Journal of Luminescence 132 (2012) 403–408

Contents lists available at SciVerse ScienceDirect

Journal of Luminescence

0022-23

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jlumin

Characterization and emissive property of microfibers doped with fluorinefunctionalized iridium complex

Wen-Li Chang a,n, Peng-Yu Yang b

a School of Mathematics, Physics and Software Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070, The People’s Republic of Chinab College of Physics, Nankai University, Tianjin 300071, The People’s Republic of China

a r t i c l e i n f o

Article history:

Received 13 March 2011

Received in revised form

5 September 2011

Accepted 12 September 2011Available online 16 September 2011

Keywords:

Iridium complex

Photoluminescence

Microfiber

Crystal

13/$ - see front matter & 2011 Elsevier B.V. A

016/j.jlumin.2011.09.015

esponding author. Tel.: þ86 931 4956005.

ail address: changwl05008@163.com (W.-L. C

a b s t r a c t

In this paper, we used two fluorine-containing ligands of 2-(2-fluorophenyl)benzo[d]thiazole (F-BT)

and 1,1,1,5,5,5-hexafluoropentane-2,4-dione (acac-F6) to construct an Ir complex of Ir(acac-F6)(F-BT)2.

The single crystal of Ir(acac-F6)(F-BT)2 is observed, which confirms its identity. Ir(acac-F6)(F-BT)2 single

crystal belongs to monoclinic system with two molecules in each unit cell. Theoretical calculation is

performed on Ir(acac-F6)(F-BT)2 to investigate its electronic nature, which suggests that the first

electronic transition owns a mixed character of metal-to-ligand-charge-transfer and ligand-to-ligand-

charge-transfer. We select a polymer matrix of poly(vinylpyrrolidone) (PVP) to explore the photo-

physical property of Ir(acac-F6)(F-BT)2 within microfibers. Results indicate that the photophysical

performances of Ir(acac-F6)(F-BT)2 within microfibers are improved compared with those of Ir(acac-

F6)(F-BT)2 powder, including microfiber sample’s higher emission energy, longer excited state lifetime,

and better photostability.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Electrospinning has been proved to be an excellent method toconstruct nano- and micro-fibers, due to the advantages offlexible construction procedure and low cost [1]. Nano- andmicro-fibers have presented some unique features such as con-trollable fiber diameter, high porosity, large surface-to-volumeratio, and strong mechanical strength, making themselves suita-ble for various applications in optoelectronic and electromecha-nical fields, such as biomedical probes, solar cells, catalystcarriers, porous electrodes, light-emitting diodes, liquid crystaldevices, and supporting matrix for optical sensors [2–7]. Besidesabove mentioned advantages, the morphology and porosity ofresulting nano- and micro-fibers can be easily modified bychanging the following three conditions of internal parameter,process parameter, and ambient parameter, which makes theresulted materials more convenient to be physically adjusted [8].

In order to obtain optical properties from nano- and micro-fibers and thereby explore their applications in optoelectronicfield, organic dyes, and luminescent metal complexes havebeen doped into the fibers to produce composite materials[9,10]. Transition metal complexes, owing to the virtues of highphotoluminescence (PL) yield, good thermal stability, and high

ll rights reserved.

hang).

photodurability, have been widely investigated [11–13]. Particu-larly, phosphorescent Ir(III) complexes have demonstrated theirapplication in various optoelectronic fields. In exploring phos-phorescent emitters with high PL efficiency and moving towardsmaterials with the required color gamut for full color displays,many efforts have been devoted to develop tricolor emittingmaterials, and a good harvest of Ir(III) complexes covering thewhole visible region has been achieved. The correlation betweenmolecular structure and photophysical parameters, includingsubstituent effect, electron drawing and pushing effect, andstructural effect has also been investigated [8–10,12–14]. Relatedtheoretical study has also been performed on typical Ir(III)complexes. It is concluded that the highest occupied molecularorbital (HOMO) has a metal Ir d character, while, the lowestunoccupied molecular orbital (LUMO) is essentially pn orbitallocalized on organic ligand [14]. The room temperature emissioncomes from the lowest triplet excited state and is thus assigned asa character of metal-to-ligand-charge-transfer 3MLCT [d(Ir)-pn(ligand)]. The emissive MLCT excited state of Ir(III) complexescan be chemically modified through molecular design and usuallyplays an important role in photoinduced electron and energytransfer process. However, to the best of our knowledge, there areno detailed reports on composite fibers doped with phosphor-escent Ir(III) complexes, neither their photophysical performancecomparison.

The introduction of fluorine atom into CN and auxiliary ligandshas been reported to be positive to improve both emissive and

Scheme 1. A synthetic procedure for Ir(acac-F6)(F-BT)2 and the corresponding Ir(acac-F6)(F-BT)2/PVP composite fibers.

W.-L. Chang, P.-Y. Yang / Journal of Luminescence 132 (2012) 403–408404

thermal characters of resulting Ir(III) complexes. In this paper, weintend to synthesize a fluorine-containing ligand and its corre-sponding Ir(III) complex. By doping this fluorine-containing com-plex into a polymer matrix, the photophysical characteristics ofresulting composite fibers doped with phosphorescent Ir(III)complex can be investigated and compared with those of pureIr(III) complex.

2. Experimental details

The fluorine-containning Ir(III) complex of Ir(acac-F6)(F-BT)2

and the corresponding Ir(acac-F6)(F-BT)2/PVP nanofibers aresynthesized according to the synthetic route shown inScheme 1. Chemical reagent such as 2-aminathiophenol, 2-fluor-obenzaldehyde, 1,1,1,5,5,5-hexafluoropentane-2, 4-dione (acac-F6), p-toluenesulfonic acid monohydrate, IrCl3 �3H2O, andpoly(vinylpyrrolidone) (PVP, K30) were purchased from AldrichChemical Co. and used as received with no further purification.Organic solvents, including 2-ethoxyethanol, 1,2-dichloroethane,ethanol, CH2Cl2, and CHCl3 were all purified through standardprocedures before use.

2.1. Ligand and complex synthesis

2-(2-Fluorophenyl)benzo[d]thiazole (F-BT). F-BT was synthe-sized according to the synthetic procedure described as follows.The mixture of 16 mmol of 2-aminathiophenol, 16 mmol of2-fluorobenzaldehyde, 1 mmol of p-toluenesulfonic acid mono-hydrate, and 20 mL of CHCl3 was heated to reflux under N2

atmosphere for 24 h. After cooling, the mixture was poured intocold water and extracted using CH2Cl2. After the evaporation ofsolvent, the residue was purified by recrystallization fromhot ethanol to give the desired product as yellow solid. 1HNMR (CDCl3, 300 MHz) d [ppm]: 7.34 (d, J¼7.5 Hz, 1 H), 7.31(d, J¼7.0 Hz, 1 H), 7.57 (d, J¼9.0 Hz, 1 H), 7.53 (m, 1 H), 7.95(d, J¼9.0 Hz, 1 H), 8.14 (m, 1 H), 8.27 (m, 2 H).

Ir(acac-F6)(F-BT)2: The Ir(III) complex of Ir(acac-F6)(F-BT)2 wassynthesized according to the synthetic procedure described asfollows. The mixture of 0.68 mmol of IrCl3 �3H2O, 1.36 mmol ofF-BT, 20 mL of 2-ethoxyethanol, and 5 mL of H2O was heated to

reflux for 48 h. After cooling, 5 mL of H2O was added into themixture to precipitate orange solid, which was then washed withethanol and hexane. The dried chloro-bridged dimmer was mixedwith 2 mmol of anhydrous sodium carbonate, 25 mL of 2-ethox-yethanol, and 2.05 mmol of acac-F6. The mixture was heated toreflux for 16 h. After cooling, 5 mL of H2O was added to pre-cipitate orange red solid, which was colleted by filtration. Thecrude product was chromatographed to give the desired productas orange needles. 1H NMR (CDCl3, 300 MHz) d [ppm]: 1.92(s, 3 H), 5.55 (s, 1H), 6.17 (d, J¼7.5 Hz, 2 H), 6.63 (d, J¼10 Hz,2 H), 6.76 (d, J¼7.0 Hz, 2H), 7.51 (m, 4H), 7.93 (m, 2H), 8.26(m, 2H). Analytically calculated for C31H18F5IrN2O2S2: C 46.44, H2.26, N 3.49. Found: C 46.54, H 2.45, N 3.31. Its identity wasfurther confirmed by single crystal XRD (CCDC entry no. 531783).

2.2. Construction of Ir(acac-F6)(F-BT)2/PVP composite fibers

We are giving a typical procedure for the electrospinningcomposite nanofibers as follows. For a typical run, 1 g of PVPwas dissolved in the mixed solvent of 1,2-dichloroethane andethanol (V:V¼1:1) to form a homogeneous solution. Then con-trolled amount of Ir(acac-F6)(F-BT)2 was added into the solutionunder stirring to form Ir(acac-F6)(F-BT)2/PVP homogeneous solu-tions. The final solutions were then electrospun to be compositenanofibers of Ir(acac-F6)(F-BT)2/PVP. For electrospinning, themixed solution was placed in a 5 mL glass syringe, with theopening end connected to a plastic needle (inner diame-ter¼0.6 mm) as the nozzle [15]. The anode terminal of a high-voltage generator was connected to a copper wire inserted intothe polymer solution in the glass syringe. A piece of Al foil wasused as the collector plate and connected to the groundingelectrode. The driving voltage was 18 kV, with the tip-to-targetdistance of 25 cm.

2.3. Measurements and methods

Single crystal data were collected on a Siemens P4 single-crystal X-ray diffractometer with a Smart CCD-1000 detector andgraphite-monochromated Mo Ka radiation, operating at 50 kVand 30 A at 298 K. All hydrogen atoms were calculated. Lumines-cence decay data were obtained with a 355 nm light generated

Table 1Selected geometric parameters of Ir(acac-F6)(F-BT)2 obtained from single

crystal XRD.

Bond length (A) Bond angle (1)

Ir–C(9) 1.982 C(9)–Ir–N(1) 81.05

Ir–N(1) 2.050 N(2)–Ir–C(22) 81.59

Ir–N(2) 2.079 O(1)–Ir–O(2) 86.72

Ir–C(22) 1.979 C(22)–Ir–C(9) 90.79

Ir–O(1) 2.143 C(22)–Ir–N(1) 95.05

Ir–O(2) 2.162 C(22)–Ir–O(2) 90.84

O(1)–Ir–N(1) 86.64

O(1)–Ir–C(9) 91.54

O(1)–Ir–N(2) 96.99

O(2)–Ir–N(1) 99.01

O(2)–Ir–N(2) 87.38

C(9)–Ir–N(2) 92.66

C(9)–Ir–O(1) 91.64

W.-L. Chang, P.-Y. Yang / Journal of Luminescence 132 (2012) 403–408 405

from pulsed Nd:YAG laser as the excitation source. The Nd:YAGlaser possesses a line width of 1.0 cm�1, pulse duration of 10 ns,and repetition frequency of 10 Hz. All photoluminescence (PL)spectra were measured with a Hitachi F-4500 fluorescencespectrophotometer. UV–vis absorption spectra that were recordedusing a Shimadzu UV-3101PC spectrophotometer. Scanning electronmicroscopy (SEM) images were obtained on a Hitachi S-4800microscope. Fluorescence microscopy image was obtained with aNikon TE2000-U fluorescence microscopy using the mercury lampas the power supply. 1H NMR spectra were obtained with a VarianINOVA 300 spectrometer. Elemental analysis was performed on aCarlo Erba 1106 elemental analyzer. All measurements were carriedout in the air at room temperature without being specified. Densityfunctional theory (DFT) and singlet excitation calculations usingtime-dependent density functional theory (TD-DFT) were performedon Ir(acac-F6)(F-BT)2 at RB3LYP/SBKJC level. The initial geometrywas obtained from its single crystal. All computations were finishedby GAMESS software package.

3. Results and discussion

3.1. Single crystal structure of Ir(acac-F6)(F-BT)2

Single crystal analysis can provide enough data on the molecularstructure of complexes and thus is a powerful tool to identify themolecular identity of Ir(acac-F6)(F-BT)2. Fortunately, Ir(acac-F6)(F-BT)2 single crystal was observed and analyzed successfully asshown in Fig. 1, which confirmed its molecular formula. It can beclearly seen that Ir center is coordinated by two F-BT ligands and abidentate auxiliary ligand of acac-F6, forming an octahedral coordina-tion sphere. The key structural parameters of Ir(acac-F6)(F-BT)2 listedin Table 1 suggest that the coordination sphere is distorted by theinhomogeneous ligands. It is interesting to see that there are twoIr(acac-F6)(F-BT)2 molecules within one unite cell. From Table 1, theIr–C bond length values are found to be similar to literature ones [16].The Ir–N bond length values are longer than Ir–C ones, and the Ir–Obond length values are even longer than both Ir–C and Ir–N ones. In

Fig. 1. Single crystal structure of Ir(acac-F6)(F-BT)2. All hydrogen atoms are

omitted for clairity.

Fig. 2. Inter-molecular p-stacking within solid state Ir(acac-F6)(F-BT)2.

addition, a slight difference between Ir–O bond lengths is observed,indicating a steric hindrance effect from F-BT ligand. The similar C–Obond length values, however, suggest that the �1 charge of acac-F6ligand is delocalized over both O atoms owing to the large conjuga-tion planar. The C(9)–Ir–N(1) (81.051), N(2)–Ir–C(22) (81.591), andO(1)–Ir–O(2) (86.721) bite angles in Ir(acac-F6)(F-BT)2 are larger thanthe corresponding N–Cu–N bite angles (�801) in tetrahedral field[17], indicating that the geometric environment around Ir(III) centeris less crowded than that round Cu(I) center. This comfortablecoordination sphere may be positive to depress the structuralrelaxation in Ir(acac-F6)(F-BT)2 excited state.

According to Zhang and coworker’s report, inter-molecular p-stacking can be found if there are large coplanar planes in ligands,and the inter-molecular p-stacking can bring a luminescenceenhancement owing to the depression of nonradiative decay[18]. Considering the large coplanar conjugation planes fromF-BT fragment, it is reasonable to see that there is inter-molecularp-stacking within solid state Ir(acac-F6)(F-BT)2. As for crystalIr(acac-F6)(F-BT)2, every two F-BT planes are bonded due to thep–p attraction between F-BT planes as shown by Fig. 2, and thetwo F-BT planes are found to be exactly parallel to each otherwith a proximate distance of �3.393 A, which confirms that thereexits face-to-face p–p stacking between the two F-BT planes. It isbelieved that above mentioned inter-molecular p-stacking canform a rigid environment round the coordination center and thussuppress the vibrational progress of excited state [18].

W.-L. Chang, P.-Y. Yang / Journal of Luminescence 132 (2012) 403–408406

3.2. Theoretical calculation on Ir(acac-F6)(F-BT)2 crystal

The onset electronic transition and frontier molecular orbitals(MOs) are usually involved in photoinduced electron and energytransfer processes. In order to get a full understanding on theelectronic configuration and frontier MOs of Ir(acac-F6)(F-BT)2,we intend to perform a DFT/TD-DFT calculation on the Ir(III)complex, which has been demonstrated to be a delicate path toexplore the electronic configuration and electronic transition oftransition metal complexes [17]. Since there is a heavy metal coreof Ir(III), we use an ECP basis of RB3LYP/SBKJC to simplify thecalculation. As shown by Table 2, the occupied frontier MOs ofHOMO and HOMO-1 of Ir(acac-F6)(F-BT)2 are composed of F-BTligand and Ir atom, admixed with small contributions from acac-F6 fragment. While, its unoccupied fronter MOs of LUMO,LUMOþ1 and LUMOþ2 are essentially pn orbitals localized onF-BT and acac-F6 ligands, with very slim contributions from Iratom. The obvious participation of acac-F6 in LUMO suggests thatthe introduction of fluorine atom into auxiliary ligand can alsoaffect the photophysical character of resulting complex. The firstelectronic transition of S0-S1 is composed of a transition fromHOMO to LUMO and is thus assigned as a highly mixed characterof intraligand-charge-transfer (ILCT), ligand-to-ligand-charge-transfer (LLCT), and metal-to-ligand-charge-trasfer (MLCT). Thefirst electronic transition energy of 2.668 eV (465 nm) is so close

Table 2Calculated percentage composition of frontier molecular orbitals and the first five

singlet excitations of Ir(acac-F6)(F-BT)2 calculated at RB3PW91/SBKJC level.

Energy (eV) Composition (%)

Ir F-BT acac-F6

LUMOþ2 (126) �2.261 11.9 80.1 8.0

LUMOþ1 (125) �2.373 4.8 93.1 2.1

LUMO (124) �2.476 5.6 32.2 62.2

HOMO (123 ) �5.919 35.8 52.8 11.4

HOMO-1 (122 ) �6.338 43.5 46.6 10.0

S0-S1 2.668 123-124(95.3)

S0-S2 2.861 123-125(86.0) and 123-126(8.3)

S0-S3 2.980 123-126(84.9) and 123-125(7.0)

S0-S4 3.062 122-124(94.1)

S0-S5 3.197 122-125(74.8) and 122-126(15.7)

Fig. 3. Absorption spectrum of Ir(acac-F6)(F-BT)2 in CH2Cl2 solution with a

concentration of 1�10�5 mol/L.

to Ir(acac-F6)(F-BT)2 onset 0–0 transition of 451 nm shown inFig. 3, confirming the correctness of TD-DFT calculation.

3.3. Morphology of Ir(acac-F6)(F-BT)2/PVP composite fibers

PVP has been proved to be an excellent supporting matrix forelectronspinning [19], and here we select PVP as the supportingmatrix. To systematically compare the photophysical characteristicdifference between pure Ir(acac-F6)(F-BT)2 and the composite fiberscontaining Ir(acac-F6)(F-BT)2, four doping concentrations of 2.0, 2.5,3.0, and 3.5 wt% have been tried to achieve the optimal performance.From the typical SEM image of the 3.0 wt% doped Ir(acac-F6)(F-BT)2/PVP composite fibers shown in Fig. 4, it can be seen that the averagediameter of the composite fibers is �870 nm. All fibers show anorderless distribution on the substrate with no branch structure,which means that they own a smooth and uniform morphology. Theporous structure of the composite fibers provides a large surface-to-volume ratio, which is reported to be two orders of magnitude biggerthan that of bulk material, making them a potential supportingmatrix for optical materials. Upon UV excitation, a greenish yellowemission is detected for the 3.0 wt% doped composite fibers, asshown in Fig. 4. The emission distribution is also uniform andhomogeneous all over the fibers, which confirms that Ir(acac-F6)(F-BT)2 complex has been successfully doped into the composite fibers.The photophysical character will be discussed in detail below.

3.4. Photophysical character comparision

3.4.1. PL spectra

Fig. 5 shows the PL spectra of the four samples and that of pureIr(acac-F6)(F-BT)2 powder. Pure Ir(acac-F6)(F-BT)2 renders agreenish yellow emission peaking at 533 nm, which is consistentwith the fluorescence microscopy as shown in Fig. 4. As for thefour composite fibers, it is observed that the PL spectra shift to�504 nm, owing to the potential interaction between Ir(acac-F6)(F-BT)2 excited state and PVP matrix. While the emission bandshapes of pure and composite fiber samples are quite similar toeach other, no other emission peaks and vibronic progressionsare detected, suggesting that the electronic transitions areunchanged.

As shown in Fig. 5, the PL intensity increases with theincreasing dopant concentration from 2.0 to 3.0 wt%, clearly dueto the increasing amount of doping Ir(acac)(F-BT)2 molecules.Upon a further higher doping concentration, however, the 3.5 wt%dope sample shows a weaker PL intensity compared with that ofthe 3.0 wt% dope sample, which means that 3.0 wt% should be theoptimal dopant concentration. Both higher and lower dopingconcentrations can compromise the emission, which can beexplained as follows. There may be at least two opposite factorsaffecting emission intensity of the composite fibers: amount ofIr(acac-F6)(F-BT)2 molecules and self-absorption of Ir(acac)(F-BT)2. When the dopant concentration is low, the emission fromIr(acac-F6)(F-BT)2 is instinctively weak. On the other hand, amuch higher dopant concentration of 3.5 wt% leads to an intenseself-absorption owing to the absorption from 450 to 600 nm,which also decreases the emission intensity. The two oppositefactors may achieve a balance in the 3.0 wt% doped sample,resulting in the maximum emission intensity.

3.4.2. Excited state lifetime

The PL decay characteristics of the four composite samples andpure Ir(acac-F6)(F-BT)2 powder are shown in Fig. 6. The emissionof pure Ir(acac)(F-BT)2 powder follows a single exponential decaypattern with excited state lifetime of 0.29 ms, suggesting thatthere is only one irradiative decay process within pure Ir(acac)

Fig. 4. SEM (left) and fluorescence microscopy (right) images of the 3.0 wt% doped Ir(acac-F6)(F-BT)2/PVP composite fibers.

Fig. 5. PL spectra of the four samples and pure Ir(acac-F6)(F-BT)2 powder.

Fig. 6. Excited state lifetime comparison between pure Ir(acac-F6)(F-BT)2 powder

and the 3.0 wt% doped sample.

Fig. 7. Emission decay comparison between pure Ir(acac-F6)(F-BT)2 powder and

the 3.0 wt% doped sample when exposed to UV radiation at 350 nm.

W.-L. Chang, P.-Y. Yang / Journal of Luminescence 132 (2012) 403–408 407

(F-BT)2 powder [18]. The long excited state lifetime also suggeststhat the emissive center owns a triplet character. As revealed bythe TD-DFT calculation result, the onset electronic transition ofIr(acac)(F-BT)2 is a mixed one of (ILþMLþLL)CT, thus we come toa conclusion that the emissive center is composed of three decaynatures with sole excited state lifetime. The excited state lifetimes

for the four doped samples are measured to be 1.30 ms for the2.0 wt% doped sample, 3.05 ms for the 2.5 wt% doped sample,4.83 ms for the 3.0 wt% doped sample, and 3.18 ms for the 3.5 wt%doped sample, respectively. All four emissions also follow a singleexponential pattern, which confirms that PVP matrix has noinfluence on the electronic transition nature of Ir(acac-F6)(F-BT)2. The single exponential decay pattern also suggests thatIr(acac-F6)(F-BT)2 molecules occupy only one kind of homoge-neous site and distribute uniformly within PVP matrix, which isconsistent with the fact revealed by the fluorescence microscopyas shown in Fig. 4. The variation of excited state lifetime uponvarious doping concentrations also confirms our hypothesisexplaining why 3.0 wt% should be the optimal dopant concentra-tion. In addition, the increased excited state lifetime suggests thatPVP matrix provides a rigid matrix for Ir(acac-F6)(F-BT)2 mole-cules. At the excited state Ir(acac-F6)(F-BT)2 molecules are stabi-lized and protected by PVP matrix, resulting in the observedsingle exponential pattern and longer excited state lifetime thanthat of pure Ir(acac-F6)(F-BT)2.

3.4.3. Photodurability

Fig. 7 shows the emission intensity variation of pure Ir(acac-F6)(F-BT)2 powder and the 3.0 wt% doped sample when exposedto a continuous UV excitation of 350 nm. Due to the photo-bleaching effect of UV excitation, the emission intensity of both

W.-L. Chang, P.-Y. Yang / Journal of Luminescence 132 (2012) 403–408408

samples decreases with increasing irradiation time, owing to thebreak down of molecular structure or photochemistry reaction.However, this decomposition procedure is largely suppressedwhen doped into PVP matrix as shown in Fig. 7. It is believedthat PVP matrix can provide a rigid matrix for the emittingmolecules and shield the negative effects causing nonradiativedecay from the surrounding environment, leading to an improvedphotodurability. All analyses suggest that Ir(acac-F6)(F-BT)2 is apromising emitting dopant for luminescent composite fibers, andPVP is an excellent matrix offering a rigid matrix towards longerexcited state lifetime and better photodurability of emittingdopant.

4. Conclusion

In this paper, we study a fluorine-containing Ir(III) complex ofIr(acac-F6)(F-BT)2 and its corresponding luminescent compositefibers constructed by the electronspinning method. The singlecrystal analysis of Ir(acac-F6)(F-BT)2 identifies the molecularstructure. Ir(III) ion occupies a distorted octahedral coordinationcenter, and the bond length and bond angle values fall in thenormal region of literature ones. Theoretical calculation onIr(acac-F6)(F-BT)2 reveals the excited state electron configurationas a mixed one of (ILþLLþML)CT. By doping Ir(acac-F6)(F-BT)2

into a polymer matrix of PVP, the morphology and photo-emis-sion characteristics of the resulted composite fibers are studiedand compared with those of the pure sample. Results suggest thatthe emission of the composite fibers shows a blue shift tendency,the excited state lifetime, and photodurability of the compositefibers become longer and better than those of pure Ir(acac-F6)(F-BT)2. The rigid framework provided by PVP matrix is responsiblefor above improvements. These findings may be useful for futurenanostructure design and fabrication.

Acknowledgment

W.L.C. was supported by ‘‘Qing Lan’’ Talent Engineering Fundsby Lanzhou Jiaotong University under Grant no. QL-08-18A.

References

[1] D. Reneker, I. Chun, Nanotechnology 7 (1996) 216.[2] G.M. Kim, A. Wutzler, H.J. Radusch, G.H. Michler, P. Simon, R.A. Sperling,

W.J. Parak, Chem. Mater. 17 (2005) 4949.[3] L. Feng, S. Li, H. Li, J. Zhai, Y. Song, L. Jiang, D. Zhu, Angew. Chem. Int. Ed. Eng.

41 (2002) 1221.[4] J. Hartgerink, E. Beniash, S. Stupp, Science 294 (2001) 1684.[5] X. Wang, C. Drew, S.-H. Lee, K.J. Senecal, J. Kumar, L.A. Samuelson, Nano Lett.

2 (2002) 1273.[6] J.A. Matthews, G.E. Wnek, D.G. Simpson, G.L. Bowlin, Biomacromolecules

3 (2002) 232.[7] Z.M. Huang, Y.Z. Zhang, M. Kotaki, S. Ramakrishna, Compos. Sci. Technol. 63

(2003) 2223.[8] A.C. Patel, S.X. Li, C. Wang, W.J. Zhang, Y. Wei, Chem. Mater. 19 (2007) 1231.[9] K.E. Erkkila, D.T. Odom, J.K. Barton, Chem. Rev. 99 (1999) 2777.

[10] C.A. Bignozzi, R. Argazzi, C.J. Kleverlaan, Chem. Soc. Rev. 29 (2000) 87.[11] 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.[12] 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.[13] F. Li, M. Zhang, J. Feng, G. Cheng, Z.J. Wu, Y.G. Ma, S.Y. Liu, Appl. Phys. Lett. 83

(2003) 365.[14] L. Zhang, B. Li, L. Shi, W. Li, Opt. Mater. 31 (2009) 905.[15] 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.[16] L. Jian, P.I. Djurovich, B.D. Alleyne, M. Yousufuddin, N.N. Ho, J.C. Thomas,

J.C. Peters, R. Bau, M.E. Thompson, Inorg. Chem. 44 (2005) 1713.[17] L. Yang, J.K. Feng, A.M. Ren, M. Zhang, Y.G. Ma, X.D. Liu, Eur. J. Inorg. Chem. 10

(2005) 1867.[18] L. Zhang, B. Li, Z. Su, Langmuir 25 (2009) 2068.[19] Y. Wang, B. Li, Y. Liu, L. Zhang, Q. Zuo, L. Shi, Z. Su, Chem. Commun. 39 (2009)

5868.