Bright Phosphorescence of a Trinuclear Copper(I) Complex: Luminescence
Thermochromism, Solvatochromism, and “Concentration Luminochromism”
H. V. Rasika Dias,*
,†
Himashinie V. K. Diyabalanage,
†
Manal A. Rawashdeh-Omary,
‡
Matthew A. Franzman,
‡
and Mohammad A. Omary*
,‡
Department of Chemistry and Biochemistry, The UniVersity of Texas at Arlington, Arlington, Texas 76019, and
Department of Chemistry, UniVersity of North Texas, Denton, Texas 76203
Received June 17, 2003; E-mail: omary@unt.edu; dias@uta.edu
Trinuclear d
10
complexes have garnered considerable interest in
recent years in large part because of their fascinating luminescence
properties.
1-5
Some of these compounds show very interesting
behavior in the presence of various reagents (solvents, heavy metals,
and Lewis acids). For example, Balch reported that a trinuclear
Au(I) complex exhibits “solvoluminescence”, a spontaneous orange
emission upon contact with solvent following irradiation with long-
wavelength UV light.
2a
Related complexes were found to form
charge-transfer complexes with nitro-9-fluorenones,
2b
and some
form hourglass figures on standing in air.
2c
Burini and Fackler
reported the formation of acid-base supramolecular stacks with
visible luminescence on interaction of nucleophilic Au(I) trinuclear
complexes with several types of electrophiles, including naked
heavy metal ions
3a
and neutral inorganic
3b
and organic
3c
Lewis acids
and electron acceptors. Gabbaı¨ and co-workers have reported that
a trinuclear Hg(II) complex forms 1:1 adducts with aromatic
hydrocarbons, which become brightly phosphorescent at room
temperature due to a mercury heavy atom effect.
4
Meanwhile,
photophysical studies for trinuclear complexes of Cu(I) and Ag(I)
did not receive as much attention as their other d
10
counterparts.
1,5
We have undertaken a systematic study of trinuclear, dinuclear,
and mononuclear complexes of Cu(I) and Ag(I) with fluorinated
pyrazolate ligands such as [3,5-(CF
3
)
2
Pz]
-
and have found that these
complexes exhibit bright, tunable luminescence. These classes of
complexes are targeted as potential candidates for emitting materials
in molecular light-emitting devices (LEDs) because fluorination
increases their volatility, thus facilitating thin-film fabrication, and
because the presence of a closed-shell transition metal should
enhance the phosphorescence.
4,6
Fluorinated ligands also endow
other beneficial properties such as improved thermal and oxidative
stability, and reduced concentration quenching of luminescence to
metal adducts.
6-8
Such ligands also enable the isolation of exceed-
ingly rare molecules such as [HB(3,5-(CF
3
)
2
Pz)
3
]MCO (where M
) Ag, Au).
9
In this paper, we describe the photophysical properties of the
trinuclear copper(I) complex {[3,5-(CF
3
)
2
Pz]Cu}
3
, henceforth re-
ferred to as Cu
3
, which exhibits multicolor bright phosphorescent
emissions that are sensitive to temperature, solvent, and concentra-
tion. The synthesis and structure of the Cu
3
complex were reported
earlier.
10
The structure (Chart 1) shows weak intramolecular
(3.221-3.242 Å) and essentially no intermolecular Cu‚‚‚Cu interac-
tions (3.879; 3.893 Å).
11
Photoluminescence emission and excitation spectra for crystals
of Cu
3
are shown in Figure 1.
12
The compound exhibits a bright
orange emission in the solid state at room temperature. Interestingly,
the emission color of Cu
3
becomes red upon cooling to intermediate
temperatures between room temperature and 77 K, but becomes
orange again at 77 K. The orange emission at 77 K is due to the
combination of two bands, a major red peak at ∼665 nm and a
yellow shoulder at ∼590 nm. The yellow shoulder disappears at
higher temperatures, leaving only the red emission, but band
broadening at room temperature leads to orange emission. The
lifetime of the major peak was 52.6 ( 0.8 µs at room temperature
and 64.4 ( 1.0 µs at 77 K. The shoulder that appears at 77 K has
a lifetime of 104 ( 2 µs.
Solutions of Cu
3
exhibit luminescence that is greatly enhanced
at cryogenic temperatures. We have discovered that the Cu
3
luminescence can be easily fine- and coarse-tuned to multiple bright
visible colors by each of the following factors: solvent, concentra-
tion, temperature, and excitation wavelength. Figure 2 shows
selected examples of some spectra and photographs illustrating these
dramatic changes in rigid frozen solutions.
The interesting luminescence behavior of crystals and rigid
solutions of Cu
3
implies a rather sophisticated photophysical
behavior that we can only speculate upon at this stage. The facts
that the bright orange-red emission of the Cu
3
crystals has a
structureless profile even at cryogenic temperature and that
microsecond lifetimes were obtained are consistent with an assign-
ment to a copper-based phosphorescence. The emission is likely
related to Cu-Cu interactions because the
3
D
3
lowest triplet sublevel
of a free Cu(I) ion has an energy of 21 930 cm
-1
(i.e., in the blue
region).
13
At first glance, one is tempted to assign the Cu
3
emission
to intramolecular and not intermolecular Cu-Cu interactions on
the basis of the crystal structural data above. However, further
insights are gained by the frozen solutions data in Figure 2, which
show structureless emissions whose energies (and colors) are
solvent-dependent and exhibit red shifts upon increasing the
concentration in some solvents to approach the solid-state behavior.
These results suggest that the emission may be related to intermo-
lecular interactions between Cu
3
units. The crystal structure (Chart
1) shows that trimer units are connected by Cu-Cu interactions to
form infinite zigzag chains. While the intermolecular Cu-Cu
distances are too long for ground-state bonding interactions, it is
not unreasonable to propose that they will contract in the excited
state and cause the low-energy visible emissions, as suggested for
†
The University of Texas at Arlington.
‡
University of North Texas.
Chart 1.
Molecular Structure and Packing Diagram of Cu
3
10
Published on Web 09/10/2003
12072
9
J. AM. CHEM. SOC. 2003,
125
, 12072-12073 10.1021/ja036736o CCC: $25.00 © 2003 American Chemical Society
other Cu(I) and d
10
systems.
14
The huge Stokes’ shifts (>18 000
cm
-1
!) suggest largely distorted excited states, consistent with this
assignment. To understand the origin of the yellow shoulder
observed at 77 K, we note that the excitation profile is similar for
this shoulder and the major red peak, while the two bands have
significantly different lifetimes. Thus, the excitation route is the
same for the two bands, but they decay independently. The two
bands are tentatively assigned to different sublevels of the emitting
“triplet” excited state that is split by spin-orbit coupling. Crosby
suggested that such sublevels behave independently and decay
differently in metal complexes even if only one emission band is
observed.
15
The remarkable luminescence behavior of Cu
3
and the sensitivity
of the emission to various factors warrant further comments. The
data are reminiscent of several novel optical phenomena that have
been reported individually for other metal complexes. These
phenomena are observed collectively for Cu
3
together with another
new phenomenon. The temperature dependence of the luminescence
color seen for both solids and solutions of Cu
3
illustrates “lumi-
nescence thermochromism”, a phenomenon reported most notably
for tetranuclear Cu(I) clusters.
5
The solvatochromism of the Cu
3
luminescence is related to a few fascinating recent observations
for several classes of d
10
and d
8
complexes.
16
The changes in the
luminescence energies in different solvents are related to both the
extent of excited-state association of Cu
3
and the different electronic
structure of various *[Cu
3
‚solvent] complexes. What is unusual
about the results here is not only the selectivity even for similar
solvents (e.g., toluene vs benzene) and the versatility of solvents
in which Cu
3
shows luminescence solvatochromism, but also the
qualitative changes in the visible emission colors and spectra when
the Cu
3
concentration is varied in the same solvent. The most
striking changes were seen in dichloromethane, in which the
luminescence was tuned to essentially all visible colors between
blue and red by varying the Cu
3
concentration. This “concentration
luminochromism”, in which multiple visible colors are emitted by
controlling the luminophore concentration, is an unprecedented
optical phenomenon, to our knowledge.
Besides the scientific significance of these results, the observation
of bright phosphorescence for crystals and sublimed thin films at
room temperature is prompting us to pursue using Cu
3
and related
fluorinated complexes as emitting materials for molecular LEDs.
Work is underway to control the photo- and electroluminescence
properties of these complexes and to investigate the coinage metal
family group trends.
Acknowledgment. This work has been supported by the Robert
A. Welch Foundation (Grant Y-1289 to H.V.R.D. and B-1542 to
M.A.O.). We thank NSF for support to M.A.F. through NSF-REU
(CHE-0243795) and H.V.R.D. (CHE-0314666).
References
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JA036736O
Figure 1.
Photoluminescence spectra of Cu
3
. All emission spectra were
scanned with λ
ex
) 300 nm, while the two excitation spectra monitored the
emission at 550 nm (bottom) and 650 nm (top).
Figure 2.
Representative emission spectra of rigid frozen solutions (77
K) of Cu
3
versus solvent and concentration. Lifetimes at λ
max
(left-to-right,
respectively) are 80 ( 3, 62 ( 1, 52 ( 2, 98 ( 2, 84 ( 1, and 21 ( 1 µs.
The photograph shows selected CH
2
Cl
2
, toluene, and CH
3
CN frozen
solutions in supracell quartz tubes exposed to UV light immediately after
removal from a liquid nitrogen bath. The temperature increase on removing
the sample from the bath changes the emission color of the CH
2
Cl
2
solution
from orange to red.
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