Study on a phosphorescent copper(I) complex and its oxygen-sensing performances upon polystyrene and MCM-41 matrixes

Article abstract of DOI:10.1016/j.saa.2012.04.011

In this paper, we synthesize a new ligand of 1-ethyl-2-(naphthalen-1-yl)- 1H-imidazo[4,5-f][1,10]phenanthroline (Phen-Np-Et) and its corresponding Cu(I) complex of [Cu(Phen-Np-Et)(POP)]BF₄, where POP is bis(2-(diphenylphosphanyl)phenyl) ether. The single-crystal structure, electronic nature and photophysical property of [Cu(Phen-Np-Et)(POP)]BF ₄ are discussed in detail. It is found that the yellow emission from [Cu(Phen-Np-Et)(POP)]BF₄ owns a long excited state lifetime of 287 μs under pure N₂ atmosphere. Theoretical calculation on [Cu(Phen-Np-Et)(POP)]⁺ suggests that the emission comes from a triplet metal-to-ligand-charge-transfer excited state. Then, [Cu(Phen-Np-Et)(POP)]BF₄ are doped into two matrixes of polystyrene and MCM-41 to investigate the oxygen-sensing performance. Finally, sensitivity maxima of 9.6 and 3.6 are achieved by the composite nanofibers of [Cu(Phen-Np-Et)(POP)]BF₄/polystyrene and the [Cu(Phen-Np-Et)(POP)] BF₄/MCM-41, respectively. Both samples are highly sensitive toward molecular oxygen, owing to the large surface-area-to-volume ratios of nanofibrous membranes and MCM-41 matrix.

Full text of DOI:10.1016/j.saa.2012.04.011

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 427–434
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Study on a phosphorescent copper(I) complex and its oxygen-sensing performances
upon polystyrene and MCM-41 matrixes
a,b,
⇑
a
b
b
Xiao-yong Xu
, Han-ning Xiao , Yi-ming Xu , Ming-jun Zhang
a
College of Materials Science and Engineering, Hunan University, Changsha 410082, China
Center for Functional Materials of Pingxiang, Pingxiang 337055, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 23 April 2012
In this paper, we synthesize a new ligand of 1-ethyl-2-(naphthalen-1-yl)-1H-imidazo[4,5-f][1,10]phenan-
throline (Phen-Np-Et) and its corresponding Cu(I) complex of [Cu(Phen-Np-Et)(POP)]BF , where POP is
4
bis(2-(diphenylphosphanyl)phenyl) ether. The single-crystal structure, electronic nature and photophys-
Keywords:
ical property of [Cu(Phen-Np-Et)(POP)]BF are discussed in detail. It is found that the yellow emission from
4
Cu(I) complex
Oxygen sensor
Molecular sieve
Polystyrene
[
Cu(Phen-Np-Et)(POP)]BF
retical calculation on [Cu(Phen-Np-Et)(POP)] suggests that the emission comes from a triplet metal-
to-ligand-charge-transfer excited state. Then, [Cu(Phen-Np-Et)(POP)]BF are doped into two matrixes of
polystyrene and MCM-41 to investigate the oxygen-sensing performance. Finally, sensitivity maxima of
.6 and 3.6 are achieved by the composite nanofibers of [Cu(Phen-Np-Et)(POP)]BF /polystyrene and the
Cu(Phen-Np-Et)(POP)]BF /MCM-41, respectively. Both samples are highly sensitive toward molecular
4 2
owns a long excited state lifetime of 287 ls under pure N atmosphere. Theo-
+
4
9
[
4
4
oxygen, owing to the large surface-area-to-volume ratios of nanofibrous membranes and MCM-41 matrix.
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
making themselves potential supporting matrix candidates for
optical sensing systems [7]. Consequently, the development of
The development of optical sensors has sparked the interest in
the synthesis and study on photoluminescent transition metal com-
plexes owing to their promising optical and electrical characters for
electro-optical applications [1,2]. Compared with traditional sen-
sors for sensing gases, metal ions and organic matters, optical sen-
sors have low requirement in instrumental implementation and
sample preparation. Therefore, research efforts have been devoted
to their development regarding their advantages such as easy-to-
take, quick and non-destructive characteristics [3,4]. Particularly,
many research efforts have been assigned to the development of
optical sensors for oxygen detection since the quantification
of molecular oxygen is usually an importance problem in various
branches of environmental monitoring, chemical industry and med-
icine, as well as biological and analytical chemistry [5,6].
To allow analyte transportation from surrounding and eliminate
self-quenching of optical sensors, sensing molecules are usually
chemically grafted or physically attached onto supporting matrixes.
Polystyrene and silica-based molecular sieves have shown their vir-
tues such as photodurability in visible spectral region, high surface-
area-to-volume ratio and easy loading of sensing molecules,
sensing molecules with appropriate emission intensity and energy,
long-lived excited state, high photodurability and good compatibil-
ity with the supporting matrixes remains to be a challenge.
Owing to their advantages of less toxic, low cost, redundant re-
source and environmental friendliness, phosphorescent Cu(I) com-
plexes have attracted much research interest as a new class of
optoelectrical material. Recently, Zhang and coworkers have re-
4
ported a series of copper(I) complexes of [Cu(N–N)(P–P)]BF that
have been proved to be superior emitters [8–10]. According to
Zhang’s report, for a typical phosphorescent Cu(I) complex, its
highest occupied molecular orbital (HOMO) and HOMO-1 have
predominant metal Cu d character, while its lowest unoccupied
⁄
molecular orbital (LUMO) and LUMO + 1 are essentially
p
orbitals
localized on the diamine ligand [8]. The photoluminescence corre-
sponds to the lowest triplet T and is thus assigned as a character
of metal-to-ligand-charge-transfer MLCT [d(Cu) ?
1
3
⁄
p
(diamine li-
gand)]. The excited state corresponding to electronic transition
from HOMO to LUMO is thus vulnerable to energy acceptors and
surrounding variations [8,10].
In this paper, we report a phosphorescent Cu(I) complex of
[
Cu(Phen-Np-Et)(POP)]BF
property and electronic nature are discussed in detail. In addition,
we dope [Cu(Phen-Np-Et)(POP)]BF into a polymer matrix of
4
. Its crystal structure, photophysical
⇑
Corresponding author at: College of Materials Science and Engineering, Hunan
University, Changsha 410082, China. Tel.: +86 018970596060.
4
polystyrene (PS) and a silica molecular sieve of MCM-41, hoping
to study and compare the oxygen-sensing performance.
E-mail address: dreamxxy01@163.com (X.-y. Xu).
1
386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.04.011

4
28
X.-y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 427–434
Experimental section
extracted with CH
2 2
Cl . After the evaporation of solvent, the residue
was further purified by recrystallization from hot methanol to give
1
A synthetic route for [Cu(Phen-Np-Et)(POP)]BF
sponding composite material of [Cu(Phen-Np-Et)(POP)]BF
shown in Scheme 1. POP, Cu(BF
MCM-41 and 1-naphthaldehyde (NP-CHO) were purchased from Al-
drich Chemical Co. and used without further purifications. The start-
ing material of 1,10-phenanthroline-5,6-dione (referred as Phen-O)
was synthesized according to the literature procedure [7,11]. All or-
ganic solvents were purified using standard procedures.
4
and the corre-
/PS is
4 2
) , EtBr, silica molecular sieve of
3
the desired product. H NMR (CDCl , 300 MHz) d [ppm]: 9.17 (m,
2H), 9.09 (d, J = 4.5 Hz, 1H), 8.65 (m, 1H), 8.09 (m, 1H), 7.99 (m,
1H), 7.72 (m, 3H), 7.67 (m, 1H), 7.54 (m, 2H), 7.49 (m, 1H), 4.42 (q,
2H), 1.435 (t, 3H).
4
Synthesis of [Cu(Phen-Np-Et)(POP)]BF
4
[Cu(Phen-Np-Et)(POP)]BF
classic literature procedure [10]. Its identity was confirmed by H
NMR, elemental analysis, IR spectrum and single-crystal XRD. 1
NMR (CDCl , 300 MHz) d [ppm]: 9.15 (m, 2H), 8.74 (d, J = 5.4 Hz,
4
was synthesized according to the
1
Synthesis of diamine ligands
H
3
Phen-Np. A typical synthetic route for Phen-Np is described as
follows. The mixture of 20 mmol of Phen-O, 22 mmol of Np-CHO,
2H), 8.10 (d, J = 6.0 Hz, 1H), 7.98 (d, J = 6.0 Hz, 1H), 7.62 (m, 7H),
7.15 (m, 8H), 6.96 (m, 12H), 6.29 (m, 2H), 4.61 (q, 2H), 1.47 (t,
1
5.40 g of NH
Then, the mixture was poured into cold water and extracted with
CH Cl . After the evaporation of solvent, the crude product was fur-
ther purified by recrystallization from EtOH. H NMR (CDCl
00 MHz) d [ppm]: 11.62 (s, 1H), 9.13 (m, 2H), 8.79 (m, 1H), 8.54
4
Ac and 30 mL of HAc was stirred at 90 °C for 4 h.
3H). Anal. Calcd for C61
Found: C, 68.81; H, 4.24; N, 5.35. IR (KBr pellet):
1587, 1547, 1425, 1249, 1048, 864, 798, 771, 718 cm .
H
46BCuF
4
N
4
OP
2
: C, 68.90; H, 4.36; N, 5.27.
m
3040, 2985,
ꢀ1
2
2
1
3
,
3
4
Fabrication of [Cu(Phen-Np-Et)(POP)]BF /PS composite nanofibers
(
m, 1H), 7.94 (m, 3H), 7.72 (m, 1H), 7.63 (m, 1H), 7.56 (m, 4H).
Phen-Np-Et. A typical synthetic route for 1-ethyl-2-(naphthalen-
A typical procedure for the electrospinning composite nanofibers
is described as follows. PS with a number-average molecular mass
of 100,000 was dissolved in DMF to form a 22 wt.% solution. Then,
1
-yl) -1H-imidazo[4,5-f][1,10]phenanthroline (referred as Phen-
Np-Et) is described as follows. The mixture of 20 mmol of NaH
and 100 mL of anhydrous N,N-dimethylformamide (DMF) was stir-
red for 1 h, and then, 10 mmol of Phen-Np and 50 mmol of EtBr were
added into above solution. The mixture was heated to reflux for
[Cu(Phen-Np-Et)(POP)]BF
ring to form [Cu(Phen-Np-Et)(POP)]BF
The final solutions were then electrospun to be composite nanofi-
bers of [Cu(Phen-Np-Et)(POP)]BF /PS. For electrospinning, the
4
was added into the solution under stir-
4
/PS homogeneous solutions.
2
4 h. After cooling, the mixture was poured into cold water and
4
4 4
Scheme 1. A synthetic route for [Cu(Phen-Np-Et)(POP)]BF and [Cu(Phen-Np-Et)(POP)]BF /PS composite nanofibers.

X.-y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 427–434
429
mixed solution was placed in a 5 ml glass syringe, with the opening
end connected to a plastic needle (inner diameter = 0.6 mm) as the
nozzle [12]. The anode terminal of a high-voltage generator was
connected to a copper wire inserted into the polymer solution in
the glass syringe. A piece of Al foil was used as the collector plate
and connected to the grounding electrode. The driving voltage
was 18 kV, with the tip-to-target distance of 25 cm.
localizes in a distorted tetrahedral coordination sphere. The two
Cu–N bond length values are slightly different from each other
and measured to be 2.035 Å and 2.074 Å, respectively. Similar cases
have been reported by literature reports, which means that the
intermolecular interactions between Cu(I) center and N atoms
are slightly different from each other owing to their different elec-
tronic nature [9,13]. As for the two Cu–P bonds, their values are
quite similar with each other. Both P atoms are found to be
ꢁ2.3 Å away from the Cu(I) center, suggesting that the intermolec-
4
Fabrication of [Cu(Phen-Np-Et)(POP)]BF /MCM-41 composite silica
ular interactions between Cu(I) center and P atoms are much sim-
ilar with each other due to the unique electronic nature of two P
atoms. The O atom in POP ligand is measured to be ꢁ2.3 Å away
from the Cu(I) center, suggesting that there may be some intermo-
lecular interaction between Cu(I) center and the O atom. However,
this distance is much longer than normal chemical bonds and
should not be considered as a coordination bond. Literatures on
POP-based Cu(I) complexes have also reported examples with sim-
ilar Cuꢂ ꢂ ꢂO separations [8,9,13].
[
Cu(Phen-Np-Et)(POP)]BF
silica molecular sieve of MCM-41 following the procedure: 1 g of
MCM-41 and 60 mg of [Cu(Phen-Np-Et)(POP)]BF were added into
0 mL of dichloromethane, and the mixture was stirred under
4
was incorporated into mesoporous
4
1
ambient condition for 24 h. The resulted suspension was filtered
to give pale yellow powder. The powder was washed with dichlo-
romethane until the solution was colorless under ultraviolet radi-
2
ation and then dried under N atmosphere.
The N–Cu–N bite angle is measured to be as large as 80.46°,
which is comparable with literature values [8,9,13], and this large
bite angle is attributed to the static congestion caused by the large
dihedral angle between N–Cu–N and P–Cu–P planes (89.62°) as
mentioned. On the other side, the P–Cu–P bite angle is measured
to be 118.06°. Since POP has a nature bite angle of 102.2° with a
flexibility ranging from 86° to 120°, the large P–Cu–P bite angle
may suggest a congestion of coordination environment round
Cu(I) center [14].
In addition, it can be seen that naphthalene moiety is uncopla-
nar with Phen ring. The free rotation of naphthalene moiety may
compromise the large conjugation system, leading to a strong
static hindrance. So, even though there are two large planes of
naphthalene moiety and Phen ring within Phen-Np-Et ligand, nei-
Methods and measurements
Density functional theory (DFT) and singlet excitation calcula-
tions using time-dependent density functional theory (TD-DFT)
were performed on [Cu(Phen-Np-Et)(POP)] at RB3PW91/SBKJC le-
+
vel. The initial geometry was obtained from its single crystal. All
computations were finished by GAMESS software package. The IR
spectrum was recorded using a Magna 560 FT-IR spectrophotome-
ter. Excited state lifetimes were obtained with a 355 nm light gener-
ated from the Third-Harmonic-Generator pumped, using pulsed
Nd:YAG laser as the excitation source. The Nd:YAG laser possesses
ꢀ1
a line width of 1.0 cm , pulse duration of 10 ns and repetition fre-
quency of 10 Hz. A Rhodamine 6G dye pumped by the same Nd:YAG
laser was used as the frequency-selective excitation source. All pho-
toluminescence (PL) spectra were measured with a Hitachi F-4500
fluorescence spectrophotometer. UV–Vis absorption spectra were
recorded using a Shimadzu UV-3101PC spectrophotometer. Scan-
ning electron microscopy (SEM) image was obtained on a Hitachi
S-4800 microscope. Small angle X-ray diffraction (SAXRD) patterns
were measured with a Rigaku-Dmax 2500 diffractometer using Cu
ther inter- nor inner-molecular
Phen-Np-Et)(POP)]BF crystal.
p-stacking is observed in [Cu
(
4
Electronic structure
According to literature reports, the MLCT excited state of [Cu(N–
N)(P–P)]BF may be vulnerable to energy acceptors and surrounding
variations [8,10]. To get a deeper understanding on the electronic
structure of [Cu(Phen-Np-Et)(POP)]BF , a DFT calculation was
4
Ka (k = 0.15405 nm) radiation at 0.02° (2h) scanning step. Fluores-
cence microscopy image was obtained with a Nikon TE2000-U fluo-
rescence microscopy using mercury lamp as the power supply. H
4
1
NMR spectra were obtained with a Varian INOVA 300 spectrometer.
Elemental analysis was performed on a Carlo Erba 1106 elemental
analyzer. Single-crystal data were collected on a Siemens P4 sin-
gle-crystal X-ray diffractometer with a Smart CCD-1000 detector
and graphite-monochromated Mo Ka radiation, operating at 50 kV
and 30 A at 298 K. All hydrogen atoms were calculated. Oxygen-
sensing performances were measured on the basis of steady emis-
sion intensity quenching. In the measurement of Stern–Volmer
plots, oxygen and nitrogen were mixed at different concentrations
via gas flow controls and passed directly into sealed gas chamber.
All measurements were carried out in the air at room temperature
without being specified.
Results and discussion
Molecular nature of [Cu(Phen-Np-Et)(POP)]BF
4
Single-crystal structure
In Fig. 1, we present the single-crystal structure of [Cu(Phen-
Np-Et)(POP)]BF , and the key structural parameters are listed in
4
Table 1. The dihedral angle between N–Cu–N and P–Cu–P planes
4
Fig. 1. Molecular structure [Cu(Phen-Np-Et)(POP)]BF obtained from its single
is measured to be 89.62°, which suggests that the Cu(I) center
crystal. All hydrogen atoms are omitted for clarity.

4
30
X.-y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 427–434
Table 1
[Cu(Phen-Np-Et)(POP)]BF
band centering at 550 nm as shown in Fig. 4. The emission band
shows no vibronic progressions with full-width-at-half-
maximum (FWHF) of 93 nm, which indicates that the emissive
state owns a charge-transfer (CT) character. The Stokes shift of
4
in solid state gives a broad emission
Selected bond lengths (Å) and angles (°) of [Cu(Phen-Np-Et)(POP)]BF
single crystal.
4
obtained from
a
Bond length
(Å)
Bond angle
(°)
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–P(1)
Cu(1)–P(2)
Cuꢂ ꢂ ꢂO
2.074
2.035
2.226
2.255
3.063
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)
80.46
109.14
122.59
104.44
113.11
118.06
4
[Cu(Phen-Np-Et)(POP)]BF between its absorption edge and emis-
sion maximum is calculated to be 90 nm, which is comparable
with literature values [8]. This large value suggests that there is
an intense structural relaxation that occurs in excited state. The
emissive energy of 550 nm is comparable with that of [Cu(Phen)
(
POP)]BF
4
[8], which means that the energy gap of [Cu(Phen-Np-
is similar with that of [Cu(Phen)(POP)]BF due to
performed on the Cu(I) complex, which has been demonstrated to
be an effective method to explore the electronic structure of transi-
tion metal complexes [8–10]. The percentage compositions of fron-
tier orbitals and the first five singlet excitation of [Cu(Phen-Np-
Et)(POP)]BF
4
4
naphthalene moiety’s inefficient conjugation with Phen ring.
The emission decay curves shown by the inset of Fig. 4 suggest
that the excited state of [Cu(Phen-Np-Et)(POP)]BF
lifetime of 287 s under pure N atmosphere, which identifies
the phosphorescence nature of the emission. Bearing above calcu-
lation results in mind, we conclude that the emission is generated
from MLCT excited state. A long-live excited state may give energy
acceptors enough time to attack and quench the excited state,
favoring sensitivity improvement and instant response [9].
Then, the potential surface crossing (PSC) from higher
state to lower MLCT state of [Cu(Phen-Np-Et)(POP)]BF
cussed as follows. As shown by the inset of Fig. 4, the emission un-
der pure N atmosphere follows a biexponential decay pattern
with = 380 s (A = 0.642) and = 80 s (A = 0.420), respec-
tively. It is reported that the observation of strong absorption in
p
p
+
4
has a long
Et)(POP)] are summarized in Table 2. The graphic presentation of
l
2
HOMO and LUMO shown in Fig. 2 suggests that HOMO of [Cu
+
(
Phen-Np-Et)(POP)] mainly localizes on Cu(I) center and P atoms,
⁄
while its LUMO is mainly composed of
p
orbital of Phen-Np-Et,
3
which is consistent with literature report [8–10]. The first electronic
transition is assigned as an electronic transition from HOMO to
LUMO, which confirms that the onset electronic transition of [Cu
⁄
p ? p
(
Phen-Np-Et)(POP)]BF
4
is a MLCT one. As shown in Table 2, LUMO
⁄
4
is dis-
is essentially
p
orbital of Phen-Np-Et ligand, and the excited state
electron localizing on LUMO may thus be attacked and quenched
by energy acceptors such as molecular oxygen, leading to a decrease
or even absence of emission intensity.
2
s
1
l
1
s
2
l
1
⁄
⁄
?
?
p
p
absorption region and a short-lived decay component from
state than those from MLCT state suggests an efficient PSC
Photophysical properties of [Cu(Phen-Np-Et)(POP)]BF
4
⁄
from the higher
p
?
p
state to MLCT state. The absorption spec-
The UV–Vis absorption spectrum of [Cu(Phen-Np-Et)(POP)]BF
4
trum of [Cu(Phen-Np-Et)(POP)]BF
4
shown in Fig. 3 exhibits a strong
is as-
ꢀ4
⁄
in CH
2
Cl
2
solution with a concentration of 1 ꢃ 10 mol/L is shown
absorption at
signed to
Under pure O
to 0.2 s, which is over 1000 times shorter than that under pure N
atmosphere. The largely decreased lifetime suggests that [Cu
(Phen-Np-Et)(POP)]BF excited state can be easily quenched by
p
?
p
absorption region. Consequently,
is assigned to MLCT decay.
atmosphere, the excited state lifetime decreases
s
2
⁄
in Fig. 3, along with those of free Phen-Np-Et ligand and POP ligand
for comparison purpose. The absorption spectrum of [Cu(Phen-
p ? p decay, and s
2
2
Np-Et)(POP)]BF
4
is mainly composed of two regions, ending at
l
2
ꢁ
460 nm. After the absorption spectrum comparison between free
ligands and [Cu(Phen-Np-Et)(POP)]BF
ing from 220 nm to 325 nm is assigned to ligand
4
, the strong absorption rang-
4
⁄
p
?
p
transitions,
molecular oxygen through a dynamic mechanism described as fol-
lows [7]:
while the weak absorption ranging from 350 nm to 450 nm is tenta-
tively attributed to MLCT transitions. After a comparison between
ꢅ
þOꢅ
½
CuðPhen-Np-EtÞðPOPÞꢄBF þO
2
!½CuðPhen-Np-EtÞðPOPÞꢄBF
4
4
2
absorption spectra of [Cu(Phen)(POP)]BF
throline) and [Cu(Phen-Np-Et)(POP)]BF , the MLCT absorption of
Cu(Phen-Np-Et)(POP)]BF
red shifts by ꢁ10 nm, owing to the large
4
(Phen = 1,10-phenan-
ð1Þ
4
[
4
⁄
where ‘‘ ’’ denotes excited state.
coplanar conjugation system in Phen-Np-Et ligand [8]. However, the
redshift tendency is limited, which may be caused by the conjuga-
tion break between naphthalene moiety and Phen ring, owing to
the free rotation of naphthalene moiety as mentioned above.
4
Characterization of [Cu(Phen-Np-Et)(POP)]BF /PS and [Cu(Phen-Np-
4
Et)(POP)]BF /MCM-41
Since its has been above confirmed that [Cu(Phen-Np-Et)
POP)]BF emission is quenchable by molecular oxygen, we decide
Table 2
(
4
Orbital percentage composition of [Cu(Phen-Np-Et)(POP)]+ calculated at RB3LYP/
SBKJC level.
to further explore its oxygen-sensing performance. For practical
sensing systems, optical probes should be embedded into support-
ing matrix to eliminate self-quenching of optical probes and to
allow analyte transportation from surrounding. There are also
some stringent criteria for supporting matrix to achieve suitable
performances [7]. Here, we select PS and MCM-41, which have
been proved to be excellent supporting materials for optical sen-
sors, as the supporting matrixes for our primitive research [7,15].
At first, three dopant concentrations of 3.0 wt.%, 3.5 wt.% and
Orbitals/
transition
Composition (%)
Energy
(eV)
LUMO + 1(174)
LUMO(173)
HOMO(172)
HOMO ꢀ 1(171)
Phen-Np-Et(93.5)
ꢀ3.935
ꢀ4.291
ꢀ7.701
ꢀ8.003
Phen-Np-Et(84.7) and POP(11.7)
Cu(28.6) and POP(65.4)
Cu(26.6) and Phen-Np(58.2) and
POP(15.3)
HOMO ꢀ 2(170)
HOMO ꢀ 3(169)
Cu(22.3) and Phen-Np(65.2) and
POP(12.3)
Cu(34.6) and Phen-Np(21.2) and
POP(44.2)
ꢀ8.038
ꢀ8.226
4
.0 wt.% are tried to achieve optimal performance. A typical SEM
image of the 3.5 wt.% doped sample is shown in Fig. 5a. [Cu
Phen-Np-Et)(POP)]BF /PS sample shows smooth and uniform
(
4
S
0
S
0
S
0
S
0
S
0
? S
? S
? S
? S
? S
1
2
3
4
5
171 ? 173(60.4) and 170 ? 173(34.5)
172 ? 173(96.3)
169 ? 173(94.7)
172 ? 174(96.8)
171 ? 174(63.2) and 170 ? 174(34.3)
2.7401
2.8074
3.0747
3.1468
3.2716
morphology with average diameter of ꢁ500 nm. The fibers are ran-
domly distributed on the substrate without any branch structures,
and this unique morphology has been reported to own a large sur-
face-area-to-volume ratio that is two orders of magnitude larger

X.-y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 427–434
431
Fig. 2. HOMO (down) and LUMO (up) of [Cu(Phen-Np-Et)(POP)]+ calculated at RB3PW91/SBKJC level.
Fig. 3. UV–Vis absorption of Phen-Np, POP and [Cu(Phen-Np-Et)(POP)]BF
solutions with a concentration of 1 ꢃ 10 mol/L.
4
in CH
2
Cl
2
Fig. 4. PL spectrum of [Cu(Phen-Np-Et)(POP)]BF
decay data of [Cu(Phen-Np-Et)(POP)]BF upon N
4
in solid state. Inset: PL intensity
ꢀ
4
4
2
and O
2
atmosphere.
than that of bulk materials, giving a high diffusion coefficient for
rapid response [15]. Correspondingly, the fluorescence
microscopy image of the 3.5 wt.% doped sample exhibits typical
yellow emission that is assigned to [Cu(Phen-Np-Et)(POP)]BF
4
,

4
32
X.-y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 427–434
Fig. 5a. SEM (left) and fluorescence microscopy (right) images of the 3.5 wt.% doped sample.
Fig. 5b. SAXRD 1 patterns of blank MCM-41 and [Cu(Phen-Np-Et)(POP)]BF4/MCM-
1. Inset: absorption spectrum of [Cu(Phen-Np-Et)(POP)]BF4/MCM-41.
4
suggesting that [Cu(Phen-Np-Et)(POP)]BF
doped into the composite nanofibers.
4
has been successfully
Fig. 6. Emission spectra of the 3.5 wt.% doped sample under various oxygen
concentrations from 0% to 100% with an interval of 10%.
To compare the influence of matrix on sensing performances,
we also prepare [Cu(Phen-Np-Et)(POP)]BF /MCM-41 with optimal
doping concentration of 60 mg/g [12,16]. The powder small angle
X-ray diffraction (SAXRD) pattern of blank MCM-41 shown in
Fig. 5b exhibits three well-resolved broad Bragg reflections of
I
0
/I100, where I
sphere and I100 is that under 100% O
values are then calculated to be 7.8 for the 3.0 wt.% doped sample,
.6 for the 3.5 wt.% doped sample and 9.1 for the 4.0 wt.% doped
0
is the luminescence intensity under 100% N
2
atmo-
4
2
atmosphere. The sensitivity
9
sample, respectively. It is observed that the 3.5 wt.% doped sample
owns a higher sensitivity than the others, and we give an explana-
tion as follows. There should be at least two opposite components
that control the sensitivity: one is the emission intensity from
d100, d110 and d200, indicating a well-ordered hexagonal mesostruc-
ture [9,10]. The well-ordered channels of MCM-41 silica result in a
large surface-area-to-volume ratio, making itself a good supporting
matrix for rapid response. After the incorporation of [Cu(Phen-Np-
[
Cu(Phen-Np-Et)(POP)]BF
4
, and the other is the self-quenching be-
molecules. When the dopant
4 4
Et)(POP)]BF , the SAXRD pattern of [Cu(Phen-Np-Et)(POP)]BF /
tween [Cu(Phen-Np-Et)(POP)]BF
4
MCM-41 is nearly identical with that of blank MCM-41, which
means that the well-ordered hexagonal arrangement of channels
in MCM-41 remains after the incorporation of [Cu(Phen-Np-Et)
concentration is low, the emission is weak, which leads to the
low sensitivity of the 3.0 wt.% doped sample. On the other hand,
a high dopant concentration may accelerate the self-quenching of
(
POP)]BF
Et)(POP)]BF
that of pure [Cu(Phen-Np-Et)(POP)]BF
4
. In addition, the absorption spectrum of [Cu(Phen-Np-
/MCM-41 shown by the inset of Fig. 5b is similar with
, showing identical absorp-
[
4
Cu(Phen-Np-Et)(POP)]BF molecules, which also decreases the
4
emission intensity and correspondingly leads to a low sensitivity
of the 4.0 wt.% doped sample. The two factors may get balanced
in the 3.5 wt.% doped sample, showing the maximum sensitivity
of 9.6.
4
tion characters. It is thus confirmed that [Cu(Phen-Np-Et)(POP)]BF
has been successfully doped into MCM-41 matrix.
4
Another finding is that the emission maximum of the 3.5 wt.%
doped sample shifts to 538 nm, compared with that of pure
4
Oxygen-sensing properties of [Cu(Phen-Np-Et)(POP)]BF /PS nanofibers
[
4
Cu(Phen-Np-Et)(POP)]BF centering at 550 nm. This decreased
Stokes shift is tentatively attributed to rigidochromism described
as follows. Zhang and coworkers have reported that the structural
Sensitivity
The luminescence response of the 3.5 wt.% doped sample under
various oxygen concentrations from 0% to 100% with an interval of
relaxation that occurs in the excited state of [Cu(N–N)(P–P)]BF
creases the energy content sharply [8–10]. When doped into PS
matrix, [Cu(Phen-Np-Et)(POP)]BF molecules are trapped in the ri-
gid grid of PS, and the structural relaxation can thus be effectively
suppressed, leading to abovementioned emission spectrum blue
shift.
4
de-
1
0% is shown in Fig. 6. The emission intensity of 538 nm decreases
sharply with increasing oxygen concentrations, which means that
Cu(Phen-Np-Et)(POP)]BF /PS is highly sensitive toward oxygen.
To evaluate the sensing performance, sensitivity is defined as
4
[
4

X.-y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 427–434
433
Fig. 8. Stern–Volmer plot of [Cu(Phen-Np-Et)(POP)]BF
4
/MCM-41 under various
oxygen concentrations from 0% to 100% with an interval of 10%. Inset: PL intensity
response under periodically varied 100% N and 100% O atmospheres.
Fig. 7. Stern–Volmer plot of the 3.5 wt.% doped sample under various oxygen
concentrations from 0% to 100% with an interval of 10%. Inset: PL intensity
2
2
responses of the 3.5 wt.% doped sample under periodically varied 100% N
atmospheres.
2
and 100%
O
2
4
Oxygen-sensing properties of [Cu(Phen-Np-Et)(POP)]BF /MCM-41
In order to further improve the photostability, as well as com-
pare the effect of supporting matrix on sensing performance, we
Stern–Volmer plot
As for an emission with single-exponential decay pattern, the
intensity form of Stern–Volmer equation with dynamic quenching
is described as follows [16].
decide to dope [Cu(Phen-Np-Et)(POP)]BF
MCM-41 with an optimal dopant concentration of 60 mg/g
4
12,16]. The Stern–Volmer plot of [Cu(Phen-Np-Et)(POP)]BF /
4
into a silica matrix of
[
MCM-41 under various oxygen concentrations is shown in Fig. 8.
Clearly, the linearity is so poor compared with that of [Cu(Phen-
I
0
=I ¼ 1 þ KSV ½O
2
ꢄ
ð2Þ
Np-Et)(POP)]BF
data suggest that [Cu(Phen-Np-Et)(POP)]BF
4
/PS, and the maximum sensitivity is only 3.6. These
cannot be uniformly
where I is steady emission intensity, I
sity at pure N
] is O concentration. A plot of I
0
denotes the emission inten-
4
2
atmosphere, KSV is the Stern–Volmer constant, and
/I versus [O ] should be linear
distributed in MCM-41 supporting matrix, and the quenching is
ineffective. However, the 60 mg/g doped sample shows a good
photodurability, and no obvious photobleaching is observed, as
[
O
2
2
0
2
with identical slopes of KSV. The Stern–Volmer plot of the 3.5 wt.%
doped sample under various oxygen concentrations is shown in
Fig. 7. It can be seen that the plot fits Eq. (2) very well and the
quenching curve shows good linear response with increasing
oxygen concentrations. We thus come to a conclusion that [Cu
shown by the inset of Fig. 8. Upon pure O
is largely quenched, but the emission intensity can always recover
to its initial value when the atmosphere changes from 100% O to
100% N , suggesting that the sensing system is not suffering from
2
atmosphere, the emission
2
2
(
Phen-Np-Et)(POP)]BF
4
molecules localize in a homogeneous micro-
oxidative photobleaching. Thus, we come to a conclusion that PS
matrix owns a virtue of uniform distribution, while MCM-41 matrix
can offer good protection for sensing probes.
environment and are uniformly distributed in PS matrix, leading to
a sooth quenching on [Cu(Phen-Np-Et)(POP)]BF excited state.
4
Conclusion
Response/recovery times and photostability
The PL signal response of the 3.5 wt.% doped sample upon peri-
odically varied 100% N and 100% O atmospheres is shown by the
inset of Fig. 7. For comparison convenience, we define 95% response
time as the time taken for a sample to lose 95% of its initial emission
In this paper, we synthesized an oxygen-sensing Cu(I) complex
2
2
of [Cu(Phen-Np-Et)(POP)]BF
nature and photophysical property are discussed in detail. By dop-
ing [Cu(Phen-Np-Et)(POP)]BF into two supporting matrixes of PS
4
. Its single-crystal structure, electronic
4
intensity when changed from 100% N
atmosphere, and 95% recovery time as the time taken to recover
5% if its final emission intensity when changed from 100% O atmo-
sphere to 100% N atmosphere [7,16]. The 3.5 wt.% doped sample
2 2
atmosphere to 100% O
and MCM-41, the oxygen-sensing performance is investigated.
The composite nanofibers show a maximum sensitivity of 9.6 with
good linearity owing to the large surface-area-to-volume ratio of
nanofibrous membranes. However, the photodurability is underde-
veloped. While the composite silica sample gives a low sensitivity of
9
2
2
owns a response time of 16 s and a recovery time of ꢁ25 s. The
instant emission quenching suggests that [Cu(Phen-Np-Et)
3
.6 with poor linearity, the photodurability is perfect. All these re-
(
POP)]BF
4
/PS is highly sensitive toward molecular oxygen. We attri-
’s vulnerable
sults suggest that phosphorescent [Cu(Phen-Np-Et)(POP)]BF
4
bute this character to both [Cu(Phen-Np-Et)(POP)]BF
4
doped nanofibrous membranes and silica molecular sieves are po-
tential candidates for low-cost and quick-response oxygen-sensing
materials. Above conclusion may be useful for the future design of
oxygen-sensing materials.
excited state and the large surface-area-to-volume ratio supplied
by PS matrix. The recovery time is found to be longer than the re-
sponse time, which has been explained by the diffusion-controlled
dynamic response and recovery behavior of a hyperbolic-type
sensor reported by Mills and coworkers [17]. In addition, the maxi-
mum emission intensity at pure N
2
atmosphere shows a decrease
Acknowledgment
tendency, which is called photobleaching. This result suggests that
PS matrix, however, is limited on protecting [Cu(Phen-Np-Et)
This work was supported by Grants from the Science Founda-
tion of Jiangxi Province, China (GJJ12661).
(
4
POP)]BF molecules from photodecomposition.

4
34
X.-y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 427–434
[
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