Novel luminescent iminephosphine complex of copper(i) with high photochemical and electrochemical stability

Article abstract of DOI:10.1039/b916782j

Luminescent heteroleptic Cu^I complexes based on asymmetrical iminephosphine ligands exhibit improved electrochemical and photochemical stability as compared to the analogous complexes based on traditional diimine or diphosphine ligands. The Roy

Full text of DOI:10.1039/b916782j

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Novel luminescent iminephosphine complex of copper(I) with high
photochemical and electrochemical stability†
Li Qin,a,c Qisheng Zhang,a Wei Sun,b,c Jingyun Wang,a,c Canzhong Lu,*a Yanxiang Cheng*b and
Lixiang Wangb
Received 5th May 2009, Accepted 24th August 2009
First published as an Advance Article on the web 2nd September 2009
DOI: 10.1039/b916782j
Luminescent heteroleptic CuI complexes based on asymmetri-
cal iminephosphine ligands exhibit improved electrochemical
and photochemical stability as compared to the analogous
complexes based on traditional diimine or diphosphine
ligands.
exhibit bright luminescence in the solid state as well as in OLEDs,
which was also suspected to have originated from their MLCT
excited state.
In this communication, we report the synthesis and
characterization of two novel mixed-ligand CuI complexes
[Cu(NP)(DPEphos)]+, where NP
=
8-diphenylphosphanyl-
quinoline (dppq) (1) or 2-methyl-8-diphenylphosphanyl-quinoline
(mdppq) (2). The chelating iminephosphine ligands of dppq and
mdppq bear both imine and phosphine donor groups that can be
looked as the structural units of phen (1,10-phenanthroline) and
dppb, respectively. An unexpected finding is that the CuI complex
based on the asymmetrical mdppq ligand processes a considerably
higher reduced state and exited state stability than the analo-
gous complexes based on symmetrical diimine or diphosphine
ligand.
Photoluminescent complexes including bi- or tridentate aromatic
N-heterocyclic ligands and low-valent metal ions like RuII, OsII,
IrIII, ReI or CuI have been exploited for practical applications
in solar energy conversion, chemical sensing, biological probing
and displays due to possessing an available low-lying metal-
to-ligand charge-transfer (MLCT) excited state.1,2 Though the
economical and environmental considerations make CuI-diimines
([Cu(NN)2]+) far more attractive than the well-known MLCT com-
plex RuII-polypyridine, the tendency to display weak emission and
short-lived excited states has limited their practical applications.2
The replacement of N-coordinating ligands with P-coordinating
ligands is often found to improve the emission properties of the
MLCT complexes because the strong p-acidity group of phosphine
can enhance the energy level of the excited states and therefore
decrease non-radiative deactivations.3 In 2002, by using mixed-
ligands, McMillin’s group reported the first example of a highly
emissive mononuclear copper complex [Cu(dmp)(DPEphos)]+
(where dmp = 2,9-dimethyl-1,10-phenanthroline and DPEphos =
bis[2-(diphenylphosphino)phenyl]ether).4 Subsequently, a great
number of similar complexes ([Cu(NN)(PP)]+) were prepared
and have been used as efficient electrophosphorescent materials
successfully in multilayer organic light-emitting diodes (OLEDs)
and simpler light emitting electrochemical cells (LECs).5 More
recently, room temperature phosphorescence was even observed
from a series of [Cu(PP)2]+ complexes.6,7 Based on the 1,2-
bis(diphenylphosphino)benzene (dppb) ligand, both homoleptic
[Cu(dppb)2]+ and heteroleptic [Cu(dppb)(DPEphos)]+ complexes
The iminephosphine ligands used herein have been used to
fabricate NiI, RuII and PdII complexes previously,8 and can be
readily prepared by the literature method.9 Complexes 1 and 2 can
be obtained from the reaction of [Cu(NCCH3)4](BF4) with one
equivalent of DPEphos and iminephosphine ligand in CH2Cl2.
Slow diffusion of diethyl ether into the ethanol or CH2Cl2 solutions
of complexes gave yellow and air-stable crystals in good yields.
As revealed by single-crystal X-ray diffraction analysis, the CuI
center in both complexes has a distorted tetrahedral geometry
(Fig. 1a).ठSince diphenylphosphanyl affords a significantly larger
steric hindrance than the quinoline group in the asymmetrical
dppq ligand, the angle between the P1–Cu1 bond and the
P2–Cu1–P3 plane (155.0◦) is much larger than that between the
N1–Cu1 bond and the P2–Cu1–P3 plane (121.8◦) in complex 1.
The introduction of sterically active substituents (methyl groups)
on the quinoline ring, by contrast, makes these two angles more
similar (140.4◦ vs. 135.6◦) in complex 2. Additionally, the methyl
groups increase the Cu–N and Cu–P (DPEphos) distances in
aState Key Laboratory of Structural Chemistry, Fujian Institute of Research
on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian,
350002, P. R. China. E-mail: czlu@fjirsm.ac.cn; Fax: +86-591-83714946;
Tel: +86-591-83705794
bState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun,
130022, P. R. China. E-mail: yanxiang@ciac.jl.cn; Fax: +86-431-85685653;
Tel: +86-431-85262106
cGraduate School of Chinese Academy of Sciences, Beijing, 100039,
P. R. China
˚
˚
complex 2 about 0.04 A and 0.02 A, respectively, consistent with
the precedent found in the phen and dmp analogues.4 Whereas,
by comparison with the [Cu(NN)(DPEphos)]+ systems (1 vs.
phen complex, 2 vs. dmp complex), the average Cu–N and Cu–P
(DPEphos) distances of the iminephosphine analogues elongate
† Electronic supplementary information (ESI) available: Experimental
procedures, electrochemistry, spectroscopy and crystallographic data.
CCDC reference numbers 730072 (1) and 730009 (2). For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b916782j
9388 | Dalton Trans., 2009, 9388–9391
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the lowest unoccupied molecular orbital (LUMO) is localized on
the p*-antibonding orbital of the quinoline ring. Thus, the lowest
excited state of 2 is attributed to metal-to-ligand charge-transfer
(MLCT) transition that mixed with a ligand-to-ligand charge-
transfer (LLCT) state.
In CH2Cl2, complexes 1 and 2 emit with maximum at
642 nm and 608 nm, respectively. Similar to those suggested
for [Cu(NN)(DPEphos)]+ complexes,4 the introduction of methyl
group on the quinoline group blue-shifted the absorption and
emission band as well as significantly increase the quantum yield
(f) and lifetime (t) from 0.0013 and 0.13 ms for 1 to 0.012 and
0.70 ms for 2, respectively. The reason for this can be explained
as that the sterically active substituent prevents the excited state
conformation change from a tetrahedral to a flattened structure,
which has a lower MLCT energy and a larger non-radiative rate
constant.7 In the rigid medium of PMMA, relaxation caused by
bond vibrations and rotations was restrained, leading to decrease
in Stokes shift and increase in quantum yield and lifetime relative
to those in solution. Though the quantum yield of 0.29 for 2 is
still smaller than that of 0.49 for [Cu(dmp)(DPEphos)]+,7 it is
comparable to the value (f = 0.28) found in a thin film of tris-(8-
hydroxyquinoline) aluminum (Alq3), a classical OLED material.
Considered the energy harvesting by phosphorescent heavy-metal
complexes is fourfold than that by fluorescent compounds in
OLEDs, 2 can be anticipated to be a useful triplet emitter for
electroluminescent applications.
Fig. 1 (a) ORTEP diagram of the complex cation in 2. Selected bond dis-
tances (A) and angles ( ): Cu1–N1 2.193(4), Cu1–P1 2.2798(14), Cu1–P2
2.2998(12), Cu1–P3 2.3170(12); N1–Cu1–P1 84.33(11), P2–Cu1–P3
◦
˚
◦
˚
108.90(4). Bond distances (A) and bond angles ( ) in the structure
of 1: Cu1–N1 2.153(2), Cu1–P1 2.2990(8), Cu1–P2 2.2909(7), Cu1–P3
2.2913(8); N1–Cu1–P1 83.25(6), P2–Cu1–P3 109.59(2). (b) The HOMO
of 2. (c) The LUMO of 2.
The electrochemical properties of 1 and 2 were investigated
by cyclic voltammetry (CV) at room temperature in dried and
argon purged CH3CN solutions, along with the comparative
complexes [Cu(dmp)(DPEphos)]+ and [Cu(dppb)(DPEphos)]+.
Similar to their dmp and dppb analogues, both 1 and 2 display
irreversible and multiple oxidation peaks that might be attributed
to multiple electron-transfer originating from the CuI center and
the P-coordinating groups (ESI, Fig. S1).† The first oxidation
peaks of 0.89 V (vs. Fc/Fc+) for 1 and 2 are close to that
for the dppb analogue but 0.10 V more positive than that for
the dmp analogue, in accordance with the greater p-acid nature
of the P-coordinating group. In CH3CN solution, 1 shows an
irreversible one-electron reduction as a consequence of the rapid
decomposition of the [Cu+(dppq)-(DPEphos)] species (ESI, Fig.
S2).† The poor electrochemical stability of 1 should be related with
the remarkable asymmetry of the steric hindrance for dppq ligand.
However, 2 displays a reversible reduction couple with the E1/2
value of -2.13 V (vs. Fc/Fc+). Furthermore, the more interesting
is that the electrochemical stability of 2 in its reduction process
is even higher than that of [Cu(dmp)(DPEphos)]+, in defiance
of the relatively weaker metal–ligand bonds of 2. As shown in
Fig. 2a, the reduction wave of 2 presents good repeatability
even after 10 scans with a variance less than 4%, while that
of the dmp analogue changed significantly from reversible to
irreversible in the first 10 cycles. We assume that the instability
of [Cu+(dmp)-(DPEphos)] species arise from the prolongation of
Cu–N bond, which inevitably promotes ligand dechelation and
substitution. In the case of [Cu+(mdppq)-(DPEphos)] species, the
negative charge is localized on the quinoline ring and mainly desta-
bilized the Cu–N bond, but the less affected Cu–P bond from one
coordinate unit will assist ligand rebinding, and hence prevent the
ligand exchange reactions. Alternatively, the large steric hindrance
of mdppq might also help reduction products against substitution
4
˚
˚
about 0.08 A and 0.04 A, respectively, indicating a more sterically
crowded coordination sphere of central copper atom in both
complexes.
The photophysical data of 1 and 2 in degassed CH2Cl2 solution
and poly(methyl methacrylate) (PMMA) film are listed in Table 1.
With reference to our previous work, the intense absorption at
ca. 280 nm is assigned to the p–p* transition of the NP ligand,
while the week shoulders (e ª 1700 M-1cm-1) at 350–360 nm can be
attributed to the MLCT transition involving the NP ligand and the
CuI cation. To better understand the nature of the frontier orbitals,
DFT calculations using the B3LYP method were carried out for
2. As shown in Fig. 1b and Fig. 1c, the electron density in the
highest occupied molecular orbital (HOMO) is mainly associated
with the CuI center and Cu–P s-bonding orbital, while that in
Table 1 Photophysical data for 1 and 2 at room temperature
a
Complex
l
max,abs/nm
l
max,em/nm
f
t/ms
Solution (in degassed CH2Cl2)
1
2
274, 360 (sh)
282, 350 (sh)
642
608
0.0013
0.012
0.13b
0.70b
Film (20 wt% in PMMA)
1
2
275, 356 (sh)
282, 350 (sh)
560
555
0.09
0.29
131c
108c
a Error 10%. b Fitted by single-exponential. c Since the decay is best-
fit by three-exponentials, a weighted-average lifetime (tav ) was used and
ꢀ
ꢀ
2
calculated by the equation tav
exponential for the lifetime ti.
=
Aiti / Aiti, where Ai is the pre-
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or IrIII systems,10 but should be associated with the asymmetrical
electronic character of mdppq ligand.
In short, the observation of high electrochemical and pho-
tochemical stability for iminephosphine complex 2 gives us a
new way to improve the stability of MLCT complexes, which
is identified as a critical fact to achieve long life devices for
display or lighting applications. The application of complex 2 in
electroluminescence devices is under way.
Acknowledgements
This work was supported by the 973 key program of the
MOST (2006CB932904, 2007CB815304), the National Natural
Science Foundation of China (20873150, 20821061, 50772113 and
20874098), the Natural Science Foundation of Fujian province
(2007F3116, 2007HZ0001-1) and the Chinese Academy of
Sciences (KJCX2-YW-M05).
Notes and references
‡ Crystal data for 1 C59H50BCuF4NO2P3: Mr = 1048.27, triclinic, space
¯
˚
group P1, a = 11.023(3), b =◦14.753(4), c = 16.362(4) A, a = 92.285(4),
3
˚
b = 91.635(5), g = 96.309(3) , V = 2641.2(12) A , T = 293 K, Z = 2,
Dcalcd = 1.318 g cm-3, m(MoKa) = 0.562 mm-1, 20 583 reflections measured,
11 808 unique (Rint = 0.0208) which were used in all calculations. The final
R(F2) was 0.0545 [I > 2s(I)].
Fig.
2 (a) Multiple scan CVs for the reduction of 2 and
[Cu(dmp)(DPEphos)]+ in acetonitrile solution at room temperature.
Scan rate 100 mV s-1 in 0.1 M TBAP. (b) Photodegradation of 2,
[Cu(dmp)(DPEphos)]+, [Cu(dppb)(DPEphos)]+ and Alq3 on the surface of
nanometer Al2O3 particles. (0.025 mmol complex mixed with 1.0 g Al2O3).
§ Crystal data for 2 C59H48BCl2CuF4NOP3: Mr = 1101.15, monoclinic,
˚
space group P21/c, a = 18.361(5), b = 12.831(4), c = 24.921(7) A, b =
◦
103.232(5) , V = 5715(3) A , T = 293 K, Z = 4, Dcalcd = 1.280 g cm-3,
3
˚
m(MoKa) = 0.612 mm-1, 34 854 reflections measured, 10 007 unique (Rint
=
0.0426) which were used in all calculations. The final R(F2) was 0.0710
[I > 2s(I)].
reactions by inhibiting the coordinating solvent from attacking
the metal center. However, since the comparative complex with
more bulky ligand, dppb, shows no electrochemical activity in the
reduction process, the second explanation can not be examined
further.
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can react with atmospheric moisture or oxygen resulting in
degradation. If either explanation mentioned above established,
it can be deduced that 2 will be more resistant to moisture or
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