Photophysical and electroluminescence properties of bis(2′,6′-difluoro-2,3′-bipyridinato-N,C4′)iridium(picolinate) complexes: effect of electron-withdrawing and electron-donating group substituents at the 4′ position of the pyridyl moiety of the cyclometalated ligand

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Materials for optical and electronic devices, 2015-01, Vol.3 (28), p.7405-7420
• Main Authors: Bejoymohandas, K. S., Kumar, Arunandan, Varughese, S., Varathan, E., Subramanian, V., Reddy, M. L. P.
• Format: Article
• Language: English
• Subjects: Devices; Doping; Electroluminescence; Emission; Ligands; Phosphorescence; Quantum efficiency; Thin films
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/C5TC01260K

Herein, we have synthesized a series of 2′,6′-difluoro-2,3′-bipyridine cyclometalating ligands by substituting electron-withdrawing (–CHO, –CF 3 , and –CN) and electron-donating (–OMe and –NMe 2 ) groups at the 4′ position of the pyridyl moiety and utilized them for the construction of five new iridium( iii ) complexes ( Ir1–Ir5 ) in the presence of picolinate as an ancillary ligand. The photophysical properties of the developed iridium( iii ) compounds were investigated with a view to understand the substituent effects. The strong electron-withdrawing (–CN) group containing the iridium( iii ) compound ( Ir3 ) exhibits highly efficient genuine green phosphorescence ( λ max = 508 nm) at room temperature in solution and in thin film, with an excellent quantum efficiency ( Φ PL ) of 0.90 and 0.98, respectively. On the other hand, the –CF 3 group substituted iridium( iii ) compound ( Ir2 ) displays a sky-blue emission ( λ max = 468 nm) with a promising quantum efficiency ( Φ PL = 0.88 and 0.84 in solution and in thin film, respectively). The –CHO substituted iridium( iii ) complex ( Ir1 ) showed greenish-yellow emission ( λ max = 542 nm). Most importantly, the strong electron-donating –NMe 2 substituted iridium( iii ) complex ( Ir5 ) gives a structureless and a broad emission profile in the wavelength region 450 to 700 nm ( λ max = 520 nm) with a poor quantum efficiency. An intense blue phosphorescence with impressive quantum efficiency, especially in thin-film noted in the case of the –OMe substituted iridium( iii ) complex ( Ir4 ). Comprehensive density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches have been performed on the ground and excited states of the synthesized iridium( iii ) complexes, in order to obtain information about the absorption and emission processes and to gain deeper insights into the photophysical properties. The combinations of a smaller Δ E S1–T1 and higher contribution of 3 MLCT in the emission process result in the higher quantum yields and lifetime values for complexes Ir1–Ir3 . Multi-layered Phosphorescent Organic Light Emitting Diodes (PhOLEDs) were designed using the phosphorescent dopants Ir2 , Ir3 and Ir4 and their elecroluminescence properties were evaluated. Compound Ir4 at a doping level of 5 wt% shows the best performance with an external quantum efficiency of 4.7%, in the nonoptimized device, and a power efficiency of 5.8 lm W −1 , together with a true-blue chromacity CIE x,y = 0.15, 0.17 recorded at t