Photoluminescence properties of Re(I) complex doped composite submicron fibers prepared by electrospinning

Article abstract of DOI:10.1016/j.saa.2010.11.011

In this paper, a Re(I) complex of Re(CO)₃(Bphen)Br, where Bphen = 4,7-diphenyl-1,10-phenanthroline is synthesized, and doped into poly(vinylpyrrolidone) submicron fibers through electrospinning technique. Their morphology, absorption, and emission spectra are investigated in detail. The composite fibers exhibit smooth and uniform morphology on the substrate, with an average diameter of ～800 nm. A bright yellow emission peaking at 543 nm is observed from these composite fibers, and this emission is attributed to the triplet emissive state of Re(CO)₃(Bphen)Br. When doped into poly(vinylpyrrolidone) matrix, the emission shows a blue-shift tendency compared with that of bulk sample, correspondingly, the photostability is also largely improved. Detailed analysis suggests that Re(CO)₃(Bphen)Br occupies a homogeneous site within poly(vinylpyrrolidone) matrix, and the matrix provides a rigid environment for Re(CO)₃(Bphen)Br.

Full text of DOI:10.1016/j.saa.2010.11.011

Spectrochimica Acta Part A 78 (2011) 469–473
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Photoluminescence properties of Re(I) complex doped composite submicron
fibers prepared by electrospinning
Wang Yia, Mao Hong-Jub, Zang Guo-Qinga,∗, Yu Yong-Shenga, Tang Zheng-Haoa
a Department of Infection, the Sixth People’s Hospital Affiliated to Shanghai Jiaotong University, Shanghai 200233, China
b Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 June 2010
Received in revised form 24 October 2010
Accepted 15 November 2010
In this paper, a Re(I) complex of Re(CO)3(Bphen)Br, where Bphen = 4,7-diphenyl-1,10-phenanthroline
is synthesized, and doped into poly(vinylpyrrolidone) submicron fibers through electrospinning tech-
nique. Their morphology, absorption, and emission spectra are investigated in detail. The composite
fibers exhibit smooth and uniform morphology on the substrate, with an average diameter of ∼800 nm.
A bright yellow emission peaking at 543 nm is observed from these composite fibers, and this emission
is attributed to the triplet emissive state of Re(CO)3(Bphen)Br. When doped into poly(vinylpyrrolidone)
matrix, the emission shows a blue-shift tendency compared with that of bulk sample, correspondingly,
the photostability is also largely improved. Detailed analysis suggests that Re(CO)3(Bphen)Br occupies
a homogeneous site within poly(vinylpyrrolidone) matrix, and the matrix provides a rigid environment
for Re(CO)3(Bphen)Br.
Keywords:
Re(I) complex
Luminescence
Electrospinning
Submicron fiber
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
nal parameters; (2) process parameters; (3) ambient parameters
[8].
In recent years, nanostructures have drawn much attention
since they may be utilized in the future to integrate nanostructures
into nanodevices needing unique optoelectronic, electrochemi-
cal, and electromechanical properties [1]. Practicable approaches,
such as drawing, template synthesis, phase separation, self-
assembling, and electrospinning, have been proposed to construct
such structures [2,3]. Among these approaches, electrospinning,
which applies electric force to drive the spinning process and
to produce fibers from solutions or melts [4], is a promising
technique for fabricating long and uniform fibers, owing to its
advantages of versatile, simple, and straightforward construction
process, as well as the low cost. Electrospun nano- and submicron-
fibers own unique properties of average fiber diameters in the
submicrometer range, high porosities, large surface areas, fully
interconnected pore structures, and sufficient mechanical strength,
making themselves attractive for a wide range of potential applica-
tions, such as in filters for optical sensors, biomedical applications,
solar cells, catalyst carriers, porous electrode, light-emitting diodes,
liquid crystal device [5–7]. In addition, the shape, diameter, sur-
face morphology, and porosity of resulted fibers can be easily
adjusted by changing the following three major factors: (1) inter-
At the same time, the development of practical components
for chemical sensors, display devices, probes of biological sys-
tems and solar-energy conversion schemes has sparked the
interest in complexes of diimine ligands with transition met-
als [9,10]. Re(I) complexes are widely studied in recent years
due to their virtues of high photoluminescence quantum yields,
and excellent thermal/photochemical stability [11–13]. However,
to the best of our knowledge, there is no related reports on
micron fibers doped with Re(I) complexes. In this paper, we pre-
pare Re(CO)3(Bphen)Br/PVP composite fibers by electrospinning
method, where PVP = poly(vinylpyrrolidone). Their photophysical
properties, as well as those of bulk Re(CO)3(Bphen)Br/PVP pow-
ders andRe(CO)3(Bphen)Br/PVPfilms, are investigated indetail. It is
found that the photostability of Re(CO)3(Bphen)Br/PVP composite
fibers is largely improved.
2. Experimental details
2.1. Starting materials
The preparation procedure for Re(CO)3(Bphen)Br/PVP compos-
ite fibers is shown in Scheme 1. Re(CO)3(Bphen)Br was synthesized
according to the literature procedure [14]. PVP (Mw = 60,000) was
purchased from Beijing Yi Li Chemical Factory and used without
purification. All other reagents are AR grade.
∗
Corresponding author. Tel.: +86 021 64369181x8673; fax: +86 021 64701361.
E-mail address: zanggq01@163.com (G.-Q. Zang).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.11.011

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Y. Wang et al. / Spectrochimica Acta Part A 78 (2011) 469–473
Scheme 1. Preparation procedure for Re(CO)3(Bphen)Br/PVP composite fibers.
2.2. Preparation of composite fibers
3. Results and discussion
A typical preparation process for Re(CO)3(Bphen)Br/PVP com-
posite fibers is described as follows. PVP was dissolved in the mixed
solvent of ethanol/dichloromethane (V:V = 1:1) to prepare a 25 wt%
solution. A controlled amount of Re(CO)3(Bphen)Br (0.67 wt%) was
added into the PVP solution under stirring for 24 h at room tem-
perature. For electrospinning, the 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 [15]. The anode ter-
minal 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 dis-
tance of 25 cm. The Re(CO)3(Bphen)Br containing composite fibers
were prepared in the air.
3.1. Morphology of the composite fibers
Fig. 1 show the SEM image of Re(CO)3(Bphen)Br/PVP compos-
ite fibers, that of pure PVP fibers is also given for comparison. It
2.3. Methods and equipments
Density functional theory (DFT) calculation was performed on
Re(CO)3(Bphen)Br at RB3LYP/SBKJC level. The initial geometry was
firstly optimized by a semiempirical method of PM6. All computa-
tions were finished by GAMESS and MOPAC 2009.
UV–vis and photoluminescence (PL) spectra were recorded on
a Shimadzu UV-3150 spectrophotometer and a Hitachi F-4500
Fluorimeter, respectively. Solid diffuse reflection spectrum was
recorded by the Hitachi F-4500 Fluorimeter using diffuse reflection
scan mode. All PL spectra were smoothened for clarity and elegancy.
The morphologies of the fibers were observed with a Hitachi S-
4800 scanning electron microscopy (SEM). Excited state lifetimes
of Re(CO)3(Bphen)Br in fibers and bulk Re(CO)3(Bphen)Br/PVP
powder were obtained with a 355 nm light generated from the
third-harmonic-generator pump, using the pulsed Nd:YAG laser
as the excitation source. Samples were placed in a quartz cham-
ber under N2 atmosphere, then pulsed 355 nm laser was used as
excitation source for PL excitation. The PL signals were recorded
and then processed by Origin 7.0 using single exponential fit mode
[16]. All measurements were performed at room temperature in
the air without being specified.
Fig. 1. SEM images of Re(CO)3(Bphen)Br/PVP composite fibers (up) and pure PVP
fibers (down).

Y. Wang et al. / Spectrochimica Acta Part A 78 (2011) 469–473
471
Fig. 2. PL spectra of the four samples.
Fig. 3. UV–vis absorption spectrum of Re(CO)3(Bphen)Br in CH2Cl2 solution with a
concentration of 1 × 10−4 mol/L. Inset: solid diffuse reflection spectrum of Sample
is observed that both samples are randomly oriented on the sub-
strates. As for pure PVP fibers, the average diameter is measured
to be ∼1000 nm, and no branch structure is observed. On the other
hand, the Re(CO)3(Bphen)Br/PVP composite fibers exhibit smooth
and uniform morphology compared with pure PVP fibers, with a
smaller diameter of ∼800 nm as shown in Fig. 1. It seems that
the addition of Re(CO)3(Bphen)Br benefits the resulting composite
fibers by smoothing the fiber surface, with the diameter decreases
from 1000 nm to 800 nm.
A.
fibers is thus confirmed: both its chemical and electronic structures
remain.
The emissions from Sample A and Sample C all follow single
exponential decay pattern shown by Eq. (1)
ꢀ
ꢁ
−t
A(t) = A0 exp
(1)
ꢀ
where A(t) stands for the emission intensity (time dependent), ꢀ
presents the decay constant, and A0 is the initial intensity. The
excited state lifetimes of ꢀ are calculated to be 4.38 ␮s for Sam-
ple A, and 4.08 ␮s for Sample C, respectively, suggesting the triplet
character of emissive states.
3.2. Photophysical properties of the composite fibers
Fig. 2 shows the PL spectra of Re(CO)3(Bphen)Br/PVP fibers
(Sample A), Re(CO)3(Bphen)Br/PVP film (Sample B), bulk
Re(CO)3(Bphen)Br/PVP (Sample C), and Re(CO)3(Bphen)Br solution
in CH2Cl2 (Sample D). All samples exhibit yellow emissions origi-
nating from Re(CO)3(Bphen)Br. Their emission peaks are measured
to be 542 nm for Sample A, 548 nm for Sample B, 548 nm for Sam-
ple C, and 580 nm for Sample D, respectively. Clearly, the excited
state energy depends on the surrounding environment largely. In
addition, it is found that the emission measured in CH2Cl2 solution
suffers a larger Stokes shift compared with that measured in solid
state, which can be explained by rigidochromism as follows [17].
In solution, all Re(CO)3(Bphen)Br molecules are surrounded by
solvent molecules, and thus free to rotate. Upon the excitation of
Re(CO)3(Bphen)Br molecules, the surrounding solvent molecules
can reorient instantly to solvate the excited state, leading to the
thermal relaxation, and consequently the energy content decrease
of Franck–Condon excited state. Since the thermally relaxed
excited state is stabilized by surrounding solvent molecules, the
emitting energy of excited state is also decreased. On the other
hand, in solid state, with the evaporation of solvent molecules,
all emitter molecules are immobilized within the rigid matrix,
the Franck–Condon excited state can thus not be fully stabilized
within its excited state lifetime, resulting in an emission from
a higher energy level and consequently the emission blue-shift.
Particularly, as for Sample A, all Re(CO)3(Bphen)Br molecules
are trapped into the framework of submicron fibers which offers
a rigid matrix for Re(CO)3(Bphen)Br molecules, leading to the
highest emitting energy of Sample A compared with those of the
other three samples.
Generally, a long excited state lifetime of 4.38 ␮s should lead to a
narrow emission band, but the PL spectrum of Sample C is as broad
as 100 nm. As for the inconsistency between the long excited state
lifetime and the broad PL spectrum, we are giving an explanation as
follows. The excited state lifetime was measured under N2 atmo-
sphere, while, PL spectra measured by F-4500 were all recorded in
the air. It is well known that molecular oxygen is an efficient killer
for excited state molecules, leading to largely decreased excited
state lifetimes [18]. It is thus reasonable to see the broad emission
band of 100 nm.
The single exponential decay pattern suggests that
Re(CO)3(Bphen)Br molecules occupy only one homogeneous
site and distribute uniformly. The increased excited state life-
time of Sample
A suggests that PVP host provides a rigid
matrix for Re(CO)3(Bphen)Br molecules, and the excited state
Re(CO)3(Bphen)Br molecules are immune from intermolecular
self-quenching or other quenching processes, leading to the
increased excited state lifetime of Sample A.
Fig. 4 shows the normalized luminescence decay curves of Sam-
ple A and Sample C when exposed to UV radiation of 350 nm. It is
found that the emission intensities of both samples decrease with
the increase of irradiation time. Decay curves reveal that Sample
A owns an improved photostability over Sample C due to the rigid
framework provided by PVP matrix. It is thus confirmed that the
decay process can be sharply slowed within Sample A, leading to a
better photostability.
Fig.
3
shows the UV–vis absorption spectrum of
Re(CO)3(Bphen)Br in CH2Cl2 solution with a concentration of
1 × 10−4 mol/L. It is observed that the absorption spectrum is
composed of a high-energy band ranging from 250 to 350 nm and
a low-energy one ranging from 350 to 500 nm. The solid diffuse
reflection spectrum of Sample A shown by the inset of Fig. 3
exhibits two absorption bands centering at 290 and 380 nm, which
are similar to those of Re(CO)3(Bphen)Br in CH2Cl2 solution as
mentioned above. The existence of Re(CO)3(Bphen)Br in composite
3.3. Theoretical calculations on Re(CO)3(Bphen)Br
In order to get a further understanding on geometric and
electronic natures of Re(CO)3(Bphen)Br, we perform a geometry
simulation using DFT calculation, and the optimized molecular
structure is shown in Fig. 5. The selected geometric parameters
listed in Table 1 suggest that Re(I) center occupies a distorted
˚
octahedral field. The Re–N bond lengths (2.08 A) are slightly

472
Y. Wang et al. / Spectrochimica Acta Part A 78 (2011) 469–473
Fig. 4. Normalized luminescence decay curves of Sample A and Sample C upon UV
radiation of 350 nm.
Fig. 6. Graphic presentations of Re(CO)3(Bphen)Br LUMO (up) and HOMO (down)
calculated at RB3LYP/SBKJC level.
Fig. 5. Optimized molecular structure of Re(CO)3(Bphen)Br in ground state calcu-
lated at RB3LYP/SBKJC level.
ous contribution from 4,7-diphenyl fragment to both HOMO and
LUMO, which is caused by the inefficient conjugation between 1,10-
phenanthroline and the two phenyl rings. The two phenyl rings may
thus serve as inert shield for excited state Re(CO)3(Bphen)Br, which
is positive towards PL performance improvement.
˚
shorter than literature ones (2.1–2.2 A), however, the calculated
N–Re–N bite angle in Re(CO)3(Bphen)Br (81.1◦) is larger than the
corresponding ones (<76◦) [16,19], indicating that the coordina-
tion sphere around the Re(I) center is less crowded. And this
decreased steric hindrance may be positive to suppress the geo-
metric relaxation in Re(CO)3(Bphen)Br excited state. The dihedral
angle between phenyl and phenanthroline rings is calculated to be
101.6◦, which suggests an inefficient conjugation between the two
rings [20].
4. Conclusions
In this paper, composite fibers containing Re(CO)3(Bphen)Br
with a diameter in the range of 800 nm are prepared by a sim-
ple electrospinning method. Their photophysical properties are
investigated and discussed in detail. The obtained composite fibers
are yellow-emitting ones with uniform morphology. The sin-
gle exponential decay pattern of the composite fibers suggests
that Re(CO)3(Bphen)Br molecules occupies only one homogeneous
site in the composite fibers. In addition, the photostability of
Re(CO)3(Bphen)Br containing composite fibers is largely improved
compared with that of bulk powers. In a word, electrospinning
is a novel and available technique to prepare Re(CO)3(Bphen)Br
containing composite fibers, which can improve luminescence per-
formances of complexes and may find potential applications in
nanodevices.
As shown in Fig. 6, the highest occupied molecular orbital
(HOMO) of Re(CO)3(Bphen)Br demonstrates a predominant Re d
character, admixed with large contribution from Br atom and
small contribution from CO fragment. While, the lowest unoccu-
pied orbital (LUMO) of Re(CO)3(Bphen)Br is essentially ␲* orbital
localized on the diamine ligand of 1,10-phenanthroline, admixed
with some contribution from Br atom. There is, however, no obvi-
Table 1
Selected geometric parameters of Re(CO)3(Bphen)Br in ground state calculated at
RB3LYP/SBKJC level.
◦
˚
A
Bond length
Bond angle
Re–Br(2)
Re–C(3)
Re–C(5)
Re–C(7)
Re–N(9)
Re–N(10)
2.533
1.906
1.943
1.941
2.083
2.085
N(9)–Re–N(10)
N(9)–Re–Br(2)
N(9)–Re–C(7)
N(10)–Re–Br(2)
N(10)–Re–C(5)
N(10)–Re–C(3)
C(3)–Re–C(5)
81.1
68.1
87.0
66.4
87.1
Acknowledgements
The authors gratefully thank the financial and instrumental sup-
ports from Shanghai Jiaotong University and Shanghai Institute of
Microsystem and Information Technology.
111.7
86.7

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473
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