Blue-to-Green Emitting Neutral Ir(III) Complexes Bearing Pentafluorosulfanyl Groups: A Combined Experimental and Theoretical Study

Abstract

A structure-property relationship study of neutral heteroleptic (1 and 2, [Ir(Cλ N)2(LλX)]) and homoleptic (3 and 4, fac-[Ir(CλN)3]) Ir(III) complexes (where LλX = anionic 2,2,6,6-tetramethylheptane-3,5-dionato- κO3,κO6 (thd) and CλN = a cyclometalating ligand bearing a pentafluorosulfanyl (-SF5) electron-withdrawing group (EWG) at the C4 (HL1) and C3 (HL2) positions of the phenyl moiety) is presented. These complexes have been fully structurally characterized, 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 pyridineC λ N-based first reduction wave that is anodically shifted at -1.38 V. The metal + CλN 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 (sigma;m/sigma;p with respect to the metal-carbon bond) of the -SF5 EWG. In degassed acetonitrile, for complexes 2-4, introduction of the -SF5 group produced a blue-shifted emission (λem 484-506 nm) in comparison to reference complexes [Ir(ppy)2(acac)] (R1, where acac = acetylacetonato) (λem 528 nm in MeCN), [Ir(CF3-ppy) (acac)] (R3, where CF3-ppyH = 2- (4-(trifluoromethyl)phenyl)pyridine) (λem 522 nm in DCM), and [Ir(CF3-ppy)3] (R8) (λem 507 nm in MeCN). The emission of complex 1, in contrast, was modestly red shifted (λem 534 nm). Complexes 2 and 4, where the -SF5 EWG is substituted para to the Ir-CC λ N bond, are efficient phosphorescent emitters, with high photoluminescence quantum yields (φPL = 58-79% in degassed MeCN solution) and microsecond emission lifetimes (τϵ = 1.35-3.02 μs). Theoretical and experimental observations point toward excited states that are principally ligand centered (3LC) in nature, but with a minor metal-to-ligand charge-transfer (3MLCT) transition component, as a function of the regiochemistry of the pentafluorosulfanyl group. The 3LC character is predominant over the mixed 3CT character for complexes 1, 2, and 4, while in complex 3, there is exclusive 3LC character as demonstrated by unrestricted density functional theory (DFT) calculations. The short emission lifetimes and reasonable φPL 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.