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 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∧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 (σm/σp 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∧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.

Introduction

Phosphorescent iridium complexes bearing arylpyridine cyclometalating (C∧N) and ancillary ligands (either anionic (L∧X) or neutral (L∧L)) have gained widespread interest among researchers because of their remarkable optoelectronic properties: e.g., good color tunability, high photoluminescence quantum yields (ΦPL), short emission lifetimes (τe), and high photo- and thermostability.[1−3] This confluence of properties render these complexes as attractive candidates as emitters for solid-state electroluminescent devices, the most common of which are organic light-emitting diodes (OLEDs) or light-emitting electrochemical cells (LEECs),[4−7] bioimaging agents,[8,9] and sensing applications.[10] With respect to their use in OLEDs, neutral Ir(III) complexes are generally more desirable in comparison to cationic Ir(III) complexes, as they can be easily vacuum deposited. High-efficiency white flat-panel displays require combined emission from red, green, and blue (RGB) emitters. While the color purity and efficiency of red and green Ir(III) emitters are satisfactory, the dearth of high-efficiency blue Ir(III) emitters remains an issue.

Unlike cationic iridium complexes, where color tuning is facile due to independent modulation of the electronics of both the C∧N and L∧L ligands, for neutral iridium complexes, bearing a nonchromophoric L∧X ligand, the emission color is governed by the electronics of the C∧N ligands. Substitution of the C∧N ligands with EWGs renders the HOMO to be more stabilized than the LUMO. This fact leads to an increased HOMO–LUMO gap, and thus blue emission is achieved. While the near-universal strategy of blue-shifting the emission by incorporation of one or two fluorine atoms in the C∧N ligand, such as 2-(4-fluorophenyl)pyridine (FppyH) or 2-(4,6-difluorophenyl)pyridine (dFppyH), is popular,[11−14] the issue of emitter degradation via defluorination negatively affects its incorporation into the emitter design.[15] Apart from fluorine atoms, other examples of EWGs used with a view to blue-shifting the emission of Ir(III) complexes consist of sulfonyl (−SO2R),[16−18] trifluoromethyl (−CF3),[19−23] trifluoromethoxy (−OCF3),[24,25] and pentafluorosulfanyl (−SF5).[26] These C∧N ligands are often used in conjunction with nonchromophoric ancillary ligands such as acac,[27] thd,[28] picolinate (pic),[29] and 3-oxo-1,3-diphenylprop-1-en-1-olate (dbm).[30] Although impressive performances have been achieved in solution, translating these performances into devices with good stability and efficiency metrics is still a challenge.[17] Thus, the design and syntheses of new blue-emitting phosphors for OLEDs and LEECs are essential.

Relatively strong intermolecular π–π interactions are generally observed for planar, nonhindered C∧N ligands, leading to the formation of small Ir(III) complex crystallites, which are responsible for unfavorable self-quenching.[31] Incorporation of bulky hydrophobic and chemically inert groups within the C∧N ligands leads to the prevention of such aggregate formation, while it also improves the photostability of these complexes in the amorphous phase. Introduction of bulky and strongly electron-withdrawing −SF5 groups (σp = 0.68)[32] instead of a trifluoromethyl (−CF3) group (σp = 0.54) should lead to significantly blue-shifted emission, concomitant with a blue-shifted absorption, due to stabilization of the C∧N-based HOMO. The −SF5 group is strongly electron-withdrawing and has been shown to be lipophilic, thermally and chemically stable, and also biologically active.[33−35] However, despite these favorable properties, as yet it is an underexplored moiety in the field of organic semiconductor materials.[26] Nevertheless, these properties make the −SF5 group an attractive candidate for replacing the commonly used C(aryl)–F motif, serving the same purpose of modulating the HOMO energy but without affecting the stability of the emitter.

The significance of the regiochemistry of the substituent on the C∧N ligand in tuning the emission wavelength has been demonstrated by molecular orbital analyses using density functional theory (DFT) calculations.[36,37] For example, the absorption and emission spectra of an Ir(III) complex bearing dFppy C∧N ligands (dFppyH = 2-(2,4-difluorophenyl)pyridine) are more blue-shifted in comparison to an analogous Ir complex with ppy C∧N ligands due to greater HOMO stabilization in comparison to LUMO stabilization. When the fluorine atom is located at the 5-position on the C∧N ligands (para to the Ir–CC bond), its inductive electron-withdrawing effect is counterbalanced by weak π donation, thereby raising the HOMO and reducing the band gap.[38]

In a recent study, we investigated the optoelectronic properties of cationic Ir(III) complexes bearing an −SF5 EWG on ppy or phenylpyrazole (ppz) C∧N ligands,[26] with the substituent position of the EWG varied so as to adopt either a para or meta relationship with respect to the Ir–CC bond. In this work, a family of neutral emissive Ir(III) complexes with the −SF5 group attached at the 4- (para with respect to the Ir–CC bond) or 5-position (meta with respect to the Ir–CC bond) of the C∧N ligands (HL1 and HL2; Chart ) is reported. To ensure neutrality of these complexes, three anionic ligands were employed: complexes 1 and 2 adopt two anionic C∧N ligands and 2,2,6,6-tetramethylheptane-3,5-dionato-κO3,κO6 (thd) as the ancillary ligand, while complexes 3 and 4 are fac-homoleptic complexes containing three C∧N ligands (Chart ). The effect of the meta/para position of the −SF5 EWG with respect to the Ir–CC bond on the optoelectronic properties of these complexes is discussed and corroborated on the basis of DFT calculations, with the results compared to several benchmark complexes (complexes R1–R10; Chart ).

Chart 1

Ligands HL1 and HL2 and Neutral Ir(III) Complexes 1–4

Chart 2

Reference Ir(III) Complexes R1–R10

Results and Discussion

Synthesis

The syntheses of the C∧N ligands and the iridium complexes 1–4 are shown in Scheme . As we reported previously, the C∧N ligands were synthesized under Stille[39] palladium-catalyzed cross-coupling conditions in good yields.[40] Ligands HL1 and HL2 were reacted with IrCl3·3H2O, and the resulting iridium dimers [Ir(L1)2(μ-Cl)]2 (D1) and [Ir(L2)2(μ-Cl)]2 (D2) were obtained in good yield and used directly in the next synthetic step.[41] Complexes 1 and 2 were isolated in good yield through cleavage of D1 and D2 with the thd ligand under basic conditions. The homoleptic complexes 3 and 4 were synthesized upon reaction of 3.1 equiv of the C∧N ligands with 1 equiv of Ir(acac)3.[27] A long reaction time (72 h) at high temperature (200 °C) favors the formation of the thermodynamically stable facial (fac) isomer in comparison to the kinetically stable meridional (mer) isomer,[27] as observed by 1H NMR spectroscopy. All the neutral complexes were purified by column chromatography. The successful syntheses of complexes 1-4 confirm the stability of the −SF5 group toward strong bases and high temperatures in organic alcoholic solvents. For both 3 and 4, 19F NMR indicated the presence of a small impurity (∼2%), which, in light of the satisfactory microanalysis, was inferred to be the mer isomer. This impurity could not be removed either by chromatography or by repeated recrystallization. In fact, the formation of an inseparable trace amount of the kinetically stable mer isomer during the synthesis of the fac isomer is already well documented.[42,43] Recrystallization of 3 and 4 on a small scale provided single crystals, which were found to be the expected fac isomer by X-ray crystallography (see Figure ). Complexes 1–4 are stable in the presence of air and moisture and are soluble in common organic solvents such as acetonitrile and dichloromethane.

Scheme 1

Synthesis of HL1 and HL2 and Complexes 1–4

Reagents and conditions. (a) 4.6–5.1 mol % of Pd(PPh3)4, N2, dry degassed PhMe, 120 °C, 48 h; (b) 2-EtOC2H4OH/H2O (3/1 v/v), 120 °C, N2, 24 h; (c) 2-MeOC2H4OH, anhydrous Na2CO3 (2.53 equiv), 110 °C, 24 h, N2; (d) (CH2OH)2, reflux, N2, 72 h.

Figure 1

Crystal structures of complexes 1–4. Thermal ellipsoids correspond to a 50% probability level. Hydrogen atoms, solvent molecules, and additional independent molecules are omitted for clarity.

All ligands, dimers and complexes were characterized by 1H, 19F and 13C NMR spectroscopy (Figures S1–S8 and S9–S28 in the Supporting Information), ESI-HRMS, melting point determination, and elemental analyses. The structures of complexes 1–4 were unequivocally determined by single crystal X-ray diffraction and corroborated the C2 (1 and 2) and C3 (3 and 4) symmetry assignments ascribed to the complexes on the basis of the solution-state 1H and 19F NMR. The downfield shift of the proton ortho to the cyclometalating carbon atom points toward the electron-withdrawing nature of the −SF5 group on the phenyl ring (Figure S6). A similar downfield shift was also found for the proton ortho to the carbon atom that is involved in the Cph–Cpyridine bond. The 19F NMR spectra exhibit a doublet for the equatorial fluorine atoms and a pentet for the axial fluorine for the −SF5 group in an intensity ratio of 4:1 as an AB4 system (Figure S7).[44] The HRMS analyses for 1–4 showed the indicative peak of the cation [M + H]+.

Crystal Structures

Crystals of 1–4 suitable for X-ray analysis were grown by slow diffusion of an antisolvent (1, ethanol; 2, methanol; 3, diethyl ether; 4, hexane) into concentrated solutions of the complexes in dichloromethane (Figure ). Table S1 in the Supporting Information contains relevant crystallographic parameters, and Table S2 in the Supporting Information compares selected bond distances and angles observed in the crystal to those predicted by DFT calculations. In all cases, the metal ion exhibits a pseudo-octahedral coordination geometry. In the case of the heteroleptic complexes 1 and 2, the pyridyl nitrogen atoms of the C∧N ligands are in a mutually trans relationship with respect to each other, as is common for many [Ir(C∧N)2(L∧X)] complexes, such as R1.[29] In the case of the homoleptic complexes, all of the nitrogen atoms are in a cis relationship. The average Ir–CC (1.995 Å) and Ir–CC (2.031 Å) bond distances in 1 and 2 are similar to those in R1 (Ir–CC, 1.991 Å; Ir–CC, 2.037 Å) (Table S2). A similar structural picture was found in [Ir(L1)2(dtBubpy)][PF6] in our previous study,[26] where dtBubpy is 4,4′-di-tert-butyl-2,2′-bipyridine (average Ir–CC distance 2.053 Å and average Ir–CC distance 2.018 Å). In 1 and 2 the average Ir–O distance (2.123 Å) was found to be longer in comparison to the Ir–CC (1.995 Å) and Ir–CC (2.031 Å) bond distances and was in accordance with the similarly longer Ir–O bond distance (2.159 Å) in the reference complex R1. The shorter average Ir–O distances in 1 and 2 in comparison to the average Ir–O distance in R1 are attributed to a strengthened bond, the result of an effective increase in the hardness of the Ir(III) center due to the presence of the electron-withdrawing −SF5 group. In the case of 3 and 4, the average Ir–NC distance (2.127 Å) was found to be longer than the Ir–CC distance (2.010 Å). In 1 and 2, the O–Ir–O bite angle (87.9(3)–88.3(1)°) is wider than that of CC–Ir–NC (80.5(3)–80.9(2)°). The bond distances and angles predicted by DFT calculations are consistent with those observed in the crystal structures (Table S2). In all of the complexes, the steric bulk of the tert-butyl (complexes 1 and 2) and −SF5 groups prohibit the formation of any significant π–π stacking interactions.

Electrochemical Properties

In order to assess the effect of the −SF5 group on the ground-state electronics of complexes 1–4, cyclic (CV) and differential pulse voltammetry (DPV) measurements were undertaken. Degassed MeCN was employed as the solvent, and the redox potentials are referenced with respect to SCE (Fc/Fc+ = 0.38 V in MeCN).[45] The relevant electrochemical data can be found in Table and Table S3 in the Supporting Information, while the CV and DPV traces are shown in Figure (the full set of redox potentials is detailed in Table S3).

Table 1

Redox Dataa of Complexes 1–4 (in Degassed MeCN) and Benchmark Complexes R1–R3 and R5–R10

							Hammett param (σ)
compd	Eox (ΔEp)/V (mV)	Ered (ΔEp)/V (mV)	ΔEredoxb/V	EHOMOc/eV	ELUMOc/eV	|ELUMO–HOMO|c/eV	σm	σp
1	1.08 (69)	–1.83 (irr)d	2.91	–5.57	–1.93	3.64	0.61
2	1.54 (75)	–1.38 (69), −1.86 (irr)d	2.92	–5.64	–1.80	3.84		0.68
3	1.10 (80)	–1.71 (irr)d	2.81	–5.66	–1.88	3.78	0.61
4	1.20 (59)	–1.88 (irr)d	3.08	–5.72	–1.74	3.98		0.68
R1e	0.86 (95)	–2.15 (125)	3.01				0	0
R2f	0.81			–5.17	–1.51	3.66	0	0
R3g	1.07						0.43
R5h	0.91 (100)						0.34
R6e	1.21 (115)	–1.99 (115)	3.20				0.34
R7i	0.76	–2.25	3.01				0	0
R8j	1.11	–2.13	3.24				0.43
R9j	1.10	–2.13	3.13				0.34
R10i	1.23	–2.06	3.29				0.34

Potentials reported vs SCE in MeCN using [n-Bu4N]PF6 as the supporting electrolyte. Measurements are recorded at room temperature at a scan rate of 100 mV s–1. The difference between the cathodic (Epc) and anodic (Epa) peak potentials, ΔEp (millivolts) for quasi-reversible redox waves is given in parentheses.

ΔEredox is |Eox – Ered|.

DFT calculated energy in eV.

Irreversible; potential is given as Epc.

From refs (29 and 46) in DMF (a correction factor of 0.05 V has been applied for direct comparison against SCE to calibrate the values in MeCN).

From ref (47) in THF (a correction factor of 0.16 V has been applied for direct comparison against SCE to calibrate the values in MeCN).

From ref (21) in DCM (measured using SCE as the standard (a correction factor of 0.06 V has been applied for direct comparison against SCE to calibrate the values in MeCN)).

From ref (38) in DMF (a correction factor of 0.05 V has been applied for direct comparison against SCE to calibrate the values in MeCN).

From ref (42) in DMF (a correction factor of 0.05 V has been applied for direct comparison against SCE to calibrate the values in MeCN).

From ref (22) in MeCN.

Figure 2

CV (solid) and DPV (dotted) traces of 1–4 in deaerated MeCN, recorded at 100 mV s–1.

At positive potential, complexes 1–4 each display oxidation waves that are quasi-reversible and single electron in nature, in the range of 1.08–1.54 V. DFT calculations indicate that incorporation of the −SF5 EWG results in a stabilization of the HOMOs of 1–4 in comparison to reference complex R2 and that the HOMOs of these complexes are comprised of almost equal contributions from the metal center and the C∧N ligands (Figure ). On the basis of the DFT calculations, and in line with literature precedent of related neutral Ir(III) complexes,[29,42] the first oxidation of complexes 1–4 is assigned to abstraction of an electron from the metal-based orbitals (the IrIII/IrIV redox couple) as well as some contribution from the C∧Nphenyl orbitals. The lower calculated HOMO energies for 1 (EHOMO = −5.57 eV), 2 (EHOMO = −5.64 eV), 3 (EHOMO = −5.66 eV), and 4 (EHOMO = −5.72 eV) in comparison to that of R2 (EHOMO = −5.17 eV) are consistent with the expected stabilization of the C∧N phenyl based orbitals by the −SF5 group in comparison to R2 (Table ). A significant anodic shift of the oxidation potential of 2 by 460 mV was observed in comparison to that of 1, which is due to the increasing electron-withdrawing nature of the −SF5 group when it is moved from a meta position to a para position with respect to the Ir–C bond of the C∧N ligands. This observation is also in agreement with the increasing Hammett parameter of the −SF5 group when it is positioned regiospecifically (σm = 0.61, σp = 0.68); this behavior is less pronounced for the fac-homoleptic complexes 3 and 4. Assuming the oxidation potentials are invariant with respect to solvent, the oxidation potentials of 1 and 2 were found to be more positive in comparison to those of R1–R3 and R5, implying that −SF5 is a stronger EWG in comparison to −F and −CF3, coincident with the smaller Hammett parameters of these substituents (Table ).[32] Assuming the fact that thd is a slightly better donor in comparison to acac, as implied by the slight cathodic shift of the oxidation potential of R2 in comparison to that of R1, complex R6 has an oxidation potential more positive than that of 1 but a value of E1/2ox lower than that of 2, suggesting that the electron-withdrawing ability follows the order L2 > dFppy > L1. Complexes 3 and 4 are harder to oxidize in comparison to the fac-homoleptic reference complexes R7–R9, which is in line with the Hammett parameters but are 0.03 V (for 4) to 0.13 V (for 3) easier to oxidize in comparison to R10.

Figure 3

DFT calculated frontier MO energies of 1–4 and R2, using B3LYP/SBKJC-VDZ for the Ir(III) metal center and 6-31G** for the atoms C, H, N, O, F, and S with CPCM(MeCN) and 0.5 eV threshold of degeneracy (isocontour of 0.03). Kohn–Sham MOs of 1–4 and R2 are also shown.

At negative potentials, multiple ligand-based multielectronic reductions can be observed for 1–4 (Figure ). While for 1, 3 and 4 the first reductions are irreversible, for 2 this reduction is found to be quasi-reversible and monoelectronic; the second reduction in 2 mirrors the behavior of the first reduction waves for 1, 3, and 4. For 1, 3, and 4, DFT calculations point to a LUMO that has predominant C∧N character with significant contribution from the −SF5 group, whereas for 2, the LUMO remains localized on the C∧N ligand but without the contribution from the −SF5 group. Therefore, and in line with our previous results for cationic iridium complexes,[26] the first reduction wave is plausibly assigned to direct reduction of the −SF5 moiety for 1, 3, and 4, where the −SF5 group, upon accepting an electron, may release a fluoride ion, thus rendering the reduction irreversible. Due to the strong electron-withdrawing nature of the −SF5 group, all of the first reduction potentials of 1–4 are anodically shifted by between 270 and 770 mV in comparison to that of R1. There is a noticeable anodic shift of 0.45 V of the first reduction potential of complex 2 in comparison to that of complex 1. This reduction wave in 2 is also distinctively reversible and as a consequence does not involve the −SF5 group and in fact represents reduction of the pyridine ring of the C∧N ligand. The DFT prediction of the LUMO energy actually aligns well with the second irreversible reduction wave (Ered2 = −1.86 (irr)). Further, the trend observed for the DFT-predicted LUMO energies matches the trend in reduction potentials for 1–4, ignoring the reversible reduction wave in 2. Extrapolated LUMO energies for complexes 1 and 2 (ELUMO(opt) = −3.61 eV for 1 and −4.65 eV for 2), where ELUMO(opt) = Eox + ΔEopt and the measured optical gap (ΔEopt) is determined from the energy of the 10% intensity of the lowest energy absorption onset; ΔEopt = 2.53 and 2.81 eV for 1 and 2, respectively (Figure ), corroborate the experimentally determined LUMO energies inferred directly from the electrochemical studies. Thus, DFT calculations for 2 do not predict the reversible pyridine-based reduction and therefore do not accurately predict the LUMO energy for this complex.

Figure 4

UV–vis absorption spectra of complexes 1–4 recorded in MeCN at 298 K (the inset spectrum shows the poorly absorbing bands of complexes 1–4 from 400 to 550 nm).

Photophysical Properties

The room-temperature UV–vis absorption spectra of 1–4 in MeCN are shown in Figure , and the data are summarized in Table S4 in the Supporting Information. Figure S29 in the Supporting Information compares the experimentally determined absorption spectra for each of the complexes with the transitions predicted by TD-DFT. The absorption spectra of complexes 1, 3, and 4 show two highly absorbing bands between 210 and 300 nm and additional, less absorptive bands beyond 300 nm. For complexes 1 and 2, prominent spin-allowed 1π → π* ligand-centered (1LC) transitions localized on the C∧N ligand and ligand-to-ligand charge transfer (1LLCT) transitions from the ancillary ligand to the C∧N ligands, as predicted by TD-DFT calculations, correlate with the high-energy bands. For complexes 3 and 4, these bands are 1π → π* ligand-centered (1LC) transitions as predicted by TD-DFT. The lower energy hypochromic bands between 270 and 300 nm are assigned to a 1LC transition for complexes 3 and 4, whereas for complexes 1 and 2 these 1LC transitions are mixed with 1MLCT transition contributions (Tables S5–S8 in the Supporting Information). The nature of the transitions between 300 and 400 nm becomes more complex with bands consisting of an admixture of 1LC and 1MLCT transitions, with varying but more significant 1MLCT content along with ligand-to-ligand transitions (1LLCT) in the case specifically for 1 and 2; the 1LLCT transitions are evidently absent for 3 and 4. For all of the complexes the band located between 412 and 456 nm is assigned as the HOMO → LUMO transition, which is principally 1MLCT (Ir(dπ) → L1/L2(π*)) in nature but mixed with 1LC (1π → π*) character (Tables S5–S8). Complexes 1–4 display a shoulder at λ >450 nm, albeit with very low molar absorptivity, which is a feature also observed for R1 at 487 nm and R2 at 468 nm (Table S4).[29,47] These bands are assigned as spin-forbidden 3MLCT and 3LLCT by direct population of the triplet state, which gains intensity by mixing with the higher lying 1MLCT through the strong spin–orbit coupling of the Ir metal center.[48] The spectra observed for 1–4 are very similar to those of the corresponding pentafluorosulfanyl-substituted cationic complex [Ir(L1)2(dtBubpy)][PF6], suggesting that the dominant absorptions in these complexes are due to the “(C∧N)2Ir” fragment.[26]

The emission spectra in deaerated MeCN solution at room temperature for complexes 1–4 are shown in Figure a, while Figure b shows the doped film (5 wt % in PMMA) emission spectra at room temperature. Table summarizes the relevant solution-phase photophysical data of 1–4 as well as the reference complexes R1–R10. The solid-state photoluminescence data are shown in Table . In MeCN solution, the emission of complexes 1–4 varies from sky blue to green, with the emission maxima in the range of 484–537 nm. The emission profiles are structured, which is an indication of an excited state that has 3LC character. Spin-unrestricted DFT calculations predict that the spin density is principally localized on a combination of the C∧N ligands and the metal center (Figure ), pointing toward a mixed 3LC and 3MLCT excited state. These predictions are in line with a variety of features that are characteristic of 3LC or 3MLCT emission: the structured vibronic features in the phosphorescence spectra and short (τe < 1.5 μs for 1, 3, and 4; τe = 3.02 μs for 2) radiative lifetimes.

Figure 5

(a) Emission spectra of complexes 1–4 in deaerated MeCN at room temperature (λexc 360 nm). (b) Normalized solid-state photoluminescence spectra of complexes 1–4 doped with 5 wt % PMMA.

Table 2

Relevant Photophysical Data for Complexes 1–4 and Some Benchmark Complexes

		emissionb
compd	absorptiona λabs/nm (ε/103 M–1 cm–1)	λem/nm (predicted λem/nm, relative error/%)	ΦPL/%	τε/μs	105kr/s–1	105knr/s–1
1	471 (0.25), 498 (0.05)	534, 563 (sh) (566, 5)	15	0.65	2.31	13.09
2	419 (0.28), 476 (0.07)	484, 516 (521, 4)	79	3.02	2.61	0.69
3	456 (0.19), 489 (0.08)	506, 537 (539, 3)	22	0.61	3.61	12.79
4	435 (0.17), 465 (0.07)	485, 506 (443, 11)	58	1.35	4.29	3.11
R1c	456 (0.23), 487 (0.09)	528	34d	1.6c
R2e	412, 468	525	43
R3f	464 (0.30)	522, 550g	32	1.72	1.86	3.95
R4h	479 (0.26)	541, 590g	43	1.14	3.77	4.99
R5i	444 (0.25), 474 (0.08)	493, 560g	40	1.5	2.67	4.00
R6c	428 (0.20), 458 (0.07)	491	62j	0.87j
R7k	455 (0.28), 488 (0.16)	518k	40	1.9	2.1	3.2
R8l	370g	507, 540g
R9l	358g	488
R10k	427 (0.16), 457 (0.03)	468	43	1.6	2.7	3.6

Absorption data are in solvents as mentioned in Table S4 in the Supporting Information.

In degassed MeCN at room temperature. Steady-state emission (in MeCN): λexc 360 nm. Time-resolved emission (in MeCN): λexc 378 nm. Solution ΦPL values were measured using quinine sulfate as the external reference (λem 450 nm in MeCN, Φr = 54.6% in 0.5 M H2SO4 as found in ref (50); the error in prediction of λem of complexes 1–4 were calculated using the equation error = |[λem(298 K) – EAE]/λem(298 K)| in eV × 100%, where EAE = adiabatic emission energy).

From ref (27) in MeCN (broad and structureless emission profile).

From ref (27) in 2-MeTHF.

From ref (51) in CHCl3 (broad and structureless emission profile).

From ref (21) in DCM (structured emission profile).

Values of vibronic bands were estimated by visual inspection of the corresponding spectra.

From ref (28) in toluene (structured emission profile).

From ref (38) in DCM (structured emission profile).

From ref (46) in DCM.

From ref (42) in 2-MeTHF (broad and structureless emission profile for R7 and only emission maximum value is reported for R10).

From ref (22) in MeCN (structured emission profile for R8, broad and structureless emission profile for R9).

Table 3

Solid-State Photophysical Data for 1–4 as 5 wt % Doped PMMA Films

compd	λem/nm	ΦPL/%	τe/μs
1	531, 559	35	1.54
2	489, 526, 558	23	1.36
3	507, 535	40	1.18
4	480, 509	51	1.15
R2a	518, 570b, 595b	33
R4a	538, 590b	49

From ref (28) as structured emission profiles for both R2 and R4, 4 wt % doped PMMA films.

Values of vibronic bands were estimated by visual inspection of the corresponding spectra.

Figure 6

TD-DFT calculated triplet spin density distributions of complexes 1–4, using UB3LYP/SBKJC-VDZ for Ir(III)) and 6-31g** for C, H, N, O, F, and S with CPCM(MeCN). Contours are at an isovalue of 0.02.

Complex 2 (where the −SF5 EWG is positioned para to the Ir–CC bond) exhibits an emission maximum that is blue-shifted in comparison to 1 (where the −SF5 EWG is positioned meta to the Ir–CC bond), which fits with the magnitude of the Hammett meta and para parameters of the −SF5 group. An analogous observation can be made for complexes 3 and 4. In line with the red shift of the absorption onset from 4 to 2 to 3 to 1 (see inset magnified spectra in Figure ), the emission maxima are also red-shifted accordingly (Table ) and this trend also follows the gradually decreasing HOMO–LUMO gap from 4 to 2 to 3 to 1, as calculated by DFT (Table and Figure ). The predicted emission maxima, EAE = E(T1) – E(S0) at the T1 optimized geometries (adiabatic electronic emission) obtained by DFT calculations[36] for 1–4, are respectively, at 566, 521, 539, and 443 nm and match closely those observed experimentally and reproduce the observed trend of red-shifted emission maxima from complex 4 to 2 to 3 to 1.

Incorporation of the −SF5 EWG on the cyclometalating phenyl rings helps to stabilize the frontier molecular orbitals (Figure ), leading to a blue shift in the observed emission color for complexes 2–4 in comparison to reference complexes R1 and R2 (Table ). For 1, slight red shifts in its emission maximum in comparison to those of R1 (6 nm, 213 cm–1) and R2 (9 nm, 321 cm–1) are observed due to a more stabilized emissive triplet state in 1 in comparison to those in R1 and R2. Considering the fact that 3LC emission is insensitive to solvent polarity,[28] the aforementioned suggestion of a more stabilized triplet state may also be applicable in justifying the observed red shift of the emission maximum of 1 in comparison to those of R3 and R5 and thus, surprisingly, are not in line with the Hammett parameters of the substituents (σm = 0.61 (−SF5), 0.43 (−CF3), 0.34 (−F)). Complex R6, bearing two electron-withdrawing fluorine atoms, displays blue-shifted emission in comparison to that of 1. Complex 2 with the −SF5 EWG positioned para to the Ir–CC bond (σp = 0.68 (−SF5)) is bluer than complexes R1–R6, where there exists meta substitution on the C∧N ligands with respect to the Ir–CC bond. Likewise, the −SF5 group promotes a more stable emissive triplet state in homoleptic complexes 3 and 4 that is responsible for their observed red shift in the emission maxima (3 vs R9 and R10 and 4 vs R10). In comparison to the emission maxima reported in our previous study involving cationic Ir(III) complexes of the form [Ir(L1)2(dtBubpy)][PF6] (λem,0–0 482 nm and λem,0–1 517 nm in degassed MeCN) and [Ir(L2)2(dtBubpy)][PF6] (λem,0–0 465 nm and λem,0–1 496 nm in degassed MeCN)[26] both the λem,0–0 and λem,0–1 peaks of 1 and 3 are red-shifted vs those of [Ir(L1)2(dtBubpy)][PF6], whereas the λem,0–0 and λem,0–1 peaks of 2 and 4 appear nearly at the same energy compared to those of [Ir(L1)2(dtBubpy)][PF6] (see Table for λem of 1–4). Both the λem,0–0 and λem,0–1 peaks of [Ir(L2)2(dtBubpy)][PF6] are, however, more blue-shifted in comparison to those of 1–4 (see Table for λem of 1–4).

The ΦPL values vary widely between 15 and 79% (Table ). The observed τe values are single component, pointing toward emission from a single species. Complexes 2 and 4, where the −SF5 EWG is positioned para to the Ir–CC bond, were found to be more brightly emissive with longer τe in comparison to those of complexes 1 and 3, where the −SF5 EWG is positioned meta to the Ir–CC bond. The calculated radiative (kr) and nonradiative (knr) decay constants (kr = ΦPL/τε and knr = (kr/ΦPL) – kr) are in the range of (2.31–4.29) × 105 and (0.69–13.09) × 105 s–1, respectively. The higher ΦPL values of 2 vs 1 and 4 vs 3 are supported by the decrease in nonradiative decay by 19 and 4 times, respectively. The low ΦPL values and high knr rates observed for complexes 1 and 3 may be explained by considering the deactivation of the emissive excited state through vibrational modes of the S–F bonds in the −SF5 group that is meta to the Ir–CC bond, as we[49] had previously observed in a cationic iridium complex containing a −CF3-substituted guanidylpyridine ancillary ligand. Frequency calculations of complexes 1 and 2 suggest that there is a strong coupling between the wagging mode of the C∧N ligand with the wagging mode of the equatorial S–F bonds of the −SF5 group in complex 1 (vibrational modes 46 and 47; Ev46 = 322.59 cm–1 and Ev47 = 324.23 cm–1; cf. Table S9 in the Supporting Information for other minor contributing vibrational modes that couple with the spin density). These couplings are found to be very weakly present in complex 2, as the −SF5 group is in the para position in this complex and is therefore farther away from the CC–Ir bond. A similar observation is found for the homoleptic complexes 3 and 4. The principal vibrational modes of deactivation of the excited state of complex 3 are 90 (Ev90 = 647.11 cm–1) and 116 (Ev116 = 865.29 cm–1), where the asymmetric stretching modes of the CC–CC bonds and Ir–NC bonds couple with the rocking mode of the equatorial S–F bonds and the equatorial out-of-plane vibration of the S atom of the −SF5 group (cf. Table S10 in the Supporting Information for other minor contributing vibrational modes that couple with the spin density). Similar to what was observed for 2, these deactivation modes are less pronounced in complex 4, where the −SF5 group is para to the CC–Ir bond.

In order to assess their potential as emitters in OLEDs, the PL properties of 1–4 were investigated as doped PMMA thin films (PMMA = poly(methyl methacrylate)). The sample films were fabricated by spin-coating 5 wt % of the emitter in PMMA in chlorobenzene solutions in air. In doped thin films, the complexes exhibit phosphorescence behavior similar to that in solution with ΦPL values of 23–51% (Figure b and Table ). The emission maxima of 1–4 in doped films are similar to those in solution. In the case of 1–4, the C∧N ligands mainly contribute to the T1 states, as shown in Figure , and thus the changes of the molecular dipole orientation are relatively small upon photoexcitation. As the PMMA molecules around the iridium complexes do not change their dipoles in the solid state, the nature of the T1 states remains unchanged, unlike the positive rigidochromic effect observed by Ikawa et al.[28] for iridium complexes containing an O∧O-based aromatic ancillary ligand where the T1 state changes in nature. Enhancement in ΦPL values of complexes 1 and 3 are observed in doped films in comparison to those in solution, while that of complex 2 decreases dramatically in the doped film. Complex 4 is virtually as efficient in the solid state (ΦPL = 51%) as in MeCN solution (ΦPL = 58%). The combination of molecular design, bulky −SF5 groups on the C∧N ligands and the tert-butyl groups of the thd ligand, and dispersion in PMMA that prevents intermolecular interactions of 1–4 makes the cause for the decrease in the ΦPL of 2 unclear at present.

Conclusions

In summary, four new neutral iridium(III) complexes bearing strongly electron withdrawing pentafluorosulfanyl groups on the cyclometalating ligands have been synthesized and structurally characterized, including by single-crystal X-ray diffraction. Their photophysical properties have been investigated, and the complexes have been found to display sky blue to blue-green emission (λem 484–537 nm). Complexes 1, 3, and 4 exhibit quasi-reversible oxidation and irreversible reduction waves, while for complex 2 the first reduction takes place on the pyridyl moiety of the C∧N ligands instead of the −SF5 groups for the other three complexes. The trend in the red shift of the optical gap is in line with the regiochemistry of the −SF5 EWG with respect to the Ir–CC bond. The nature of the emission of these complexes is an admixture of 3LC and 3MLCT, corroborated by DFT calculations, and the predicted emission maximum follows the trend of the experimental values of respective complexes. While complexes 1 and 3, bearing −SF5 EWG meta to the Ir–CC bond of the C∧N ligand, exhibit red-shifted emission in comparison to mono-/difluoro analogues (1 vs R5/R6 and 3 vs R9/R10), complexes 2 and 4, which are substituted by −SF5para to the Ir–CC bond of the C∧N ligand, display blue-shifted λem,0–0 in comparison to their mono-/difluoro analogues (2 vs R5/R6 and 4 vs R9). Thus, this study demonstrates the value of introduction of a pentafluorosulfanyl group onto the C∧N ligands para to the Ir–CC bond to promote a greater blue shift in the emission in comparison to the commonly employed dFppy ligands. The photophysical data of these emitters (λem 480–531 nm; ΦPL = 23–51%) on dispersion at a concentration of 5 wt % in PMMA thin films suggest that these complexes would be of interest as sky blue emitter replacements for commonly used phosphors, such as FIrpic, [Ir(dFppy)(pic)]. Current efforts are underway to evaluate them in OLEDs.

Experimental Section

General Synthetic Procedures

Commercial chemicals were used without further purification. Ligands HL1 and HL2 were synthesized using a literature procedure.[26] All reactions were performed using standard Schlenk techniques under an inert (N2) atmosphere with reagent grade solvents. Flash column chromatography was performed using silica gel (60 Å, 40–63 μm). Silica plates with aluminum backings (250 μm with indicator F-254) were used for analytical thin layer chromatography (TLC). Compounds were visualized under UV irradiation. 1H (for ligands and dimers), 13C, and 19F NMR spectra were recorded on a Bruker Avance spectrometer at 400, 125, and 376 MHz, respectively. The following abbreviations have been used for multiplicity assignments: “s” for singlet, “d” for doublet, “t” for triplet, “p” for pentet, “m” for multiplet, and “br” for broad. Deuterated chloroform (CDCl3) and deuterated dichloromethane (CD2Cl2) were used as the solvents of record. 1H and 13C NMR spectra were referenced with respect to the NMR solvent peaks. An Electrothermal melting point apparatus was used to record melting points (mps). Mps were recorded in open-ended capillaries and are uncorrected. Thermogravimetric analysis (TGA) data were collected on a TA Instruments SDT 2960 apparatus. High-resolution mass spectra (HRMS) were recorded at the EPSRC UK National Mass Spectrometry Facility at Swansea University on a quadrupole time-of-flight (Q-TOF) instrument using a Model ABSciex 5600 Triple TOF in positive electrospray ionization (pESI) mode, and spectra were recorded using sodium formate solution as the calibrant. Elemental analyses were performed by Mr. Stephen Boyer, London Metropolitan University.

Syntheses of Precursor [Ir(C∧N)2(μ-Cl)]2 Dimers D1 and D2

The dimers were synthesized following literature procedures.[25]

Tetrakis[2-(4-(pentafluoro-λ6-sulfanyl)phenyl)pyridinato-N,C2′]bis(μ-chloro)diiridium(III), [Ir(L1)2(μ-Cl)]2 (D1)

IrCl3·3H2O (0.2 g, 0.57 mmol, 1 equiv) and HL1 (0.36 g, 1.3 mmol, 2.28 equiv) in a degassed mixture of 2-ethoxyethanol (6 mL) and water (2 mL) affords the dimer D1 as yellow solid. Yield: 0.41 g, 91%. 1H NMR (400 MHz, CDCl3, main component) δ (ppm): 9.23–9.19 (m, 4H), 8.01–7.97 (m, 4H), 7.92 (td, J = 7.4, 1.5 Hz, 4H), 7.58 (d, J = 8.6 Hz, 4H), 7.22 (d, J = 8.6, 2.2 Hz, 4H), 6.97 (ddd, J = 7.3, 5.8, 1.5 Hz, 4H), 6.12 (d, J = 2.2 Hz, 4H). 19F{1H} NMR (377 MHz, CDCl3, main component) δ (ppm): 84.9 (p, J = 151 Hz, 4F), 61.7 (d, J = 151 Hz, 16F). The characterization data match those previously reported.[26]

Tetrakis[2-(3-(pentafluoro-λ6-sulfanyl)phenyl)pyridinato-N,C2′]bis(μ-chloro)diiridium(III), [Ir(L2)2(μ-Cl)]2 (D2)

IrCl3·3H2O (0.2 g, 0.57 mmol, 1 equiv) and HL2 (0.36 g, 1.3 mmol, 2.28 equiv) in a degassed mixture of 2-ethoxyethanol (6 mL) and water (2 mL) affords the dimer D2 as a yellow solid. Yield: 0.38 g, 85%. 1H NMR (400 MHz, CDCl3, main component) δ (ppm): 9.17 (dd, J = 5.8, 0.8 Hz, 4H), 8.01 (d, J = 7.8 Hz, 4H), 7.91 (td, J = 7.5, 1.5 Hz, 4H), 7.86 (d, J = 2.4 Hz, 4H), 6.97 (dd, J = 8.7, 2.4 Hz, 4H), 6.93 (ddd, J = 7.3, 5.8, 1.4 Hz, 4H), 5.96 (d, J = 8.6 Hz, 4H). 19F{1H} NMR (377 MHz, CDCl3, main component) δ (ppm): 85.9 (p, J = 151 Hz, 4F), 63.0 (d, J = 150 Hz, 16F). The characterization data match those previously reported.[26]

Syntheses of [Ir(C∧N)2(thd)] (1 and 2) and [Ir(C∧N)3] (3 and 4) Complexes

Bis[2-(4-(pentafluoro-λ6-sulfanyl)phenyl)pyridinato-N,C2′](2,2,6,6-tetramethylheptane-3,5-dionato-κO3,κO6)iridium(III), [Ir(L1)2(thd)] (1)

The reaction was performed under nitrogen. The precursor dimer complex [Ir(L1)2(μ-Cl)]2 (150 mg, 0.095 mmol, 1 equiv), anhydrous Na2CO3 (25 mg, 0.24 mmol, 2.53 equiv), and 2,2,6,6-tetramethyl-3,5-heptanedione (54 mg, 0.29 mmol, 3.05 equiv) were stirred in degassed 2-methoxyethanol (4 mL) at 110 °C for 24 h to give an orange mixture. The mixture was cooled to room temperature, and water (50 mL) was added. The suspension was stirred for 10 min and filtered. The solid product was washed with water and with methanol/water (2/1, v/v) and was then purified by column chromatography on silica (17 g; 40–63 μm). The elution was performed with hexane/dichloromethane (1/1, v/v) to give a yellow fraction. The solution was evaporated to dryness. The product still contained the starting ligand. Therefore, the product was dissolved in methanol (17 mL), and it was precipitated on stirring with water (8.5 mL). The product was filtered and washed with methanol/water (2/1, v/v). Orange solid. Yield: 113 mg, 64%. Rf: 0.51 (hexanes/DCM: 3/2, v/v, on silica). Mp: 254–255 °C. TGA: >350 °C (5% decomposition). 1H NMR (400 MHz, CD2Cl2) δ (ppm): 8.39 (ddd, J = 5.7, 1.5, 0.8 Hz, 2H), 7.96 (d, J = 7.9 Hz, 2H), 7.89–7.83 (m, 2H), 7.67 (d, J = 8.6 Hz, 2H), 7.30–7.24 (m, 4H), 6.56 (d, J = 2.3 Hz, 2H), 5.57 (s, 1H), 0.90 (s, 18H). 13C NMR (125 MHz, CD2Cl2) δ (ppm): 195.60, 166.77, 149.26, 149.11, 148.88, 138.39, 129.45, 128.96, 123.92, 123.64, 120.10, 118.87, 90.83, 41.60, 28.32. 19F{1H} NMR (377 MHz, CD2Cl2) δ (ppm): 86.3 (p, J = 149 Hz, 2F), 62.2 (d, J = 149 Hz, 8F). HR APCIMS: [M + H]+ (100%) calcd 937.1524, (C33H34F10IrN2O2S2+); found 937.1543. Anal. Calcd for C33H33F10IrN2O2S2: C, 42.35; H, 3.55; N, 2.99. Found: C, 42.45; H, 3.50; N, 3.03.

Bis[2-(3-(pentafluoro-λ6-sulfanyl)phenyl)pyridinato-N,C2′](2,2,6,6-tetramethylheptane-3,5-dionato-κO3,κO6)iridium(III), [Ir(L2)2(thd)] (2)

The reaction was performed under nitrogen. The precursor dimer complex [Ir(L2)2(μ-Cl)]2 (150 mg, 0.095 mmol, 1 equiv), anhydrous Na2CO3 (25 mg, 0.24 mmol, 2.53 equiv), and 2,2,6,6-tetramethyl-3,5-heptanedione (60 mg, 0.33 mmol, 3.47 equiv) were stirred in degassed 2-methoxyethanol (4 mL) at 110 °C for 24 h to give a yellow mixture. The mixture was cooled to room temperature, and water (50 mL) was added. The suspension was stirred for 10 min and filtered. The solid product was washed with water and with methanol/water (2/1, v/v) and was then purified by column chromatography on silica (17 g, 40–63 μm). The elution was performed with hexane/dichloromethane (1/2, v/v) to give a yellow fraction. The solution was evaporated to dryness. The glassy product was suspended in methanol (10 mL) and sonicated for 1 min. The product is not very soluble in methanol. Water was added (5 mL). The mixture was sonicated again for 1 min. The product was filtered and washed with methanol/water (2/1, v/v). Yellow solid. Yield: 149 mg, 84%. Rf: 0.57 (hexanes/DCM: 3/2, v/v, on silica). Mp: 324–325 °C dec. TGA: 98 °C (5% decomposition). 1H NMR (400 MHz, CD2Cl2) δ (ppm): 8.38 (ddd, J = 5.7, 1.5, 0.8 Hz, 2H), 7.97–7.91 (m, 4H), 7.89–7.83 (m, 2H), 7.25 (ddd, J = 7.3, 5.7, 1.4 Hz, 2H), 7.03 (dd, J = 8.5, 2.4 Hz, 2H), 6.44 (d, J = 8.5 Hz, 2H), 5.59 (s, 1H), 0.90 (s, 18H). 13C NMR (125 MHz, CD2Cl2) δ (ppm): 195.66, 167.10, 156.07, 148.95, 145.97, 138.35, 133.75, 128.96, 125.63, 123.29, 120.91, 119.46, 90.82, 41.61, 28.33. 19F{1H} NMR (377 MHz, CD2Cl2) δ (ppm): 87.4 (p, J = 149 Hz, 2F), 63.7 (d, J = 149 Hz, 8F). HR APCI MS: [M + H]+ (100%) calcd 937.1524, (C33H34F10IrN2O2S2+); found 937.1535. Anal. Calcd for C33H33F10IrN2O2S2: C, 42.35; H, 3.55; N, 2.99. Found: C, 42.21; H, 3.44; N, 2.91.

General Procedure for the Synthesis of fac-[Ir(C∧N)3] Complexes

In a round-bottom flask containing Ir(acac)3 (1.0 equiv) and C∧N ligand (3.1 equiv) was placed ethylene glycol to give a suspension with a concentration of ca. 0.3 M. The mixture was sealed and then degassed by repeated vacuum–N2 cycles, before being placed under N2. The reaction mixture was heated to reflux for 72 h. The mixture was then cooled to room temperature, and water was added. The mixture was then extracted with DCM and dried over MgSO4 before filtering under reduced pressure. The material was then purified by column chromatography on silica gel, using first a hexanes/Et2O mixture (80/20, v/v) to remove the ligand and then DCM or DCM/MeOH (95/5, v/v) to elute the complex. For both complexes 19F NMR indicated the presence of a small impurity, which was assigned as the mer isomer given the microanalysis.

fac-Tris[2-(4′-pentafluorosulfanyl)-pyridinato-N,C2′]iridium(III), fac-[Ir(L1)3] (3)

Column conditions: DCM. Yellow powder. Yield: 0.111 g, 42%. Rf: 0.80 (hexanes/DCM: 2/3, v/v, on silica). Mp: 375 °C dec. TGA: 217 °C (5% decomposition). 1H{19F} NMR (400 MHz, CD2Cl2) δ (ppm): 8.01 (d, J = 8.2, Hz, 3H), 7.80 (td, J = 8.1, 1.6 Hz, 3H), 7.79 (d, J = 8.6 Hz, 3H), 7.61 (dd, J = 5.6, 0.8 Hz, 3H), 7.33 (dd, J = 8.6, 2.4 Hz, 3H), 7.11 (ddd, J = 7.0, 5.6, 1.3 Hz, 3H), 7.01 (d, J = 2.4 Hz, 3H). 13C NMR (125 MHz, CD2Cl2) δ (ppm): 164.7, 159.2, 147.9, 147.4, 137.8, 132.8, 124.4, 124.3, 120.8, 118.5 (one quaternary 13C NMR signal was found to be missing). 19F{1H} NMR (377 MHz, CD2Cl2) δ (ppm): 85.85 (p, J = 148.77 Hz, 3F), 61.90 (d, J = 148.77 Hz, 12F). HR-MS (TOF MS ASAP+): [M + H]+ (100%) calcd 1034.0367, (C33H22N3F15S3Ir+); found 1034.0363. Anal. Calcd for C33H21N3F15S3Ir (MW 1032.93): C, 38.37; H, 2.05; N, 4.07. Found: C, 38.61; H, 2.36; N, 3.86 (average of two runs).

fac-Tris[2-(5′-pentafluorosulfanyl)-pyridinato-N,C2′]iridium(III), fac-[Ir(L2)3] (4)

Column conditions: DCM/MeOH (95/5, v/v). Yellow powder. Yield: 0.163 g, 78%. Rf: 0.87 (hexanes/DCM: 2/3, v/v, on silica). Mp: 345 °C dec. TGA: 157 °C (5% decomposition). 1H{19F} NMR (400 MHz, CD2Cl2) δ (ppm): 8.03–7.98 (m, 6H), 7.80 (ddd, J = 8.2, 7.4, 1.6 Hz, 3H), 7.55 (ddd, J = 5.5, 1.7, 0.8 Hz, 3H), 7.16 (dd, J = 8.5, 2.4 Hz, 3H), 7.08 (ddd, J = 7.3, 5.6, 1.2 Hz, 3H), 6.84 (d, J = 8.5 Hz, 3H). 13C NMR (125 MHz, CD2Cl2) δ (ppm): 165.7, 164.8, 147.8, 144.6, 137.9, 136.8, 126.6, 124.1, 121.3, 120.1 (one quaternary 13C NMR signal was found to be missing). 19F{1H} NMR (377 MHz, CD2Cl2) δ (ppm): 87.57 (p, J = 149.53 Hz, 3F), 63.63 (d, J = 149.53 Hz, 12F). HR-MS (TOF MS NSI+): [M + H]+ (100%) calcd 1034.0367 (C33H22N3F15S3Ir+); found 1034.0364. Anal. Calcd for C33H21N3F15S3Ir (MW 1032.93): C, 38.37; H, 2.05; N, 4.07. Found: C, 38.61; H, 2.36; N, 3.86 (average of two runs).

X-ray Crystallography

Single crystals were grown by diffusion of an anti-solvent (1, ethanol; 2, methanol; 3, diethyl ether; 4, hexane) into concentrated solutions of the complexes in dichloromethane (CCDC: 1527433–1527436). Crystallographic techniques similar to those found in the study of Pal et al. was adopted to determine the solid-state structures.[25] Structures were solved by Patterson (PATTY; 1, 3),[52] direct (SIR2004; 2),[53] or dual-space (SHELXT; 4)[54] methods and refined by full-matrix least squares against F2 (SHELXL-2013).[54]

Photophysical Measurements

HPLC grade MeCN was used to prepare sample solutions with varying concentrations in the order of micromoles. Detailed techniques of UV–vis spectroscopy, steady-state, time-resolved photoluminescence spectroscopy, determination of photoluminescence quantum yields by optical dilution method,[55,56] and respective instruments used were the same as found in the study of Pal et al.[25]

Electrochemistry Measurements

Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements were performed following the procedure in the study of Pal et al.[25]

Estimated uncertainties on measurements: UV–vis absorption spectra, ± 2 nm; molar extinction coefficients, 10%; CV and DPV redox potentials, ±10 mV; steady-state emission spectra, ±3 nm; excited state lifetimes, ±10%; photoluminescence quantum yields, ±5%.