Siloxanol-functionalized copper iodide cluster as a thermochromic luminescent building block

Article abstract of DOI:10.1021/ic200672r

A copper iodide cluster bearing reactive silanol groups exhibits thermochromic luminescence properties sensitive to its chemical environment and is thus a suitable building block for the synthesis of optically active materials.

Full text of DOI:10.1021/ic200672r

Article
pubs.acs.org/IC
Siloxanol-Functionalized Copper Iodide Cluster as a Thermochromic
Luminescent Building Block
,
†
†
†
‡
§
́
Sandrine Perruchas,* Nicolas Desboeufs, Sebastien Maron, Xavier F. Le Goff, Alexandre Fargues,
§
†
,†
Alain Garcia, Thierry Gacoin, and Jean-Pierre Boilot*
†
Laboratoire de Physique de la Matier
̀
e Condensee
ents et Coordination (DCPH), CNRS, Ecole Polytechnique, 91128 Palaiseau Cedex, France
Institut de Chimie de la Matiere Condensee de Bordeaux (ICMCB), CNRS, 87 Avenue du Dr A. Schweitzer, 33608 Pessac Cedex,
́
(PMC), CNRS, Ecole Polytechnique, 91128 Palaiseau Cedex, France
‡
́ ́ ́ ́
Laboratoire Heteroelem
§
̀
́
France
*
S Supporting Information
ABSTRACT: A copper iodide cluster bearing reactive silanol groups exhibits thermochromic
luminescence properties sensitive to its chemical environment and is thus a suitable building block for
the synthesis of optically active materials.
INTRODUCTION
(OH) (CH ) PPh ) ] (2), obtained through hydrolysis and
2 2 2 2 2
condensation reactions of the corresponding alkoxysilane
■
Photoluminescent materials are currently an active domain of
research driven by their broad range of applications in light-
cluster [Cu I (PPh (CH ) Si(OEt) ) ] (1). In this original
4
4
2
2 2
3 4
1
disiloxane cluster (2), the ligands exhibit intracluster con-
densation, but silanol groups are also formed and isolated
without intercluster condensation. Temperature-dependent
luminescence measurements of 2 revealed emissions sensitive
to the chemical environment. Constraints in the solid state due
to an efficient hydrogen-bonding network involving the silanol
groups are probably responsible for this phenomenon. Finally,
the reactivity of this siloxanol cluster toward further
condensation reaction with the usual sol−gel precursors is
demonstrated, making this cluster an unprecedented building
block for the synthesis of thermochromic luminescent
materials.
emitting devices. In this prospect, transition-metal coordina-
tion compounds have attracted considerable interest because of
2
,3
their various photophysical properties. Among them, first-
row transition-metal compounds are attractive from an
economical point of view, while they are less toxic and more
easily available than the noble-metal complexes. The molecular
tetracopper(I) iodide clusters formulated as [Cu I L ] (L =
4
4 4
4
−6
organic ligand)
rich photoluminescence properties,
to access materials exhibiting original optical properties.
are particularly appealing because of their
5
,7,8
which can be exploited
9
,10
In
this context, the sol−gel process presents several advantages to
design materials for optical applications because of its chemical
and processing versatilities. Numerous silica- and/or siloxane-
11
EXPERIMENTAL SECTION
■
based hybrid organic−inorganic emissive materials have thus
All manipulations were performed with standard air-free techniques
using Schlenk equipment, unless otherwise noted. Solvents were
distilled from the appropriate drying agents and degassed prior to use.
Copper(I) iodide and tetramethoxysilane (TMOS) were purchased
from Aldrich and used as received. Diphenylethyltriethoxysilanephos-
phine [PPh (CH ) Si(OCH CH ) ] was synthesized by the method
12
been developed. Compounds containing silanol groups
(
SiOH) represent an interesting class of precursors for these
13
kinds of materials. Indeed, upon condensation reaction,
strong siloxane (Si−O−Si) bonds are formed, which can lead to
polysiloxane polymeric derivatives, supramolecular organo-
silicon compounds, as well as discrete assemblies like
2
2
2
2
3 3
1
7
reported in the literature.
Synthesis of 2. To a suspension of CuI (0.6 g, 3.1 mmol) in
dichloromethane (20 mL) was added the phosphine ligand
PPh (CH ) Si(OCH CH ) (1.05 g, 2.8 mmol). The solution was
1
4−16
metallasiloxanes.
While characterization of the precursors
is essential to the comprehension of the properties exhibited by
the final material, isolation of these functionalized compounds,
prior to their polymerization, is often difficult mainly because of
the high reactivity of the silanols toward condensation.
2
2
2
2
3 3
stirred for 12 h at room temperature. The mixture was filtered, and
after evaporation of the solvent, the product was recovered as a
colorless oil. The latter was dissolved in tetrahydrofuran (THF; 12
Here, we report on the synthesis and structural and optical
characterization of a siloxanol-functionalized luminescent
cluster, namely, [Cu I (PPh (CH ) Si(OH) OSi-
Received: April 1, 2011
Published: December 23, 2011
4
4
2
2
2
2
©
2011 American Chemical Society
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Inorganic Chemistry
Article
mL), and H O−HCl (pH 2.5, 1 mL) was added. The clear solution
2
was left to stand at room temperature, and after 2 days, colorless
crystals of the formula [Cu I (PPh (CH ) Si(OH) OSi-
4
4
2
2
2
2
(
OH) (CH ) PPh ) ]·4THF were obtained. Yield: 66% (1 g, 0.46
2 2 2 2 2
1
mmol). Liquid-state NMR: H (300.06 MHz, DMF) δ 0.98−1.03 (2
H, m, CH CH Si), 2.52−2.55 (2 H, m, PCH CH ), 6.48 (2 H, s,
2
2
2
2
31
SiOH), 7.44−7.52 (6 H, m, Ph), 7.82−7.78 (4 H, m, Ph); P (145.77
29
MHz, DMF) δ −24.8 (br); Si (71.54 MHz, DMF) δ −51.8 (d).
31
29
Solid-state MAS (magic angle spinning) NMR: P: δ −26.3 (q); Si:
δ −50.6 (d). EDX anal. Calcd (atom %) for Cu I P Si : Cu, 25; I, 25;
4
4
4
4
P, 25; Si, 25. Found: Cu, 24.3 (0.6); I, 26.8 (0.2); P, 22.0 (0.2); Si,
2
6.9 (0.6). Anal. Calcd (wt %) for C H O Si P Cu I + 4C H O: C,
56 64 10 4 4 4 4 4 8
3
9.60; H, 4.43. Found: C, 39.40; H, 4.58.
Synthesis of 3. To TMOS (2.2 mL, n = 15 mmol) was added
Si
−4
cluster 2 (400 mg, 1.8 × 10 mol, nSi = 1.5 mmol). The white
suspension was left to stir for 2 days at room temperature until the
solution became clear. A white precipitate was obtained by the
addition of EtOH to the solution. After filtration, the solid was washed
with EtOH and dried under a vacuum (m = 440 mg). Several
Figure 1. Molecular structure of 2 in a thermal ellipsoid plot
(probability factor 50%).
Among the 12 initial alkoxysilane SiOCH CH functions, 4
2
3
1
experiments gave reproducible results. Liquid-state NMR: H (300.06
have been condensed into Si−O−Si bonds and 8 have been
hydrolyzed into SiOH groups. The cluster is thus formulated as
[Cu I (PPh (CH ) Si(OH) OSi(CH ) (OH) PPh ) ] (2)
MHz, CDCl ) δ 0.77−1.26 (2 H, m, CH CH Si), 2.14−2.54 (2 H, m,
3
2
2
31
PCH CH ), 3.46 (10 H, m, CH ), 7.00−7.78 (10 H, m, Ph);
P
2
2
3
4 4
2
2
2
2
2
2
2
2 2
3
1
(
145.77 MHz) δ−24.9 (br). Solid-state MAS NMR: P δ −21.5 (br)1;
with the ligands now chelating. Elemental analyses and Fourier
transform infrared (FTIR) and NMR characterizations confirm
this formula (see the SI). The silanol protons are revealed in
2
9
2
3
Si δ −58.2 (br, T Si
), −67.7 (br, T Si
), −86.0 (br, Q
cluster
2
cluster
TMOS), −93.9 (br, Q TMOS). EDX anal. Calcd (atom %) for
Cu I P Si : Cu, 18.2; I, 18.2; P, 18.2; Si, 45.4. Found: Cu, 19.1 (0.7);
1
4
4
4
10
the liquid-state H NMR spectra as a singlet at 6.48 ppm. As
I, 19.4 (0.5); P, 15.7 (0.3); Si, 45.8 (0.9). Anal. Calcd (wt %) for
expected, in the FTIR spectrum, the Si−OH stretching band is
C H O Si P Cu I + 5Si(OCH ) + 1OSi(OCH ) : C, 33.69; H,
56
59 10
4
4
4 4
3
3
3
2
−1
observed at 825 cm and the O−H ones appear as a broader
4.26. Found: C, 33.89; H, 4.20.
−1
peak centered at 3400 cm . Concerning the Si−O−Si bonds,
the Si−O antisymetric stretching vibrations correspond to a
RESULTS AND DISCUSSION
Cluster 2 is synthesized in solution starting from compound 1
see the Experimental Section). The latter was previously
■
(
−1
broad band centered at 1100 cm .
The ligands present a chelating geometry relatively rare for
these cubane copper iodide clusters. The only other example
reported with the bis(ethylamidophosphine) ligand leads to a
obtained from the reaction of CuI with the alkoxysilane
phosphine ligand PPh (CH ) Si(OCH CH ) . As we have
2
2 2
2
3 3
18
relatively distorted [Cu I ] cluster core. The Cu−I [2.736(1),
4
4
reported, 1 is recovered as oil due to uncontrolled partial
2
.665(1), and 2.675(1) Å] and Cu−P [2.240(1) Å] bond
hydrolysis and condensation of the ligand by air exposure, thus
10
distances in cluster 2 are within the range of reported values for
copper iodide clusters coordinated with diphenylphosphine
derivatives. The siloxane bond presents an expected value for
the Si−O distance of 1.609(1) Å, but the Si−O−Si angle is
more unusual, with a high value of 175.5(3)°. The almost
linearity of the Si−O−Si angle has also been reported for [t-
BuSi(OH) ] O and has been attributed to the absence of
preventing structural characterization. In order to induce on
purpose hydrolysis and condensation of the alkoxysilane
groups, 1 was reacted in an acidic medium (THF/H O−HCl,
2
pH 2.5), leading to cluster 2 as a crystalline powder in high
yield (66%), as illustrated in Scheme 1.
2
2
Scheme 1. Cluster 2 Synthesis
intramolecular hydrogen bonding involving the silanol
19
groups. The Cu−Cu distances [2.704(1) and 2.743(1) Å]
are the shortest reported in the literature for similar clusters
with phosphine ligands, which generally lie in the range 2.8−3.1
20
Å. These distances are even comparable to those reported for
pyridine derivatives, although previous studies showed that
Cu−Cu bonds in [Cu I L ] clusters based on phosphine
4
4 4
21
ligands are generally significantly longer. This is probably due
to the steric constraints induced by condensation of the ligands.
This “chelating effect” is less significant for the larger
bis(ethylamidophosphine) ligand, which induces less con-
From single-crystal X-ray diffraction analysis (see the
Supporting Information, SI), 2 crystallizes in the tetragonal
18
straints. These Cu−Cu values, shorter than the sum of the
22
van der Waals radii of copper(I) (2.80 Å), imply strong
metal−metal bonding interactions in cluster 2. This is an
important point because Cu−Cu interactions have been
reported to greatly influence the luminescence properties of
space group I4 /a, with the molecular structure depicted in
1
Figure 1. This cluster presents a [Cu I ] cubane structure
4
4
formed by four copper atoms and four iodine atoms, which
occupy alternatively the corners of a distorted cube. The
phosphine ligands are coordinated to each copper atom by the
phosphorus atom, and the hydrolysis and condensation
5
the copper iodide clusters.
The crystal structure of 2 can be described as an assembly of
chains of clusters running along the c axis (see Figure 2 and the
unit cell content in Figure S2 in the SI). THF solvent molecules
are incorporated within the structure and are located between
the cluster chains. In these chains, adjacent clusters are linked
reactions of two alkoxysilane Si(OEt) groups result in the
3
formation of two intracluster siloxane Si−O−Si bonds.
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Inorganic Chemistry
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Figure 2. Hydrogen-bonding network in the crystal structure of 2.
by hydrogen bonds between the SiOH groups (SiOH···OSi =
Figure 4. Temperature-dependent luminescence spectra of 2 from 20
to 290 K (a) in the solid state and (b) in a DMF solution. Emission
2
.009 Å). The hydrogen-bonding network is shown in Figure 2
(
λex = 300 nm) and excitation spectra are solid and dotted lines,
2
2
23
and corresponds to the R (10) motif in the Etter coding.
respectively.
One of the two SiOH groups is also involved in hydrogen
bonding, with the oxygen atoms of the THF molecules acting
as hydrogen-bond acceptors (SiOH···OTHF = 1.858 Å). FTIR
analyses confirm this hydrogen-bonding network involving all
of the silanol groups by the large peak observed at 3400 cm .
Although here the SiOH groups are in close contact (SiO···OSi
due to weaker rigidity in solution compared to the solid state, as
was already observed for similar clusters. This phenomenon,
named the “rigidochromic effect”, has been attributed to the
−1
3
effect of medium rigidity due to distortion of the CC state
5
relative to the ground state. The internal luminescence
=
2.731 Å), they are not condensed into a Si−O−Si bond
quantum yield of the cluster was measured at 290 K in the
because of steric hindrance. Indeed, the condensation of the
eight SiOH groups two by two would result in too high
constraint in the tetrahedral geometry of the silicon atom.
At room temperature, 2 is a white powder under ambient
light (Figure 3a), which under UV excitation emits an intense
solid state, and a high value of 60% was obtained for λ = 360
ex
nm. By lowering of the temperature of the cluster in powder,
down to 20 K, the intensity of the CC band progressively
increased along with the narrowing of the bandwidth (Figure
4
a). No shift of the band was observed. Around 80 K, a new
emission band of low intensity appeared at higher energy, λmax
420 nm, corresponding to the halide-to-ligand charge-transfer
XLCT) transition classically observed for these clusters at low
=
(
24
temperature. For the cluster in solution, by lowering of the
temperature, the intensity of the CC emission progressively
increased to 80 K (Figure 4b). A blue shift of the band was
observed with λmax = 595 nm at 290 K and λmax = 560 nm
below 150 K. This phenomenon is due to solidification of the
solvent after the glass transition (TDMF = 212 K) and is also
attributed to the “rigidochromic effect” previously mentioned.
At 180 K, the XLCT emission band appeared at λmax = 420 nm,
and below 80 K, its intensity progressively increased with the
concomitant decrease of the CC band. At 80 K, the intensities
of the two emission bands were similar, giving the purple
emission observed in liquid nitrogen by the addition of the
yellow and blue lights (Figure 3d). At 20 K, the XLCT band
was largely predominant, opposite to the cluster in the solid
state.
Figure 3. Photographs of 2: in powder at room temperature (a) under
ambient light and (b) under UV irradiation at 312 nm (UV lamp) and
dissolved in DMF (c) at room temperature and (d) in liquid nitrogen
at 77 K.
yellow light in powder and in solution (Figures 3b,c). The
thermochromic luminescence properties are revealed by
dipping the samples into liquid nitrogen. The yellow emission
of the cluster in solution turns bright purple (Figure 3d) and is
progressively recovered upon warming to room temperature,
indicating a completely reversible thermochromic lumines-
cence. This is different for these clusters in the solid state
whose the emission color does not change by lowering of the
temperature.
Emission and excitation spectra recorded for 2 in the solid
state and in solution from 290 K down to 20 K are shown in
Figure 4. At 290 K, the emission spectra display a single broad
emission band centered at 570 nm for the cluster in powder
Cluster 2 in solution thus exhibits the thermochromic
luminescence classically observed for [Cu I L ] clusters with
4
4 4
the two emission bands CC and XLCT, whose relative
5
intensities vary in temperature. This contrasts with the
luminescent behavior of the cluster in crystalline powder
because the appearance of the XLCT emission band seems to
be prevented. The communication between the two emission
states, whose populations are normally in thermal equilibrium,
seems to be perturbed in the solid state. This particular
emissive behavior could originate from the cluster assembly,
which induces “constraints” in the solid state, thus preventing
the communication between the two excited states. This can be
related to the efficient hydrogen-bonding network observed
between the clusters in the crystal structure, but the exact effect
responsible for the XLCT band quenching is difficult to
evaluate at this stage. Another unexpected emissive behavior is
and at 595 nm in solution (λ = 300 nm). This is in agreement
ex
with the yellow light observed at room temperature, which can
be attributed to the “cluster-centered” (CC) emission band
24
characteristic of these clusters. The 25 nm shift observed is
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Inorganic Chemistry
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the absence of CC band shift upon lowering of the temperature
both in the solid state and in solution. This is surprising
because such cubane clusters usually present a red shift due to
shortening of the Cu−Cu bond at low temperature. This
phenomenon can be attributed to the chelating character of the
ligand, which limits the flexibility of the cluster, thus preventing
Cu−Cu bond variation.
Finally, cluster 2 was reacted with an alkoxysilane derivative
in order to demonstrate the reactivity of the silanol groups
toward further condensation. Upon reaction with TMOS, a
white solid (3) was precipitated and characterized (NMR,
FTIR, EDX, and elemental analysis in the Experimental Section
glass substrate, a transparent and emissive “sol−gel” thin film
can be obtained with the thermochromic luminescence
properties of 3 preserved (corresponding spectra reported in
Figure S4 in the SI).
25
CONCLUSION
■
In conclusion, this siloxanol cluster, perfectly characterized, is a
neat building block for the synthesis of “sol−gel” materials. The
presence of eight SiOH groups per molecular unit assures the
reactivity of the cluster toward further condensation reactions
and thus the grafting of molecules of potential interest. This
highly luminescent cluster whose emission properties vary with
the temperature and chemical surroundings has thus great
potential for the synthesis of light-emitting materials, in sensor
applications for instance. In this prospect, the preparation of
functional luminescent materials using this cluster as an
emissive building block is currently in progress.
2
3
1
2
and SI). The appearance of new peaks (T , T , Q , and Q ) in
the solid-state 2 Si NMR spectra along with P signals
identical with those of 2, is consistent with the reaction of the
silicon atoms without cluster alteration (spectra in the SI).
These new Si peaks correspond to the condensation product
of one (T ) and two (T ) TMOS molecules per Si(OH) group
9
26
31
29
2
3
2
27
(
details in the SI). This leads to a mixture of condensation
ASSOCIATED CONTENT
■
products for 3 with six TMOS molecules on average per cluster.
These results confirm the covalent attachment of the
alkoxysilane derivative to the cluster via the Si−O−Si bond
and thus the reactivity of the SiOH groups.
The photoluminescent properties of 3 were investigated. The
internal luminescence quantum yield measured at 290 K is 17%
*
S
Supporting Information
Characterization details and X-ray crystallographic data in CIF
format. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
(
λex= 340 nm). The temperature-dependent luminescence
■
spectra recorded for 3 are shown in Figure 5a. A behavior quite
Corresponding Author
E-mail: sandrine.perruchas@polytechnique.edu. Phone: (+33)
0)1 69 33 46 85. Fax: (+33) (0)1 69 33 47 99.
*
(
ACKNOWLEDGMENTS
■
The authors thank the CNRS for funding.
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■
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2
ex
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2
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(
8
(
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1
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7
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(
(
1
(
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7
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