Crystal-to-crystal transformation between three CuI coordination polymers and structural evidence for luminescence thermochromism

Article abstract of DOI:10.1002/anie.200704349

Temperature-dependent Cu...Cu distances lead to the luminescence thermochromism of copper(I) coordination polymers with cubane-like Cu ₄X₄ cluster nodes (X = halogen) according to a study on three copper(I) coordination polymers with a bis-thioether ligand (L). The picture shows solid-state luminescence spectra and photographs of the two-dimensional network polymer [Cu₄I₄L₂] _n at 298 and 77 K. (Graph Presented)

Full text of DOI:10.1002/anie.200704349

Angewandte
Chemie
DOI: 10.1002/anie.200704349
Coordination Polymers
Crystal-to-Crystal Transformation between Three CuI Coordination
Polymers and Structural Evidence for Luminescence
Thermochromism**
Tae Ho Kim, Yong Woon Shin, Jong Hwa Jung, Jae Sang Kim, and Jineun Kim*
Recently, there has been considerable interest in the struc-
tural and photophysical properties of mono- and polynuclear
complexes of transition metals in oxidation states having the
d10 electronic configuration.[1] Particularly interesting among
these are complexes of CuI, which not only show a great
variety of structural forms but are also often brightly
luminescent even at room temperature. The luminescence
associated with attractive Cu···Cu (cuprophilic) interactions[2]
has been of great interest, both from experimental[3] and
theoretical[4] perspectives. Such complexes show an unusual
wealth of geometries and stoichiometries because of the
relatively small energy difference between the various
polymorphs, depending on synthetic conditions.[5] In addition,
a number of CuI coordination polymers based on copper(I)
iodide and bidentate ligands have been synthesized by self-
assembly reactions.[6] The structures of self-assembled coor-
dination polymers depend on a delicate balance between
alternative conformations of the organic ligands and on the
nature of the metals and anions,[7] as well as the solvent
employed.[5c,8] Our interest in CuI coordination chemistry is
mainly focused on thioether ligands.[6] The continuing interest
in the S-donor ligands and the scant research on their
copper(I) complexes prompted us to investigate the possibil-
ity of diverse structures and photophysical properties of
copper(I) complexes with a new bis-thioether ligand, namely,
2-(cyclohexylthio)-1-thiomorpholinoethanone (L). Although
a number of studies on the luminescence thermochromism of
cubane-like clusters Cu4X4 (X = halogen) have been repor-
ted,[1a,3a–d] there was no direct evidence that a change in
Cu···Cu distance is responsible for luminescence thermo-
chromism. Herein we report on the synthesis, crystal struc-
tures, crystal-to-crystal transformation,[9] and luminescent
properties of coordination polymers based on CuI and L.
Ligand L was synthesized by the literature method
(Scheme S1 in the Supporting Information; crystal data of L
are listed in Table S1, and an ORTEP view is shown in
Figure S1).[6b]
Self-assembly reaction between CuIand L under appro-
priate conditions produced three coordination polymers:
yellow nonluminescent 1, orange luminescent 2, and green
luminescent 3 (Scheme 1). The reaction of CuIand L in 1:1
molar ratio at room temperature yielded a product with the
formula [Cu2I2L2]n (1). Crystalline [Cu4I4L2]n (2) was also
[*] Dr. T. H. Kim, Prof. J. H. Jung, Prof. J. S. Kim, Prof. J. Kim
Department of Chemistry (BK21) and
Research Institute of Natural Science
Gyeongsang National University
900 Gajwa-dong, Jinju 660-701(South Korea)
Fax: (+82)55-758-5532
E-mail: jekim@gnu.ac.kr
Dr. Y. W. Shin
Test&Analytical Laboratory
Korea Food&Drug Administration
123-7 Yongdang-dong, Busan 608-829 (South Korea)
[**] T.H.K. was supported by the BK21 program.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Scheme 1. Syntheses and structural transformations of 1, 2, and 3.
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obtained as a minor product after polymer 1 was crystallized
acetonitrile solution with excess L under static or sonication
conditions, respectively. After 3 sank to the bottom from the
boundary surface (Figure S2c in the Supporting Information),
3 transformed into 1 or 2 depending on the solution
composition (n = 1 and n ꢀ 2, respectively), and the reverse
transformation was not possible. Besides the transformation
in solvent, crystal-to-crystal transformation without solvent
on heating 1, 2, or 3 (3’) at above 1808C resulted in polymer 2.
Powder X-ray diffraction (PXRD) patterns for 1, 2, and 3 (3’)
before and after heating are shown in Figure 1d and e,
respectively. After heating, the PXRD patterns of 1 and 3’ are
the same as that of 2, and no change occurred for 2. I n
addition, 1, 2, and 3’ after heating emitted orange light
(Figure 1c) like 2. The change in photoluminescence (Fig-
ure 1a–c) agrees with the change in PXRD patterns. After
heating, the luminescence spectra from all samples are the
same as the spectrum of 2 before heating. Therefore, it is
concluded that 2 has the highest thermal stability. (Lumines-
cence spectra before and after heating are shown in Figure S9
in the Supporting Information.)
from a solution of CuIand L in 2:1 molar ratio. Polymer 2 was
also prepared by addition of diethyl ether under conditions of
excess CuI. Crystals of [{Cu4I4L2}·CH3CN·n-C6H14]n (3) grew
from the boundary between immiscible acetonitrile solution
and a layer of n-hexane (Figures S2 and S3 in the Supporting
Information). In this case, 1 was formed in acetonitrile
solution with 1:1 ratio, while 2 was formed in acetonitrile
solution with n:1 ratio (n ꢀ 2; crystallographic data and
perspective views of the structures of 1–3 are given in the
Supporting Information, Tables S2–S4 and Figures S4–S6,
respectively).
In 1, rhombohedral Cu2I2 clusters are linked by the ligands
to form a polymeric one-dimensional (1D) loop chain.
Structural analysis of 2 revealed 2D undulating polymeric
networks with Cu4I4 cluster nodes. Although the quality of the
single-crystal X-ray data of 3 was lower due to molecules of
solvation (acetonitrile and n-hexane), the structure of 3 was
also determined to be a 1D zigzag loop-chain polymer with
Cu4I4 cluster nodes. Crystals of 3 lose the solvent at room
temperature to yield green luminescent [Cu4I4L2]n (3’). The
1D loop zigzag polymer 3 transforms into 2D network
polymer 2. Two S donor atoms of L in 2 and 3 are on the
same side (syn conformation) and opposite sides (anti
conformation) of the amide bond plane, respectively (see
Figure S7 in the Supporting Information). It seems that
sonication and loss of solvent from 3 led to a change in sulfur
conformation from anti to syn. Thus, S2 (or S4) in 3 can bind
to neighboring cubane clusters of other loop chains to form
the 2D network structure 2 (Figures S5, S6, and S8 in the
Supporting Information). Note that Cu2, Cu4, I2, and I4
positions in 2 are exchanged with respect to those of 3. As
shown in Scheme 1, polymer 1 was transformed into polymer
2 in acetonitrile solution with an excess of CuIat about 70 8C
under sonication conditions. On the other hand, polymer 2
was slowly or immediately transformed into polymer 1 in
Copper ions in 1 have distorted tetrahedral environment
with two iodido ligands and two sulfur donors in the
coordination shell. The Cu···Cu distance (2.98 Š) is longer
than the sum of the van der Waals radii (2.80 Š),[10] and thus
À
implies no cuprophilic interaction. The Cu S (2.278–2.531 Š)
À
and Cu Ibond lengths (2.503–2.972 Š) are within the range
of known values.[11]
The single-crystal X-ray data for 2 and 3 were collected at
four different temperatures to elucidate the relation between
luminescence thermochromism and Cu···Cu distance. Crys-
talline 3 was unstable at room temperature owing to loss of
solvent from the crystals. The Cu···Cu distances of 2 and 3 at
different temperature are listed in Table 1. A significant
structural feature of 2 is the relatively short Cu···Cu distances,
which are shorter than the sum of the van der Waals radii. The
Cu···Cu distances decrease with decreasing temperature,
Figure 1. Photographs of 1, 2, and 3’ before heating without (a) and with (b) UV irradiation, and after heating with UV irradiation (c). Powder X-
ray diffraction patterns of 1, 2, and 3’ before (d) and after (e) heating.
686
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Chemie
Table 1: Cu···Cu distances [Š] in 2 and 3 at four different temperatures.
298 K[a]
223 K
173 K
123 K
2
Cu1···Cu3
Cu1···Cu2
Cu3···Cu4
Cu2···Cu3
Cu1···Cu4
Cu2···Cu4
2.833(2)
2.789(2)
2.755(2)
2.740(2)
2.625(2)
2.631(2)
2.8052(17)
2.7642(17)
2.7446(17)
2.7267(17)
2.6213(18)
2.6169(17)
2.7878(13)
2.7513(13)
2.7400(13)
2.7182(13)
2.6221(14)
2.6107(13)
2.7733(10)
2.7403(11)
2.7410(10)
2.7111(11)
2.6292(11)
2.6079(11)
3
Cu2···Cu4
Cu1···Cu2
Cu3···Cu4
Cu1···Cu4
Cu2···Cu3
Cu1···Cu3
2.814(3)
2.815(3)
2.760(3)
2.748(3)
2.736(3)
2.672(3)
2.7930(13)
2.7871(13)
2.7469(13)
2.7321(12)
2.7234(13)
2.6594(12)
2.7809(11)
2.7752(11)
2.7344(11)
2.7175(11)
2.7091(11)
2.6501(10)
2.7713(10)
2.7652(10)
2.7235(10)
2.7030(10)
2.6981(10)
2.6432(10)
[a] Two runs (600 frames for each run) were collected due to crystal
decay.
À
whereas the Cu S bond lengths are nearly constant (see
Table S5 in the Supporting Information). In addition, some
À
À
À
À
other bond lengths (e.g., C15 C16, C3 C4, C5 C6, C1 C6,
À
À
À
C1 C2, S2 C11, and N1 C8) increase, though the lattice
constants decrease with decreasing temperature. Surprisingly,
longer Cu···Cu distances (Cu1···Cu3, Cu1···Cu2) are substan-
tially contracted, whereas shorter Cu···Cu distances
(Cu3···Cu4, Cu1···Cu4) show almost no change. This finding
can be explained in terms of fast contraction of the cubane-
Figure 2. Solid-state luminescence spectra and photographs of 2 (a)
and 3’ (b) at 298 (solid line) and 77 K (dashed line).
À
like Cu4I4 cluster and distances such as Cu···Cu, C2 C3, and
C14 C15, which causes substantial elongation of other bonds
Cu···Cu distances. According to theoretical works,[4a–e] the
À
À
À
À
À
À
À
À
(e.g., C15 C16, C3 C4, C5 C6, C1 C6, C1 C2, S2 C11, and
Cu Cu bonds in the excited state (LUMO) are of bonding
À
N1 C8) in 2. This effect may be related to the change in the
character. As the temperature decreases, the Cu···Cu dis-
tances become shorter, the bonding character increases, and
thus the energy levels are lowered. The energy difference
between the excited states and the ground state becomes
smaller. Thus, shorter-wavelength parts of the emission
spectra correspond to transition from excited state of longer
Cu···Cu bonds, which disappear with bond shortening. The
remaining longer-wavelength parts (600–650 and 525–600 nm
regions for 2 and 3’, respectively) of the emission spectra
correspond to transition from excited states of shorter Cu···Cu
photoluminescence spectrum of 2. Similar phenomena were
observed for 3, but the biggest change was observed for Cu I
À
distances (see Table S6 in the Supporting Information).
Coordination polymer 1 did not emit visible light under
UV irradiation. Solid-state emission spectra of powders of 2
and 3’ are shown in Figure 2. The emission bands could be
assigned to a combination of ligand-to-metal charge-transfer
(LMCT) and d–s transitions due to Cu···Cu interaction within
Cu4I4 clusters according to previous literature.[4a–e,12] The
maxima of the emission bands of 2 were observed at 538 and
599 nm (lex = 350 nm) at room temperature and 77 K, respec-
tively. The band shift of about 60 nm for 2 made detection of
the color change by the naked eye possible (Figure 2a, inset).
The maxima of the emission bands of 3’ were measured at 526
and 538 nm (lex = 286 nm) in the solid state at room temper-
ature and 77 K, respectively. The shift of the emission peak
(10 nm) and the bandwidth of 3’ are smaller than those of 2, so
there was no substantial color change (Figure 2b, inset). The
band widths are also related to the spread of Cu···Cu distances
(0.208 and 0.143 Š for 2 and 3, respectively), which vary from
2.625(2) and 2.672(3) Š to 2.833(2) and 2.815(3) Š at 298 K
for 2 and 3, respectively (Table 1). Hence, 2 shows broader
spectra than 3’. With decreasing temperature, the intensity of
the short-wavelength part (less than ca. 575 and ca. 525 nm for
2 and 3’, respectively) of the emission spectrum at 298 K
decreases. The peak shifts are related to shortening of the
bonds. Therefore, the wavelength of the emitted light (lmax
)
increases with decreasing temperature. This is direct evidence
that luminescence thermochromism of Cu4I4 compounds is
caused by temperature-dependent Cu···Cu distances. It seems
À
that no relation exists between Cu Idistances and lumines-
À
cence thermochromism, since the changes in Cu Idistance in
2 and 3 (from 0.017 and 0.133 Š to À0.005 and À0.102 Š,
respectively) are reversed in comparison with the shifts in
emission spectra (Tables S5 and S6 in the Supporting Infor-
À
mation). However, the Cu Idistances may affect the Cu···Cu
distances. The emission intensity at the long-wavelength side
does not change for 2, while it is increased for 3’. Nonradiative
and radiative processes may be involved in 2 and 3’,
respectively. These may be related to population transfer
À
mediated by Cu Ibonds, since the biggest difference between
À
2 and 3’ is the change in Cu Idistances with temperature
(Tables S5 and S6).
Angew. Chem. Int. Ed. 2008, 47, 685 –688
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In conclusion, we have shown how the solvent and molar
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ratio of reactants play important roles in the assembly of
different coordination polymers from CuIand L. Crystal-to-
crystal transformations with or without solvent have been
discussed. Also, we have demonstrated that temperature-
dependent variation of Cu···Cu distance is responsible for the
luminescence thermochromism of Cu4I4 coordination poly-
mers. Further study of these Cu4I4 compounds as well as other
CuIcoordination polymers is in progress.
Experimental Section
Steady-state luminescence spectra were acquired with a Perkin–
Elmer LS 50B spectrophotometer. The excitation and emission
spectra were corrected for the wavelength-dependent lamp intensity
and detector response, respectively. The pulsed excitation source was
generated using the 350-nm line of the xenon lamp for 2 and 3’.
Cooling in temperature-dependent measurements for the solid
materials was performed by using a liquid nitrogen tank.
X-ray diffraction experiments were performed in transmission
mode on a Bruker GADDS diffractometer equipped with graphite-
monochromated CuKa radiation (l = 1.54073 Š). Each diffraction
frame was collected at 258 intervals in the 2q range of 5–608 for 30 s at
a detector distance of 15 cm. The two frames were integrated from 58
to 608 and merged.
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Single-crystal diffraction data for L and 1 at 173 K and for 2 and 3
at four different temperatures (123, 173, 223, and 298 K) were
collected on a Bruker SMART CCD diffractometer equipped with
graphite-monochromated MoKa radiation (l = 0.71073 Š). The cell
parameters for the compounds were obtained from a least-squares
refinement of the spot (from 45 collected frames) using the SMART
program. The intensity data were processed using the Saint Plus
program. All of the calculations for the structure determination were
carried out using the SHELXTL package (version 5.1).[13] Absorption
corrections were applied by using XPREP and SADABS.[14] In most
cases, hydrogen positions were input and refined in a riding manner
along with the attached carbon atoms. CCDC 660759–660768 contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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[11] The values were retrieved from the 2006 edition of the Cam-
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Received: September 20, 2007
Revised: November 9, 2007
Published online: December 13, 2007
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Keywords: copper · luminescence · metal–metal interactions ·
S ligands · thermochromism
.
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