White-light phosphorescence from binary coordination polymer nanoparticles

Materials Chemistry and Physics 139 (2013) 345e349

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White-light phosphorescence from binary coordination polymer nanoparticles Lijie Qin a, Yachao Zhu a, Hong Yang a, *, Liang Ding a, Feng Sun b, Mei Shi c, *, Shiping Yang a a

The Key Laboratory of Resource Chemistry of Ministry of Education & Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Normal University, 100 Guilin Road, Shanghai 200234, PR China b Shanghai Nanotechnology Promotion Centre, 245 Jianchuan Road, Shanghai 200237, PR China c Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, PR China

h i g h l i g h t s < Phosphorescent nanoscale coordination polymer nanoparticles (NCPs) were conveniently synthesized. < The phosphorescent carboxyl-functionalized iridium complex was a building block with rare earth Y(III) ions as metallic nodes. < Multi-color emission from blue to orange was obtained.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 June 2012 Received in revised form 4 January 2013 Accepted 30 January 2013

Phosphorescent nanoscale coordination polymer nanoparticles (NCPs) were conveniently synthesized by phosphorescent carboxyl-functionalized iridium complexes as a building block and rare earth Y(III) ions as metallic nodes. They reveal to be uniform nanospheres with average diameter around 200 nm. Multicolor emission from blue to orange was obtained by tuning the ratios of two iridium complexes with energy transfer between them. Furthermore, the white-light emission with CIE coordinates of (0.319, 0.388) was performed. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Organometallic compounds Nanostructures Luminescence Photoluminescence spectroscopy Composite materials

1. Introduction White-light emission has received much attention not only for full-color flat-panel displays with color filters, but also for backlight panels with alternative lighting sources [1e6]. The strong spin-orbit coupling of the heavy-metal atoms such as Pt(II), Ir(III) and Ru(II) allows for efficient intersystem crossing (ISC) between singlet and triplet states, which results in a high quantum yield of phosphorescent emission [7e9]. In fact, white-light phosphorescent emission is widely considered to be the most attractive due to their high quantum efficiency [10,11]. Generally, white-light phosphorescence can be obtained from multiple doped phosphorescent materials with the RGB wavelength region from 400 to 700 nm [12,13], single chain phosphorescent polymer with the RGB chromophores [14e16], hybrid organic/inorganic structures and so on [17,18]. On the other hand, nanoscale coordination polymers (NCPs), which consist of metallic nodes and polydentate bridging ligands, have rapidly gained much attention due to their unique and highly * Corresponding authors. E-mail address: [email protected] (H. Yang). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.01.053

chemically tailorable properties [19]. Up to now, scientists have deliberately designed many functional NCPs for the applications in the fields of catalysis [20e22], chemical sensing [23e25], gas storage [26,27], nonlinear optics [28], magnetic resonance imaging [29e31] and drug delivery [32,33]. Very recently, Lin group [34] reported phosphorescent NCPs for their application in vitro optical imaging. Our group also reported magnetophosphorescent NCPs using phosphorescent carboxyl-functionalized iridium complexes as a building block and magnetic Gd(III) ions as metallic nodes for their application in vitro magnetic resonance imaging (MRI) and phosphorescent imaging [35]. In this paper, we extended our previous work and reported binary coordination polymer phosphorescent nanoparticles (NCPs) prepared by two carboxyl-functionalized iridium complexes (Scheme 1) as a building block and Y(III) ions as metallic nodes. Considering that 4f orbitals of Y3þ are empty, no fef transitions occur which would emit in the ultraviolet region, therefore, only the phosphorescent emission from iridium complexes was observed. By varying the doping ratio of these two complexes, efficient resonance energy transfer mediated phosphorescent emission can be fine tuned, and NCPs exhibited multiple colors from blue to orange under

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Scheme 1. The molecular structure of two iridium complexes and schematic representation of energy transfer in NCPs.

one single wavelength excitation. More interestingly, white-light phosphorescent NCPs were obtained. 2. Materials and methods 2.1. Materials Reagents used were of commercially available reagent quality unless otherwise stated. The compound [(ppy)2Ir(H2dcppy)]PF6 (Ir-2) [ppy ¼ 2-phenyl pyridine, H2dcppy ¼ 2,20 -bipyridine-4,40 dicarboxylic acid] [36] and the chloride-bridged dimmer [(dfppy)2Ir(m-Cl)2(dfppy)2] [dfppy ¼ 2-(40 ,60 -difluoro)-phenylpyridine] [37] were synthesized according to the literature. 2.2. General method TEM images were determined at 200 kV at a JEOL JEM-2100 Transmission Electron Microscope. All SEM images were determined at a Hitachi S-4800 Scanning Electron Microscope. TGA measurements were carried out using a modulated DSC2910-1090B (TA instrument, America) in a nitrogen atmosphere at a heating rate of 10  C min1 in the temperature range of 20e800  C. The concentration of Ir and Y elements in the nanoparticles was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The absorption and emission spectra were recorded using UV-7502PC Xinmao spectrophotometer and Varian Cary Eclipse Fluorescence Spectrophotometer (America), respectively. Lifetime studies were performed with an Edinburgh FL 900 photo-counting system with a hydrogen-filled excitation source. 1H NMR spectra were recorded with Bruker DMX400. Mass spectra were tested by the Shimadzu AXIMA-CFRTM plus matrix-assisted laser desorption ionization time-flight mass spectrometry (MALDI-TOF). 3. Experimental Preparation of iridium complex (dfppy)2Ir(Hpyc)]PF6 (Ir-1): To a solution of 4-pyridinecarboxylic acid (37.9 mg, 0.308 mmol) in methanol (10 mL) was added [(dfppy)2Ir(m-Cl)2(dfppy)2] (93.6 mg, 0.077 mmol in 10 mL dichloromethane) via a cannula. The mixture solution was refluxed for 2 h, and then the saturated sodium acetate methanol solution (2 mL) was added. The mixture solution was refluxed for a further 1 h and then cooled to room temperature. A saturated ammonium hexafluorophosphate methanol solution

(2 mL) was added, and the mixture solution was stirred for a further 30 min. The solvent was removed under reduced pressure, hydrochloric acid (1 mol L1, 10 mL) was added, and the suspension was stirred for 10 min. The product was filtered, washed with water, and extracted into methanol. A saturated ammonium hexafluorophosphate methanol solution (2 mL) was added again, and the mixture was stirred for a further 30 min. The solvent was removed under reduced pressure, and the residue was extracted into dichloromethane and filtered. The solvent was removed under reduced pressure to give Ir-1 as a yellow powder 112.8 mg in the yield of 76%. 1H NMR (400 MHz, DMSO-d6, 298 K): d (ppm) 5.74e5.77 (2 H, dd, J ¼ 2.0 and 8.4 Hz), 6.79e6.85 (2 H, m), 7.55e 7.58 (2 H, dt, J ¼ 1.2 and 6.6 Hz), 7.79e7.80 (4 H, d, J ¼ 6.4 Hz), 8.09e8.13 (2 H, t, J ¼ 7.8 Hz), 8.18e8.20 (2 H, d, J ¼ 8.4 Hz), 8.59e 8.60 (4 H, d, J ¼ 6.4 Hz), 8.79e8.81 (2 H, d, J ¼ 5.6 Hz). HR-MS (MALDI-TOF, DMF) calcd for C34H22IrN4O4F4, 819.12064; found, 819.1208 (M  PF6). Preparation of nanoscale coordination polymers (NCPs): For preparation of nanoscale coordination polymer with Ir-1 (denoted as NCP-1), to a methanol solution of Ir-1 (3 mM, 10 mL) in round flask, 4 mmol L1 aqueous solution of Y(OAc)3 (5 mL) was added and stirred at room temperature for 2 h. The formed precipitates were collected by centrifugation. The obtained material was washed with pure methanol (15 mL) for three times. For preparation for nanoscale coordination polymers from NCP-2 to NCP-10, the method was similar to that of NCP-1 except that different ratio of Ir-1 and Ir-2 (99.9:0.1 for NCP-2, 99.8:0.2 for NCP-3, 99.7:0.3 for NCP-4, 99.6:0.4 for NCP-5, 99.5:0.5 for NCP-6, 99:1 for NCP-7, 96:4 for NCP-8, 90:10 for NCP-9 and 0:100 for NCP-10). 4. Results and discussion NCPs were obtained by the addition of Y(OAc)3 aqueous solution to the CH3OH solution with different ratio between Ir-1 and Ir-2 for 2 h at room temperature. Ir-1 and Ir-2 have similar molecular structures, so the morphology of NCPs is not seriously influenced with different doping ratio. The typical FE-SEM (Fig. 1a, b) and TEM (Fig. 1c, d) images of NCPs were shown in Fig. 1, respectively. The asobtained nanoparticles are of near-spherical shape with an average diameter of about 200 nm. The chemical composition of NCPs was determined by energy dispersive X-ray (EDX, Fig. 1e). The ratio of iridium to yttrium ions is w1.5, confirmed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The phosphorescent color in the ethanol solution changes from blue to orange with increasing Ir-2 content (Fig. 2a). As shown in the Commission Internationale de l’Eclairage (CIE) chromaticity diagram (Fig. 2b), the emission colors changed from blue with CIE coordinates of (0.16, 0.28) to orange (0.55, 0.43). The blue-light emission from NCP-1 was almost completely quenched by about 10 mol% Ir-2 doping (Fig. 2a). More interestingly, NCP-5 showed pseudo white-light emission with CIE coordinates of (0.319, 0.388) when the doping concentration of Ir-2 was 0.4 mol% of Ir-1 (Fig. 2). To investigate the energy transfer in NCPs, the UVevis absorption and phosphorescence spectra of Ir-1 and Ir-2 in ethanol solution were measured. As presented in Fig. 3, the absorption peak located at w400 and w450 nm can be assigned to 1MLCT and 3 MLCT based transitions, respectively [38]. The maximum emission bands for Ir-1 and Ir-2 are 485 and 595 nm, respectively, which is likely to be 3MLCT based transitions. Obviously, the phosphorescence band of Ir-1 fairly overlapped with the 3MLCT absorption of Ir-2, which indicates that there may be good Föster-type energy transfer from Ir-1 to Ir-2 [39]. To avoid the energy transfer from the iridium complex to metal ions, Y3þ with the high excited level was chosen as metallic nodes.

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Fig. 1. Reprehensive low (a) and high (b) magnification FE-SEM images, low (c) and high (d) magnification TEM images of NCPs and EDX pattern of NCPs (e).

Fig. 2. (a) Phosphorescent photographs, from 1 to 10, doping ratio of Ir-1 and Ir-2: 100:0, 99.9:0.01, 99.8:0.02, 99.7:0.03, 99.6:0.04, 99.5:0.05, 99:0.1, 96:0.4, 90:10, 0:100. (b) CIE 2000 chromaticity coordinates of the corresponding NCPs from NCP-1 to NCP-10, dispersed in the ethanol solution under 365 nm UV illumination.

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Fig. 5. Fluorescence decay curves (Ex ¼ 371 nm) of NCPs monitored at 478 nm for NCP-1, NCP-3, NCP-5, NCP-7. All of the samples were dispersed in ethanol.

Fig. 3. (a) The absorption and phosphorescent spectra of Ir-1 (solid) and Ir-2 (dash), respectively, (b) normalized emission spectra from NCP-1 to NCP-10 under 365 nm UV illuminations.

The tunable phosphorescence spectra of NCPs were observed as function of Ir-2 content excited by 365 nm (Fig. 3b). For NCP-1, bluegreen phosphorescence centered at 478 nm was observed. For NCP-10, the emission attributed to the phosphorescence of Ir-2

appeared around 589 nm. By gradually increasing the doping concentration of Ir-2 from 0.1% to 10%, the phosphorescence of Ir-1 was partially quenched, and the phosphorescence of Ir-2 was significantly enhanced. As the doping ratio of Ir-1 and Ir-2 was increased to 90:10, the phosphorescence of Ir-1 was almost quenched completely, indicating the triplet energy of Ir-1 was efficiently transferred to Ir-2. The phosphorescence quantum yields of NCP-1, NCP-3. NCP-5, NCP-7 and NCP-10 in ethanol solution were measured to be 7.2%, 9.1%, 7.1%, 7.9% and 15.4%, respectively. For comparison, the mixed iridium complex solutions with the same molar ratio of Ir-1 and Ir-2 (90:10), neither quenching of the phosphorescence from Ir-1 nor enhanced phosphorescence from Ir2 can be observed (Fig. 4), which indicates that coordination assembly process are indispensable for the occurrence of efficient energy transfer. Another comparison experiment was performed by physically mixing NCP-1 and NCP-10 with the same molar ratio (90:10). Only the decrease of the phosphorescence of NCP-10 was observed (Fig. 4). These facts suggest the coordination assembly process undergoes in the donor, but the acceptor and metal ions can promote the energy transfer process. To further investigate the energy transfer process of NCPs, timeresolved fluorescence decay was investigated. As shown in Fig. 5, the luminescence lifetime of Ir-1 at 478 nm is 373 ns. With the enhancement of the concentration of Ir-2, the luminescence lifetime decreased gradually, which is the strong evidence of FRET from Ir-1 to Ir-2. For example, with the enhancement of the concentration of Ir-2, the luminescence lifetime decreased gradually. For example, when Ir-2 doping concentration changed from 0.2% to 0.4% and 1%, respectively, the luminescence lifetime of Ir-1 detected at 478 nm was decreased from 186 ns to 129 ns and 87 ns, respectively. 5. Conclusions

Fig. 4. The emission spectra of complexes of Ir-1 (-), Ir-2 (C) and Ir-1:Ir-2 ¼ 90:10 (,); NCP-1 (B), NCP-10 (>) and NCP-1:NCP-10 ¼ 90:10 (A), excited by 365 nm.

In conclusion, we have demonstrated a facile and versatile strategy to synthesize phosphorescent nanoscale coordination polymers with efficient and color-tunable visible-light emission. Excitation energy transfer between two iridium complexes occurred and the emission colors of the NCPs were gradually tuned from blue to yellow by varying ratios of the compounds. Careful tuning of the composition led to pseudo white-light emission with

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CIE coordinates of (0.319, 0.388). It is believed that this simple strategy is the first report to use phosphorescent NCPs to achieve white light emission, which expands the application of NCPs and suggests a new strategy to obtain white light emission. Acknowledgments This project has been partially supported by the Leading Academic Discipline Project of Shanghai Normal University (DZL806), the National Natural Science Foundation of China (20971086), the Shanghai Municipal Education Commission (10ZZ84). References [1] J. Kido, M. Kimura, K. Nagai, Science 267 (1995) 1332e1334. [2] Y. Sun, N.C. Giebink, H. Kanno, B. Ma, M.E. Thompson, S.R. Forrest, Nature 440 (2006) 908e912. [3] X. Gong, W. Ma, J.C. Ostrowski, G.C. Bazan, D. Moses, A.J. Heeger, Adv. Mater. 16 (2004) 615e619. [4] S.K. Lee, D.H. Hwang, B.J. Jung, N.S. Cho, J. Lee, J.D. Lee, H.K. Shim, Adv. Funct. Mater. 15 (2005) 1647e1655. [5] X. Gong, S. Wang, D. Moses, G.C. Bazan, A.J. Heeger, Adv. Mater. 17 (2005) 2053e2058. [6] P.I. Shih, Y.H. Tseng, F.I. Wu, A.K. Dixit, C.F. Shu, Adv. Funct. Mater. 16 (2006) 1582e1589. [7] K. Yuichiro, G. Kenichi, B. Jason, J.B. Julie, S. Hiroyuki, A. Chihaya, Appl. Phys. Lett. 86 (2005) 071104. [8] M.A. Baldo, S. .Lamansky, P.E. Burrows, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 75 (1999) 4e6. [9] M.A. Baldo, D.F. O’Brien, M.E. Thompson, S.R. Forrest, Phys. Rev. B 60 (1999) 14422e14428. [10] C. Ulbricht, B. Beyer, C. Friebe, A. Winter, U.S. Schubert, Adv. Mater. 21 (2009) 4418e4441. [11] Z.-Q. Chen, Z.Q. Bian, C.-H. Huang, Adv. Mater. 22 (2010) 1534e1539. [12] J. Kido, H. Shionoya, K. Nagai, Appl. Phys. Lett. 67 (1995) 2281e2283. [13] K. Joo Hyun, H. Petra, K. Mun-Sik, K.Y.J. Alex, T. Ya-Hsien, S. Ching-Fong, Appl. Phys. Lett. 85 (2004) 1116e1118. [14] D.A. Poulsen, B.J. Kim, B. Ma, C.S. Zonte, J.M.J. Fréchet, Adv. Mater. 22 (2010) 77e82.

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