Blue-to-Green Emitting Neutral Ir(III) Complexes Bearing Pentafluorosulfanyl Groups: A Combined Experimental and Theoretical Study
A structure−property relationship study of neutral heteroleptic (1 and 2, [Ir(C ∧ N) 2 (L ∧ X)]) and homoleptic (3 and 4, fac-[Ir(C ∧ N) 3 ]) Ir(III) complexes (where L ∧ X = anionic 2,2,6,6-tetramethylheptane-3,5-dionato-κO 3 ,κO 6 (thd) and C ∧ N = a cyclometalating ligand bearing a pentafluorosulfanyl (−SF 5 ) electron-withdrawing group (EWG) at the C 4 (HL1) and C 3 (HL2) positions of the phenyl moiety) is presented. These complexes have been fully structurally characterized, including by
... zed, including by single-crystal X-ray diffraction, and their electrochemical and optical properties have also been extensively studied. While complexes 1 ([Ir(L1) 2 (thd)]), 3 (Ir(L1) 3 ), and 4 (Ir(L2) 3 ) exhibit irreversible first reduction waves based on the pentafluorosulfanyl substituent in the range of −1.71 to −1.88 V (vs SCE), complex 2 ([Ir(L2) 2 (thd)]) exhibits a quasi-reversible pyridine C ∧ N -based first reduction wave that is anodically shifted at −1.38 V. The metal + C ∧ N ligand oxidation waves are all quasi-reversible in the range of 1.08−1.54 V (vs SCE). The optical gap, determined from the lowest energy absorption maxima, decreases from 4 to 2 to 3 to 1, and this trend is consistent with the Hammett behavior (σ m /σ p with respect to the metal−carbon bond) of the −SF 5 EWG. In degassed acetonitrile, for complexes 2−4, introduction of the −SF 5 group produced a blue-shifted emission (λ em 484−506 nm) in comparison to reference complexes [Ir(ppy) 2 (acac)] (R1, where acac = acetylacetonato) (λ em 528 nm in MeCN), [Ir(CF 3 -ppy) (acac)] (R3, where CF 3 -ppyH = 2-(4-(trifluoromethyl)phenyl)pyridine) (λ em 522 nm in DCM), and [Ir(CF 3 -ppy) 3 ] (R8) (λ em 507 nm in MeCN). The emission of complex 1, in contrast, was modestly red shifted (λ em 534 nm). Complexes 2 and 4, where the −SF 5 EWG is substituted para to the Ir−C C ∧ N bond, are efficient phosphorescent emitters, with high photoluminescence quantum yields (Φ PL = 58−79% in degassed MeCN solution) and microsecond emission lifetimes (τ ε = 1.35−3.02 μs). Theoretical and experimental observations point toward excited states that are principally ligand centered ( 3 LC) in nature, but with a minor metal-to-ligand charge-transfer ( 3 MLCT) transition component, as a function of the regiochemistry of the pentafluorosulfanyl group. The 3 LC character is predominant over the mixed 3 CT character for complexes 1, 2, and 4, while in complex 3, there is exclusive 3 LC character as demonstrated by unrestricted density functional theory (DFT) calculations. The short emission lifetimes and reasonable Φ PL values in doped thin film (5 wt % in PMMA), particularly for 4, suggest that these neutral complexes would be attractive candidate emitters in organic light-emitting diodes.