Anionic Cyclometalated Iridium(III) Complexes with a Bis-Tetrazolate

Aug 22, 2017 - Instituto de Ciencia Molecular, Universidad de Valencia, C/J. Beltran 2, 46980 Paterna, Spain ... the bis-tetrazolate unit does not con...
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Anionic Cyclometalated Iridium(III) Complexes with a Bis-Tetrazolate Ancillary Ligand for Light-Emitting Electrochemical Cells Elia Matteucci,† Andrea Baschieri,*,‡ Andrea Mazzanti,† Letizia Sambri,† Jorge Á vila,§ Antonio Pertegás,§ Henk J. Bolink,*,§ Filippo Monti,*,∥ Enrico Leoni,∥,⊥ and Nicola Armaroli*,∥ †

Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy Dipartimento di Chimica “Giacomo Ciamician”, Università di Bologna, Via San Giacomo 11, 40126 Bologna, Italy § Instituto de Ciencia Molecular, Universidad de Valencia, C/J. Beltran 2, 46980 Paterna, Spain ∥ Istituto per la Sintesi Organica e la Fotoreattività, Consiglio Nazionale delle Ricerche, Via P. Gobetti 101, 40129 Bologna, Italy ⊥ Laboratorio Tecnologie dei Materiali Faenza, ENEA, Via Ravegnana 186, 48018 Faenza (RA), Italy ‡

S Supporting Information *

ABSTRACT: A series of monoanionic Ir(III) complexes (2−4) of general formula [Ir(C^N)2(b-trz)](TBA) are presented, where C^N indicates three different cyclometallating ligands (Hppy = 2-phenylpyridine; Hdfppy = 2-(2,4difluoro-phenyl)pyridine; Hpqu = 2-methyl-3-phenylquinoxaline), b-trz is a bistetrazolate anionic N^N chelator (H2b-trz = di(1H-tetrazol-5-yl)methane), and TBA = tetrabutylammonium. 2−4 are prepared in good yields by means of the reaction of the suitable b-trz bidentate ligand with the desired iridium(III) precursor. The chelating nature of the ancillary ligand, thanks to an optimized structure and geometry, improves the stability of the complexes, which have been fully characterized by NMR spectroscopy and high-resolution MS, while X-ray structure determination confirmed the binding mode of the b-trz ligand. Density functional theory calculations show that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are mainly localized on the metal center and the cyclometalating ligands, while the bis-tetrazolate unit does not contribute to the frontier orbitals. By comparison with selected classes of previously published cationic and anionic complexes with high ligand field and even identical cyclometallating moieties, it is shown that the HOMO−LUMO gap is similar, but the absolute energy of the frontier orbitals is remarkably higher for anionic vs cationic compounds, due to electrostatic effects. 2−4 exhibit reversible oxidation and reduction processes, which make them interesting candidates as active materials for light emitting electrochemical cells, along with red, green, and blue emission, thanks to the design of the C^N ligands. Photoluminescence quantum yields range from 28% (4, C^N = pqu, red emitter) to 83% (3, C^N = dfppy, blue emitter) in acetonitrile, with the latter compound reaching 95% in poly(methyl methacrylate) (PMMA) matrix. In thin films, the photoluminescence quantum yield decreases substantially probably due to the small intersite distance between the complexes and the presence of quenching sites. In spite of this, surprisingly stable electroluminescence was observed for devices employing complex 2, demonstrating the robustness of the anionic compounds.

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INTRODUCTION Iridium-based ionic transition-metals complexes (Ir-iTMCs) are one of the most widely used classes of emitters in solid-state electroluminescent devices such as organic light-emitting diodes (OLEDs)1,2 and light-emitting electrochemical cells (LECs),3,4 to be used for flat panel displays and lighting.3 Iridium(III) complexes have drawn much attention thanks to their unique properties including short excited-state lifetimes (often 70 h).

Article

EXPERIMENTAL SECTION

General Information. Analytical grade solvents and commercially available reagents were used as received, unless otherwise stated. Chromatographic purifications were performed using aluminum oxide. 1 H, 19F, and 13C NMR spectra were recorded on Varian Inova (300 and 600 MHz for 1H) and Varian Mercury (400 MHz for 1H) spectrometers. Chemical shifts (δ) are reported in ppm relative to residual solvent signals for 1H and 13C NMR (1H NMR: 7.26 ppm for CDCl3, 1.94 ppm for CD3CN; 13C NMR: 77.0 ppm for CDCl3, 1.32 ppm for CD3CN). 13C NMR spectra were acquired with 1H broad band decoupled mode. Coupling constants are given in Hz. The highresolution mass spectra (HRMS) were obtained with a ESI-QTOF (Agilent Technologies, model G6520A) instrument, and the m/z values are referred to the monoisotopic mass. ESI-MS analyses were performed by direct injection of acetonitrile solutions of the compounds using a WATERS ZQ 4000 mass spectrometer. Di(1H-tetrazol-5-yl)methane39 (1) was prepared according to reported methods. Caution: Although we experienced no difficulties in handling these nitrogen-rich compounds, small scale and best safety practices are strongly encouraged. Synthesis of complex [Ir(ppy)2(b-trz)]TBA (2) and [Ir(dfppy)2(btrz)]TBA (3). General Procedure. A solution of compound 1 (2.5 eq, 11.4 mg, 0.075 mmol) and KOH (5 eq, 8.4 mg, 0.15 mmol) in EtOH (2 mL) was stirred for 4 h at room temperature to give compound 1a. Ir(III) dimer 5−6 (0.03 mmol) was dissolved in DCM (6 mL), and the resulting solution was slowly added to a solution of compound 1a. The resulting mixture was stirred at room temperature for further 24 h. After this time, solvent was evaporated and the crude was purified by flash chromatography on Al2O3 using a mixture of dichloromethane/ methanol (8:2) to give the expected product. The obtained product was then dissolved in a 3:1 solution of DCM/EtOH (20 mL) and TBABr (3.0 equiv) was added. The resulting mixture was stirred at room temperature for 24 h. After this time, H2O was added and the mixture was extracted with DCM (3 × 20 mL). The collected organic phases were washed with brine (20 mL), dried over Na2SO4, and concentrated. The desired products 2−3 were obtained after recrystallization from a DCM/Et2O solution. Complex 2. 44.5 mg, Yield = 83%. 1H NMR (CD3CN, 400 MHz) δ 7.97−7.95 (m, 4H), 7.74 (ddd, JHH = 8.4 Hz, JHH = 7.2 Hz, JHH = 1.6 Hz, 2H), 7.65 (dd, JHH = 7.8 Hz, JHH = 1.6 Hz, 2H), 7.03−6.99 (m, 2H), 6.85 (ddd, JHH = 7.8 Hz, JHH = 7.2 Hz, JHH = 1.4 Hz, 2H), 6.73 (dt, JHH = 7.4 Hz, JHH = 1.4 Hz, 2H), 6.32 (ddd, JHH = 7.8 Hz, JHH = 1.4 Hz, JHH = 0.4 Hz, 2H), 4.52 (s, 2H), 3.10−3.05 (m, 8H), 1.64− 1.55 (m, 8H), 1.35 (sest, JHH = 7.4 Hz, 8H), 0.97 (t, JHH = 7.4 Hz, 12H). 13C NMR (CDCl3, 100 MHz) δ 169.2 (C), 156.1 (C), 154.3 (C), 149.6 (CH), 144.3 (C), 136.2 (C), 132.3 (CH), 129.0 (CH), 123.5 (CH), 121.3 (CH), 120.2 (CH), 118.4 (CH), 58.7 (CH2), 23.8 (CH2), 22.6 (CH2), 19.6 (CH2), 13.5 (CH3). HRMS (ESI-QTOF) m/ z: calcd. for C25H18IrN10−: 649.1327; found 649.1327 [(M C16H36N)−]. ESI-MS: 651 [M − C16H36N]−. Complex 3. 27.2 mg, Yield = 47%. 1H NMR (CD3CN, 600 MHz) δ 8.24 (d, JHH = 8.8 Hz, 2H), 7.90 (d, JHH = 5.8 Hz, 2H), 7.81 (dt, JHH = 7.8 Hz, JHH = 1.4 Hz, 2H), 7.08−7.05 (m, 2H), 6.49 (ddd, JHH = 12.8 Hz, JHH = 9.6 Hz, JHH = 2.4 Hz, 2H), 5.79 (dd, JHH = 9.0 Hz, JHH = 2.4 Hz, 2H), 4.55 (s, 2H), 3.09−3.05 (m, 8H), 1.63−1.56 (m, 8H), 1.35 (sest, JHH = 7.4 Hz, 8H), 0.96 (t, JHH = 7.4 Hz, 12H). 19F NMR (CDCl3, 376 MHz) δ −109.0 (q, JCF = 9.2 Hz, 2F), −111.4 (t, JCF = 11.0 Hz, 2F). 13C NMR (CD3CN, 100 MHz) δ 166.1 (C, d, JCF = 7.2 Hz), 163.8 (C, dd, JCF = 253.0 Hz, JCF = 12.2 Hz), 161.9 (C, dd, JCF = 257.4 Hz, JCF = 12.8 Hz), 160.4 (C, d, JCF = 6.6 Hz), 156.5 (C), 150.7 (CH), 139.1 (CH), 129.6 (C, t, JCF = 3.2 Hz), 123.8 (CH), 123.5 (CH), 114.7 (CH, dd, JCF = 16.8 Hz, JCF = 2.8 Hz), 97.5 (CH, t, JCF = 27.4 Hz), 59.3 (CH2), 24.3 (CH2), 22.8 (CH2), 20.3 (CH2), 13.8 (CH3). HRMS (ESI-QTOF) m/z: calcd. for C25H14F4IrN10−: 721.0950; found 721.0961 [(M − C16H36N)−]. ESI-MS: 723 [M − C16H36N]−. Synthesis of Complex [Ir(pqu)2(b-trz)]TBA (4). Compound 1 (2.2 equiv, 10.1 mg, 0.066 mmol) and TBAOH in MeOH (40% w/w) (5 10585

DOI: 10.1021/acs.inorgchem.7b01544 Inorg. Chem. 2017, 56, 10584−10595

Article

Inorganic Chemistry equiv, 0.15 mmol, 107 μL) in DCM/EtOH 3:1 solution (8 mL) was stirred for 4 h at room temperature to give compound 1b. After this time, Ir(III) dimer 7 (39.9 mg, 0.03 mmol) was added, and the resulting solution was stirred for additional 24 h. H2O was then added and the solution was extracted with DCM (3 × 10 mL). The collected organic phases were washed with brine (20 mL), dried over Na2SO4, and concentrated. The desired product 4 was obtained after recrystallization from a DCM/Et2O solution (57.1 mg, yield = 93%). 1 H NMR (CD3CN, 600 MHz) δ 8.24 (d, JHH = 8.2 Hz, 2H), 7.82 (t, JHH = 8.6 Hz, 4H), 7.49 (t, JHH = 7.4 Hz, 2H), 7.07−7.04 (m, 2H), 7.02 (t, JHH = 7.8 Hz, 2H), 6.74−6.69 (m, 4H), 3.39 (s, 2H), 3.19 (s, 6H), 3.09−3.05 (m, 8H), 1.63−1.56 (m, 8H), 1.35 (sest, JHH = 7.4 Hz, 8H), 0.97 (t, JHH = 7.4 Hz, 12H). 13C NMR (CDCl3, 100 MHz) δ 167.1 (C), 156.4 (C),155.4 (C), 151.9 (C), 146.6 (C), 141.3 (C), 139.4 (C), 135.1 (CH), 129.4 (CH), 129.2 (CH), 128.9 (CH), 128.4 (CH), 127.7 (CH), 126.9 (CH), 121.2 (CH), 58.7 (CH2), 27.1 (CH3), 23.8 (CH2), 21.9 (CH2) 19.5 (CH2), 13.5 (CH3). HRMS (ESI-QTOF) m/z: calcd. for C33H24IrN12−: 779.1858; found 779.1862 [(M − C16H36N)−]. ESI-MS: 781 [M − C16H36N]−. Electrochemistry. Voltammetric experiments were performed using a Metrohm AutoLab PGSTAT 302N electrochemical workstation in combination with the NOVA 2.0 software package. All the measurements were carried out at room temperature in acetonitrile solutions with a sample concentration approximately 1 mM and using 0.1 M tetrabutylammonium hexafluorophosphate (electrochemical grade, TBAPF6) as the supporting electrolyte. Oxygen was removed from the solutions by bubbling argon for 20 min. All the experiments were carried out using a three-electrode setup (BioLogic VC-4 cell, volume range: 1−3 mL) using a platinum or a glassy carbon working electrode (both having an active surface disk of 1.6 mm in diameter), the Ag/AgNO3 redox couple (0.01 M in acetonitrile, with 0.1 M TBAClO4 supporting electrolyte) as the reference electrode and a platinum wire as the counter electrode. At the end of each measurement, ferrocene was added as the internal reference. Cyclic voltammograms (CV) were recorded at different scan rates, from 100 to 1000 mV s−1. Osteryoung square-wave voltammograms (OSWV) were recorded with scan rate of 125 mV s−1, a SW amplitude of ±20 mV, and a frequency of 25 Hz. Computational Details. Density functional theory (DFT) calculations were carried out using the D.01 revision of the Gaussian 09 program package40 in combination with the M06 global-hybrid meta-GGA exchange-correlation functional.41,42 The fully relativistic Stuttgart/Cologne energy-consistent pseudopotential with multielectron fit was used to replace the first 60 inner-core electrons of the iridium metal center (i.e., ECP60MDF)43 and was combined with the associated triple-ζ basis set (i.e., cc-pVTZ-PP basis).43 On the other hand, the Pople 6-31G(d,p) basis was adopted for all other atoms.44 All the reported complexes were fully optimized, both in the ground state (S0) and in their lowest triplet state (T1), by using the polarizable continuum model (PCM) to simulate acetonitrile solvation effects.45−47 Frequency calculations were always used to confirm that every stationary point found by geometry optimizations was actually a minimum on the corresponding potential-energy surface (no imaginary frequencies). The structural overlap between the X-ray crystal structure of complex 4 and its ground-state theoretically computed one was obtained using the VMD program48 by minimizing the root-mean-square deviation (RMSD) of all the atomic positions, except hydrogen atoms. Time-dependent DFT calculations (TDDFT),49−51 carried out at the same level of theory used for geometry optimizations, were used to simulate the electronic absorption spectra of the investigated molecules in their optimized S0 geometry. The first 100 singlet and 25 triplet vertical excitations were computed for all the complexes. Natural transition orbitals (NTOs) transformations were adopted to obtain a clear and compact orbital representation for the electronic transition density matrix in the case of complex multiconfigurational excitations.52 To investigate the nature of the T1 state, geometry optimizations and frequency calculations were performed at the spin-unrestricted UM06 level of theory, imposing a spin multiplicity of 3. The emission energy from the lowest triplet excited state was estimated by subtracting the SCF energy of the T1 state in its

minimum conformation from that of the singlet ground state having the same geometry of T1. All the pictures showing molecular orbitals and spin-density surfaces were created using GaussView 5.53 Photophysical Measurements. The spectroscopic investigations were carried out in spectrofluorimetric grade acetonitrile. The absorption spectra were recorded with a PerkinElmer Lambda 950 spectrophotometer. For the photoluminescence experiments, the sample solutions were placed in fluorimetric Suprasil quartz cuvettes (10.00 mm) and dissolved oxygen was removed by bubbling argon for 30 min. The uncorrected emission spectra were obtained with an Edinburgh Instruments FLS920 spectrometer equipped with a Peltiercooled Hamamatsu R928 photomultiplier tube (PMT) (spectral window: 185−850 nm). The emission spectra of complex 4, when reported al λ > 800 nm, were recorded using a supercooled Hamamatsu R5509-72 PMT, cooled down to −80 °C by liquid nitrogen (spectral window: 800−1700 nm). An Edinburgh Xe 900 with 450 W xenon arc lamp was used as the excitation light source. The corrected spectra were obtained via a calibration curve supplied with the instrument. The photoluminescence quantum yields (ΦPL) in solution were obtained from the corrected spectra on a wavelength scale (nm) and measured according to the approach described by Demas and Crosby,54 using an air-equilibrated water solution of quinine sulfate in 1 N H2SO4 as reference (ΦPL = 0.546).55 The emission lifetimes (τ) were measured through the time-correlated single photon counting (TCSPC) technique using an HORIBA Jobin Yvon IBH FluoroHub controlling a spectrometer equipped with a pulsed SpectraLED (λexc = 373 nm; fwhm = 11 nm) as the excitation source and a red-sensitive Hamamatsu R-3237-01 PMT (185−850 nm) as the detector. The analysis of the luminescence decay profiles was accomplished with the DAS6 Decay Analysis Software provided by the manufacturer, and the quality of the fit was assessed with the χ2 value close to unity and with the residuals regularly distributed along the time axis. To record the 77 K luminescence spectra, samples were put in quartz tubes (2 mm inner diameter) and inserted into a special quartz Dewar flask filled with liquid nitrogen. The poly(methyl methacrylate) (PMMA) films containing 1% (w/w) of the complex and neat film samples were drop-cast from dichloromethane solutions. The thickness of the films was not controlled. Solid-state ΦPL values were calculated by corrected emission spectra obtained from an Edinburgh FLS920 spectrometer equipped with a barium sulfatecoated integrating sphere (diameter of 4 in.) following the procedure described by Würth et al.56 Experimental uncertainties are estimated to be ±8% for τ determinations, ± 10% for ΦPL, ± 2 nm and ±5 nm for absorption and emission peaks, respectively. LEC Preparation. All materials were used as received. Poly(3,4ethylenedioxythiophene)/poly-styrenesulfonate (PEDOT/PSS CLEVIOS P VP AI 4083) was purchased from Heraeus. The ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) and the solvent dichloromethane were purchased from Sigma-Aldrich. The photolithography-patterned ITO glass substrates were purchased from Naranjo Substrates (www.naranjosubstrates. com). The substrates were cleaned with sonication and soap, then deionized water, isopropanol, and UV−O3 lamp for 20 min. The thickness of the films was determined with an Ambios XP-1 profilometer. 80 nm of (PEDOT/PSS) was coated at 1000 rpm for 60 s and then was dried at 150 °C for 15 min. The emitting layer (200 nm) was prepared by spin-coating of a dichloromethane solution consisting of the 2−4 with the addition of the ionic liquid ([Bmim][PF6]) in a 4:1 molar ratio. The devices were then transferred to an inert atmosphere glovebox ( 300 nm; Figure 4, inset), several weaker and broader bands are present, but their attribution is less straightforward. Therefore, we compared the experimental spectra of 2−4 with (i) TD-DFT calculated ones and (ii) to the absorption profiles of a series of cationic complexes with identical cyclometallating ligands and a high field ancillary moiety, whose simplest representative is B.60 As already observed for the above-mentioned diisocyanidebased series60 and in line with theoretical predictions, the absorption spectrum of 4 is the most red-shifted of the group, showing broad absorption features spreading up to 650 nm. The main responsible for this behavior is the lowest singlet electronic transition, centered at 546 nm and involving an almost pure HOMO → LUMO excitation (Figure 2 and S15). In 4, the presence of the quinoxaline moiety in the C^N ligands is able to strongly stabilize the LUMO and, as a consequence, lower the energy of the S0 → S1 electronic transition. On the other hand, the absorption profiles of 2 and 3 extend only up to 500 and 475 nm, respectively. The observed blue shift in the spectrum of the fluorinated system 3 is attributable to the remarkable HOMO stabilization exerted by the electronwithdrawing substituents on the phenyl moieties of the cyclometalating ligand, where the HOMO is centered. This photophysical behavior is commonly observed in fluorinated cationic iridium(III) complexes,3,67 and is confirmed in the present anionic systems (Figures 2 and 4). It is worth noting that the spin-forbidden S0 → T1 direct transition is clearly detectable in the spectra of 2 and 3 (i.e., at 474 nm with ε = 4.5 × 102 M−1 cm−1 for 2, and at 452 nm with ε = 3.8 × 102 M−1 cm−1 for 3; Figure 4, inset). In both cases, TD-DFT calculations indicate that this transition is a strongly multiconfigurational excitation that can be hardly visualized in terms of a compact and clear series of occupied/virtual Kohn− Sham molecular-orbital couples. For this reason, this S0 → T1 transition is reported in Figure S16 and S17 in terms of two couples of natural transition orbitals (NTO)52 for both 2 and 3, respectively. This compact representation allows one to understand that, in both complexes, the direct population of the lowest triplet can be essentially achieved by promotion of one electron from the iridium d orbitals and the π orbitals on both the cyclometalating ligands to the π* orbitals of the C^N ligands themselves. As a consequence, the emission from these complexes is expected to arise from a 3LC state (centered on the cyclometalating ligands) with a mixed 3MLCT contribution. As expected, the bis-tetrazolate ligand 12− is not directly involved in the transition, thus acting as a real ancillary ligand. The emission spectra of 2−4 are collected in Figure 5, in acetonitrile at 298 K (full lines) and 77 K (dashed lines); luminescence properties are summarized in Table 3. As expected, the spectra of complexes 2 and 3 are rather structured, suggesting an emission from a predominantly LC state. This scenario is corroborated by the small Stokes shift of 1020 and 710 cm−1 observed for 2 and 3, respectively. In agreement with the absorption spectra, the emission of 3 is blue-shifted by about 0.17 eV compared to 2 (Figure 5). Upon lowering the temperature to 77 K, the vibronic progression of the emission spectra of 2 and 3 becomes more pronounced and only a minor blue shift is observed (approximately 0.06 eV, Table 3), corroborating the hypothesis of a LC state with some 3MLCT contribution. On the contrary, the room-temperature emission spectrum of 4 is broad and unstructured, which, at first sight might indicate

Table 2. Electrochemical Data of 2−4 Determined by Cyclic Voltammetry in Room-Temperature Acetonitrile Solution +0.1 M TBAPF6a Electrochemical datab [V] complex

Eox

2 3

+0.52 (+0.52) +0.85 (+0.85)

4

+0.67 (+0.66)

[Ir(ppy)2(bpy)]+d Ae Bf Cg

+0.87 +1.42 (+1.2irr.) +0.51

Ered −2.64 (−2.64) −2.51, −2.8irr. (−2.50, −2.8) −1.75, −2.04 (−1.76, −2.05) −1.78, −2.60 −1.82; −1.98 (−2.3irr.; −2.5irr.) −2.62

ΔEredoxc 3.16 (3.16) 3.36 (3.35) 2.42 (2.42) 2.65 3.24 (3.5) 3.13

a

Data in brackets refer to OSWV experiments; ferrocene is used as internal reference. For the sake of comparison, electrochemical data of A−C and of the archetypal complex [Ir(ppy)2(bpy)]+ are reported. b All redox processes are reversible, unless otherwise stated (irr.). c ΔEredox = Eox − Ered. dData from refs 66 and 3. eData from ref 38. f Data from ref 60. gData from ref 37.

Figure 2); (iii) both oxidation and reduction potentials of 2 are shifted by about −0.9 V if compared to the cationic counterpart A, as already suggested by DFT (theoretically estimated shift: 1.3 ± 0.2 eV); (iv) the redox properties of 2 are virtually identical to those of C, a similar anionic cyanide-based complex with the same overall charge and a comparable electronic distribution. In addition to that, it should be mentioned that the experimental results found by both CV and OSWV experiments show a good correlation with those estimated by DFT, with the HOMO−LUMO energy gap systematically higher than the redox gap of 1.0 ± 0.1 eV for all the reported complexes, including those of Chart 1. Photophysical Properties and Excited-State Calculations. All the investigated complexes 2−4 are stable in acetonitrile solution for several months and do not show degradation under standard laboratory conditions. The roomtemperature electronic absorption spectra of 2−4 are reported in Figure 4 and compared with the theoretically calculated ones in Figures S13−S15. The spectral window between 230 and 300 nm shows intense absorption bands (ε ≈ 2÷5 × 104 M−1 cm−1) that are assigned to spin-allowed ligand-centered (LC) π−π* transitions involving both the cyclometalating and the ancillary ligand 12−, with some likely contribution from 3LC forbidden transitions,

Figure 4. Absorption spectra of 2−4 at 298 K in acetonitrile solution. The lowest-energy transitions are magnified in the inset. 10590

DOI: 10.1021/acs.inorgchem.7b01544 Inorg. Chem. 2017, 56, 10584−10595

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Inorganic Chemistry

Figure 6. Spin-density distribution for the fully relaxed lowest triplet state (T1) of complexes 2−4, computed in acetonitrile (isovalue: 0.002 e bohr−3). Figure 5. Corrected and normalized emission spectra of 2−4 in acetonitrile at 298 K (full lines) and at 77 K (dashed lines). Excitation wavelength: 360 nm.

As far as photoluminescence quantum yields (ΦPL) are concerned, the best emitter is 3, with ΦPL = 83% in roomtemperature acetonitrile solution; also the nonfluorinated analogue 2 shows a remarkable emission efficiency (ΦPL = 75%, Table 3). On the contrary, a lower value is measured for 4 (ΦPL = 28%), probably due to both the energy-gap law and the distorted excited state geometry of the quinoxaline ligands (see above). Anyway, all of these ΦPL values are about twice as much those of a similar cationic series (whose archetype is B, Chart 1)60 and are similar those of the already reported cyanide-based anionic iridium(III) complexes whose model compound is C, (Chart 1).37 The excited-state properties of 2 and 3 are also comparable in terms of emission energies, to similar anionic iridium(III) complexes based on differently substituted tetrazolate ligands.19,23 However, in the first case,19 such ligands are monodentate and lead to coordination compounds with lower emission quantum yields and, apparently, to relevant stability problems, as evidenced by the presence of degradation products and/or impurities in the NMR spectra. In the other case,23 probably to prevent the latter issue, a chelating bistetrazolate is used, but quantum yields remain around 50%. Such limitations have been overcome with the present bistetrazolate chelator 12−. Since the anionic iridium(III) complexes 2−4 have been ultimately created for their potential application as active materials in light-emitting electrochemical cells (LECs), we also investigated their emission properties in solid state under different conditions: (i) in a poly(methyl methacrylate) (PMMA) matrix at a concentration of 1% by weight or (ii) as neat films. Figure S19 shows the solid-state emission spectra of 2−4 in both experimental conditions and Table S1 summarizes the associated luminescence properties and photophysical parameters. The emission spectra in PMMA matrix are virtually superimposable to those recorded in acetonitrile (Figure S19), indicating that all complexes deactivate from the same triplet

emission from a pure 3MLCT state. However, its radiative decay constant (kr) is comparable to that of 2 and 3 (Table 3), and its kr is rather low compared to other charged iridium(III) complexes emitting from 3MLCT states.3 Moreover, no spectral shift is observed between the room-temperature emission spectrum of 4 vs 77 K. All these experimental evidence indicate that the emission of 4 can also arise from a predominantly 3LC state, likewise 2 and 3. In order to get a deeper insight on the emitting states of all the investigated complexes, spin-unrestricted DFT calculations were carried out to optimize the lowest triplet state (T1) of 2− 4. Theoretical results indicate that, after relaxation from the Franck−Condon region, all the complexes emit from a predominantly 3LC state with a 3MLCT contribution from the iridium d orbitals. As clearly indicated by the T1 spindensity distributions reported in Figure 6, in all the complexes the unpaired electrons are mainly located on the cyclometalating ligand facing the nonplanar bis-tetrazolate ancillary unit, whereas the iridium metal center shares only 0.43 ± 0.03 e on its dπ orbitals. Therefore, the unstructured emission of 4 might arise from the highly distorted conformation adopted by the phenylquinoxaline ligand (pqu) that prevents the observation of any vibronic progression in the emission spectrum. A similar behavior has been reported for the corresponding cationic analogue of 4.60 The TD-DFT calculated emission energies are computed to be 2.31 eV (536 nm), 2.47 eV (502 nm), and 1.69 eV (733 nm) for 2, 3, and 4, respectively. These values are in excellent agreement with the mean-photon energy of the emission spectra recorded in room-temperature acetonitrile solution and reported in Figure S18 (i.e., 2.37, 2.50, and 1.77 eV for 2, 3, and 4, respectively).

Table 3. Luminescence Properties and Photophysical Parameters of 2−4 in Acetonitrile Solution oxygen-free liquid solution, 298 K λem [nm]

a

sh

ΦPLb

[%]

rigid matrix, 77 K −1

−1

τ [μs]

kr [10 s ]

knr [10 s ]

c

d

5

e

5

λema

[nm]

2

498, 520

75

2.09

3.6

1.2

485, 516

3

467, 494

83

2.27

3.7

0.75

457, 488

4

676

28

1.37

2.0

5.3

666, 710sh

τc [μs] 4.31 0.79 3.61 1.14 5.66 1.51

(93%) (7%) (74%) (26%) (77%) (23%)

λexc = 360 nm; sh shoulder. bλexc = 360 nm, quinine sulfate in 1 N H2SO4 aqueous solution as reference. cλexc = 373 nm. dkr = Φ/τ. eknr = 1/τ − kr. 10591

DOI: 10.1021/acs.inorgchem.7b01544 Inorg. Chem. 2017, 56, 10584−10595

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Figure 7. (a) LEC architecture. (b, c) Luminance vs time registered respectively for L2 and L3 (LEC operated with a pulsed current of 100 A m−2, 1 kHz, 50% duty cycle and block wave). (d) Electroluminescence spectrum of L2 and L3.

Table 4. Device Performance of the LEC: ITO/PEDOT:PSS/iTMC:[Bmim][PF6]/Al Operated with a Pulsed Current of 100 A m−2a device

Lummaxb (cd m−2)

tmaxc (h)

t1/2d (h)

efficacye (cd A−1)

PEf (lm W−1)

EQEg (%)

λEL (nm)

L2 L3

3 67

43 0.1

>70 0.4

0.06 0.66

0.01 0.12

0.03 0.22

584 553

a

1 kHz, 50% duty cycle and block wave. bMaximum luminance. cTime to reach maximum luminance. dTime to reach one-half of the maximum luminance. eMaximum efficacy. fMaximum power efficiency. gMaximum external quantum efficiency.

(PEDOT/PSS) layer was deposited by spin-coating. The LEC active layer (200 nm) was deposited from dichloromethane solution, which consisted of the emitter mixed with the ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) in a 4:1 molar ratio (complex/IL). As a top contact electrode aluminum was thermally evaporated. For simplicity, the LECs containing complexes 2−4 are referred to as L2−4. LECs are typically characterized by applying a bias and monitoring the emitted light over time. Under a bias, the ions present in the active layer dissociate and migrate toward the electrodes. Once injection overcomes, the resistance of the active layer decreases due to formation of p- and n-doped regions. Yet it was found that their performance strongly improves when driven by a pulsed current.68 Under this driving, the pulsed current is applied and the voltage drops reaching a minimum. Therefore, the devices were characterized by applying an average pulsed current (1 kHz, 50% duty cycle) of 100 A m−2 in inert atmosphere. The LEC characteristics for L2−3 (luminance and electroluminescence) are depicted in Figure 7b−d and summarized in Table 4. The electroluminescence of L4 was unnoticeable under the driving conditions selected, which could be attributed to the low photoluminescence quantum yield in neat film (4%) and the low solubility of 4 in different solvents, leading to films with

state responsible for the emission in diluted solution. Notably, in PMMA matrix, the emission quantum yields of 2 and 3 approach 100%, with complex 3 being always the best emitter of the series (i.e., ΦPL = 95%, Table S1). On the other hand, in neat films, the distance between the iridium complexes is small and exciton diffusion becomes possible, in particular due to their long lifetimes. This very often leads to a reduction in the photoluminescence quantum yield, as excited states can decay nonradiatively when encountering trapping sites (both intrinsic and extrinsic). Additionally, it is possible that triplet−triplet annihilation processes play a role. Hence, ΦPL are dramatically reduced and emission lifetimes are no longer monoexponential (Table S1). For all of the complexes, an emission red shift is observed with respect to solution, due to band broadening and change of the relative intensities of the luminescence peaks (Figure S19). This behavior is typically observed also in cationic iridium(III) complexes whose ligands are not equipped with bulky substituents, which limits aggregation-induced detrimental effects on luminescence.61 LEC Devices. LECs were prepared to evaluate the electroluminescence properties of complexes 2−4 in a sandwich architecture (Figure 7a). The devices were fabricated on cleaned glass ITO-patterned substrate, where a 60 nm poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) 10592

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complexes were easily prepared, and the films were sandwiched in between two electrodes allowing for the evaluation of the electroluminescence properties. Probably because of the small intersite distance, the photoluminescence of the complexes is strongly reduced in the films, and this inevitably hampers electroluminescence. However, surprisingly stable electroluminescence was observed for devices employing complex 2, demonstrating the robustness of the anionic compounds. These device results hold promise for further improvements in the area of luminescent anionic complexes for LECs.

a poor morphology quality. Only L2 and L3 exhibit electroluminescence under the tested conditions, displaying a broad and not well-defined band corresponding to yelloworange and green light with a peak at 584 and 553 nm, respectively. However, the required voltages to sustain the low current densities applied are rather high (around 16 V during the pulse, close to the compliance of the setup) (Figure S20). A small but notable decrease in average driving voltage can be observed, indicating that the devices do operate as LECs; in fact, charge injection is improved as a function of driving time. It would appear, however, that the transport of carriers through the film is problematic as the voltage required is significantly higher than what is observed for state of the art LECs. The luminance of L2 and L3 is low, which is not surprising considering the low ΦPL observed in neat films. Additionally, the high driving voltage also indicates a nonideal transport of charge carriers, and therefore, it is likely that the mobility of electrons and holes are not balanced leading to a nonideal position of the emission zone. In agreement with the fast decrease in driving voltage, the luminance in L2 increases in a few minutes, and then it remains almost constant for a long time until it reaches a maximum luminance after 43 h. In the case of L3 the luminance rises during the first 5 min and then rapidly decreases. For this device, the driving voltage decreases slightly over the same initial time frame after which it starts to increase. The increase in driving voltage usually indicates a permanent degradation in the emitting film, which would explain the rapid decrease in luminance. Similar trends in decreasing operation lifetimes have been observed in LECs using cationic iridium complexes with an increasing number of F-substituents on the ppy.69 However, the operation stability of L2 is rather good, which demonstrates the robustness of the compound as corroborated by the reversible oxidation and reduction in solution. In perspective, preparation of host−guest systems in which the exciton is confined to the lower bandgap material or ligand functionalization with bulky groups that avoid intermolecular interaction in solid state, should strongly improve the ΦPL observed in neat films and, as a consequence, the LEC performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01544. NMR and ESI mass spectra for complexes 2−4. Supplementary figures and tables including comparison of the experimental X-ray geometry of 4 with its DFT calculated one, square-wave voltammograms of 2−4 in room-temperature acetonitrile solutions, simulated absorption spectra of 2−4 in acetonitrile, NTO representations of the S0 → T1 transition of 2 and 3, emission spectra of 2−4 in 1% PMMA matrix and related luminescence properties, voltage vs time graph of L2 and L3. Cartesian coordinates of the DFT optimized geometries of 2−4 in their singlet and triplet ground states (i.e., S0 and T1). X-ray data for complex 4 (PDF) Accession Codes

CCDC 1555727 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

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*(A.B.) E-mail: [email protected]. *(F.M.) E-mail: fi[email protected]. *(H.J.B.) E-mail: [email protected]. *(N.A.) E-mail: [email protected].

CONCLUSIONS We have reported a series of anionic iridium(III) complexes equipped with the bistetrazolate ligand 12−. The chelating nature of such a ligand improves the stability of the complexes, thanks to an optimized structure and geometry, which allows a better coordination of the metal center, without causing distortions of bond angles and structural stress. This chelator is particularly simple, as it has only a methylene bridge between the two tetrazolate moieties. This severely limits the occurrence of undesired nonradiative processes promoted by rotational or vibrational motions, which may compromise the emission performance. All the complexes emit from a predominantly ligand centered state in the red, green, and blue regions of the visible spectrum. Accordingly, DFT calculations show that the HOMO and LUMO are mainly localized on the metal center and on cyclometalating ligands, while the bis-tetrazolate unit does not contribute to the frontier orbitals. By comparison with selected classes of previously published cationic and anionic complexes with high ligand field and even identical cyclometallating moieties, it is shown that the HOMO−LUMO gap is similar. However, the absolute energy of HOMOs and LUMOs is remarkably higher for anionic vs cationic compounds, due to electrostatic effects. Neat films of the

ORCID

Andrea Baschieri: 0000-0002-2108-8190 Andrea Mazzanti: 0000-0003-1819-8863 Henk J. Bolink: 0000-0001-9784-6253 Filippo Monti: 0000-0002-9806-1957 Enrico Leoni: 0000-0001-6190-1708 Nicola Armaroli: 0000-0001-8599-0901 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Italian Ministry of Research (MIUR), the National Research Council of Italy (through the CNR Projects PHEEL and N−CHEM), the European Commission. Financial support by the Spanish Ministry of Economy and Competitiveness (MINECO) of Spain (MAT2014-55200 and Unidad de Excelencia Mariá de Maeztu MDM-2015-0538) and Generalitat Valenciana (Prometeo2016/135) is gratefully acknowledged. J.A. thanks the 10593

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Spanish Ministry of Education, Culture and Sport for his predoctoral (FPU) grant.

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