Photophysical performance comparison between bulk Cu(I) complex and its electrospinning fibers: Synthesis and characterization

Article abstract of DOI:10.1016/j.saa.2016.06.017

In this report, a diamine ligand having an electron-pulling group in its conjugation plane was designed. A methyl group was connected with this diamine ligand, hoping to further increase its steric hindrance. Its Cu(I) complex was synthesized and characte

Full text of DOI:10.1016/j.saa.2016.06.017

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 169 (2016) 30–37
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Photophysical performance comparison between bulk Cu(I) complex and
its electrospinning fibers: Synthesis and characterization
a
b,
a
Wan Pu , Zhao Yuqing ⁎, Wang Lisha
a
Zhaotong University, Zhaotong, Yunnan, PR China
School of Civil Engineering and Communication, North China University of Water Resources and Electric Power, Zhengzhou 450045, Henan, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 April 2016
Received in revised form 24 May 2016
Accepted 8 June 2016
Available online 14 June 2016
In this report, a diamine ligand having an electron-pulling group in its conjugation plane was designed. A methyl
group was connected with this diamine ligand, hoping to further increase its steric hindrance. Its Cu(I) complex
was synthesized and characterized by NMR, single crystal analysis and photophysical analysis. There was a
distorted tetrahedral coordination field in this Cu(I) complex. Its onset electronic transition owned a mixed char-
acter of metal-to-ligand-charge-transfer which suffered from bad geometric relaxation. To limit this geometric
relaxation and improve emissive performance, this Cu(I) complex was doped into a polymer host through
electrospinning technique. Photophysical comparison between solid state sample, solution sample and compos-
ite samples indicated that excited state geometric relaxation was effectively limited by polymer immobilization
effect, resulting in improved emissive performance, such as emission blue shift, long emission decay lifetime and
better photostability.
Keywords:
Cu complex
Photoluminescence
Microfiber
Crystal
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
unoccupied FMOs are essentially π* orbitals of diamine ligands [18]. In
this case, onset electronic transitions between occupied FMOs and un-
occupied ones are assigned as a mixture of metal-to-ligand-charge-
transfer (MLCT) and ligand-to-ligand-charge-transfer (LLCT). Such
MLCT excited state is usually vulnerable to environment surrounding.
Its structural relaxation is a major non-radiative decay path for
[Cu(N\\N)(P\\P)] excited state, leading to decreased energy content
and compromised emission quantum yield [18–20]. By introducing
electron-pulling groups and large conjugation planes into diamine li-
gand, non-radiative decay probability of [Cu(N\\N)(P\\P)] emissive
state can be decreased, which consequently improves its emissive per-
formance [18–20].
One-dimensional nanocomposite materials, such as nanorods, nano-
wires and nanofibers, have shown their wide application in optoelec-
tronic and optical sensing systems and thus drawn much attention [1].
Among these samples, fibrous samples prepared by electrospinning
have been usually proposed as supporting matrix for various optical
dopants owing to their inexpensive preparation route, controllable
morphology and good mechanical strength [2–8]. To satisfy various de-
mands in practical application, a number of dopants and sensitizers
have been doped into these electrospinning fibers to give composite fi-
brous samples [9–15].
Phosphorescent Cu(I) complexes with a general molecular formula
of [Cu(N\\N)(P\\P)] have shown promising virtues and thus are usual-
ly applied as dopant in optical systems, including long emission decay
lifetime, high emission quantum yield, promising photostability and
low cost. Here N\\N and P\\P denote a diamine ligand and a phospho-
rous ligand, respectively [14–17]. Theoretical analysis on typical
Aiming at further improved photophysical performance from
[Cu(N\\N)(P\\P)] complexes, such structural relaxation in MLCT excit-
ed state should be effectively limited. Some precursive methods include
modifying ligands with steric hindrance groups, being doped into rigid
supporting matrixes and being loaded into polymer frameworks [18–
20].
[
Cu(N\\N)(P\\P)] complexes suggests that their occupied frontier mo-
Guided by above analysis, in this paper, a diamine ligand having an
electron-pulling group in its conjugation plane is designed, as shown
in Scheme 1. A methyl group is connected with this diamine ligand,
hoping to increase its steric hindrance. Its Cu(I) complex is synthesized
and doped into poly(vinylpyrrolidone) (PVP) fibers using
lecular orbitals (FMOs) consist of metal d orbitals, while their
⁎
Corresponding author.
E-mail addresses: puwan01@163.com (W. Pu), zhao_yuqing1@163.com (Z. Yuqing).
http://dx.doi.org/10.1016/j.saa.2016.06.017
386-1425/© 2016 Elsevier B.V. All rights reserved.
1

W. Pu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 169 (2016) 30–37
31
3 2 4 3 2 4
Scheme 1. Design and synthesis strategy for our diamine ligand PYO, Cu(I) complex [Cu(PYO)(PPh ) ]BF and electrospinning fibers ([Cu(PYO)(PPh ) ]BF /PVP).
electrospinning technique. Photophysical performance comparison be-
tween bulk Cu(I) complex and its electrospinning fibers is carried out.
The correlation between structural relaxation in MLCT excited state
and PVP framework is analyzed and discussed in detail.
NaN
30 min. During this time, ZnCl
tion was stirred at room temperature for 1 h and then at 80 °C for anoth-
er 10 h under N protection. Plenty of crushed ice was added. Crude
product was purified on a silica gel column (n-hexane:CH Cl
): δ 7.72 (1H, m), 8.07 (1H, m), 8.23
1H, d, J = 6.0), 8.76 (1H, m). Anal. Calcd for C : C, 48.98; H,
3.43; N, 47.60. Found: C, 48.84, H, 3.53; N, 47.50. MS m/z: [m + 1]
calc. for C , 147.0; found, 148.3.
3
(10 mmol) in DMF (20 mL) was stirred at room temperature for
2
(1 g) was slowly added. The final solu-
2
2
2
=
1
2
. Experimental details
50:1). H NMR (300 MHz, CDCl
3
(
6 5 5
H N
+
2
.1. General information
6 5 5
H N
Design strategy and synthesis route for our diamine ligand 2-
Then TP (5 mmol), 4-methyl-benzoyl chloride (6 mmol) and pyri-
dine (20 mL) were mixed together and heated at 120 °C for 2 days
(
[
pyridin-2-yl)-5-(p-tolyl)-1,3,4-oxadiazole (PYO), Cu(I) complex
Cu(PYO)(PPh ]BF and electrospinning fibers ([Cu(PYO)(PPh ]BF
PVP) are shown in Scheme 1. Related chemicals, including polymer
PVP (K30), NaN , Cu(BF , Cu powder, ZnCl , triphenylphosphane and
)
3 2
4
)
3 2
4
/
under N
2
protection. After cooling, this mixture was dispersed with
cold water. Crude product was purified on a silica gel column (n-
1
3
)
4 2
2
hexane:CH
J = 6.0), 7.51 (1H, t), 7.95 (1H, t), 8.14 (2H, d, J = 6.0), 8.33 (1H, d,
J = 6.0), 8.87 (1H, d, J = 3.6). Anal. Calcd. For C14 O: C, 70.87; H,
4.67; N, 17.71. Found: C, 70.99; H, 4.74; N, 17.61. MS m/z: [m + 1]
calc. for C14 O, 239.1; found, 240.3.
2 2 3
Cl = 50:1). H NMR (CDCl ): δ 2.49 (3H, s), 7.37 (2H, d,
picolinonitrile, were commercially provided by Shanghai Chemical
Co. (China) and used with no further purifications. Organic solvents,
11 3
H N
+
including CH
2
Cl
2
,
CHCl
3
,
MeCN and N,N′-dimethylformamide
(
DMF), were bought from Tianjin Chemical Co. (China) and purified be-
11 3
H N
fore use with standard procedures. Solvent water was deionized before
use.
3 2 4
2.3. Synthesis of [Cu(PYO)(PPh ) ]BF
A Varian INOVA 300 spectrometer and a Agilent 1100 MS spectrom-
eter (COMPACT) were responsible for data collection of NMR and mass
spectra. Elemental analysis was performed on a Carlo Erba 1106 ele-
mental analyzer. A Siemens P4 single-crystal X-ray diffractometer
equipped with a Smart CCD-1000 detector was applied to collect single
crystal data, using graphite-monochromated Mo Kα radiation at 298 K.
All H atoms were calculated. A HP 8453 UV-Vis-NIR diode array spectro-
photometer and a Hitachi F-4500 fluorescence spectrophotometer were
used to take UV-Vis absorption and emission spectra. A two-channel
TEKTRONIX TDS-3052 oscilloscope was responsible for emission decay
lifetime analysis, using pulsed YAG laser (355 nm) as excitation source.
A Hitachi S-4800 microscope and a Nikon TE2000-U fluorescence mi-
croscopy were applied to take scanning electron microscopy (SEM)
and fluorescence microscopy pictures. Density functional theory (DFT)
calculation was finished with GAMESS at RB3LYP/SBKJC level in vacuum
using single crystal structure as initial geometry. Graphical plotting for
FMOs was generated by wxMacmolplt software package with contour
value of 0.025.
[Cu(CH
Cu(BF
[Cu(PYO)(PPh
[19]. A mixture of [Cu(CH
CH Cl (10 mL) was stirred at room temperature for 30 min. Then
3
CN)
and Cu powder as starting chemicals [19].
]BF was synthesized following a literature procedure
CN) ]BF (2 mmol), PPh (4 mmol) in
4 4
]BF was firstly prepared in MeCN solution with
4 2
)
3
)
2
4
3
4
4
3
2
2
PYO (2 mmol) was added. The resulting solution was stirred at room
temperature for another 30 min. Solvent was extracted by thermal
evaporation. Crude product was purified by recrystallization from tetra-
1
hydrofuran/ether. H NMR: δ 2.42 (3H, s), 7.25 (19H, m), 7.29 (5H, m),
7.41 (8H, m), 7.66, (1H, t), 8.09 (2H, d, J = 6.0), 8.27 (2H, t), 8.36 (1H, t).
4 3 2
Anal. Calcd. for C50H41BCuF N OP : C, 65.84; H, 4.53; N, 4.61. Found: C,
+
4 3 2
65.77; H, 4.40; N, 4.52. MS m/z: [m] calc. For C50H41BCuF N OP ,
911.2; found, 911.5.
3 2 4
2.4. Construction of composite fibers [Cu(PYO)(PPh ) ]BF /PVP
Composite fibers [Cu(PYO)(PPh
doping [Cu(PYO)(PPh ]BF into PVP polymer through electrospinning
technique [14]. First, PVP host was stirred and dissolved in DMF (5 mL,
0 wt%). After this solution become transparent, [Cu(PYO)(PPh ]BF
3 2 4
) ]BF /PVP were constructed by
)
3 2
4
2
.2. Synthesis of diamine ligand PYO
2
)
3 2
4
2
-(2H-tetrazol-5-yl)-pyridine (TP) was firstly synthesized following
was weighted and added into this PVP solution. This solution was
then transferred into a glass syringe (5 mL) with a plastic needle
a literature procedure [15]. A mixture of picolinonitrile (5 mmol) and

32
W. Pu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 169 (2016) 30–37
3 2 4 3 2 4
Fig. 1. A. Single crystal structure of [Cu(PYO)(PPh ) ]BF . B. Inner-molecular π stacking of [Cu(PYO)(PPh ) ]BF .
(
inner diameter = 0.6 mm). A copper wire was inserted into this glass
consequently increase its emissive performance. Face-to-face π-π at-
traction between PYO conjugation plane and PPh phenyl rings allows
a more organized geometry in [Cu(PYO)(PPh ]BF . It is observed
that one of PPh phenyl rings aligns nearly parallel to PYO plane with
face-to-face distance of 3.396 Å and intersection angle of only 0.88°, re-
spectively. Similar cases have been reported for [Cu(N\\N)(P\\P)] com-
plexes having large conjugation planes in their ligands [19,20]. This
syringe and connected with the anode terminal of a high-voltage gener-
ator. A piece of Al foil was connected to grounding electrode and placed
under this glass syringe, working as collecting board with tip-to-target
distance of 25 cm and driving voltage of 18 kV.
3
3
)
2
4
3
3
. Results and discussion
3
.1. Single crystal structure of [Cu(PYO)(PPh
) ]BF
3 2 4
Table 1
3 2 4
Selected structural parameters of [Cu(PYO)(PPh ) ]BF .
3 2 4
Single crystal structure of [Cu(PYO)(PPh ) ]BF is demonstrated in
Bond length
Å
Bond angle
°
Fig. 1A. A representative tetrahedral coordination field is observed for
each Cu center which is coordinated by two PYO N atoms and two
PPh P atoms. An electron-pulling oxadiazole ring is connected to a pyr-
3
idine ring and a methylbenzene ring through σ bonds, showing a large
conjugation plane of steric hindrance in PYO ligand which may slow
3 2 4
down non-radiative decay of [Cu(PYO)(PPh ) ]BF excited state and
Cu \\ N(2)
Cu \\ N(3)
Cu \\ P(1)
Cu \\ P(2)
2.129
2.177
2.230
2.274
N(2) \\ Cu \\ N(3)
N(2) \\ Cu \\ P(1)
N(3) \\ Cu \\ P(1)
N(2) \\ Cu \\ P(2)
N(3) \\ Cu \\ P(2)
P(1) \\ Cu \\ P(2)
78.57
123.41
112.67
98.57
99.49
130.41

W. Pu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 169 (2016) 30–37
33
highly organized array has been found as a rigid structure and positive
to restrict geometric relaxation in excited state [19]. Despite of the
above mentioned inner-molecular π stacking, there is, however, no
distorted owing to its heterogeneous ligands, including N-based PYO
and P-based PPh [18]. Its two Cu\\P bonds are different from each
3
other but still comparable to literature values (~2.27 Å) [15,18]. The two
Cu\\N bonds are much longer than literature values (~2.08 Å), suggesting
that coordination affinity of PYO N atoms is decreased by its electron-
pulling effect [15,18]. It seems that coordination affinity of the N atom
in pyridine ring, N(3), is even weaker than that of the N atom in
oxadiazole ring, N(2). This result can be explained by the fact that there
3 2 4
inter-molecular π stacking in [Cu(PYO)(PPh ) ]BF crystal, as shown
in Fig. 1B. This should be attributed to the free rotation of PPh
rings.
3
phenyl
Table 1 shows selected structural parameters of [Cu(PYO)(PPh
3
)
2
]BF
4
.
Similar to literature cases, this tetrahedral coordination field is slightly
Fig. 2. SEM images of our composite samples (a, 7 wt%; b, 9 wt%; c, 11 wt%; d, 13 wt%) and fluorescence microscopy image of 13 wt% doped sample (e).

34
W. Pu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 169 (2016) 30–37
are a short conjugation chain and electron-donating N and O atoms in
oxadiazole ring, which increases N(2)'s coordination affinity.
N\\Cu\\N bite angle of [Cu(PYO)(PPh
ature values [15,18]. While, its P\\Cu\\P bite angle is larger than litera-
ture values (~120°), suggesting that PPh ligand is distorting itself to
3 2 4
) ]BF is comparable to liter-
3
satisfy inner-molecular π stacking. For most bond angles listed in
Table 1, they are close to but not exactly equal to 109°, indicating a
structural distortion in [Cu(PYO)(PPh ) ]BF triggered by its heteroge-
3 2 4
neous ligands. Such structural distortion should be restricted since it in-
creases non-radiative decay probability and decreases energy content of
MLCT excited state [14,19,20]. With the help of inner-molecular π stack-
ing and the large steric hindrance of conjugation plane in PYO ligand,
structural relaxation in [Cu(PYO)(PPh
ed, showing promising emissive performance. Electronic structure of
Cu(PYO)(PPh ]BF is analyzed through its DFT calculation, which
3 2 4
) ]BF may be effectively restrict-
[
3
)
2
4
confirms its MLCT/LLCT excitation nature. See Supporting Information
for a detailed explanation.
3
3 2 4
.2. Morphology analysis on [Cu(PYO)(PPh ) ]BF /PVP composite fibers
Fig. 3. Solid state diffuse reflection spectrum (ref.) of bulk [Cu(PYO)(PPh
absorption (abs.) spectra of pure PVP, [Cu(PYO)(PPh ]BF (in CH Cl , 2 μM) and the
four composite samples. Inset: UV-Vis absorption spectra of PYO and PPh in CH Cl
2 μM).
3 2 4
) ]BF , UV-Vis
3
)
2
4
2
2
Although the electron-pulling oxadiazole and its steric hindrance in
PYO ligand may improve [Cu(PYO)(PPh ]BF emissive performance
3
2
2
(
)
3 2
4
by restricting its structural relaxation, we still have to figure out another
way to further restrict its structural relaxation. In this effort,
3
a new one and different from absorption spectra of PPh and TP ligands.
[
Cu(PYO)(PPh
3
)
2
]BF
4
is immobilized in a polymer matrix through
It is tentatively attributed to electronic absorption of MLCT, as suggested
by above DFT calculation result. This MLCT transition energy and corre-
sponding absorption edge (450 nm) are found much higher than those
of other [Cu(N\\N)(P\\P)] complexes. We attribute its causation to the
electron-pulling effect of PYO ligand and its short conjugation chain
[18–20].
As for PVP, there are three π → π* absorption bands peaking at
260 nm, 300 nm and 350 nm, respectively. These bands are traced in
all absorption spectra of our composite samples, with no obvious spec-
tral shift. There is a broad absorption band ranging from 350 nm to
490 nm in each composite absorption spectrum, which can be attribut-
electrospinning technique, hoping to minimize its structural relaxation
through polymer rigid framework. Owing to its virtues of controllable
morphology, proper mechanical strength and compatibility with vari-
ous dopants, PVP is here selected as supporting matrix [14]. Aiming at
a full comparison between bulk sample and PVP-based fibrous samples,
various doping levels are tried, including 7 wt%, 9 wt%,11 wt% and
1
3 wt%, respectively.
These PVP-based fibrous samples and their morphology are firstly
evaluated through their SEM images, as shown in Fig. 2. For all samples,
their fibers are randomly distributed on substrates with smooth surface
and uniform morphology. There are no branch structures or knots in
these fibers. They align cross each other, resulting in a porous structure
with its surface-area-to-volume ratio two orders of magnitude higher
than those of bulk materials [14]. Since there are no heterogeneous in-
terface or phase separation, it is tentatively concluded that
ed to MLCT absorption of [Cu(PYO)(PPh
found enhanced in PVP composite samples, compared to that of
[Cu(PYO)(PPh ]BF in CH Cl solution, along with a slight red shift.
This result confirms that [Cu(PYO)(PPh ]BF MLCT excited state is
3 2 4
) ]BF dopant. This MLT band is
3
)
2
4
2
2
3
)
2
4
vulnerable to surrounding environment, which is consistent with liter-
ature reports [19,20]. Despite of their different absorption intensity
values, absorption wavelength and band shape of our composite sam-
[
3 2 4
Cu(PYO)(PPh ) ]BF molecules have been successfully and uniformly
doped into PVP matrix with good compatibility with it. Mean diameters
of these composite fibers are determined as ~0.9 μm and similar to each
other. Dopant concentration variation has slim effect on sample diame-
ter or morphology, which should be explained by the low doping con-
centrations in PVP matrix. Fig. 2 gives a fluorescence microscopy
image of a representative sample (13 wt% doped). Under Hg lamp exci-
tation, homogeneous green emission comes out from all fibers, further
ples are similar to those of [Cu(PYO)(PPh
without any new absorption bands. We thus come to a conclusion
that [Cu(PYO)(PPh ]BF dopant is simply immobilized in PVP frame-
3 2 4 2 2
) ]BF in CH Cl solution,
)
3 2
4
work, with its excited state well protected by PVP framework. There is
no strong interaction between dopant molecules and PVP matrix.
3 2 4
Since bulk [Cu(PYO)(PPh ) ]BF sample is too thick for light pene-
confirming that [Cu(PYO)(PPh
distributed in PVP matrix with no phase separation or condensed
aggregation.
3
)
2
]BF
4
molecules have been uniformly
tration, we here use its solid state diffuse reflection (ref.) spectrum to
replace its absorption spectrum. This reflection spectrum has two low
reflection regions peaking at 283 nm and 427 nm, respectively, ending
at 458 nm. These two regions are consistent with the strong absorption
3
.3. Photophysical comparison between bulk [Cu(PYO)(PPh
3
)
2
]BF
4
and
bands of [Cu(PYO)(PPh
ples. The slight red shift in this reflection spectrum, compared to ab-
sorption spectrum of [Cu(PYO)(PPh ]BF in CH Cl solution, should
3 2 4 2 2
) ]BF in CH Cl solution and composite sam-
[Cu(PYO)(PPh ]BF /PVP
)
3 2
4
3
)
2
4
2
2
3
.3.1. UV-Vis absorption and solid state diffuse reflection spectra
UV–Vis absorption (abs.) spectra of pure PVP, [Cu(PYO)(PPh
in CH Cl , 2 μM) and the four composite samples are shown in Fig. 3.
In solution, [Cu(PYO)(PPh ]BF has two strong absorption bands,
be attributed to solid state aggregation. In this case, it is further con-
firmed that PVP matrix only immobilizes and protects
) ]BF
3 2 4
(
2
2
[Cu(PYO)(PPh
transition.
3 2 4
) ]BF molecules, without changing its electronic
3
)
2
4
peaking at 233 nm and 270 nm, respectively. There is still a broad ab-
sorption band ranging from 350 nm to 450 nm. The first strong absorp-
3.3.2. Emission spectra
Emission (em.) spectra of [Cu(PYO)(PPh
bulk [Cu(PYO)(PPh ]BF and the four composite samples are shown
in Fig. 4. In solution, [Cu(PYO)(PPh ]BF shows a broad and weak
tion band is quite similar to PPh
π → π* absorption of PPh , as shown by Fig. 3 inset. The second strong
absorption band is considered as an absorption conduct of PPh and
3
absorption and thus attributed to
3 2 4 2 2
) ]BF (in CH Cl , 2 μM),
3
)
3 2
4
3
3
)
2
4
PYO ligands. This assignment is consistent with their LLCT nature, as
suggested by above DFT calculation result. The weak band, however, is
emission band peaking at 580 nm with FWHM of 76 nm, where
FWHM means full width at half maximum. There are no vibronic

W. Pu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 169 (2016) 30–37
35
shift between absorption edge and emission peak. Emission quantum
yields (Ф) of these samples are listed in Table 2. After a systematical
comparison between [Cu(PYO)(PPh ) ]BF in CH Cl solution, in solid
3 2 4 2 2
state and in PVP matrix, it is found that PVP immobilization limits
MLCT geometric relaxation the most effectively due to its rigid frame-
3 2 4
work. In the meanwhile, [Cu(PYO)(PPh ) ]BF excited state is effective-
ly restricted, showing small FWHM values of [Cu(PYO)(PPh
3 2 4
) ]BF /PVP
samples.
3.3.3. Excited state decay lifetimes
3 2 4 2 2
Excited state decay lifetimes of [Cu(PYO)(PPh ) ]BF (in CH Cl ,
2
3 2 4
μM), bulk [Cu(PYO)(PPh ) ]BF and the four composite samples are
shown in Fig. 5. All samples follow biexponential decay pattern with
mean decay lifetimes of 1.26 μs for [Cu(PYO)(PPh ]BF solution,
06.0 μs for bulk [Cu(PYO)(PPh ]BF , 156.1 μs for the 7 wt% doped
3
)
2
4
1
3
)
2
4
sample, 168.6 μs for the 9 wt% doped sample, 175.9 μs for the 11 wt%
doped sample and 150.5 μs for the 13 wt% doped sample, respectively.
Their detailed fitting parameters are listed in Table 2. Phosphorescent
nature of these samples are thus confirmed by their long-lived emissive
components [19–22]. Taking the above DFT calculation result into ac-
Fig. 4. Emission spectra (λex = 355 nm) of [Cu(PYO)(PPh
3 2 4 2 2
) ]BF in CH Cl (2 μM), bulk
[
Cu(PYO)(PPh ]BF and the four composite samples.
3
)
2
4
3
progressions, which means that its emissive center owns a charge trans-
fer character. This finding is consistent with its MLCT electronic nature.
There is a large Stokes shift (130 nm) between absorption edge
count, their emissive state can be assigned as (MLCT&LLCT), which is
consistent with literature cases [19–20].
2 2 3 2 4
In CH Cl solution, [Cu(PYO)(PPh ) ]BF shows the shortest lifetime
(
[
450 nm) and emission peak (580 nm), suggesting that
Cu(PYO)(PPh ]BF excited state suffers from bad geometric relaxa-
solution which is attributed to solvent molecule attack
]BF
emission peaking at 552 nm is obviously enhanced with FWHM of
8 nm. Its Stokes shift between absorption edge (458 nm) and emission
of only 1.26 μs. This observation is consistent with our above conclusion
that it suffers from bad geometric relaxation and solvent molecule at-
tack, owing to the limited steric hindrance of PYO ligand. In solid
)
3 2
4
2 2
tion in CH Cl
and limited PYO steric hindrance. In bulk state, [Cu(PYO)(PPh
3
)
2
4
state, solvent effect is eliminated, [Cu(PYO)(PPh
ly increased to 106.0 μs. Clearly, the highly ordered arrangement in solid
state [Cu(PYO)(PPh ]BF forms a rigid environment. With its geomet-
ric relaxation restricted, [Cu(PYO)(PPh ]BF lifetime is obviously in-
3 2 4
) ]BF lifetime is great-
9
)
3 2
4
peak (552 nm) is 94 nm. This decreased Stokes shift value, along with its
emission blue shift, suggests that the geometric relaxation of
)
3 2
4
creased. After being immobilized in PVP matrix, its geometric
relaxation is further depressed by polymer framework, showing an
3 2 4 2 2
[Cu(PYO)(PPh ) ]BF in bulk state is weaker than that in CH Cl solu-
tion, owing to its inner-molecular π stacking [19,20]. Its effect, however,
even longer lifetime than that of solid state [Cu(PYO)(PPh
3 2 4
) ]BF . Tak-
is limited owing to the limited steric hindrance of PYO ligand.
ing above emission spectrum analysis into account, we come to a con-
In PVP matrix, [Cu(PYO)(PPh
band peaking 505 nm which is clearly blue shifted compared to emis-
sion spectra of [Cu(PYO)(PPh ]BF in CH Cl solution and in solid
state. Composite emission intensity generally increases with increasing
dopant concentration. FWHM values are found as 80 nm for the 7 wt%
doped sample, 78 nm for the 9 wt% doped sample, 80 nm for the
3
)
2
]BF
4
shows a Gauss-liked emission
clusion that MLCT geometric relaxation in [Cu(PYO)(PPh
excited state can be effectively restricted by PVP framework.
3 2 4
) ]BF
3
)
2
4
2
2
There is an interesting fact worthy of more explanation words. Com-
posite sample lifetime firstly increases from 156.1 μs (7 wt%) to 175.9 μs
(11 wt%). Then composite sample lifetime is decreased to 150.5 μs at
doping concentration of 13%. There seems an optimal doping concentra-
tion (11 wt%) for our composite samples, which can be explained as fol-
lows. It is assumed that there are two or more factors that affect excited
state decay lifetime: (1) PVP immobilization effect which is positive for
lifetime and (2) self-quenching/absorption between dopant molecules
which is negative for lifetime. If doping concentration is quite low,
self-quenching/absorption between dopant molecules can be ignored.
PVP immobilization effect is increased with increasing doping concen-
tration, with composite sample lifetime gradually increased. On the
other hand, if dopant molecules are more than enough, PVP immobiliza-
tion effect gets to its ceiling, self-quenching/absorption tends to domi-
nate composite samples, compromising lifetime. There should be both
strong PVP immobilization effect and limited self-quenching/absorption
in the optimal composite sample, showing its longest excited state
decay lifetime.
1
1 wt% doped sample and 81 nm for the 13 wt% doped sample, respec-
tively. Clearly, these values are smaller than those of
Cu(PYO)(PPh ]BF in CH Cl solution and in solid state. These de-
[
3
)
2
4
2
2
creased FWHM values are consistent the highly ordered arrangement
in our composite samples. Stokes shift of our composite samples be-
tween absorption edge (490 nm) and emission peak (505 nm) is only
1
5 nm. It is not usual to see a green emission with small Stokes shift
from a Cu(I) complex with N/P ligands, and we are giving an explana-
tion as follows. According to a literature report, the geometric relaxation
of MLCT excited state in [Cu(N\\N)(P\\P)] complexes acts as a major
energy loss and non-radiative decay path [18–20]. After being doped
into PVP matrix, dopant molecules are dispersed and immobilized in
polymer framework with their geometric relaxation greatly limited.
Corresponding energy loss is thus decreased, with minimal Stokes
Table 2
2
2
Fitting parameters of [Cu(PYO)(PPh
(A + A ).
) ]BF
3 2 4
in CH Cl
2 2
(2 μM), bulk [Cu(PYO)(PPh ) ]BF
3 2 4
and the four composite samples. y = A
1
∗ exp(−x / t
1
) + A
2
2 0 1 1 2 2
∗ exp(−x / t ) + y , τ = (A t + A t )
/
t
1 1
2 2
t
Sample
Ф
A
1
t
1
(s)
A
2
t
2
(s)
R2
τ (s)
1.26
Solution
Bulk
0.089
0.55
0.40
0.52
0.60
0.58
0.3111
1.12674E−7
1.13131E−5
1.14853E−5
1.45892E−5
1.59409E−5
1.55636E−5
0.46272
0.25775
0.28594
0.31326
0.37050
0.21727
1.32709E−6
1.19502E−4
1.72553E−4
1.85179E−4
1.9267E−4
1.71843E−4
0.999
0.996
0.993
0.992
0.994
0.996
0.38843
0.48915
0.42806
0.47067
0.37931
106.0
7
9
1
1
wt%
wt%
1 wt%
3 wt%
156.1
168.6
175.9
150.5

36
W. Pu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 169 (2016) 30–37
Fig. 5. Excited state decay lifetimes of [Cu(PYO)(PPh
3 2 4 2 2
) ]BF (in CH Cl , 2 μM), bulk
Fig. 6. Emission intensity monitoring under continuous radiation is performed on
Cu(PYO)(PPh ]BF (in CH Cl , 2 μM), bulk [Cu(PYO)(PPh ]BF and the four
composite samples (λex = 355 nm).
[
Cu(PYO)(PPh ]BF and the four composite samples (λex = 355 nm).
3
)
2
4
[
3
)
2
4
2
2
3
)
2
4
3
.3.4. Photostability under continuous radiation
Based on our above analysis, it is confirmed that
Cu(PYO)(PPh ]BF dopant molecules have been immobilized and
doped into PVP fibers through electrospinning technique. Detailed anal-
ysis and comparison between solid state sample, solution sample and
composite samples suggested that MLCT geometric relaxation was ef-
fectively restricted by PVP immobilization effect. With its geometric re-
[
3
)
2
4
well protected in PVP matrix. Their geometric relaxation in excited
state is effectively restricted. With this perfect protection from PVP
framework, we assume that photostability of our dopant may be im-
proved as well in our composite samples. Emission intensity monitoring
3 2 4
laxation greatly limited, emissive performance of [Cu(PYO)(PPh ) ]BF
was obviously improved, including emission blue shift, long emission
decay lifetime and better photostability. MLCT geometric relaxation is
widely existed in transition metal complexes. In most cases, this geo-
metric relaxation compromises their emissive centers more or less.
Polymer immobilization effect can effectively limit such geometric re-
laxation. Thus, it is reasonable to assume that similar polymer immobi-
lization effect should be found for other composite samples. This finding
should be helpful for later design of one dimensional nanostructures for
photoelectronic applications.
under continuous radiation is performed on [Cu(PYO)(PPh
CH Cl , 2 μM), bulk [Cu(PYO)(PPh ]BF and the four composite sam-
ples to confirm this hypothesis. It is observed in Fig. 6 that
Cu(PYO)(PPh ]BF has the lowest photostability in solid state. Only
0% of its initial emission intensity is recovered after 40 min of continu-
3 2 4
) ]BF (in
2
2
)
3 2
4
[
)
3 2
4
6
ous radiation. Photo-induced oxidization and deconstruction during
this time should be responsible for this emission intensity decrease. In
CH Cl solution, photostability of [Cu(PYO)(PPh ) ]BF is greatly im-
2 2 3 2 4
proved. This is because dopant molecules are dispersed in solution
and thus protected by solvent molecules. After being immobilized in
Acknowledgement
PVP matrix, [Cu(PYO)(PPh
which can be explained as follows. In composite samples, all
Cu(PYO)(PPh ]BF molecules are dispersed in PVP framework
3 2 4
) ]BF shows an even better photostability,
The authors thank our universities for their equipment and financial
support.
[
3
)
2
4
which offers a protective environment for them. This framework may
absorb excitation energy first and then transfer its energy to dopant
molecules, resulting in an indirect energy transfer mechanism. Since
there is no direct exposure to excitation light, it is thus rational to see
their greatly improved photostability of our composite samples.
In addition, it is found that photostability of our composite samples
is decreased by increasing doping concentration, which can be ex-
plained by the increasing aggregation between dopant molecules.
Never the less, photostability of our composite samples is obviously im-
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.saa.2016.06.017.
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