Ir ( C ^ N ) 2 ( N ^ N ) ] + emitters containing a naphthalene unit within a linker between the two cyclometallating ligands †

The synthesis of four cyclometallated [Ir(C^N)2(N^N)][PF6] compounds in which N^N is a substituted 2,2’-bipyridine (bpy) ligand and the naphthyl-centred ligand 2,7-bis(2-(2-(4-(pyridin-2-yl)phenoxy)ethoxy) ethoxy)naphthalene provides the two cyclometallating C^N units is reported. The iridium(III) complexes have been characterized by H and C NMR spectroscopies, mass spectrometry and elemental analysis, and their electrochemical and photophysical properties are described. Comparisons are made with a model [Ir(ppy)2(N^N)][PF6] compound (Hppy = 2-phenylpyridine). The complexes containing the naphthyl-unit exhibit similar absorption spectra and excitation at 280 nm leads to an orange emission. The incorporation of the naphthalene unit does not lead to a desirable blue contribution to the emission. Density functional theory calculations were performed to investigate the geometries of the complexes in their ground and first triplet excited states, as well as the energies and compositions of the highestoccupied and lowest unoccupied molecular orbital (HOMO and LUMO) manifolds. Trends in the HOMO– LUMO gaps agree with those observed electrochemically. The energy difference between the LUMO and the lowest unoccupied MO located on the naphthyl unit (LUMO+7) is large enough to explain why there is no contribution from the naphthyl-centred triplet excited state to the phosphorescence emission. Singlet excited states were also investigated. Light-emitting electrochemical cells (LECs) using the [Ir(C^N)2(N^N)][PF6] and [Ir(ppy)2(N^N)][PF6] complexes in the emissive layer were made and evaluated. The presence of the naphthyl-bridge between the cyclometallating units does not significantly alter the device response.


Introduction
Light-emitting electrochemical cells (LECs) are promising electroluminescent devices for highly efficient and low-cost applications in ultrathin and flexible lighting.The charged active layer sandwiched between two electrodes is either a conjugated light-emitting polymer 1 or an ionic transition-metal complex (iTMC). 2 The ionic nature of the active material allows the charged species to migrate towards the electrodes when a bias is applied, forming doped zones and thus lowering the injec-tion barrier facilitating efficient electron and hole injection. 3ue to their particular stability, extremely high efficiencies and ability to tune the emission color, iTMCs incorporating iridium(III) (Ir-iTMCs) are by far the most versatile active materials used in iTMC-LECs. 4,5Ir-iTMCs are typically of the form [Ir(C^N) 2 (N^N)] + , where C^N is a cyclometallating ligand and N^N is an N,N-chelate.Since the high spin-orbit coupling of Ir-iTMCs permits intersystem-crossing from singlet to triplet states, iridium(III)-based materials achieve spin-forbidden phosphorescence emissions approaching photoluminescence quantum yields of 100%. 6,7[6][7]9 A major challenge that remains is to attain white light emission.
1][12][13] Other strategies (not uniquely based on iTMCs) include multifluorophoric conjugated polymers, 14,15 a combined polymer-composite blue-light emitting layer with an orange ionic iridium complex, 16,17 or employ a colour conversion layer. 18,19However, all these LEC devices only operate at low luminance values.The reason for the small number and poor performances of white-emitting LECs reported is largely ascribed to the lack of highly efficient and stable deepblue-emitting complexes required for colour-mixing.
We now explore a possible approach to dual-emitting iTMCs for white-light emission which follows the principle of combining complementary colours by combining a blueemitting naphthalene domain and orange-emitting [Ir (C^N) 2 (N^N)] + complexes.The naphthalene component is accommodated within a linker between the two cyclometallating units in ligand H 2 1 (Scheme 1).

Experimental
General 1 H, 13 C and 31 P NMR spectra were recorded at room temperature using a Bruker Avance III-600, III-500 or III-400 NMR spectrometer. 1 H and 13 C NMR chemical shifts were referenced to residual solvent peaks with respect to δ(TMS) = 0 ppm and 31 P NMR chemical shifts with respect to δ(85% aqueous H 3 PO 4 ) = 0 ppm.Solution absorption and emission spectra were measured using an Agilent 8453 spectrophotometer and a Shimadzu RF-5301PC spectrofluorometer, respectively.
Electrospray ionization (ESI) mass spectra were recorded on a Bruker esquire 3000plus instrument.Quantum yields in CH 2 Cl 2 solution and powder were measured using a Hamamatsu absolute photoluminescence (PL) quantum yield spectrometer C11347 Quantaurus-QY.Emission lifetimes and powder emission spectra were measured with a Hamamatsu Compact Fluorescence lifetime Spectrometer C11367 Quantaurus-Tau, using an LED light source with λ exc = 280 nm.Quantum yields and PL emission spectra in thin films were recorded using a Hamamatsu absolute quantum yield C9920.The preparation of the thin film samples consisted of deposition on a quartz plate (1 cm 2 ) of the complex with addition of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [Bmim][ PF 6 ].These samples were excited using a light source with λ exc = 320 nm at room temperature under ambient conditions.

Computational details
Dispersion-corrected density functional calculations (DFT-D) were carried out with the D.01 revision of the Gaussian 09 program package. 24The Becke's three-parameter B3LYP exchange-correlation functional 25,26 was used, together with the 6-31G** basis set for C, H, N and O, 27 and the "double-ζ" quality LANL2DZ basis set for the Ir atom. 28Relativistic effects were accounted for by means of an effective core potential (ECP), which was used to replace the inner core electrons of Ir.Intramolecular non-covalent interactions are expected to play a relevant role in the studied systems.Consequently, to get a better description of their molecular geometry, the Grimmés D3 dispersion term with Becke-Johnson damping was added to the B3LYP functional (B3LYP-D3). 29,30The geometries of both the singlet ground electronic state (S 0 ) and the lowest-energy triplet state (T 1 ) were fully optimized.No symmetry restrictions were imposed.The geometry of T 1 was calculated at the spin-unrestricted UB3LYP-D3 level using a spin multiplicity of three.All the calculations were performed in the presence of the solvent (CH 2 Cl 2 ).2][33] Time-dependent DFT (TD-DFT) [34][35][36] calculations of the lowest-lying 30 singlet excited states and the lowest-lying 10 triplets of all the complexes were performed in the presence of the solvent at the minimum-energy geometry optimized for the ground state.The geometry of the first naphthyl-centred triplet excited state of each system was first optimized at TD-DFT level, and then reoptimized at the UB3LYP-D3 level to compare with the results obtained for T 1 from DFT calculations.

Device preparation
LECs , 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 the luminance versus time by 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.Compound H 2 1 was isolated in 74.6% yield.The base peak in the electrospray mass spectrum (m/z 643.6) arose from the [M + H] + ion, and elemental analysis was in accord with the expected composition.The 1 H and 13 C NMR spectra were assigned by 2D methods (COSY, NOESY, HMQC and HMBC) and the spectra were consistent with the symmetrical structure shown in Scheme 1 for H 2 1.

Results and discussion
The conventional methodology 39 for preparing complexes of the type [Ir(C^N) 2 (N^N)][PF 6 ] is treatment of a [Ir (C^N) 2 (μ-Cl) 2 ] dimer with two equivalents of an N^N ligand followed by anion exchange.However, the unfavourable effect that residual chloride ion has on the performance of LEC devices 40 has led us to develop an alternative strategy in which the chlorido dimer is converted to an intermediate solvento complex 41 by treatment of the dimer with AgPF 6 in methanol (Scheme 2). 42The dimer [Ir 2 (1)   23,43 ) and from its proximity to one or more oxygen atoms of the polyethyleneoxy-chain.Unfortunately, no X-ray quality single crystals of any of the four complexes could be grown.In Fig. 2, the broadened signal at δ 6.55 ppm for protons H G2 (the orthoprotons on the 6-and 6′-phenyl substituents of ligand 2) is consistent with hindered rotation of the phenyl rings as observed in other [Ir(C^N) 2 (N^N)] + complexes with 6-phenyl-2,2′-bipyridine or 6,6′-diphenyl-2,2′-bipyridine N^N ligands. 44,45A similarly broadened resonance for H G2 appears at δ 6.58 ppm in the 1 H NMR spectrum of [Ir(1)( 4 4)][PF 6 ]) leading to a slightly destabilized HOMO.These findings are consistent with the oxidation process being centred on the iridium and C^N ligand, as discussed previously. 42he reduction processes centred on the N^N ligand are similar for [Ir(1)( 4 5)][PF 6 ], electron-releasing tert-butyl groups are introduced into the N^N domain and there is a significant shift of E red 1=2 to more negative potentials.This is consistent with the LUMO of the complex being localized on the N^N ligand as discussed below and in agreement with, for example, [Ir( ppy) 2 ( 2)][PF 6 ]. 43
Upon excitation at 280 nm, the complexes all exhibit orange emission.The solution emission spectra are shown in Fig. 6 and emission maxima, quantum yields and lifetimes are given in Tables 2 and 3 4)][PF 6 ]) consistent with the emission originating from a mixture of MLCT and LLCT transitions 6 involving the Ir(III) metal centre and the ppy units of the C^N ligands in the HOMO and the N^N ligand in the LUMO as discussed below.This is consistent with the electrochemical data (Table 1).In degassed solutions, the [Ir(1)(N^N)][PF 6 ] orange emissions exhibit photoluminescence quantum yields (PLQYs) in the range 6-18% (Table 2) with lifetimes of between 135 and 370 ns (Table 3).Fig. 7 depicts the emission spectra of solid-state samples of the complexes.For [Ir(1)( 2)][PF 6 ] and   [Ir(1)( 3)][PF 6 ], going from solution to solid leads to a blue-shift in λ max em , whereas for the complexes containing the tert-butyl substituents, λ max em is little affected.Both the PLQY and emission lifetimes are enhanced from solution to solid state (Tables 2 and 3).We note that the solution PLQY recorded for the reference complex [Ir( ppy) 2 ( 4)][PF 6 ] of 1% (Table 2) is consistent with the 0.07% previously reported. 23he emission spectra of the [Ir(1)(N^N)][PF 6 ] complexes as amorphous thin films with the composition used in LEC devices were also recorded (Fig. S1 †).The spectra are similar and follow the same trends discussed above for the powder spectra with the emission maxima slightly red-shifted ([Ir(1) ( 2 2).
A comparison of Fig. 6 and 7 reveals that each complex in solution, most noticeably [Ir(1)( 4)][PF 6 ], exhibits a second emission at higher energies than the dominant band, and that this emission band is absent in the powder samples.Since the aim of introducing the naphthyl domain into the [Ir (C^N) 2 (N^N)] + complex was to generate a dual emitter, the appearance of the second band, albeit weak, demanded further investigation.The absence of the band in the solid state samples suggested that the high-energy emission might arise from dissociated ligand.Fig. 8 shows an overlay of the normalized emission spectra of [Ir(1)( 4)][PF 6 ], [Ir ( ppy) 2 ( 4)][PF 6 ], H 2 1 and 4. Both [Ir(1)( 4)][PF 6 ] and [Ir ( ppy) 2 ( 4)][PF 6 ] exhibit a band with λ max em ∼ 420 nm (Table 2), suggesting that the origin of the emission is not the naphthyl domain.Note that the previously reported emission spectrum of [Ir( ppy) 2 ( 4)][PF 6 ] did not extend to below 450 nm. 23    a λ exc = 280 nm; lifetimes were measured using a Hamamatsu Quantaurus-Tau (see Experimental section) and some values of λ max em differ slightly from those in Table 2 where spectra were recorded on a Shimadzu RF-5301PC spectrofluorometer.b Biexponential fit using the equation τ av = ∑A i τ i /∑A i where A i is the pre-exponential factor of the lifetime.similar to the absorption spectra shown in Fig. 5, but, as expected, lack the naphthyl band at 237 nm.The 425 or 420 nm emission results from two absorption bands at ∼258 and ∼358 nm (Fig. 9, black and cyan spectra).The excitation spectra for [Ir(1)( 4)][PF 6 ] and [Ir( ppy) 2 ( 4)][PF 6 ] resemble that of 4 (Fig. 9).In contrast, the excitation spectrum of H 2 1 for the emission at 410 nm (Fig. 9, green spectrum) is broader.The similarity between the excitation spectra of [Ir(1)( 4)][PF 6 ], [Ir ( ppy) 2 ( 4)][PF 6 ] and free ligand 4 indicate that the high-energy emission around 420 nm arises from a fluorescent excited state of coordinated ligand 4.Although an extremely low concentration impurity could be responsible for such observations, the method of measuring and the reproducibility of recorded solutions of independently synthesized compounds makes this scenario unlikely.
The PLQY increases from 1% to 9% on going from [Ir ( ppy) 2 ( 4)][PF 6 ] to [Ir(1)( 4)][PF 6 ], but this is not necessarily a consequence of partial energy transfer from the naphthyl unit (PLQY = 15% for H 2 1) to the Ir(III) coordination sphere.Excited-state lifetime measurements of [Ir(1)( 4)][PF 6 ] (τ av = 144 ns for the 564 nm and τ av = 4 ns for the 420 nm emission band, Table 3) corroborate the fact that the higher-energy emission arises from a fluorescent excited state being in the same range as for H 2 1 (τ av = 6 ns for the 420 nm emission band).

Theoretical calculations
To gain a better understanding of the electrochemical and photophysical properties of complexes [Ir(1)(N^N)] + , (N^N = 2-5), a combined DFT/TD-DFT theoretical investigation was undertaken at the B3LYP-D3/(6-31G** + LANL2DZ) level in the presence of the solvent (CH 2 Cl 2 ) (see the Experimental section for full computational details).Calculations were also performed for the reference complex [Ir( ppy) 2 (4)] + for comparison purposes.
The geometry of the complexes in their ground electronic state (S 0 ) was optimized without imposing any symmetry restriction.The calculated geometries reproduce the trends observed typically on this type of complexes, showing a distorted octahedral coordination of the iridium atom. 6,48 S1 † summarizes the values of the geometrical parameters defining the iridium coordination sphere and of selected distances and dihedral angles.For all the complexes, the pendant phenyl rings introduced as R and R 1 substituents in the N^N ligand (Scheme 2) present intracation face-to-face π-stacking interactions with the phenyl rings of the closest ppy in the cyclometallating ligand.The calculated centroid-centroid distances between rings A and G in Scheme 2 range from 3.40 to 3.56 Å, in good agreement with the X-ray values reported for complexes [Ir( ppy) 2 (2)] + and [Ir( ppy) 2 (3)] + . 48The steric hindrance produced by the phenyl substituents induces a twisting between the rings of the bpy ligand.The twisting angle is higher for [Ir(1)( 2   (24.9°) because in the former two pendant phenyl rings are introduced in R and R 1 positions.For all the [Ir(1) 2 (N^N)] + complexes, the cyclometallating ligand 1 remains in an expanded disposition (Fig. 10), featuring a distance from the centroid of the naphthalene group to the iridium atom of 10.6-10.8Å.
The geometry of the complexes in their first triplet excited state (T 1 ) was also optimized using the spin-unrestricted UB3LYP-D3/(6-31G**+LANL2DZ) approach.It is worth to note that the face-to-face intracation interactions observed in the ground state are preserved in T 1 , and that the naphthaleneiridium distance remains almost constant.The most important changes are found for the bpy ligand that becomes more planar in T 1 (twisting angles in the 13-18°range, Table S1, ESI †).This points to a higher electron density on the bpy ligand in T 1 , which would stabilize the planar form as it favors the electronic delocalization.
Fig. 11 displays the isovalue contours calculated for the highest-occupied (HOMO) and lowest-unoccupied (LUMO) molecular orbitals of complex [Ir(1)(2)] + as a representative example.Orbitals HOMO−1 and LUMO+7, which respectively correspond to the highest-occupied and lowest-unoccupied molecular orbitals centred on the naphthalene group of ligand 1, are also displayed.The topology of the corresponding MOs of all the [Ir(1)(N^N)] + complexes fully reproduces that of the selected example.The table inserted in Fig. 11 summarizes the MO energies calculated for the [Ir(1)(N^N)] + complexes and compares them with those obtained for the HOMO and LUMO of [Ir( ppy) 2 (4)] + .As it is usually found for ppy-based cyclometallated Ir-iTMCs, 6,44,45 the HOMO results from a mixture of d π orbitals of Ir(III) and phenyl π orbitals, with some contribution from the pyridine rings, of the cyclometallating ligand, whereas the LUMO is located over the bpy of the ancillary ligand.The similar energy values estimated for the HOMO of [Ir(1)(N^N)] + are in good agreement with the experimental E ox 1=2 values (Table 1), which do not change greatly along the series.
The nature of the low-lying triplet states was first studied by performing TD-DFT calculations at the optimized geometry of the ground state (S 0 ).The vertical excitation energies and electronic descriptions computed for the lowest-lying triplet states of the [Ir(1)(N^N)] + complexes and those of [Ir( ppy) 2 (4)] + are given in Table S2.† All the complexes present a first triplet excited state (T 1 ) mainly defined by the HOMO → LUMO excitation, which implies an electron transfer from the Ir-ppy environment, where the HOMO is localized, to the bpy ligand, where the LUMO resides (see Fig. 11).The T 1 state therefore shows a mixed 3 MLCT/ 3 LLCT character, and the calculated excitation energies ([Ir( ppy) 2 (4)] + : 2.56 eV > [Ir(1)(4)] + : 2.55 eV > [Ir(1)(5)] + : 2.53 eV > [Ir(1)(2)] + : 2.48 eV > [Ir(1)(3)] + : 2.43 eV) follow the trend expected on the basis of the HOMO-LUMO gap.The T 1 state was further examined by optimizing its structure at the spin-unrestricted UB3LYP-D3 level as described above.The unpaired-electron spin density distribution calculated for this state (Fig. 12a) matches the topology of the HOMO → LUMO excitation and therefore corroborates the electron transfer from the Ir-ppy environment to the N^N ligand, thus confirming the 3 MLCT/ 3 LLCT character of T 1 .The electronic nature predicted for T 1 is in good agreement with the broad an unstructured shape of the emission bands observed experimentally (Fig. 6 and 7).
Back to the TD-DFT calculation of triplet states, the first excited state involving the naphthyl group of ligand 1 is found 0.3-0.4eV above T 1 (Table S2 †).As was to be expected, it is mainly described by the HOMO−1 → LUMO+7 excitation, and its vertical excitation energy from S 0 remains almost constant (2.85-2.86eV) for all the [Ir(1)(N^N)] + complexes, as the structural differences between these complexes concern the N^N ligand from which the naphthyl is far apart.The spin-density distribution calculated for this state is shown in Fig. 12b and clearly reflects the localization of the electronic transition on  5)] + , respectively.This energy difference is large enough to discard the contribution of the naphthyl-centred triplet excited state to the phosphorescence emission, which is predicted to occur from the lowest-energy, HOMO → LUMO, 3 MLCT/ 3 LLCT T 1 state.TD-DFT calculations were also performed for singlet excited states to investigate the nature of the bands appearing in the experimental absorption spectra (Fig. 5).Singlet excited states with mainly MLCT character and small oscillator strengths are found in the 400 nm region, followed by ligand-centred (LC) π → π* states involving both the ppy and bpy moieties with some MLCT/LLCT character between 400 and 300 nm.No excited state implying the naphthyl moiety of ligand 1 is found below 300 nm.According to the theoretical results, it is unlikely that emission, fluorescent or phosphorescent, takes place from naphthyl-centred excited states.4).The LECs were prepared on ITO-patterned glass substrates in a double-layer structure.They consist of a PEDOT:PSS layer (80 nm) and the electroluminescent active layer (100 nm) sandwiched between two electrodes.Aluminium was thermally evaporated as the top cathode (70 nm).The LECs were characterized under inert conditions using a pulsed current driving of 25 A m −2 (average current density), a frequency of 1000 Hz and duty cycles of 50%.This operation method provides faster response and longer lifetimes than constant voltage operation. 49In LECs, the initial limited injection is assisted by the ion accumulation at the electrode interface and the formation of electrochemical p-and n-doped regions.Therefore, the behaviour of LECs is mainly governed by the ion mobility in the amorphous thin film.Even though iTMCs are intrinsically ionic, additional ions are usually added in the active layer to speed up the device response and to balance the carriers injected.In this work, the ionic liquid (IL) 1-butyl-3-imidazolium hexafluoridophosphate [Bmim][PF 6 ] was added to the active layer at a molar iTMC : IL ratio of 4 : 1. [50][51][52] Fig. 13 displays the luminance versus time curves for LECs A-E, and Table 4 collects the parameters that summarize the   device performances.The turn-on time (t max ) is defined as the time to reach the maximum luminance (L max ) and the device lifetime (t The main differentiator between the complexes [Ir(1) ( 5)][PF 6 ] and [Ir( ppy)( 4)][PF 6 ] used to prepare LECs D and E is the naphthyl-bridged functionalization of the ppy ligands in the case of LEC D. Yet both the device responses t max and t 1/2 are comparable for these two devices, which indicates that the ppy functionalization does not influence the ionic movement in the films.Therefore, the longer t max and t 1/2 found for LECs A-C should not be ascribed to the large size of ligand 1.Moreover, the [Ir(1)(2)][PF 6 ] complex, used in the fabrication of LEC A, exhibits the highest PLQY in thin film (22.6%), whereas the device efficiency is superior for D incorporating a complex with a slightly lower PLQY (21.9%).Considering these PLQY values and a typical outcoupling of 20%, the theoretical maximum EQEs predicted for LECs A and D when all injected electrons and holes combine have very similar values of 4.5% and 4.4%, respectively.As the EQE achieved for D (2.1%) is slightly closer to the theoretical value than that obtained for A (1.8%), we can hypothesize that the lower exciton-quenching for D could be the result of a better-balanced carrier injection due to the faster response.Hence, for the devices with slower response (devices A-C), lower efficiencies were achieved.Comparable characteristics have been observed for similar orange complexes reported in LECs. 43he electroluminescence (EL) spectrum of all the LECs was registered during the operation of the devices (Fig. S3 †).All the LECs show orange electroluminescence with maxima in the 580-590 nm range but for LEC C ([Ir(1)(4)][PF 6 ]) which emits at 575 nm.The EL spectra are similar to the PL spectra recorded in powder (Fig. 7) and in thin film (Fig. S1 †).

Conclusions
We have prepared and characterized a series of cyclometallated [Ir(C^N) 2 (N^N)][PF 6 ] compounds in which the two cyclometallating C^N units are connected by a naphthyl-containing linker.The N^N ligand (2-5) is a 2,2′-bipyridine functionalized with phenyl and tert-butyl groups.The electrochemical and photophysical properties were compared with those of [Ir( ppy) 2 ( 4)][PF 6 ].The complexes containing the naphthyl-unit exhibit similar absorption spectra, which differ from that of [Ir( ppy) 2 ( 4)][PF 6 ] only in the presence of an intense absorption at ∼236 nm arising from naphthyl-centred π → π* transitions.Excitation at 280 nm leads to an orange emission for solutions of each complex, and going from solution to powder or thin film leads to little change or to a small blue shift in the emission.The incorporation of the naphthalene unit does not lead to a desirable blue contribution to the emission, and DFT/ TD-DFT calculations were performed to understand this observation.The energy difference between the LUMO and the lowest-unoccupied MO centred on the naphthyl moiety (LUMO+7) is large enough to explain why there is no contribution from the naphthyl-centred triplet excited state to the phosphorescence emission.Singlet excited states were also investigated.LECs using the [Ir(1)(N^N)][PF 6 ] and [Ir( ppy) 2 ( 4)][PF 6 ] complexes in the emissive layer led to long living devices with modest turn-on times.The presence of the naphthyl-bridge between the cyclometallating units does not significantly alter the device response, indicating that it does not play a significant role in the ionic transport.

Fig. 13
Fig. 13 Luminance vs. time for the LECs driven using a pulsed current of 25 A m −2 (average current density) at a frequency of 1000 Hz and duty cycles of 50%.

Table 3
Emission lifetimes for [Ir(1)(N^N)][PF 6 ] complexes in solution and as powder samples a

Table 4
Performance of ITO/PEDOT:PSS/active layer/Al LECs measured using a pulsed current driving (average current density 25 A m −2 , 1000 Hz, 50% duty cycle).Active layer = Ir-iTMC : [Bmim][PF 6 ] 4 : 1 molar ratio a Time to reach the maximum luminance L max .b Time to reach one-half of the maximum luminance.c Maximum efficacy.d Maximum power conversion efficiency.e Maximum external quantum efficiency.f Photoluminescence quantum yield in thin film using the same composition than for the device active layer (λ exc = 320 nm).
49,53 is the time to reach one-half of L max after this value is achieved.LEC devices can be divided in two groups with LECs D and E, which incorporate complexes [Ir(1)(5)][PF 6 ] and [Ir( ppy)(4)][PF 6 ], showing shorter t max and faster luminance decays than LECs A-C (Table4).LECs D and E show t max , and t 1/2 below 3.5 and 35 hours, respectively, whereas devices A-C have longer turn-on times above 45 hours and a much more stable behaviour with t 1/2 values ranging from 581 to 785 hours.In contrast, LECs D and E achieve higher luminance values above 100 cd m −2 (133 and 146 cd m −2 , respectively) compared with LECs A-C, which show maximum luminances of 91, 67 and 54 cd m −2 , respectively.The maximum values obtained for the efficacy, the power conversion efficiency (PCE) and the external quantum efficiency (EQE) of LEC D (5.4 cd A −1 , 2.2 lm W −1 and 2.1%, respectively) are slightly lower than those found for LEC E (6.4 cd A −1 , 3.7 lm W −1 and 2.6%), and are similar to those reported for other orange-emitting Ir-iTMC-LECs under pulsed current operation.49,53Theefficiency,PCEandEQEvalues found for LECs A-C are smaller than those obtained for D, and decrease in passing from A to B and to C (Table4).This trend is in agreement with the trend of the PLQY for the amorphous thin film (device environment) of the complexes [Ir(1)(2)][PF 6 ], [Ir(1)(3)][PF 6 ] and [Ir(1)(4)][PF 6 ].