Regioisomerism in cationic sulfonyl-substituted [ Ir ( C ^ N ) 2 ( N ^ N ) ] + complexes : its in fl uence on photophysical properties and LEC performance †

A series of regioisomeric cationic iridium complexes of the type [Ir(C^N)2(bpy)][PF6] (bpy = 2,2’-bipyridine) is reported. The complexes contain 2-phenylpyridine-based cyclometallating ligands with a methylsulfonyl group in either the 3-, 4or 5-position of the phenyl ring. All the complexes have been fully characterized, including their crystal structures. In acetonitrile solution, all the compounds are green emitters with emission maxima between 493 and 517 nm. Whereas substitution meta to the Ir–C bond leads to vibrationally structured emission profiles and photoluminescence quantum yields of 74 and 77%, placing a sulfone substituent in a para position results in a broad, featureless emission band, an enhanced quantum yield of 92% and a shorter excited-state lifetime. These results suggest a larger ligand-centred (LC) character of the emissive triplet state in the case of meta substitution and a more pronounced charge transfer (CT) character in the case of para substitution. Going from solution to the solid state (powder samples and thin films), the emission maxima are red-shifted for all the complexes, resulting in green-yellow emission. Data obtained from electrochemical measurements and density functional theory calculations parallel the photophysical trends. Light-emitting electrochemical cells (LECs) based on the complexes were fabricated and evaluated. A maximum efficiency of 4.5 lm W at a maximum luminance of 940 cd m was observed for the LEC with the complex incorporating the sulfone substituent in the 4-position when operated under pulsed current driving conditions.


Introduction
Ionic transition metal complexes (iTMCs), 1,2 in particular bis-cyclometallated cationic iridium(III) complexes of the type [Ir(C^N) 2 (N^N)] + , are promising candidates for application in light-emitting electrochemical cells (LECs) and have therefore been intensely studied as emissive materials. In this class of iridium emitters, high quantum yields are possible with emission colours spanning the whole visible spectrum, combined with high stability of the complexes. 3,4 Emission colour tuning is possible by varying the cyclometallating (C^N) ligands as well as the ancillary (N^N) ligand. The frontier orbitals in these types of complexes are spatially separated, with the HOMO being located mainly on the iridium centre and the C^N ligands and the LUMO on the N^N ligand. [5][6][7] An increase in the HOMO-LUMO gap and a resulting blue-shift in the emission maximum are therefore achieved by the combination of electron-withdrawing substituents on the C^N ligands and electron-donating groups on the N^N ligand. 3,4,8 While fluorine substituents have been widely used as electron-withdrawing groups on the cyclometallating ligands, [9][10][11][12] the use of sulfone groups has remained limited. Most examples of sulfone-substituted cyclometallating ligands in iridium complexes are those of neutral emitters used in organic light-emitting diodes (OLEDs). [13][14][15][16][17][18][19] Recently, we have reported the synthesis and characterization of cationic [Ir(C^N) 2 (N^N)] + complexes containing sulfone-substituted phenylpyrazole 20 or phenylpyridine ligands, [21][22][23] which give promising performances in LECs. In comparison with the archetypal [Ir( ppy) 2 (bpy)][PF 6 ] 24 (Hppy = 2-phenylpyridine, bpy = 2,2′-bipyridine), replacement of Hppy by 2-(4-methylsulfonylphenyl)pyridine (H1) leads to a 92 nm blue-shift of the emission maximum in MeCN solution (complex [Ir(1) 2 6 ] in Scheme 1). 22 With the aim of achieving blue emission, we recently reported the synthesis of a series of iridium complexes with 2-(4-methylsulfonylphenyl)pyridine (H1) and 2-(3-methylsulfonylphenyl) pyridine (H2) as cyclometallating ligands and electron-rich pyrazolylpyridine as the diimine N^N ligand. 23 Surprisingly, a difference of ∼30 nm in the emission maximum was observed on changing the substitution position of the sulfone group.

(bpy)][PF
There have been few investigations on the influence of the substitution position of ligand functionalities on the properties of [Ir(C^N) 2 (N^N)] + complexes. Examples include the introduction of diphenylamino, 25 fluoro, 26 methyl, 27 trifluoromethyl, 28,29 methylpyridinium, 30 bromo and fluorenyl 31 and benzylsulfonyl 15 groups in the 3-, 4-and 5-positions of the cyclometallating phenyl ring. In all of these studies, except for the one investigating bromo and fluorenyl groups, 31 it was found that substitution in the 4-position, i.e. para to the Ir-C bond, has the largest influence on emission colour. This observation was supported by density functional theory (DFT) calculations. 27 In most of the cases, however, cyclometallating ligands with substituents in the 5-position, meta to the Ir-C bond, have been used. Only a few examples of iridium complexes with substituents in the 4-position of the cyclometallating ligands have been reported, 32,33 and even fewer with substituents in the 3-position. 34 Herein, we report the synthesis of a series of [Ir-(C^N) 2 (bpy)][PF 6 ] complexes with methylsulfonyl groups in the 3-, 4-and 5-positions of the phenyl ring of the cyclometallating ligands (Scheme 1). The effect of the substitution position on the photophysical and electrochemical properties was investigated and further analysed with the help of DFT calculations. The electroluminescence properties of the complexes have been investigated in LEC devices operated under pulsed driving conditions.

General
Microwave reactions were carried out in a Biotage Initiator 8 reactor. 1 H and 13 C{ 1 H} NMR spectra as well as 2D NMR spectra were recorded on a Bruker Avance III-500 spectrometer at 295 K; chemical shifts were referenced to residual solvent peaks with δ (TMS) = 0 ppm. For electrospray ionization (ESI) and LC-ESI mass spectra, a Bruker Esquire 3000 plus spectrometer and a combination of Shimadzu (LC) and Bruker AmaZon X instruments were used, respectively. FT-IR spectra were recorded on a Perkin Elmer Spectrum Two UATR instrument. Absorption spectra were recorded on an Agilent 8453 spectrophotometer and solution emission spectra on a Shimadzu 5301PC spectrofluorophotometer. Solution and powder photoluminescence quantum yields were recorded on a Hamamatsu absolute PL quantum yield spectrometer, C11347 Quantaurus QY. Emission spectra of powder samples as well as solution and powder excited-state lifetime measurements were carried out using a Hamamatsu Compact Fluorescence Lifetime spectrometer, C11367 Quantaurus Tau. The photoluminescence (PL) properties (spectra and quantum yields) in thin films were measured using a Hamamatsu absolute quantum yield C9920. Electrochemical measurements were performed using cyclic and square wave voltammetry on a CH Instruments 900B potentiostat with glassy carbon working and platinum auxiliary electrodes; a silver wire was used as a pseudo-reference electrode. Dry, purified CH 3 CN was used as the solvent and 0.1 M [ n Bu 4 N][PF 6 ] as the supporting electrolyte. Ferrocene as an internal reference was added at the end of each experiment.

Synthesis
The synthesis of [Ir(1) 2 (bpy)][PF 6 ], 22  General procedure for the synthesis of iridium(III) complexes Iridium dimers and ancillary ligand were suspended in MeOH (15 mL) in a microwave vial and heated at 120°C for 1 h in a microwave reactor (14 bar). The resulting yellow solution was filtered through cotton and concentrated under reduced pressure. The residue was dissolved in a little MeOH, an excess of solid NH 4 PF 6 was added and the resulting suspension was stirred for 5 min at room temperature. The yellow precipitate was filtered off and redissolved in CH 2 Cl 2 . The solvent was removed under reduced pressure, the crude product was purified by column chromatography (silica) and the solvent was removed under reduced pressure. The residue was dissolved in a little CH 2 Cl 2 , precipitated with Et 2 O and left in the fridge overnight. The resulting precipitation was filtered off, washed with MeOH and Et 2 O and dried under vacuum.

Crystallography
Single crystal data were collected on a Bruker APEX-II diffractometer; for data reduction, solution and refinement, APEX was used. 37

Computational details
DFT calculations were carried out with the D.01 revision of the Gaussian 09 program package 39 using Becke's three-parameter B3LYP exchange-correlation functional 40,41 together with the 6-31G** basis set for C, H, N, S and O 42 and the "double-ζ" quality LANL2DZ basis set for the Ir element. 43 An effective core potential (ECP) replaces the inner core electrons of Ir leaving the outer core [(5s) 2 (5p) 6 ] electrons and the (5d) 6 valence electrons of Ir(III). The geometries of the singlet ground state (S 0 ) and those of the lowest-energy triplet states (T 1 to T 3 ) were fully optimized without imposing any symmetry restriction. The geometry of the triplets was calculated at the spin-unrestricted UB3LYP level with a spin multiplicity of three. Phosphorescence emission energies were estimated as the vertical difference between the energy of the minimum of the lowest-energy triplet state and the energy of S 0 at the T 1 optimized geometry. The calculation of the energy of S 0 at that geometry was performed as an equilibrium single-point calculation with respect to the solvent reaction field/solute electronic density polarization process. All the calculations were performed in the presence of the solvent (acetonitrile). Solvent effects were considered within the self-consistent reaction field (SCRF) theory using the polarized continuum model (PCM) approach. [44][45][46] Time-dependent DFT (TD-DFT) calculations of the lowest lying singlets and triplets were performed in the presence of the solvent at the minimum-energy geometry optimized for the ground state.

Device preparation
LECs were prepared on top of a patterned indium tin oxide (ITO, 15 Ω per square) coated glass substrate (http://www. naranjosubstrates.com) previously cleaned as follows: (a) sonication with soap, (b) deionized water, (c) isopropanol and (d) UV-O 3 lamp for 20 min. An Ambios XP-1 profilometer was used to determine the film thickness. First, 80 nm of poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (CLEVIOS™ P VP AI 4083, aqueous dispersion, 1.3-1.7% solid content, Heraeus) was coated in order to avoid the formation of pinholes and to improve the reproducibility of the cells. Subsequently, the emitting layer (100 nm) was deposited by spin-coating from a MeCN solution of the emitting compound with the addition of the ionic liquid 1-butyl-3-methylimidazolium hexafluoridophosphate [Bmim][PF 6 ] (>98.5%, Sigma-Aldrich) in a 4 to 1 molar ratio. The devices were then transferred to an inert atmosphere glovebox (<0.1 ppm O 2 and H 2 O, MBraun), where a layer (70 nm) of aluminium (the top electrode) was thermally evaporated onto the devices using an Edwards Auto500 evaporator integrated in the inert atmosphere glovebox. The area of the device was 6.5 mm 2 . The devices were not encapsulated and were characterized inside the glovebox at room temperature.

Device characterization
The device lifetime was measured by applying a pulsed current and monitoring the voltage and luminance versus time by using a True Colour Sensor MAZeT (MTCSiCT sensor) with a Botest OLT OLED Lifetime-Test System. The average current density is determined by multiplying the peak current density by the time-on time and dividing by the total cycle time. The average luminance is directly obtained by taking the average of the obtained photodiode results and correlating it to the value of a luminance meter. The current efficiency is obtained by dividing the average luminance by the average current density. The electroluminescence (EL) spectra were measured using an Avantes AvaSpec-2048 fiber optic spectrometer during device lifetime measurements.

Ligand synthesis and characterization
Cyclometallating ligands 2-(4-methylsulfonylphenyl)pyridine (H1) and 2-(3-methylsulfonylphenyl)pyridine (H2) 23 were prepared in three steps starting from fluorophenylboronic acid and 2-bromopyridine. The resulting fluorophenylpyridines were converted into the corresponding methylsulfonylphenylpyridines via nucleophilic aromatic substitution with sodium thiomethoxide and subsequent oxidation with H 2 O 2 /Na 2 WO 4 ·2H 2 O. The synthesis and characterization of these ligands has already been reported. 22 2-(2-Methylsulfonylphenyl)pyridine (H3) was synthesized according to a method described in the literature. 36 2 , however, the microwave-assisted reaction of IrCl 3 ·xH 2 O and H3 in a mixture of 2-ethoxyethanol and water proved to be more successful (Scheme 2). The dimer was isolated as an orange solid by filtration and proved to be sufficiently pure for subsequent reactions, as determined by 1 H NMR spectroscopy.
[Ir(3) 2 Cl] 2 was characterized by 1 H NMR and IR spectroscopy and elemental analysis. The base peak at m/z 657.1 in the LC-ESI mass spectrum of a methanolic solution of [Ir (3)  the eluent of the LC column; this has already been observed for analogous dimers. 22

Synthesis and characterization of [Ir(C^N) 2 (bpy)][PF 6 ] complexes
The synthesis and characterization of [Ir(1) 2 (bpy)][PF 6 ] have been reported previously. 22 The complexes [Ir (2) 6 ] − ion containing atom P88 resides on a special position, leading to the ion being shared equally between two unit cells. The asymmetric unit of [Ir(3) 2 (bpy)][PF 6 ] contains half a cation and half an anion. The second half is generated by a C 2 axis through the central Ir atom, parallel to the b axis of the unit cell.
All the complexes crystallize in achiral space groups, with both enantiomers present in the unit cell.
The bpy ligand is nearly planar in 2{[Ir(1) 2 (bpy)][PF 6 ]}·7CH 2 Cl 2 , with an angle of 3.0°between the pyridyl ring planes. By moving the sulfone group from the 5-to 4-position, the deviation from planarity becomes larger, with angles between the ring planes of 6.4 and 7.8°in the two independent cations in 2{[Ir (2) 6 ], with the sulfone substituent in the 3-position, the bpy ligand is substantially twisted, with an angle of 13.6°between the pyridine ring planes.   The presence of intramolecular CH aryl ⋯OS hydrogen bonds has been observed in a series of alkyl-aryl 47  and −30.2°(O2-S1-C19-C20). This leads to an optimization of intramolecular CH⋯O contacts in the range 2.  (2)     show vibrational structures in their emission profiles, indicating larger 3 LC character of the emissive state. This assumption is further supported by the radiative decay rate constants (k r = PLQY/τ) of the compounds (Table 1). In general, the higher the rate constant, the smaller the contribution of the 3 LC state to the emissive triplet state. 3 In this series of complexes, [Ir(2) 2 (bpy)][PF 6 ] has a k r (7.2 × 10 5 s −1 ) more than double that of the other two complexes (3.2 and 2.6 × 10 5 s −1 ). This observation is therefore in accordance with the supposed higher charge transfer character of the emissive state of [Ir (2) 31 For both series, they showed that the largest influence was exerted by a substituent in the 5-position of the phenyl ring. This observation is in accordance with the trend in emission maxima in the present series of regioisomeric complexes. Concerning the nature of the triplet emissive state, the present data can be compared with a series of SF 5 -functionalized [Ir(C^N) 2 (N^N)] + complexes. 33 In both series, moving the substituent in the phenyl ring of the C^N ligand from the 5-to 4-position (meta to para to the Ir-C bond) leads to a decrease in 3 LC and an increase in the CT character of the emissive state, observed by a broadening of   the emission spectrum and a shorter excited-state lifetime. Replacement of the bpy ancillary ligand by pyrazolylpyridines in combination with the 4-and 5-substituted sulfonyl cyclometallating ligands gives a different trend, with the para-substituted complexes resulting in the largest blue-shift in the emission maximum. 23 This observation can be explained by a change in the nature of the emissive state, from a larger charge transfer (bpy) to a more ligand-centred character ( pzpy), causing a structured emission band and a 54 nm blueshift. The emission colour is therefore strongly dependent on both cyclometallating and ancillary ligands, making the comparison within the herein presented series difficult due to different contributions of ligand-centred and charge transfer states to the triplet emissive state. Photoluminescence quantum yields (PLQY,  (2) (Table 1). Again, much shorter excited state lifetimes are observed for the non-de-aerated solutions (0.144 to 0.310 μs), which can be attributed to quenching due to oxygen present in the system.
The photoluminescence spectra shown in Fig. 8 6 ] is almost completely lost in the solid state spectra. For all the complexes, powder emission is redshifted compared to solution emission (Fig. 7). The largest red-shift is observed for [Ir(1) 2 (bpy)][PF 6 ] (42 nm), and the smallest for [Ir(2) 2 (bpy)][PF 6 ] (25 nm). In powder samples, the difference between the emission maxima of the complexes is smaller (7 nm) than that in solution (24 nm).
PLQYs of powder samples are significantly lower than solution quantum yields, ranging from 6-7% for [Ir(1) 2 6 ] has the highest quantum yield. Excited-state lifetimes in powder samples are shorter than those in solution (Table 1); biexponential fits were used for τ of all three complexes. Shorter lifetimes, lower quantum yields and less vibrational structure indicate strong luminescence quenching in the solid state due to intermolecular interactions. 11 The photoluminescence properties in the amorphous thin film configuration used in LEC devices, where the complex is mixed with the ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluoridophosphate [Bmim][PF 6 ] in a complex : IL 4 : 1 molar ratio, were also investigated (Fig. S1 †). The band maximum is positioned at 533 nm for [Ir(1) 2 Table 2 and cyclic voltammograms are shown in Fig. S2. † In complexes of the type [Ir(C^N) 2 (N^N)][PF 6 ], the first reduction is based on the N^N ligand, while the first oxidation is metalbased with a contribution from the C^N ligands. 32 In our series, the first reduction waves are reversible and the poten-   The geometry of the complexes in their ground electronic state (S 0 ) was fully optimized without imposing any symmetry restriction. The calculations reproduce the main trends observed in the experiments. They predict a near octahedral coordination of the Ir metal where the ancillary bpy ligands remain mostly planar, with N-C-C-N dihedral angles of 3.2, 3.5 and 2.8°. In accord with the X-ray structures, the ppy cyclometallating ligands deviate from planarity in passing from [Ir(1) 2 (bpy)] + and [Ir(2) 2 (bpy)] + (inter-ring angle of 0.8-0.9°) to [Ir(3) 2 (bpy)] + for which the ring planes form an angle of 12.8°. In [Ir(1) 2 (bpy)] + and [Ir(2) 2 (bpy)] + , the SO 2 Me groups adopt a conformation in which the methyl is perpendicular to the phenyl ring and the oxygen atoms form CH⋯O contacts of about 2.60 Å. In [Ir(3) 2 (bpy)] + the SO 2 Me groups form closer CH⋯O contacts of 2.02 and 2.22 Å with adjacent pyridyl and phenyl rings, respectively. Fig. 9 shows the energy and electron density contours calculated for the highest-occupied (HOMO) and lowest-unoccupied molecular orbitals (LUMO and LUMO+1) of [Ir(1) 2 (bpy)] + , [Ir(2) 2 (bpy)] + and [Ir(3) 2 (bpy)] + , which are compared with those obtained for the archetypal complex [Ir( ppy) 2 (bpy)] + . The topology of the MOs is the same for all three complexes and reproduce that of the MOs of the reference complex [Ir( ppy) 2 (bpy)] + . The LUMO+2, which is not displayed in Fig. 9, is close in energy to the LUMO+1 and shows a similar topology.
The introduction of the sulfone groups stabilizes the HOMO by 0.4-0.5 eV because, as expected, this orbital is located on the Ir atom and the phenyl rings of the C^N ligands to which the electron-withdrawing SO 2 Me groups are attached ( Fig. 9). Calculations confirm the larger stabilization of the HOMO by ∼0.1 eV for complex [Ir(2) 2 (bpy)] + in good agreement with the higher oxidation potential measured for [Ir(2) 2 (bpy)] + (1.29 V) when compared with [Ir(1) 2 (bpy)] + (1.18 V) and [Ir(3) 2 (bpy)] + (1.20 V). The stabilization is larger for [Ir(2) 2 (bpy)] + because the carbon in the 4-position, to which the SO 2 Me group is linked, contributes to the HOMO to a higher degree than the carbons in 5-and 3-positions. This enhances the electron-withdrawing effect of the sulfone groups compared with [Ir(1) 2 (bpy)] + and [Ir(3) 2 (bpy)] + . A similar result was observed when changing the sulfone position from 5 to 4 in a closely related family of complexes with pyrazolylpyridine N^N ligands. 23 Regarding the unoccupied MOs, the introduction of the sulfone groups especially stabilizes the orbitals localized on the C^N ligands. In this way, the LUMO+1 of [Ir( ppy) 2 (bpy)] + (−1.76 eV) decreases in energy by 0.42 and 0.45 eV in passing to [Ir(1) 2 (bpy)] + and [Ir(3) 2 (bpy)] + , respectively. The stabilization is smaller for [Ir(2) 2 (bpy)] + (0.28 eV) because of the minor participation of the carbon in the 4-position to which the sulfone group is attached. Similar trends are found for LUMO+2, the partner of LUMO+1 also located on the C^N ligands. The LUMO, which is mainly concentrated on the bpy ligand, undergoes a smaller stabilization of about 0.10 eV for all three complexes (Fig. 9). The energies predicted for the LUMO are in good agreement with similar values recorded for the first reduction potential of [Ir(C^N) 2 (bpy)][PF 6 ] (C^N = [1] − to [3] − ) that are ∼0.10 V less negative than that reported for [Ir( ppy) 2 (bpy)] + (−1.77 V) ( Table 2). 3 The higher HOMO-LUMO gap predicted for [Ir(2) 2 (bpy)] + (3.59 eV) compared with [Ir(1) 2 (bpy)] + (3.51 eV) and [Ir(2) 2 (bpy)] + (3.48 eV) is in accord with the blue shift observed for the lowest energy band of [Ir(2) 2 (bpy)] + in the absorption spectrum (Fig. 5). This band is actually due to a set of electronic transitions involving singlet excited states of Hydrogen atoms are omitted. 3 MLCT/ 3 LLCT nature calculated at ∼350 nm, but also to the lower-energy 3 LC states located on the sulfone-substituted ppy ligands (Table S1 †). The theoretical simulation obtained from the TD-DFT calculation of the singlet excited states (Fig. S6 †) correctly predicts the shape of the experimental absorption spectra (Fig. 5).
To investigate the nature of the lowest-energy triplet excited states, a TD-DFT study was first performed for [Ir(C^N) 2 (bpy)] + (C^N = [1] − to [3] − ) at the optimized geometry of S 0 . Table 3 compares the excitation energies and electronic nature computed for the three lowest triplet states and includes those obtained for the reference complex [Ir( ppy) 2 (bpy)] + . The nature of the three triplets is the same for all four complexes but the energy ordering changes. For [Ir( ppy) 2 (bpy)] + , the lowest lying triplet (T 1 ) results from the HOMO → LUMO excitation and therefore implies a charge transfer from the metal and the phenyl rings of the cyclometallating ligands, where the HOMO is located, to the ancillary ligand, where the LUMO resides (see Fig. 9). The T 1 state therefore shows a mixed 3 MLCT/ 3 LLCT character. The T 2 and T 3 triplets are well above T 1 by ∼0.3 eV and mainly imply excitations from the HOMO to the LUMO+1 and LUMO+2 located on the cyclometallating ppy − ligands. T 2 and T 3 therefore correspond to 3 LC states with some contribution from the metal.
To verify the predicted trends and to obtain additional information about the emitting state, the geometries of the lowest triplet excited states of [Ir(C^N) 2 (bpy)] + (C^N = [1] − to [3] − ) and [Ir( ppy) 2 (bpy)] + were optimized using the spin-unrestricted UB3LYP approach. In all cases, we were able to locate the minimum energy geometries of the three lowest triplet states by carefully selecting the starting point for the optimization process. Fig. 10a summarizes the adiabatic energy difference (ΔE), calculated as the difference between the total energies of S 0 and T 1 , T 2 or T 3 at their respective minimumenergy structures, and the emission energy (E em ), estimated as the vertical energy difference between T 1 and S 0 at the optimized minimum-energy geometry of T 1 . Fig. 10b shows the unpaired-electron spin density distributions calculated for T 1 to T 3 at their optimized geometries.
The T 1 and T 2 states of [Ir(1) 2  a spin-density distribution mainly centred on one of the cyclometallating ligands (∼1.7 unpaired electrons) with a small contribution from the metal (∼0.3e) (Fig. 10b). In contrast, T 3 features a spin density distribution spreading the ppy-Ir environment and the bpy ligand (Ir ∼ 0.5e, C^N ligands ∼ 0.5e, N^N ligand ∼ 1.0e) that perfectly matches the topology of the HOMO → LUMO MLCT/LLCT excitation. Calculations therefore confirm the predominant 3 LC nature of the lowest-energy triplet state of [Ir(1) 2 (bpy)] + and [Ir(3) 2 (bpy)] + in accord with the structured shape of the emission band observed experimentally for these two complexes (Fig. 6).

Electroluminescence
The electroluminescent behaviour of complexes [Ir-(C^N) 2 (bpy)][PF 6 ] (C^N = [1] − to [3] − ) was tested by incorporating them into LEC devices. The preparation of the devices consisted in the deposition in air of an 80 nm layer of PEDOT:PSS on top of a patterned indium tin oxide coated glass substrate followed by a 100 nm emitting layer. The emitting layer was prepared by mixing one of the complexes with the ionic liquid [Bmim][PF 6 ] at a molar ratio of 4 : 1. After this, the devices were annealed at 100°C for one hour under an inert atmosphere and the top aluminium contact was deposited by thermal evaporation.
The devices were operated using a block-wave pulsed current driving mode (average current density: 100 A m −2 , frequency: 1000 Hz, duty cycle: 50%). This driving method was selected in order to enhance the device response. Under these conditions, the voltage required to maintain the current density decreases versus time due to the formation of p-and n-doped regions, which reduces the resistance of the active layer. The LEC behaviour is depicted in Fig. 11 6 ].
The electroluminescence (EL) spectra recorded for the LECs prepared show emission maxima in the 550-560 nm range for all three complexes (Fig. S7 †). The emission band is redshifted compared with the photoluminescence spectra in solution, powder and thin film, and all the LEC devices exhibit green colour.

Conclusions
Three regioisomeric iridium(III) complexes containing methylsulfonyl-functionalized cyclometallating ligands were prepared to study the effect of the substituent's position on the photophysical, electrochemical and LEC device properties. Structural data for the complexes showed the expected core structures and the influence of the steric hindrance induced by the SO 2 Me group in the 3-position of the cyclometallating ligand which leads to ligand distortions.
The complexes are green emitters in solution and greenyellow emitters as powder samples. Vibrationally structured emission bands were observed in MeCN solution for complexes with sulfone groups in the 3-and 5-position of the phenyl ring (meta to the Ir-C bond). The two complexes showed similar quantum yields and lifetimes. An enhanced PLQY of 92%, a shorter excited-state lifetime and a broad unstructured emission profile were obtained for the 4-substituted compound (substituent para to the Ir-C bond). The lack of a vibrational emission profile suggests a more pronounced charge transfer character of the emissive triplet state compared to the other two complexes. In the solid state ( powder), emission maxima are red-shifted, vibrational structure is lost and quantum yields and lifetimes are decreased, indicating excited-state quenching due to intermolecular interactions.
The para-substituted complex (4-SO 2 Me) again exhibits the highest quantum yields of 27% as powder samples and 45% in thin films. Electrochemical data parallel the photoluminescence trends and show that a methylsulfone substituent in the para position to the Ir-C bond has the largest influence on the oxidation potential. DFT calculations rationalize the different effect exerted by the sulfone group depending on the substitution position, and support the experimentally gained results. They confirm the different nature of the emissive triplet state of the meta-( 3 LC) versus para-substituted ( 3 MLCT/ 3 LLCT) complexes.
Green electroluminescence with maxima ranging from 550 to 560 nm is observed for LECs with all the complexes. Maximum luminance levels, power conversion efficiencies and EQEs are similar for complexes with the sulfone substituent meta to the Ir-C bond. For the complex containing the methylsulfonyl group in the 4-position ( para to the Ir-C bond), significantly higher luminance (940 cd m −2 ) and efficiencies (PCE = 4.4 lm W −1 and EQE = 2.6%) are obtained. The increased efficiency correlates with the higher solid state quantum yield. Lifetimes are rather short and comparable for all the devices, ranging from 0.7 to 1.5 h.
We have shown that while the influence on the emission colour is negligible on changing the substitution position of a sulfone group on the cyclometallating ligand, the nature of the emissive triplet state (ligand-centred or charge transfer) is sensitive to the substitution pattern. This leads to significantly enhanced quantum yields in the case of substitution para to the Ir-C bond and, as a consequence, superior device performance.