Neutral cuprous complexes as ratiometric oxygen gas sensors

Article abstract of DOI:10.1039/c1dt11777g

Four neutral mononuclear Cu(i) complexes, [Cu(pyin)(PPh₃) ₂] (1a), [Cu(pyin)(DPEphos)] (1b), [Cu(quin)(PPh₃) ₂] (2a) and [Cu(quin)(DPEphos)] (2b) (Hpyin = 2-(2-pyridyl)indole, Hquin = 2-(2-quinolyl)indole and DP

Full text of DOI:10.1039/c1dt11777g

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PAPER
Neutral cuprous complexes as ratiometric oxygen gas sensors†
Xiaohui Liu,a,c Wei Sun,a,c Luyi Zou,a Zhiyuan Xie,a Xiao Li,*a Canzhong Lu,b Lixiang Wanga and
Yanxiang Cheng*a
Received 19th September 2011, Accepted 24th October 2011
DOI: 10.1039/c1dt11777g
Four neutral mononuclear Cu(I) complexes, [Cu(pyin)(PPh3)2] (1a), [Cu(pyin)(DPEphos)] (1b),
[Cu(quin)(PPh3)2] (2a) and [Cu(quin)(DPEphos)] (2b) (Hpyin = 2-(2-pyridyl)indole, Hquin =
2-(2-quinolyl)indole and DPEphos = bis(2-(diphenylphosphino)phenyl)ether) have been synthesized.
X-Ray crystal structure analysis revealed that the central Cu(I) ion in all complexes is in a distorted
tetrahedral coordination environment. All four complexes display the typical metal-to-ligand charge
transfer (MLCT) absorption band at 371, 363, 413 and 402 nm, respectively. No emission was observed
from any complexes in the solid state due to triplet–triplet annihilation. However, the complexes show
unusual dual-emission originating from intraligand charge-transfer (ILCT) and MLCT transitions,
when dispersed in a rigid matrix (e.g. PMMA) or in frozen CH2Cl2. The oxidation potential of
Cu(I)/Cu(II) in these neutral complexes, ~0.5 V (vs. Ag/AgCl), is lower than those of cationic Cu(I)
complexes. Films containing 10 wt% of these complexes in PMMA shows ratiometric fluorescent
oxygen gas sensing property with a response ratio of 0.3–3.2 and response time of 3–4 s. Complex 2b
acts as a ratiometric oxygen gas sensor with good reversibility through energy and electron transfer
mechanisms under the loss of a counteranion.
Luminescent transition metal complexes should be more suit-
able oxygen gas sensors than organic dyes because these complexes
The determination of oxygen levels is necessary in many fields,
usually show strong phosphorescent characteristics and oxygen
Introduction
molecules are always in the triplet ground state,3 which are
advantageous for triplet–triplet annihilation. Noble-metal com-
plexes such as of Ru(II) or Ir(III),4 can act as oxygen gas sensors
with high sensitivity, reversibility and specificity. In particular,
inexpensive Cu(I) complexes have also been applied in oxygen
gas sensing because they possess analogous MLCT transitions
to Ru(II) complexes.5 However, the square-planar geometry in
excited states for ionic Cu(I) complexes means that they are easily
attacked by nucleophilic reagents from the environment, including
counter-anions, which results in non-radiative decay, decreased
emission quantum yield and sensitivity.6 One of the improvement
methods is to synthesize heteroleptic Cu(I) complexes containing a
large counter-anion with low nucleophilicity. Such cationic Cu(I)
complexes show excellent sensing ability to molecular oxygen,5c
even better than Ru(II) complexes. However, a better alternative,
we suggest, is to develop neutral Cu(I) complexes, which can
completely avoid this effect of the counter-anion.6
The previous work we reported recently has demonstrated that
neutral Cu(I) complexes possess a similar luminescent perfor-
mance to ionic ones, based on completely identical skeletons, and
are more effective phosphorescent emitters.7 However, both the
neutral and ionic complexes can simply interconvert from one
to another in the presence of acid or base. Therefore, “pure”
neutral Cu(I) complexes are expected to be superior potential
such as industrial production, environmental surveillance, medical
analysis and in daily life due to its importance as an active
species in these processes. Generally, oxygen detection methods
are divided into chemical systems, such as the Clark electrode and
chemiluminescence,1 and physical measurement. In contrast to the
chemical methods, the physical ones, such as optical sensors, can
be used in wider areas and have a long-lived lifetime and good
reproducibility because no reactions occur, although they are at
the high level of cost as instrumental analytical methods.2 A large
number of luminescent compounds, especially recently reported
phosphorescent complexes, offer more chances to improve the
measuring ability of optical sensors.
aState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun,
130022, P. R. China. E-mail: xiaoli@ciac.jl.cn, yanxiang@ciac.jl.cn;
Fax: +86-431-85262572; Tel: +86-431-85262106
bState Key Laboratory of Structural Chemistry, Fujian Institute of Research
on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002,
P. R. China
cThe Graduate School, Chinese Academy of Sciences, Beijing, 100039, P. R.
China
† Electronic supplementary information (ESI) available: Fig. S1–S3 and
crystallographic data. CCDC reference numbers CCDC 828213–828216.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c1dt11777g
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candidates as new phosphorescent materials. In this paper, we
report four novel neutral Cu(I) complexes, in which the ligands
used are 2-(2¢-pyridyl)indole (Hpyin)8 and 2-(2¢-quinolyl)indole
(Hquin) as well as auxiliary triphenylphosphine (PPh3) and bis(2-
(diphenylphosphino)phenyl) ether (DPEphos). Unlike the ligands
used in the previous work, Hpyin or Hquin is the sole heterocyclic
ligand used to prepare the present neutral Cu(I) complexes. More
interestingly, these complexes were found to have oxygen gas
sensing ability during the measurement of their photophysical
properties.
Result and discussion
Synthesis and characterization
Scheme 1 shows the structures of four neutral Cu(I) complexes (1a,
1b, 2a, 2b), which were readily synthesized in good yields by the
elimination of NaI from the reaction of sodium salt of the ligand
(Na+pyin- or Na+quin-), CuI and phosphine in the ratio of 1 : 1 : 2
under an argon atmosphere in absolute tetrahydrofuran (THF).
The reactions were easily monitored by the quenching of the bright
green fluorescence of the sodium salt of the ligand. Complexes 1a
and 1b are sensitive to air in solution but are stable solids, and
complexes 2a and 2b are relatively stable both in solution and
the solid state. From thermogravimetric analysis (TGA) data, the
decomposition temperatures of 1b (362 ◦C) and 2b (378 ◦C) (with
DPEphos) are higher than that of 1a (253 ◦C) and 2a (264 ◦C)
(with PPh3). Hence DPEphos as an auxiliary ligand increases the
thermal stability of its derived complexes.
Fig. 1 Crystal structure of complex 1a with thermal ellipsoids at 30%
probability. H atoms are omitted for clarity.
Scheme 1 The molecular structures of the complexes 1a, 1b, 2a and 2b.
Crystals of complexes 1a, 1b, 2a and 2b suitable for X-ray
diffraction analysis were obtained by diffusion of methanol to
the dichloromethane solution of complexes. Complexes 1a and 1b
were obtained as yellow crystalline solids, while complexes 2a and
2b are red crystals. The crystallographic data are reported in Table
1, and views of the structures of the complexes 1a, 1b, 2a and
2b are shown in Fig. 1–4, respectively. Selected bond lengths and
angles for complexes 1a, 1b, 2a and 2b are listed in Table 2.
X-Ray diffraction analysis shows that the Cu(I) center in
all four complexes is located in a typically pseudo-tetrahedral
coordination environment. Bond lengths for P–Cu, Cu–N(pyridyl
or quinolyl moiety) and angles for P–Cu–P are comparable to
those reported in the literature for similar heteroleptic Cu(I)
complexes.9–11 However, the bond lengths of Cu–N(indolyl moi-
Fig. 2 Crystal structure of complex 1b with thermal ellipsoids at 30%
probability. Solvent molecules and H atoms are omitted for clarity.
that anionic indolyl moiety has more negative charge than the
pyridyl or quinolyl moieties.10 The dihedral angles between the
N–Cu–N and the P–Cu–P planes of the four complexes are 86.36
(1a), 86.99 (1b), 84.44 (2a) and 85.42◦ (2b), respectively, which are
in the normal range compared with that found in the mixed-ligand
Cu(I) complexes of [Cu(N,N)(P,P)]+ systems.9,11 The N–Cu–N
˚
bond angles are similar in all the complexes (80.76–81.93 A) with
little variation. The P–Cu–P bond angle of 2a (122.90◦) is larger
than that of 1a (116.82◦), which indicates that the steric hindrance
of quinolyl in quin is greater than pyridyl in pyin. Intramolecular
˚
eties) (2.006–2.042 A) are obviously shorter than those of Cu–
˚
N(pyridyl or quinolyl moieties) (2.101–2.125 A), which indicates
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Table 1 Crystallographic data for complexes 1a, 1b, 2a and 2b
Complex
[Cu(pyin)(PPh3)2] (1a)
[Cu(pyin)(POP)]·CH2Cl2 (1b)
[Cu(quin)(PPh3)2]·CH2Cl2 (2a)
[Cu(quin)(POP)]·CH3OH (2b)
Empirical formula
Mr
C49H39CuN2P2
781.30
Orthorhombic
C49H37CuN2OP2·CH2Cl2
C53H41CuN2P2·CH2Cl2
C53H39CuN2OP2·CH3OH
880.21
916.28
877.38
Crystal system
Triclinic
Triclinic
Triclinic
¯
¯
¯
Space group
Pna21
P1
P1
P1
˚
a/A
20.386(3)
21.293(3)
9.1912(11)
9.586(4)
11.370(4)
20.412(8)
96.330(7)
93.891(7)
103.119(7)
2143.5(14)
2
10.9902(8)
12.2055(8)
19.3749(13)
75.6690(10)
81.1440(10)
66.0970(10)
2297.9(3)
2
11.7152(9)
12.1017(9)
16.7936(12)
74.3790(10)
77.3600(10)
70.7180(10)
2142.1(3)
2
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
3989.6(9)
4
1.301
0.664
52.12
21912
7863
0.0656
487
0.0581, 0.0896
0.1003, 0.1030
1.006
Z
Dc/g cm-3
m/mm◦-1
1.364
0.749
52.30
12192
8309
0.0363
523
0.0695, 0.1498
0.1275, 0.1819
1.001
1.324
0.700
51.44
12689
8560
0.0206
550
0.0549, 0.1441
0.0719, 0.1595
1.003
1.360
0.630
52.10
12056
8233
0.0301
558
0.0634, 0.1519
0.0899, 0.1707
1.003
2qmax
/
No. reflns measd
No. reflns used
Rint
No. params
R1,a wR2b [I > 2s(I)]
R1,a wR2b (all data)
GOF on F2
ꢀ
ꢀ
ꢀ
ꢀ
2
2
a R1 = [|Fo| - |Fc|]/ |Fo|. b wR2 = { [w(Fo - Fc2)]/ (wFo2)}1/2; w = 1/[s (Fo2) + (0.075P)2], where P = [max(Fo2, 0) + 2Fc2]/3.
Fig. 4 Crystal structure of complex 2b with thermal ellipsoids at 30%
probability. Solvent molecules and H atoms are omitted for clarity.
Fig. 3 Crystal structure of complex 2a with thermal ellipsoids at 30%
probability. Solvent molecules and H atoms are omitted for clarity.
Photophysical and electrochemical properties
The absorption spectra of free ligands and their corresponding
complexes in CH2Cl2 are shown in Fig. 5 and detailed results are
summarized in Table 3. These complexes exhibited weak bands at
long wavelength, ranging from 350 to 450 nm for 1a and 1b (e =
5.07 ¥ 103 and 6.69 ¥ 103 M-1 cm-1), and from 400 to 500 nm for
2a and 2b (e = 9.00 ¥ 103 and 9.39 ¥ 103 M-1 cm-1), corresponding
to electronic excitation of MLCT, in addition to intense p–p* and
ILCT absorptions. It is important to note that, in the MLCT
region (Fig. 5), the absorption spectra of 1b and 2b exhibited a
p–p interactions between indolyl or quinolyl and phenyl rings
˚
˚
(from DPEphos) of 3.455 A (C19–N2) in 1b and 3.423 A (C53–
N1) in 2b, respectively, with DPEphos as the second ligand, were
observed due to the restricted bite angle of the bidentate ligand,7,12
which result in more distorted tetrahedral geometries in 1b and 2b.
A further consequence might be the higher stability of complexes
1b and 2b.
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◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for 1a, 1b, 2a and 2b
Complex
1a
1b
2a
2b
Cu–N(2)
Cu–N(1)
Cu–P(2)
Cu–P(1)
2.017(4)
2.111(4)
2.259 (14)
2.244(13)
2.006(4)
2.112(4)
2.286(16)
2.236(16)
2.012(3)
2.125(2)
2.254(9)
2.242(9)
2.042(3)
2.101(3)
2.311(11)
2.242(12)
N(2)–Cu–N(1)
N(2)–Cu–P(2)
N(1)–Cu–P(2)
N(2)–Cu–P(1)
N(1)–Cu–P(1)
P(2)–Cu–P(1)
81.00(15)
80.83(16)
80.76(11) 81.93(13)
119.64(11) 120.42(12) 116.53(8) 123.58(10)
113.88(12) 119.41(12) 110.67(7) 122.46(10)
114.96(11) 107.57(12) 105.38(8) 110.74(10)
103.44(12) 112.81(11) 112.90(7) 102.16(9)
116.82(5)
112.07(5)
122.90(3) 111.48(4)
N(1)–C(5)–C(6)–N(2) 6.7(6)
N–Cu–N/P–Cu–P 86.36
6.9(5)
86.99
-2.3(4)
84.44
-4.3(6)
85.42
Table 3 Absorbance peak (labs , molar absorptivity) in dichloromethane;
emission peak (lem), quantum yield (U) and lifetime (t) of Cu(I) complexes
in PMMA at 298 K
Absorbance (DCM) Emission (PMMA film)
l
abs /nm, e/M-1 cm-1 lem (nm)
t
U (%)
[Cu(pyin)-
(PPh3)2] (1a)
232, 4.05 ¥ 104
324, 1.55 ¥ 104
371, 5.07 ¥ 103
562
1.17 ms 8.0
474 (weak) 4.66 ns
[Cu(pyin)-
231, 3.24 ¥ 104
566
1.20 ms 5.0
(DPEphos)] (1b) 324, 1.49 ¥ 104
363, 6.69 ¥ 103
475 (weak) 4.78 ns
414, 2.01 ¥ 103
[Cu(quin)-
(PPh3)2] (2a)
231, 5.31 ¥ 104
354, 1.56 ¥ 104
413, 9.00 ¥ 103
608
550 (sh)
1.32 ms 4.2
5.04 ns
Fig. 5 Room-temperature electronic absorption spectra of complexes 1a,
1b with their free ligand Hpyin (a) and complexes 2a, 2b with their free
ligand Hquin (b) in degassed CH2Cl2.
[Cu(quin)-
(DPEphos) (2b)
231, 3.81 ¥ 104
354, 1.39 ¥ 104
402, 9.39 ¥ 103
463, 2.87 ¥ 103
610
550 (sh)
1.59 ms 4.4
4.89 ns
shoulder at 414 nm (e = 2.01 ¥ 103 M-1 cm-1) and 463 nm (e =
2.87 ¥ 103 M-1 cm-1) (that was not obvious for 1a and 2a), which
may be assigned to the 3MLCT charge transition,13 indicating that
these neutral Cu(I) complexes (1a, 1b, 2a and 2b) could possess
different photophysical properties relative to the classical cationic
complexes.
No emission was observed for all complexes in the solid state
at ambient temperature because of triplet–triplet annihilation,
but they showed emission when they were dispersed in a rigid
matrix (10 wt% in PMMA, Fig. 6). The maximum emission peak
of complexes 1a, 1b, 2a or 2b are at 562, 566, 608 or 610 nm,
respectively, with the efficiency from 4.4–8.0% in PMMA at 298
K. The main peak is assigned to the emission from the MLCT
excited state based on the large Stokes shift, while the weak
feature at ~470 nm (1a and 1b) or shoulder at ~550 nm (2a and
2b) is presumed to be the emission originated from the ILCT
transition in the pyin or quin ligands at 298 K. In order to confirm
this hypothesis, the emission spectra of complex 2b, as a typical
example, were measured as a function of temperature (Fig. 7).
The intensity of the short wavelength emission decreased and
eventually vanished with decreasing temperature, accompanied
Fig. 6 The emission spectra of 1a, 1b, 2a and 2b in 10 wt% PMMA films
at 298 K; lex = 365 nm.
with an increase of the main peak emission and slight blue shift
from 610 to 600 nm, since the low temperature restricted vibration
of the chromophore. The short wavelength emission still persisted
and exhibited the same temperature dependence even at a lower
concentration of 0.5 wt% in PMMA or in a polystyrene (PS) or
polycarbonate (PC) matrix at 298 K. Conclusive evidence was
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the electron-rich Cu(I) center also raises the lowest unoccupied
molecular orbital (LUMO) energy level of the neutral complexes.
DFT calculations support this interpretation, in which the HOMO
of 2b is localized both on the indole moiety and the copper center,
while HOMO-1 is distributed over the metal centered orbital;
even the HOMO-2 orbital is also mainly copper-centered. The
band gaps obtained from DFT calculations (3.40, 3.26, 2.78 and
2.84 eV for 1a, 1b, 2a and 2b, respectively) are consistent with the
experimental MLCT absorption band (the optical band gap using
the edge of the lowest-energy absorption band was determined to
be 2.92, 2.77, 2.55 and 2.40 eV for complexes 1a, 1b, 2a and 2b,
respectively). The relatively low oxidation potential of each Cu(I)
complex readily results in combination with an oxidant, such as
oxygen.18 Thus, these neutral Cu(I) complexes exhibit a useful
oxygen-sensing feature.
Fig. 7 Emission spectra of complex 2b in PMMA as a function of
temperature from 294 to 77 K; lex = 365 nm.
Molecular oxygen sensing
based on the emission from the sodium salts of the ligands (Fig.
S1 and S2, ESI†), which displayed similar emissive positions of
shoulders and broad features as the complexes. DFT calculations
also demonstrate that the emission involves MLCT and ILCT
mechanisms (Fig. 8). This is the first time that dual emission in
mononuclear Cu(I) complexes has been observed, although similar
phenomena were reported for Ru(II),14 Pt(II),15 and dinuclear
Cu(I)16 complexes.
The oxygen gas-sensing experiments were performed on disper-
sions of the complexes into PMMA. Fig. 9 shows the emission
spectra of a PMMA film containing a loading level of 10 wt%
of complex 2b under several different oxygen concentrations in
argon, ranging from 375 to 700 nm after excitation at 365 nm.
The emission from MLCT of complex 2b with its maximum at
610 nm was easily quenched by oxygen; this was simultaneously
accompanied by a fluorescence increase at 410 nm corresponding
to emission from the ligand centered (LC) state of the free ligand
quin (Fig. S1 and S2, ESI†). The ratios of emission intensities
at l = 411 and 610 nm ranged from 0.3 to 3.2, indicating
that this complex acts as a ratiometric sensor for oxygen gas-
sensing. Furthermore, the clear change of emission color from
orange to blue upon excitation of UV light is desirable for
ratiometric fluorescent probes, and was easily visible. Response
time and relative fluorescence intensity changes of complex 2b
are recorded in Fig. 10 upon repeatedly switching environments
between vacuum and 100% oxygen. Two different steps on the
recovery curve by vacuum show that the removal of oxygen
absorbed on the surface of a film was facile, but difficult when it was
incorporated in a polymer film. The Stern–Volmer curve, fitted by
Fig. 8 Partial Kohn–Sham orbital energy-level diagram of 2b from
density functional theory (DFT) calculation.
The electrochemical behavior of the complexes was investi-
gated by cyclic voltammetry (CV). Relative to the ionic Cu(I)
complexes,17 these neutral complexes in degassed CH2Cl2 solution
displayed a lower irreversible oxidation peak at 0.55 (1a), 0.45 (1b),
0.60 (2a) and 0.51 V (2b), respectively (Ag/AgCl as reference),
which is attributed to the oxidation of Cu(I) to Cu(II). The unusual
oxidation potentials probably originate from the strong electron-
donating effect of anionic indole units, which significantly increase
the electron density on the Cu(I) center and hence raise the highest
occupied molecular orbital (HOMO) energy level. In CH3CN,
no obvious reduction peak was observed, which indicates that
Fig. 9 Emission spectra of complex 2b under different oxygen concen-
trations, lex = 365 nm. The inset shows the ratiometric calibration curve
I
411 nm/I610 nm as a function of oxygen concentration.
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is due to charge transfer from the anionic ligand to an oxygen
molecule (except for energy transfer when sensing), as shown in
Fig. 12, although no direct experimental evidence is available.
Some indirect information, however, supports the above postulate:
(1) the sample after O2-sensing still exhibited an MLCT absorption
band (Fig. S3, ESI†), attributed to the presence of the Cu–N
bond (from the pyridine ring); (2) the blue emission is proposed
to originate mainly from the contribution of the indole moieties,
based on the near identical emissive position of both the free
ligands Hbyin and Hquin; thus the cleavage of Cu–N (indole)
bond may have occurred giving rise to the indole radical; (3) DFT
calculations also show that the ILCT transition increases when
the MLCT transition still remains after the central Cu(I) ion is
attacked by gaseous oxygen (Fig. 13). (4) The excellent reversibility
for oxygen gas-sensing proves that no Cu–O bond is formed during
these procedures.20 In addition, the neutral Cu(I) complex does not
contain any counter-anion, which always attacks a Cu(I) ion in the
excited states as a nucleophilic reagent so inhibiting the recovery of
its excited state geometry from square-planar.6 So the neutral Cu(I)
complexes readily recover their tetrahedral geometry after removal
of oxygen molecules with concomitant loss of the counteranion.
Hence, the mechanism should incorporate both charge and energy
transfer, resulting in the formation of a radical.
Fig. 10 Response time and relative fluorescence intensity changes at
610 nm for complex 2b in PMMA coating upon switching between vacuum
and oxygen repeatedly, lex = 365 nm.
Fig. 12 The possible sensing mechanism of complex 2b towards O2.
Fig. 11 Stern–Volmer plots for 10 wt% 2b doped sample in PMMA under
various oxygen concentrations from 0 to 100% at 298 K. Solid line is fitted
using the Demas model: I0/I = [f 01/(1 + KSV1[O2]) + f 0 /(1 + KSV2[O2])]-1;
2
(f 01 + f 02 = 1), I0/I100 = 2.65, KSV1 (O2V%-1) = 0.33547, KSV2 (O2 vol%-1) =
0.00139, f 01 = 0.59393, f 01 = 0.40607, R2 = 0.99922.
the Demas model, was nonlinear (Fig. 11),19 which also shows that
the complex molecules were in varying microenvironments with
heterogeneous active sites originating from the depth variation
of the complex molecules in the PMMA film due to the coating
process.20 The four complexes all showed a rapid response time
at 3–4 s in reducing their intensity by 95% response; however,
only compound 2b displayed perfect reversibility under reduced
pressure or in a flow of inert gas. Its high reproducibility is ascribed
to the stronger coordination ability of the quin and the bidentate
phosphine ligands relative to the other three complexes as well as
to the absence of a counteranion.
Unlike the ionic Cu(I) complexes, in which only the quenching
of the MLCT emission is observed as a result of the energy
transfer,3 the appearance of a high energy emissive peak indicates
that dissociation of the ligand, or decomposition of the complex,
might have occurred.4 However, the rapid recovery of the MLCT
emission remains unexplained. The intensity of the ILCT emission
(Fig. 9) decreased more slowly than that of MLCT, and remained
stronger than triplet emission, which confirms that emissions,
including from LC, ILCT and MLCT transitions, are from a single
molecule and share similar excitation energy. The sole difference
is that of their relative intensity ratio. Thus, we suggest that this
Fig. 13 LUMO, HOMO and HOMO-1 orbitals of 2b+(O2)-.
Conclusion
In summary, we have reported the first example of neutral
heteroleptic Cu(I) complexes based on indole derivatives as oxygen
gas sensors. These neutral Cu(I) complexes exhibit a ILCT and
MLCT (singlet and triplet) dual emission from a single complex
molecule. The enriched electronic density of neutral complexes
ligated to the negative ligands results in atypically low oxidation
potential in contrast to cationic Cu(I) complexes. The high oxygen
concentration-dependent luminescence quenching property, in
conjunction with a remarkable enhancing of blue fluorescence
emission, makes these complexes plausible candidates as ratio-
metric oxygen sensors. The sensing ability obviously correlates
with the rigidity of the complex molecule and the absence of
counterion. The sensing mechanism of these neutral molecules is
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attributed to the synergetic effect of the energy and charge transfer.
Further studies to develop new neutral Cu(I) complexes with high
photoluminescent quantum yields, by improving the rigidity of
the anionic ligands, are in progress.
absorption corrections were performed using SADABS program.21
The crystal structure was solved using the SHELXTL program
and refined using full-matrix least squares.22 All non-hydrogen
atoms were refined anistropically. The positions of the hydrogen
atoms attached to carbon atoms were fixed at their ideal positions.
The crystallographic data are reported in Table 1.
DFT calculations were performed using GAUSSIAN 03 W
software package using a spin-restricted formalism at the B3LYP
level. The basis sets 6-311++G** was used for all other atoms.23
Experimental
General
Starting materials were purchased from Sigma–Aldrich, Acros
or Fluka and used without further purification. Solvents were
freshly distilled over appropriate drying reagents under an argon
atmosphere. All experiments were carried out under a dry argon
atmosphere using standard Schlenk techniques unless otherwise
stated.
Synthesis
[Cu(pyin)(PPh3)2] (1a). A typical procedure is as follows. A
degassed mixture of ligand Hpyin (0.194 g, 1.0 mmol) and NaH
(0.12 g, 5.0 equiv.) were stirred in 10 ml newly evaporated THF for
2 h under an argon atmosphere in flask A. CuI (0.190 g, 1.0 mmol)
and triphenylphosphine (0.532 g, 2.0 mmol) were degassed, and
stirred in 10 ml THF for 2 h under the argon atmosphere in flask
B. The solution of sodic ligand in flask A was transferred to Cu(I)
solution in flask B. The reaction mass was stirred for 2 h, then
the solvent was evaporated under vacuum to dryness. The solid
residue was extracted with 10 ml absolute dichloromethane under
the argon atmosphere while the extract was filtered and transferred
to an argon-protected flask. 10 ml methanol was layered above the
resulting solution afforded yellow crystals of complex 1a,◦which
Characterization. 1H NMR and 13C NMR with TMS as
internal reference and 31P NMR spectra with 85% H3PO4 as
external reference were recorded on a Bruker Avance 600 MHz
spectrometer at room temperature. Elemental analyses for C, H
and N were performed with a BioRad elemental analysis system.
Thermogravimetric analysis (TGA) was performed on a Perkin–
Elmer series 7 analysis systems under N2 at a heating rate of
10 ◦C min-1. Absorption spectra were recorded on a Perkin–Elmer
Lambda35 UV/Vis Spectrometer. Photoluminescence spectra
were recorded on SHIMADZU RF-5301PC Spectrofluoropho-
tometer. The measurements at low temperature (77 K) were
performed in quartz tubes inserted in a liquid-nitrogen-filled
quartz Dewar. The HOMO and LUMO energies were determined
using XRD geometries to approximate the ground state. The PL
quantum yields were determined by the integrating sphere with
409 nm excitation of a HeCd laser. Luminescence lifetimes were
measured with a Lecroy Wave Runner 6100 digital oscilloscope
(1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns)
for the excitation (Continuum Sunlite OPO). The samples for all
the steady optical spectra were in sealed quartz cuvettes under
nitrogen atmosphere. Cyclic voltammetry was performed using a
on Chi660b electrochemical ananlyzer with a three-electrode cell
in 0.1 M tetrabutylammonium perchlorate (Bu4NClO4), in dry
dichloromethane solution under an argon atmosphere at a scan
rate of 100 mV s-1 and using ferrocene as standard, in which a
glassy carbon working electrode, a Pt auxiliary electrode, and a
Ag/AgCl reference electrode were employed.
The films for oxygen sensing was made as follows: 5 mg Cu(I)
complex and 45 mg PMMA (MW = 12 000) powder were dissolved
in 0.5 ml absolute dichloromethane under an argon atmosphere,
then the viscous mixture solution was filmed and dried on the wall
of a cylindrical cuvette by gradual vacuum avoiding bubbles. The
cuvette was fixed on a modified appurtenance in the darkroom
of luminescence spectrometer, and a tube was piped between the
cuvette and the pressure system. Oxygen sensitivity was measured
by monitoring the fluorescence intensity of the coating film upon
switching between vacuum and certain oxygen concentration in
the quartz cuvette.
1
were washed with methanol, yield 0.65 g (84%); Mp169 C; H
NMR (600 MHz, CDCl3) d 7.88–7.81 (m, J = 7.3 Hz, 2H), 7.67–
7.63 (m, 1H), 7.50 (t, J = 7.5 Hz, 1H), 7.30–7.08 (m, 32H), 6.86–
6.81 (m, 2H), 6.71 (t, J = 7.5 Hz, 1H); 13C NMR (151 MHz, CDCl3)
d 156.44, 148.29, 147.11, 145.46, 136.25, 134.65, 134.51, 133.66,
133.56, 132.39, 129.17, 128.38, 128.32, 120.14, 119.95, 119.59,
119.32, 117.36, 116.54, 98.75; 31P NMR (243 MHz, CDCl3) d
-0.75. Elemental analysis for C49H39CuN2P2: calc.: C, 73.83; H,
5.33; N: 3.44; found: C, 73.76; H, 5.30; N, 3.37%.
The procedures for the synthesis of 1b, 2a and 2b were similar to
the described as 1a. Only the quantities of ligand that were used,
the product yields, the melting points, NMR data, and elemental
analyses are given.
[Cu(pyin)(DPEphos)] (1b). Using Hpyin (0.194 g, 1.0 mmol)
and bis(2-(diphenylphosphino)phenyl)ether (0.538 g, 1.0 mmol)
afforded 0.57 g yellow crystals of 1b (72%). Mp 210 ◦C; 1H NMR
(600 MHz, ppm, CDCl3) d 7.86 (d, J = 4.9 Hz, 1H), 7.77 (d,
J = 8.0 Hz, 1H), 7.59 (d, J = 7.6 Hz, 1H), 7.43 (t, J = 7.6 Hz,
1H), 7.27–6.95 (m, 26H), 6.88 (t, J = 7.5 Hz, 2H), 6.80–6.72 (m,
4H), 6.57 (t, J = 7.5 Hz, 1H); 13C NMR (151 MHz, CDCl3) d
158.63, 156.25, 148.14, 147.18, 145.39, 135.64, 134.27, 133.69,
133.15, 133.05, 132.96, 132.20, 130.65, 128.99, 128.10, 128.07,
128.04, 126.31, 124.29, 120.20, 119.82, 119.63, 119.19, 118.62,
117.52, 116.05, 97.78; 31P NMR (243 MHz, CDCl3) d -14.43.
Elemental analysis for C49H37CuN2OP2·CH2Cl2: calc.: C, 68.22;
H, 4.47; N, 3.18; found: C, 68.09; H, 4.35; N, 3.06%.
[Cu(quin)(PPh3)2] (2a). Using Hquin (0.244 g, 1.0 mmol) and
triphenylphosphine (0.532 g, 2.0 mmol) afforded 0.68 g red crystals
of 2a (81%). Mp 157 ◦C; 1H NMR (600 MHz, CDCl3) d 8.00 (q, J =
8.6 Hz, 2H), 7.66 (d, J = 7.4 Hz, 1H), 7.59 (dd, J = 12.7, 8.2 Hz, 2H),
7.32 (s, 1H), 7.30–7.21 (m, 18H), 7.19 (t, J = 7.4 Hz, 1H), 7.17–7.10
(m, 12H), 7.00–6.94 (m, 2H), 6.85–6.77 (m, 2H); 13C NMR (151
MHz, CDCl3) d 156.85, 148.17, 146.77, 146.53, 136.58, 135.24,
X-Ray crystallography. The crystal data was collected on a
Bruker Smart APEX diffractometer with CCD detector and
˚
graphite monochromated Mo-Ka radiation (l = 0.71073 A).
The intensity data were recorded with w scan mode (298 K).
Lorentz polarization factors were made for the intensity data and
1318 | Dalton Trans., 2012, 41, 1312–1319
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134.96, 134.22, 134.02, 132.74, 132.56, 132.44, 129.58, 129.22,
129.09, 128.83, 128.72, 127.76, 127.48, 124.98, 121.01, 120.59,
120.04, 117.99, 117.19, 102.21; 31P NMR (243 MHz, CDCl3)
d -0.72. Elemental analysis for C53H41CuN2P2·CH2Cl2: calc.: C,
70.78; H, 4.73; N, 3.06; found: C, 70.57; H, 4.46; N, 2.89%.
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[Cu(quin)(DPEphos)] (2b). Using Hquin (0.244 g, 1.0 mmol)
and bis(2-(diphenylphosphino)phenyl)ether (0.538 g, 1.0 mmol)
◦
1
afforded 0.64 g of 2b (76%). Mp 196 C; H NMR (600 MHz,
CDCl3) d 7.90 (d, J = 8.6 Hz, 1H), 7.82 (d, J = 8.6 Hz, 1H), 7.77
(d, J = 8.4 Hz, 1H), 7.62 (d, J = 7.8 Hz, 1H), 7.52 (t, J = 8.7 Hz, 2H),
7.49–7.45 (m, 1H), 7.32 (dt, J = 23.0, 7.1 Hz, 1H), 7.22–6.85 (m,
30H), 6.84–6.79 (m, 2H), 6.70 (t, J = 7.6 Hz, 1H); 13C NMR (151
MHz, CDCl3) d 158.85, 155.94, 147.68, 146.48, 146.39, 135.34,
134.32, 133.16, 132.27, 130.68, 128.99, 128.64, 127.98, 127.87,
126.96, 126.96, 126.9, 126.84, 126.73, 124.36, 124.17, 120.55,
119.82, 119.53, 119.42, 117.7, 116.14, 100.86; 31P NMR (243 MHz,
CDCl3) d -13.7. Elemental analysis for C53H39CuN2OP2·CH3OH:
calc.: C, 73.92; H, 4.94; N, 3.19; found: C, 73.62; H, 4.76; N, 3.07%.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Nos. 20874098, 51073152 and 51073152)
and the 973 Project (2009CB623600).
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©