When Cu4I4 cubane meets Cu3(pyrazolate) 3 triangle: Dynamic interplay between two classical luminophores functioning in a reversibly thermochromic coordination polymer

Article abstract of DOI:10.1039/c1cc14303d

A supramolecular dual emissive system incorporating two classical copper(i)-cluster-based luminophores, namely, Cu₄I₄ and Cu₃Pz₃ (Pz = pyrazolate), is reported. The targeted luminescent coordination polymer exhi

Full text of DOI:10.1039/c1cc14303d

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Chemical Communications
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Volume 47 | Number 46 | 14 December 2011 | Pages 12405–12560
ISSN 1359-7345
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D. Li et al.
When Cu I cubane meets Cu (pyrazolate) triangle: dynamic interplay
4
4
3
3
between two classical luminophores functioning in a reversibly
thermochromic coordination polymer
1359-7345(2011)47:46;1-6

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When Cu I cubane meets Cu (pyrazolate) triangle: dynamic interplay
4
4
3
3
between two classical luminophores functioning in a reversibly
thermochromic coordination polymerw
Shun-Ze Zhan, Mian Li, Xiao-Ping Zhou, Jun-Hao Wang, Ju-Rong Yang and Dan Li*
Received 17th July 2011, Accepted 16th August 2011
DOI: 10.1039/c1cc14303d
A supramolecular dual emissive system incorporating two
classical copper(I)-cluster-based luminophores, namely, Cu
and Cu Pz (Pz = pyrazolate), is reported. The targeted
excimeric [Cu Pz ] dimer of trimers (*Cu ) is usually responsible
3
3 2
6
9c,12
4
I
4
for the bright phosphorescence,
I
and they both involve
I
3
3
close Cu ÀCu interactions (also known as cuprophilicity) as
luminescent coordination polymer exhibits reversible thermo-
chromism spanning from green to orange-red.
crucial stabilizing forces; this offers a thermodynamic possibility
for the dynamic interplay between *Cu
studies indicate that *Cu usually emits at the wavelength of
540–580 nm upon excitation at 350–400 nm, whereas *Cu
usually exhibits higher-energy excitation of 270–320 nm and
4 6
and *Cu . (ii) Previous
4
1
0a
The role of coordination compounds in the territory of
supramolecular photochemistry is assured by the presence of
a number of classical, well investigated photofunctional metal
complexes that promise applications ranging from opto-
6
9
c
lower-energy emission of 630–720 nm; this avoids the coupling
of the two excited states, and ensures that they can be excited
1
–3
electronic devices to biomedical probes. In the burgeoning
4
field of luminescent coordination polymers, one vital quest will
by different wavelengths. (iii) *Cu
and *Cu are both triplet,
4 6
phosphorescent excited states with comparable microsecond
1
0a,13a
be to develop well-defined microenvironments for immobilizing
multi-cluster-based luminophores, and to realize the bimolecular
1
processes involving electron and/or energy transfer between
lifetimes,
which make them distinguishable by normal
instrumental probes.
Reported herein is a dual emissive supramolecular coordi-
different luminophores that lead to intriguing properties such
nation polymer, namely, [Cu
5-p-tolyl-pyrazolate), in which two classical coordination
luminophores, Cu and Cu Pz , are integrated via coordination
4 4 3 3 3 n
I (NH )Cu L ] (L = 3-(4-pyridyl)-
5,6
as dual emission. The scarcity of molecule-based dual emissive
5
compounds may be due to Kasha’s rule which permits only the
I
4 4
3
3
lowest excited state to emit, whereas in supramolecular systems it
would be possible to simultaneously incorporate different
luminophores. Nevertheless, it might be difficult to achieve
the dynamic interplay between different luminophores in a supra-
molecular system in light of thermodynamic consideration—
the thermal activation barriers are generally much higher than
those in molecular dual emissive systems.
bonds and stabilized via cuprophilic interactions. The crystalline
sample of the complex was prepared from the solvothermal
reaction of copper(I) iodide and the ligand in ethanol mixed
with a small amount of aqua ammonia at 180 1C, and it
%
crystallized in the R3 space group (see ESIw for details). The
overall structure exhibits a two-dimensional layer (Fig. 1)
constructed from Cu
building units, which are linked via the pyridyl groups in L.
The Cu cluster adopts a distorted cubane configuration
4 4 3 3
I and Cu Pz clusters as secondary
We have been interested in the construction of luminescent
coordination polymers by utilizing some classical copper(I)
4 4
I
7–9
clusters as essential and functional components. Among others,
7
,10,11
9,12,13
3
tetranuclear Cu
4
I
4
and trinuclear Cu
3
Pz
have been
extensively studied during past decades. These two classical
copper(I) clusters might be a pair of luminophores suitable
qfor demonstrating the above supramolecular dual emission
protocol due to the following reasons. (i) The major contri-
bution to the luminescent origin of Cu
4
I
4
1
is believed to be
0a,11a
whereas the
the cluster-centered excited states (*Cu
4
),
Department of Chemistry, Shantou University, Guangdong 515063,
P. R. China. E-mail: dli@stu.edu.cn; Fax: +86 754-82902767;
Tel: +86 754-82902741
w Electronic supplementary information (ESI) available: Synthetic
and crystallographic details, additional measurements, figures and
descriptions. CCDC 829986 (293 K) and 829987 (100 K). For ESI
and crystallographic data in CIF or other electronic format see DOI:
Fig. 1 Representation of the double-layer (blue and orange) packing
patterns of the reported coordination polymer immobilized with Cu
and Cu Pz as separated luminophores. [Cu (pyridyl) (NH )]: left circle,
Cu Pz : right circle. Cu –Cu interactions: green and dashed lines. Color
codes: Cu in red, I in purple, N in blue, C in black; all H atoms are omitted
except for the ones in coordinated NH shown in the left circle.
4 4
I
3
3
4
I
4
3
3
I
I
[
3
3 2
]
10.1039/c1cc14303d
3
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Chem. Commun., 2011, 47, 12441–12443 12441

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(
Fig. 1, left inserted circle) with Cu–Cu distances of 2.6490(5)
7
Three of the four
,11
˚
to 2.8043(6) A, similar to those reported.
Cu sites in Cu I are filled by the linking pyridyl groups in L,
4
4
and the remaining site is completed by an NH molecule
3
À1
(
confirmed by the IR vibration of 3367 cm and elemental
analysis). The Cu Pz triangle is constructed from three
3
3
I
pyrazolate groups edge-bridging three linear-coordinated Cu
ions (Fig. 1, right inserted circle). Compared with other
Cu Pz trimers, the present triangle is twisted, but the intra-
trimeric and inter-trimeric Cu–Cu distances (3.2104(6) and
3
3
9
,12,13
˚
3
.6459(7) A, respectively) both lie in normal ranges.
Viewed from the c axis, these Cu Pz triangles are connected
by the C -symmetry Cu cubanes through the pyridyl
3
3
3
4 4
I
groups, extending to a 2-D layer parallel to the ab plane.
Two adjacent 2-D layers are assembled into a double-layer
I
arrangement via cooperative, inter-layer Cu –Cu interactions
I
in the [Cu Pz ] units (Fig. 1, right inserted circle). The [Cu Pz ]
3 2
3
3 2
3
Fig. 2 Solid-state luminescent spectra upon excitation at 370 nm
(a) and 270 nm (b) and varying the temperatures from 50 to 298 K.
Photographs (c) showing luminescence thermochromism from green
to orange-red of the solid sample upon heating and cooling when
exposed to UV lamp (365 nm). Note the chromic processes are fast
dimers of trimers exhibit a centrosymmetric staggered stacking
9
c
mode, which is believed to be the energetically favored
1
2b
conformation indicated by theoretical calculation,
recently documented in a ligand-assisted [Cu Pz
Within the double-layer, one Cu Pz
faces another offset Cu Pz
while the Cu cubanes always face the voids of the adjacent
layer, and the coordinated NH groups attached to Cu I clusters
and is
3
3
]
2
discrete
3
triangle
1
3b
compound.
3
(
within 1 minute) and reversible.
3
3
triangle from the adjacent layer,
4
I
4
chromic luminescence at different excitation bands, and also
suggests that *Cu and *Cu are separately responsible for the
3
4 4
4
6
8
–13
penetrate into these voids. Further, these double layers pack
with each other along the c axis, but the stacking mode between
two double layers is different from that within one double-layer.
HE and LE emissions, consistent with previous studies.
Thus, the luminescence chromism (from bright yellow to dark
orange) of this dual emissive system is dependent on excitation
wavelength at room temperature.
Between two double layers, the Cu
of the adjacent double layer, while one Cu
another Cu from the adjacent double layer (see Fig. S2 and S3
in ESIw for additional figures and structural description). The
3
Pz
3
triangles face the voids
4
I
4
cubane faces
Furthermore, this dual emissive system shows luminescence
thermochromism that may originate from the dynamic interplay
4 4
I
4 6
between *Cu and *Cu . When excited at 370 nm and decreasing
distances between two neighbouring Cu
3
Pz
3
triangles from
the temperature from 293 K to 77 K, the luminescence changes
correspondingly from yellow to brighter green (Fig. 2c). This
can be explained by examining the temperature-varied emission
spectra (Fig. 2a), wherein the intensity of the HE band
dramatically increases (IHE : ILE = 10 at 50 K) and gradually
becomes the dominant contribution of the overall luminescence.
The LE band also decreases in intensity, especially in the
temperature range below 200 K where the LE peaks can hardly
be observed in the spectra. In comparison, when increasing the
temperature from 293 K to 413 K, the sample undergoes a
chromic process from bright yellow to dark orange-red (Fig. 2c),
which is believed to originate from the prevailing LE band in the
higher temperature range. The chromic process is very fast and
reversible upon cooling to room temperature, indicating that
such luminescence thermochromism may be due to a physical
process involving energy transfer through thermal activation,
other than a chemical process involving phase transition, as
evidenced by powder X-ray diffraction (Fig. S4 in ESIw).
Upon the excitation at 270 nm, the intensity increment
of the HE band upon lowering the temperature is much
smaller (IHE : ILE = 2.5 at 50 K) than that at 370 nm, and
the LE band intensity does not change significantly except for
reasonable thermal fluctuation (Fig. 2b). This is probable
because the population of the LE excited states mainly origi-
nates from the 270 nm excitation, which disfavours the
accumulation of the HE excited states at the first place of this
excitation.
˚
adjacent double layers are very long (B4.94 A), suggesting no
I
I
obvious Cu –Cu interaction between them. Cryogenic single
crystal structure measurement (100 K) and temperature varied
powder X-ray diffraction (293–523 K, Fig. S4 in ESIw) indicate
no crystalline phase transition upon varying temperatures.
The photophysical behavior of this bi-cluster-based lumines-
cent coordination polymer is interesting. The solid-state crystal-
line sample (purified, see ESIw) presents two major excitation
bands at ca. 270 nm and 370 nm (Fig. S5 in ESIw). The excitation
spectra are superposed by the solid-state diffuse reflectance UV-
Vis spectrum (Fig. S6 in ESIw) that covers a broad range from
200 nm to 450 nm, suggesting that more than a single excited state
may be populated. The excitation band at 270 nm is consistent
9c
with the excitation wavelengths of the *Cu series, whereas the
6
10a
excitation band at 370 nm corresponds to the *Cu excitations;
4
this implies that both luminophores are excited simultaneously.
As expected, dual emission, with lmax at 530 nm (higher-
energy, HE) and 700 nm (lower-energy, LE), is detected upon
excitation at both 370 nm (Fig. 2a) and 270 nm (Fig. 2b).
Nevertheless, the yellow emission excited at 370 nm is much
brighter than the orange emission excited at 270 nm (Fig. S7 in ESIw).
Careful examination of the room-temperature (298 K)
emission spectra reveals that the relative intensity of the HE
band is stronger than that of the LE band (IHE : ILE = 1.4)
when excited at 370 nm, whereas the situation is reversed
(
IHE : ILE = 0.83) when excited at 270 nm. This explains the
1
2442 Chem. Commun., 2011, 47, 12441–12443
This journal is c The Royal Society of Chemistry 2011

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4 4
whereas the intra-tetrameric Cu–Cu distances in Cu I are
11a
˚
reduced by less than 0.3 A at the excited state. This speculation
À1
is verified by the much larger Stokes shift of LE (22 751 cm
À1
)
than that of HE (8159 cm ) in this work.
Taken together, we have demonstrated and interpreted in
this work a supramolecular dual emissive and thermochromic
system incorporating two classical coordination luminophores,
4 4 3 3
Cu I and Cu Pz . The tunable dual emissive behavior of this
luminescent coordination polymer is the consequence of a thermal
equilibrium between two separated, competitive excited states.
The strategy presented herein will be taken advantage by us to
fabricate diverse photofunctional coordination polymers by finely
tuning the structure and property of each separated luminophore.
This work is financially supported by National Basic Research
Program of China (973 Program, 2012CB821700), the National
Natural Science Foundation for Distinguished Young Scholars of
China (20825102) and the National Natural Science Foundation
of China (20771072).
Fig. 3 Proposed potential energy diagram for the dual emission.
Notes: Dd : the deviation of inter-trimeric Cu–Cu distances in the
Cu Pz unit (LE) between the excited state and ground state; Dd
the deviation of intra-tetrameric Cu–Cu distances in the Cu unit
HE) between the triplet excited state and ground state; E: energy.
1
[
3
3
]
2
2
:
4 4
I
(
The emission lifetime measurements (see Table S3 in ESIw)
reveal a more complicated situation for the components and
Notes and references
the interplay of the excited states. At all detection wavelengths
1
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respectively) and various temperatures (from 50 K to 293 K),
the luminescence decays require fitting to triexponential equations
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9
c
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3
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6
1
0a
monoexponential decay, and literature
showing that the
4
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*
Cu exhibits relatively shorter lifetime (also at ms scale) and
4
its cluster-centered excited state is attributed to a combination
of iodide to copper charge transfer and d–s transitions, we
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1
tentatively assign the shorter t
1
and t
2
to *Cu
4
, and the longer
at various
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and N. D. McClenaghan, J. Am. Chem. Soc., 2007, 129, 8688;
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temperatures are larger when excited at 260 nm and monitored
3
to *Cu . The fractional contributions of t
6
3
(d) M. Nishikawa, K. Nomoto, S. Kume, K. Inoue, M. Sakai,
at 700 nm than those when excited at 360 nm and monitored at
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6
540 nm; this matches with the above excitation and emission data
and testifies our assignment of t to *Cu and t and t to *Cu .
3
6
1
2
4
We shall propose a photophysical model (Fig. 3) to illustrate
the dynamic interplay between *Cu and *Cu functioning in
4
6
this dual emissive and thermochromic complex. Upon excitation,
the two excited clusters of HE (*Cu , with two coupled
components) and LE (*Cu ) emit their characteristic maximum
8
4
6
4
7, 1711; (c) M. Li, Z. Li and D. Li, Chem. Commun., 2008, 3390.
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9
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1
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1
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9
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(
270 nm) can populate a major number of the LE excited state
and a minor number of the HE excited state, and the conversion
from LE to HE excited states is unlikely because the energy
1
barrier DE
of view, the difference of the photophysical behaviors between
Cu and *Cu derives from the different geometric flexibility
of the excited states of Cu and [Cu Pz . Previous theoretical
studies indicate that the inter-trimeric Cu–Cu distances in
2
1
is much higher than DE . From the structural point
*
4
6
I
4 4
3
3 2
]
9
c,12
˚
Cu Pz ] can contract up to 1.07 A at the excited state,
3 3 2
[
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 12441–12443 12443