Bright and stable light-emitting electrochemical cells based on an intramolecularly π-stacked, 2-naphthyl-substituted iridium complex

Department of Chemistry, University of B Switzerland. E-mail: catherine.housecro@u Instituto de Ciencia Molecular, Universidad Spain. E-mail: enrique.orti@uv.es † Electronic supplementary informatio [Ir(ppy)2(Naphbpy)][PF6]; Table S1 experim of [Ir(ppy)2(Naphbpy)] ; Fig. S2 optimi connecting conformers 1 and 2; Fig. S3 the pendant 2-naphthyl group. CCDC 972 in CIF or other electronic format see DOI Cite this: J. Mater. Chem. C, 2014, 2, 7047


Introduction
The successful market entry of displays based on organic lightemitting diodes (OLEDs) testies the maturity of the technology in terms of both stability and efficiency.2][3] These materials are stacked in multilayer structures obtained through vacuum deposition, and use low work function cathodes that have to be protected from the environment through rigorous encapsulation to avoid fast degradation. 4The corresponding costs are compatible with the display market but need to be substantially reduced for lighting applications.][7] Light-emitting electrochemical cells (LECs) are also solution processed and consist of a single electroactive layer of an ionic organic semiconductor placed between two electrodes. 8The high concentration of ions assists both charge injection and transport, allowing the use of stable metal electrodes and thus paving the way to a new generation of low-cost lighting sources. 9,10onic transition-metal complexes (iTMCs) are among the most studied active species for LECs. 11They exhibit high photoluminescence quantum yield (PLQY) and the emission can be easily tuned through ligand design.iTMCs can be deposited as pure layers in electroluminescent devices since the complex by itself assists charge injection and transport and is the active luminescent species. 12The simplest LEC consists of a single active layer composed entirely of an iTMC balanced by small mobile counter anions such as [PF 6 ] À or [BF 4 ] À .
Cationic heteroleptic iridium(III) complexes with the structure [Ir(C^N) 2 (N^N)] + , where C^N is an anionic cyclometallated ligand and N^N a neutral diimine ancillary ligand, represent the most promising candidates for long-living, stable LECs.While the optoelectronic properties can be directly tuned by varying the type of ligands and the electron affinity of their substituents, the performances in devices (i.e., the stability, efficiency and turn-on time) cannot be straightforwardly predicted.Some trends, however, exist and a few general design rules have been proposed.The introduction of bulky substituents on the periphery of the iTMC allows for an increased spatial separation between the emissive centers in a pure lm and thereby for a reduction of the exciton quenching. 13,14This effect enhances the PLQY of the lm and therefore the overall device efficiency.On the other hand, the device lifetime has been correlated with the reactivity of the complex in the excited state, where nucleophilic molecules are able to react with the iTMCs leading to the formation of quenching species and to fast device degradation. 15,16This phenomena can be limited by introducing intramolecular p-p interactions within the ligands, which close the structure of the complex and protect the metal center from nucleophilic substitution with water or residual solvent molecules in the lm. 17An illustrative example is the incorporation of a pendant phenyl ring adjacent to one N-donor of the chelating N^N ligand, which results in a face-to-face p-stacking with one of the cyclometallating C^N ligands. 18This structural feature has an enormous impact on the device lifetime, which increases from a few hours to several months.The control of both inter-and intramolecular interactions in iTMCs has led to a new generation of LECs with enhanced lifetime and efficiency, with potential practical application in solid-state lighting. 19ithin this perspective, we have synthesized and characterized the hexauoridophosphate [PF 6 ] À salt of a heteroleptic iridium(III) complex with the formula [Ir(ppy) 2 (Naphbpy)][PF 6 ], where Hppy is 2-phenylpyridine and Naphbpy is the 6-(2naphthyl)-2,2 0 -bipyridine ancillary ligand.The purpose of the 2naphthyl substituent on the bpy ligand is twofold: (i) to p-stack with the ppy À ligands, stabilizing the geometry of the complex in the excited state, and (ii) to act as a bulky group on the periphery of the complex thus reducing the intermolecular interactions.The [Ir(ppy) 2 (Naphbpy)][PF 6 ] complex was used to prepare single-layer LECs and its structural features resulted in a high device stability and efficiency.

Experimental section
General A Bruker Avance III-500 NMR spectrometer was used to record 1 H and 13 C NMR spectra; chemical shis are referenced to residual solvent peaks with respect to d(TMS) ¼ 0 ppm.Electrospray ionization (ESI) mass spectra were recorded using a Bruker esquire 3000 plus instrument.Solution electronic absorption spectra were recorded on an Agilent 8453 spectrophotometer, emission spectra using a Shimadzu RF-5301 PC spectrouorometer and FT-IR spectra with a Shimadzu 8400S instrument with Golden Gate accessory (solid samples).Solution quantum yields were measured using a Hamamatsu absolute PL quantum yield spectrometer C11347 Quantaurus-QY, and lifetimes were measured using a Hamamatsu Compact Fluorescence Lifetime Spectrometer C11367 Quantaurus-Tau; an LED light source with excitation wavelength of 280 nm was used.Microwave reactions were carried out in a Biotage Initiator 8 reactor.
Electrochemical measurements were recorded using a CH Instruments 900B potentiostat using glassy carbon, a platinum wire and a silver wire as the working, counter and reference electrodes, respectively.Samples were dissolved in HPLC grade MeCN (z10 À4 mol dm À3 ) containing 0.1 mol dm À3 [ n Bu 4 N][PF 6 ] as the supporting electrolyte; all solutions were degassed with argon.Cp 2 Fe was used as an internal reference.

Crystallography
Data were collected on a Bruker-Nonius KappaAPEX diffractometer with data reduction, solution and renement using APEX2 23 and SHELXL97. 24The ORTEP-style diagram was generated using Mercury v. 3.0, and this program was also used for structural analysis. 25,26r(ppy) 2 ( Naphbpy

Computational details
Density functional calculations (DFT) were carried out with the D.01 revision of the Gaussian 09 program package 27 using Becke's three-parameter B3LYP exchange-correlation functional 28,29 together with the 6-31G** basis set for C, H, and N, 30 and the "double-z" quality LANL2DZ basis set for the Ir element. 31The geometries of the singlet ground state and of the lowest-energy triplet states were fully optimized without imposing any symmetry restriction.All the calculations were performed in the presence of the solvent (CH 2 Cl 2 ).3][34] Time-dependent DFT (TD-DFT) calculations of the lowest-lying 20 triplets were performed in the presence of the solvent at the minimum-energy geometry optimized for the ground state.The geometries of the two lowest-energy triplets (T 1 and T 2 ) were rst optimized at the spin-unrestricted UB3LYP level and aerwards reoptimized at the more accurate TD-DFT level.The ligand Naphbpy was prepared by Kröhnke methodology, 35 rather than by the Jameson literature procedure. 21  This journal is © The Royal Society of Chemistry 2014 deformation is associated with a face-to-face p-interaction between the 2-naphthyl unit and the phenyl ring of one of the cyclometallated ligands (Fig. 2).

Dynamic behaviour of [Ir(ppy) 2 (Naphbpy)] + in solution
8][39][40] In these complexes, the 1 3).This is consistent with dynamic behaviour involving the ring C and G domains (see Scheme 1 for ring labels).Rings C and G correspond to those involved in the intra-cation face-toface p-stacking (Fig. 1 and 2), and their resonances for most of the 13 C nuclei are not resolved in the 13 C NMR spectrum at 295 K, but are resolved at 210 K.In contrast, at 295 K, the signals for protons in the bipyridine (rings E and F) and phenylpyridine rings A, B and D are well resolved.
On cooling from 295 K to 210 K, signals for the protons in bpy rings E and F and orthometallated ring A remain largely unaffected, with the exception of the effects of signal overlap (Fig. 3).The broad signals for orthometallated ring C are resolved at 210 K into two sets of signals with relative integrals 2 : 1.This is illustrated in Fig. 4; the signal for the minor component of H C6 coincides with the signal at d 5.32 ppm for residual CDHCl 2 .Fig. 3

Electrochemical and photophysical properties
The electrochemical behaviour of [Ir(ppy) 2 (Naphbpy)][PF 6 ] in CH 2 Cl 2 solution was investigated by cyclic voltammetry.A quasi-reversible oxidation, assigned to an iridium-centred process, occurs at +0.83 V (with respect to Fc/Fc + ), which is similar to those reported for [Ir(ppy) 2 (bpy)][PF 6 ] (+0.84 V) 39 and [Ir(ppy) 2 (Phbpy)][PF 6 ] (+0.81 V) 14 in DMF.A second unidentied species is observed aer oxidation of the metal complex, the   origins of which are unknown (see Fig. S1 in the ESI †).On the other hand, the quasi-reversible reduction wave at À1.85 V is assigned to a ligand-based process.The electrochemical band gap (E ox 1/2 À E red 1/2 ) of 2.68 V compares with 2.61 V for [Ir(ppy) 2 (bpy)][PF 6 ] 39 and 2.60 V for [Ir(ppy) 2 (Phbpy)][PF 6 ]. 14 The absorption spectrum of [Ir(ppy) 2 (Naphbpy)][PF 6 ] (Fig. 6a) is dominated by a broad and intense, ligand-centred (LC) band at 264 nm assigned to spin-allowed p / p* transitions involving both the cyclometallating and the ancillary ligands.The less intense broad absorption at 387 nm is attributed to transitions with mixed metal-to-ligand (MLCT) and ligand-to-ligand (LLCT) charge transfer character.Finally, the weak band observed in the spectra above 450 nm is due to direct spin-forbidden transitions from the singlet ground state (S 0 ) to the rst triplet excited states of the complex, enabled by the high spin-orbit coupling constant of the iridium metal core (z Ir ¼ 3909 cm À1 ). 19xcitation at 275 nm produces a broad, structureless emission centred at 598 nm (Fig. 6b).The band shape is consistent with there being signicant MLCT character in the emitting state. 19The emission maximum recorded for [Ir(ppy)  39 Photoluminescence in the solid state was measured on a 100 nm thick lm (Fig. 7b) obtained by spin coating a solution of the complex mixed with the ionic liquid (IL) 1-butyl-3-methylimidazolium hexauoridophosphate [BMIM][PF 6 ] on a quartz substrate (the molar ratio complex:IL was 4 : 1).The IL was added to mimic the formulation of the active layer in a LEC (see below).The emission spectrum in thin lm consists of a broad emission centred at 596 nm that mostly matches the band observed in solution, thus conrming the charge transfer nature of the luminescence.The measured PLQY was 10.2%, a value smaller than those obtained for the analogous complexes [Ir(ppy) 2 (bpy)][PF 6 ] (34%) 39 and [Ir(ppy) 2 (Phbpy)][PF 6 ] (21%). 14he origin of this reduction in PLQY is commented upon in the next section.

Theoretical calculations
To gain further insight into the structural, electrochemical and photophysical properties, the molecular and electronic structures of the [Ir(ppy) 2 (Naphbpy)] + cation, in both ground and excited states, were investigated by performing DFT calculations at the B3LYP/(6-31G**+LANL2DZ) level in the presence of the solvent (CH 2 Cl 2 ).
Geometry optimization of [Ir(ppy) 2 (Naphbpy)] + in the ground electronic state (S 0 ) leads to the two conformers depicted in Fig. 5. Conformer 1 corresponds to that observed in the crystal structure (Fig. 1).Even considering the differences to be expected between the calculated geometry (in CH 2 Cl 2 solution) and the crystal structure due to the different media, calculations  predict geometric parameters in good accord with the experimental X-ray data (see Table S1 in the ESI †).For instance, the values computed for the bite angle of the ancillary (73.9 ) and the cyclometallating (80.1 and 80.0 ) ligands agree well with the X-ray values (75.60 (18), 79.9(2) and 80.4(2) , respectively).In agreement with experiment, the ppy À ligands are close to planar whereas the bpy ligand presents a dihedral N(1)-C( 5)-C(6)-N(2) angle of 17.5 (X-ray value ¼ 14.4 ) to accommodate the pendant 2-naphthyl group.The 2-naphthyl unit is twisted around the C(10)-C(11) bond by 57.5 (N(2)-C(10)-C(11)-C(20) dihedral angle) slightly underestimating the X-ray value (61.1 ).Calculations correctly reproduce the face-to-face p-stacking between the 2-naphthyl group and the phenyl ring of the adjacent ppy À ligand.The inuence of the crystal packing could explain the difference between the value calculated for the bond distance Ir-N(2) (2.356 Å) and the X-ray value (2.215(5) Å), as N(2) belongs to the pyridine to which the pendant 2-naphthyl group is attached.
The almost identical HOMO energies are in good agreement with the oxidation potentials measured for [Ir(ppy) 2 (Naphbpy)] + (+0.83 V) and [Ir(ppy) 2 (bpy)] + (+0.84 V), and the slightly higher (0.04 eV) HOMO-LUMO gap predicted for the former is in accord with the slightly greater (0.07 V) electrochemical gap reported for this complex.For both cations, the HOMO is composed of a mixture of Ir(III) d p orbitals (t 2g ) and phenyl p orbitals with some contribution from the pyridine rings of the cyclometallating ligands, whereas the LUMO corresponds to the p* LUMO of the bpy ligand.Therefore, if emission originates from the triplet state associated with the HOMO / LUMO excitation, similar wavelengths and photophysical properties are to be expected for both complexes.However, it is worth mentioning the presence in [Ir(ppy) 2 (Naphbpy)] + of two MOs, the HOMOÀ1 and the LUMO+2, that are mainly localized on the pendant 2-naphthyl group.Transitions originating from the HOMOÀ1 of [Ir(ppy) 2 (Naphbpy)] + have no equivalence in [Ir(ppy) 2 (bpy)] + and may appear at lower energies than those resulting from the HOMOÀ1 in the latter.
The nature of the low-lying triplet states was rst investigated by performing time-dependent DFT (TD-DFT) calculations at the optimized geometry of the ground state (S 0 ).Table 1 lists the vertical excitation energies and electronic descriptions computed for the four lowest-lying triplet excited states of [Ir(ppy) 2 (Naphbpy)] + .A close correspondence, both in energy and electronic nature, can be made between states T 1 (2.46 eV), T 3 (2.71eV) and T 4 (2.78 eV) of [Ir(ppy) 2 (Naphbpy)] + and the T 1 , T 2 and T 3 states of [Ir(ppy) 2 (bpy)] + calculated at 2.43, 2.75 and 2.80 eV, respectively.For both systems, the lowest-lying triplet state (T 1 ) is mainly dened by the HOMO / LUMO excitation, which has a mixture of metal-to-ligand and ligand-to-ligand charge transfer ( 3 MLCT/ 3 LLCT) character.States T 3 and T 4 of [Ir(ppy) 2 (Naphbpy)] + are dened by transitions from the HOMO to the LUMO+1 and LUMO+2 and have ligand-centred ( 3 LC) character involving the C^N ligands as it is also the case for the T 2 and T 3 states of [Ir(ppy) 2 (bpy)] + .
The T 2 state of [Ir(ppy) 2 (Naphbpy)] + is singular as it has no correspondence in the archetype [Ir(ppy) 2 (bpy)] + cation.Despite its large multicongurational character, it mainly involves electron excitations between MOs centred on the pendant 2naphthyl group.It therefore has a 3 LC character largely localized on the 2-naphthyl substituent of the ancillary ligand.The presence of this singular T 2 state reduces the energy difference between the rst and second triplet states to 0.17 eV in [Ir(ppy) 2 (Naphbpy)] + .As the energy of both states will decrease aer geometrical relaxation, the small gap between them opens the possibility that T 2 becomes competitive with T 1 .
To study this possibility, the two lowest triplet states of [Ir(ppy) 2 (Naphbpy)] + were reexamined by optimizing their geometries at the TD-DFT level.The optimized structure of T 1 presents small but noticeable differences compared with that obtained for the ground state (Table S1 †).The Ir-N(2) bond shortens from 2.356 to 2.249 Å in passing from S 0 to T 1 and the bpy domain becomes more planar (the N(1)-C(5)-C(6)-N(2) dihedral angle changes from 17.5 to 11.0 ).These changes point to a stronger interaction between the Ir core and the ancillary ligand in T 1 .For the T 2 state, the coordination sphere of the iridium center remains mostly unaffected compared to S 0 (Table S1 †), and the most important changes concern the pendant 2-naphthyl group (Fig. S3 †).The intramolecular pstacking between the 2-naphthyl group and the phenyl ring of the adjacent ppy À ligand is preserved in both excited states.
Fig. 8 displays the unpaired-electron spin density distributions calculated for the T 1 and T 2 states aer full-geometry relaxation.For T 1 , the spin density distribution (Ir: 0.48, ppy: 0.49, bpy: 1.01, Naph: 0.02 e) perfectly matches the topology of the HOMO / LUMO excitation (Fig. 7) and indicates an electron transfer from the Ir(ppy) 2 moiety to the bpy ligand.It therefore corroborates the mixed 3 MLCT/ 3 LLCT character of the T 1 state.For T 2 , the spin density distribution (Ir: 0.01, ppy: 0.03, bpy: 0.15, Naph: 1.81 e) is mostly localized over the 2-naphthyl unit and conrms the 3 LC (2-naphthyl) character predicted above for this triplet (Table 1).The different electronic nature obtained for T 1 and T 2 justies the changes computed for their respective geometries.The important point is that, aer fullgeometry relaxation at the TD-DFT level, the 3 MLCT/ 3 LLCT T 1 state continues to be the lowest-lying triplet, although it is only 0.09 eV below the T 2 state (adiabatic energy difference ¼ electronic energy difference between the excited states at their respective mínimum-energy equilibrium geometries).
Calculations therefore suggest that the emitting T 1 state of [Ir(ppy) 2 (Naphbpy)] + has a mixed 3 MLCT/ 3 LLCT nature in good agreement with the broad and unstructured aspect of the emission band (Fig. 6b).However, the 3 LC T 2 triplet resides at energies low enough to compete with T 1 during the population process.This could be the reason for the lower PLQY measured for [Ir(ppy) 2 (Naphbpy)] + compared to [Ir(ppy) 2 (bpy)] + , for which no low-energy state equivalent to T 2 state is available.The vertical emission energy calculated for T 1 at its optimized minimum-energy geometry (621 nm) is in reasonably good accord with the emission maximum observed experimentally (598 nm).

Electroluminescent devices
The electroluminescence (EL) of the [Ir(ppy) 2 (Naphbpy)][PF 6 ] complex was investigated in double layer LECs consisting of a 100 nm thick layer of the complex spin-coated from acetonitrile (2 wt%) onto an 80 nm thick lm of poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS).The PEDOT:PSS layer was deposited on a pre-patterned indium-tinoxide (ITO)-coated glass substrate with the twofold purpose of attening the ITO anode surface and enhancing hole injection from the ITO to the HOMO of the complex.Prior to spin-coating, the IL [BMIM][PF 6 ] was added to the [Ir(ppy) 2 (Naphbpy)][PF 6 ] solution (the molar ratio complex : IL was 4 : 1), in order to enhance the ionic mobility of the emitting layer and thus reducing the turn-on time of the device. 41Finally, an aluminum cathode (100 nm) was vacuum-deposited onto the emitting layer through a shadow mask.Spin-coating was done in ambient atmosphere; the base pressure during the cathode evaporation was 10 À6 mbar.The thickness of the lms was determined with an Ambios XP-1 prolometer.Thin lm photoluminescence spectra and quantum yields were measured with a Hamamatsu C9920-02 absolute pl quantum yield measurement system.Lifetime data were obtained by applying pulsed currents and monitoring the voltage and simultaneously the luminance by a true colour sensor MAZeT (MTCSICT Sensor) using a lifetime test system designed by Botest (Botest OLT OLED Lifetime-Test System).The photocurrent was calibrated using a Minolta LS-100 luminance meter.Electroluminescence spectra were recorded using an Avantis ber optics photospectrometer.Devices were not encapsulated and were characterized inside a glovebox.The LECs were driven using a pulsed current (block wave, 1000 Hz; duty cycle 50%; peak current per pulse 100 A m À2 ; average current 50 A m À2 ) and their performances were recorded continuously over time (Fig. 9a-b).
Due to the pulsed current driving, the LECs turn on instantly delivering more than 100 cd m À2 aer being biased for 1 s.In the rst measured hour the device luminance reaches 200 cd m À2 and then grows steadily until reaching $330 cd m À2 where it remains stable for the duration of the experiment (350 hours) (Fig. 9a).The average voltage follows the opposite trend, starting from values above 5 V at the beginning of the measurements, and progressively decaying and subsequently stabilizing at about 2.7 V (Fig. 9a).This indicates a reduction of the device resistance, which is consistent with the formation of doped zones in the emitting layer near the electrodes. 42As mentioned above, the device lifetime was monitored for more than 350 hours, showing high luminance and essentially no degradation.Accelerated lifetime testing in LECs is unfortunately not trivial as higher temperature testing would inuence the ionic motion and therefore the local distribution of ions, which alters the device performance.Testing at higher luminance levels can also  not be easily modeled to lower luminance driving conditions because with higher luminance the current density is increased and leads to an increase in the doped region and, as a result, to an increase in the luminescence quenching.Due to this we were not able to extrapolate the lifetime but it is expected to be at least 10 times this initial phase leading to a stability of 3000 hours.Such a very good stability is consistent with an efficient intramolecular interaction able to stabilize the excited state of the complex and thus enhancing the device lifetime. 14The corresponding efficacy was stable at an average value of 3.2 cd A À1 and a power conversion efficiency of 1.8 lm W À1 when considering the measured potential drop at the device contacts (Fig. 9b).A series of devices was prepared and measured at different peak current density, from 25 to 100 A m À2 .The device efficacy is essentially independent of the luminance level and scales linearly with the applied current density over this range, meaning that the LEC can be used at the desired light output without altering its efficiency (Fig. 9c).The electroluminescent spectra appears as a broad peak centered at 588 nm (Fig. 9d), very similar to what is observed in the photoluminescence spectra (Fig. 6b) and in analogous complexes reported previously. 14

Conclusions
A new charged bis-cyclometallated iridium(III) complex, [Ir(ppy) 2 (Naphbpy)][PF 6 ], incorporating a 2-naphthyl substituent in the 6-position of the ancillary 2,2 0 -bipyridine ligand has been presented.The pendant 2-naphthyl group p-stacks with the adjacent cyclometallating ligand and protects the complex from undesirable degradation reactions.The complex presents a broad, structureless emission band centred around 595-600 nm both in solution and in thin lm that points to an emission from a triplet state with a large charge transfer character.Theoretical calculations conrm the MLCT/LLCT nature of the lowest-energy emitting triplet and predict a second low-lying triplet state of LC nature associated with the 2-naphthyl unit.The presence of this triplet is invoked as a plausible reason for the relatively low photoluminescence quantum yields displayed by the complex.Despite this shortcoming, the complex gives rise to bright and very stable solid-state light-emitting electrochemical cells.Instantaneous light turn-on and luminances above 300 cd m À2 are obtained for LECs using this complex.Furthermore, no decay in luminance is observed for more than 350 hours, demonstrating their extraordinary stability.
H NMR spectra at 295 K show broad signals only for the ortho-and meta-protons of the 6phenyl substituent and, upon cooling, only these signals are affected.At 295 K, the 1 H NMR spectrum of a CD 2 Cl 2 solution of [Ir(ppy) 2 (Naphbpy)][PF 6 ] exhibits very broad signals for the 2naphthyl protons (not all are observed) and broadened signals for the protons of the orthometallated ring C (top spectrum in Fig. reveals that the two multiplets for H B5 and H D5 initially broaden upon cooling.Analysis of the COSY, HMQC and HMBC spectra recorded at 295 K and 210 K reveals that each of the signals for H B6 , H D6 and H D3 splits into two doublets (relative integrals 2 : 1).The spectroscopic data are consistent with the [Ir(ppy) 2 -(Naphbpy)] + cation existing in two conformations at low temperature by virtue of the position of the 2-naphthyl substituent with respect to the cyclometallated ring C. The 2D VT-NMR spectra were not consistent with rotation of the 2naphthyl group, and we propose that the dynamic behaviour involves a change in conformation by internal rotation of the bpy ligand.Conformer 1 in Fig. 5a corresponds to the crystallographically determined structure, with the 2-naphthyl unit (rings G) lying over the cyclometallated ring C. Twisting around the inter-ring bond of the bpy ligand generates conformer 2 in which the 2-naphthyl unit retains the stacking interaction over ring C but adopts a different relative p-stacked arrangement.Fig. 5b shows the optimized geometry calculated for conformer 2 using DFT calculations (see below).