the ring roundabout : [ Ir ( C ^ N ) 2 ( N ^ N ) ] emitters with sulfonyl-substituted cyclometallating ligands †

Department of Chemistry, University of B Switzerland. E-mail: catherine.housecro@u Instituto de Ciencia Molecular, Universida Spain. E-mail: enrique.orti@uv.es † Electronic supplementary information details; syntheses of ligands, and IR spec results of TD-DFT calculations. Fig. S1 Fig. S5: PL spectra of complexes in th crystallographic data in CIF or o 10.1039/c5ra07940c Cite this: RSC Adv., 2015, 5, 42815


Introduction
Cationic iridium(III) complexes of the type [Ir(C^N) 2 (N^N)] + , in which C^N is a cyclometallated ligand and N^N is a diimine ligand, are well-established emissive materials in light-emitting electrochemical cells (LECs). 1,2Prototype LECs incorporated conjugated polymers in the active layer, 3 but the development of LECs with emissive ionic transition-metal complexes (iTMCs) has led to single-component devices. 1 The application of a bias across the device causes the ions in the active layer to migrate towards the electrodes where their accumulation results in a drop in potential near the electrode surface.The formation of ionic junctions lowers the barrier for electron and hole injection and, as a consequence, the device is independent of the work function of the electrode material. 4,5Hence, it is not a prerequisite that LECs use low work-function metals, as is necessary in organic light-emitting diodes (OLEDs), and air-stable electrodes (e.g.Al) are employed in LECs.This has a benecial consequence: the rigorous exclusion of air, which is essential in the fabrication of OLEDs, is unnecessary in LECs.Another advantage of LECs over OLEDs is their far simpler architecture and the fact that they can be assembled using solution processing. 1 Despite the fabrication advantages of LECs, there is a lack of wide bandgap iTMCs to produce the blue iTMC-LECs needed to combine with orange emitters for white light.In [Ir(C^N) 2 (N^N)] + complexes, the HOMO contains contributions from the Ir(III) dp orbitals and phenyl p-orbitals of the cyclometallated C^N ligands, and the LUMO is usually localized on the N^N ligand. 1 The localized character of the HOMO-LUMO manifold lends itself perfectly to colour tuning the emission of [Ir(C^N) 2 (N^N)] + complexes by introducing N^N ligands that destabilize the LUMO and/or C^N ligands that stabilize the HOMO.In 2006, Nazeeruddin et al. showed that [Ir(ppy) 2 (4,4 0 -(Me 2 N) 2 bpy)] + (Hppy ¼ 2-phenylpyridine, 4,4 0 -(Me 2 N) 2 bpy ¼ 4,4 0 -bis(dimethylamino)-2,2 0 -bipyridine) was an efficient blue-green emitter, 6 and pointed to the potential use of 1-phenylpyrazole or uorinated 2-phenylpyridine ligands to stabilize the HOMO of [Ir(C^N) 2 (N^N)] + .This approach to blue emission was demonstrated in 2005 by Thompson and coworkers in a landmark paper. 7The quest for blue emitters [8][9][10][11][12][13][14][15][16][17][18][19][20][21] continues, with utilization of the structure-property 'rules', leading to the replacement of 1-phenylpyrazolyl-by 2-methyl-5-phenyl-2Htetrazolyl-functionalized C^N ligands. 22The number and position of uoro groups in Hppy have only a small inuence on the photophysical properties of [Ir(C^N) 2 (4,4 0 -t Bu 2 bpy)] + complexes (4,4 0 -t Bu 2 bpy ¼ 4,4 0 -di-tert-butyl-2,2 0 -bipyridine), 23 although increasing the number of uorine atoms leads to shorterlived LECs. 23Indeed, the use of uorinated C^N ligands in iridium(III) complexes in OLEDs has been questioned due to the instability of the C phenyl -F bond; 24 hence, uorine-free blue [Ir(C^N) 2 (N^N)] + emitters are desirable.It must be mentioned that blue photoluminescence (PL) in solution, thin-lm or powder, is no guarantee of blue electroluminescence (EL) under device conditions, and signicant shis in emission maxima on going from PL to EL are common. 25ollowing studies of blue emitting complexes for OLEDs, 26,27 Evariste et al. 28 demonstrated the use of cyclometallated methyland methoxy-substituted 2,3 0 -bipyridine ligands as the C^N domain in blue-emitting [Ir(C^N) 2 (4,4 0 -t Bu 2 bpy)] + complexes.An alternative approach is to replace the uoro-substituents in the C^N ligands by electron-withdrawing sulfone groups, a design strategy previously demonstrated in OLEDs, 29 and one that we have applied in [Ir(C^N) 2 (N^N)] + complexes (Scheme 1). 30,31owever, the complexes in Scheme 1 are green emitters and we consider here further modication to achieve a shi in emission to higher energies.An appealing strategy is the use of sulfonefunctionalized C^N ligands and derivatives of 1-phenylpyrazole as the N^N ligand, combining the effects of the electronwithdrawing sulfone groups with those of the electron-rich pyrazole ring, leading to larger HOMO-LUMO gaps in [Ir(C^N) 2 (N^N)] + complexes.Stereoisomerism in [Ir(C^N) 3 ] emitters in OLEDs is important for the enhancement of phosphoresence quantum yield and electrochemical stability, with the fac-isomer being superior to the mer-isomer, 32 and we have now investigated the effect of positional isomerization in the sulfone-functionalized ligands as it is known that the emission behaviour of some iridium complexes [33][34][35] is sensitive to the positions of the substituents in the C^N ligands.Density functional theory (DFT) calculations have shown that photophysical properties of cationic [Ir(ppy) 2 (P^P)] + complexes vary with the position of functionalization of the [ppy] À ligands. 36o further blue-shi the emission, we have synthesized a series of complexes combining sulfone-substituted H1 and H2 and pyrazolylpyridine ancillary ligands (Scheme 2).The complex [Ir(ppy) 2 (pzpy)][PF 6 ] previously prepared by He et al. 8 is used as a reference compound for the comparison of the photophysical and electrochemical properties of the new complexes (Scheme 3), which are further analysed through DFT calculations.The electroluminescent properties of the complexes have been investigated in LEC devices operated under pulsed driving conditions.

Experimental
General experimental, ligand synthesis and IR spectroscopic data for the complexes are given in the ESI.†  General procedure for the synthesis of iridium(III) complexes.The iridium(III) dimer and the ancillary ligand were suspended in MeOH (15 mL) in a vial and heated at 120 C for 1 h in a microwave reactor (14 bar).The resulting yellow solution was ltered through cotton and concentrated under reduced pressure.The residue was dissolved in 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 removed by ltration and redissolved in CH 2 Cl 2 /CH 3 CN.The solvent was removed under reduced pressure, the crude product was puri-ed by column chromatography (silica) and the solvent removed again.

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-z" quality LANL2DZ basis set for Ir. 43The geometries of the singlet ground state (S 0 ) and of the lowest-energy triplet state (T 1 ) were fully optimized without imposing any symmetry restriction.The geometry of T 1 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.All the calculations were performed in the presence of the solvent (acetonitrile).5][46] The calculation of the energy of S 0 at the T 1 geometry was performed as an equilibrium singlepoint calculation with respect to the solvent reaction eld/ solute electronic density polarization process.Timedependent DFT (TD-DFT) [47][48][49] calculations of the lowest lying 15 triplets were performed in the presence of the solvent at the minimum-energy geometry optimized for the ground state.

Device preparation and characterization
Electroluminescent LEC devices were made as follows.First, glass substrates with sputtered ITO contact (Naranjo Substrates) were cleaned by sequentially washing and sonication with soap, deionized water, isopropanol and a UV-O 3 lamp for 20 min.Then, an 80 nm layer of poly(3,4ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) (CLEVIOS™ P VP AI 4083, aqueous dispersion, 1.3-1.7%solid content, Heraeus) was spin-coated on the ITO substrates to improve the reproducibility of the devices and to prevent the formation of pinholes.Aer this layer deposition, a 100 nm transparent lm of the iTMC and the ionic liquid (IL) 1-butyl-3methylimidazolium hexauoridophosphate ([Bmim][PF 6 ]) (>98.5%,Sigma-Aldrich) in a 4 : 1 molar ratio were spin-coated from a 20 mg mL À1 acetonitrile solution.The devices were transferred into an inert atmosphere glovebox (<0.1 ppm O 2 and H 2 O, M. Braun).The Al electrode (70 nm) was thermally vapourdeposited using a shadow mask under a vacuum (<1 Â 10 À6 mbar) with an Edwards Auto500 evaporator integrated in the glovebox.The nal device conguration was ITO/PEDOT:PSS/ iTMC:IL/Al.The device characteristics were measured by applying a pulsed current (average current density 100 A m À2 , 50% duty cycle, 1 kHz, block wave) and monitoring the current and luminance versus time by a True Colour Sensor MAZeT (MTCSiCT Sensor) with a Botest OLT OLED Lifetime-Test System.The electroluminescent spectra were measured using an Avantes AvaSpec-2048 Fiber Optic Spectrometer.

Ligand synthesis and characterization
The sulfone-substituted cyclometallating ligand H2 was prepared in three steps in an analogous manner to the synthesis of 2-(4-methylsulfonyl)phenylpyridine (H1), 31 starting from 2bromopyridine and 3-uorophenylboronic acid.The rst step was a Suzuki coupling in EtOH/H 2 O with PdCl 2 , 50 followed by a nucleophilic aromatic substitution with NaSMe to yield the corresponding thioether derivative.Oxidation of sulfur using H 2 O 2 /Na 2 WO 4 in MeOH to give H2 (Scheme 4) was performed under mild conditions.High yields were achieved in all three steps.Compound H2 was characterized using multinuclear NMR spectroscopy, mass spectrometry, infrared spectroscopy and elemental analysis.The base peak in the LC-ESI mass spectrum corresponded to [M + H] + with an isotope pattern matching that calculated. 1 H and 13 C NMR signals were assigned using COSY, HMQC and HMBC methods.Pyrazolylpyridine ancillary ligands were synthesized according to literature. 51

Synthesis and characterization of [Ir(C^N) 2 Cl] 2 dimers
Iridium complexes [Ir(C^N) 2 (N^N)] + are usually synthesized by cleavage of the chlorido-bridged iridium dimer with the corresponding ancillary ligand.These dimers are typically obtained by reaction of IrCl 3 $xH 2 O and the cyclometallating ligand under reux conditions. 52However, for [Ir(C^N) 2 Cl] 2 dimers containing sulfone-substituted cyclometallating ligands, we have found that a route starting from [Ir(cod)Cl] 2 53 (cod ¼ cycloocta-1,5diene) is more convenient (Scheme 5), and [Ir(2) 2 Cl] 2 was therefore prepared in an analogous manner to [Ir(1) 2 Cl] 2 . 31The dimers were isolated in high yields as yellow solids.[Ir(2) 2 Cl] 2 proved to be poorly soluble in common organic solvents but could be characterized by 1 H NMR spectroscopy.In the LC-ESI mass spectrum of a MeOH solution of the dimer, the base peak at m/z 657.1 was assigned to the [Ir(2) 2 ] + ion.Further intense peaks were observed at m/z 698.1 and 739.The [Ir(2) 2 (dmpzpy)] + cations pack into sheets that are coparallel with the bc-plane (Fig. 3).
A preliminary structure of [Ir(1) 2 (dmpzpy)][PF 6 ] was determined with crystals grown from a MeCN solution of the complex overlaid with Et 2 O.This revealed the octahedral metal centre as well as the presence of the three bidentate ligands, conrming the expected structure of the complex.
The photoluminescence spectra of the complexes were recorded in MeCN solution both at room temperature and at 77 K and are shown in Fig. 5; emission maxima are summarized in Table 1.The emission prole is independent of the excitation wavelength.In complexes of the type [Ir(C^N) 2 (N^N)] + , emission occurs from the lowest-lying triplet state (T 1 ), which comprises contributions from metal-to-ligand/ligand-to-ligand charge-    transfer 3 MLCT/ 3 LLCT and ligand-centred 3 LC states.In general, the higher the charge-transfer character, the broader and less structured the emission prole.The vibrational structure observed in Fig. 5a for the spectra at room temperature indicates a large 3 LC character of the emissive state.The spectra at 77 K conrm this assignment because the vibrational peaks preserve their position showing almost no rigidochromic shi with respect to room temperature (Fig. 5b and Table 1).
Only a negligible hypsochromic shi of 2 nm in l max em occurs when the bpy ligand in [Ir(1) 2 (bpy)][PF 6 ] (493 nm) 31 is replaced with the electron-rich pzpy or dmpzpy.This suggests that the emissive state of [Ir(C^N) 2 (N^N)] + complexes with cyclometallating ligand H1 is relatively independent of the N^N ligand and has a high 3  Photoluminescence quantum yields (PLQY, Table 1) of the complexes in de-aerated MeCN solution are in the range 53-77% and are comparable to the PLQY of [Ir(1) 2 (bpy)][PF 6 ] (74%). 31[Ir(ppy) 2 (pzpy)][PF 6 ], however, has a lower quantum yield of 23%. 8Shavaleev et al. have reported a series of iridium complexes with alkylated pyrazolylpyridine ligands. 57Strong phosphorescence quenching in solution was observed when the ancillary ligands contained a methyl group in the 5-position of the coordinated pyrazole ring; this may induce steric hindrance upon coordination to iridium.In the latter work, incorporation of the 5-Me substituent leads to a three-fold increase in the nonradiative decay rate (k nr ¼ (1 À PLQY)/s), and the authors relate this to reversible dissociation of the sterically hindered Ir-N bond.This effect is only seen in solution and a visible emission from powdered samples of all their complexes was observed.In our series of complexes, however, no such large drop in PLQY is observed.Quantum yields of [Ir(C^N) 2 ( dmpzpy 1 to avoid the first harmonic of the excitation in the spectrum; apart from this, emission spectra are independent of the excitation wavelengths used.

Table 1 Photophysical properties of complexes [Ir(C^N) 2 (N^N)][PF 6 ] (C^N ¼ [1]
À and [2] À , N^N ¼ pzpy and dmpzpy) in MeCN solution and as powder samples.Quantum yields were measured in de-aerated solution; lifetimes were measured in de-aerated solution under argon atmosphere.Second-order fits were used for solid-state lifetime measurements; percentage of population is given in parentheses.k r ¼ PLQY/s; ], with a value of 3.2 Â 10 5 s À1 . 31A higher k r indicates a lower 3 LC character of the emissive state. 1,8xcitation of powder samples of [Ir(C^N) 2 (N^N)][PF 6 ] (C^N ¼ [1] À and [2] À , N^N ¼ pzpy and dmpzpy) results in the emission spectra shown in Fig. 6, and PL data are summarized in Table 1.With respect to the solution spectra (Fig. 5), the solid-state emission maxima are red-shied by about 30 nm and vibrational structure is less pronounced, suggesting strong intermolecular interactions in the solid state. 8Such a red-shi of the emission maximum on going from the solution to the solid seems to be common for blue or blue-green emitting iridium complexes.The extent of the red-shi can, however, vary between #15 nm 8,28,58,59 and $33 nm. 28,60,61n our previously reported sulfone-substituted [Ir(C^N) 2 (bpy)][PF 6 ] complexes, red-shis in the emission maxima of z40 nm were observed on going from solution to solid.In contrast, the yellow emitting uorineand thioethercontaining [Ir(C^N) 2 (bpy)][PF 6 ] in this series showed blue-shis of 10-19 nm from solution to powder. 31Excited-state lifetimes (determined using second-order ts) in the solid state are much lower than in MeCN solution.In addition to the dramatically decreased PLQYs of powdered samples (#11%), this suggests strong quenching of the excited state in powder samples.
Calculations predict a near-octahedral structure for the complexes in their ground electronic state (S 0 ).The ancillary ligand is mostly planar in the pzpy complexes, the inter-annular N-N-C-N dihedral angle having a value of 3.4 .The presence of the methyl group in 5-position of the coordinated dmpzpy ligand introduces some steric hindrance and the N-N-C-N angle increases to 11.5 .This loss of planarity does not significantly affect the Ir-N py and Ir-N pz coordination distances with the ancillary ligand, that change from 2.23 and 2.19 Å in the complexes with N^N ¼ pzpy to 2.21 and 2.21 Å in those with N^N ¼ dmpzpy, respectively.Fig. 7 compares the energy and electron density contours calculated for the highest occupied (HOMO) and lowestunoccupied molecular orbitals (LUMO and LUMO+1) of [Ir(ppy) 2 (pzpy)] + with those obtained for [Ir(C^N) 2 (N^N)] + (C^N ¼ [1] À and [2] À , N^N ¼ pzpy and dmpzpy).The introduction of the sulfone groups leads to a stabilization of the HOMO, which is located, as expected, on the Ir atom and the phenyl rings of the cyclometallating ligands.The stabilization is slightly larger (0.08 eV) for the complexes with C^N ¼ [2] À because of the higher participation of the carbon in 4-position to which the sulfone group is attached.The attachment of methyl groups to the N^N ligand has no relevant effect on the energy of the HOMO because this ligand does not contribute to the orbital.These trends explain the higher oxidation potentials measured for [Ir(2) 2 (N^N)] + (+1.27V) compared with [Ir(1) 2 (N^N)] + (+1.20 V) and the negligible changes observed when substituting the pzpy ligand by dmpzpy (Table 2).The oxidation potential reported for [Ir(ppy) 2 (pzpy)] + (+0.88 V) 8 is signicantly smaller in accord with the higher energy of its HOMO.
To disentangle this apparent contradiction between theory and experiment and to obtain more information about the emitting state, the geometry of the lowest-energy triplet excited state T 1 was optimized using the spin-unrestricted UB3LYP approach.Fig. 8a summarizes the adiabatic energy difference between S 0 and T 1 (DE) 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. 8b shows the unpaired-electron spin density distributions calculated for the lowest-energy triplet of complexes [Ir(1) 2 (pzpy)] + and [Ir(2) 2 (pzpy)] + .Almost identical spin-density plots are obtained for complexes with N^N ¼ dmpzpy and for [Ir(bpy) 2 (pzpy)] + .Therefore, independently of the nature of the LUMO, the spindensity distribution calculated for T 1 is mainly centred on one of the cyclometallating ligands ($1.65 unpaired electrons) with some contribution from the iridium atom ($0.29e).This indicates that the lowest-energy triplet state has a predominant LC nature with some MLCT character in accord with the structured shape of the emission band observed experimentally.
In agreement with experiment (Table 1), calculations nd almost identical emission energies for complexes with C^N ¼ [1] À (E em $ 2.25 eV) and C^N ¼ [2] À (E em $ 2.41 eV).The emission energy is not affected by the addition of methyl groups to the ancillary ligand because the emission is centred on the C^N ligands (Fig. 8).Compared with [Ir(ppy) 2 (pzpy)] + (E em ¼ 2.35 eV), the calculated emission energies correctly reproduce the red-shied and blue-shied emission observed experimentally for complexes [Ir(1) 2 (N^N)] + and [Ir(2) 2 (N^N)] + , respectively.
Time dependent DFT (TD-DFT) calculations conrm the 3 LC nature of the lowest-lying triplet (Table S1 †).The rst triplet À and [2] À , N^N ¼ pzpy and dmpzpy) were tested in LEC devices, which were operated using a pulsed current driving mode with a frequency of 1 kHz and a duty cycle of 50%. 63,64The LEC performance parameters measured using an average current density of 100 A m À2 are given in Table 3.Under these conditions, all the devices showed a typical LEC behaviour (Fig. S1-S4 †).The voltage needed to maintain the established current density starts at a high value due to the initially high injection barriers and rapidly drops as a result of ion migration towards the electrodes forming an electric double-layer that reduces the injection barriers.The time needed to reach the maximum luminance (t on ) is relatively short ranging from 30 to 78 seconds.The relatively fast response matches with the rather poor stability of the devices with lifetimes (t 1/2 ) of a few minutes (Table 3).The results are comparable to those obtained for other blue-green LECs reported in the literature, 12,16,22 for which a limited lifetime was observed especially when operated using a pulsed driving mode.In these cases, a relatively fast t on was also measured what is consistent with the values reported here.
The maximum luminance achieved under pulsed current density of 100 A m À2 was 141 cd m À2 for the LEC prepared using [Ir(1) 2 (pzpy)][PF 6 ].The device performance is therefore not very high due to the moderate efficiencies.The trends obtained for the power conversion efficiency (PCE) and the external quantum efficiency (EQE) follow the photoluminescence quantum yields measured for thin lms of the complexes with the same composition of the active layer in the device (Fig. S5 †).Complex [Ir(2) 2 (pzpy)][PF 6 ] with the lowest efficiency values (Table 3) show the smallest PLQY (6.9%), and the other three complexes with comparable higher efficiencies have PLQY values ranging from 13.0 to 16.9%.Fig. 9 shows the electroluminescent (EL) spectra recorded for the LECs prepared.All the LECs exhibit structured electroluminescent emission bands coherent with the LC character of the emissive state.The position of the peaks forming the EL band are similar to the PL spectra depicted in Fig. 5 (compare Tables 1 and 3), but their relative intensity drastically changes.For complexes [Ir(2) 2 (N^N)][PF 6 ], the EL spectra show bluer weak emission peaks at 466 and 496 nm and the maximum emission is at 546 nm (Fig. 9).For complexes [Ir(1) 2 (N^N)][PF 6 ], an intense peak is observed at 492 nm but the maximum emission appears around 530 nm.The relative intensity of the EL peaks determines that the LEC devices exhibit a green colour for all the four complexes.

Conclusions
A series of [Ir(C^N) 2 (N^N)] + complexes with 2-phenylpyridinebased cyclometallating and pyrazolylpyridine ancillary ligands has been synthesized.Two different cyclometallating ligands, H1 and H2, with a sulfonyl substituent in either the 3-or 4position of the phenyl ring of the ppy domain were combined with 2-(1H-pyrazol-1-yl)pyridine (pzpy) and 2-(3,5-dimethyl-1Hpyrazol-1-yl)pyridine (dmpzpy) N^N ligands.These ligand combinations were designed to blue-shi the emission of the [Ir(C^N) 2 (N^N)] + complex.In practice, green or blue emissions were observed, the colour depending on the sulfone substituent position in the cyclometallating ligand.).The marked vibrational structure and the absence of a rigidochromic shi in the low temperature spectra indicate signicant 3 LC contributions to the emissive state.Theoretical calculations show that the sulfone substituents strongly stabilize the orbitals located over the cyclometallating ligands and conrm the 3 LC nature of the lowest-lying triplet state.PLQY values in de-aerated MeCN solution are quite high for all four complexes (between 53 and 77%), whereas the PLQY values of powdered samples are considerably lower than in solution (#11%), suggesting strong quenching effects due to intermolecular interactions.Unfortunately, all four complexes performed poorly when tested in LEC conguration.
Methyl groups on the pyrazolylpyridine ligand did not have a signicant effect on the properties of the complexes since both the emission and electrochemical properties are mainly determined by the cyclometallating ligands.On the other hand, changing the substitution position of the sulfone group on the cyclometallating ligand produced a shi of z30 nm in the emission maximum.This shi is reproduced in the theoretical study and is due to the different effect that sulfone substitution has on the frontier molecular orbitals depending on the substitution position.We are currently investigating the effect of substituental isomerism on the cyclometallating ligand in relation to colour tuning in more detail.