime of the
emission is consistent with the nanosecond scale of fluorescence
(Table 1). As the temperature is decreased to 77 K, a blue shift
of 8 nm is observed with respect to the emission band of 1 at
room temperature, whereas its full width at half-maximum
(fwhm) undergoes no change. Surprisingly, the intensity of the
Fig. 4 Relative emission spectra of framework 1 with the variation of
its fluorescent quantum yields collected under excitation at 318 nm in
the solid state at 77 and 298 K. Inset: fluorescent image of the bright
blue luminescing framework 1 at 298 K.
Fig. 2 Packing arrangement representation of the two adjacent layers in 2.
532 Chem. Commun., 2012, 48, 531–533
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Table 1 PL data of frameworks 1 and 2 at 10, 77 and 298 K
on the fluorescent C3N6 core. The two frameworks both
display (4,4)-connected 2D networks with fascinating architectures.
In particular, they show bright blue fluorescence both under
ambient and freezing conditions, and drastic fluorescence
enhancement when the temperature is lowered.
lema, fwhmb/nm
10 K 77 K
Ffc (%)
Compd
td/ns
298 K
10 K 77 K 298 K
1
2
424, 70 424, 70 432, 72 74.5
431, 76 432, 75 432, 74 25.8
74.0
25.4
40.3
13.7
5.49
6.47
This work was supported by grants from the 973 Program
(2007CB815301), the National Science Foundation of China
(21073192, 20733003, 20801055 and 20871114), the Science
Foundation of CAS (KJCX2-YW-H20) and of Fujian Province
(2009HZ0006-1 and 2006L2005).
a
b
lem = emission band. fwhm = full width at half-maximum.
c
d
Ff = quantum yield. t = lifetime at 298 K.
Notes and references
z Crystal data for 1: CdC29H33N7O8, Mr = 719.77, monoclinic, space
group P21/c, a = 12.943(4) A, b = 12.329(3) A, c = 23.867(5) A, b =
108.881(12)1, V = 3603.6(16) A3, Z = 4, T = 293(2) K, rcalcd
=
1.327 g cmꢁ3, R1 = 0.0522 and wR2 = 0.1645 for 27 237 reflections
collected, 6911 observed reflections (I 4 2s(I)) of 7817 (Rint = 0.0346)
unique reflections and 381 parameters, GooF = 0.975. Crystal data
for 2: CdC29H31N7O7, Mr = 702.01, monoclinic, space group C2/c,
a = 29.28(2) A, b = 14.524(10) A, c = 20.084(15) A, b =
115.414(11)1, V = 7714(10) A3, Z = 8, T = 293(2) K, rcalcd
=
1.209 g cmꢁ3, R1 = 0.0758 and wR2 = 0.2071 for 25 354 reflections
collected, 5749 observed reflections (I 4 2s(I)) of 8737 (Rint = 0.0666)
unique reflections and 393 parameters, GooF = 1.104.
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Fig. 5 Relative emission spectra of framework 2 with the variation of
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blue luminescing framework 2 at 298 K.
emission response sharply rises to Ff = 74.0%, suggesting its
high sensitivity to temperature. Upon further cooling to 10 K, the
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The emission features of 1 and 2 closely match to that of the
fluorophore ligand (Fig. S7, ESIw), indicating the metal-
perturbed intraligand charge transfers. Thus, the intense fluores-
cence of the two frameworks should be mainly contributed from
the well planar and p-conjugated C3N6 cores of ligands. Till
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intensity has been observed for the luminescent spectra of each
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for the rigidity of ligands, thereby the radiationless decay of the
intraligand (pꢀ ꢀ ꢀp*) excited state could be reduced to some extent
and the increase of the quantum yields should be observed
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In summary, two novel microporous MOFs have been
hydrothermally synthesized from a fluorophore ligand based
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 531–533 533