Study on polymer fibers doped with phosphorescent Re(I) complex via electrospinning: Synthetic strategy, structure, morphology and photophysical features

Article abstract of DOI:10.1016/j.saa.2013.11.022

In the following paper, a Re(I) complex with electron-withdrawing oxadiazole group in its diamine ligand (denoted as N-N) was synthesized. Its single crystal structure analysis confirmed the successful synthesis of both ligand and complex. The central Re(

Full text of DOI:10.1016/j.saa.2013.11.022

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 605–612
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Study on polymer fibers doped with phosphorescent Re(I) complex via
electrospinning: Synthetic strategy, structure, morphology
and photophysical features
b
b
Wang Dejin a, , Liu Qiang , Wang Fei
⇑
a School of Resources and Environment, Anqing Normal University, Anqing 246052, PR China
b Shanghai University, Shanghai 200444, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
A Re(I) complex owing oxadiazole
derived ligand was synthesized.
Its single crystal structure and
electronic nature were investigated.
It was doped into a polymer matrix to
construct composite fibers.
The photophysical features of pure
sample and doped sample were
compared.
ꢀ
The doped sample showed better
performance by restraining geometric
relaxation.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 October 2013
Received in revised form 2 November 2013
Accepted 5 November 2013
Available online 13 November 2013
In the following paper, a Re(I) complex with electron-withdrawing oxadiazole group in its diamine ligand
(denoted as N–N) was synthesized. Its single crystal structure analysis confirmed the successful synthesis
of both ligand and complex. The central Re(I) ion localized in a traditional octahedral coordination
environment. The diamine ligand 2-(pyridin-2-yl)-5-(p-tolyl)-1,3,4-oxadiazole (denoted as PPOZ) took
a coplanar structure and the corresponding face-to-face
p–p attraction between PPOZ ligands made
the Re(I) complex molecules adjust a highly ordered arrangement which was positive to improve emis-
sive performance. In order to repress the excited state geometric relaxation and further improve the
emissive performance, the Re(I) complex was doped into a polymer host poly(vinylpyrrolidone) via
electrospinning, resulting in composite fibers. The morphology of those composite fibers was analyzed
by electron microscopy. The photophysical comparison between bulk sample and composite fibers indi-
cated that the composite fibers showed emission blue shift, longer excited state lifetime and improved
photostability. Further analysis suggested that the excited state geometric relaxation could be effectively
repressed when the Re(I) complex molecules were immobilized in the polymer matrix, leading to above
variations.
Keywords:
Re complex
Photoluminescence
Microfiber
Crystal
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
the developed techniques for constructing those nanostructures,
electrospinning has been proved to be a charming one because of
One-dimensional nanostructures such as nanowires and nanof-
ibers have been demonstrated to be promising candidates owing to
their potential optoelectronic and mechanical features [1]. Among
its low cost, simple and easy-to-go constructing route. The ob-
tained electrospinning fibers usually own large surface-to-volume
ratio, controllable morphology and feasible mechanical strength,
making themselves a potential supporting host for various opto-
electronic systems [2–6].
⇑
Corresponding author. Tel.: +86 0556 5501280.
E-mail address: wang_dj01@163.com (W. Dejin).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.11.022

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To achieve satisfactory photophysical performance, some opto-
Re(CO)3(PPOZ)Br and the composite fibers (denoted as
PVP@Re(CO)3(PPOZ)Br). The initial reagent 2-(2H-tetrazol-5-
yl)pyridine (denoted as TYP) was obtained through a literature
procedure [16]. The other salts and organic chemicals, including
4-methylbenzoyl chloride, sodium azide, poly(vinylpyrrolidone)
(PVP, K30), zinc bromide and Re(CO)5Br, were bought from Alfresa
Pharma Corporation and used as received without further purifica-
tions. The organic solvents used in this work, including CH2Cl2,
CHCl3, ethanol and N,N0-dimethyl formamide (DMF), were bought
from Shanghai Chemical Cooperation and redistilled prior to use.
The solvent water used in this work was deionized.
electronic dopants such as organic dyes and transition metal com-
plexes have been doped into the electropinning fibers, which
widens the application of these composite fibers [7,8]. As a class
of typical phosphorescent material, Re(I) complexes with general
molecular formula of Re(CO)3(N–N)X have been widely studied
since they own good photoluminescence (PL) yield, controllable
emissive wavelength and enough stability [9–11], where N–N
and X mean diamine ligand and halogen atom, respectively. Theo-
retical calculation on them suggests that the photoinduced excita-
tions and emissions involve electronic transitions between frontier
molecular orbitals (FMOs). The occupied FMOs are usually com-
posed of metal ion and halogen atom, while the unoccupied FMOs
are p* orbitals in nature. The excited state and the corresponding
emissive state are thus assigned as a mixed character of metal-
to-ligand-charge-transfer and ligand-to-ligand-charge-transfer
(ML&LLCT) [11–15].
The systematical analysis by Zhang and coworkers on the
MLCT excited state of a series of transition metal complexes sug-
gests that the MLCT excited state, as well as the emissive perfor-
mance of corresponding metal complexes, is related with the
geometric relaxation of MLCT excited state [12–15]. Both emis-
sive energy and emission quantum yield can be improved upon
effective repression of geometric relaxation. Some approved solu-
tions include using N–N ligands with large steric hindrance and
the immobilization into solid matrixes such as polymer matrix
or silica molecular sieves [12–15]. In addition, it has been found
that the existence of electron-withdrawing group in N–N ligand
is positive to improve emissive performance of corresponding
Re(I) complexes [15]. It is thus expected that the emissive perfor-
mance of Re(I) complex with electron-withdrawing group in its
N–N ligand can be further improved when it is immobilized in so-
lid matrix.
PPOZ diamine ligand
The diamine ligand PPOZ was synthesized following a literature
procedure described as follows [16]. 10 mmol of TYP was slowly
added into 20 mL of cold anhydrous pyridine under ice bath. Then
11 mmol of 4-methylbenzoyl chloride was also added into above
solution slowly. The mixture was stirred for 20 min under ice bath
and another 10 min at room temperature. Then the solution was
heated to reflux under N2 protection for two days. The solution
was cooled to room temperature and then poured into 400 g of
water ice. The resulting solid product was collected and further
purified on
a silica gel column (n-hexane: ethyl acetate
(V:V) = 30:1). Yield: 50%. 1HNMR (CDCl3): d 2.48 (3H, s), 7.37 (2H,
d, J = 6.0), 7.49 (1H, t), 7.92 (1H, t), 8.14 (2H, d, J = 6.0), 8.33 (1H,
d, J = 6.0), 8.86 (1H, d, J = 3.6). Anal. Calcd. For C14H11N3O: C,
70.87; H, 4.67; N, 17.71. Found: C, 70.94; H, 4.83; N, 17.54.
Re(CO)3(PPOZ)Br complex
The Re(I) complex Re(CO)3(PPOZ)Br was synthesized following
a literature procedure described as follows [17]. 0.5 mmol of PPOZ
ligand and 0.5 mmol of Re(CO)5Br were added into 15 mL of anhy-
drous toluene. The solution was heated to reflux under N2 protec-
tion overnight. Then the solvent was vaporized. The resulting solid
product was collected and further purified on a silica gel column
(n-hexane: ethyl acetate (V:V) = 30:1). Yield: 47%. 1HNMR (CDCl3):
d 2.51 (3H, s), 7.39 (2H, d, J = 6.0), 7.52 (1H, t), 7.95 (1H, t), 8.17 (2H,
d, J = 6.0), 8.37 (1H, d, J = 6.0), 8.88 (1H, d, J = 3.6). Anal. Calcd. For
Encouraged by above results and considerations, in this paper,
we design and synthesize a N–N ligand 2-(pyridin-2-yl)-5-(p-
tolyl)-1,3,4-oxadiazole (denoted as PPOZ) which owns an
electron-withdrawing group in its molecular structure and its cor-
responding Re(I) complex Re(CO)3(PPOZ)Br. The composite elec-
trospinning fibers are also constructed using a polymer as the
host and Re(CO)3(PPOZ)Br as the photo-active dopant. Re(CO)3
(PPOZ)Br and the composite fibers are carefully characterized
and studied. Their photophysical comparison is also performed to
analyze the geometric relaxation of MLCT excited state and its
correlation with emissive performance.
C17H11N3O4: C, 34.76; H, 1.89; N, 7.15. Found: C, 34.57; H, 1.83; N,
7.34. Its identity is further confirmed by single crystal XRD analysis
which will be discussed below.
PVP@Re(CO)3(PPOZ)Br
Experimental details
PVP@Re(CO)3(PPOZ)Br composite fibers were prepared through
electropinning technique as follows. Transparent solution of PVP
dissolved in DMF was firstly prepared. Then finely measured
Re(CO)3(PPOZ)Br was added into this solution to give
Scheme
1 demonstrates the synthetic and preparation
procedure for the diamine ligand PPOZ, the Re(I) complex
Scheme 1. The synthetic and preparation procedure for PPOZ, Re(CO)3(PPOZ)Br and PVP@Re(CO)3(PPOZ)Br.

W. Dejin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 605–612
607
homogeneous solution which was then transferred into a 5 mL
glass syringe equipped with plastic needle (inner diame-
benzene group are connected to 1,3,4-oxadiazole group via two
bonds. The free rotation of both bonds allows the three groups
a
r
r
ter = 0.6 mm) [18]. The end of a copper wire was connected to
the anode terminal of a high-voltage generator, with the other
end inserted into the polymer solution. A piece of Al foil was con-
nected to the grounding electrode and placed beneath the plastic
needle, serving as the collecting substrate. The tip-to-plate dis-
tance was 25 cm and the driving voltage was 18 kV.
to form a large conjugation plane. The central Re(I) ion is sur-
rounded by three CO groups, two N atoms from a PPOZ ligand
and a Br atom, which means that it occupies the center of a dis-
torted octahedral coordination environment. The selected struc-
tural parameters of this coordination sphere are summarized in
Table 1, which confirms the distortion of the coordination environ-
ment around Re(I) center. The three bond length values shown in
Table 1 are comparable with literature ones [17], however, they
are slightly different from each other. It can be observed that Re–
C(1) and Re–C(2) bond length values are similar to each other,
while Re–C(2) bond is longer than both ones, which means that
the Br atom localized at the opposite site of C(2) may slightly de-
crease the coordination ability of C(2). There is also a slight differ-
ence between the two Re–N bonds. Since Re–N(1) bond (2.165 Å) is
shorter than Re–N(2) bond (2.228 Å), the coordination ability of
N(1) atom from 1,3,4-oxadiazole group is then higher than that
of N(2) atom from pyridine group. It is expected that the restricted
conjugation chain and the electron-pushing O atom in 1,3,4-oxadi-
azole group are responsible for the increased coordination ability
of N(1). In other words, the restricted conjugation chain limits
the delocalization of N(1) lone pair, and its coordination ability is
further enhanced by the neighboring electron-pushing O atom. In
addition, the structural distortion may also be partially responsible
for the bond length difference [17].
As for the bond angles listed in Table 1, their values also indi-
cate the distortion of the coordination field. The bite angle of
N–Re–N is only 73.25° which is quite smaller than literature values
in tetrahedral fields (>80°) [14–17], indicating that the coordina-
tion environment in Re(CO)3(PPOZ)Br is less congested than that
in tetrahedral fields. We attribute the causation to the small steric
hindrance of CO and Br ligands. Since the geometric relaxation of
MLCT excited state is responsible for the major energy loss path,
the less crowded coordination field may face more severe geomet-
ric relaxation [12–15]. On the positive side, the immobilization
effect from polymer host can be confirmed with the elimination
of ligand steric hindrance.
Methods and measurements
1H NMR spectra were measured by a Varian INOVA 300 spec-
trometer. The elemental analysis was finished on a Carlo Erba
1106 elemental analyzer. The single crystal analysis was finished
by a Siemens P4 single-crystal X-ray diffractometer loaded with a
Smart CCD-1000 detector at 298 K, using graphite-monochromat-
ed Mo K
tion and emission spectra were measured in CH2Cl2 solutions
(1 M) by a HP 8453 UV–Vis–NIR diode array spectrophotometer
a radiation. All hydrogen atoms were calculated. Absorp-
l
and a Hitachi F-4500 fluorescence spectrophotometer, respec-
tively. Emission decay dynamics were detected by a two-channel
TEKTRONIX TDS-3052 oscilloscope, using pulsed Nd:yttrium alu-
minium garnet (YAG) laser (355 nm, the third-harmonic-generator
pump) as the excitation source. Scanning electron microscopy
(SEM) and fluorescence microscopy images were measured on a
Hitachi S-4800 microscope and a Nikon TE2000-U fluorescence
microscopy powered by a mercury lamp, respectively. Theoretical
analysis on Re(CO)3(PPOZ)Br, including the composition of the
FMOs and the first ten singlet excitations, was performed by time
dependent density functional theory (TD-DFT) using GAMESS at
RB3PW91/SBKJC level in vacuum, with its crystal structure as the
initial structure. The graphical presentation was generated by
wxMacmolplt software package with contour value of 0.025. All
measurements were carried out in the air at room temperature
without any specifications.
Results and discussion
As above mentioned, PPOZ ligand takes a coplanar structure.
Single crystal structure of Re(CO)3(PPOZ)Br
The face-to-face
p–p attraction between PPOZ ligands makes
Re(CO)3(PPOZ)Br molecules take a highly ordered array as shown
in Fig. 1B, which is consistent with the literature report by Zhang
and coworkers [12,15]. In this case, each PPOZ ligand aligns nearly
parallel to each other, correspondingly, Re(CO)3(PPOZ)Br mole-
As mentioned in the Introduction part, a N–N ligand owing a
typical electron-withdrawing group in its molecular structure
was designed to improve the emissive performance of Re(CO)3
(PPOZ)Br. The successful synthesis of the Re(I) complex, as well
as that of PPOZ ligand, can be proved by the single crystal structure
shown in Fig. 1A. It can be seen that the pyridine group and the
cules also adjust their orientation to meet the
p–p attraction
between PPOZ ligands. The intersection angle and the face-to-face
Fig. 1A. The single crystal structure of Re(CO)3(PPOZ)Br. All hydrogen atoms are omitted for clarity.

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Table 1
Table 2
Selected structure parameters of Re(CO)3(PPOZ)Br obtained from single crystal.
Percentage composition of FMOs as well as the first ten onset electronic transitions.
Bond length
Å
Bond angle
°
MO/transition
Energy (eV)
Contribution (%)
Re CO
0.6 1.2
Re–C(1)
Re–C(2)
Re–C(3)
Re–Br(1)
Re–N(1)
Re–N(2)
1.886
1.920
1.890
2.615
2.165
2.228
N(1)–Re–N(2)
N(1)–Re–C(1)
N(2)–Re–C(3)
N(1)–Re–Br(1)
N(2)–Re–Br(1)
N(1)–Re–C(2)
N(2)–Re–C(2)
73.25
99.41
99.48
84.80
86.37
95.49
91.86
PPOZ
Br
0.3
0.5
2.0
42.1
48.2
0.7
31.7
33.7
LUMO + 2(73)
LUMO + 1(72)
LUMO(71)
ꢁ1.894
ꢁ2.297
ꢁ3.157
ꢁ5.676
ꢁ5.796
ꢁ6.634
ꢁ6.947
ꢁ7.064
1.7671
1.9952
2.6692
2.7377
2.8585
3.0595
3.1813
3.2899
3.3230
3.6564
98.0
96.6
89.6
4.5
9.9
9.7
1.2
4.2
1.7
4.1
HOMO(70)
35.1
27.7
61.7
33.5
36.5
18.3
14.1
27.9
14.5
16.7
HOMO-1(69)
HOMO-2(68)
HOMO-3(67)
HOMO-4(66)
20.2
13.1
S0 ? S1
S0 ? S2
S0 ? S3
S0 ? S4
S0 ? S5
S0 ? S6
S0 ? S7
S0 ? S8
S0 ? S9
S0 ? S10
70 ? 71(98.6)
69 ? 71(98.0)
68 ? 71(98.5)
70 ? 72(96.2)
69 ? 72(97.6)
67 ? 71(86.6)
70 ? 73(94.5)
69 ? 73(53.4)+66 ? 71(37.1)
66 ? 71(45.3)+ 69 ? 73(44.6)
68 ? 72(97.6)
Fig. 1B. The face-to-face p–p attraction between PPOZ ligands in Re(CO)3(PPOZ)Br
crystal.
distance between PPOZ ligands are only 1.72° and 3.257 Å, con-
firming the existence of face-to-face attraction between PPOZ
p–p
ligands. According to Zhang’s statement [12,15], this highly or-
dered array can be considered as a rigid structure, where the MLCT
excited state geometric relaxation can be effectively repressed,
showing improved emissive performance such as increased emis-
sive energy, longer excited state lifetime and higher emission yield.
It is thus expected that Re(CO)3(PPOZ)Br crystal may also presents
an improved emissive performance compared with that of diluted
Re(CO)3(PPOZ)Br molecule.
Theoretical calculation and analysis on Re(CO)3(PPOZ)Br
Since the photoinduced excitations and emissions involve elec-
tronic transitions between FMOs, it is thus necessary to get a clear
understanding on the FMOs as well as the onset electronic transi-
tions of Re(CO)3(PPOZ)Br. TD-DFT calculation has been proved to
be a powerful tool to reveal the electronic structure of transition
metal complexes [12–15]. The percentage composition of the
FMOs as well as that the first ten onset electronic transitions are
summarized in Table 2. Similar to literature cases [12–15], the
occupied FMOs of MOs 66–70 all own predominant metal charac-
ter, admixed with some contributions from Br atom and CO groups.
On the other hand, the unoccupied FMOs of MOs 71–73 are essen-
Fig. 2. The graphical presentations of two representative MOs of HOMO (up) and
LUMO (down).
(LLCT). The excited state is usually derived from the onset elec-
tronic transition and thus should also be assigned as a mixed char-
acter of MLCT and LLCT. In addition, the electron-withdrawing
group 1,3,4-oxadiazole is clearly involved in the photoinduced
electronic transitions, and the emissive performance of Re(CO)3
(PPOZ)Br is expected to be improved [15].
tially
p
* orbitals of PPOZ ligand. To get a visual understanding, two
SEM and fluorescence microscopy images of PVP@Re(CO)3(PPOZ)Br
representative MOs of HOMO and LUMO are plotted as Fig. 2.
Clearly, the electron cloud mainly distributes on Br atom and Re
center on ground state. Upon excitation, the electron cloud local-
izes mainly on PPOZ ring, suggesting a charge transfer process.
The first ten singlet excitations nearly all consist of electronic tran-
sitions from occupied FMOs to unoccupied ones, which means that
they own a mixed character of metal-to-ligand-charge-transfer
As for the supporting host, it need to own enough mechanical
strength, as well as good electrospinning property to meet the
requirements for optoelectronic dopants such as porous structure
and controllable morphology. Among the proposed candidates for
electrospinning hosts, PVP has been widely used due to its advan-
tages of uniform morphology, practicable mechanical strength and
good compatibility with optoelectronic dopants [19]. In this paper,
(MLCT [d(Re) ? p
*(PPOZ)]) and ligand-to-ligand-charge-transfer

W. Dejin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 605–612
609
PVP is selected as the supporting host for Re(CO)3(PPOZ)Br doped
composite fibers. Four doping concentrations, 6 wt%, 10 wt%,
14 wt% and 16 wt%, are tried to systematically study the photo-
physical features of those composite fibers and correspondingly
seek the optimal doping concentration. First, the morphology of
those composite fibers can be directly seen from their SEM images.
As shown in Fig. 3A, all electronspinning fibers are randomly dis-
tributed on the substrates, showing orderless arrangement. No
branches have been observed for those composite fibers, showing
smooth surface. The average diameter vales for those fibers are
90 nm for the 6 wt% sample, 100 nm for the 10 wt% sample,
110 nm for the 14 wt% sample and 100 nm for the 16 wt% sample,
respectively. It is observed that the diameter is nearly independent
of the doping concentration variation. No detectable phase separa-
tion can be observed, indicating PVP’s good compatibility with
Re(CO)3(PPOZ)Br. Those fibers intercross with each other, resulting
in a fibrous porous structure whose surface-to-volume ratio is two
orders of magnitude higher than those of bulk materials, making
them a shining host for optoelectronic dopants [19].
Fig. 3B. The representative fluorescence microscopy image of the 14 wt% sample.
The successful doping and good compatibility with the dopant
can also be confirmed by the representative fluorescence micros-
copy image of the 14 wt% sample shown in Fig. 3B. Clearly, there
is yellow emission coming out from the composite fibers under
Hg-lamp excitation, which means that the optoelectronic dopant
Re(CO)3(PPOZ)Br has been successfully doped into PVP host. We
and pure PVP are measured and shown in Fig. 4. Bulk Re(CO)3
(PPOZ)Br, however, is hard to take its absorption spectra since it
is too thick for light to penetrate. Alternatively, its solid state dif-
fuse reflection spectrum is then measured to evaluate its electronic
transition. As shown in Fig. 4, the absorption spectrum for
Re(CO)3(PPOZ)Br dilute solution consists of two major regions,
which are the high energy one ranging from 230 nm to 350 nm
with triple peaks at 260 nm, 290 nm and 330 nm, and the low
energy one ranging from 365 nm to 500 nm with broad peak
around 395 nm, respectively. Considering their similar absorption
energy and extinction coefficient with those of free ligands
[16,17,19], the first one can be identified as the spin-allowed
have above mentioned that there is face-to-face
Re(CO)3(PPOZ)Br crystal. With the universal distribution in PVP
host, the stacking can be eliminated in those composite fibers,
p–p stacking in
p–p
further confirming the immobilization effect from PVP host, which
will be discussed in detail below.
p
? p
* transitions of PPOZ ligand. While the latter one can be ten-
Photophysical features of Re(CO)3(PPOZ)Br and PVP@Re(CO)3(PPOZ)Br
tatively attributed to absorption of MLCT and LLCT according to
above theoretical calculation result. This assignment is consistent
with MLCT and LLCT transitions’ low extinction coefficients
[13–16].
The four composite fibers present similar absorption spectra
with that of Re(CO)3(PPOZ)Br dilute solution, peaking at 240 nm,
Absorption and diffuse reflection spectra
The electronic transition of Re(CO)3(PPOZ)Br and the composite
fibers can be presented by their absorption spectra. As a conse-
quence, the absorption spectra (abs.) of Re(CO)3(PPOZ)Br solution
290 nm and 337 nm, respectively, along with
a broad band
in CH2Cl2 with concentration of 1 lM, the four composite fibers
Fig. 3A. The SEM images of the 6 wt% sample (up left), the 10 wt% doped sample (up right), the 14 wt% doped sample (down left) and the 16 wt% doped sample (down right).

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W. Dejin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 605–612
large as 87 nm. Here, FWHM means full-width-at-half-maximum.
The emission spectrum is broad, showing no vibronic progression,
suggesting the charge transfer nature of the emissive state. This
observation is in consistent with above theoretical calculation re-
sult and literature reports [12,14,15,17]. In solid state, Re(CO)3
(PPOZ)Br emission moves to 572 nm with FWHM value of 93 nm
The red shift can be attributed to the aggregation of the excited
states [15]. It has been above pointed out that there is p–p stacking
within solid Re(CO)3(PPOZ)Br crystal, which is believed to be effec-
tive on repressing MLCT excited state relaxation and thus improv-
ing emissive performance [15]. Compared with the emission
spectrum in solution, it can be observed, however, that such
p-p
stacking may be plain in repressing the excited state relaxation
since the crystal emissive state shows a decreased energy and wid-
ened spectrum.
After being doped into PVP host, Re(CO)3(PPOZ)Br emission
shifts to 573 nm for the 6 wt% sample, 568 nm for the 10 wt% sam-
ple, 562 nm for the 14 wt% sample and 563 nm for the 16 wt% sam-
ple, respectively. Their FWHM values are 115 nm, 114 nm, 116 nm
and 118 nm, respectively. The emission blue shift and the widened
spectra indicate that the immobilization in PVP matrix can repress
the MLCT geometric relaxation and stabilize the emissive state. As
suggested by literature reports, the geometric relaxation is the
main causation for energy loss and inradiative decay path of MLCT
excited states [12,15]. When Re(CO)3(PPOZ)Br molecules are doped
into PVP host, they are also immobilized in the polymer matrix and
not free to distort their structures. With the excited state geomet-
ric relaxation effectively repressed, it is not surprised to see the
emission blue shift from PVP doped fibers. Since the excited state
can be stabilized by PVP host, the emission spectra are conse-
quently widened. It seems that the immobilization in polymer ma-
trix is even more effective on repressing the geometric relaxation
Fig. 4. The absorption spectra of Re(CO)3(PPOZ)Br solution in CH2Cl2 with
concentration of 1 lM, the four composite fibers and pure PVP, along with the
solid state diffuse reflection spectrum of bulk Re(CO)3(PPOZ)Br.
extending to ꢂ450 nm. The first band at 240 nm is quite similar to
the absorption peak of pure PVP, while the latter two ones can be
recognized as the absorption peaks of Re(CO)3(PPOZ)Br. After being
doped into PVP host, Re(CO)3(PPOZ)Br shows nearly identical
absorption feature with that of dilute solution without any new-
ly-generated peaks or shoulder bands. It is thus concluded that
there is no strong interaction between dopant molecules and PVP
host. Re(CO)3(PPOZ)Br molecules are merely trapped and immobi-
lized in PVP matrix.
As for bulk Re(CO)3(PPOZ)Br, its solid state diffuse reflection
spectrum owns low reflection over a wide region ranging from
230 nm to 400 nm, with a typical valley at 290 nm, which is in con-
sistent with the strongest absorption of Re(CO)3(PPOZ)Br dilute
solution. Similarly, no newly-generated bands are observed. It is
thus concluded that the electronic transitions of Re(CO)3(PPOZ)Br
are independent of its existing status: diluted in solution, con-
densed in solid state or immobilized in PVP host.
than
p–p stacking does, where the small steric hindrance of CO
group and Br atom should be blamed to be responsible.
Excited state decay dynamics
Aiming at a deeper understanding on the immobilization influ-
ence on the excited state, its lifetimes upon various existing sta-
tuses are measured and discussed as follows. The excitation
spectrum of Re(CO)3(PPOZ)Br is firstly recorded to ensure effective
excitation. As shown in Fig. 6, the excited state of Re(CO)3(PPOZ)Br
follows single exponential decay mode regardless of its existing
Emissive spectra
The emissive spectra for the four composite samples, Re(CO)3
(PPOZ)Br solution in CH2Cl2 with concentration of 1 lM and bulk
Re(CO)3(PPOZ)Br are shown in Fig. 5. In solution, Re(CO)3(PPOZ)Br
exhibits yellow emission centering at 563 nm with FWHM value as
status, with decay times of 1.80
l
s for Re(CO)3(PPOZ)Br solution,
s for the 6 wt% sample,
3.13 s for bulk Re(CO)3(PPOZ)Br, 4.47
l
l
Fig. 6. The emission decay characters of the four composite fibers, Re(CO)3(PPOZ)Br
solution in CH2Cl2 with concentration of 1 M and bulk Re(CO)3(PPOZ)Br. Inset: the
excitation spectrum of Re(CO)3(PPOZ)Br solution.
Fig. 5. The emissive spectra for the four composite samples, Re(CO)3(PPOZ)Br
solution in CH2Cl2 with concentration of 1 lM and bulk Re(CO)3(PPOZ)Br.
l

W. Dejin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 605–612
611
4.79
4.42
l
s for the 10 wt% sample, 4.88
l
s for the 14 wt% sample and
photostability of the composite fibers is improved compared with
that of bulk Re(CO)3(PPOZ)Br, which can be explained as follows.
When being doped into polymer host, Re(CO)3(PPOZ)Br molecules
are protected by the surrounding PVP framework which may act as
an energy-absorbing antenna. The excitation energy can be firstly
absorbed by the energy-absorbing antenna without directly excit-
ing Re(CO)3(PPOZ)Br molecules. Thus, the photo-oxidization and
photochemical breakdown can be avoided, leading to the improved
photostability. In addition, it can be seen that the photostability of
the composite fibers decreases with the increasing doping concen-
tration. It is expected that the increasing aggregation between
Re(CO)3(PPOZ)Br molecules may compromise the photostability
of the composite fibers. On the positive side, the photostability of
the composite fibers is found to be largely improved compared
with that of bulk sample, suggesting that PVP can be developed
as a promising host for composite fibers by repressing the MLCT
excited state geometric relaxation and improving the
photostability.
ls for the 16 wt% sample, respectively. The single exponential
decay mode suggests that all excited states localize in homogenous
environment. As for Re(CO)3(PPOZ)Br solution, the molecules in di-
lute solution are surrounded by solvent molecules. They are thus
free to distort their structure and to lose excited state energy,
resulting in the shortest excited state lifetime of 1.80
densed state, the excited state geometric relaxation of Re(CO)3
(PPOZ)Br may be partially repressed by the stacking above
mentioned, leading to a longer excited state lifetime of 3.13 s.
ls. In con-
p–p
l
When Re(CO)3(PPOZ)Br is doped into polymer host, the immobili-
zation in PVP matrix effectively represses the excited state geomet-
ric relaxation, causing the even longer excited state lifetimes of the
four composite fibers. Above comparison provides an evidence for
the fact that the excited state geometric relaxation can be effec-
tively repressed by doping Re(CO)3(PPOZ)Br into PVP matrix.
Although the excited state lifetimes of the four composite fibers
are similar to each other, it seems that the excited state lifetime
firstly increases with the increasing doping concentration, showing
a maximum value when the doping concentration is as high as
14 wt%. Upon an even higher doping concentration of 16 wt%, the
excited state lifetime tends to decrease. Apparently, 14 wt% is the
optimal doping concentration. Bothe higher or lower doping con-
centrations can decrease the excited state lifetime, which may be
explained as follows. The emissive intensity of the composite fibers
clearly depends on the doping amount of Re(CO)3(PPOZ)Br mole-
cules, which means that the higher the doping concentration is,
the stronger the emissive intensity becomes. However, when the
doping concentration is more than enough, the aggregation also
becomes serious, correspondingly, the strong self-quenching and
self-absorption may quench the excited state, leading to the de-
creased excited state lifetime.
Conclusion
To sum up, we synthesized and characterized a Re(I) complex
Re(CO)3(PPOZ)Br whose diamine ligand was composed of an elec-
tron-withdrawing oxadiazole group. Its geometric and electronic
structures were confirmed and revealed by single crystal structure
analysis and DFT calculation. Re(CO)3(PPOZ)Br took a traditional
coordination pattern similar to literature cases. The excited state
owned a mixed character of MLCT and LLCT which suffered from
geometric relaxation. In order to repress the geometric relaxation
and thus improve the emissive performance, Re(CO)3(PPOZ)Br
was doped into PVP host via electrospinning to construct compos-
ite fibers. The photophysical features of the final composite fibers
were compared with those of bulk sample. Data suggested that
the composite fibers owned blue-shifted emission, longer excited
state lifetime and better photostability, owing to the effective
repression of the geometric relaxation. In other words, the immo-
bilization in PVP matrix was effective on repressing the MLCT ex-
cited state geometric relaxation, showing improved emissive
performance. Considering that MLCT geometric relaxation is the
major energy loss of excited state, it is expected that the polymer
immobilization effect might be an universal phenomenon. This
conclusion may be used for future design of one dimensional nano-
structures for optoelectronic applications. For further efforts, the
combination of other transition metal complexes and polymer host
should be carried out to testify the universality of this effect.
Photostability
To get a further understanding on the photostability difference
between bulk sample and composite fibers, the emission intensity
variations of the four composite fibers and the bulk sample upon
continuous radiation are given in Fig. 7. The emission of bulk
Re(CO)3(PPOZ)Br decreases greatly with increasing radiation time,
suggesting that the photostability of bulk Re(CO)3(PPOZ)Br is poor
since bulk material directly accept the excitation energy. We attri-
bute the emission intensity decrease to the photo-oxidization and
photochemical breakdown of Re(CO)3(PPOZ)Br molecular struc-
ture. As for the four composite fibers, their emission intensity re-
mains well with increasing radiation time, suggesting that the
Acknowledgments
The author gratefully thank the financial support from the Uni-
versity Provincial Natural Science Research Key Project of Anhui
Province, China (Grant No. KJ2011Z231).
References
[1] G.M. Kim, A. Wutzler, H.J. Radusch, G.H. Michler, P. Simon, R.A. Sperling, W.J.
Parak, Chem. Mater. 17 (2005) 4949–4957.
[2] L. Feng, S. Li, H. Li, J. Zhai, Y. Song, L. Jiang, D. Zhu, Angew. Chem. Int. Ed. 41
(2002) 1221–1223.
[3] J. Hartgerink, E. Beniash, S. Stupp, Science 294 (2001) 1684–1688.
[4] D. Reneker, I. Chun, Nanotechnology 7 (1996) 216–223.
[5] J.A. Matthews, G.E. Wnek, D.G. Simpson, G.L. Bowlin, Biomacromolecules 3
(2002) 232–238.
[6] A.C. Patel, S.X. Li, C. Wang, W.J. Zhang, Y. Wei, Chem. Mater. 19 (2007) 1231–
1238.
[7] K.E. Erkkila, D.T. Odom, J.K. Barton, Chem. Rev. 99 (1999) 2777–2796.
[8] C.A. Bignozzi, R. Argazzi, C.J. Kleverlaan, Chem. Soc. Rev. 29 (2000) 87–96.
[9] V.W.W. Yam, B. Li, Y. Yang, B.W.K. Chu, K.M.-C. Wong, K.K. Cheung, Eur. J. Inorg.
Chem. 22 (2003) 4035–4042.
Fig. 7. The emission intensity variations of the four composite fibers and the bulk
Re(CO)3(PPOZ)Br upon continuous radiation.

612
W. Dejin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 605–612
[10] S. Ranjan, S.Y. Lin, K.C. Hwang, Y. Chi, W.L. Ching, C.S. Liu, Y.T. Tao, C.H. Chien,
S.M. Peng, G.H. Lee, Inorg. Chem. 42 (2003) 1248–1255.
[11] F. Li, M. Zhang, J. Feng, G. Cheng, Z.J. Wu, Y.G. Ma, S.Y. Liu, Appl. Phys. Lett. 83
(2003) 365–367.
[12] L. Zhang, B. Li, Z. Su, Langmuir 25 (2009) 2068–2074.
[13] L. Zhang, B. Li, J. Lumin. 129 (2009) 1304–1308.
[14] L. Yang, J.K. Feng, A.M. Ren, M. Zhang, Y.G. Ma, X.D. Liu, Eur. J. Inorg. Chem. 10
(2005) 1867–1879.
[15] L. Zhang, S. Yue, B. Li, D. Fan, Inorg. Chim. Acta 384 (2012) 225–232.
[16] L. Shi, B. Li, S. Yue, D. Fan, Sensor Actuat. B Chem. 137 (2009) 386–392.
[17] Z. Si, X. Li, X. Li, H. Zhang, J. Organomet. Chem. 694 (2009) 3742–3748.
[18] H.Q. Yu, H.W. Song, G.H. Pan, S.W. Li, Z.X. Liu, X. Bai, T. Wang, S.Z. Lu, H.F. Zhao,
J. Lumin. 124 (2007) 39–44.
[19] Y. Wang, B. Li, Y. Liu, L. Zhang, Q. Zuo, L. Shi, Z. Su, Chem. Commun. 39 (2009)
5868–5870.