Mechanochromic and thermochromic luminescence of a copper iodide cluster

Article abstract of DOI:10.1021/ja103431d

The mechanochromic and thermochromic luminescence properties of a molecular copper(I) iodide cluster formulated [Cu₄I₄(PPh ₂(CH₂CH=CH₂))₄] are reported. Upon mechanical grinding in a mortar, its solid-state emission properties are drastically modified as well as its thermochromic behavior. This reversible phenomenon has been attributed to distortions in the crystal packing leading to modifications of the intermolecular interactions and thus of the [Cu ₄I₄] cluster core geometry. Notably, modification of the Cu-Cu interactions seems to be involved in this phenomenon directly affecting the emissive properties of the cluster.

Full text of DOI:10.1021/ja103431d

Published on Web 07/26/2010
Mechanochromic and Thermochromic Luminescence of a Copper Iodide Cluster
,
†
‡
†
†
§
Sandrine Perruchas,* Xavier F. Le Goff, S e´ bastien Maron, Isabelle Maurin, Fran c¸ ois Guillen,
§
†
,†
Alain Garcia, Thierry Gacoin, and Jean-Pierre Boilot*
Laboratoire de Physique de la Mati e` re Condens e´ e (PMC), CNRS - Ecole Polytechnique, 91128 Palaiseau Cedex, France,
Laboratoire H e´ t e´ ro e´ l e´ ments et Coordination (DCPH), CNRS - Ecole Polytechnique, 91128 Palaiseau Cedex, France, and
Institut de Chimie de la Mati e` re Condens e´ e de Bordeaux (ICMCB) - CNRS, 87 AVenue du Docteur A. Schweitzer,
33608 Pessac Cedex, France
Received April 22, 2010; E-mail: sandrine.perruchas@polytechnique.edu; jean-pierre.boilot@polytechnique.edu
(
LE, around 550 nm) attributed to a combination of a halide-to-
Abstract: The mechanochromic and thermochromic lumines-
cence properties of a molecular copper(I) iodide cluster formulated
metal charge transfer (XMCT) and copper-centered d f s, p
transitions. This emission is called “cluster centered” (CC) as it
involves a [Cu I ] cluster centered triplet excited state. At low
4 4
temperature, this band is extremely weak and the emission is
dominated by a higher energy one (HE, around 420 nm) attributed
to a triplet halide-to-ligand charge-transfer (XLCT) excited state.
Here, we report on the mechanochromic luminescence properties
[
4 4 2 2 2 4
Cu I (PPh (CH CHdCH )) ] are reported. Upon mechanical
grinding in a mortar, its solid-state emission properties are
drastically modified as well as its thermochromic behavior. This
reversible phenomenon has been attributed to distortions in the
crystal packing leading to modifications of the intermolecular
4 4
interactions and thus of the [Cu I ] cluster core geometry. Notably,
4 4 2 2 2 4
of the [Cu I (PPh (CH CHdCH )) ] cluster whose photoemission
modification of the Cu-Cu interactions seems to be involved in
this phenomenon directly affecting the emissive properties of the
cluster.
properties are modified upon mechanical grinding. This molecular
cluster undergoes a great change of its solid state emission properties
and in particular shows modification of its thermochromic lumi-
nescence properties. This phenomenon is reversible since, upon
exposure to solvent or heating, this compound recovers its original
crystalline phase and emission properties. Study of the optical
properties and structure by X-ray diffraction and NMR analysis of
the cluster before and after the grinding process suggests that the
mechanochromic behavior originates from local distortions in the
crystal packing where the intermolecular arrangement is modified,
Luminescent materials based on transition-metal complexes have
been receiving increasing attention due to their wide field of
1
applications in, for instance, detection, sensing, biological labeling,
2
and display visualization devices. Controlling or tuning the
photoluminescence properties of these compounds by external
stimuli is particularly attractive in order to obtain photofunctional
materials. In this context, modification of the luminescence color
in response to mechanical or thermal stimuli is one strategy to
achieve this goal. Such materials exhibiting luminescence mecha-
4 4
thus affecting the Cu-Cu distances of the [Cu I ] cluster core. This
copper-based coordination cluster enlarges the small family of
photoluminescent mechanochromic compounds and represents the
first example of a mechano-switchable emissive compound com-
bining both mechanochromic and thermochromic luminescence
properties.
3
4
nochromism (piezochromism) or thermochromism have potential
applications in optical recording, memory, sensing pressure, or
5
motion systems.
In the literature, examples of luminescent mechanochromic materials
based on transition-metal complexes or organic compounds are
6
relatively limited. These materials are characterized by emission
wavelength change upon mechanical grinding or pressing of the solid
sample. The subsequent reversion to their original solid-state emission
is usually achieved upon thermal treatment or recrystallization.
Concerning transition-metal complexes, examples reported so far are
7
-12
mainly based on multinuclear gold(I) complexes.
Examples of a
13
14
15
zinc complex, one silver derivative, and two platinum complexes
can be also found in the literature.
During the course of studies on luminescent hybrid materials
4 4 2 2 2 4
Figure 1. Molecular structure of [Cu I (PPh (CH CHdCH )) ].
4 4 4
based on copper(I) iodide clusters namely [Cu I L ] (L ) phosphine
1
6
ligands), we have discovered the mechanochromic luminescence
The copper iodide cluster was synthesized in solution from CuI
and allyldiphenylphosphine and was obtained as a white crystalline
powder (95% yield see Experimental section in the Supporting
Information (SI)). Colorless crystals suitable for single crystal X-ray
diffraction analysis were also obtained, and the resulting molecular
structure is depicted in Figure 1 (view of the unit cell in Figure
S1). The cluster presents the classical cubane structure formed by
four copper atoms and four iodine atoms which alternatively occupy
the corners of a distorted cube. The phosphine ligands are
coordinated to each copper atom by the phosphorus atom. Intramo-
properties of one of them. Copper iodide clusters are well-known
1
7
for their thermochromic luminescence properties. This phenom-
enon is characterized by two emission bands of different energy
1
8
whose relative intensities vary in temperature. At room temper-
ature, the luminescence is usually dominated by a low energy band
†
Laboratoire de Physique de la Mati e` re Condens e´ e (PMC), CNRS - Ecole
Polytechnique.
‡
Laboratoire H e´ t e´ ro e´ l e´ ments et Coordination (DCPH), CNRS
- Ecole
Polytechnique.
§
Institut de Chimie de la Mati e` re Condens e´ e de Bordeaux (ICMCB) - CNRS.
10.1021/ja103431d  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 10967–10969 9 10967

C O M M U N I C A T I O N S
lecular bond lengths and angle values are close to those previously
reported for other copper iodide clusters coordinated with phosphine
is probably high because small constraints imposed on crystallites
by the manual grinding lead to a drastic change of the optical
properties of the whole sample.
Elemental analysis of the cluster both before and after grind-
ing gave the expected composition (based on the formula
19
derivatives, but relatively longer Cu-Cu distances (3.3904(7) and
.1792(6) Å) are observed.
The [Cu (PPh (CH CHdCH
3
4
I
4
2
2
2 4
)) ] cluster is a white powder
under ambient light (Figure 2). At room temperature and under
UV lamp irradiation (peak wavelength at 365 nm), a weak green
emission is observed as shown in Figure 2a. The mechanochromic
luminescence properties are revealed by grinding the solid sample
using a pestle. The weak green emission is then converted into a
much more intense yellow one (Figures 2b and 2c show partially
and completely crushed samples, respectively), while the color of
the sample remains white (Figure 2e). Upon dissolution of the
crushed powder in dichloromethane and subsequent evaporation
of the solvent, the compound is reverted to the initial pale green
emissive phase. The yellow emission is recovered when the solid
is ground one more time which indicates a completely reversible
phenomenon. After grinding, the weak green emission is also
restored by heating the crushed powder at 100 °C for 30 min. The
grinding process also affects the thermochromic luminescence
properties of the compounds. As shown in Figure 2d, at 77 K in
liquid nitrogen, the green emission of the pristine crystalline cluster
turns into a bright blue one under UV excitation. This is different
for the crushed compound, for which the room temperature yellow
emission is replaced by a purple one at 77 K (Figure 2f). When
both samples are progressively warmed up, the room temperature
emission is recovered, showing a completely reversible thermo-
chromic luminescence mechanism.
[Cu
4 4 2 2 2 4
I (PPh (CH CHdCH )) ] see SI) indicating that no chemical
reaction related to adsorption or removal of atmospheric molecules
3
1
1
was induced by the grinding. P and H liquid NMR spectra,
UV-vis. absorption spectra (λmax ) 300 nm in Figure S4), and
luminescence spectra at room temperature under UV excitation (λmax
) 615 nm in Figure S5) are similar whether the cluster is crystalline
or crushed before the dissolution, showing that the molecular structure
of the cluster is preserved in the crushed compound. Solid-state 31
MAS NMR analysis shows identical spectra for the compounds before
and after grinding with a single quartet originating from the
coupling of the four equivalent phosphorus atoms of the cluster (Figure
S3). This suggests that the yellow emissive compound is in low
concentration and not detectable.
P
63,65
31
Cu- P
4 4 2 2 2 4
Figure 3. PXRD (Cu KR) of [Cu I (PPh (CH CHdCH )) ] before and after
grinding measured at room temperature and calculated from the single crystal
data recorded at 150 K.
Solid state emission and excitation spectra were recorded for
the cluster before and after grinding from room temperature down
to 8 K (Figure 4a and 4b respectively and Table 1). At 275 K, for
the cluster before grinding, a broad emission band is observed at
Figure 2. Photos showing luminescence changes of [Cu
4 4 2
I (PPh -
(
CH CHdCH )) ] upon grinding and at different temperatures. (a) Cluster
2
2
4
before grinding and (b-c) cluster upon grinding under 365 nm (UV lamp)
at room temperature. Cluster (d) before and (f) after grinding under 365
nm (UV lamp) at 77 K. (e) Corresponding photo of (b) under ambient light
at room temperature.
λmax ) 530 nm under excitation at λex ) 360 nm (absolute quantum
yield Φ365 nm ) 2%). This is in agreement with the pale green
emission observed in Figure 2a. After grinding, a broad emission
band is observed at λmax ) 580 nm (λex ) 360 nm at 275 K, Figure
4b) in agreement with the yellow emission observed in Figure 2c.
This yellow band is much more intense as compared to the one of the
pristine crystalline powder (absolute quantum yield Φ365 nm ) 14%).
For both uncrushed and crushed clusters, these green and yellow
emission bands can be attributed to the CC low energy band (LE)
Powder X-ray diffraction (PXRD) diagrams were recorded for
the compound before and after grinding (Figure 3). The diffraction
peaks correspond to those calculated from the diffraction data of
the single crystal X-ray structure analysis confirming the purity of
the powdered sample. No new diffraction peak appears so no new
crystalline phase is formed during the grinding process. However,
the intensities of the diffraction peaks decrease and their widths
increase. The Rieltveld analysis indicates that the grinding process
induces a reduction of the crystalline domains down to 150 nm
whereas the microstrain contribution is negligible (details in SI).
After dissolution and crystallization of the crushed powder or after
heating at 100 °C for 30 min (Figure S2), the compound presents
an identical diagram of the initial phase confirming the reversibility
of the phenomenon. Differential scanning calorimetry (DSC)
measurement shows an exothermic peak occurring between 100
and 200 °C only for the compound after grinding which can be
assigned to this recrystallization phenomenon. As the heating
treatment anneals the grinding ‘effect’, one can suggest that the
yellow emissive species result from local distortions in the whole
crystalline structure. Note that their photoluminescence efficiency
4 4 4
common for [Cu I L ] type clusters (see introductary discussion).
At 8 K, for the cluster before grinding, an intense emission band
(30 times higher compared to the one at 275 K) is observed at λmax
) 440 nm. This emission band can be attributed to the XLCT high
energy band (HE) of these clusters. As shown in Figure 4a, by
increasing the temperature, the intensity of this blue band (HE)
progressively decreases with a concomitant red shift to progressively
reach λmax ) 530 nm at 275 K (LE). For the crushed cluster, two
emission bands with comparable intensities are observed at 8 K
(λex ) 360 nm): a structured one with a maximum at 470 nm (HE)
and a second less intense band at λmax ) 580 nm (LE) corresponding
to the one observed at 275 K. By increasing the temperature from
8 to 275 K, the intensity of the HE band progressively decreases
with a concomitant increase of the LE one without significant
wavelength shifts. Note that the blue emission band at 440 nm of
1
0968 J. AM. CHEM. SOC. 9 VOL. 132, NO. 32, 2010

C O M M U N I C A T I O N S
the one observed in the solid state for the cluster after grinding (λmax
)
580 nm) compared to the crystalline one (λmax ) 530 nm) and which
shows a similar luminescence thermochromic behavior to that of the
crushed solid. Thus, the mechanochromism luminescence observed
in this study seems to be related to cuprophilic interactions. This is
not surprising since most of the mechanochromic luminescent transi-
tion-metal complexes reported in the literature involve modulation of
7-11
aurophilic interactions.
4 4
In order to explain the origin of the modification of the [Cu I ]
core and thus of the Cu-Cu interactions, the crystalline structure
of the pristine powder has been analyzed in detail. However, no
intermolecular interactions directly involving the [Cu I ] core are
4 4
observed. Despite the presence of numerous phenyl groups in the
crystal structure, no typical π-π stacking interactions exist either.
Only several short CH · · ·H contacts involving the allyl groups of
ligands are detected (2.85 Å in Figure S7). This suggests that the
mechanical constraints induced by the grinding modify these
interligand interactions leading to a different cluster packing in the
crushed compound. These local distortions allow the relaxation of
Figure 4. Solid-state luminescence spectra of (a) uncrushed and (b) crushed
Cu (PPh (CH CHdCH )) ] recorded between 275 and 8 K.
[
I
4 4
2
2
2
4
the crystalline phase is also present in the spectrum of the crushed
compound at low temperature, in accordance with the remaining
crystalline cluster shown by the PXRD analysis.
4 4 2 2 2 4
Thus, the [Cu I (PPh (CH CHdCH )) ] cluster presents two
emission bands (LE and HE) with temperature dependent intensities,
particularly evident for the crushed compound. For the crystalline
cluster, it appears that the LE emission is somehow quenched as
its intensity is very low and does not increase upon heating. In this
case, the two different excited states (XLCT and CC) seem to be
not coupled anymore with independent population of the two states.
However, upon grinding, the LE emission is no longer quenched
and a thermal activated mechanism seems to be recovered with
possible energy transfer between the two excited states (this is even
more evident when luminescence spectra are recorded at λex ) 315
nm as shown in Figure S6). By exhibiting the typical thermochromic
luminescence of copper iodide clusters, this study constitutes another
4 4
the [Cu I ] core, which is probably sterically constrained in the
crystalline phase, and drastically modify the luminescence proper-
ties. This phenomenon clearly appears as a competition between
the crystalline cohesion and the intracluster interactions. Note that
this copper cluster is the first compound presenting both mecha-
nochromic and thermochromic luminescence, and further photo-
physical studies combined with structural analysis are currently in
progress to confirm our mechanism assumption.
Acknowledgment. The authors thank C. Roch (Institut Lavoisier
de Versailles UMR 8180 CNRS-Universit e´ de Versailles Saint-
Quentin en Yvelines) for the DSC measurements.
confirmation of the preservation of the [Cu
in the yellow emissive crushed compound.
4 4 4
I L ] cubane structure
Supporting Information Available: Experimental procedures for
the cluster synthesis, characterization data, and single crystal X-ray
diffraction crystallographic file (CIF format). This material is available
free of charge via the Internet at http://pubs.acs.org.
Table 1. Luminescence Properties of [Cu
4
I
4
(PPh
2
(CH
2
2 4
CHdCH )) ]
a
before and after Grinding
em
λ
max (nm)
ex
4 4 2 2 2 4
[Cu I (PPh (CH CHdCH )) ]
HE
LE
λ
max (nm)
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crushed
8 K
440 (1)
-
470 (1)
-
-
360
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360-315
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18
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4 4
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typical thermochromic luminescence properties observed for this cluster
21
family. The ‘relaxation’ of the Cu-Cu interactions in the crushed
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(
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615 nm in Figure S5) which takes place at a wavelength closer to
JA103431D
J. AM. CHEM. SOC. 9 VOL. 132, NO. 32, 2010 10969