Cuprous Iodide Pseudopolymorphs Based on Imidazole Ligand and Their Luminescence Thermochromism

Article abstract of DOI:10.1021/acs.cgd.6b00114

Two cuprous iodide pseudopolymorphs, formulated as [(Cu₄I₄)(MBI)₂]_∞ (MBI = 1,1′-methylene-bis(imidazole)) with an irregular cubane-like Cu₄I₄ cluster as tetrahedrally coordinated secondary building unit and imidazole derivative as bridging ligand, have been synthesized and characterized by single-crystal X-ray diffraction analysis. Both of two compounds only exhibited a single broad low-energy cluster-centered (³CC) triplet emission band between room temperature and 77 K. Of particular interest, these two Cu₄I₄-imidazole pseudopolymorphs still displayed thermochromic luminescence originating from a red shift of such tunable single cluster-centered triplet emission, being different from that observed in the previously reported Cu₄I₄-pyridine system by balancing temperature-dependent multiple emissions (high-energy and low-energy) derived from their energetically distinct triplet states.

Full text of DOI:10.1021/acs.cgd.6b00114

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Article
Cuprous Iodide Pseudo-Polymorphs Based on Imidazole
Ligand and Their Luminescence Thermochromism
zhixing fu, Jian Lin, Le Wang, Cheng Li, Wenbo Yan, and Tao Wu
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00114 • Publication Date (Web): 21 Mar 2016
Downloaded from http://pubs.acs.org on March 22, 2016
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Cuprous Iodide Pseudoꢀpolymorphs Based
on Imidazole Ligand and Their
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Luminescence Thermochromism
Zhixing Fu,§ Jian Lin,§ Le Wang, Cheng Li, Wenbo Yan, and Tao Wu*
State and Local Joint Engineering Laboratory for Novel Functional Polymeric
Materials, College of Chemistry, Chemical Engineering and Materials Science,
Soochow University, Jiangsu 215123, China
ABSTRACT: Two cuprous iodide pseudoꢀpolymorphs, formulated as
[(Cu4I4)(MBI)2]∞ (MBI=1,1’ꢀmethyleneꢀbis(imidazole)) with irregular cubaneꢀlike
Cu4I4 cluster as tetrahedrally coordinated secondary building unit and imidazole
derivative as bridging ligand, have been synthesized and characterized by
singleꢀcrystal Xꢀray diffraction analysis. Both of two compounds only exhibited
single broad lowꢀenergy clusterꢀcentered (3CC) triplet emission band between room
temperature and 77 K. Of particular interest, these two Cu4I4ꢀimidazole
pseudoꢀpolymorphs still displayed thermochromic luminescence originating from red
shift of such tunable single clusterꢀcentered triplet emission, being different from that
observed in the previously reported Cu4I4ꢀpyridine system by balancing
temperatureꢀdependent multiple emissions (highꢀenergy and lowꢀenergy) derived
from their energetically distinct triplet states.
ꢀ INTRODUCTION
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Complexes based on cuprous iodide are currently at the forefront of coordination
chemistry and crystal engineering research because of the quest for active materials in
optoelectronics.1ꢀ6 Such research interests mainly stem from the increasing demand
for more affordable complexes in preference to other luminescent metal complexes
based on precious (i.e. the platinum group) and rareꢀearth metals, which are often
quite expensive and environmentally problematic.7ꢀ9 In fact, CuIꢀbased complexes or
coordination polymers possess several following advantages, which further stress
their research interests and potential or practical applications as luminescent
materials: 1) they have a large variety of coordination geometries, arising from many
possible combinations of coordination numbers available for Cu(I) (two, three and
four) and geometries adopted by the iodides (from terminal to ꢁ2ꢀ and up to
ꢁ8ꢀbridging);10ꢀ15 2) they present different luminescent levels, which can be of a
ligand centred, charge transfer in the case of polynuclear compounds, or even
metalꢀcentred nature;5,16,17 3) most of cuprous iodide complexes are characterized by a
remarkably high photoluminescent quantum yield in solid state;18 4) the reagents are
cheap and it is relatively easy to obtain the final products.7,11
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Photoluminescence (PL) tunability is always the focus of most concern for
CuIꢀbased complexes. They are generally highly luminescent at the room temperature,
and the emissions are strikingly sensitive to the temperature.6,11,19 External stimulus
(such as temperature, pH, solvent vapor, and mechanical grinding) can tune
photophysical properties by balancing stimuliꢀresponsive multiple emissions derived
from their energetically distinct triplet states in CuIꢀbased complexes.20ꢀ23 For
example, the typical Cu4I4ꢀpyridine system shows two unique triplet emissions at
room temperature and at 77 K, respectively.17 The lowꢀenergy (LE) emission band at
around 550ꢀ650 nm is dominant at room temperature, attributed to clusterꢀcentered
(3CC) triplet emission consisting of a mixture of iodideꢀtoꢀcopper charge transfer
(3XMCT) and metal d → s, p transitions. The highꢀenergy (HE) emission band in the
blue region (400ꢀ450 nm) is dominant at 77 K, assigned to iodideꢀtoꢀligand charge
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transfer (3XLCT) and metalꢀtoꢀligand charge transfer (3MLCT) transitions. The PL
intensity ratio of LE and HE band at different temperature could effectively control
the emission color of CuIꢀbased complexes. In addition, it is generally accepted that
terminal or bridging ligands with variable substituted groups play vital roles in
controlling the size and geometry of CuIꢀbased clusters and their assembled structures,
which further tune their luminescent properties by altering cuprophilic interaction
owing to its sensitivity with respect to distortions within the Cu4I4 cluster.7,16,18,22
In most of previous studies on cuprous iodide coordination compounds, ligands
were focused on pyridine,17,24,25 alkylphosphine2,20,22 or triethylenediamine26ꢀ31. Few
investigations have been performed on imidazole and its derivatives as terminal or
bridging ligand.22,23 Whether could the CuIꢀimidazole complexes show similar
thermochromism luminescence with CuIꢀpyridine system? In fact, although the
thermal stimuliꢀresponsive behavior of CuI clusters is fairly studied through various
examples, their emission shifts, colors, and shift directions are still unpredictable. In
this paper, we aimfully synthesized Cu4I4 clusterꢀbased structures with imidazole
derivative as terminal and bridging ligands to probe their thermochromic
luminescence. The current work for the first time reveals that Cu4I4ꢀimidazole system
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only displays a single broad CC excited state emission band, which still generates
remarkable thermochromic luminescence realized through dramatic red shift of the
single broad clusterꢀcentered (3CC) triplet emission.
ꢀ EXPERIMENTAL SECTION
General Procedures. Copper (I) iodide (CuI, AR, 99.5%), potassium iodide (KI,
AR, 99%), 1ꢀmethylimidazole (C4N2H6, AR 99.5%), N, Nꢀdimethylformamide (DMF,
AR, 95%), N, Nꢀ dimethylacetamide (DMA, AR, 99%)，acetonitrile (CH3CN, AR,
99%) were purchased from Aladdin Industrial, Inc. All chemicals were used as
received without further purification. Powder Xꢀray diffraction (XRD) measurements
on the purity of samples were carried out with a Bruker D2 phaser diffractometer
using CuꢀKα (λ=1.54056 Å) radiation operated at 30 kV and 10 mA (Figure S1). The
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PL spectra were collected on a HORIBA scientific Fluorologꢀ3 steady state and
timeꢀresolved fluorescence spectrophotometer equipped with a 450 W xenon lamp.
Single crystal Xꢀray diffraction (SCXRD) data were performed on Agilent
diffractometers at room temperature (295 K) with graphite monochromated Mo Kα
(λ=0.71073 Å) radiation. The structures were solved by direct methods using
SHELXSꢀ97 and the refinements against all reflections of the compounds were
performed using SHELXLꢀ97.
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Synthesis of ligand 1, 1’ꢀmethylene bis(imidazole) (MBI). NaOH (17.98 g,
0.4495 mol), imidazole (4.750 g, 0.0698 mol), tetraethylammonium bromide (440 mg,
2.095 mmol), H2O (25 g, 1.389 mol), dichloromethane (100 mL) were mixed in a 250
mL roundꢀbottomed flask. The mixture was refluxed for 24 hours under 45 °C, then
white needleꢀlike crystals were obtained with the yield of 3.254 g.
Synthesis of [(Cu4I4)(MBI)2]∞ꢁ(Guest) (1). CuI (0.5929 mmol, 0.1152 g), MBI
(0.1973 mmol, 0.0292 g) were mixed in the presence of acetonitrile (3.0 mL) and
DMA (4.0 mL). The mixture was stirred at room temperature for 10 min to give
precipitate, and then this heterogeneous mixture was placed in a 15 mL Teflonꢀlined
steel autoclave and heated to 150 °C for three days. The resulting colorless block
crystals were obtained by filtration, washed by acetonitrile, and airꢀdried for the
subsequent measurements.
Synthesis of [(Cu4I4)(MBI)2]∞ꢁ(C2H3N)ꢁ8(H2O) (2). A solution of CuI (0.016 g,
0.084 mmol) in saturated potassium iodide (KI) solution (2 mL) was mixed with a
solution of MBI (0.028 g, 0.188 mmol) in acetonitrile (2 mL) and H2O (1 mL). Then
this mixture was placed in a 15 mL Teflonꢀlined steel autoclave and heated to 150 °C
for five days. The resulting colorless needlelike crystals were obtained by filtration,
washed by ethanol, and airꢀdried.
Synthesis of [(Cu4I4)(MIM)4] (3). CuI (0.209 mmol, 0.040 g), 1ꢀmethylimidazole
(MIM, 0.026 mmol, 2.13 mg) were combined together in the presence of acetonitrile
(0.6 g) and ethanol (1.6 g). Then the mixture was placed in a 3 mL glass tube and
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heated to 120 °C for three days. The resulting colorless rodꢀlike crystals were
obtained by filtration, washed by ethanol, and airꢀdried.
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ꢀ RESULTS AND DISCUSSIONS
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ꢀ
Figure 1. (a) Fourꢀcoordinated Cu4I4 cubane unit simplified into a tetrahedron in
compounds 1 and 2. (b) Tetrahedrally coordination mode of Cu4I4 cluster by linear
bridging ligand. (c) Twoꢀdimensional layered structure in compound 1. (d)
Threeꢀdimensional single net in compound 2. I, purple ball; Cu, light blue ball; C,
gray ball; N, blue ball; and Cu4I4 cluster was represented by green tetrahedron.
Singleꢀcrystal Xꢀray diffraction analysis revealed that compound 1 crystallizes in
monoclinic system with P21/c space group, and the crystallographically asymmetric
unit consists of four independent Cu(I) ions, four iodide ions, and two MBI ligand
moieties (Figure S2 and Table S1). Each Cu(I) adopts tetrahedral coordination
geometry bonded to three iodide ions and one nitrogen atom from MBI ligand. The
Cu−I bond lengths locate in the range of 2.677(4) and 2.758(3) Å and I−Cu−I bond
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angles are in the range of 109.477(3) and 117.296(3) °. The Cu···Cu distances range
from 2.634(6) to 2.750(1) Å, being shorter than the sum of Vander Waals radii of
Cu(I) (2.8 Å) and implying strong metalꢀmetal cuprophilic interaction in the
cluster.20,22 Compound 1 possesses twoꢀdimensional layered (4, 4)ꢀconnected
coordination network constructed by imidazole derivative (MBI) as linear bridging
ligand and Cu4I4 cubane cluster as secondary building unit. Each Cu4I4 unit is bridged
by four organic ligands MBI to form a two dimensional structure (Figure1aꢀ1c).
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Compound 2 crystallizes in hexagonal system with R3c space group, and the
crystallographically asymmetric unit also consists of four independent copper (I) ions,
four iodide ions, and two MBI ligands (Figure S3). Similar to copper (I) coordination
environment occurring in compound 1, each Cu in compound 2 also tetrahedrally
bonds to three iodide ions and one nitrogen donor from MBI ligand. Cu−I bond
lengths locate in the range of 2.641(6) and 2.801(4) Å and I−Cu−I bond angles are in
the range of 109.316(2) and 117.337(2) °. The Cu···Cu distances range from 2.646(8)
to 2.721(4) Å, being less than the sum of the Vander Waals radii of Cu(I) (2.8 Å) and
implying strong metalꢀmetal cuprophilic interactions. Like compound 1, each Cu4I4
cubic unit in compound 2 is connected to four ligands to form fourꢀconnected network.
However, due to different distortion configuration of flexible MBI ligand, compound
2 adopts twoꢀfold interpenetrated 3D network (Figure 1d and Figure S4). Topological
analysis shows that single network in compound 2 possesses an unprecedented
fourꢀconnected selfꢀpenetrating topological net with short and long Schläfli symbol of
86 and 87.87.87.87.86.86, respectively, when treating Cu4I4 as 4ꢀconnected node and
MBI ligand as linear linker (Figure S5). To better understand the structure of
compound 2, its network can be parsed in different way, as shown in Figure 2. The
Cu4I4 cubane unit with four MBI ligands (Figure 2a) could form single helix (Figure
2b and 2d) and double helix (Figure 2c and 2e) along cꢀaxis. The adjacent single helix
and double helix fuse together to form 3D network with trigonal symmetry (Figure
2f).
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Although compounds 1 and 2 are pseudoꢀpolymorph with the same Cu4I4 cubane
unit and bridging ligand, the different distortion configuration of flexible MBI ligand
still results in subtle variation in the Cu−I bond length and I−Cu−I bond angle (Table
S2 and S3). To further probe the effect of imidazole ligand on the configuration of
Cu4I4 cluster, we purposely synthesized compound 3, formulated as [(Cu4I4)(MIM)4]∞,
consisting of discrete Cu4I4 cluster with 1ꢀmethyl imidazole (MIM) as terminal ligand
(Figure S6). The Cu−I bond lengths, I−Cu−I bond angles as well as Cu···Cu distances
are all in the rational range (Table S4), being similar with that observed in compounds
1 and 2.
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Figure 2. Structure analysis of compound 2. (a) Cu4I4 cluster bonded to four MBI
ligands. (b, d) oneꢀdimensional helical structure viewed from bꢀaxis and cꢀaxis,
respectively. (c, e) double helical structure viewed from bꢀaxis and cꢀaxis,
respectively. (f) single network in twoꢀfold interpenetrated threeꢀdimensional
framework. (g) twoꢀfold interpenetrated threeꢀdimensional framework. I, purple ball;
Cu, light blue ball; C, gray ball; N, blue ball; and Cu4I4 cluster was represented by
green tetrahedron.
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Emission spectra of compounds 1ꢀ3 at room temperature, as shown in the Figure 3,
all display a single broad lowꢀenergy emission band centered at 600 nm for 1 (λex=
315 nm), 645 nm for 2 (λex= 350 nm), and 636 nm for 3 (λex= 350 nm) in agreement
with the observed orangeꢀred light. Obviously, the origin of the emission signatures in
1ꢀ3 at 298 K can be attributed to 3CCꢀtransitions, as the Cu···Cu distances in them do
not exceed 2.80 Å. Cu···Cu distance is the decisive factor in control of fluorescence
properties of CuIꢀbased compounds. In addition, the internal quantum yields of the
compounds were determined to be 1.72%, 1.71%, and 5.85% for 1, 2, and 3,
respectively (Table S5).
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Figure 3. Photoluminescent excitation and emission spectra of compounds 1, 2 and 3
at room temperature.
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Figure 4. Top: Temperatureꢀdependent excitation and emission spectra of compound
1 from 10ꢀ298 K in the solid state (λex=315 nm). Down: Photos of sample 1 and 1*
(ground) under ambient light and UV light.
To investigate the thermochromic properties of compounds 1ꢀ3,
temperatureꢀdependent solidꢀstate excitation and emission spectra from room
temperature down to 10 K were recorded, as shown in Figure 4ꢀ6. For compound 1,
with the temperature decreasing to 150 K, the LE emission intensity increases with
emission peak slightly redꢀshifted. However, when below 98 K, the LE emission
continues red shifting and begins to weaken. At the range of 68 K to 10 K, LE band at
around 648 nm is still dominated, being accompanied by a new weak emission band
at approximate 510 nm. The new emission band is not in blue light region, so it
cannot be attributed to the imidazoleꢀinvolved highꢀenergy excited state, such as
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iodineꢀtoꢀligand chargeꢀtransfer (XLCT) and metalꢀtoꢀimidazole chargeꢀtransfer
(MLCT). This weak green emission is possibly assigned as originating from a
ligandꢀtoꢀcopper transition(LMCT).17 For compound 2, with the temperature
decreasing from room temperature to 10 K, the LE emission band is progressively
redꢀshifted from 645 to 685 nm and accompanied with a shoulder at 550 nm. All the
same, no obvious HE emission band at around 400ꢀ500 nm is observed during the
temperature decreasing. This dramatic red shift of about 40 nm makes it easy to detect
the color change by naked eye (Figure 5). Similar to compound 2, upon lowering the
temperature to 10 K, luminescence in compound 3 is also dominated by
clusterꢀcentered (3CC) emission. The single broad LE emission band is marginally
redꢀshifted to 668 nm, which is accompanied by widening of the bandwidth.
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Figure 5. Top: Temperatureꢀdependent excitation and emission spectra of compound
2 from 10ꢀ298 K in the solid state (λex=370 nm). Down: photos of compounds 2 and
2* (ground) under ambient light and UV light.
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Figure6. Top: Temperatureꢀdependent excitation and emission spectra of compound 3
from 10ꢀ298 K in the solid state (λex=370 nm). Down: photos of compound 3 under
ambient light and UV light.
Above data show that the origin of thermochromic luminescence in 1ꢀ3 is
originated only from a single broad tunable LE emission, being quite different from
typical Cu4I4(pyridine)4 clusterꢀbased compounds where relative intensities of LE
(3CC) and HE (XLCT) emissions at different temperature are responsible for
thermochromic luminescence. The absolute predomination of LE emission at room
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temperature and the absence of HE emission at low temperature in compounds 1ꢀ3
possibly are the synergistic effects coming from Cu4I4 cluster with strong cuprophilic
interaction in it and imidazole ligand with unmatchable π* molecular orbital for
ligandꢀinvolved triplet transition. As displayed in structural data, the relatively
flexible bridging ligand MBI and terminal ligand MIM cannot cause the serious
distortion of Cu4I4 cluster. The Cu···Cu distances in compounds 1ꢀ3 is in the narrow
range and always less than the sum of the Vander Waals radii of Cu(I) (2.8 Å). Only
strong stimulus, such as pretty low temperature, can cause a subtle change in the
Cu···Cu distances, which further results in the large positive Stoke’s shift in its PL
spectrum. Weak stimulus, such as mechanical attrition treatment, can not lead to
further alteration in the Cu···Cu distances. Thus, compared with the unground sample,
the similar emission color for ground sample is observed, as shown in Figure 4ꢀ6.
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ꢀ CONCLUSION
In
summary,
two
cuprous
iodide
pseudoꢀpolymorphs
with
1,1’ꢀmethyleneꢀbis(imidazole) as bridging ligand were successfully synthesized and
structurally characterized by singleꢀcrystal Xꢀray diffraction analysis. Although
compounds 1 and 2 have the same Cu4I4 cubane unit and bridging ligand, they
exhibited totally different topological network due to different distortion
configuration of ligand. Different from compound 1 with a typical 2D (4,4) net,
compound 2 displayed unprecedented 3D twoꢀfold interpenetrated net with
selfꢀpenetrating 4ꢀconnected topological framework. The average Cu···Cu distances
in those compounds are shorter than the sum of the Vander Waals radii of copper, so
compounds 1 and 2 only exhibited a single broad lowꢀenergy clusterꢀcentered (3CC)
triplet emission band at room temperature and 77 K. Even so, these two
pseudoꢀpolymorphs still displayed consistent thermochromic luminescence
originating from the single clusterꢀcentered triplet emission, which is redꢀshifted upon
lowering temperature. Current study indicates that organic ligand bonded to Cu4I4
cluster can effectively control thermochromic behavior. This work promotes us to
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carry out further investigation on other type of organic ligands for enriching
luminescent thermochromism behavior from CuIꢀligand systems.
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ASSOCIATED CONTENT
Supporting Information
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The Supporting Information is available free of charge on the ACS Publications
website at DOI:10.1021/???????.
Crystallographic parameters, experimental and calculated powder XPD
diffraction diagrams (PDF)
Single crystal Xꢀray diffraction crystallographic file (CCDC
1445345ꢀ1445347) (CIF)
ꢀ AUTHOR INFORMATION
Corresponding Author
*(T.W.) Eꢀmail: wutao@suda.edu.cn
§These authors contributed equally.
Notes
The authors declare no competing financial interest.
ꢀ ACKNOWLEDGMENTS
This work was supported by National Natural Science Foundation of China (No.
21271135), a startꢀup fund (Q410900712) from Soochow University, the Priority
Academic Program Development of Jiangsu Higher Education Institutions (PAPD),
Young Thousand Talented Program.
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Two cuprous iodide pseudoꢀpolymorphs, formulated as [(Cu4I4)(MBI)2]∞
(MBI=1,1’ꢀmethyleneꢀbis(imidazole)) with irregular cubaneꢀlike Cu4I4 cluster as
tetrahedrally coordinated secondary building unit and imidazole derivative as
bridging ligand, exhibited single broad lowꢀenergy clusterꢀcentered (3CC) triplet
emission band between room temperature and 77 K, and their thermochromic
luminescence behavor originated from red shift of such tunable single
clusterꢀcentered triplet emission.
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