Synthesis, characterization, photophysical and oxygen-sensing properties of a copper(I) complex

Article abstract of DOI:10.1007/s11243-010-9370-1

In this paper, we report the synthesis, crystal structure, photophysical properties, and electronic nature of a phosphorescent Cu(I) complex of [Cu(TBT)(POP)]BF₄, where TBT and POP stand for 4, 5, 9, 14-tetraaza-benzo[b]-triphenylene and bis(2-(diphenylphosphanyl)phenyl) ether, respectively. [Cu(TBT)(POP)]BF₄ renders a red phosphorescence peaking at 622 nm, with a long excited-state lifetime of 13.2 μs. Density functional calculation reveals that the emission comes from a triplet metal-to- ligandcharge-transfer excited state. We electrospun composite nanofibers of [Cu(TBT)(POP)]BF₄ and polystyrene, hoping to explore the possibility of replacing precious-metal-based oxygen sensors with cheap Cu-based ones. The finally obtained samples with average diameter of ～700 nm exhibit a maximum sensitivity of 5.8 toward molecular oxygen with short response/recovery time (5/13 s) due to the large surface-area-to-volume ratio of nanofibrous membranes. No photobleaching is detected in these samples. All these results suggest that phosphorescent Cu(I) complexes doped nanofibrous membranes are promising candidates for low-cost and quick-response oxygen-sensing materials. Springer Science+Business Media B.V. 2010.

Full text of DOI:10.1007/s11243-010-9370-1

Transition Met Chem (2010) 35:605–611
DOI 10.1007/s11243-010-9370-1
Synthesis, characterization, photophysical and oxygen-sensing
properties of a copper(I) complex
Chuanqi Zhou Qingshuang Wang
•
Received: 22 January 2010 / Accepted: 7 May 2010 / Published online: 26 May 2010
Ó Springer Science+Business Media B.V. 2010
Abstract In this paper, we report the synthesis, crystal
structure, photophysical properties, and electronic nature of
a phosphorescent Cu(I) complex of [Cu(TBT)(POP)]BF4,
where TBT and POP stand for 4,5,9,14-tetraaza-benzo[b]-
triphenylene and bis(2-(diphenylphosphanyl)phenyl) ether,
Introduction
During the past decade, a great interest in transition metal
complexes, especially heavy metal complexes such as
Ru(II) and Re(I) complexes, has been sparked by the
development of practical components for chemical sensors,
display devices, biological probes, phototherapy, and solar-
energy conversion [1, 2]. At the same time, strong appeal
of replacing the expensive compounds based on precious
metal complexes with cheap ones, as well as the need for a
deeper understanding on the correlation between molecular
structures and photophysical properties, has pushed a
continuous progress in the design of luminescent first-row
transition metal complexes. Phosphorescent Cu(I) com-
plexes, as a new class of optoelectrical material, have
drawn much attention due to their advantages of lower
toxicity, low cost, abundant resource, and environmental
friendliness [3].
respectively. [Cu(TBT)(POP)]BF renders a red phospho-
4
rescence peaking at 622 nm, with a long excited-state
lifetime of 13.2 ls. Density functional calculation reveals
that the emission comes from a triplet metal-to-ligand-
charge-transfer excited state. We electrospun composite
nanofibers of [Cu(TBT)(POP)]BF and polystyrene, hoping
4
to explore the possibility of replacing precious-metal-based
oxygen sensors with cheap Cu-based ones. The finally
obtained samples with average diameter of *700 nm
exhibit a maximum sensitivity of 5.8 toward molecular
oxygen with short response/recovery time (5/13 s) due to
the large surface-area-to-volume ratio of nanofibrous
membranes. No photobleaching is detected in these sam-
ples. All these results suggest that phosphorescent Cu(I)
complexes doped nanofibrous membranes are promising
candidates for low-cost and quick-response oxygen-sensing
materials.
Generally, emission signals from charge-transfer excited
states of copper(I) complexes are typically weak and short
lived because the lowest energy charge-transfer state of a
1
0
d
system involves excitation from a metal–ligand dr*
orbital [3, 4]. An important consequence is that the excited
state typically prefers a tetragonally flattened geometry,
whereas the ground state usually adopts a tetrahedral-like
coordination geometry appropriate for a closed-shell
structure. Aside from reducing energy content, the geo-
metric relaxation that occurs in excited states facilitates
relaxation back to the ground state [5, 6]. McMillin,
Cuttell, Walton, and coworkers first reported this type of
exciplex quenching, and by now, many other studies have
confirmed the mechanism [3, 7, 8].
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-010-9370-1) contains supplementary
material, which is available to authorized users.
C. Zhou (&)
Beijing Institute of Genomics, Chinese Academy of Sciences,
No. 7 Beitucheng West Road, Chaoyang District,
100029 Beijing, People’s Republic of China
e-mail: chuanqizhou@gmail.com
Q. Wang
School of Life Science and Technology, Changchun University
of Science and Technology, 7089 Weixing Road, Changchun,
Mixed-ligand systems involving triphenylphosphane
seem to be promising because they exhibit long excited-
state lifetimes in solid state and frozen solutions [9, 10].
1
30022 Jilin, People’s Republic of China
123

6
06
Transition Met Chem (2010) 35:605–611
A series of new mixed-ligand copper(I) complexes such
?
as [Cu(N–N)(POP)] [POP=bis(2-(diphenylphosphanyl)-
0.05 mmol of 4-methylbenzenesulfonic acid was heated at
80 °C for 10 h, the crude product was filtered off and then
recrystallized from ethanol to give the pure desired prod-
phenyl) ether] which are superior emitters have been
synthesized. It is found that the exciplex quenching is
relatively inefficient for the charge-transfer excited state in
this POP system. In addition, the introduction of sterically
blocking ligands can impede geometric relaxation as well
as solvent attack [11]. Here, steric effects cooperate to
effectively constrain the excited state close to the ground-
state geometry, which has been proved by the theoretical
1
uct. Yield 1.11 g (79%). H NMR (300 Hz, CDCl , 25 °C):
3
d 9.63(d, 2H, J = 8.0 Hz), 9.21(d, 2H, J = 8.0 Hz),
8.37(d, 2H, J = 6.4 Hz), 7.87(d, 2H, J = 6.4 Hz), 7.78(m,
2H). Anal. Calcd. for C H N : C, 76.60; H, 3.55; N,
1
8 10 4
19.86. Found: C, 76.71; H, 3.67; N, 19.72.
Synthesis of the Cu(I) complex
?
studies by Feng and coworkers on [Cu(N–N)(P–P)] sys-
tem [12]. For a typical phosphorescent Cu(I) complex, the
highest occupied molecular orbital (HOMO) has a pre-
dominant metal Cu d character, while the lowest unoccu-
pied orbital (LUMO) is essentially p* orbitals localized on
the diimine ligand. The photoluminescence corresponds to
[Cu(TBT)(POP)]BF was synthesized according the liter-
4
ature procedures except that the diimine was replaced by
TBT [7, 11]. Its identity was confirmed by NMR, elemental
analysis, and single-crystal XRD (CCDC 731780 which
can be obtained free of charge from The Cabridge Crys-
tallographic Data Center via www.ccdc.cam.ac.uk/data_
the lowest triplet T and is thus assigned as a character of
1
3
metal-to-ligand-charge-transfer MLCT [d(Cu) ? p*(dii-
request/cif, please also see Supplementary Material for
1
mine ligand)]. The MLCT excited states of cuprous diimine
compounds are often luminescent and play important roles
in photoinduced electron and energy transfer process.
In this paper, we report a phosphorescent Cu(I) com-
crystal data). H NMR (300 Hz, CDCl , 25 °C): d 9.61(d,
3
2H, J = 8.0 Hz), 9.25(d, 2H, J = 8.0 Hz), 8.31(d, 2H,
J = 6.4 Hz), 7.92–7.77(m, 22H), 7.35–7.31(m, 10H). Anal.
Calcd. for C H BCuF N OP : C, 66.78; H, 3.94; N, 5.77.
5
4
38
4
4
2
plex, namely [Cu(TBT)(POP)]BF , where TBT stands for
4
Found: C, 66.70; H, 3.81; N, 5.81.
4
,5,9,14-tetraaza-benzo[b]triphenylene. Its crystal struc-
ture, photophysical properties, and electronic nature are
discussed in detail. In addition, we electrospun composite
Fabrication of [Cu(TBT)(POP)]BF /polystyrene
4
nanofibrous membranes
nanofibers of [Cu(TBT)(POP)]BF /polystyrene, hoping to
4
explore the possibility of replacing precious-metal-based
oxygen sensors with cheap Cu-based ones. The resulting
samples exhibit a maximum sensitivity of 5.8 toward
molecular oxygen with short response/recovery time (5/
A typical procedure for the electrospinning of composite
nanofibers is described as follows. Polystyrene with a
number-average molecular mass of 100,000 was dissolved
0
in N,N -dimethyl formamide (DMF) to form a 22 wt%
solution. Then [Cu(TBT)(POP)]BF was added into the
1
3 s), and no photobleaching is detected in these samples.
4
solution under stirring to form [Cu(TBT)(POP)]BF /poly-
4
Experimental section
styrene homogeneous solutions. The final solutions were
then electrospun to give composite nanofibrous membranes
A synthetic route for the diimine ligand of TBT and its
corresponding Cu(I) complex with POP as the auxiliary
ligand is shown in Scheme 1. Bis[2-(diphenylphosphino)-
phenyl] ether (POP, 99% purity), Cu(BF ) (AR grade),
of [Cu(TBT)(POP)]BF /polystyrene.
4
Methods and measurements
4
2
1
,10-phenanthroline (AR grade), benzene-1,2-diamine (AR
grade), and polystyrene (melt flow rate = 3.4 g/10 min)
were purchased from Aldrich Chemical Co. and used without
further purifications. Organic solvents were purified through
standard procedures. 1,10-phenanthroline-5,6-dione and
Density functional theory (DFT) and singlet excitation
calculations using time-dependent density functional the-
?
ory (TD-DFT) were performed on [Cu(TBT)(POP)] at
RB3LYP/SBKJC level. The initial geometry was obtained
from its single crystal. All computations were done with
the GAMESS software package. Excited-state lifetimes
were obtained with a 355-nm light generated from the
Third-Harmonic-Generator pumped, using pulsed Nd:YAG
laser as the excitation source. The Nd:YAG laser possesses
[
Cu(CH CN) ]BF was synthesized according to the litera-
3 4 4
ture procedures [11, 13].
Synthesis of the diimine ligand
-
1
4
,5,9,14-tetraaza-benzo[b]triphenylene (TBT) was synthe-
a line width of 1.0 cm , pulse duration of 10 ns and
repetition frequency of 10 Hz. A Rhodamine 6G dye
pumped by the same Nd:YAG laser was used as the fre-
quency-selective excitation source. All photoluminescence
sized by a modification of a literature method [14]. A
mixture of 5 mmol of 1,10-phenanthroline-5,6-dione,
5
.5 mmol of benzene-1,2-diamine, 25 mL of ethanol, and
1
23

Transition Met Chem (2010) 35:605–611
607
Scheme 1 A synthetic route for
TBT and [Cu(TBT)(POP)]BF
4
(
PL) spectra were measured with a Hitachi F-4500 fluo-
the coordinated N atoms, indicating a weak interaction
between O atom and Cu(I) center. Similar CuꢀꢀꢀO separa-
tions have also been reported in POP-based Cu(I) com-
plexes that contain 1,10-phenanthroline derived ligands,
which means that the Cu(I) center tends to achieve a five-
coordinated environment [5, 6, 11, 12].
rescence spectrophotometer. UV–Vis absorption spectra
were recorded using a Shimadzu UV-3101PC spectropho-
tometer. Scanning electron microscopy (SEM) images
1
were obtained on a Hitachi S-4800 microscope. H NMR
spectra were obtained with a Varian INOVA 300 spec-
trometer. Elemental analysis was performed on a Carlo
Erba 1106 elemental analyzer. Single-crystal data were
collected on a Siemens P4 single-crystal X-ray diffrac-
tometer with a Smart CCD-1000 detector and graphite-
monochromated Mo Ka radiation, operating at 50 kV and
Unlike the literature values of *80°, the N–Cu–N bond
angle of [Cu(TBT)(POP)]BF (81.2°) is somewhat greater,
4
clearly due to the reduced static congestion caused by the
large dihedral angle between N–Cu–N and P–Cu–P planes
of [Cu(TBT)(POP)]BF (89.58°) as mentioned [11, 12]. On
4
3
0 A at 298 K. All hydrogen atoms were calculated.
the other hand, the P–Cu–P bond angle is as large as
113.2°, as shown in Table 1. Considering POP’s natural
bite angle of 102.2°, with a flexibility range from 86° to
120°, the large P–Cu–P bond angle in [Cu(TBT)(POP)]BF4
Oxygen-sensing performances were measured on the basis
of steady emission intensity quenching. All measurements
were carried out in the air at room temperature without
being specified.
Results and discussion
Crystal structure of [Cu(TBT)(POP)]BF4
Figure 1 shows the molecular structure of [Cu(TBT)(POP)]-
BF obtained from its single crystal. Geometric parameters
4
listed in Table 1 suggest that the Cu(I) center has a dis-
torted tetrahedral coordination sphere. The dihedral angle
between N–Cu–N and P–Cu–P planes is 89.6°. Cu–N bond
lengths in [Cu(TBT)(POP)]BF occupy a narrow region of
4
˚
.05–2.06 A which are comparable to literature values
2
[
5, 6, 11, 12]. Similarly, the two Cu–P bond lengths are
˚
exactly the same with a distance of 2.24 A away from the
Cu(I) center. The O atom of the POP ligand lies at a dis-
Fig. 1 Molecular structure [Cu(TBT)(POP)]BF
-
4
obtained from its
moiety and all hydrogen atoms are omitted for
single crystal. BF
clarity
4
˚
tance of *3 A away from the Cu(I) center and opposite to
123

6
08
Transition Met Chem (2010) 35:605–611
Table 1 Selected geometric parameters of [Cu(TBT)(POP)]BF
obtained from single-crystal data
4
As shown in Fig. 2, the [Cu(TBT)(POP)]BF4 doped
poly(methyl methacrylate) (PMMA) film (10 wt%) renders a
broad emission band peaking at 620 nm, without giving any
vibronic progressions, suggesting a charge-transfer emissive
character. The emission owns a long excited-state lifetime of
˚
A
Bond length
Bond angle
°
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–P(1)
Cu(1)–P(2)
CuꢀꢀꢀO
2.051(5)
2.062(5)
2.2421(18)
2.2435(18)
3.08
N(1)–Cu–N(2)
N(1)–Cu–P(1)
N(2)–Cu–P(1)
N(1)–Cu–P(2)
N(2)–Cu–P(2)
P(1)–Cu–P(2)
81.20(2)
117.21(15)
112.82(14)
116.71(14)
111.51(14)
113.26(7)
13.2 ls, indicating the presence of a triplet state emissive
character. The large Stokes shift between absorption and
emission spectra of [Cu(TBT)(POP)]BF (120 nm) reveals a
4
severe geometric relaxation that occurs in the excited state.
Similarly, the emission peak also demonstrates a red shift
compared with that of [Cu(Phen)(POP)]BF4 peaking at
suggests a crowded coordination environment around the
Cu(I) center. Correspondingly, N–Cu–N and P–Cu–P
planes tend to spread out to minimize the crowed envi-
ronment [15].
*
narrower energy gap than that of [Cu(Phen)(POP)]BF due
550 nm, confirming that [Cu(TBT)(POP)]BF owns a
4
4
to TBT’s larger coplanar conjugation system [11].
?
Theoretical calculations on [Cu(TBT)(POP)]
Photophysical properties of [Cu(TBT)(POP)]BF4
As mentioned earlier, the MLCT excited states of cuprous
diimine compounds are often luminescent and play
important roles in photoinduced electron and energy
transfer process. In order to get a further understanding on
Figure 2 shows the UV–Vis absorption spectrum of
[
Cu(TBT)(POP)]BF in CH Cl solution with a concen-
4 2 2
-
tration of 1 9 10 mol/L, along with those of TBT and
4
POP ligands. The [Cu(TBT)(POP)]BF absorption spec-
4
the electronic nature of [Cu(TBT)(POP)]BF , we per-
4
trum is composed of a strong absorption band ranging from
formed a DFT/TD-DFT calculation, which has been proved
to be a powerful tool to investigate electronic properties
2
50 to 330 nm and a weak low-energy absorption band
?
of transition metal complexes, on [Cu(TBT)(POP)] at
ranging from 350 to 500 nm. From the comparison
between those absorption spectra, the former absorption
band should correspond to ligands p ? p* transitions,
while the latter one is experimentally assigned as MLCT
transition absorptions [14]. Compared with that of
B3LYP/SBKJC level [12]. As shown by Fig. 3 and
?
Table 2, the HOMO of [Cu(TBT)(POP)] has an evident
metal Cu character, admixed with large contributions from
the phosphorous ligand, while its LUMO is essentially dii-
mine ligand p* orbitals of TBT. The calculated onset tran-
sition of S ? S corresponds to an electronic transition
[
Cu(Phen)(POP)]BF4 (Phen=1,10-phenanthroline), it is
observed that the MLCT absorption of [Cu(TBT)(POP)]-
0
1
BF red shifts by *50 nm, owing to the enlarged coplanar
4
from HOMO to LUMO as shown by Table 3, and the onset
excitation transition (510.66 nm) correlates quite well with
the experimentally recorded absorption edge shown in
Fig. 2 (502 nm). It is thus confirmed that the onset elec-
conjugation system in the TBT ligand [11].
tronic transition of [Cu(TBT)(POP)]BF is a MLCT one.
4
According to Feng’s report, the photoluminescence of
[Cu(N–N)(P–P)] ± complexes corresponds to the lowest
triplet T , which consists of the transition from HOMO to
1
LUMO or LUMO ? 1, and is thus assigned as having mixed
character between MLCT [d(Cu) ? p*(phen)] [12]. Cor-
respondingly, the emissive state of [Cu(TBT)(POP)]BF is
4
also expected to be a MLCT one. Combined with the long
excited-state lifetime as mentioned, we come to a conclusion
that the red emission of [Cu(TBT)(POP)]BF originates
4
3
from MLCT excited state, which is consistent with a pre-
vious report [12].
Oxygen-sensing properties of [Cu(TBT)(POP)]BF4/
polystyrene nanofibrous membranes
Fig. 2 UV–Vis absorption of TBT, POP, and [Cu(TBT)(POP)]BF
CH Cl
2 2
4
in
solutions with a concentration of 1 9 10 mol/L. Inset: PL
spectrum of [Cu(TBT)(POP)]BF doped in PMMA
-
4
Considering the long excited-state lifetime of [Cu(TBT)-
(POP)]BF which makes the excited [Cu(TBT)(POP)]BF
4
4
4
1
23

Transition Met Chem (2010) 35:605–611
609
SEM image of the 2 wt% doped sample. The composite
fibers with average diameter of *700 nm are randomly
distributed on the substrate, showing a smooth and uniform
morphology. No branch structure is observed for the
composite fibers. Those composite fibers own a large sur-
face-area-to-volume ratio which is two orders of magnitude
larger than that of continuous thin films, providing an
excellent matrix with high diffusion coefficient for a rapid
response, which will be discussed later [18].
Sensitivity of [Cu(TBT)(POP)]BF /polystyrene
4
nanofibrous membranes
Figure 4 shows the emission spectra of the 2 wt% doped
sample under various oxygen concentrations from 0 to
100% with an interval of 10%. With increasing oxygen
concentrations, the emission intensity at 560 nm decreases
significantly. The sensitivity (I0/I100, where I is the lumi-
0
nescence intensity under 100% N atmosphere and I is
2
100
that under 100% O atmosphere) values of the four samples
2
with various dopant concentrations are 5.1 for 1 wt% doped
[
Cu(TBT)(POP)]BF /polystyrene sample, 5.5 for 1.5 wt%
4
Fig. 3 LUMO (up) and HOMO (down) presentations of [Cu(TBT)-
?
POP)] calculated at RB3LYP/SBKJC level with the contour value
doped [Cu(TBT)(POP)]BF /polystyrene sample, 5.8 for 2
4
(
of 0.022
wt% doped [Cu(TBT)(POP)]BF /polystyrene sample, and
4
4.4 2.5 wt% doped [Cu(TBT)(POP)]BF /polystyrene sam-
4
ple, respectively. Generally, a sensor with sensitivity
higher than 3 can be used for actual applications, which
means that all four samples are qualified enough to serve as
actual sensors [19]. The 2 wt% doped sample exhibits a
higher sensitivity than the others, which can be explained
as follows. There may be at least two opposite factors
affecting sample’s sensitivity: emission intensity from
probe molecules and adverse interaction between probe
molecules (aggregation, for example). When the dopant
concentration is low, emission from the probe is weak,
leading to a low sensitivity. On the other hand, a much
higher dopant concentration may accelerate the intermo-
lecular aggregation which also decreases the sensitivity.
The two opposite factors may achieve a balance in the 2
wt% doped sample, resulting in the maximum sensitivity of
5.8.
?
Table 2 Orbital percentage composition of [Cu(TBT)(POP)] cal-
culated at RB3LYP/SBKJC level
Orbitals
Composition (%)
Energy (eV)
LUMO ? 1
LUMO
TBT(81.2)
-4.34
-4.88
-7.79
-8.15
TBT(98.7)
HOMO
Cu(24.1) POP(70.6)
Cu(41.7) POP(20.1)
HOMO - 1
molecule sensitive and vulnerable to any electron/energy
acceptors, we intend to investigate its PL response toward
oxygen and thus explore the possibility of replacing pre-
cious-metal-based oxygen sensors with cheap Cu-based
ones. For practical applications in optical oxygen-sensing
devices, it is necessary to embed the sensors into a solid
matrix acting as a supporting medium, allowing oxygen
transportation from the surroundings, and the support may
also have quite stringent criteria for suitable performances.
Here, we select polystyrene that has been proved to be an
excellent host material for electrospinning as the support-
ing matrix for our earlier research [16, 17].
In addition, it is observed that the PL spectra of the 2
wt% doped sample, peaking at *560 nm, exhibit a blue
shift tendency compared with the PL spectrum of
[Cu(TBT)(POP)]BF4 centering at 620 nm. Correspond-
ingly, the excited-state lifetimes of samples under 100% N2
atmosphere are 100 ls for 1 wt% doped sample, 110 ls for
1.5 wt% doped sample, 120 ls for 2 wt% doped sample,
Morphology of [Cu(TBT)(POP)]BF /polystyrene
4
and 90 ls for 2.5 wt% doped sample. The much longer
excited-state lifetimes compared with that of pure
nanofibrous membranes
[Cu(TBT)(POP)]BF4 (13.2 ls) suggest a more stable
To begin with, we tried four dopant concentrations of 1,
emissive state, indicating that the geometric relaxation in
1
.5, 2, and 2.5 wt%. The inset of Fig. 4 shows a typical
excited state is largely and efficiently suppressed by the
123

6
10
Transition Met Chem (2010) 35:605–611
Table 3 The first four singlet
transitions of [Cu(TBT)(POP)]
calculated at RB3LYP/SBKJC
level
?
Transitions
Composition (%)
Energy (nm)
Oscillator strength
S
0
S
0
S
0
? S
? S
? S
1
2
3
HOMO ? LUMO(95.9)
510.66
456.13
436.75
0.0038128
0.0000309
0.0358045
HOMO - 1 ? LUMO(89.4)
HOMO - 1 ? LUMO ? 1(66.5)
HOMO ? LUMO ? 1(24.3)
HOMO ? LUMO ? 1(66.7)
HOMO - 1 ? LUMO ? 1(25.1)
S
0
? S
4
429.86
0.1068919
Fig. 4 Emission spectra of the 2 wt% doped sample under various
oxygen concentrations from 0 to 100% with an interval of 10%. Inset:
a SEM image of the 2 wt% doped sample
Fig. 5 Stern–Volmer plots of the four samples under various oxygen
concentrations from 0 to 100% with an interval of 10%. Solid lines
are fitted using nonlinear fitting method. Inset: PL intensity responses
rigid environment provided by polystyrene matrix. What’s
more, the 2.5 wt% doped sample’s shorter excited-state
lifetime compared with that of the 2 wt% doped sample
confirms that a higher dopant concentration may accelerate
the intermolecular aggregation as mentioned. On the other
hand, compared to the sharp decrease at 620 nm, the
emission at 517 nm seems to be more insensitive toward
oxygen, suggesting its fluorescence nature. The ratio of
fluorescence to total emission, however, is so small that the
total emission is still highly sensitive toward oxygen.
of the 2 wt% doped sample under periodically varied 100% N and
2
1
2
00% O atmospheres
However, all the Stern–Volmer plots are nonlinear, which
means that the microenvironments around [Cu(TBT)-
(POP)]BF molecules are inhomogeneous, and thus the
4
quenching behavior within the four samples should not be
described by Expression 1. As mentioned earlier, the
samples are uniform and homogeneous, which means that
the inhomogeneous microenvironments are caused by the
probe molecules themselves.
Stern–Vomler plots of [Cu(TBT)(POP)]BF4/
polystyrene nanofibrous membranes
According to Li’s report, in a hydrophobic environment,
?
all [Cu(N–N)(P–P)] complexes with p surfaces tend to
form a dual-molecule structure, where N–N and P–P stand
for diimine and phosphorous ligands [20]. With the for-
mation of this dual-molecule structure, two molecules are
bonded head-to-head (the diimine moiety is defined as the
Figure 5 shows the Stern–Volmer plots of the four samples
at various oxygen concentrations. Generally, in a homo-
geneous media with a single-exponential decay, the
intensity form of Stern–Volmer equation with dynamic
quenching is described as follows [19].
‘‘head’’), resulting in a somewhat rigid structure and con-
sequently suppressed geometric relaxation. Considering the
large p surface of TBT and the hydrophobicity matrix of
polystyrene, we believe [Cu(TBT)(POP)]BF4 molecules
also experience this kind of intermolecular aggregation,
leading to the inhomogeneous microenvironments. In this
case, we assume that there are two primary luminophore
I0=I ¼ 1 þ KSV ½O2ꢁ
ð1Þ
where I is luminescent intensity. The subscript 0 denotes a
value in the absence of quencher, KSV is the Stern–Volmer
constant, and [O ] is O concentration. A plot of I /I versus
2
2
0
[
O ] should be linear with identical slopes of K .
2 SV
1
23

Transition Met Chem (2010) 35:605–611
611
Table 4 Values of sensitivity and nonlinear fitting results of the four
samples
Conclusion
2
In this paper, we report the synthesis, crystal structure,
photophysical properties, and electronic nature of a phos-
Sample
I
I
0
/
K
SV1
K
SV2
f
1
f
2
R
-
1
)
-1
100 (O
2
%
2
(O % )
phorescent Cu(I) complex of [Cu(TBT)(POP)]BF . We
4
1
1
2
2
wt%
doped
5.1 0.2087
5.5 0.2201
5.8 0.2092
4.4 0.1485
0.0034
0.0009
0.0023
0.0023
0.783 0.217 0.9993
0.834 0.166 0.9993
0.827 0.173 0.9996
0.783 0.217 0.9993
fabricated electrospinning composite nanofibers of [Cu-
(TBT)(POP)]BF and polystyrene, hoping to explore the
4
.5 wt%
doped
possibility of replacing precious-metal-based oxygen sen-
sors with cheap Cu-based ones. The resulted samples exhibit
a maximum sensitivity of 5.8 toward oxygen with short
response/recovery time due to the large surface-area-to-
volume ratio of nanofibrous membranes, and no photoble-
aching is detected in these samples. All these results suggest
that phosphorescent Cu(I) complexes doped on nanofibrous
membranes are promising candidates for low-cost and quick-
response oxygen-sensing materials.
wt%
doped
.5 wt%
doped
sites in the matrix, mono-molecule and dual-molecule sites,
and the Stern–Volmer equation should be described as:
I0
I
1
¼
ð2Þ
f1
f2
þ
1
þKSV1pO2
1þKSV2pO2
Acknowledgments The authors gratefully thank the financial and
experimental supports from Beijing Institute of Genomics, Chinese
Academy of Sciences.
where f and f are the fractional contributions from each
2
1
oxygen accessible site (f ? f = 1), K and KSV2 are the
1
2
SV1
associated Stern–Volmer quenching constants for each
oxygen accessible site [19]. The nonlinear fitting results are
summarized in Table 4 and shown in Fig. 5. It can be
observed that the two-site model is applicable for the four
samples, confirming the correctness of our hypothesis that
the inhomogeneous microenvironments are caused by the
probe molecules themselves.
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123