Synthesis, photophysical and electroluminescent properties of green organic light emitting devices based on novel iridium complexes containing benzimidazole ligands

Journal of Organometallic Chemistry 761 (2014) 74e83

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Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem

Review

Synthesis, photophysical and electroluminescent properties of green organic light emitting devices based on novel iridium complexes containing benzimidazole ligands Jayaraman Jayabharathi*, Karunamoorthy Jayamoorthy, Venugopal Thanikachalam Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 December 2013 Received in revised form 25 February 2014 Accepted 1 March 2014

Synthesis, photophysical and electroluminescent analysis of cyclometalated iridium (III) complexes Ir(bpb)2(pic) are reported. The strongly allowed phosphorescence is the result of significant spin-orbit coupling of the Ir center. The lowest energy excited sate is a mixture of metal to ligand charge transfer (MLCT) and pep* states. Weak bands at longer wavelength were assigned to 1MLCT ) S0 and 3 MLCT ) S0 transitions. These complexes were doped into emissive region of multilayer organic light emitting diode (OLED) by vapor-deposition emit green electroluminescence with similar currentevoltage characteristics. These Ir(bpb)2(pic) doped OLEDs show appreciable efficiencies as compared with other iridium complex based OLEDs in the literature, which result from efficient trapping and radiative relaxation of singlet and triplet excitons formed in electroluminescent process. Devices with Ir(bpb)2(pic) shows the better performance in terms of brightness and power efficiency with brightness of 130301 cd/ m2 at 14.5 V and 26.2 lm/W at 5.5 V, respectively. Ó 2014 Elsevier B.V. All rights reserved.

Keywords: OLED Electroluminescence Green emitter Iridium complex

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Optical measurements and compositions analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Device fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 General procedure for the synthesis of iridium complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 Absorption and emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 Mixing of excited states (LC and MLCT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Electronic transition theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Effect of substituents on tuning wavelength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 0 Description of the structure of Iridium(III)bis(1-benzyl-2-phenyl-1H-benzimidazolato-N,C2 )(picolinate), [Ir(bpb)2 (pic)] (1) . . . . . . . . . . . . . . . . . . . 79 Thermal studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 Electrochemical analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Electroluminescent properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

* Corresponding author. Tel.: þ91 9443940735. E-mail address: [email protected] (J. Jayabharathi). http://dx.doi.org/10.1016/j.jorganchem.2014.03.002 0022-328X/Ó 2014 Elsevier B.V. All rights reserved.

J. Jayabharathi et al. / Journal of Organometallic Chemistry 761 (2014) 74e83

Introduction Third-row transition-metal complexes are well-known for their ability to achieve high-efficiency phosphorescence at room temperature [1,2] and has 100% internal quantum efficiency, because they can effectively harvest both singlet and triplet excitons [3]. As a result, Ru (II)-, Os (II)-, Ir (III)- and Pt (II)- based cyclometalated complexes were significantly developed which exhibited wide application in organic light-emitting devices (OLEDs) [4e7]. Among these, iridium (III) complexes display best electrophosphorescence with an external quantum efficiency as high as 27% in OLEDs and are considered to be a class of promising electrophosphorescent materials due to their non-planar configuration and short phosphorescent lifetime [8e14]. However, there is a crucial issue of phase separation between iridium (III) complexes and host materials that influence the doped device performance. This is overcome by increasing the bulkiness of the iridium (III) complexes which improved the dispersibility and high-efficiency emission results. Attempts were made to enhance the efficiencies of OLEDs by increasing the solubility and dispersibility of iridium (III) complexes [15e20]. Emission color of Ir (III) complexes is strongly governed by the nature of cyclometalating ligand and also by ancillary ligands [21e23]. The emission colors of Ir(dfppy)2(LX) complexes are tuned from blue to red by changing ancillary ligands [24,25]. The non-radiative decay of Ir(ppz)3 at ambient temperature are possible through higher thermally activated metal-to-ligand charge transfer (MLCT) states or MLCT to metal dd state conversion [26,27]. The picolinate complex Ir(ppz)2(pic) shows relatively stronger green emission (526 nm) at room temperature whereas the emission colors change from blue (422 nm) to orange-red (587 nm) at low temperature (77 K) [28]. Despite elegant research on green phosphores [29e32], there are only few reports on room temperature green phosphores. The energy gap has been tuned by incorporating the substituents in the ligand to obtain the desired emission. The purpose of the present study is the molecular design of a highly efficient greenphosphorescent complexes viz., iridium (III) bis (1-benzyl-20 phenyl-1H-benzimidazolato-N,C2 )(picolinate), [Ir(bpb)2(pic)] (1), iridium (III) bis(1-(4-fluorobenzyl)-2-(4-fluorophenyl)-1H-benzi0 midazolato-N,C2 ) (picolinate) [Ir(fbfpb)2(pic)] (2), iridium (III) bis(1-(4-methybenzyl)-2-p-tolyl-1H-benzimidazolato0 N,C2 )(picolinate) Ir(mbtb)2(pic)] (3) and iridium (III) bis(1-(4methoxybenzyl)-2-(4-methoxyphenyl)-1H-benzimidazolato0 N,C2 )(picolinate) [Ir(mbmpb)2(pic)] (4) suitable for green OLED devices. In addition to high phosphorescence efficiency, the complexes should possess high thermal stability for device fabrication and stable device performance. We particularly focused our attention on designing metal complexes that provide green emission from the MLCT excited state. Experimental Optical measurements and compositions analysis The ultravioletevisible (UVevis) spectra of the phosphorescent iridium complexes were measured in an UVevis spectrophotometer (Perkin Elmer Lambda 35) and corrected for background absorption due to solvent. Photoluminescence (PL) spectra were recorded on a (Perkin Elmer LS55) fluorescence spectrometer. NMR spectra were recorded on Bruker 400 MHz NMR spectrometer. The mass spectra of the samples were obtained using a Thermo Fischer LC-Mass spectrometer. Cyclic voltammetry (CV) analysis were performed by using CHI 630A potentiostat electrochemical analyzer. Measurements of oxidation and reduction were undertaken using 0.1 M tetra(n-butyl)ammonium- hexafluorophosphate

75

as the supporting electrolyte, at scan rate of 0.1 VS1. The potentials were measured against an Ag/Agþ (0.01 M AgNO3) reference electrode using ferrocene/ferrocenium (CP2Fe/CP2Feþ) as the internal standard. The onset potentials were determined from the intersection of two tangents drawn at the rising current and background current of the cyclic voltammogram. All calculations were performed using density functional theory (DFT) as implemented in the with Gaussian-03 program using the Becke3-Lee-Yang-Parr (B3LYP) functional supplemented with the standard 6-31G (d, p) basis set [33].

Device fabrication The EL devices based on the iridium (III) complexes were fabricated by vacuum deposition of the materials at 5  106 torr onto a clean glass precoated with a layer of indium tin oxide (ITO) as the substrate. The glass was cleaned by sonication successively in a detergent solution, acetone, methanol and deionized water before use. Organic layers were deposited onto the substrate at a rate of 0.1 nm s1. LiF and Alq3 were thermally evaporated onto the surface of organic layer. The thickness of the organic materials and the cathode layers were controlled using a quartz crystal thickness monitor. A series of devices (I, II, III and IV) with the multilayer configuration ITO/NPB (30 nm)/iridium complex: CBP (7%) (30 nm)/ BCP (10 nm)/Alq (40 nm)/Mg:Ag was fabricated. Measurements of current, voltage and light intensity of these devices (IeIV) were made simultaneously using a Keithley 2400 sourcemeter. The EL spectra and luminance of the devices were carried out in ambient atmosphere without further encapsulations.

General procedure for the synthesis of iridium complexes A mixture of corresponding aldehyde (2 mmol), o-phenylenediamine (1 mmol) and ammonium acetate (2.5 mmol) has been refluxed at 80  C in ethanol which yields the benzimidazole derivatives. The benzimidazole based cyclometalated iridium complexes have been synthesized via Nonoyama route [34a] (Scheme 1).

Iridium(III)bis(1-benzyl-2-phenyl-1H-benzimidazolato0 N,C2 )(picolinate), [Ir(bpb)2 (pic)], (1) Yield: 88%. 1H NMR (400 MHz, CDCl3): d 5.72 (d, 1H, J ¼ 8.0 Hz), 5.81e5.97 (m, 3H), 6.35 (d, 1H, J ¼ 7.2 Hz), 6.32e6.73 (m, 3H), 6.78e 6.88 (m, 2H), 7.16e7.54 (m, 16H), 7.86e7.95 (m, 2H), 8.14 (s, 1H), 8.25 (d, 1H, J ¼ 7.6 Hz). 13C NMR (100 MHz, CDCl3): d 48.12, 48.37, 109.58, 110.29, 114.45, 117.68, 120.67, 121.39, 122.88, 123.51, 123.74, 124.62, 124.78, 124.86, 125.94, 126.01, 127.33, 127.68, 128.09, 128.02, 129.27, 129.33, 129.49, 129.83, 134.26, 134.36, 134.60, 135.06, 135.23, 135.28, 135.69, 137.28, 139.80, 149.17, 149.94, 153.57, 163.28, 164.68, 173.55. Anal. calcd. for C46H34IrN5O2: C, 62.71; H, 3.89; N, 7.95. Found: C, 62.67; H, 3.90; N, 7.98. MS: m/z 882.1, calcd. 881.23.

Iridium(III)bis(1-(4-fluorobenzyl)-2-(4-fluorophenyl)-1H0 benzimidazolato-N,C2 ) (picolinate), [Ir(fbfpb)2(pic)], (2) 1 Yield: 90%. H NMR (400 MHz, CDCl3): d 5.65 (d, 1H, J ¼ 8.4 Hz), 5.83e5.92 (q, 5H), 6.23 (d, 1H, J ¼ 10.4 Hz), 6.46e6.60 (m, 2H), 6.84e7.49 (m, 22H), 7.93 (d, 2H, J ¼ 8.4 Hz), 8.09 (s, 1H), 8.28 (d, 1H, J ¼ 7.2 Hz). 13C NMR (100 MHz, DMSO): d 46.36, 46.59, 108.34, 111.58, 111.93, 113.21, 115.62, 115.68, 115.84, 115.90, 119.17, 123.77, 124.18, 124.27, 126.88, 128.04, 128.85, 130.94, 132.30, 135.11, 135.34, 138.52, 139.03, 149.08, 152.07, 152.82, 160.28, 161.65, 162.48, 162.68, 172.17. Anal. calcd. for C46H30F4IrN5O2: C, 57.95; H, 3.18; N, 7.37. Found: C, 57.98; H, 3.17; N, 7.35. MS: m/z 954.1, calcd. 953.20.

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J. Jayabharathi et al. / Journal of Organometallic Chemistry 761 (2014) 74e83

Scheme 1. Synthetic route of iridium (III) complexes 1e4.

Iridium(III)bis(1-(4-methybenzyl)-2-p-tolyl-1H-benzimidazolato0 N,C2 ) (picolinate), [Ir(mbtb)2(pic)], (3) Yield: 86%. 1H NMR (400 MHz, CDCl3): d 1.95 (s, 3H), 2.04 (s, 3H), 2.30 (s, 6H), 5.66e5.82 (m, 3H), 5.93 (d, 2H, J ¼ 17.2 Hz), 6.15 (s, 1H), 6.42 (s, 1H), 6.57 (dd, 2H, J ¼ 8 Hz, 8 Hz), 6.80e6.84 (t, 1H, J ¼ 15.6 Hz), 7.02e7.44 (m, 17H), 7.85e7.88 (t, 1H, J ¼ 15.2 Hz), 7.98 (d, 1H, J ¼ 5.2 Hz), 8.08e8.10 (t, 1H, J ¼ 8 Hz), 8.24 (d, 1H, J ¼ 7.6 Hz). 13 C NMR (100 MHz, CDCl3): d 21.76, 21.71, 21.76, 47.70, 47.94, 109.38, 110.09, 114.29, 117.44, 121.82, 122.52, 123.10, 123.46, 124.40, 124.55, 125.85, 125.95, 127.23, 127.58, 129.87, 129.89, 131.54, 131.93, 132.27, 132.37, 135.12, 135.57, 135.63, 137.12, 137.74, 137.89, 139.29, 139.75, 139.80, 139.86, 149.32, 149.99, 151.59, 153.59, 163.23, 164.63, 173.54. Anal. calcd. for C50H42IrN5O2: C, 64.08; H, 4.52; N, 7.47. Found: C, 64.07; H, 4.51; N, 7.49. MS: m/z 938.1, calcd. 937.30. Iridium(III)bis(1-(4-methoxybenzyl)-2-(4-methoxyphenyl)-1H0 benzimidazolato-N,C2 ) (picolinate), [Ir(mbmpb)2(pic)], (4) Yield: 91%. 1H NMR (400 MHz, DMSO): d 3.37 (s, 3H), 3.41 (s, 3H), 3.68 (s, 6H), 5.55 (d, 1H, J ¼ 8 Hz), 5.64 (s, 1H), 5.76 (s, 1H), 5.95e6.11 (m, 4H), 6.39e6.46 (m, 2H), 6.82e6.95 (m, 5H), 7.04e7.12 (dd, 4H, J ¼ 8 Hz, J ¼ 8 Hz), 7.25e7.33 (m, 3H), 7.68e7.81 (m, 6H), 7.90 (d, 1H), 8.02e8.12 (m, 2H). 13C NMR (100 MHz, DMSO): d 46.33, 54.09, 54.16, 54.99, 105.54, 105.72, 111.04, 111.36, 112.85, 114.15, 114.20, 115.14, 119.14, 119.41, 122.59, 122.85, 123.51, 123.61, 126.56, 126.72, 126.81, 126.91, 127.23, 128.00, 128.06, 128.51, 135.07, 135.32, 138.46, 138.87, 138.92, 148.93, 151.14, 152.39, 154.33, 158.57, 159.47, 159.65, 162.49, 163.37, 172.21. Anal. calcd. for C50H42IrN5O6: C, 59.99; H, 4.23; N, 7.80. Found: C, 59.97; H, 4.24; N, 7.81. MS: m/z 1001.9, calcd. 1001.12.

absorption bands of complexes (1e4) show three kinds of bands. The intense band around 302 nm in the ultraviolet part of the spectrum can be assigned to the allowed ligand centered (pep*) transitions [34b,35] and somewhat weaker bands also observed in the lower part of energy. The band position, size and extinction coefficient of the bands in the range 425e482 nm suggest that these are MLCT transitions (1MLCT and 3MLCT) [36,37]. Similar to our earlier reports [38e44], weak bands located at longer wavelength were assigned to the 1MLCT ) S0 and 3MLCT ) S0 transitions of iridium complexes. Thus the broad absorption shoulders at 448 and 481 nm observed for complex 1 are likely to be ascribed to the 1MLCT ) S0 and 3MLCT ) S0 transitions, respectively. The intensity of the 3MLCT ) S0 transition is close to that of 1 MLCT ) S0 transition, suggesting that 3MLCT ) S0 transition is strongly allowed by SeT mixing of spin-orbit coupling [45] and similar observations are made for complexes 2e4. Absorption around 440 nm for complexes 1e4 corresponds to the transition of

Results and discussion Absorption and emission spectra Fig. 1 depicts the absorption and emission spectra of iridium (III) complexes 1e4 in dichloromethane at room temperature. The

Fig. 1. Absorption and emission spectra of iridium complexes 1e4 in CH2Cl2.

J. Jayabharathi et al. / Journal of Organometallic Chemistry 761 (2014) 74e83

77

the 1MLCT state as evident from its extinction coefficient of the order 103. The absorption like long tail toward lower energy and higher wavelength around 485 nm is assigned to 3MLCT transition and gains intensity by mixing with the higher lying 1MLCT transition through the spin-orbit coupling of iridium (III) [39]. Both singlet MLCT (1MLCT) and triplet MLCT (3MLCT) bands are typically observed for these complexes in all solvents. In order for these iridium (III) complexes to be useful as phosphors EL devices, strong spin-orbit coupling must be present to efficiently mix the singlet and triplet excited states. Clear evidences for mixing of the singlet and triplet excited states are seen in the absorption of these complexes. Mixing of excited states (LC and MLCT) Phosphorescence of mononuclear metal complexes originates from the ligand-centered excited state (LC), metal-centered excited state and MLCT excited state. For the cyclometalated iridium complexes, the wave function of the excited triplet state FT, responsible for the phosphorescence is expressed as,






FT ¼ aFT p  p* þ bFT ðMLCTÞ

(1)

where ‘a’ and ‘b’ are the normalized co-efficient, FT (pep*) and FT (MLCT) are the wave function of 3(pep*) and 3(MLCT) excited states, respectively. For these iridium complexes, the wave function of the triplet state (FT) responsible for the phosphorescence and equation (1) implies that the excited triplet state of these iridium complexes are mixture of FT (pep*) and FT (MLCT) [36]. The triplet state is attributed to dominantly 3pep* excited state when a > b and dominantly 3MLCT excited state when b > a. The phosphorescence spectra of these complexes 1e4 obtained at room temperature show significant broad shape. According to our previous studies [40e42], phosphorescence spectra from ligand centered 3pep* state display vibronic progressions, while those from the 3MLCT state are broad in shape. Complexes 1, 3 and 4 were excited state with large contribution of 3MLCT whereas complex 2 was excited state with large contribution of 3pep*. All these complexes show emission at 526, 512, 521 and 515 nm in dichloromethane, respectively (Table 1). The emission spectra of complexes 2e4 are blue shifted when compared with complex 1 and this may be due to the electronic effect of the substituents. The emission spectrum of complex 2 is more blue shifted in comparison with that of complex 1 which is the result of the introduction of electron withdrawing fluoro substituent into the phenyl ring attached to C(2) carbon of the imidazole ring. Electronic transition theory The time correlated single photon counting (TCSPC) results fit to single exponentials decay shown in Fig. 2, DAS6 software was used for the fit and the c2 values are around 1. The phosphorescence lifetime of all iridium(III) complexes were measured in degassed CH2Cl2 solution at room temperature and the signal was measured

Fig. 2. Lifetime decay spectra of iridium complexes 1e4 in CH2Cl2.

at the emission wavelength of individual complexes. The absolute PL quantum yields were measured by comparing phosphorescence intensities (integrated areas) of a standard sample (Coumarin 46) and the unknown sample [42] using the formula Funk ¼ Fstd(Iunk/ Istd)(Astd/Aunk)(hunk/hstd)2where, Funk is the phosphorescence quantum yield of the sample, Fstd is the quantum yield of the standard; Iunk and Istd are the integrated emission intensities of the sample and the standard, respectively. Aunk and Astd are the absorbances of the sample and the standard at the excitation wavelength, respectively. hunk and hstd are the indexes of refraction of the sample and standard solutions [44]. The radiative and nonradiative decays of the excited state of iridium complexes were obtained using the quantum yields and lifetimes and are listed in Table 1. Moreover, radiative lifetime of these complexes fall in the range of 1.60e1.79 ms. The formula employed to calculate the radiative (kr) and non-radiative (knr) rate constants is Fp ¼ FISC {kT/ (kr þ knr)}; kr ¼ Fp/s; knr ¼ (1/s)  (Fp/s); s ¼ (kr þ knr)1, where, Here, FISC is the intersystem-crossing yield. For the iridium complexes FISC is safely assumed to be 1.0 because of the strong spinorbit interaction caused by heavy atom effects of iridium [45]. The kr and knr are the radiative and non-radiative deactivation; sf is the lifetime of the excited state. Perusal of the radiative and nonradiative rate constants shows that the nonradiative emission is predominant over radiative transitions. Experimentally observed result indicates that the kr values of complexes increases with an increase of lemi. According to the theory of electronic transition, the kr is proportional to square of electric dipole transition moment (MTeS). First-order perturbation theory gives an approximate expression for MTeS [46]

MTS ¼

X

bn h1 Fn jMj1 F0 i

(2)

where 1Fn and 1F0 are the wave functions of Sn and So states, respectively and M is the electric dipole vector with the use of the

Table 1 Absorption (labs), emission (lemi), fluorescence quantum yield (V), lifetime (s), radiative rate constant (kr), nonradiative rate constant (knr) and electrochemical behavior of iridium complexes 1e4. Complex

labs (nm)

lemi (nm)

F

s (ms)

kr (s1)

knr (s1)

lonset (nm)

E1/2oxi (V)

HOMO (eV)

LUMO (eV)

Eg (eV)

1 2 3 4

481.7, 448.8, 310.6 470.91, 425.57, 307.2 482.24, 450.50, 309.4 470.91, 430.10, 302.16

526 512 521 515

0.48 0.32 0.42 0.38

1.68 1.60 1.79 1.71

0.29 0.20 0.23 0.22

0.31 0.43 0.30 0.33

467 462 471 476

0.45 0.58 0.35 0.30

5.25 5.38 5.15 5.10

2.60 2.70 2.52 2.50

2.65 2.68 2.63 2.60

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J. Jayabharathi et al. / Journal of Organometallic Chemistry 761 (2014) 74e83

spin-orbit coupling operator (Hso) and the wave function of the lowest excited triplet state (3F1), bn is formulated as

bn ¼ h1 Fn jHSO j3 F1 i=ð1 En  3 E1 Þ

(3)

where 1En and 3E1 are the energies of Sn and lowest excited triplet states, respectively. Now, equation (2) is simply expressed as

MTS ¼ fh1 Fn jHSO j3 F1 ih1 F1 jMj1 F0 ig=ð1 E1  3 E1 Þ ¼ a=ð1 E1  3 E1 Þ Equation

(4)

(4) predicts

that,

when

a ¼ h1 Fn jHSO j3 F1 ih1 F1 jMj1 F0 i is approximately constant, kr in-

creases with a decrease in the energy difference (1E1  3E1). In the present study, kr increases with an increase in Eemi. This fact is explained by assuming that the S1 energy does not differ significantly among the complexes and the energy difference (1E1  3E1) increases with decrease of Eemi. Actually in these complexes the S1 ) S0 absorption bands are located at 302e310 nm but phosphorescence S0 ) T1 exhibits large bathochromic shift ranging from 515 to 526 nm. These data provide the evidence that the energy difference (1E1  3E1) increases with the decrease of Eemi and therefore, kr increases with an increase in lemi [46]. The nonradiative transition (knr) is explained in terms of coupling of vibronic states on an initial potential energy surface to isoenergetic levels of the final state (Eg) [47aef], [47g]. The relation between knr and the energy gap (E0) is given by

    gE0 E0 ; g ¼ ln 1 knr fexp ZuM SM ZuM

(5)

where, the energy gap (E0) is related to the zero-point energy (ZPE) separations of two coupled surfaces. The -uM describes, in the limit of a single configuration coordinates model, the average energy of the vibrational mode(s) that couples the final state to the initial state. The degree to which the two surfaces are vibronically coupled is given by the Huang-Rhys factor (SM),

SM ¼

  1 M uM ðDQe Þ2 2 Z

Fig. 3. Optimized structure of Iridium(III)bis(1-benzyl-2-phenyl-1H-benzimidazolato0 N,C2 )(picolinate).

occupied molecular orbital (HOMO) stability and emission energy gap are controlled by the nature and number of substituent and its inductive influence on aromatic ring. The photophysical study of these complexes demonstrates that the electron withdrawing fluoro substituent increases the absorption and emission energies of complexes by stabilizing the HOMO level. Besides increasing the emission energy, the lower HOMO energies decrease the energy separation between the 1MLCT and 3LC states, which in turn modified the excited state properties of the iridium complexes.

(6)

where, M is the reduced mass of the oscillator, uM is the fundamental frequency and DQe is the difference between the ground and excited-state equilibrium geometries with respect to the specified nuclear (i.e., vibrational) coordinate. Decrease in energy gap (E0) at constant DQe results in an increase in the vibrational overlap between the lowest energy vibrational state of the excited state and isoenergetic vibrational states of the ground state surface. Effect of substituents on tuning wavelength Substituent effect of the d orbital (t2g) stabilizes iridium (III) through the carbon atom-iridium bonding and this identifies with the inductive effect of the substituents. Therefore, the highest

Table 2 Selected bond lengths ( A), bond angles ( ) and torsional angles ( ) of [Ir(bpb)2 (pic)], (1) by DFT/B3LYP/6-31 (d,p). Bond length

DFT/B3LYP/6-31(d,p)

Bond angles ( )

DFT/B3LYP/6-31(d,p)

Ir(1)eO(1) Ir(1)eC(1) Ir(1)eN(1)) Ir(1)eN(2) Ir(1)eC(26) Ir(1)eC(46)

2.14 2.14 2.05 2.08 2.00 2.07

O(1)eIr(1)eN(1) O(1)eIr(1)eN(2) O(1)eIr(1)e(26) O(1)eIr(1)e(46) N(1)eIr(1)eN(2) N(1)eIr(1)e(26)

86.5(15) 96.9(15) 177.5(2) 93.3(19) 87.6(19) 95.5(2)

(4) (5) (5) (4) (6) (5)

Fig. 4. (a) TGeDTA curves of the iridium complexes 1e4; (b) DSC curves of the iridium complexes 1e4.

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Fig. 5. (a) Cyclic voltammogram curves of the iridium complexes 1e4; (b) HOMO LUMO energy levels of the iridium complexes 1e4; (c) the HOMO-LUMO orbital picture of complexes 1 and 2.

The redox potentials of the cyclometalated iridium complexes were measured relative to an internal ferrocene reference (Cp2Fe/ Cp2Feþ ¼ 0.45 V versus SCE in CH2Cl2 solvent) [48e50]. The reduction occurs primarily on the more electron accepting heterocyclic portion of the cyclometalated benzimidazole ligands (lowest unoccupied molecular orbital (LUMO) contribution) whereas the oxidation process is to largely involve in the Ir-phenyl center (HOMO contribution). The calculated energies of HOMO and LUMO along with energy gap are given in Table 1. The iridium complexes show reversible oxidation behavior, HOMO and LUMO energies were calculated based on the HOMO energies and lowestenergy absorption edges of the absorption spectra [48]. From the energy gap values it was concluded that all the reported dopants (1e4) are green emitters.

Description of the structure of Iridium(III)bis(1-benzyl-2-phenyl0 1H-benzimidazolato-N,C2 )(picolinate), [Ir(bpb)2 (pic)] (1) The selected bond lengths and bond angles of Ir(bpb)2 (pic) (1) are presented in Table 2 and the optimization has been obtained using density functional theory (DFT/B3LYP/6-31 (d,p)). This complex exhibits an octahedral geometry around metal iridium and prefers cis eC,C and trans-N,N chelate disposition instead of trans-

C,C and trans-N,N chelate. Electron rich phenyl rings normally exhibit very strong influence and trans effect. Therefore, the transC,C arrangement is expected to be thermodynamically higher in energy and kinetically more labile. This well known phenomenon has been referred to as “transphobia” [49]. The IreC bonds of the complex 1, i.e IreCav ¼ 2.038  A is shorter than IreN bonds i.e., Ire Nav ¼ 2.060  A. The IreO bond length [2.138  A] is longer than the mean IreO bond length (2.088 A) reported [51] and these observations reflect the trans influence of the phenyl groups. All other bond lengths and bond angles are analogous to the similar type of complexes (Fig. 3).

Thermal studies The thermal properties of the iridium complexes were investigated by thermal gravimetric analyses (TGA) under nitrogen atmosphere. The iridium complexes are amorphous in nature possess high thermal stability. Decomposition of Ir(bpb)2 (pic) and (Ir(mbtb)2(pic) begins at about 382  C and 363  C respectively and proceeds in three stages. whereas Ir(fbfpb)2(pic) decomposition begins at about 390  C and proceeds in four stages. Decomposition of Ir(mbmpb)2(pic) begins at about 368  C as shown in Fig. 4a. These complexes exhibits a very complicated decomposition

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Fig. 6. (a) General structure of multilayer OLED device; (b) schematic energy diagram of LUMO/HOMO for devices IeIV; (c) color of the green OLED device of Ir(bpb)2(pic) 1.

Table 3 Performances of electroluminescence devices 1e4. Device

Vd (V)

Lmax (cd/m2)

hext (%)

hc cd/A

hp (lm/W)

ELmax (nm)

C.I.E@8V (x,y)

I II III IV

3.0 4.6 3.0 3.0

130301, 14.5 V 38156, 16 V 103156, 18 V 83882, 18 V

10.1, 6 V 6.2, 9.5 V 12.0, 8 V 13.8, 8 V

37.3, 20.8, 46.2, 47.8,

26.2, 5.5 V 7.2, 9.5 V 21.2, 7 V 22.9, 7 V

519 505 518 509

(0.28, (0.23, (0.30, (0.29,

6V 9.5 V 8V 8V

Vd: driving voltage; Lmax: luminous efficiency; h: current efficiency; ELmax: electroluminescence maxima.

0.62) 0.60) 0.62) 0.61)

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Electroluminescent properties

Fig. 7. Electroluminescence spectra of complexes 1e4 in CH2Cl2.

process and mass loss calculations are not in agreement with an intermediate of definite composition. However, the total mass loss between 363 and 1000  C corresponds to the presence of metallic iridium as the final decomposition product as identified by chemical analysis and also from its lustrous appearance. We have investigated the thermal properties of the complexes by differential scanning calorimetry (DSC). Fig. 4b indicates that these iridium complexes undergo a glass transition around 92  C, followed by crystallization around 154  C and crystalline melting at 381, 395, 392 and 378  C for complexes, respectively and there was no phase transition signal observed from 30 to 300  C. Electrochemical analysis In order to investigate the HOMO and LUMO energy levels, cyclic voltammetry studies were carried out. On the basis of the onset potential of oxidation, HOMO energies of the iridium complexes can be estimated with regard to the energy level of the ferrocenium/ferrocene redox couple (Fig. 5a) [52]. The HOMO energies were calculated to be 5.25, 5.38, 5.15 and 5.10 eV for complexes 1e4 respectively. This is consistent with the theoretical calculations that the one-electron oxidation of such d6 complexes would mainly occur at the metal site, together with a minor contribution from the surrounding chelates [53]. The HOMO and LUMO energy levels are shown in Fig. 5b. The 3D orbitals of HOMO and LUMO of complexes 1 and 2 are shown in Fig. 5c.

To illustrate the electroluminescent properties of iridium complexes, typical OLED devices using the iridium complexes (1e4) as dopants were fabricated (Fig. 6a). The devices (IeIV) had a multilayer configuration, ITO/NPB (30 nm)/iridium complex: CBP (7%) (30 nm)/BCP (10 nm)/Alq3 (40 nm)/Mg:Ag, in which ITO (indium tin oxide) was used as the anode, NPB (4,40 -bis[N-(1-naphthyl)-Nphenylamino]biphenyl) was used as the hole-transporting material, CBP (4,40 -N,N0 -dicarbozolebiphenyl) as the host, the iridium complexes as dopant, BCP (2,9-dimethyl-4,7-dipheny-1,10phenanthroline) as the hole blocker, Alq (tris(8hydroxyquinolinato)aluminum) as the electron transporter and Mg:Ag as the cathode. The electroluminescent characteristics are displayed in Table 3. The relative energies of these materials are depicted in Fig. 6b. The devices IeIV emit strong green light (Fig. 6c) with an emission maximum at 519, 505, 518 and 509 nm, respectively (Fig. 7), which are well matched with those of the PL spectra in solution (512e526 nm). There are no characteristic emission peaks from CBP or Alq3, indicating that the emission originates mostly from the dopant. These results imply that effective energy transfer from the host (CBP) to the dopant (Ir complex) occurs in the emissive layer. The turn-on voltage of devices IeIV, are 3.0, 4.6, 3.0 and 3.0 eV, respectively. The intermolecular pep stacking interactions in the solid state and are considered as the important factors in determining the operating voltage. The strong electronic coupling and intermolecular interactions contribute to effective injection and transport of charges may lead to lower driving voltages [54,55]. The dopant molecules in green devices can be deep trap sites from both of electrons and holes. Electron and hole movements may follow the deeply trapped charge transport mechanism. Usually, carrier mobility in deeply trapped organic layers is very slow. Deeply trapped dopants could delay carrier mobility by several orders [56]. However, owing to HOMO and/or LUMO levels of dopants, easier electron injection contributes some voltage reduction. When a guest with a narrow Eg is doped into a host with a wide Eg [Eg of dopants 1e 4 is about 0.85 eV lower than that of CBP], the difference in HOMO and/or LUMO levels between the guest and the host significantly increases. Then, the guest becomes a deep trap for hole and/or electron transport in the emitting layer and reduction of the driving voltage is the results. Upon reduction of the driving voltage the power efficiency of the OLED was greatly improved [57]. Fig. 8a and b shows the brightnessevoltage and the external quantum yielde current density characteristics of the devices, respectively. All devices show quite appreciable efficiencies and brightness when compared to device with benzimidazole based acetylacetanato 0 complex, iridium(III)bis(1,2-diphenyl-1H-benzimidazolato-N,C2 ) (acetylacetanato) [(pbi)2Ir(acac)], as dopant show brightness of

Fig. 8. (a) Plot of brightness ns voltage; (b) plot of external quantum yield ns current density.

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Fig. 9. (a) Plot of current efficiency ns current density; (b) plot of power efficiency ns current density.

46393 cd/m2, external quantum efficiency 12.7%, current efficiency 46 cd/A and power efficiency of 12 lm/W [58]. Devices I and II show the better performance than others in terms of brightness of 130301 cd/m2 at 14.5 V,103156 cd/m2 at 18 V, respectively. Device (I) with of Ir(bpb)2(pic) as dopant, shows high power (Fig. 9a) as well as current (Fig. 9b) efficiencies, 26.2 Im/w at 5.5 V and 37.3 cd/A at 6 V, respectively (Table 3). Conclusions We have synthesized a series of iridium(III) complex dopants using various substituted benzimidazole ligands. These complexes exhibit different quantum efficiencies in solution depending upon the nature of substituents. The wavelength can be tuned by adjusting the electronic properties of the substituents in the ligand. Some of the complexes discussed here showed 3MLCT predominant mixing states for their lowest excited triplet states. But the degree of mixing between 3MLCT and 3pep* states of the excited states varied. We have fabricated the OLED that can be driven with a low voltage by using the phosphorescent iridium complexes as dopants. All devices show quite appreciable efficiencies and brightness when compared to device with benzimidazole based acetylacetanato complex, iridium (III) bis(1,2-diphenyl -1H-benzimidazolato0 N,C2 ) (acetylacetanato) [(pbi)2Ir(acac)], as dopant. The OLED with (Ir(bpb)2(pic) as dopant exhibited a substantially lower driving voltage and the reduction in the driving voltage is because of the narrower HOMOeLUMO energy gap. Acknowledgments One of the authors Prof. J. Jayabharathi is thankful to Department of Science and Technology, Ministry of Science and Technology, India [No. SR/S1/IC-73/2010], Defence Research and Development Organisation, India (NRB-213/MAT/10-11), University Grants Commission, India (F. No. 36-21/2008 (SR)) and Council of Scientific and Industrial Research, India (NO. 3732/NS-EMRII) for providing funds to this research study. References [1] C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610e3616. [2] F. So, B. Krummacher, M.K. Mathai, D. Poplavskyy, S.A. Choulis, V.E. Choong, J. Appl. Phys. 102 (2007) 091101e091103. [3] B. Tong, Q. Mei, S. Wang, Y. Fang, Y. Meng, B. Wang, J. Mater. Chem. 18 (2008) 1636e1639. [4] Y.L. Tung, L.S. Chen, Y. Chi, P.T. Chou, Y.M. Cheng, E.Y. Li, G.H. Lee, C.F. Shu, F.I. Wu, A. Carty, J. Adv. Funct. Mater. 16 (2006) 1615e1626. [5] P.T. Chou, Y. Eur Chi, J. Inorg. Chem. (2006) 3319e3332. [6] P.L. Burn, S. Lo, I.D.W. Samuel, Adv. Mater. 19 (2007) 1675e1688. [7] S.Y. Chang, Y.M. Cheng, Y. Chi, Y.C. Lin, C.M. Jiang, G.H. Lee, P.T. Chou, Dalton Trans. (2008) 6901e6911.

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