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A rhenium complex with diamine ligand containing oxadiazole group and fluorine atom: Synthesis, characterization, photoluminescence and electroluminescence performances
Optical Materials 35 (2013) 940–947
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
A rhenium complex with diamine ligand containing oxadiazole group and fluorine atom: Synthesis, characterization, photoluminescence and electroluminescence performances Wensheng Yang a, Wan Yang b, Weisheng Liu a, Wenwu Qin a,⇑ a
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province and State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China College of Marxism, Northwest Normal University, Lanzhou 730000, China
b
a r t i c l e
i n f o
Article history: Received 10 September 2012 Received in revised form 9 November 2012 Accepted 18 November 2012 Available online 27 December 2012 Keywords: Re(I) complex Single crystal Photophysical study Electroluminescence
a b s t r a c t In this paper, a diamine ligand of 2-(4-fluorophenyl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (FPYOZ), which owned both enlarged conjugation chain with electron-pulling group and fluorine atom, was synthesized. Its corresponding Re(I) complex was also synthesized and studied in detail, including single crystal analysis, electronic structure, photophysical performance, thermal stability and electrochemical property. Single crystal analysis suggested that there was a coordination ability difference between the N atom from pyridine ring and the one from oxadiazole moiety. Theoretical calculation on the complex suggested that the onset electronic transition owned a mixed character of metal-to-ligand-charge-transfer and ligand-to-ligand-charge-transfer. Upon photoexcitation of 375 nm, this complex showed a yellow emission peaking at 537 nm with excited state lifetime of 8.35 ls. Cyclic voltammetry result suggested that this complex owned HOMO and LUMO energy levels of 5.37 eV and 3.04 eV. The decomposition temperature of this complex was as high as 300 °C, as revealed by thermogravimetric analysis data. The optimal electroluminescence device using this complex as the emitting dopant showed an electroluminescence peaking at 562 nm, with a maximum luminance of 6250 cd/m2 and a maximum current efficiency of 7.3 cd/A. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Owing to their advantages of promising optical characters and photoluminescence (PL) performance, phosphorescent Re(I) complexes with molecular formula of Re(CO)3(N–N)X (N–N = diamine ligand, X = halogen atom) have shown their potential in optoelectronic applications such as solar cells, optical sensors and organic light-emitting diodes (OLEDs) [1–3]. In addition, theoretical calculation results on typical Re(CO)3(N–N)X complexes have suggested that the occupied frontier molecular orbitals (MOs) have predominant metal d character, while the unoccupied frontier MOs are essentially p of N–N ligand [4,5]. The onset electronic transitions correspond to electronic excitations from occupied frontier MOs to unoccupied ones and thus have been assigned to metal-to-ligand-charge-transfer (MLCT) character. Since these MLCT-based electronic transitions are usually involved in photo-induced optoelectronic processes, it is then more convenient to adjust occupied
⇑ Corresponding author. Tel./fax: +86 931 8912382. E-mail address: [email protected] (W. Qin). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.11.020
frontier MOs or unoccupied ones individually without affecting irrelevant MOs. Among the numerous applications, being the emitting material in OLEDs has been considered to be the most attractive one because OLEDs have been demonstrated to be a promising candidate for the next generation flat-panel displays [1,6]. Ever since the first report of electroluminescence (EL) from Re(CO)3(N–N)X complexes, more and more efforts have been devoted to the exploration for superior emitters, aiming at good PL performance, short excited state lifetime, suited energy levels and high stability [7]. Many substituent groups have been introduced into N–N ligands, and the correlation between molecular structure and PL performance has also been investigated. For example, Zhang et al. introduced a series of electron–donor/acceptor groups into N–N ligands and discussed their influence on the PL performance of resulted Re(I) complexes [4,8]. It was found that enlarged conjugation chains with electron-pulling groups were positive to improve PL performance, including improving PL quantum yield and decreasing excited state lifetime. A similar work reported by Zhang et al. suggested, however, that oversized conjugation chains in N–N ligands could quench the emissive state through a thermally-activated energy transfer from the emissive center to a non-emissive
W. Yang et al. / Optical Materials 35 (2013) 940–947
ligand triplet state [5]. In other words, the conjugation chains in N–N ligands should be limited to avoid the negative effects. The research work done by Grushin and coworkers suggested that the introduction of fluorine atom into N–N ligands could improve the EL performance of resulted Re(I) complexes by improving thermal stability and film-forming ability, which were critical for EL device construction [9]. Guided by above results, we decide to design and synthesize a diamine ligand of 2-(4-fluorophenyl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (FPYOZ) possessing both enlarged conjugation chain with electron-pulling group and fluorine atom in its molecular structure, aiming to realize a promising emitter with good PL and EL performance. Since commercially available Re(I) complexes are usually chlor ones and expensive, in this paper, we select an economical one of Re(CO)3(FPYOZ)Br. Its single crystal structure, photophysical features, electrochemical and thermal property are discussed in detail. In addition, its potential for EL application is also investigated. 2. Experimental details Scheme 1 shows the synthetic procedure for the diamine ligand of FPYOZ and its Re(I) complex of Re(CO)3(FPYOZ)Br. The staring reagent of 2-(2H-tetrazol-5-yl)-pyridine (TYP) was synthesized according to a literature procedure [10]. 4-fluorobenzoyl chloride, NaN3, ZnBr2, 2-cyanopyridine 4,40 ,400 -tris[3-methylphenylphenylamino] triphenylamine (m-MTDATA), 4,4-bis[N-(1-naphthyl)N-phenylamino]biphenyl (NPB), 4,40 -dicarbazolyl-1,10 -biphenyl (CBP), 4,7-diphenyl-1,10-phenanthroline (Bphen), tris(8-hydroxyquinoline)aluminum (Alq3) and Re(CO)5Br were commercially obtained from Aldrich Chemical Co. and used without further purifications. All organic solvents were purified using standard procedures. 2.1. Synthesis of FPYOZ ligand FPYOZ was synthesized according to a literature procedure described as follows [10]. First, 10 mmol of TYP and 11 mmol of 4-fluorobenzoyl chloride were dissolved in 25 mL of pyridine. The transparent solution was heated to reflux under N2 protection for 2 days. After cooling, the solution was poured into plenty of cold water. The resulted solid product was recrystallized in hot ethanol. The obtained crude product was further purified on a silica gel column. Yield: 45%. 1H NMR (300 MHz, CDCl3): d 7.43 (1H, m),
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7.54 (2H, m), 7.92 (1H, m), 8.21 (1H, t), 8.23 (1H, t), 8.34 (1H, d, J = 6.0), 8.81 (1H, d, J = 3.6). Anal. Calcd. For C13H8N3OF: C, 64.73; H, 3.34; N, 17.42. Found: C, 64.62; H, 3.21; N, 17.64. 2.2. Synthesis of Re(CO)3(FPYOZ)Br complex The Re(I) complex of Re(CO)3(FPYOZ)Br was synthesized according to a literature procedure described as follows [4]. Firstly, 20 mmol of FPYOZ and 20 mmol of Re(CO)5Br were dissolved in 25 mL of toluene. The mixture was heated to reflux under N2 protection for 12 h. After cooling, the solvent was removed by evaporation. The residue was firstly purified by recrystallization from the mixed solvent of n-hexane:CH3Cl = 1:1 (V:V). The crude product was further purified on a silica gel column to give the desired complex as yellow powder. Yield: 72%. 1H NMR (300 MHz, CDCl3): d 7.58 (1H, m), 7.71 (2H, m), 7.84 (1H, m), 8.32 (1H, t), 8.34 (1H, t), 8.45 (1H, d, J = 6.0), 8.89 (1H, d, J = 3.6). Anal. Calcd. for C16H8BrN3O4FRe: C, 32.50, H, 1.36, N, 7.11. Found: C, 32.67, H, 1.47, N, 7.03. This molecular structure was then further proved by single crystal analysis. 2.3. EL device construction In this work, all functional layers in EL devices were deposited onto ITO glass substrates (resistivity <30 ohm/square, active area = 4 mm2) through resistive heating method under chamber pressure of 3 104 Pa. EL spectra were obtained by a PR650 spectrascan spectrometer. The luminance-current–voltage characteristics of the EL devices were measured simultaneously with a Keithley 2400 source meter. 2.4. Methods and measurements Time dependent density functional theory (TD-DFT) calculation was run on Re(CO)3(FPYOZ)Br at RB3PW91/SBKJC level in vacuum. The single crystal structure of Re(CO)3(FPYOZ)Br was used as the initial structure. Graphical presentation for the frontier MOs was plotted by wxMacMolPlt with contour value of 0.025. 1H NMR spectra were obtained with a Varian INOVA 300 spectrometer. Elemental analysis was performed on a Carlo Erba 1106 elemental analyzer. The single crystal data were collected on a Siemens P4 single crystal X-ray diffractometer with a Smart CCD-1000 detector and graphite-monochromated Mo Ka radiation at 298 K, with operating voltage and current of 50 kV and 30 A, respectively. All
Scheme 1. The synthetic procedure for FPYOZ ligand and its Re(I) complex of Re(CO)3(FPYOZ)B.
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hydrogen atoms were calculated. UV–Vis absorption spectra were taken on a Shimadzu UV-3101PC spectrophotometer. PL and excitation spectra were measured with a Hitachi F-4500 fluorescence spectrophotometer. PL quantum yield was measured according to a literature method [11]. Luminescence decay data were measured by a 355 nm light generated from the third-harmonic-generator pump, using pulsed Nd:yttrium aluminum garnet (YAG) laser as the excitation source. Thermogravimetric analysis (TGA) on Re (CO)3(FPYOZ)Br was performed with a thermal analysis instrument (SDT2960, TA Instruments, New Castle, DE) with heating rate of 10 °C/min. Electrochemical measurement of Re(CO)3(FPYOZ)Br was performed on a CHI830b electrochemical workstation (CH Instruments, Shanghai Chenhua Instrument Corporation, China) in a conventional three-electrode cell, with a platinum-sheet working electrode, a platinum-wire counter electrode and a silver/silver nitrate (Ag/Ag+) reference electrode. The voltammogram was recorded in CH3CN solution with 103 M sample and 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte. Before electrochemical measurement, the solution was bubbled with nitrogen for 5 min to eliminate the dissolved O2. All measurements were carried out in the air at room temperature without being specified. 3. Results and discussions 3.1. Molecular structure analysis on Re(CO)3(FPYOZ)Br 3.1.1. Geometric structure The desired diamine ligand possessing both enlarged conjugation chain with electron-pulling group and fluorine atom in its molecular structure could be confirmed by the single crystal structure of Re(CO)3(FPYOZ)Br. As shown in Fig. 1A, pyridine group and 4-fluorophenyl group were connected to a 1,3,4-oxadiazole ring through two r bonds. The three rings were coplanar with each other, resulting in an enlarged conjugation chain with electronpulling group (1,3,4-oxadiazole) and fluorine atom, which was expected to be helpful for PL and EL improvement. In addition, the p–p attraction between FPYOZ ligands made Re(CO)3(FPYOZ)Br molecules adopt a highly-ordered arrangement, as shown by Fig. 1B. In this case, the FPYOZ planes of Re(CO)3(FPYOZ)Br molecules aligned almost parallel to each other, with FPYOZ–FPYOZ intersection angle of only 0.07° and mean distance of 3.560 Å,
which confirmed that there was face-to-face p–p attraction between FPYOZ planes. It has been reported by Zhang and coworkers that such highly-ordered arrangement can be considered as a rigid one and can effectively depress the geometric relaxation that happens in complex excited state, leading to improved PL performance [12]. As for the coordination structure of Re(CO)3(FPYOZ)Br, the Re(I) ion was surrounded by two N atoms from one FPYOZ ligand, three C atoms from CO groups and one Br atom, showing a distorted octahedral coordination environment, as suggested by Fig. 1A and Table 1. The two Re–C bond length values of Re–C(1) and Re–C(3) were similar with each other, owing to their similar coordination nature. While, Re–C(2) bond was slightly longer than the other two Re–C bonds, which might be caused by the coordinated Br atom localizing on the opposite site of C(2) atom. The Re–Br bond length value was comparable with literature values [4]. As for the two Re–N bonds, there was also a slight difference between them: the bond length value of Re–N(1) was longer than that of Re–N(2), suggesting that the coordination attraction between Re(I) ion and the N atom from pyridine [N(1)] was weaker than that between Re(I) ion and the N atom from 1,3,4-oxadiazole group [N(2)]. The electron-donating N(3) and O(4) atoms in 1,3,4-oxadiazole group should be responsible for the coordination attraction difference. The distortion of coordination sphere around Re(I) ion might also be partially be responsible for the difference between the two Re–N bonds, which could be confirmed by the following fact. The N(1)–Re–N(2) bite angle was found to be obviously smaller than those in tetrahedral coordination cases (80°) [5]. In other words, the coordination distortion was triggered by the small steric hindrance from CO and Br ligands, leading to the variation of Re–N bond length.
3.1.2. Electronic structure of frontier MOs As mentioned above, the occupied frontier MOs usually own predominant metal d character, while the unoccupied frontier MOs are essentially p of N–N ligand. Since frontier MOs of transition metal complexes are usually involved in photo-induced optoelectronic processes, the composition of Re(CO)3(FPYOZ)Br frontier MOs should be analyzed [4,5]. With the crystal structure of Re(CO)3(FPYOZ)Br on hand, we decided to run a TD-DFT calculation on Re(CO)3(FPYOZ)Br at RB3PW91/SBKJC level, which had been
Fig. 1A. Single crystal structure of Re(CO)3(FPYOZ)Br.
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Fig. 1B. The p–p attraction Re(CO)3(FPYOZ)Br molecules.
Table 1 Selected bond length and bond angle values of Re(CO)3(FPYOZ)Br obtained from single crystal. Bond length
Å
Bond angle
Degree
Re–C(1) Re–C(2) Re–C(3) Re–Br(1) Re–N(1) Re–N(2)
1.896 1.926 1.891 2.611 2.224 2.164
N(1)–Re–N(2) N(1)–Re–C(2) N(2)–Re–C(2) N(1)–Re–Br(1) N(2)–Re–Br(1) C(1)–Re–Br(1) C(3)–Re–Br(1)
73.24 91.92 95.55 86.36 84.81 92.62 90.19
proved to be a powerful tool to reveal the electronic structure of transition metal complexes [4,5]. The composition of frontier MOs and the first 10 singlet excitations of Re(CO)3(FPYOZ)Br were listed in Table 2. The graphical
presentation for HOMO and LUMO was also shown in Fig. 2 to give a clear view on the presentative MOs. As suggested by Table 2 and Fig. 2, the occupied frontier MOs of HOMO, HOMO-1, HOMO-2, HOMO-3 and HOMO-4 were composed of contributions from Re atom, Br atom and CO group, showing a mixed character. The unoccupied MOs of LUMO, LUMO + 1 and LUMO + 2 were essentially p of FPYOZ ligand owing to its dominant contribution. As shown by Fig. 2, LUMO mainly localized on the conjugation plane of FPYOZ ligand. This large conjugation plane was expected to be positive to help charge transportation and trapping during EL process. The first ten excitations arose from electronic transitions from occupied frontier MOs to unoccupied ones. These transitions were thus assigned as a mixed character of metal-to-ligand-chargetransfer and ligand-to-ligand-charge-transfer (ML&LLCT) [4,5,12]. Considering that the occupied frontier MOs were basically independent of N–N ligand variation due to its slim contribution to these MOs, it was thus theoretically confirmed that the chemical
Table 2 Composition of frontier MOs and first 10 singlet excitations calculated at RB3PW91/SBKJC level. MO/transition
LUMO + 2(73) LUMO + 1(72) LUMO(71) HOMO(70) HOMO-1(69) HOMO-2(68) HOMO-3(67) HOMO-4(66) S0 ? S1 S0 ? S2 S0 ? S3 S0 ? S4 S0 ? S5 S0 ? S6 S0 ? S7 S0 ? S8 S0 ? S9 S0 ? S10
Energy (eV)
2.240 2.601 3.437 5.845 5.968 6.808 7.140 7.238 1.6678 1.8994 2.5758 2.6346 2.7636 2.9657 2.9718 3.0892 3.2180 3.5634
Contribution (%)
Nature
Re
FPYOZ
CO
Br
0.3 1.7 4.2 35.2 27.8 61.7 37.1 36.0
98.8 94.9 90.4 4.5 10.1 10.0 14.2 13.3
0.9 2.5 3.6 18.1 13.9 27.6 15.5 16.7
0.1 0.9 1.8 42.2 48.2 0.7 33.2 34.0
70 ? 71(97.9) 69 ? 71(97.7) 68 ? 71(97.9) 70 ? 72(95.4) 69 ? 72(96.0) 70 ? 73(88.5) & 66 ? 73(5.5) 67 ? 71(87.4) & 70 ? 73(5.9) & 66 ? 71(4.5) 69 ? 73(96.7) 66 ? 71(88.5) & 67 ? 71(6.0) & 70 ? 73(3.4) 68 ? 72(95.7)
FPYOZ FPYOZ FPYOZ Re & Br Br & Re Re & CO Re & Br Re & Br ML/LLCT ML/LLCT ML/LLCT ML/LLCT ML/LLCT ML/LLCT ML/LLCT ML/LLCT ML/LLCT ML/LLCT
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Fig. 2. The graphical presentation for HOMO (down) and LUMO (up) of Re(CO)3(FPYOZ)Br.
modifications on N–N ligand could adjust the excited state character without affecting irrelevant MOs. In other words, the PL performance of Re(CO)3(FPYOZ)Br was expected to be improved by introducing enlarged conjugation chain and F atom into its N–N ligand, without compromising the occupied frontier MOs. 3.2. Photophysical character of Re(CO)3(FPYOZ)Br 3.2.1. UV–Vis absorption and excitation The UV–Vis absorption spectra of Re(CO)3(FPYOZ)Br and free FPYOZ ligand measured in CHCl3 solutions (10 lM) were shown in Fig. 3. The absorption spectrum of free FPYOZ ligand was com-
Fig. 3. The UV–Vis absorption spectra of Re(CO)3(FPYOZ)Br and free FPYOZ ligand in CHCl3 solutions (10 lM), along with the absorption spectrum of free FPYOZ.
posed of five absorption bands of 235 nm, 263 nm, 295 nm, 308 nm and 345 nm, ending at 356 nm. While, there were four absorption bands for the absorption spectrum of Re(CO)3 (FPYOZ)Br, peaking at 235 nm, 285 nm, 342 nm and 400 nm, respectively, with absorption edge of 500 nm. The high energy absorption bands of 235 nm, 285 nm and 342 nm were similar to those of free FPYOZ ligand, suggesting that they were ligand p–p transitions in nature. The slight difference in spectral shape might be caused by the electronic delocalization during coordination. The low energy band of 400 nm, however, was a newly-generated one compared with the absorption spectrum of free FPYOZ ligand. Taking above TD-DFT calculation result into account, this low energy absorption band could be attributed to the electronic transitions of ML&LLCT [4]. A literature report had suggested that the MLCT absorption of Re(CO)3(Phen)Br (Phen = 1,10-phenanthroline) localized at 405 nm, ending at 500 nm [4]. After the comparison between the absorption spectra of Re(CO)3 (FPYOZ)Br and Re(CO)3(Phen)Br, it was observed that the large conjugation chain in FPYOZ ligand did not move the MLCT transition energy of Re(CO)3(FPYOZ)Br to low energy region. This observation conflicted with Zhang’s report that large conjugation system in diamine ligand could decrease the energy required for the onset electronic transition of metal complexes, narrowing the optical edge (band gap) [5]. The electron-pulling effect from 1,3,4-oxadiazole group and F atom in FPYOZ ligand might be responsible for this phenomenon. In other words, electron-pulling substituent groups in diamine ligands could depress the negative effect caused by oversized conjugation system in diamine ligand. The excitation spectrum of Re(CO)3(FPYOZ)Br was composed of three bands at 340 nm, 388 nm and 430 nm. It was also observed that the optimal excitation window localized in the low energy region from 365 nm to 465 nm, whose corresponding absorption intensity was rather weak as shown in Fig. 3. The intense absorption ranging from 200 nm to 350 nm was ineffective on exciting the emissive state of Re(CO)3(FPYOZ)Br, which could be explained as follows. As mentioned above, the intense absorption was attributed to p–p transitions of FPYOZ ligand, while the weak absorption was assigned to the mixture of ML&LLCT. Although the molar extinction coefficient of ML&LLCT transitions was much lower than that of p–p transitions [11], the energy transfer efficiency of ML&LLCT excited state to the emissive state was much higher than that of p–p excited state due to the fact that the emissive state of Re(CO)3(FPYOZ)Br derived from the first singlet transition of S0 ? S1. The p–p excited state had to experience a series of energy-exhausting potential surface crossing procedures to finally transfer its energy to the emissive center, leading to its poor efficiency of exciting the emissive center. 3.2.2. PL spectrum and quantum yield The PL spectrum of Re(CO)3(FPYOZ)Br measured in CHCl3 solution (10 lM) was shown in Fig. 4. The emission band showed a maximum at 537 nm, without giving any vibronic progressions, and the full-width-at-half-maximum (FWHM) value of this emission band was as large as 75 nm. Above characters indicated that the emissive state owned a charge transfer (CT) character. Considering that the emissive state derived from the singlet transition of S0 ? S1 which owned CT character, it was reasonable to observe that the emissive state also owned a CT character. The Stokes shift between its absorption edge (500 nm) and emission peak (537 nm) was only 37 nm. Thus, the geometric relaxation that occured in Re(CO)3(FPYOZ)Br excited state had been effectively suppressed, which might be caused by the face-to-face p–p attraction between Re(CO)3(FPYOZ)Br molecules. A literature report had suggested that the emission of Re(CO)3(Phen)Br localized at 554 nm, with FWHM value of 90 nm [4]. After the comparison between the emission spectra of Re(CO)3(FPYOZ)Br and Re(CO)3(Phen)Br, it
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two components all contributed to the emissive center with identical decay times. 3.3. Thermal stability and electrochemical property of Re(CO)3(FPYOZ)Br
Fig. 4. The PL spectrum of Re(CO)3(FPYOZ)Br measured in CHCl3 solution (10 lM, kex = 375 nm). Inset: the emission decay characteristic of Re(CO)3(FPYOZ)Br.
was concluded that the enlarged conjugation chain with electronpulling group (1,3,4-oxadiazole group) and fluorine atom could move the emission to high energy region, as well as narrowing the emission band. PL quantum yield, which is defined as the ratio of the number of photons emitted to the number of photons absorbed, is an important factor for both PL and EL applications. This parameter can be determined by the following formula, where the subscripts s and r refer to sample and reference standard solution respectively, n is the refractive index of the solvent, D is the integrated intensity, and U is PL quantum yield.
Us ¼ Ur ðBr =Bs Þðns =nr Þ2 ðDs =Dr Þ
ð1Þ
Considering that all functional layers in EL devices are usually deposited by resistive heating method, the emitting material of Re(CO)3(FPYOZ)Br should be thermally stable enough to experience vacuum evaporation during device construction. The TGA curve of Re(CO)3(FPYOZ)Br was shown in Fig. 5. It was observed that the TGA curve was composed of two weight loss regions. The first region ranging from 320 °C to 410 °C was attributed to the thermal release of FPYOZ ligand, where the theoretical weight loss was 40.8% and the measured one was 42.8%. The second region ranging from 410 °C to 600 °C should be assigned to the thermal decomposition and release of ligands such as CO and Br ligands. Never the less, the smooth TGA curve below 320 °C indicated that Re(CO)3(FPYOZ)Br was thermally stable enough below 300 °C, making itself thermally stable enough to experience EL device construction through vacuum evaporation. On the other hand, the energy levels of functional materials are critical when designing EL device structures. Thus, the electrochemical property of Re(CO)3(FPYOZ)Br had been determined by cyclic voltammetry (CV) at room temperature. As shown in Fig. 6, Re(CO)3(FPYOZ)Br exhibited one irreversible oxidation peak with onset potential of 0.97 V and one reduction wave with onset potential of 1.36 V, respectively. The energy level values of HOMO (EHOMO) and LUMO (ELUMO) were calculated according to the followRed ing equations, where EOxd onset and Eonset denoted the onset potential values of oxidation and reduction peaks, respectively.
EHOMO ¼ ðEOxd onset þ 4:4ÞeV
ð2Þ
ELUMO ¼ ðERed onset þ 4:4ÞeV
ð3Þ
The quantity B is calculated by B = 1 10AL, where A is the absorption coefficient at the excitation wavelength and L is the optical length. With its PL spectrum and absorption data on hand, the PL quantum yield of Re(CO)3(FPYOZ)Br was measured to be 0.11. This value, however, seemed to be not satisfactory enough compared with literature reports whose quantum yields were usually higher than 0.50 [13,14]. It seemed that the enlarged conjugation chain with electron-pulling group (1,3,4-oxadiazole group) and F atom could compromise the PL quantum. We attributed the causation to the strong electron-pulling effect of 1,3,4oxadiazole group and F atom which might decrease the radiative constant of emissive state [15].
The corresponding values of Re(CO)3(FPYOZ)Br were calculated to be 5.37 eV for EHOMO and 3.04 eV for ELUMO, respectively. The energy gap between HOMO and LUMO was then observed to be 2.33 eV. This value, however, was smaller than the optical edge of 2.48 eV (500 nm) but quite similar to the emissive energy of 2.31 eV (537 nm), suggesting that the solvent effect distorted Re(CO)3(FPYOZ)Br molecular structure and decreased the band gap [17]. This complex with such a narrow energy gap of 2.33 eV was suitable to be used as a guest in host–guest EL devices. Guided by above data, in this work, CBP was chosen to be the host for EL
3.2.3. Excited state lifetime The excited state lifetime (s) of an emitting material is another crucial factor for EL application. According to literature reports by Zhang and coworkers [11,16], a long excited state lifetime of emitting material could lead to emission saturation of EL devices at high current densities, compromising both EL efficiency and brightness. Consequently, short excited state lifetime is desired for the emitting materials in EL application to minimize the efficiency roll-off at high current densities. The emission decay characteristic of Re(CO)3(FPYOZ)Br in CHCl3 solution (10 lM) was shown by the inset of Fig. 4. The yellow emission followed monoexponential decay pattern with s = 8.35 ls. This long-lived excited state indicated its phosphorescent nature. Above TD-DFT calculation result suggested that the emissive state owned a character of ML&LLCT. Thus, the emissive state could be described as 3 (ML&LLCT). In addition, it was observed that even though the emissive state owned a mixed character, the emissive state followed monoexponential decay pattern, suggesting that the
Fig. 5. The TGA curve of Re(CO)3(FPYOZ)Br.
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Fig. 6. CV curve of Re(CO)3(FPYOZ)Br.
devices because of its wide energy gap (3.3 eV), as well as its higher LUMO level (2.7 eV) and lower HOMO level (6.0 eV) compared with those of Re(CO)3(FPYOZ)Br [18]. 3.4. EL performance of Re(CO)3(FPYOZ)Br Above results suggested that Re(CO)3(FPYOZ)Br could be developed as an emitting dopant for EL application, owing to its promising characters of proper emission energy, good thermal stability and well-suited energy levels. In addition, it has been mentioned that enlarged conjugation chain with electron-pulling group and fluorine atom were introduced into the Re(I) complex to improve PL and EL performance. In order to evaluate this hypothesis, we investigated the EL performance of Re(CO)3(FPYOZ)Br. In this initial effort, a classic structure of ITO/m-MTDATA (30 nm)/NPB (25 nm)/ CBP:Re(CO)3(PTO)Br (30 nm)/Bphen (20 nm)/Alq3 (30 nm)/LiF/Al was used to explore the potential of Re(CO)3(FPYOZ)Br in EL application. Here, m-MTDATA was the hole-injection layer, NBP was the hole-transporting layer, Bphen and Alq3 were exciton-blocking layer and electron-transporting layer, respectively. Firstly, four dopant concentrations were tried to optimize device performance. The EL spectra of the four devices under applied voltage of 8 V were shown in Fig. 7. When the dopant concentration was as low as 10%, a slight blue emission peaking at 440 nm was observed. Clearly, there were excess excitons within the emitting layer, and the diffusion of these excitons made them be captured by neighbor functional layers, leading to the blue emission.
Fig. 7. The EL spectra of the four devices under applied voltage of 8 V.
Since there was an exciton-blocking layer which prevented exciton penetration in these EL devices, the excess excitons might localize in CBP or NBP layers. According to literature reports [18–20], the fluorescence from CBP peaked at 390 nm. Thus, the blue emission of 440 nm should be assigned to NPB emission. In other words, the generated excitons in the emitting layer were not completely captured by the emitting dopant of Re(CO)3(FPYOZ)Br. The excess excitons moved into NBP layer, leading to NPB emission. With the increasing dopant concentrations, NBP emission disappeared, with only pure emission from Re(CO)3(FPYOZ)Br left. It was then observed that the EL spectra of Re(CO)3(FPYOZ)Br peaking at 562 nm gave a red shift tendency compared with its PL spectrum peaking at 537 nm. It seemed that Re(CO)3(FPYOZ)Br excited state could be stabilized by CBP host, leading to spectral red shift, and this observation was consistent with literature reports [18–20]. The energy transfer mechanism between CBP and Re(CO)3 (FPYOZ)Br was then analyzed as follows. Considering that there was an obvious overlap between CBP emission (390 nm) and Re(CO)3(PTO)Br absorption/excitation [19], there might be two possible pathways. One was Förster energy transfer from singlet CBP excited state to ML&LLCT excited state of Re(CO)3(FPYOZ)Br, the other was charge trapping energy transfer since the energy gap of Re(CO)3(FPYOZ)Br fell in that of CBP. To identify the dominating energy transfer mechanism, the current density versus voltage characteristics of the four EL devices were shown in Fig. 8. It could be seen that the current density value generally increased with increasing dopant concentrations, suggesting that high Re(CO)3(FPYOZ)Br concentration in CBP layer was helpful to enhance charge carrier transportation within those EL devices. Above fact denied the possibility of charge trapping energy transfer mechanism, and Förster energy transfer mechanism should be responsible for the major energy transfer mechanism. The enlarged conjugation chain with electron-pulling group and fluorine atom in FPYOZ ligand might be helpful to achieve charge carrier balance, as expected above. The inset of Fig. 8 showed the current efficiency versus current density characteristics of the four EL devices. It was observed that the current efficiency firstly increased with increasing current densities within low current density region. After reaching the maximum values, the current efficiency decreased sharply with increasing current densities. This efficiency roll-off was attributed to triplet–triplet and triplet-polar annihilations [21]. To avoid such efficiency roll-off, the excited state lifetime of Re(CO)3(FPYOZ)Br should to be further shortened to achieve high efficiency at high
Fig. 8. The current density versus voltage characteristics of the four EL devices. Inset: the current efficiency versus current density characteristics of the four EL devices.
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ML&LLCT. Further analysis revealed that Re(CO)3(FPYOZ)Br has HOMO and LUMO energy levels at 5.37 eV and 3.04 eV, respectively. TGA data suggested that it owned a high thermal decomposition temperature of 300 °C. Using Re(CO)3(FPYOZ)Br as the emissive dopant, an electroluminescence peaking at 562 nm was realized, with a maximum luminance of 6250 cd/m2 and a maximum current efficiency of 7.3 cd/A. The low emission yield and the long excited state lifetime, however, compromised its EL performance. Future efforts may be devoted to diamine ligand structure design and device optimization for performance improvement. Acknowledgments
Fig. 9. The luminance-current density characteristics of the 12% doped device under various voltages.
current density. The maximum current efficiency of 7.3 cd/A was achieved by the 12% doped device at 155 mA/cm2. Fig. 9 showed the luminance-current density characteristics of the 12% doped device under various voltages. Upon driving voltage of 15 V, a maximum brightness of 6250 cd/m2 was achieved with current efficiency of 1.32 cd/A and current density of 427 mA/cm2. Even higher driving voltages compromised the brightness, owing to the triplet-polar annihilation at high current densities. The maximum brightness of this work, however, was not satisfactory enough. Considering the suited energy levels and good thermal stability of Re(CO)3(FPYOZ)Br, the low PL quantum yield and the long excited state lifetime should be the causation for the underdeveloped brightness, indicating that the diamine ligand structure should be further modified to improve the EL performance. 4. Conclusions In this paper, a diamine ligand with enlarged conjugation chain with electron-pulling group and fluorine atom and its corresponding Re(I) complex were synthesized and studied, including the molecular structure, electronic structure, photophysical performance, thermal stability and electrochemical property. Re(CO)3 (FPYOZ)Br was a yellow emitter peaking at 537 nm with excited state lifetime of 8.35 ls. Single crystal analysis and theoretical calculation suggested that the emission owned a mixed character of
We are thankful to the ‘Scientific Research Fund of Lanzhou University’. This study was supported in part by the ‘Key Program of National Natural Science Foundation of China’ (20931003). References [1] L. Cambot, M. Cantuel, Y. Leydet, G. Jonusauskas, D.M. Bassani, N.D. McClenaghan, Coord. Chem. Rev. 252 (2008) 2572–2584. [2] K.E. Erkkila, D.T. Odom, J.K. Barton, Chem. Rev. 99 (1999) 2777–2796. [3] D.R. McMillin, K.M. McNett, Chem. Rev. 98 (1998) 1201–1209. [4] Z. Si, X. Li, X. Li, H. Zhang, J. Organomet. Chem. 694 (2009) 3742–3748. [5] L. Zhang, S. Yue, B. Li, D. Fan, Inorg. Chim. Acta 384 (2012) 225–232. [6] D.V. Scaltrito, D.W. Thompson, J.A. O’Callaghan, G.J. Meyer, Coord. Chem. Rev. 208 (2000) 243–266. [7] C. Adachi, M.A. Baldo, S.R. Forrest, S. Lamansky, M.E. Thompson, R.C. Kwong, Appl. Phys. Lett. 78 (2001) 1622–1624. [8] X. Li, D. Zhang, W. Li, B. Chu, L. Han, T. Li, Z. Su, J. Zhu, Y. Chen, Z. Hu, P. Lei, Z. Zhang, Opt. Mater. 31 (2009) 1173–1176. [9] V.V. Grushin, N. Herron, D.D. LeCloux, W.J. Marshall, V.A. Petrov, Y. Wang, Chem. Commun. (2001) 1494–1495. [10] L. Shi, B. Li, S. Yue, D. Fan, Sensor. Actuator. B: Chem. 137 (2009) 386–392. [11] L. Zhang, B. Li, J. Lumin. 129 (2009) 1304–1308. [12] L. Zhang, B. Li, Z. Su, Langmuir 25 (2009) 2068–2074. [13] L. Zhang, B. Li, L. Shi, W. Li, Opt. Mater. 31 (2009) 905–911. [14] P. Chen, L. Zhao, Y. Duan, Y. Zhao, W. Xie, G. Xie, S. Liu, L. Zhang, B. Li, J. Lumin. 131 (2011) 2144–2147. [15] W. Bai, L. Sun, J. Lumin. 132 (2012) 2697–2703. [16] L. Zhang, B. Li, L. Zhang, Z. Su, ACS Appl. Mater. Int. 1 (2009) 1852–1855. [17] L. Zhang, B. Li, L. Zhang, S. Yue, Q. Xue, S. Liu, J. Electrochem. Soc. 158 (2011) J243–J245. [18] G.B. Che, Z.S. Su, W.L. Li, B. Chu, M.T. Li, Appl. Phys. Lett. 89 (2006) 103511-1– 103511-3. [19] L. Zhang, B. Li, J. Electrochem. Soc. 156 (2009) J174–J178. [20] L. Zhang, P. Chen, B. Li, Z. Hong, S. Liu, J. Phys. D: Appl. Phys. 41 (2008) 245101. [21] M.A. Baldo, C. Adachi, S.R. Forrest, Phys. Rev. B 62 (2000) 10967–10977.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
A rhenium complex with diamine ligand containing oxadiazole group and fluorine atom: Synthesis, characterization, photoluminescence and electroluminescence performances Wensheng Yang a, Wan Yang b, Weisheng Liu a, Wenwu Qin a,⇑ a
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province and State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China College of Marxism, Northwest Normal University, Lanzhou 730000, China
b
a r t i c l e
i n f o
Article history: Received 10 September 2012 Received in revised form 9 November 2012 Accepted 18 November 2012 Available online 27 December 2012 Keywords: Re(I) complex Single crystal Photophysical study Electroluminescence
a b s t r a c t In this paper, a diamine ligand of 2-(4-fluorophenyl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (FPYOZ), which owned both enlarged conjugation chain with electron-pulling group and fluorine atom, was synthesized. Its corresponding Re(I) complex was also synthesized and studied in detail, including single crystal analysis, electronic structure, photophysical performance, thermal stability and electrochemical property. Single crystal analysis suggested that there was a coordination ability difference between the N atom from pyridine ring and the one from oxadiazole moiety. Theoretical calculation on the complex suggested that the onset electronic transition owned a mixed character of metal-to-ligand-charge-transfer and ligand-to-ligand-charge-transfer. Upon photoexcitation of 375 nm, this complex showed a yellow emission peaking at 537 nm with excited state lifetime of 8.35 ls. Cyclic voltammetry result suggested that this complex owned HOMO and LUMO energy levels of 5.37 eV and 3.04 eV. The decomposition temperature of this complex was as high as 300 °C, as revealed by thermogravimetric analysis data. The optimal electroluminescence device using this complex as the emitting dopant showed an electroluminescence peaking at 562 nm, with a maximum luminance of 6250 cd/m2 and a maximum current efficiency of 7.3 cd/A. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Owing to their advantages of promising optical characters and photoluminescence (PL) performance, phosphorescent Re(I) complexes with molecular formula of Re(CO)3(N–N)X (N–N = diamine ligand, X = halogen atom) have shown their potential in optoelectronic applications such as solar cells, optical sensors and organic light-emitting diodes (OLEDs) [1–3]. In addition, theoretical calculation results on typical Re(CO)3(N–N)X complexes have suggested that the occupied frontier molecular orbitals (MOs) have predominant metal d character, while the unoccupied frontier MOs are essentially p of N–N ligand [4,5]. The onset electronic transitions correspond to electronic excitations from occupied frontier MOs to unoccupied ones and thus have been assigned to metal-to-ligand-charge-transfer (MLCT) character. Since these MLCT-based electronic transitions are usually involved in photo-induced optoelectronic processes, it is then more convenient to adjust occupied
⇑ Corresponding author. Tel./fax: +86 931 8912382. E-mail address: [email protected] (W. Qin). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.11.020
frontier MOs or unoccupied ones individually without affecting irrelevant MOs. Among the numerous applications, being the emitting material in OLEDs has been considered to be the most attractive one because OLEDs have been demonstrated to be a promising candidate for the next generation flat-panel displays [1,6]. Ever since the first report of electroluminescence (EL) from Re(CO)3(N–N)X complexes, more and more efforts have been devoted to the exploration for superior emitters, aiming at good PL performance, short excited state lifetime, suited energy levels and high stability [7]. Many substituent groups have been introduced into N–N ligands, and the correlation between molecular structure and PL performance has also been investigated. For example, Zhang et al. introduced a series of electron–donor/acceptor groups into N–N ligands and discussed their influence on the PL performance of resulted Re(I) complexes [4,8]. It was found that enlarged conjugation chains with electron-pulling groups were positive to improve PL performance, including improving PL quantum yield and decreasing excited state lifetime. A similar work reported by Zhang et al. suggested, however, that oversized conjugation chains in N–N ligands could quench the emissive state through a thermally-activated energy transfer from the emissive center to a non-emissive
W. Yang et al. / Optical Materials 35 (2013) 940–947
ligand triplet state [5]. In other words, the conjugation chains in N–N ligands should be limited to avoid the negative effects. The research work done by Grushin and coworkers suggested that the introduction of fluorine atom into N–N ligands could improve the EL performance of resulted Re(I) complexes by improving thermal stability and film-forming ability, which were critical for EL device construction [9]. Guided by above results, we decide to design and synthesize a diamine ligand of 2-(4-fluorophenyl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (FPYOZ) possessing both enlarged conjugation chain with electron-pulling group and fluorine atom in its molecular structure, aiming to realize a promising emitter with good PL and EL performance. Since commercially available Re(I) complexes are usually chlor ones and expensive, in this paper, we select an economical one of Re(CO)3(FPYOZ)Br. Its single crystal structure, photophysical features, electrochemical and thermal property are discussed in detail. In addition, its potential for EL application is also investigated. 2. Experimental details Scheme 1 shows the synthetic procedure for the diamine ligand of FPYOZ and its Re(I) complex of Re(CO)3(FPYOZ)Br. The staring reagent of 2-(2H-tetrazol-5-yl)-pyridine (TYP) was synthesized according to a literature procedure [10]. 4-fluorobenzoyl chloride, NaN3, ZnBr2, 2-cyanopyridine 4,40 ,400 -tris[3-methylphenylphenylamino] triphenylamine (m-MTDATA), 4,4-bis[N-(1-naphthyl)N-phenylamino]biphenyl (NPB), 4,40 -dicarbazolyl-1,10 -biphenyl (CBP), 4,7-diphenyl-1,10-phenanthroline (Bphen), tris(8-hydroxyquinoline)aluminum (Alq3) and Re(CO)5Br were commercially obtained from Aldrich Chemical Co. and used without further purifications. All organic solvents were purified using standard procedures. 2.1. Synthesis of FPYOZ ligand FPYOZ was synthesized according to a literature procedure described as follows [10]. First, 10 mmol of TYP and 11 mmol of 4-fluorobenzoyl chloride were dissolved in 25 mL of pyridine. The transparent solution was heated to reflux under N2 protection for 2 days. After cooling, the solution was poured into plenty of cold water. The resulted solid product was recrystallized in hot ethanol. The obtained crude product was further purified on a silica gel column. Yield: 45%. 1H NMR (300 MHz, CDCl3): d 7.43 (1H, m),
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7.54 (2H, m), 7.92 (1H, m), 8.21 (1H, t), 8.23 (1H, t), 8.34 (1H, d, J = 6.0), 8.81 (1H, d, J = 3.6). Anal. Calcd. For C13H8N3OF: C, 64.73; H, 3.34; N, 17.42. Found: C, 64.62; H, 3.21; N, 17.64. 2.2. Synthesis of Re(CO)3(FPYOZ)Br complex The Re(I) complex of Re(CO)3(FPYOZ)Br was synthesized according to a literature procedure described as follows [4]. Firstly, 20 mmol of FPYOZ and 20 mmol of Re(CO)5Br were dissolved in 25 mL of toluene. The mixture was heated to reflux under N2 protection for 12 h. After cooling, the solvent was removed by evaporation. The residue was firstly purified by recrystallization from the mixed solvent of n-hexane:CH3Cl = 1:1 (V:V). The crude product was further purified on a silica gel column to give the desired complex as yellow powder. Yield: 72%. 1H NMR (300 MHz, CDCl3): d 7.58 (1H, m), 7.71 (2H, m), 7.84 (1H, m), 8.32 (1H, t), 8.34 (1H, t), 8.45 (1H, d, J = 6.0), 8.89 (1H, d, J = 3.6). Anal. Calcd. for C16H8BrN3O4FRe: C, 32.50, H, 1.36, N, 7.11. Found: C, 32.67, H, 1.47, N, 7.03. This molecular structure was then further proved by single crystal analysis. 2.3. EL device construction In this work, all functional layers in EL devices were deposited onto ITO glass substrates (resistivity <30 ohm/square, active area = 4 mm2) through resistive heating method under chamber pressure of 3 104 Pa. EL spectra were obtained by a PR650 spectrascan spectrometer. The luminance-current–voltage characteristics of the EL devices were measured simultaneously with a Keithley 2400 source meter. 2.4. Methods and measurements Time dependent density functional theory (TD-DFT) calculation was run on Re(CO)3(FPYOZ)Br at RB3PW91/SBKJC level in vacuum. The single crystal structure of Re(CO)3(FPYOZ)Br was used as the initial structure. Graphical presentation for the frontier MOs was plotted by wxMacMolPlt with contour value of 0.025. 1H NMR spectra were obtained with a Varian INOVA 300 spectrometer. Elemental analysis was performed on a Carlo Erba 1106 elemental analyzer. The single crystal data were collected on a Siemens P4 single crystal X-ray diffractometer with a Smart CCD-1000 detector and graphite-monochromated Mo Ka radiation at 298 K, with operating voltage and current of 50 kV and 30 A, respectively. All
Scheme 1. The synthetic procedure for FPYOZ ligand and its Re(I) complex of Re(CO)3(FPYOZ)B.
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hydrogen atoms were calculated. UV–Vis absorption spectra were taken on a Shimadzu UV-3101PC spectrophotometer. PL and excitation spectra were measured with a Hitachi F-4500 fluorescence spectrophotometer. PL quantum yield was measured according to a literature method [11]. Luminescence decay data were measured by a 355 nm light generated from the third-harmonic-generator pump, using pulsed Nd:yttrium aluminum garnet (YAG) laser as the excitation source. Thermogravimetric analysis (TGA) on Re (CO)3(FPYOZ)Br was performed with a thermal analysis instrument (SDT2960, TA Instruments, New Castle, DE) with heating rate of 10 °C/min. Electrochemical measurement of Re(CO)3(FPYOZ)Br was performed on a CHI830b electrochemical workstation (CH Instruments, Shanghai Chenhua Instrument Corporation, China) in a conventional three-electrode cell, with a platinum-sheet working electrode, a platinum-wire counter electrode and a silver/silver nitrate (Ag/Ag+) reference electrode. The voltammogram was recorded in CH3CN solution with 103 M sample and 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte. Before electrochemical measurement, the solution was bubbled with nitrogen for 5 min to eliminate the dissolved O2. All measurements were carried out in the air at room temperature without being specified. 3. Results and discussions 3.1. Molecular structure analysis on Re(CO)3(FPYOZ)Br 3.1.1. Geometric structure The desired diamine ligand possessing both enlarged conjugation chain with electron-pulling group and fluorine atom in its molecular structure could be confirmed by the single crystal structure of Re(CO)3(FPYOZ)Br. As shown in Fig. 1A, pyridine group and 4-fluorophenyl group were connected to a 1,3,4-oxadiazole ring through two r bonds. The three rings were coplanar with each other, resulting in an enlarged conjugation chain with electronpulling group (1,3,4-oxadiazole) and fluorine atom, which was expected to be helpful for PL and EL improvement. In addition, the p–p attraction between FPYOZ ligands made Re(CO)3(FPYOZ)Br molecules adopt a highly-ordered arrangement, as shown by Fig. 1B. In this case, the FPYOZ planes of Re(CO)3(FPYOZ)Br molecules aligned almost parallel to each other, with FPYOZ–FPYOZ intersection angle of only 0.07° and mean distance of 3.560 Å,
which confirmed that there was face-to-face p–p attraction between FPYOZ planes. It has been reported by Zhang and coworkers that such highly-ordered arrangement can be considered as a rigid one and can effectively depress the geometric relaxation that happens in complex excited state, leading to improved PL performance [12]. As for the coordination structure of Re(CO)3(FPYOZ)Br, the Re(I) ion was surrounded by two N atoms from one FPYOZ ligand, three C atoms from CO groups and one Br atom, showing a distorted octahedral coordination environment, as suggested by Fig. 1A and Table 1. The two Re–C bond length values of Re–C(1) and Re–C(3) were similar with each other, owing to their similar coordination nature. While, Re–C(2) bond was slightly longer than the other two Re–C bonds, which might be caused by the coordinated Br atom localizing on the opposite site of C(2) atom. The Re–Br bond length value was comparable with literature values [4]. As for the two Re–N bonds, there was also a slight difference between them: the bond length value of Re–N(1) was longer than that of Re–N(2), suggesting that the coordination attraction between Re(I) ion and the N atom from pyridine [N(1)] was weaker than that between Re(I) ion and the N atom from 1,3,4-oxadiazole group [N(2)]. The electron-donating N(3) and O(4) atoms in 1,3,4-oxadiazole group should be responsible for the coordination attraction difference. The distortion of coordination sphere around Re(I) ion might also be partially be responsible for the difference between the two Re–N bonds, which could be confirmed by the following fact. The N(1)–Re–N(2) bite angle was found to be obviously smaller than those in tetrahedral coordination cases (80°) [5]. In other words, the coordination distortion was triggered by the small steric hindrance from CO and Br ligands, leading to the variation of Re–N bond length.
3.1.2. Electronic structure of frontier MOs As mentioned above, the occupied frontier MOs usually own predominant metal d character, while the unoccupied frontier MOs are essentially p of N–N ligand. Since frontier MOs of transition metal complexes are usually involved in photo-induced optoelectronic processes, the composition of Re(CO)3(FPYOZ)Br frontier MOs should be analyzed [4,5]. With the crystal structure of Re(CO)3(FPYOZ)Br on hand, we decided to run a TD-DFT calculation on Re(CO)3(FPYOZ)Br at RB3PW91/SBKJC level, which had been
Fig. 1A. Single crystal structure of Re(CO)3(FPYOZ)Br.
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Fig. 1B. The p–p attraction Re(CO)3(FPYOZ)Br molecules.
Table 1 Selected bond length and bond angle values of Re(CO)3(FPYOZ)Br obtained from single crystal. Bond length
Å
Bond angle
Degree
Re–C(1) Re–C(2) Re–C(3) Re–Br(1) Re–N(1) Re–N(2)
1.896 1.926 1.891 2.611 2.224 2.164
N(1)–Re–N(2) N(1)–Re–C(2) N(2)–Re–C(2) N(1)–Re–Br(1) N(2)–Re–Br(1) C(1)–Re–Br(1) C(3)–Re–Br(1)
73.24 91.92 95.55 86.36 84.81 92.62 90.19
proved to be a powerful tool to reveal the electronic structure of transition metal complexes [4,5]. The composition of frontier MOs and the first 10 singlet excitations of Re(CO)3(FPYOZ)Br were listed in Table 2. The graphical
presentation for HOMO and LUMO was also shown in Fig. 2 to give a clear view on the presentative MOs. As suggested by Table 2 and Fig. 2, the occupied frontier MOs of HOMO, HOMO-1, HOMO-2, HOMO-3 and HOMO-4 were composed of contributions from Re atom, Br atom and CO group, showing a mixed character. The unoccupied MOs of LUMO, LUMO + 1 and LUMO + 2 were essentially p of FPYOZ ligand owing to its dominant contribution. As shown by Fig. 2, LUMO mainly localized on the conjugation plane of FPYOZ ligand. This large conjugation plane was expected to be positive to help charge transportation and trapping during EL process. The first ten excitations arose from electronic transitions from occupied frontier MOs to unoccupied ones. These transitions were thus assigned as a mixed character of metal-to-ligand-chargetransfer and ligand-to-ligand-charge-transfer (ML&LLCT) [4,5,12]. Considering that the occupied frontier MOs were basically independent of N–N ligand variation due to its slim contribution to these MOs, it was thus theoretically confirmed that the chemical
Table 2 Composition of frontier MOs and first 10 singlet excitations calculated at RB3PW91/SBKJC level. MO/transition
LUMO + 2(73) LUMO + 1(72) LUMO(71) HOMO(70) HOMO-1(69) HOMO-2(68) HOMO-3(67) HOMO-4(66) S0 ? S1 S0 ? S2 S0 ? S3 S0 ? S4 S0 ? S5 S0 ? S6 S0 ? S7 S0 ? S8 S0 ? S9 S0 ? S10
Energy (eV)
2.240 2.601 3.437 5.845 5.968 6.808 7.140 7.238 1.6678 1.8994 2.5758 2.6346 2.7636 2.9657 2.9718 3.0892 3.2180 3.5634
Contribution (%)
Nature
Re
FPYOZ
CO
Br
0.3 1.7 4.2 35.2 27.8 61.7 37.1 36.0
98.8 94.9 90.4 4.5 10.1 10.0 14.2 13.3
0.9 2.5 3.6 18.1 13.9 27.6 15.5 16.7
0.1 0.9 1.8 42.2 48.2 0.7 33.2 34.0
70 ? 71(97.9) 69 ? 71(97.7) 68 ? 71(97.9) 70 ? 72(95.4) 69 ? 72(96.0) 70 ? 73(88.5) & 66 ? 73(5.5) 67 ? 71(87.4) & 70 ? 73(5.9) & 66 ? 71(4.5) 69 ? 73(96.7) 66 ? 71(88.5) & 67 ? 71(6.0) & 70 ? 73(3.4) 68 ? 72(95.7)
FPYOZ FPYOZ FPYOZ Re & Br Br & Re Re & CO Re & Br Re & Br ML/LLCT ML/LLCT ML/LLCT ML/LLCT ML/LLCT ML/LLCT ML/LLCT ML/LLCT ML/LLCT ML/LLCT
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Fig. 2. The graphical presentation for HOMO (down) and LUMO (up) of Re(CO)3(FPYOZ)Br.
modifications on N–N ligand could adjust the excited state character without affecting irrelevant MOs. In other words, the PL performance of Re(CO)3(FPYOZ)Br was expected to be improved by introducing enlarged conjugation chain and F atom into its N–N ligand, without compromising the occupied frontier MOs. 3.2. Photophysical character of Re(CO)3(FPYOZ)Br 3.2.1. UV–Vis absorption and excitation The UV–Vis absorption spectra of Re(CO)3(FPYOZ)Br and free FPYOZ ligand measured in CHCl3 solutions (10 lM) were shown in Fig. 3. The absorption spectrum of free FPYOZ ligand was com-
Fig. 3. The UV–Vis absorption spectra of Re(CO)3(FPYOZ)Br and free FPYOZ ligand in CHCl3 solutions (10 lM), along with the absorption spectrum of free FPYOZ.
posed of five absorption bands of 235 nm, 263 nm, 295 nm, 308 nm and 345 nm, ending at 356 nm. While, there were four absorption bands for the absorption spectrum of Re(CO)3 (FPYOZ)Br, peaking at 235 nm, 285 nm, 342 nm and 400 nm, respectively, with absorption edge of 500 nm. The high energy absorption bands of 235 nm, 285 nm and 342 nm were similar to those of free FPYOZ ligand, suggesting that they were ligand p–p transitions in nature. The slight difference in spectral shape might be caused by the electronic delocalization during coordination. The low energy band of 400 nm, however, was a newly-generated one compared with the absorption spectrum of free FPYOZ ligand. Taking above TD-DFT calculation result into account, this low energy absorption band could be attributed to the electronic transitions of ML&LLCT [4]. A literature report had suggested that the MLCT absorption of Re(CO)3(Phen)Br (Phen = 1,10-phenanthroline) localized at 405 nm, ending at 500 nm [4]. After the comparison between the absorption spectra of Re(CO)3 (FPYOZ)Br and Re(CO)3(Phen)Br, it was observed that the large conjugation chain in FPYOZ ligand did not move the MLCT transition energy of Re(CO)3(FPYOZ)Br to low energy region. This observation conflicted with Zhang’s report that large conjugation system in diamine ligand could decrease the energy required for the onset electronic transition of metal complexes, narrowing the optical edge (band gap) [5]. The electron-pulling effect from 1,3,4-oxadiazole group and F atom in FPYOZ ligand might be responsible for this phenomenon. In other words, electron-pulling substituent groups in diamine ligands could depress the negative effect caused by oversized conjugation system in diamine ligand. The excitation spectrum of Re(CO)3(FPYOZ)Br was composed of three bands at 340 nm, 388 nm and 430 nm. It was also observed that the optimal excitation window localized in the low energy region from 365 nm to 465 nm, whose corresponding absorption intensity was rather weak as shown in Fig. 3. The intense absorption ranging from 200 nm to 350 nm was ineffective on exciting the emissive state of Re(CO)3(FPYOZ)Br, which could be explained as follows. As mentioned above, the intense absorption was attributed to p–p transitions of FPYOZ ligand, while the weak absorption was assigned to the mixture of ML&LLCT. Although the molar extinction coefficient of ML&LLCT transitions was much lower than that of p–p transitions [11], the energy transfer efficiency of ML&LLCT excited state to the emissive state was much higher than that of p–p excited state due to the fact that the emissive state of Re(CO)3(FPYOZ)Br derived from the first singlet transition of S0 ? S1. The p–p excited state had to experience a series of energy-exhausting potential surface crossing procedures to finally transfer its energy to the emissive center, leading to its poor efficiency of exciting the emissive center. 3.2.2. PL spectrum and quantum yield The PL spectrum of Re(CO)3(FPYOZ)Br measured in CHCl3 solution (10 lM) was shown in Fig. 4. The emission band showed a maximum at 537 nm, without giving any vibronic progressions, and the full-width-at-half-maximum (FWHM) value of this emission band was as large as 75 nm. Above characters indicated that the emissive state owned a charge transfer (CT) character. Considering that the emissive state derived from the singlet transition of S0 ? S1 which owned CT character, it was reasonable to observe that the emissive state also owned a CT character. The Stokes shift between its absorption edge (500 nm) and emission peak (537 nm) was only 37 nm. Thus, the geometric relaxation that occured in Re(CO)3(FPYOZ)Br excited state had been effectively suppressed, which might be caused by the face-to-face p–p attraction between Re(CO)3(FPYOZ)Br molecules. A literature report had suggested that the emission of Re(CO)3(Phen)Br localized at 554 nm, with FWHM value of 90 nm [4]. After the comparison between the emission spectra of Re(CO)3(FPYOZ)Br and Re(CO)3(Phen)Br, it
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two components all contributed to the emissive center with identical decay times. 3.3. Thermal stability and electrochemical property of Re(CO)3(FPYOZ)Br
Fig. 4. The PL spectrum of Re(CO)3(FPYOZ)Br measured in CHCl3 solution (10 lM, kex = 375 nm). Inset: the emission decay characteristic of Re(CO)3(FPYOZ)Br.
was concluded that the enlarged conjugation chain with electronpulling group (1,3,4-oxadiazole group) and fluorine atom could move the emission to high energy region, as well as narrowing the emission band. PL quantum yield, which is defined as the ratio of the number of photons emitted to the number of photons absorbed, is an important factor for both PL and EL applications. This parameter can be determined by the following formula, where the subscripts s and r refer to sample and reference standard solution respectively, n is the refractive index of the solvent, D is the integrated intensity, and U is PL quantum yield.
Us ¼ Ur ðBr =Bs Þðns =nr Þ2 ðDs =Dr Þ
ð1Þ
Considering that all functional layers in EL devices are usually deposited by resistive heating method, the emitting material of Re(CO)3(FPYOZ)Br should be thermally stable enough to experience vacuum evaporation during device construction. The TGA curve of Re(CO)3(FPYOZ)Br was shown in Fig. 5. It was observed that the TGA curve was composed of two weight loss regions. The first region ranging from 320 °C to 410 °C was attributed to the thermal release of FPYOZ ligand, where the theoretical weight loss was 40.8% and the measured one was 42.8%. The second region ranging from 410 °C to 600 °C should be assigned to the thermal decomposition and release of ligands such as CO and Br ligands. Never the less, the smooth TGA curve below 320 °C indicated that Re(CO)3(FPYOZ)Br was thermally stable enough below 300 °C, making itself thermally stable enough to experience EL device construction through vacuum evaporation. On the other hand, the energy levels of functional materials are critical when designing EL device structures. Thus, the electrochemical property of Re(CO)3(FPYOZ)Br had been determined by cyclic voltammetry (CV) at room temperature. As shown in Fig. 6, Re(CO)3(FPYOZ)Br exhibited one irreversible oxidation peak with onset potential of 0.97 V and one reduction wave with onset potential of 1.36 V, respectively. The energy level values of HOMO (EHOMO) and LUMO (ELUMO) were calculated according to the followRed ing equations, where EOxd onset and Eonset denoted the onset potential values of oxidation and reduction peaks, respectively.
EHOMO ¼ ðEOxd onset þ 4:4ÞeV
ð2Þ
ELUMO ¼ ðERed onset þ 4:4ÞeV
ð3Þ
The quantity B is calculated by B = 1 10AL, where A is the absorption coefficient at the excitation wavelength and L is the optical length. With its PL spectrum and absorption data on hand, the PL quantum yield of Re(CO)3(FPYOZ)Br was measured to be 0.11. This value, however, seemed to be not satisfactory enough compared with literature reports whose quantum yields were usually higher than 0.50 [13,14]. It seemed that the enlarged conjugation chain with electron-pulling group (1,3,4-oxadiazole group) and F atom could compromise the PL quantum. We attributed the causation to the strong electron-pulling effect of 1,3,4oxadiazole group and F atom which might decrease the radiative constant of emissive state [15].
The corresponding values of Re(CO)3(FPYOZ)Br were calculated to be 5.37 eV for EHOMO and 3.04 eV for ELUMO, respectively. The energy gap between HOMO and LUMO was then observed to be 2.33 eV. This value, however, was smaller than the optical edge of 2.48 eV (500 nm) but quite similar to the emissive energy of 2.31 eV (537 nm), suggesting that the solvent effect distorted Re(CO)3(FPYOZ)Br molecular structure and decreased the band gap [17]. This complex with such a narrow energy gap of 2.33 eV was suitable to be used as a guest in host–guest EL devices. Guided by above data, in this work, CBP was chosen to be the host for EL
3.2.3. Excited state lifetime The excited state lifetime (s) of an emitting material is another crucial factor for EL application. According to literature reports by Zhang and coworkers [11,16], a long excited state lifetime of emitting material could lead to emission saturation of EL devices at high current densities, compromising both EL efficiency and brightness. Consequently, short excited state lifetime is desired for the emitting materials in EL application to minimize the efficiency roll-off at high current densities. The emission decay characteristic of Re(CO)3(FPYOZ)Br in CHCl3 solution (10 lM) was shown by the inset of Fig. 4. The yellow emission followed monoexponential decay pattern with s = 8.35 ls. This long-lived excited state indicated its phosphorescent nature. Above TD-DFT calculation result suggested that the emissive state owned a character of ML&LLCT. Thus, the emissive state could be described as 3 (ML&LLCT). In addition, it was observed that even though the emissive state owned a mixed character, the emissive state followed monoexponential decay pattern, suggesting that the
Fig. 5. The TGA curve of Re(CO)3(FPYOZ)Br.
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Fig. 6. CV curve of Re(CO)3(FPYOZ)Br.
devices because of its wide energy gap (3.3 eV), as well as its higher LUMO level (2.7 eV) and lower HOMO level (6.0 eV) compared with those of Re(CO)3(FPYOZ)Br [18]. 3.4. EL performance of Re(CO)3(FPYOZ)Br Above results suggested that Re(CO)3(FPYOZ)Br could be developed as an emitting dopant for EL application, owing to its promising characters of proper emission energy, good thermal stability and well-suited energy levels. In addition, it has been mentioned that enlarged conjugation chain with electron-pulling group and fluorine atom were introduced into the Re(I) complex to improve PL and EL performance. In order to evaluate this hypothesis, we investigated the EL performance of Re(CO)3(FPYOZ)Br. In this initial effort, a classic structure of ITO/m-MTDATA (30 nm)/NPB (25 nm)/ CBP:Re(CO)3(PTO)Br (30 nm)/Bphen (20 nm)/Alq3 (30 nm)/LiF/Al was used to explore the potential of Re(CO)3(FPYOZ)Br in EL application. Here, m-MTDATA was the hole-injection layer, NBP was the hole-transporting layer, Bphen and Alq3 were exciton-blocking layer and electron-transporting layer, respectively. Firstly, four dopant concentrations were tried to optimize device performance. The EL spectra of the four devices under applied voltage of 8 V were shown in Fig. 7. When the dopant concentration was as low as 10%, a slight blue emission peaking at 440 nm was observed. Clearly, there were excess excitons within the emitting layer, and the diffusion of these excitons made them be captured by neighbor functional layers, leading to the blue emission.
Fig. 7. The EL spectra of the four devices under applied voltage of 8 V.
Since there was an exciton-blocking layer which prevented exciton penetration in these EL devices, the excess excitons might localize in CBP or NBP layers. According to literature reports [18–20], the fluorescence from CBP peaked at 390 nm. Thus, the blue emission of 440 nm should be assigned to NPB emission. In other words, the generated excitons in the emitting layer were not completely captured by the emitting dopant of Re(CO)3(FPYOZ)Br. The excess excitons moved into NBP layer, leading to NPB emission. With the increasing dopant concentrations, NBP emission disappeared, with only pure emission from Re(CO)3(FPYOZ)Br left. It was then observed that the EL spectra of Re(CO)3(FPYOZ)Br peaking at 562 nm gave a red shift tendency compared with its PL spectrum peaking at 537 nm. It seemed that Re(CO)3(FPYOZ)Br excited state could be stabilized by CBP host, leading to spectral red shift, and this observation was consistent with literature reports [18–20]. The energy transfer mechanism between CBP and Re(CO)3 (FPYOZ)Br was then analyzed as follows. Considering that there was an obvious overlap between CBP emission (390 nm) and Re(CO)3(PTO)Br absorption/excitation [19], there might be two possible pathways. One was Förster energy transfer from singlet CBP excited state to ML&LLCT excited state of Re(CO)3(FPYOZ)Br, the other was charge trapping energy transfer since the energy gap of Re(CO)3(FPYOZ)Br fell in that of CBP. To identify the dominating energy transfer mechanism, the current density versus voltage characteristics of the four EL devices were shown in Fig. 8. It could be seen that the current density value generally increased with increasing dopant concentrations, suggesting that high Re(CO)3(FPYOZ)Br concentration in CBP layer was helpful to enhance charge carrier transportation within those EL devices. Above fact denied the possibility of charge trapping energy transfer mechanism, and Förster energy transfer mechanism should be responsible for the major energy transfer mechanism. The enlarged conjugation chain with electron-pulling group and fluorine atom in FPYOZ ligand might be helpful to achieve charge carrier balance, as expected above. The inset of Fig. 8 showed the current efficiency versus current density characteristics of the four EL devices. It was observed that the current efficiency firstly increased with increasing current densities within low current density region. After reaching the maximum values, the current efficiency decreased sharply with increasing current densities. This efficiency roll-off was attributed to triplet–triplet and triplet-polar annihilations [21]. To avoid such efficiency roll-off, the excited state lifetime of Re(CO)3(FPYOZ)Br should to be further shortened to achieve high efficiency at high
Fig. 8. The current density versus voltage characteristics of the four EL devices. Inset: the current efficiency versus current density characteristics of the four EL devices.
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ML&LLCT. Further analysis revealed that Re(CO)3(FPYOZ)Br has HOMO and LUMO energy levels at 5.37 eV and 3.04 eV, respectively. TGA data suggested that it owned a high thermal decomposition temperature of 300 °C. Using Re(CO)3(FPYOZ)Br as the emissive dopant, an electroluminescence peaking at 562 nm was realized, with a maximum luminance of 6250 cd/m2 and a maximum current efficiency of 7.3 cd/A. The low emission yield and the long excited state lifetime, however, compromised its EL performance. Future efforts may be devoted to diamine ligand structure design and device optimization for performance improvement. Acknowledgments
Fig. 9. The luminance-current density characteristics of the 12% doped device under various voltages.
current density. The maximum current efficiency of 7.3 cd/A was achieved by the 12% doped device at 155 mA/cm2. Fig. 9 showed the luminance-current density characteristics of the 12% doped device under various voltages. Upon driving voltage of 15 V, a maximum brightness of 6250 cd/m2 was achieved with current efficiency of 1.32 cd/A and current density of 427 mA/cm2. Even higher driving voltages compromised the brightness, owing to the triplet-polar annihilation at high current densities. The maximum brightness of this work, however, was not satisfactory enough. Considering the suited energy levels and good thermal stability of Re(CO)3(FPYOZ)Br, the low PL quantum yield and the long excited state lifetime should be the causation for the underdeveloped brightness, indicating that the diamine ligand structure should be further modified to improve the EL performance. 4. Conclusions In this paper, a diamine ligand with enlarged conjugation chain with electron-pulling group and fluorine atom and its corresponding Re(I) complex were synthesized and studied, including the molecular structure, electronic structure, photophysical performance, thermal stability and electrochemical property. Re(CO)3 (FPYOZ)Br was a yellow emitter peaking at 537 nm with excited state lifetime of 8.35 ls. Single crystal analysis and theoretical calculation suggested that the emission owned a mixed character of
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