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(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-κ O ³ ,κO ⁶ (thd) and C ^∧N = a cyclometalating ligand bearing a pentafluorosulfanyl (−SF ₅) electron-withdrawing group (EWG) at the C ₄ (H L1) and C ₃ (H L2) 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 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 ( 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.

Abstract

Two heteroleptic ( 1 and 2) and two homoleptic ( 3 and 4) neutral Ir(III) complexes bearing a strongly electron withdrawing −SF ₅ group on the cyclometalating ligand at different regiospecific positions have been synthesized and optoelectronically characterized. Complexes 2 and 4, where the −SF ₅ group is para to the Ir−C _ppy bond, exhibit blue-shifted emission in comparison to 1 and 3. In general, the −SF ₅ group serves as an effective electron-withdrawing group for blue-shifting the emission of neutral iridium complexes.