Article pubs.acs.org/cm
A Simple Design for Strongly Emissive Sky-Blue Phosphorescent Neutral Rhenium Complexes: Synthesis, Photophysics, and Electroluminescent Devices Wing-Kin Chu,† Chi-Chiu Ko,*,† Kin-Cheung Chan,† Shek-Man Yiu,† Fu-Lung Wong,‡ Chun-Sing Lee,‡ and V. A. L. Roy*,‡ †
Department of Biology and Chemistry, and ‡Center of Super Diamond and Advanced Films (COSDAF), Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong, People’s Republic of China S Supporting Information *
ABSTRACT: A simple design strategy for a new class of stable, vacuum-sublimable, and strongly emissive sky-blue neutral phosphorescent Re(I) phenanthroline complexes {Re(R2phen)(CO)3[CNB(C6F5)3]} is reported. These complexes show intense bluish green emission in CH2Cl2 solution with the highest emission quantum yield and bluest emission ever reported for the neutral Re(I) diimine complexes. In the solid state, they display sky-blue emission. The electroluminescent properties of devices containing these complexes have also been investigated.
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INTRODUCTION Over the past few decades, tricarbonyl Re(I) diimine complexes have been extensively studied1 and found applications in various areas2−4 such as photocatalysis,2 photosensitization,3 chemosensing, and biological labeling.4 Despite the well-known phosphorescence of the Re(I) diimine complexes,1 their applications in light-emitting devices remain relatively less explored.5 In contrast, applicability of neutral dinuclear Re(I) complexes for efficient electroluminescent (EL) device applications has been reported in recent years.6 This lack of research is probably due to the poor processability of many of the highly emissive tricarbonyl Re(I) diimine complexes, which are largely unsublimable, as many of them are cationic complexes. Although sublimable neutral tricarbonyl Re(I) diimine complexes with various anionic ancillary ligands have also been reported, the emission energy and quantum efficiency of most of these neutral Re(I)−diimine emitters are usually much lower as compared to those of the cationic complexes.1k,7 To improve the emission properties of the neutral tricarbonyl Re(I) diimine complexes, we proposed to incorporate a strong π-accepting anionic ligand. With reference to our recent work on various series of isocyano rhenium(I) diimine complexes, [Re(N−N)(CO)n(CNR)4−n]+ where n = 0−2, which possessed excellent tunability and emission properties due to the strong and tunable π-accepting ability of the isocyanide ligands,8 we proposed to design neutral Re(I) diimine complexes with enhanced luminescent properties by incorporating an anionic isocyanide ligand. Herein, we report the synthesis, structures, photophysical, and electrochemical properties of the tricarbonyl Re(I) phenanthroline complexes with isocyanotris(pentafluorophenyl)borate ligand. This work also demonstrates © 2014 American Chemical Society
the high thermal stability of the isocyanotris(pentafluorophenyl)borato complexes. Because of the stability upon sublimation and their reversible electrochemical behavior, electroluminescent devices using these complexes as emissive dopants have also been investigated.
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RESULTS AND DISCUSSION Synthesis and Characterizations. Reactions of the cyano complexes P1 and P2 with B(C6F5)3 in dichloromethane solution under reflux produce highly stable, sublimable, and strong sky-blue phosphorescent complexes 1 and 2, fac{Re(R2phen)(CO)3[CNB(C6F5)3]} [R2phen = 1,10-phenanthroline (phen) and 4,7-dimethyl-1,10-phenanthroline (Me2phen)] (Scheme 1). All complexes had been characterized by 1H and 19F NMR spectroscopy, IR spectroscopy, mass spectrometry, and gave satisfactory elemental analysis. Complexes 1 and 2 were characterized by three strong CO stretches (1920−2040 cm−1) and one CN stretch at ca. 2200 cm−1 in their IR spectra, typically observed in other facial Scheme 1. Synthetic Routes to 1 and 2
Received: November 22, 2013 Revised: March 21, 2014 Published: March 23, 2014 2544
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tricarbonyl isocyano Re(I) diimine complexes.9 Although a stronger π-backbonding in [Re−CNB(C6F5)3] is expected on the formation of the Lewis adduct with the borane, the higher stretching frequencies of CN stretches in 1 and 2 (∼2200 cm−1) as compared to those of the cyano complexes P1 and P2 (∼2120 cm−1) were attributed to the Lewis acid−base interaction and electron-withdrawing properties of B(C6F5)3, which significantly reduced the electron population in the antibonding orbital of CN bond, thus strengthening the C N bond. The much stronger π-backbonding in [Re−CNB(C6F5)3] than in [Re−CN] is also reflected in the higher stretching frequency of CO stretches and more anodic potentials for the ReI/II oxidation in 1 and 2 as compared to their cyano precursors (see below). X-ray Crystal Structure. The structures of 1 and 2 have been determined by X-ray crystallography. Perspective drawings of 1 and 2 are depicted in Figure 1. The experimental
Figure 2. Overlaid (a) UV−vis absorption and (b) corrected emission spectra of P1, P2, 1, and 2 in CH2Cl2 solution upon excitation at 380 nm at 298 K.
Table 1. UV−Vis Absorption Data of P1, P2, 1, and 2 in CH2Cl2 Solution complex P1 P2 1 2
absorption λabs/nm (ε/M−1 cm−1) 262 (28 550), 289 260 (29 250), 281 255 (35 265), 269 355 sh (5810) 260 (34 675), 266 350 sh (6660)
(12 960), 357 (4480), 397 sh (3130) (15 980), 349 (5575), 383 sh (4005) (33 920), 273 (33 990), 325 (8410), (32 570), 273 (34 165), 321 (10 010),
shoulder of these complexes followed the order of 2 (350 nm) > 1 (355 nm) ≫ P2 (383 nm) > P1 (397 nm), which is in line with the π-accepting ability of the ancillary ligands (isocyanoborate > cyanide) and the decreasing π-accepting ability of the phenanthroline ligands (Me2phen < phen). This suggested the assignment of a predominant MLCT [dπ(Re) → π*(R2phen)] mixed with the LLCT {π[CNB(C6F5)3] → π*(R2phen)} transitions. The significant blue-shift (>2400 cm−1) of the lowest-energy absorption for 1 and 2 versus the corresponding cyano complexes P1 and P2 indicated that the π-accepting ability of the isocyanoborate ligand was significantly stronger than that of the cyanide ligand. All of the complexes were photostable and exhibited longlived phosphorescence with the corrected emission maxima in the range of 488−575 nm (Figure 2b, Table 2) and quantum efficiency (Φem) from 0.12−0.54 in degassed CH2Cl2 solution (∼0.028 mM) at room temperature upon excitation with λ < 400 nm. These emissions were independent of the excitation wavelength. The structureless emissions of P1, P2, and 1 were ascribed to the 3MLCT [dπ(Re) → π*(R2phen)] phosphorescence mixed with LLCT {π[CNB(C6F5)3] → π*(R2phen)} character. The different emission profile of 2 including the structured emission band and the considerably longer emission lifetime (τo ≈ 35 μs) suggested the significant mixing of 3LC character in the emissive excited state. A substantial blue-shift of the emission and considerable enhancement of the emission quantum yield (0.52−0.54 vs 0.12−0.2) were clearly noted on the conversion of cyano complexes P1 and P2 to their B(C6F5)3 Lewis adduct. To the best of our knowledge, the solution emissions of 1 and 2 are among the bluest and of the highest quantum efficiency for all neutral tricarbonyl Re(I) bipyridine or phenanthroline complexes [Re(CO)3(N−N)X] reported in the literature1k [N−N = phen: X = Cl (606 nm; Φem = 1.3%),1d,7f X = Br (570 nm),7g X = 3,5-bis(trifluoromethyl)pyrazol-1-ide (568 nm; Φem = 2.2%);5b N− N = bpy: X = Cl (622 nm; Φem = 0.5%),7a X = OTf (579 nm; Φem = 2.2%),7e X = NCS (635 nm; Φem = 0.3%),2c X = C Cpy (627 nm; Φem = 0.4%),7e X = O2CCH3 (640 nm; Φem = 0.4%),7c X = CCR (640−670 nm)7d] and the neutral dinuclear Re(I) complexes [Re2(μ-Cl)2(CO)6(diazine)] (547−
Figure 1. Perspective drawings of (a) 1 and (b) 2 with atom numbering. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 30% probability level.
details for the crystal structure determinations and selected bond distances and bond angles are summarized in Supporting Information Tables S1 and S2, respectively. These complexes adopted a distorted octahedral geometry with a slightly bent isocyanide ligand. The derivation of the CN−B bond angles, 166.4° for 1 and 174.0° for 2, from the linearity is the result of the π-backbonding interaction from the rhenium metal center.8,10 The bond lengths of Re−CO and Re−CNB(C6F5)3 were in the range of 1.905−1.969 and 2.096−2.099 Å, respectively, which are close to those in other carbonyl and isocyano rhenium(I) complexes.8,10 Comparing the bonding parameter of the X-ray crystal structure of CH3CNB(C6F5)3,11 the isocyanoborate ligands in 1 and 2 showed a slightly longer CN bond distance (1.15 vs 1.12 Å). This could be attributable to the π-backbonding interaction between the rhenium metal center and isocyanoborate ligand, which rendered a weaker CN bond. Photophysical Properties. The electronic absorption properties of all of the complexes in CH2Cl2 solution were investigated (Figure 2a), and the data are summarized in Table 1. All of the complexes displayed intense ligand-centered (LC) π → π* absorption in the high-energy UV region (λ < 300 nm). In addition to intense LC absorption, moderately intense absorption with molar absorptivity on the order of 103 at λ ≥ 320 nm was observed. With reference to the spectroscopic studies of the Re(I) diimine complexes,1,8 this moderately intense absorption was ascribed to the mixing of the metal-toligand charge transfer (MLCT) transitions of {dπ(Re) → π*[CNB(C6F5)3]}, [dπ(Re) → π*(R2phen)] and the ligand-toligand charge transfer (LLCT) transition of {π[CNB(C6F5)3] → π*(R2phen)}. The energy of the lowest-energy absorption 2545
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glassy medium were further blue-shifted as compared to those in the solid state. As the emission of P2 in 77 K glassy medium became highly structured with exceptionally long-lived lifetime, the emission was assigned to 3LC excited-state origin. To provide insights into the doped electroluminescence (EL) device applications of 1 and 2 (see below), the emission properties of 1 and 2 in thin films (5 wt % in SPP013 and PMMA) have also been investigated (Table 2, Supporting Information Figure S3). Electrochemistry. The electrochemical behaviors of all complexes were investigated by cyclic voltammetry. The first oxidation and reduction potentials corresponding to the metalcentered oxidation (ReI/II)1,8 and the phenanthroline-based reduction1,3d,8b−e were determined and summarized in Table 3.
Table 2. Emission Data of P1, P2, 1, and 2 P1
P2
1
2
medium (T/K)
emissiona λem/nm (τo/μs)
Φemb
CH2Cl2 (298) solid (298) glassc (77) CH2Cl2 (298) solid (298) glassc (77) CH2Cl2 (298) solid (298) glassc (77) thin film (298) 5% in PMMA 5% in SPP013 CH2Cl2 (298) solid (298) glassc (77) thin film (298) 5% in PMMA 5% in SPP013
575 (2.0) 534 (0.5) 508 (18.2) 559 (5.6) 557 (0.6) 472, 506, 541 (208.3, 55.8) 538 (6.6) 463, 491, 520 (17.5) 461, 493, 527 (150.0, 46.2)
0.12 0.04 −d 0.20 0.07 −d 0.54 0.85 −d
504 (12.2) 513 (3.4) 488, 517 (34.5) 474, 508, 542 (14.0) 468, 502, 538 (735.3, 270.0)
0.42 0.16 0.52 0.23 −d
477, 508 (28.2) 483, 515 (5.1)
Table 3. Electrochemical Data of P1, P2, 1, and 2 in Acetonitrile Solution (0.1 M nBu4NPF6) at 298 Ka oxidation,b E1/2/V vs SCE [ΔEp/mV]d (Epa/V vs SCE)
0.22 0.12
P1 P2 1 2
a
Excitation at 380 nm. Emission maxima are corrected values. bFor CH2Cl2 solutions, the luminescence quantum yields were measured using quinine sulfate as a standard; for the samples in solid state (powder) and thin films, absolute luminescence quantum yields were measured with excitation at 350 nm. cEtOH/MeOH (4:1 v/v) glass at 77 K. dNot measured.
(1.48)c (1.44)c 1.75 [85]d 1.72 [81]d
reduction,b E1/2/V vs SCE [ΔEp/mV]d (Epc/V vs SCE) −1.36 −1.50 −1.31 −1.45
[91]d [69]d [85]d [97]d
Working electrode, glassy carbon; scan rate, 100 mV s−1. bE1/2 is (Epa + Epc)/2; Epa and Epc are the anodic and cathodic peak potentials, respectively. cIrreversible oxidation; anodic peak potential (Epa/V vs SCE). dΔEp is |Epa − Epc|. a
730 nm; Φem = 8−53%)].6 This can also be ascribed to the strong π-accepting isocyanoborate ligand. In the solid state, due to the rigidochromic behavior of these complexes,12 blue-shift of the emissions (Table 2, Supporting Information Figure S1) was observed. As a result, complex 1 displayed a sky-blue emission in the solid state (Figure 3). The
The representative cyclic voltammograms of 1 are shown in Figure 4. The cyano complexes P1 and P2 revealed an
Figure 4. Cyclic voltammograms of (a) oxidative scan and (b) reductive scan of 1 in acetonitrile solution (0.1 M nBu4NPF6). Scan rate: 100 mV s−1. Figure 3. Normalized emission spectra of 1 in solid and 5 wt % thin film doped in PMMA.
irreversible oxidation at +1.48 and +1.44 V vs SCE, respectively (Supporting Information Figures S4−S7), whereas their Lewis adducts complexes 1 and 2 (Figure 4, Supporting Information Figures S8−S10) displayed an electrochemically quasi-reversible oxidative couple at much more anodic potentials of +1.75 and +1.72 V vs SCE, respectively, with the peak-to-peak separations of ca. 85 mV. The considerably higher metalcentered oxidation potentials for 1 and 2 as compared to their cyano precursors were due to the much stronger π-accepting ability of the isocyanoborate ligand as compared to the cyanide ligand, which rendered a more stabilized dπ(Re) orbital in 1 and 2. The quasi-reversible reduction couples in the range of −1.31 to −1.50 V vs SCE with peak-to-peak separations ranging from 69 to 97 mV, attributable to phenanthroline-based reduction,1,3d,8b−e were observed in the reductive scan. The
emission bands of the cyano complexes, P1 and P2, in the solid state were also structureless and ascribed to MLCT excitedstate origin, whereas those of 1 and 2 became structured. In light of the highly structured emission bands in the solid state of 1 and 2, they were assigned to be derived from the mixing of the 3LC and 3MLCT excited state. The absolute luminescence quantum yields for the complexes in the solid state have also been determined (Table 2). In 77 K EtOH/MeOH (4:1, v/v) glassy medium, the emission bands of 1 and 2 were very similar to those in their solid state (Supporting Information Figure S2). This is supportive of the predominant 3LC character. In contrast, the emissions of the cyano complexes in the 77 K 2546
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Table 4. Electroluminescent Data for Doped Devices with 1 and 2 Doped at Different Concentrationsa doped device 1 1 1 2 2 2
(5%) (10%) (15%) (5%) (10%) (15%)
turn on/Vb 5 5.5 5.5 4.5 5.0 6
max CE (cd/A)c 8 9.4 7.7 5.7 7.9 5.7
± ± ± ± ± ±
0.1 0.4 0.1 0.1 0.3 0.3
max PE (lm/W)d
max EQE/%e
± ± ± ± ± ±
3.1 3.7 3.0 2.3 3.1 2.2
(5.5 V) (6 V) (6 V) (5 V) (5 V) (5.5 V)
5 5.3 4.0 3.6 5.0 3.2
0.3 0.2 0.1 0.1 0.3 0.2
λ/nm at 9 V 428, 518, 518, 440, 430, 492,
518, 535 535 492, 492, 515,
535
515, 530 515, 530 530
a
All of the devices with various doping concentrations were fabricated twice to ensure the reproducibility. The data were reported on the basis of the measurements of eight device pixels. bTurn on (V): the voltage applied to the device to achieve brightness of 1 cd/m2. cCE: Current efficiency. dPE: Power efficiency at the same voltage as max CE. eEQE: External quantum efficiency at the same voltage as max CE.
The CE and PE against the current density of the 10% doped devices are shown in Supporting Information Figures S12 and S13. The EL spectra of the 10% doped devices showed slightly structured bands with λmax at 535 nm (CIE of 0.3, 0.48) and 515 nm (CIE of 0.31, 0.51) at around 7−9 V for devices with 1 and 2, respectively. As the emission of SPP013 is peaking at 365 nm,13b the EL in the high energy region from 400 to 500 nm observed in 5% doped devices was attributed to the emissions of neighboring organic layers. As doping concentration increased to 10% or 15%, more efficient energy transfer between the host (SPP013) and dopants (1 and 2) occurred. This enhanced the exciton formation at the doped layer, suppressing the emissions from neighboring organic layers. As a result, the high-energy EL component significantly decreased or disappeared in 10% or 15% doped EL devices. The details of the EL performance of the 10% doped devices with 1 and 2 are shown in Supporting Information Figures S14−S17. The j−V curves of both devices with 1 and 2 exhibited typical diode characteristic. The PE−V−EQE curves of both devices with 1 and 2 exhibited gradual roll off of power efficiencies and external quantum efficiencies. Although the EL performance of these devices is not outstanding as compared to other previously published works with tricarbonyl rhenium(I) complexes,5k,l,6c,d their EL are of higher energy with maxima of 515 nm, which is a novel discovery of Re complexes in bluish green light-emitting applications.
high sensitivity of the reduction potential to the substituents on the phenanthroline ligand and the much less sensitivity to the change of the ancillary ligand from cyanide to isocyanoborate are in agreement with phenanthroline-based reduction assignment. Electroluminescent Properties. These Lewis adducts (1 and 2) showed exceptionally high thermal stability with no decomposition up to 270 °C under N2 atm as reflected in the TGA thermogram of 1 (Supporting Information Figure S11). They are sublimable at T ≈ 250 °C under vacuum of 10−6 Torr. The identities of sublimed complexes have been characterized by the 1H and 19F NMR spectroscopy. In view of the outstanding emission properties and the high thermal and electrochemical stability of 1 and 2, EL devices using 1 and 2 with a device configuration of ITO/HATCN (10 nm)/NPB (45 nm)/TCTA (5 nm)/SPP013: 1 or 2 (20 nm)/TmPyPB (20 nm)/BPhen (20 nm)/LiF (1 nm)/Al (150 nm) were fabricated by vacuum sublimation. On the basis of the electrochemical properties and emission energies of 1 and 2 (Supporting Information Table S3), SPP013 with HOMO, LUMO, and triplet energy of 6.56, 2.9, and 2.73 eV, respectively,13 was chosen as a suitable host material to match the energy levels and triplet energy of these complexes in each device group. Thus, efficient energy transfer between the host and the Re(I) complexes could be achieved. The results of the EL measurements of devices with 1 and 2 doped at 5%, 10%, and 15% are summarized in Table 4. The EL spectra of these devices are revealed in Figure 5. These devices
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CONCLUSION We have developed a new strategy for designing strongly emissive and vacuum-sublimable neutral Re(I) diimine complexes using the anionic isocyanotris(pentafluorophenyl)borate ligand. Because of the exceptionally strong π-accepting ability of the isocyanoborate ligand, the emission energy and quantum yields of these complexes are higher than or comparable to most of their cationic Re(I) complex analogues with neutral ligands including phosphines, pyridine, and acetonitrile. It is anticipated that this strategy should also be applicable for developing new series of neutral and sublimable phosphorescent materials from many of the well-known cationic triplet emitters. Given the excellent processability, high thermal stability, and the good electrochemical properties of the isocyanotris(pentafluorophenyl)borato complexes 1 and 2, electroluminescent devices using 1 and 2 as emissive dopants have been fabricated. The 10% doped devices showed higher efficiencies than the 5% and the 15% doped devices. This report confirmed the stability and feasibility of using isocyanotris(pentafluorophenyl)borato complexes for device application. By suitable modification of the diimine and isocyanoborate ligands of the current complex system together with device
Figure 5. EL spectra of doped devices containing (a) 1 and (b) 2 with various dopant concentrations.
exhibited turn on voltage from 4.5 to 6.0 V. Comparing devices with different concentrations of dopants, the 10% doped devices of 1 and 2 showed the best performance with maximum current efficiency (CE) and power efficiency (PE) of 9.4 cd/A and 5.3 lm/W for device with 1; and 7.9 cd/A and 5.0 lm/W for device with 2, respectively. The maximum efficiencies of these devices are recorded at low brightnesses (1−10 cd/m2). 2547
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(UVO3) cleaner. After the UVO3 treatment, the ITO glass substrates were loaded into a vacuum deposition system with a base pressure of around 7 × 10−7 Torr. Complexes 1 and 2 were purified by sublimation and characterized prior to using for device fabrication. Two groups of devices were fabricated according to the device configuration of ITO/HATCN (10 nm)/NPB (45 nm)/TCTA (5 nm)/SPP013: 1 or 2 (20 nm)/TmPyPB (20 nm)/BPhen (20 nm)/ LiF (1 nm)/Al (150 nm). The organic, inorganic, and metal layers were sequentially deposited on the ITO glass substrates by thermal evaporation. HATCN, NPB, TCTA, SPP013, TmPyPB, BPhen, LiF, and aluminum were accountable for hole injection layer (HIL), hole transport layer 1 (HTL-1), HTL-2, emitting host layer (EHL), electron transport layer 2 (ETL-2), ETL-1, electron injection layer (EIL), and cathode, respectively. In each device group, 5, 10, and 15 vol % of 1 or 2 was doped in SPP013 layer. Therefore, three devices for 1 and another set of three devices for 2, each with four 0.1 cm2 light-emitting pixels (Supporting Information Figure S18), were fabricated for device optimization and comparison. After device fabrication, all of the emitting spots in the devices were then taken to ambient for EL measurement. For the measurement, the voltage− current−brightness (I−V−B) characteristics, the International Commission on Illumination (CIE) coordinates, and the EL emission spectra were measured by a Spectra PR650 CCD camera with a computer controller power supply. The current and power efficiencies of the device then were calculated from the measured data. All of the device groups with different doping concentrations were fabricated twice to ensure the reproducibility of the experimental data. Crystal Structure Determination. The crystal structures were determined on an Oxford Diffraction Gemini S Ultra X-ray singlecrystal diffractometer using graphite monochromatized Cu−Kα (λ = 1.54178 Å) or Mo−Kα (λ = 0.71073 Å) radiation. The structure was solved by direct methods employing the SHELXL-97 program15 on PC. Re and many non-H atoms were located according to the direct methods. The positions of other non-hydrogen atoms were found after successful refinement by full-matrix least-squares using the SHELXL97 program15 on PC. In the final stage of least-squares refinement, all non-hydrogen atoms were refined anisotropically. H atoms were generated by the program SHELXL-97.15 The positions of H atoms were calculated on the basis of the riding mode with thermal parameters equal to 1.2 times that of the associated C atoms, and participated in the calculation of final R-indices. Synthesis. Cyanotricarbonyl(1,10-phenanthroline) Rhenium(I), [Re(phen)(CO)3(CN)] (P1). This complex was synthesized according to a modified procedure for related cyano Re(I) diimine complexes.4h To a solution of {[Re(phen)(CO)3(MeCN)]OTf} (200 mg, 0.31 mmol) in 40 mL of EtOH/H2O (1:1 v/v) was added KCN (2.03 g, 31.2 mmol, 100 mol equiv). The reaction mixture was refluxed under an inert atmosphere of argon for 16 h, during which the color of the solution changed from yellow to orange. After that, the reaction mixture was extracted with dichloromethane (50 mL × 3) and subsequently washed with copious amounts of water (100 mL × 3). The dichloromethane extract was then dried over MgSO4. After filtration and removal of solvent under reduced pressure, the residue was purified by column chromatography on silica gel using dichloromethane/acetone (1:1 v/v) as eluent. Further purification was achieved by recrystallization through slow diffusion of diethylether vapor into a concentrated dichloromethane solution of the complex that gave P1 as a yellow crystalline solid. Yield: 110 mg, 0.23 mmol, 74%. 1H NMR (400 MHz, CDCl3, 298 K): δ 7.89 (dd, 2H, J = 8.2, 5.1 Hz, 3,8-phen H’s), 8.06 (s, 2H, 5,6-phen H’s), 8.57 (dd, 2H, J = 8.2, 1.4 Hz, 4,7-phen H’s), 9.42 (dd, 2H, J = 5.1, 1.4 Hz, 2,9-phen H’s). IR (KBr disc, v/cm−1): 1904, 1939, 2023 v(CO), 2120 v(CN). ESI− MS: m/z 476 [M]+. Anal. Calcd for P1: C, 40.33; H, 1.69; N, 8.82. Found: C, 40.17; H, 1.77; N, 8.80. Cyanotricarbonyl(4,7-dimethyl-1,10-phenanthroline) Rhenium(I), [Re(Me2phen)(CO)3CN)] (P2). To a solution of {[Re(Me2phen)(CO)3(MeCN)]OTf} (200 mg, 0.30 mmol, 1.0 mol equiv) in 40 mL of EtOH/H2O (1:1 v/v) was added KCN (1.94 g, 30.0 mmol, 100 mol equiv). The reaction mixture was refluxed under an inert atmosphere of argon for 16 h. The reaction mixture was then extracted with
optimization, the emission energy, efficiency, and the electroluminescence could be further fine-tuned and improved.
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EXPERIMENTAL SECTION
Materials and Reagents. Rhenium carbonyl, 1,10-phenanthroline (phen), 4,7-dimethyl-1,10-phenanthroline (Me2phen), potassium cyanide (KCN), and tris(pentafluoro)phenylborane [B(C6F5)3] were purchased from Acros Organic Chemical Co. and used without further purification. [Re(R 2 phen)(CO) 3 (MeCN)] + (R 2 phen = phen, Me2phen) was synthesized according to a literature method.6a N,N′Bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB), dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN), 2,7-bis(diphenylphosphoryl)-9,9′-spirobi[fluorene] (SPP013), 4,4′,4″tris(carbazol-9-yl)triphenylamine (TCTA), 1,3,5-tri(3-pyrid-3-ylphenyl)benzene (TmPyPB), and 4,7-diphenyl-1,10-phenanthroline (BPhen) were purchased from Luminescence Technology Corp. All other reagents and solvents were of analytical reagent grade, and commercial deuterated solvents were used without further purification. Reactions for the preparations of 1 and 2 were performed under strictly anaerobic conditions in an inert atmosphere of argon using standard Schlenk technique with deoxygenated and dried dichloromethane solution. Physical Measurements and Instrumentation. 1H and 19F NMR spectra were recorded on a Bruker AV400 (400 MHz) FT-NMR spectrometer. Chemical shifts (δ, ppm) were reported relative to tetramethylsilane (Me4Si). All positive ESI mass spectra were recorded on a PE-SCIEX API 150 EX single quadrupole mass spectrometer. Mass spectra of 1 and 2 were performed in acetonitrile solution with a small quantity of potassium iodide for ionization. The elemental analyses were performed on an Elementar Vario MICRO Cube elemental analyzer. Steady-state emission and excitation spectra at room temperature and at 77 K were recorded on a Horiba Jobin Yvon Fluorolog-3-TCSPC spectrofluorometer using a 450 W xenon lamp as the excitation source and R928P photomultiplier tube. Solutions were rigorously degassed on a high-vacuum line in a two-compartment cell with no less than four successive freeze−pump−thaw cycles. Measurements of the EtOH/MeOH (4:1 v/v) glass samples at 77 K were carried out with the dilute EtOH/MeOH sample solutions contained in a quartz tube inside a liquid-nitrogen-filled quartz optical Dewar flask. Luminescence quantum yields of samples in CH2Cl2 solutions were determined according to the method described by Crosby et al.14 using quinine sulfate in aqueous sulfuric acid (0.5 M) as reference standard. Absolute luminescence quantum yields of thin films and solids (powder samples) were measured on a Hamamatsu C9920-03 absolute PL quantum yield measurement system. Luminescence lifetimes of the samples were measured using timecorrelated single photon counting (TCSPC) technique on the TCSPC spectrofluorometer in a Fast MCS mode with a NanoLED-375LH excitation source, which has its excitation peak wavelength at 375 nm and pulse width shorter than 750 ps. The photon counting data were analyzed by Horiba Jobin Yvon Decay Analysis Software. Cyclic voltammetric measurements were performed by using a CH Instruments, Inc. model CHI 620 Electrochemical Analyzer. Electrochemical measurements were performed in acetonitrile solutions with 0.1 M nBu4NPF6 as the supporting electrolyte at room temperature. The reference electrode was a Ag/AgNO3 (0.01 M in acetonitrile) electrode, and the working electrode was a glassy carbon electrode (CH Instruments, Inc.) with a platinum wire as the counter electrode. The working electrode surface was polished with a 1 μm α-alumina slurry (Linde) and then a 0.3 μm α-alumina slurry (Linde) on a microcloth (Buehler Co.). The ferrocenium/ferrocene couple (FeCp2+/0) was used as the internal reference. All solutions for electrochemical studies were deaerated with prepurified argon gas prior to measurements. Electroluminescent Device. Patterned 15 Ω/sq. ITO glass substrates, each with four 0.1 cm2 light-emitting pixels, were routinely cleaned by Decon 90 detergent and deionized water. They were then blown dry by dust filtered nitrogen gas and were kept in an 110 °C oven for 3 h before a 25 min surface treatment in an ultraviolet ozone 2548
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dichloromethane (50 mL × 3) and subsequently washed with copious amounts of water (100 mL × 3). The dichloromethane extract was then dried over MgSO4. After filtration and removal of solvent under reduced pressure, the residue was purified by column chromatography on silica gel using dichloromethane/acetone (1:1 v/v) as eluent. Analytically pure crystalline solid was obtained by slow diffusion of diethylether vapor into a concentrated dichloromethane solution of the complex. Yield: 103 mg, 0.20 mmol, 68%. 1H NMR (400 MHz, CDCl3, 298 K): δ 2.95 (s, 6H, methyl H’s), 7.66 (d, 2H, J = 5.3 Hz, 3,8-phen H’s), 8.19 (s, 2H, 5,6-phen H’s), 9.23 (d, 2H, J = 5.3 Hz, 2,9phen H’s). IR (KBr disc, v/cm−1): 1883, 1914, 2019 v(CO), 2121 v(CN). ESI−MS: m/z 506 [M]+. Anal. Calcd for P2: C, 42.85; H, 2.40; N, 8.33. Found: C, 42.72; H, 2.37; N, 8.01. Isocyanotris(pentafluorophenyl)boratotricarbonyl(1,10-phenanthroline) Rhenium(I), {Re(phen)(CO)3[CNB(C6F5)3]} (1). A mixture of [Re(phen)(CO)3(CN)] (50 mg, 0.11 mmol, 1.0 mol equiv) and B(C6F5)3 (65 mg, 0.13 mmol, 1.2 mol equiv) was dissolved in dried and degassed dichloromethane. The resulting solution was refluxed overnight, during which the color of the solution changed from yellow to pale yellow. After the solvent was removed under reduced pressure, the residue was purified by column chromatography on silica gel using dichloromethane as eluent. Further purification was achieved by slow diffusion of diethylether vapor into a concentrated dichloromethane solution of the complex that gave 1 as pale yellow crystalline solid. Yield: 58 mg, 0.58 mmol, 56%. 1H NMR (400 MHz, CDCl3, 298 K): δ 7.96 (dd, 2H, J = 8.2, 5.2 Hz, 3,8-phen H’s), 8.11 (s, 2H, 5,6-phen H’s), 8.67 (dd, 2H, J = 8.2, 1.4 Hz, 4,7-phen H’s), 9.37 (dd, 6H, J = 5.2, 1.4 Hz, 2,9-phen H’s). 19F NMR (376 MHz, CDCl3, 298 K): δ −134.80 (dd, 6F, J = 23.0, 8.2 Hz, o-phenyl F’s), −158.95 (t, 3F, J = 23.0 Hz, p-phenyl F’s), −164.80 (td, 6F, J = 23.0, 8.2 Hz, m-phenyl F’s). IR (KBr disc, v/cm−1): 1927, 1946, 2033 v(CO), 2199 v(C N). ESI−MS: m/z 1029 [M + K]+. Anal. Calcd for 1: C, 41.31; H, 0.82; N, 4.25. Found: C, 41.42; H, 1.06; N, 4.47. Isocyanotris(pentafluorophenyl)boratotricarbonyl(4,7-dimethyl1,10-phenanthroline) Rhenium(I), {Re(Me2phen)(CO)3[CNB(C6F5)3]} (2). A mixture of [Re(Me2phen)(CO)3(CN)] (50 mg, 0.10 mmol, 1.0 mol equiv) and B(C6F5)3 (61 mg, 0.12 mmol, 1.2 mol equiv) was dissolved in dried and degassed dichloromethane. The resulting solution was refluxed overnight. After the solvent was removed under reduced pressure, the residue was purified by column chromatography on silica gel using dichloromethane as eluent. Further purification was achieved by slow diffusion of diethylether vapor into a concentrated dichloromethane solution of the complex that gave 2 as a pale yellow crystalline solid. Yield: 52 mg, 0.05 mmol, 52%. 1H NMR (400 MHz, CDCl3, 298 K): δ 2.92 (s, 6H, methyl H’s), 7.65 (dd, 2H, J = 5.3, 0.8 Hz, 3,8-phen H’s), 8.15 (s, 2H, 5,6-phen H’s), 9.08 (d, 2H, J = 5.3 Hz, 2,9-phen H’s). 19F NMR (376 MHz, CDCl3, 298 K): δ −134.67 (dd, 6F, J = 20.6, 8.2 Hz, o-phenyl F’s), −159.17 (t, 3F, J = 20.6 Hz, pphenyl F’s), −165.06 (td, 6F, J = 20.6, 8.2 Hz, m-phenyl F’s). IR (KBr disc, v/cm−1): 1921, 1951, 2036 v(CO), 2208 v(CN). ESI−MS: m/z 1057 [M + K]+. Anal. Calcd for 2: C, 42.54; H, 1.19; N, 4.13. Found: C, 42.30; H, 1.51; N, 4.32.
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*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been supported by the Hong Kong University Grants Committee Area of Excellence Scheme (AoE/P-03-08), a General Research Fund (Project No. CityU 101212), and a grant (Project No. T23-713/11) from the Research Grants Council of the Hong Kong SAR, China. W.-K.C. acknowledges receipt of a University Postgraduate Studentship and a Research Tuition Scholarship administrated by City University of Hong Kong. We sincerely thank Professor V. W.-W. Yam of The University of Hong Kong for access to the equipment for luminescence quantum yield measurements of our solid and thin film samples and for her helpful discussion.
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ASSOCIATED CONTENT
S Supporting Information *
CIF files and experimental details for the X-ray crystal structures, selected bonding parameters for 1 and 2, overlaid emission spectra of selected complexes in solid, 77 K EtOH/ MeOH (4:1 v/v) glassy medium, and thin film, cyclic voltammograms and TGA thermogram of selected complexes, and graphs showing the EL characteristics of the devices (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*Tel.: 852-34426958. Fax: 852-34420522. E-mail: vinccko@ cityu.edu.hk. 2549
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