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-κO³,κO⁶ (thd) and C^N = a cyclometalating ligand bearing a pentafluorosulfanyl (-SF₅) electron-withdrawing group (EWG) at C4 (HL1) and C3 (HL2) positions of the phenyl moiety] is presented. These complexes have been fully structurally characterised, 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 V 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 behaviour (σ_m/σ_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 (λ_em = 484-506 nm) compared to reference complexes [Ir(ppy)₂(acac)], R1 (where acac = acetylacetonato) (λ_em = 528 nm in MeCN), [Ir(CF₃-ppy)(acac)], R3 (where CF₃-ppy = 2-(4-(trifluoromethyl)phenyl)pyridine) (λ_em = 522 nm in DCM) and [Ir(CF₃-ppy)₃], R8 (λem = 507 nm in MeCN). The emission of complex 1, by contrast, was modestly red-shifted (λem = 534 nm). Complexes 2 and 4, where the –SF₅ EWG is substituted para to the Ir-C_ppy 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 towards 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 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.