Cu4I4 clusters supported by PλN- type ligands: New structures with tunable emission colors

Article abstract of DOI:10.1021/ic2015226

A series of Cu₄I₄ clusters (1-5) supported by two P^λN-type ligands 2-[(diRphosphino)methyl]pyridine (1, R = phenyl; 2, R = cyclohexyl; 3, R = tert-butyl; 4, R = iso-propyl; 5, R = ethyl) have been synthesized. Single crystal X-ray analyses show that all five clusters adopt a rare "octahedral" geometry. The central core of the cluster consists of the copper atoms arranged in a parallelogram with μ⁴-iodides above and below the copper plane. The copper atoms on the two short edges of the parallelogram (Cu-Cu = 2.525(2)-2.630(1) A) are bridged with μ²-iodides, whereas the long edges (Cu-Cu = 2.839(3)-3.035(2) A) are bridged in an antiparallel fashion by the P ^λN ligands. This Cu₄I₄ geometry differs significantly from the "cubane" and "stairstep" geometries reported for other Cu₄I₄L₄ clusters. Luminescence spectra of clusters 3 and 4 display a single emission around 460 nm at room temperature that is assigned to emission from a triplet halide-to-ligand charge-transfer (³XLCT) excited state, whereas clusters 1, 2, and 5 also have a second band around 570 nm that is assigned to a Cu₄I₄ cluster-centered (³CC) excited state. The structural and photophysical properties of a dinuclear Cu₂I ₂(P^λN)₂ complex obtained during the sublimation of cluster 3 is also provided.

Full text of DOI:10.1021/ic2015226

Article
pubs.acs.org/IC
Cu4I4 Clusters Supported by P∧N-type Ligands: New Structures with
Tunable Emission Colors
Zhiwei Liu, Peter I. Djurovich, Matthew T. Whited, and Mark E. Thompson*
Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States
S
* Supporting Information
ABSTRACT: A series of Cu4I4 clusters (1−5) supported by two P∧N-type ligands
2-[(diRphosphino)methyl]pyridine (1, R = phenyl; 2, R = cyclohexyl; 3, R = tert-
butyl; 4, R = iso-propyl; 5, R = ethyl) have been synthesized. Single crystal X-ray
analyses show that all five clusters adopt a rare “octahedral” geometry. The central
core of the cluster consists of the copper atoms arranged in a parallelogram with μ4-
iodides above and below the copper plane. The copper atoms on the two short
edges of the parallelogram (Cu−Cu = 2.525(2)−2.630(1) Å) are bridged with μ2-
iodides, whereas the long edges (Cu−Cu = 2.839(3)−3.035(2) Å) are bridged in an
antiparallel fashion by the P∧N ligands. This Cu4I4 geometry differs significantly
from the “cubane” and “stairstep” geometries reported for other Cu4I4L4 clusters.
Luminescence spectra of clusters 3 and 4 display a single emission around 460 nm
at room temperature that is assigned to emission from a triplet halide-to-ligand charge-transfer (3XLCT) excited state, whereas
clusters 1, 2, and 5 also have a second band around 570 nm that is assigned to a Cu4I4 cluster-centered (3CC) excited state. The
structural and photophysical properties of a dinuclear Cu2I2(P∧N)2 complex obtained during the sublimation of cluster 3 is also
provided.
INTRODUCTION
■
Copper(I) complexes have attracted considerable interest
because of their rich structural and photophysical properties1−3
and potential applications as inexpensive, abundant materials in
optoelectronics,4,5 catalysis,6,7 and biological systems.8 In
particular, complexes based on copper(I) iodide and pyridine
(py) ligands display remarkable structural diversity. For
example, complexes with nuclearities ranging from mono-
cluster, and (d) a new type of Cu4I4 cluster, based on crystallographic
Figure 1. Illustrations of the copper iodide cores of (a) Cu2I2 cluster,
(b) cubane-like Cu4I4 cluster, (c) repeat unit of a stairstep (CuIL)∞
metallic (e.g., CuI(3-Mepy)3 (3-Mepy = 3-methylpyridine)) to
polymetallic (e.g., (CuIpy)∞) have been prepared from the
simple combination of copper(I) iodide and pyridine-type
ligands in different ratios, although the most common are
bimetallic (Cu2I2L4), tetrametallic (Cu4I4L4), and polymeric
((CuIL)∞) clusters.9−12 Among these complexes, Cu4I4L4
clusters have been extensively investigated because of their
peculiar photoluminescence properties.1,13−17
The most common motif for Cu4I4L4 clusters reported in the
literature is a cubane-like structure consisting of a copper
tetrahedron with iodides capping the four faces (Figure 1b).
This geometry can also be viewed as a pair of Cu2I2 fragments
with their Cu−Cu axes oriented perpendicular to each other
(Figure 1a). Generally, the cubane cluster exhibits two distinct
emission bands, one at high energy (ca. 450 nm) and the other
at low energy (ca. 600 nm), arising from halide-to-ligand
charge-transfer (3XLCT) excited states and triplet cluster-
centered (3CC), respectively.18−20 The former band is quite
prominent at low temperature and is strongly influenced by the
electronic structure of ligand. The latter band dominates at
room temperature in clusters with Cu−Cu distances less than
2.8 Å (the van der Waals radii of Cu is 1.4 Å,21 although it is
structural data, Cu, cyan; I, purple. The solid lines show the bonding
within the Cu2I2 fragment, and the dashed lines in (b), (c), and (d)
illustrate the bonding between Cu2I2 fragments (based on close atom−
atom contacts in the crystal structures).
noteworthy that the value was recently re-evaluated to be 1.92
Å22−24).
The Cu4I4L4 cluster has also been found in a “stairstep” or
“distorted stairstep” configuration, a motif constructed from the
association of two Cu2I2 fragments along Cu−I edges (Figure
1c).25−36 In the stairstep structure, Cu−Cu distances are
typically longer than 2.8 and hence there are no substantial
interactions between Cu ions. Thus, such stairstep complexes
exhibit luminescence only from the 3XLCT excited state.
Although Cu4I4L4 clusters with distorted stairstep structures
and short Cu−Cu distances in the 2.65−2.8 Å range have been
described in the literature,25−29 no reports of emission with
3CC characteristics have appeared for these derivatives.
Received: July 18, 2011
Published: December 1, 2011
© 2011 American Chemical Society
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In this paper, we report a series of Cu4I4(P∧N)2 clusters,
supported by bridging 2-[(diorganophosphino)methyl]pyridine
(P∧N) ligands, that have a rare “octahedral” geometry. This
structure can be thought of as being built by association of two
Cu2I2 fragments along Cu−Cu−I faces, leading to a parallel
orientation of the Cu−Cu axes in the two fragments (Figure
1d). This geometric arrangement differs significantly from the
aforementioned cubane and stairstep Cu4I4L4 clusters. Both
short (2.52−2.63 Å) and long (2.84−3.04 Å) Cu−Cu
interactions are found in these clusters. In the solid state at
room temperature, it is found that the emission color of these
Cu4I4(P∧N)2 clusters can be tuned from blue to white by
altering the organic groups on the phosphine moiety in the
P∧N ligand.
N, 2.09. Found: C, 32.27; H, 4.16; N, 2.33. Cluster 3: Pale yellow
crystals (28%) were obtained in 3 h; Anal. Calcd. for
C28H48Cu4I4N2P2: C, 27.20; H, 3.91; N, 2.27. Found: C, 27.26; H,
3.74; N, 2.24. Cluster 4: Pale yellow crystals (30%) were obtained in 3
h; Anal. Calcd. for C24H40Cu4I4N2P2: C, 24.42; H, 3.42; N, 2.37.
Found: C, 24.53; H, 3.13; N, 2.41. Cluster 5: White crystals (17%)
were obtained in 2 days; 1H NMR (400 MHz, CDCl3): δ 9.06 (d, J =
5.2 Hz, 2H), 7.72 (t, J = 8.0 Hz, 2H), 7.30 (t, J = 7.6 Hz, 2H), 7.21 (d,
J = 7.6 Hz, 2H), 3.53 (d, J = 7.2 Hz, 4H), 1.73 (m, 8H), 1.15 (m,
12H). Anal. Calcd. for C20H32Cu4I4N2P2: C, 21.37; H, 2.87; N, 2.49.
Found: C, 21.39; H, 2.55; N, 2.45.
Synthesis of Complex 3a. The complex 3a was obtained by
gradient sublimation of the cluster 3 under high temperature (∼ 200
1
°C) and low pressure (10−6 Torr). Yield: ca. 20%. H NMR (400
MHz, CDCl3): δ 8.91 (d, J = 4.8 Hz, 2H), 7.68 (t, J = 7.6 Hz, 2H),
7.32 (d, J = 8.0 Hz, 2H), 7.25 (t, J = 6.0 Hz, 2H), 3.23 (d, J = 7.6 Hz,
4H), 1.28 (d, J = 13.6 Hz, 36H). 31P NMR (400 MHz, CDCl3): δ 28.4.
Anal. Calcd. for C28H48Cu2I2N2P2: C, 39.31; H, 5.66; N, 3.27. Found:
C, 39.37; H, 5.64; N, 3.28.
X-ray Crystallography. Clusters 1−5 and complex 3a were
obtained as crystals and found suitable for X-ray measurement. The
crystals were mounted on a glass fiber with Paratone-N oil. X-ray
diffraction data were collected on a Bruker SMART APEX
diffractometer using graphite-monochromated Mo Kα radiation, and
structures were determined using direct methods with standard
Fourier techniques using the Bruker AXS software package. In some
cases, Patterson maps were used in place of the direct methods
procedure. Detailed crystal parameters and structure refinements are
given in the Supporting Information.
Photophysical Measurements. Photoluminescence spectra
were measured using a PTI QuantaMaster model C-60SE
spectrophotometer equipped with a 928 PMT detector. Phosphor-
escent lifetimes were measured by time-correlated single-photon
counting using an IBH Fluorocube instrument equipped with a 331
nm LED excitation source. Quantum yield measurements were carried
out using a Hamamatsu C9920 system equipped with a xenon lamp,
calibrated integrating sphere and model C10027 photonic multi-
channel analyzer.
EXPERIMENTAL SECTION
■
General Procedures. Chemicals were received from commercial
resources (i.e., CuI and n-BuLi were purchased from Aldrich,
chlorodiorganophosphines were purchased from TCI) and used as
received. Solvents were degassed prior to use, and tetrahydrofuran
(THF) was additionally dried over sodium and distilled. All reactions
were performed under N2 atmosphere. 1H NMR spectrum was
recorded on a Varian-400MR NMR spectrometer. Chemical shift data
for each signal are reported in ppm units with CDCl3 as reference,
where δ (CDCl3) = 7.26 ppm. Elemental analyses were performed by
the SCS Microanalysis Laboratory at the University of Illinois using a
Model CE 440 CHN Analyzer.
Synthesis of P∧N-Type Ligands. The P∧N-type ligands were
synthesized as shown in Scheme 1, similar to the literature
Scheme 1. Synthesis of P∧N-Type Ligands and
Corresponding Cu4I4 Clusters 1−5
Computational Methods. All of the molecular orbital calcu-
lations were performed with the Titan (version 1.0.7) software
package at the B3LYP level using a LACVP** basis set. Geometric
coordinates taken from X-ray crystal structures were used for single-
point, density functional theory (DFT) calculations.
RESULTS AND DISCUSSION
■
The P∧N-type ligands were synthesized from a reaction of
lithio-2-picoline (obtained by metalation of 2-methylpyridine)
with chlorodiorganophosphine using a modified literature
procedure.37,38 Considering that P∧N-type compounds are
readily oxidized in air,39 the ligands were used directly without
further purification in the subsequent reaction. The five clusters
1−5 were obtained as pale yellow to yellow crystals that
precipitated from mixed solutions (CuI in CH3CN and ligand
in CH2Cl2) over a period of hours to days. Clusters 1−4 are
poorly soluble in common solvents (toluene, CH2Cl2, THF),
whereas 5 is soluble enough that NMR spectra can be obtained
in CDCl3.
procedure.37,38 To a stirred solution of 2-picoline (0.93 g, 10 mmol)
in THF (30 mL) was added dropwise n-BuLi (1.6 M in hexanes, 6.25
mL, 10 mmol) at −78 °C. The mixture was allowed to warm to room
temperature for 2 h and then cooled to −78 °C, followed by slow
addition of R2PCl (10 mmol) in THF (10 mL). The mixture was
stirred overnight at room temperature. Degassed water (30 mL) was
added, though the water did not mix with the THF because of the high
concentration of LiCl. The organic layer was separated and dried with
Na2SO4. The THF was removed by distillation under reduced
pressure, and the resulting oily compound was dissolved in 5 mL of
degassed CH2Cl2. The ligands were used in the next reaction step
without further purification.
Synthesis of Clusters 1−5. Solutions of CuI in CH3CN and
P∧N-type ligand in CH2Cl2 were filtered through syringe filter (0.45
μm) and the two solutions mixed. The molar ratio of CuI to P∧N
ligand was 4: 1. The solutions were left standing at room temperature.
Clusters were obtained as crystals in times ranging from hours to
several days with yields averaging 30% over the two steps (based on 2-
picoline). Cluster 1: Pale yellow crystals (43%) were obtained in 12 h;
Anal. Calcd. for C36H32Cu4I4N2P2: C, 32.85; H, 2.45; N, 2.13. Found:
C, 32.78; H, 2.09; N, 2.10. Cluster 2: Yellow crystals (31%) were
obtained in 6 h; Anal. Calcd. for C36H56Cu4I4N2P2: C, 32.25; H, 4.21;
Samples of 1−5 isolated directly from the reaction mixture
were examined by single crystal X-ray analysis. The copper
clusters have a common structural motif that differs from either
the cubane or the stairstep configurations, and is similar to that
reported for a related complex with bridging P∧P-type ligands,
Cu4I4(dcpm)2 (dcpm = bis(dicyclohexylphosphino)-
methane).40 Figure 2 shows the structure of cluster 3 as a
representative example. Unlike cubane-type Cu4I4L4 clusters
(cf. Figure 1b), derivatives 1−5 all exhibit rigorous Ci
crystallographic symmetry. The four copper atoms are arrayed
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shorter than those found in Cu2I2 clusters coordinated with
monodentate N-heteroaromatic ligands (2.872−3.303 Å)41 or
diphosphine ligands (2.898 Å),42 but are comparable with those
in bimetallic complexes supported by 1,2,3-triazole (2.530 Å),25
benzimidazoles (2.546 Å),43 2-benzoyl pyridines (2.587 Å),44
1,8-naphthyridine (2.61−2.63 Å),45 and related nitrogen donor
ligands.46−51 The steric constraints imposed by the bridging
P∧N-ligands also influence the separation between A and B
fragments in the five Cu4I4 cores. Compounds 3 and 4 have the
shortest distance between Cu1A−Cu2B (2.839(3) and
2.880(1) Å, respectively), whereas the longest separation is
found in 5 (3.035(2) Å). The latter value is similar to that
reported for Cu4I4(dcpm)2 with a P∧P bridging ligand (Cu−Cu
= 3.005(1) Å).40 Likewise, short Cu(A)−I(B) distances are
found in 3 (Cu1A−I2B = 2.647(1) Å) and 4 (Cu1A−I2B =
2.804(1) Å) while the longest value is seen in 5 (Cu2A−I2B =
3.249(2) Å).
Figure 2. ORTEP drawing of the cluster 3 with ellipsoids at the 40%
probability level, hydrogen atoms are omitted for clarity.
The absorption spectra of the compounds 1−4, recorded as
saturated solutions in dichloromethane, as well as that of
compound 5 are shown in Figure 4a. Compounds 2 and 3 have
extremely poor solubility; their saturated solutions have
maximum absorbance of only about 0.1, while those of the
compounds 1 and 4 are about 0.9. The spectra, which resemble
those reported for cubane Cu4I4L4 clusters (L = pyridyl or
organophosphine),18,52 are characterized by a broad absorption
band at 280 nm, assigned to ligand-centered transitions, and
featureless transitions at longer wavelengths (>300 nm) that
gradually decrease in intensity, assigned to halide-to-ligand
charge-transfer transitions. The low energy transitions can be
observed down to 430 nm in the excitation spectra recorded
from neat solids (Figure 4a). Note though that the lineshapes
of the excitation spectra do not parallel the absorption spectra
at shorter wavelengths because of the strong self-absorption
that occurs from the high molar concentration in the solid state
samples.
in a parallelogram with obtuse angles that vary from 90.81° for
1 to 98.50° for 5. The Cu4 array is μ4-capped by two iodine
atoms, which forms a distorted octahedron, and has two
additional iodine atoms μ2-bound on each short side of the
parallelogram. The P∧N-type ligands are coordinated to
separate metals on opposing long sides of the Cu4 array in a
C2-symmetric arrangement. The Cu−N bond lengths range
between 1.999−2.018 Å (average = 2.007(3) Å), while the Cu−
P bond lengths fall between 2.209−2.237 Å (average =
2.220(1) Å) and are similar to the values observed in
Cu4I4(dcpm)2 (Cu−P = 2.218−2.224 Å).40
The Cu4I4 core structures of 1−5 are shown in Figure 3
along with selected bond distances. The clusters can be viewed
as being composed of two Cu2I2 dimers (see Figure 1a),
denoted A and B. The dimers are associated along their Cu−
Cu−I faces such that their respective Cu−Cu axes are oriented
in a parallel fashion (see Figure 1d) and have fold angles that
range between 48.43° for 3 to 55.28° for 4. The average
distance between Cu and μ2-bound iodide (2.651(1) Å) is
markedly shorter than to the μ4-bound iodide (2.725(1) Å).
The Cu1A−Cu2A distances vary from 2.525(2) to 2.630(1) Å
and are comparable to the equivalent Cu−Cu distance (2.576
Å) found in Cu4I4(dcpm)2.40 These Cu−Cu lengths are much
Luminescence spectra for the clusters 1−5 recorded in the
solid state at room temperature and 77 K, and in 2-MeTHF
solution at 77 K, are shown in Figure 4b−d; data are
summarized in Table 1. At room temperature, the clusters
display varied luminescence with quantum yields (ΦPL) ranging
Figure 3. Illustration of Cu4I4 core structure of the clusters 1−5: Cu, cyan; I, purple. Inset table: selected bond distances (Å) in the five clusters.
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Figure 4. (a) Absorption spectra in dichloromethane solution for 1−5 along with excitation spectra in the solid state monitored at λem = 460 nm,
(b−d) are normalized emission spectra at (b) room temperature in the solid state, (c) 77 K in the solid state, and (d) 77 K in 2-MeTHF solution. An
excitation wavelength of 350 nm was used to record spectra (b−d).
Table 1. Photoluminescent Quantum Yield (ΦPL), Peak Emission Wavelengths (λem), and Lifetime (τ) for Clusters 1−5
crystalline powder (298 K)
crystalline powder (77 K)
λem (nm) τ (μs)
16.4
2Me-THF solution (77 K)
a
a
ΦPL
λem (nm)
τ (μs)
λem (nm)
τ (μs)
1
2
3
4
5
0.43
0.12
0.56
0.13
0.08
460/580
470/560
460
3.4/3.5
470
480
470
470
460
470/560
470
26.6/17.9
13.6
b
c
d
0.3, 1.4 /0.4, 5.9
6.6, 18.8
20.3
1.6
480/560
480/600
470/560
29.4/31.4
24.9/23.0
24.9/29.3
e
460
0.6, 5.5
15.9
f
450/570
0.5/0.5, 2.4
19.1
a
The maxima for the emission band at lower energy were determined by Gaussian fitting. In the biexponential decays, the percentage of the longer
b
c
d
lifetime is as found in the following five footnotes. Percentage of the longer lifetime is 55%. Percentage of the longer lifetime is 86%. Percentage
of the longer lifetime is 81%. Percentage of the longer lifetime is 24%. Percentage of the longer lifetime is 26%.
e
f
18
from 0.08 to 0.56 in microcrystalline powders. The five clusters
can be divided into two categories on the basis of their emission
spectra in the solid state (Figure 4b): the first group shows only
a high energy (HE) emission band around 450−470 nm (3 and
4), while the second shows an additional low energy (LE)
emission band (or shoulder) near 560−580 nm (1, 2, and 5).
In the five clusters, both the HE and LE luminescence
transients decay on the microsecond time scale (Table 1),
implying that these transitions originate from triplet excited
states. The emission spectra of 1−5 differ from that of
observed here is related to that displayed by the Cu4I4(py)4
40
and Cu4I4(dcpm)2 complexes, except that 1 shows dual
emission at room temperature, whereas Cu4I4(py)4 and
Cu4I4(dcpm)2 display only LE emission under the same
conditions. Moreover, solid Cu4I4(dcpm)2 displays almost
equal amounts of HE (λem = 490 nm) and LE (λem = 650
nm) emission at 77 K.40 In 2-MeTHF solution clusters 1−5 are
nonemissive at room temperature, but bright luminescence is
observed at 77 K (Figure 4d). Clusters 3 and 4 display broad
emission centered at 500 nm with shoulders near 560 and 600
nm, respectively. Clusters 1 and 5 show dual emission bands
around 470 and 560 nm that are red-shifted and blue-shifted,
respectively, compared with their emission in the solid state at
room temperature. The cluster 2 gives an HE emission band at
470 nm along with a much weaker LE emission shoulder near
570 nm.
Cu4I4(dcpm)2, which only displays a LE emission band (λem
=
590 nm, ΦPL = 0.12, τ = 9.7 μs) at room temperature.40 When
the temperature is decreased to 77 K, no LE emission can be
observed from clusters 1, 2, and 5 (Figure 4c). The luminescent
thermochromism is most obvious for the cluster 1 (see
Supporting Information), giving a blue emission at 77 K and
white emission at room temperature that can be easily
distinguished by the naked eye. The thermochromic behavior
To gain insight into the emission properties, we performed
single-point DFT calculations of 1−5 using coordinates
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Figure 5. HOMO (transparent) and LUMO (mesh) frontier orbitals plots for the cluster 3 (a), and for the HOMO (b) and LUMO (c) of
octahedral Cu4I4.
obtained from the X-ray analyses. Calculations were also
performed on bare Cu4I4 clusters that were energy minimized
in both the cubane and the octahedral geometries. Theoretical
investigations of this type have proven to be very helpful in
establishing the nature of the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) of related copper complexes and aided in the
assignment of spectral transitions.13,17,52−54 The calculations
were performed with the Titan software package using a
B3LYP/LACVP** model. The five clusters have similar
frontier orbitals; Figure 5a shows representative HOMO and
LUMO plots for 3 (see Supporting Information for MO images
of the other clusters). As shown in the figure, the HOMOs are
mainly localized on the iodide 5p (48%) and copper 3d (43%)
orbitals, while the LUMOs are almost exclusively on the pyridyl
groups (82%) with minor participation from orbitals on the Cu
(7.7%) and P (4.4%) atoms. The contribution of these atomic
orbitals to the electronic structure of the HOMO is similar to
investigation of cubane Cu4I4L4 (L = pyridyl, organo-
phosphine) clusters proposed that both 3XLCT and 3CC states
can still be populated upon irradiation at room temper-
ature.13,52 Excitation to the XLCT state leaves the cluster
3
structure relatively unperturbed, whereas the Cu4I4 core in the
3CC configuration is significantly distorted from the ground
state geometry since this transition is characterized by a net
transfer of an electron from a set of antibonding 3d-orbitals to a
bonding combination of 4s-orbitals.52−54 The dual emission,
thermochromic, and rigidochromic behavior of the Cu4I4L4
clusters was explained by suggesting that the two electronic
configurations are separated by an energy barrier and therefore,
poorly coupled in the excited state at low temperatures. The
3CC state can only be thermally populated at higher
temperatures, which then leads to the observation of dual
emission. Thus, by analogy, the HE emission bands at 450−470
nm observed in clusters 1−5 can be ascribed to transitions from
an 3XLCT configuration. The LE emissions observed from 1, 2,
and 5 in the solid state at room temperature, and from 3 and 4
at 77 K in 2-MeTHF solution, are assigned to a distorted Cu4I4
that described for cubane derivatives, Cu4I4(PH3)4.53
A
calculation of the bare octahedral Cu4I4 cluster shows that
the energy, minimized without constraints, is 65.85 kcal/mol
higher than that of the bare Cu4I4 cubane. The Cu−I distances
for the μ2 (2.662 Å) and μ4 (2.873 and 2.888 Å) interactions
compare favorably to the corresponding bonds found in 1−5,
while the Cu−Cu distances similarly have short (2.479 Å, μ2-
bound) and long (2.986 Å) separations. The HOMO of the
bare octahedral Cu4I4 (Figure 5b) has an electronic structure
equivalent to that of 3 and is composed primarily of iodide 6p
orbitals (53.3%) and Cu 3d orbitals (41.3%) arranged in an
antibonding configuration. The LUMO (Figure 5c) consists of
orbitals on the iodide (6s, 36.2%; 6p, 17.5%) and Cu (4s,
34.9%; 4p, 10.7%; 3d, 0.07%) atoms and shows a net bonding
interaction between the Cu pair with long separation. For the
calculated structure of the triplet state of the bare octahedral
Cu4I4, the Cu−Cu distances for the μ2-bound copper atoms
expand to 2.682 Å, whereas the nonbridged Cu−Cu distances
contract to 2.566 Å. On the other hand, distances for the Cu−I
μ2 (2.676 Å) and μ4 (2.895 and 2.899 Å) interactions are only
slightly longer than in the singlet ground state. The change in
the Cu4 framework in the bare cluster is likely due to
population of σ-bonding 4s and 4p orbitals of the LUMO in the
cluster centered triplet (3CC) state.
3
based CC configuration. The same assignments can be made
to the equivalent HE and LE transitions reported for
Cu4I4(dcpm)2.40
While the luminescence from the 1−5 displays characteristics
3
3
similar to the XLCT and CC emission reported for cubane
Cu4I4L4 complexes, the thermochromic and ridigochromic
response of these two types of clusters are notably different.
The predominant blue luminescence observed from com-
pounds 1−5 at room temperature is in sharp contrast from the
near exclusive green-to-orange emission seen from cubane
Cu4I4L4 complexes under the same conditions. The variation in
the emissive behavior of 1−5 can be attributed to the effects of
unequal Cu−Cu bond lengths in the Cu4 array and to the rigid
coordination environment enforced by the bidentate P∧N
3
ligands that stabilizes the molecular geometry in the XLCT
state. It has been shown that the Cu−Cu interactions in cubane
Cu4I4L4 clusters, as estimated from Cu−Cu distances, are the
key parameter that influences the luminescence properties of
the 3CC transition.52,55 For cubanes with pyridine based ligands
that show strong 3CC transitions, the reported Cu−Cu
distances are in the range of 2.56−2.90 Å.10,56 However,
cubane derivatives with pyridyl57,58 or phosphine52 based
3
For clusters 1−5 the π-orbitals of the pyridyl ligands have
lower energies than the 4s, 4p σ-bonding combinations; thus a
halide-to-ligand charge transfer (XLCT) transition is the lowest
energy excited state when the complexes are in the ground state
geometry. Unoccupied molecular orbitals with significant 4s
character on the Cu atoms (8−10%) only appear in LUMO+4.
However, a previous DFT and time-dependent DFT (TDDFT)
ligands and Cu−Cu distances >2.90 Å can also display CC
transitions in the solid state at room temperature. Likewise,
there is a correlation between the presence of the 3CC emission
band in clusters 1−5 and the Cu−Cu distances in the crystal
structures. The three compounds that have the shortest Cu1A−
Cu2A distances within the dimer unit (2.525(2), 2.574(1), and
3
2.562(2) Å for 1, 2, and 5, respectively) show XLCT and
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Inorganic Chemistry
Article
3
variable amounts of CC emission at both room temperature
Complex 3a displays a strong green luminescence centered at
530 nm (ΦPL = 0.32, τ = 13 μs) in the solid state at room
temperature (Figure 7). This emission is significantly red-
and 150 K (the temperature at which the crystal structures were
collected, see Supporting Information). However, these
derivatives also have the longest Cu1A−Cu2B separation
between dimer units (2.943(1), 2.930(1), and 3.035(2) Å for 1,
2, and 5, respectively). Moreover, changes in just one pair of
Cu−Cu distances are insufficient to account for the LE
emission since clusters 3 and 4, which exhibit little-to-no LE
emission, also have short Cu1A−Cu2A separations that fall
within the range found for 3CC transitions. Since both
theoretical13 and experimental55 studies of the cubane clusters,
along with calculations of bare octahedral Cu4I4 (vida supra),
show that the Cu(A)−Cu(B) distances significantly decrease
3
when promoted to the CC excited state, the overall geometry
of the Cu4I4 core in 1, 2, and 5 must undergo some kind of
structural relaxation to produce the LE emission. For example,
a contraction of the longer Cu(A)−Cu(B) distances in the Cu4
array of 1, 2, and 5 would entail a concomitant deformation of
the bridging P∧N ligand to accommodate the shortened Cu−
Cu bonds. Any distortion of the P∧N ligand would likely be
influenced by the steric bulk of the organophosphine moiety.
Thus, the presence of LE emission in 1, 2, and 5, and
conversely the relative absence in 3 and 4 (with bulky tert-butyl
and iso-propyl groups), appears to be controlled by the extent
to which the P∧N ligand can warp and thereby allow the Cu4
framework to relax during formation of the excited state.
The thermal stability of 1−5 with respect to vapor deposition
under vacuum was examined by gradient sublimation at low
pressure (10−6 Torr). None of the clusters could be sublimed
intact. However, it is interesting to note that a binuclear
complex 3a is obtained during the sublimation of the cluster 3.
Moreover, sublimation of the complex 3a gave a mixture of 3
and 3a, indicating that there is a thermal equilibrium between
the two species in the gas phase. Figure 6 shows an ORTEP of
Figure 7. Emission spectra of 3a in solid state at room temperature
and 77 K. Inset: HOMO (transparent) and LUMO (mesh) plots for
the dimeric complex 3a.
shifted compared with luminescence from cluster 3 (λem = 460
nm), but comparable to that from the dimeric complex
Cu2I2(py)4 (λem = 517 nm).18 At 77 K, the emission maximum
of 3a is unchanged but the bandwidth narrows and the lifetime
increases to 38 μs. On the basis of DFT calculations (Figure 7,
inset) and similarity to the HE photophysical properties from
clusters 1−5, the emission of the complex 3a is assigned to a
3XLCT transition.
SUMMARY
■
Five tetranuclear copper(I) iodide complexes with the formula
Cu4I4(P∧N)2 (P∧N = 2-[(diRphosphino)methyl]pyridine; 1, R
= phenyl; 2, R = cyclohexyl; 3, R = t-butyl; 4, R = i-propyl; 5, R
= ethyl) have been synthesized. Structural characterization
shows that 1−5 adopt a octahedral geometry for the Cu4I4 core,
which has been rarely reported, that differs from the well-
known cubane and stairstep configurations. The Cu−Cu bond
lengths in the core of the Cu4 array of 1−5 have both short
(bridged by an μ2-iodide) and long (flanked by the P∧N
ligands) separations. Photophysical studies show that the
emission from this type cluster can be tuned from blue to
white in the solid state at room temperature by utilizing P∧N-
type ligands with differing steric demands of the organo-
phosphine group. DFT calculations suggest that P∧N ligands
serve to constrain relaxation of the long Cu−Cu distances in
the Cu4I4 core during formation of the excited state. Thermal
decomposition of cluster 3 occurs during sublimation and leads
to the formation of a dinuclear complex, Cu2I2(P∧N)2 3a,
which has been structurally and photophysically characterized.
Figure 6. ORTEP drawing of the complex 3a with ellipsoids at the
40% probability level. Hydrogen atoms are omitted for clarity. Selected
distances (Å): Cu1−Cu2, 2.795(1); Cu1−P1, 2.234(1); Cu1−N1,
2.124(3); Cu1−I1, 2.636(1); Cu1−I2, 2.595(1); Cu2−N2, 2.126(3);
Cu2−P2, 2.242(1); Cu2−I1, 2.641(1); Cu2−I2, 2.660(1).
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data in CIF format and summary of X-ray data
collection and refinement of the clusters 1−5, and the dimer
complex 3a; temperature dependent photoluminescence
spectra for the cluster 1; the DFT calculated HOMOs and
LUMOs of clusters 1, 2, 4, and 5; photoluminescence spectra
for the clusters 1−5 at 150 K. This material is available free of
charge via the Internet at http://pubs.acs.org.
the dimeric complex 3a. Each Cu ion in 3a has a distorted
tetrahedral geometry and is coordinated by the bidentate P∧N
ligand along with two bridging μ2-I atoms. The Cu−Cu
(2.795(1) Å) and Cu−N (2.124(3) Å) distances are
significantly longer than the respective values in cluster 3
(2.630 (1) and 2.018 (3) Å).
235
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Inorganic Chemistry
Article
(31) Cariati, E.; Roberto, D.; Ugo, R.; Ford, P. C.; Galli, S.; Sironi, A.
Inorg. Chem. 2005, 44, 4077−4085.
AUTHOR INFORMATION
Corresponding Author
*E-mail: met@usc.edu.
■
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25, 1791−1801.
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693−704.
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10043−10045.
ACKNOWLEDGMENTS
We wish to acknowledge Universal Display Corporation for
financial support of this work.
■
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