DOI: 10.1002/chem.201304500
Full Paper
& Luminescence
Understanding Electrogenerated Chemiluminescence Efficiency in Blue-Shifted Iridium(III)-Complexes: An Experimental and Theoretical Study Gregory J. Barbante,[a] Egan H. Doeven,[a] Emily Kerr,[a] Timothy U. Connell,[b] Paul S. Donnelly,[b] Jonathan M. White,[b] Thais Lpes,[a, d] Sarah Laird,[c] David J. D. Wilson,[c] Peter J. Barnard,[c] Conor F. Hogan,*[c] and Paul S. Francis*[a]
Chem. Eur. J. 2014, 20, 3322 – 3332
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Full Paper Abstract: Compared to tris(2-phenylpyridine)iridium(III) ([Ir(ppy)3]), iridium(III) complexes containing difluorophenylpyridine (df-ppy) and/or an ancillary triazolylpyridine ligand [3-phenyl-1,2,4-triazol-5-ylpyridinato (ptp) or 1-benzyl-1,2,3triazol-4-ylpyridine (ptb)] exhibit considerable hypsochromic shifts (ca. 25–60 nm), due to the significant stabilising effect of these ligands on the HOMO energy, whilst having relatively little effect on the LUMO. Despite their lower photoluminescence quantum yields compared with [Ir(ppy)3] and [Ir(dfppy)3], the iridium(III) complexes containing triazolylpyridine ligands gave greater electrogenerated chemiluminescence (ECL) intensities (using tri-n-propylamine (TPA) as a co-reactant), which can in part be ascribed to the more energetically favourable reactions of the oxidised complex (M + ) with both TPA and its neutral radical oxidation product. The calculated iridium(III) complex LUMO energies were shown to
Introduction Electrogenerated chemiluminescence (ECL) is a form of luminescence produced by high-energy reactions between electrogenerated precursors,[1] in which the electronically excited states responsible for the emission of light can be generated through the annihilation between oxidised and reduced forms of the same species, or by using a sacrificial co-reactant. The application of a co-reactant ECL as a highly sensitive mode of detection has been predominantly based on the use of tris(2,2’-bipyridine)ruthenium(II) ([Ru(bpy)3]2 + ), and related polyimine–ruthenium(II) complexes, with characteristic orange/ red emissions (lmax ca. 590–700 nm).[1] Over the last decade, however, numerous researchers have begun to explore chemiluminescence and ECL reactions with cyclometalated iridium(III) complexes exhibiting a wide range of electrochemical properties and emission maxima that can be tuned through subtle changes in the structure of one or more ligands.[2] These [a] Dr. G. J. Barbante, Dr. E. H. Doeven, E. Kerr, T. Lpes, Prof. P. S. Francis Centre for Chemistry and Biotechnology, Faculty of Science Engineering and Built Environment, Deakin University Waurn Ponds, Victoria 3216 (Australia) Fax: (+ 61) 3-5227-2356 E-mail: [email protected] [b] T. U. Connell, Dr. P. S. Donnelly, Prof. J. M. White School of Chemistry and Bio21 Molecular Science and Biotechnology Institute, University of Melbourne Melbourne 3010 (Australia) [c] S. Laird, Dr. D. J. D. Wilson, Dr. P. J. Barnard, Dr. C. F. Hogan Department of Chemistry La Trobe Institute for Molecular Sciences, La Trobe University Victoria 3086 (Australia) E-mail: [email protected] [d] T. Lpes Present address: Department of Chemistry Federal University of Santa Maria Santa Maria, Rio Grande do Sul 97.105-900 (Brazil) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304500. Chem. Eur. J. 2014, 20, 3322 – 3332
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be a good predictor of the corresponding M + LUMO energies, and both HOMO and LUMO levels are related to ECL efficiency. The theoretical and experimental data together show that the best strategy for the design of efficient new blue-shifted electrochemiluminophores is to aim to stabilise the HOMO, while only moderately stabilising the LUMO, thereby increasing the energy gap but ensuring favourable thermodynamics and kinetics for the ECL reaction. Of the iridium(III) complexes examined, [Ir(df-ppy)2(ptb)] + was most attractive as a blue-emitter for ECL detection, featuring a large hypsochromic shift (lmax = 454 and 484 nm), superior co-reactant ECL intensity than the archetypal homoleptic green and blue emitters: [Ir(ppy)3] and [Ir(df-ppy)3] (by over 16-fold and threefold, respectively), and greater solubility in polar solvents.
complexes have created new possibilities for multiplexed ECL detection systems.[2d,g,r,s] However, in contrast to the vast range of orange/red-emitting metal complex electrochemiluminophores,[1b,c,g, 3] relatively few blue emitters are available, and the most effective design of blue-emitting complexes for ECL detection is yet to be fully elucidated. We have therefore used electrochemical, spectroscopic and computational techniques to explore a series of blue-emitting iridium(III) complexes that exhibit various potentially attractive structural attributes for conventional and multiplexed ECL detection. Derived from the extensive development of iridium-complexes for other photonic applications,[4] a common strategy to create luminophores with higher energy emission than the classic green-emitter tris(2-phenylpyridine)iridium(III) ([Ir(ppy)3]; lmax ca. 520 nm),[2d,e,s] is to incorporate electron-withdrawing fluorine substituents on the cyclometalating ring of ppy, which stabilise the HOMO level to a much greater extent than the LUMO.[2g,n,s] Numerous alternative ligands have also been explored to manipulate properties such as emission wavelength or ECL efficiency, to allow more facile preparation or to introduce functionality suitable for labelling or immobilisation.[2f–i,l,n,p,q,u,v, 5] These ligands have a variety of influences on the frontier molecular orbitals[4b, 6] and, therefore, the electrochemical and spectroscopic properties relevant to ECL detection. In an important recent example, Zanarini et al.[2n] reported several green and blue electrochemiluminophores based on cationic iridium complexes with two anionic ppy or difluorophenylpyridine (df-ppy) ligands and a neutral 1,2,3-triazol-4-ylpyridine ligand, functionalised at the 1-nitrogen position by using simple “click” chemistry.[7] Neutral iridium(III) complex analogues incorporating anionic 1,2,4-triazol-5-ylpyridinato ligands (functionalised at the 3-carbon position) have been developed as efficient high-energy emitters for photonic applications,[8] but have not yet been utilised for ECL. Herein, we directly compare the properties of four iridium(III) complexes containing ppy or df-ppy and substituted 1,2,3-triazol-4-ylpyridine or 1,2,4-triazol-5-ylpyridinato ligands (Figure 1), against
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Figure 1. Iridium(III) complexes.
the archetypal [Ru(bpy)3]2 + , [Ir(ppy)3], and [Ir(df-ppy)3] complexes, and two additional homoleptic species with nitrogen heterocyclic ligands: tris(1-phenyl-3-methylimidazolin-2-ylidene-C,C2’)iridium(III) ([Ir(pmi)3])[6] and tris(1-phenylpyrazolylN,C2’)iridium(III) ([Ir(ppz)3])[6, 9] that have found recent application in the emission and electron-blocking layers of light-emitting devices.[10]
with the heteronuclear single quantum coherence (gHSQC) and heteronuclear multiple bond correlation (gHMBC) spectra were used for the complete assignment of the hydrogen and carbon atoms of bicyclic phenylpyridine ligands. A comparison of the 13C spectra of each compound shows the resonances due to the carbon atoms bonded to fluorine in [Ir(df-ppy)2(ptb)]BF4 are shifted down-field when compared with the analogous carbon atoms in [Ir(ppy)2(ptb)]Cl; d = 163.3 and 161.2 ppm and d = 130.1 and 124.4 ppm, respectively (Figure S12 in the Supporting Information), due to the large inductive effect of the fluorine substituent. Conversely, the resonances assigned to the carbon atoms ortho to the fluorine atoms are shifted up-field; d = 114.4, 99.1 and 127.8 ppm compared with d = 132.0, 122.3 and 143.7 ppm in the non-fluorinated cyclometalating ligand, presumably due to mesomeric effects. The carbon atom bound to the iridium is meta relative to both fluorine atoms, and the resonance assigned to this atom is shifted down-field, d at about 153.5 ppm (cannot resolve between two signals) in [Ir(df-ppy)2(ptb)]BF4 compared with d = 146.6 ppm in [Ir(ppy)2(ptb)]Cl (Figure S12 in the Supporting Information). The meta position is typically influenced by inductive effects, which account for the down-field shift observed. The increased electron deficiency at this carbon has been proposed as intrinsic to the hypsochromic shift observed in iridium(III) complexes with fluorinated cyclometalating ligands.[7, 8] Both [Ir(ppy)2(ptb)]Cl and [Ir(df-ppy)2(ptb)]BF4 were characterised by single-crystal X-ray crystallography (Figure 2
Results and Discussion Preparation of iridium(III) complexes The homoleptic iridium(III) complexes were obtained from commercial sources and the [Ir(ppy)2(ptp)] and [Ir(df-ppy)2(ptp)] species were synthesised as previously described.[8a,c] However, the synthesis of [Ir(df-ppy)2(ptb)]PF6 involved modification of a previous procedure,[11] and here we also prepared the novel [Ir(ppy)2(ptb)]PF6 analogue. The synthesis and characterisation of these two species thus warrant some elaboration. The hexafluorophosphate salts of these complexes were prepared (by means of anion metathesis with NH4PF6) from the corresponding chloride and tetrafluoroborate salts. Synthesis of [Ir(ppy)2(ptb)]Cl and [Ir(df-ppy)2(ptb)]BF4 was based on the procedure of Mydlak and co-workers,[11] as described in detail in the Experimental Section. The complexes were characterised by 1H, 13 C homo- and heteronuclear experiments, as well as 19F NMR spectroscopy where relevant (Figures S1–S11 in the Supporting Information). The 1H and 19F NMR spectra of [Ir(df-ppy)2(ptb)]BF4 were consistent with those previously reported.[11] Integration of peaks in the 1H NMR spectrum showed the expected number of signals, but significant overlap made total assignment difficult. Correlation spectroscopy (gCOSY), along Chem. Eur. J. 2014, 20, 3322 – 3332
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Figure 2. An ORTEP representation of the complex cations a) [Ir(df-ppy)2(ptb)] + (7) and b) [Ir(ppy)2(ptb)] + (4) with thermal ellipsoids at the 40 % probability level. Hydrogen atoms and solvent molecules omitted for clarity.
and Table 1). The iridium cations in both complexes are in a distorted octahedral C2N4 environment and the Npyridyl atoms are in a meridional arrangement, which demonstrates the geometry of the starting material is retained at the temperatures used during synthesis. Bond distances between atoms that make up the coordination sphere are typical of other isolated iridium(III) complexes with triazole-containing ancillary ligands.[7, 8c, 11, 12] Spectroscopic properties The absorption spectra of the iridium(III) complexes typically show intense bands between 240 and 300 nm, which can be ascribed to spin-allowed p!p* ligand-centred (LC) transitions, and weaker bands above 300 nm that have been assigned to both allowed and spin-forbidden metal-to-ligand charge trans-
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Full Paper due to efficient (thermally activated) population of a non-radiative metal-centred (MC) ligand-field state involving rupture of a metal–ligand bond.[4c, 9b] Like [Ir(ppz)3], [Ir(pmi)3] has highenergy MLCT absorption bands, but the non-radiative excited state is destabilised by the strong bonding between the metal and phenylimidazole carbene ligand, resulting in relatively intense near-UV photoluminescence.[6] The overall emissions from these iridium(III) complexes, ranging from near-UV to green, are characterised in Figure 3. Absolute quantum yields of the phenylpyridine-based complexes were in reasonable agreement with recent reports[8a,c, 9b, 11, 14] and found to decrease in the order: [Ir(ppy/df-ppy)3] > [Ir(ppy/df-ppy)2(ptp)] > [Ir(ppy/ df-ppy)2(ptb)] + (Table 2). The quantum yield of [Ir(pmi)3] found here, however, was significantly higher than the previously reported value.[6]
Table 1. Crystallography data. [Ir(df-ppy)2(ptb)]BF4·2 CHCl3 [Ir(ppy)2(ptb)]Cl·2CH2Cl2 formula Mr color and habit crystal size [mm3] system space group T [K] a [] b [] c [] a [8] b [8] g [8] V [3] Z 1calcd [Mg m3] l [] m [mm1] F(000) reflns measured independent reflns R [I > 2s(I)] wR(F2) (all data)
C38H26N6IrCl6BF8 1134.36 pale yellow block 0.56 0.38 0.23 triclinic P1¯ 130.0(1) 10.8801(2) 12.9381(3) 14.1902(3) 83.380(2) 86.396(2) 88.246(2) 1979.73(7) 2 1.903 0.71070 3.855 1104.0 24 648 15 076 0.0376 0.0891
C38H32N6IrCl5 942.14 yellow plate 0.44 0.25 0.07 triclinic P1¯ 130.0(1) 13.0177(3) 13.5889(3) 13.9571(3) 98.458(2) 110.482(2) 117.030(2) 1916.61(8) 2 1.633 0.71070 3.868 928 35 374 8781 0.0333 0.0948
fer (MLCT) transitions (Table 2 and Figure S13 in the Supporting Information).[9a] Strong spin-orbit coupling of the metal centre promotes mixing of these charge-transfer transitions with the higher energy spin-allowed ligand-centred transitions.[13] The MLCT bands of [Ir(ppz)3] and [Ir(pmi)3] are significantly higher in energy than those of the ppy/df-ppy-based iridium(III) complexes, indicative of greater separation between the d and p* orbitals.[9a] With the exception of [Ir(ppz)3], each of the iridium(III) complexes exhibited intense photoluminescence in acetonitrile at room temperature (Table 2). Substitution of a ppy ligand of [Ir(ppy)3] with the 1,2,4- or 1,2,3-triazolylpyridine ligand resulted in more structured emission bands that were blue-shifted by approximately 22 and 34 nm. Replacement of the ppy ligands with df-ppy resulted in further blue-shifts of 29 and 24 nm (Figure S14 in the Supporting Information). [Ir(ppz)3] is essentially non-emissive at room temperature (Fp < 0.01),[9b]
Figure 3. CIE 1931 (2-degree observer) chromaticity characterisation of the luminescence of [Ru(bpy)3]2 + (1), [Ir(ppy)3] (2), [Ir(ppy)2(ptp)] (3), [Ir(ppy)2(ptb)] + (4), [Ir(df-ppy)3] (5), [Ir(df-ppy)2(ptp)] (6), [Ir(df-ppy)2(ptb)] + (7) and [Ir(pmi)3] (9). CIE color space representation generated with efg’s Computer Lab software (http://www.efg2.com/Lab).
Table 2. Selected spectroscopic (br = broad, s = shoulder) and electrochemical data for the iridium(III) complexes in acetonitrile: [Ir(ppy)3] (2), [Ir(ppy)2(ptp)] (3), [Ir(ppy)2(ptb)] + (4), [Ir(df-ppy)3] (5), [Ir(df-ppy)2(ptp)] (6), [Ir(df-ppy)2(ptb)] + (7), [Ir(ppz)3] (8) and [Ir(pmi)3] (9). E0’ vs. Fc0/ + [V] ox red 2 3 4 5 6 7 8 9
0.33 0.67,[i] 0.86 0.87 0.68 0.96,[i] 1.22 1.17 0.38 0.22
2.67 2.48 2.19 2.53 2.39 2.14[j] – –
Absorbance lmax [nm]
lmax[nm][a]
380(br), 280, 242, 204 350(br), 269 380(br), 256 350(br), 273, 235 350(br), 254, 205 367(br), 248, 205 322, 296, 260(s), 242, 210 300, 230(s), 209
527 492, 478, 495 463, 454, – 384,
Photoluminescence Fp (ref.)[b]
517 508 489 484, 510(s) 405, 423(s)
ECL
99 5 (90,[c] 98,[d] 89[e])[9b, 14a,b] 74 4 (45[f])[8a] 37 2 99 5 (98,[d] 97[e])[9b, 14b] 52 1 (37[g])[8c] 21 1 (22[h])[11] ( 0.43 V vs. Fc;[2s] Eq. (3)]. Secondly, the reduction potential of M should not be significantly more negative than that of the reductant derived from the coreactant; in this case the neutral radical, TPAC (2.09 V vs Fc).[2s] This ensures that the LUMO level of M + is below that of the reductant, so that excited state formation is thermodynamically feasible; moreover, this also allows for other ECL pathways to exist, such as that embodied in Equations (8) and (9). The data in Table 2 show that the complexes producing the three lowest ECL intensities, [Ir(pmi)3], [Ir(ppz)3] and [Ir(ppy)3], do not satisfy the first criterion. The radical TPAC + , however, is also generated at the electrode surface [Eq. (2)], enabling the relatively weak ECL from [Ir(ppy)3]. There is an evident correlation between the first oxidation potential and the ECL intensity among the iridium complexes with [Ir(df-ppy)2(ptb)] + being the brightest ECL emitter (despite having the lowest photoluminescence quantum yield). Unlike [Ru(bpy)3]2 + and the efficient red-emitting ECL iridium(III) complexes described by Kim et al.,[2h] each of the investigated iridium(III) complexes had reduction potentials beyond the TPAC donor, negating Equations (8) and (9) as a significant light-producing pathway. However, the LUMO energy levels calculated for the oxidised (M + ) complexes (Figure 7) are consistently lower in energy (by ca. 0.5 eV) compared with the parent complexes. This suggests that the pathway represented by Equation (5) should be efficient for the complexes containing ptb ligands and, to a lesser extent, those containing ptp ligands, with [Ir(ppy)3] as a borderline case. No efficient pathways are apparent for [Ir(ppz)3] and [Ir(pmi)3], for which no ECL was observed. The overall trend of increasing ECL intensity with less negative first reduction potential (and lower calculated LUMO level) can be understood in the context of the Marcus–Hush theory of electron transfer kinetics; that is, the rate of reaction in Equation (5) increases with increasing exergonicity because this reaction (leading to the excited state product) occurs in the so called “normal” region. On the other hand, the increase in ECL intensity with first oxidation potential (and lower HOMO level) is a consequence of the rate of the competing reaction leading to the ground state product [Eq. (6)] being progressively more inhibited deeper into the Marcus inverted region. As illustrated in Figure S17 in the Supporting Information, values of DG for Equations (5) and (6) computed using the DFT calculated HOMO and LUMO levels for M + show similar trends with ECL intensity.
Conclusion Of the iridium(III) complexes studied, [Ir(df-ppy)2(ptb)] + was most attractive as a blue-emitter for ECL detection, featuring a considerable hypsochromic shift (ca. 60 nm) and over 16-fold greater co-reactant ECL intensities than the parent [Ir(ppy)3] complex, good solubility in polar solvents, and convenient synChem. Eur. J. 2014, 20, 3322 – 3332
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thesis of functionalised analogues that would be suitable for immobilisation or ECL-labelling experiments.[19] As demonstrated in this study, the use of df-ppy and triazolylpyridine ligands provides considerable hypsochromic shifts compared to [Ir(ppy)3] due to the greater stabilising effect on the HOMO energy than the LUMO. Moreover, the co-reactant ECL intensities are increased by the more favourable reaction of the oxidised complex M + with both TPA and TPAC (indicated by the relative oxidation and reduction potentials in Table 2 and the M + LUMO energies in Figure 7). Overall it is clear that the best strategy when seeking to design efficient blue-shifted ECL emitters is to aim to significantly stabilise the HOMO, while moderately stabilising the LUMO, thereby increasing the energy gap but ensuring favourable thermodynamics and kinetics for the ECL reaction. The iridium(III) complex (M) LUMO energies were shown to be a good predictor of M + LUMO energies. However, although the LUMO levels and the HOMO levels were reasonably good predictors of the co-reactant ECL efficiency, these studies show that neither oxidation potentials, reduction potentials nor photophysical properties should be considered in isolation in the design of new ECL emitters.
Experimental Section General procedures Reagents and solvents were purchased from commercial sources and used without further purification unless otherwise stated. 1 H NMR spectra were recorded at 500 MHz, and 13C NMR spectra were collected at 125 MHz on a Varian FT NMR 500 spectrometer. All chemical shifts were referenced to residual solvent peaks and are quoted in ppm relative to TMS. ESI-MS spectra were recorded on an Agilent 6510 ESI-TOF LC/MS mass spectrometer.
Absorption and photoluminescence emission spectra UV-visible absorption spectra were collected using a Cary 300 Bio UV/Vis spectrophotometer (Varian Australia, Mulgrave, Vic., Australia) with 1 cm path length quartz cells. Photoluminescence spectra were collected with a Cary Eclipse Spectrofluorimeter (Varian Australia), using a 1 cm quartz cuvette (5 nm bandpass, 1 nm data interval, PMT voltage: 800 V). Emission spectra correction factors were established using an Optronic Laboratories spectral irradiance standard (model OL 245 m) with constant current source (model OL 65A). UV/Vis and photoluminescence spectra were recorded at 1 and 10 mm respectively in acetonitrile. Absolute quantum yield measurements and CIE data were obtained using a Horiba JY Nanolog 3 fluorescence spectrophotometer equipped with iHR-320 emission monocromator (100 g mm1 grating), 450 W xeon arc (1200 g mm1 grating) and NanoLED excitation sources, Fluorohub single-photon counting controller and TBX Picosecond photon detection module/Symphony II (1LS-256-OE) LN2 cooled CCD detectors. Absolute quantum yields and CIE data were collected using the xeon arc lamp as the excitation source at 330, 350 or 450 nm, 2.5 nm emission and excitation slits, a 150 mm QuantaPhi integrating sphere and the Symphony II LN2 cooled CCD detector. Quantum yields and CIE coordinates were calculated using the supplied Fluorescence (Horiba JY) software, and are the average of three replicates. Samples for quantum yield measurements were prepared in an oxygen free environment (glovebox) at concentrations between 1 and 5 mm using freshly purified (solvent press) dry/de-
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Full Paper gassed acetonitrile before being sealed in a quartz cuvette and rapidly transferred to the instrument for analysis.
Electrochemistry and electrogenerated chemiluminescence Experiments were performed using Autolab PGSTAT 101 (for cyclic voltammograms) and Autolab PGSTA12 (for chronoamperometrey and cyclic voltammograms with ECL detection) potentiostats (Metrohm Autolab B.V., Netherlands). The electrochemical cell consisted of a cylindrical glass cell with a quartz window base and Teflon cover with spill tray. The cell was encased in a custom-built lighttight faraday cage. A conventional three-electrode configuration, consisting of a glassy carbon 3 mm diameter working electrode, shrouded in Teflon (CH Instruments, Austin, TX, USA), reference electrode and gold wire counter electrode. Potentials were referenced to the ferrocene/ferrocenium couple measured in situ (1 mm) in each case. The presented CVs were not corrected for the capacitive (non-faradaic) current. ECL spectra were obtained using an Ocean Optics CCD, model QE65pro, interfaced with our electrochemical cell using an optic fibre (1.0 m, 1.0 mm core diameter), collimating lens, and custom cell holder, and the acquisition was triggered using a HR 4000 Break-Out box in conjunction with the potentiostat. The complexes were prepared at a concentration of 0.25 mm for voltammetric measurements, and 0.1 mm for ECL measurements in freshly distilled acetonitrile, with 0.1 m tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte, with a co-reactant concentration of 10 mm TPA for ECL experiments. Prior to each experiment, the working electrode was polished using 0.3 mm and then 0.05 mm alumina with water on a felt pad, sonicated in MilliQ water (1 min), rinsed in freshly distilled acetonitrile and dried with a stream of N2. The working electrode was then positioned at an appropriate distance (ca. 2 mm) from the bottom of the cell for detection of the ECL signal, and the solution was purged with grade 5 argon for 15 min prior to measurement. ECL spectra were recorded using a 45 s integration time, and single 40 s chronoamperometry pulse at 1.4 V with the exception of [Ir(ppy)3], for which 0.4 V was used due to the inhibition of ECL at high overpotentials.[2r,s] Spectra were integrated to determine the relative ECL intensities.
Computational methods Density functional theory (DFT) calculations were carried out within the Gaussian 09 suite of programs.[20] Ground state geometries were optimised in the absence of solvent with the mPW1PW91[21] functional in conjunction with the def2-SVP basis set and associated core potential.[22] The mPW1PW91 functional has previously been demonstrated to yield reliable results for such systems.[1g, 23] Geometries optimised with LANL2DZ and 6–31 + G(d) basis sets and effective core potentials were equivalent. Singlepoint energy calculations were carried out with the def2-TZVP basis set and core potential.[22] The polarisable continuum model (PCM)[24] self-consistent reaction field (SCRF) was used to model solvent effects at the gas-phase optimised geometries with a solvent of acetonitrile, consistent with the experimental system. Frontier MO energies were calculated using DFT MOs with mPW91PW91, PBE,[25] B3LYP,[26] BP86,[26a, 27] and wB97XD.[28] An SCF convergence criteria of 108 a.u. was employed throughout. Molecular orbital analysis was carried out with the AOMix program.[29]
Chemicals fac-Tris[2-(2-pyridinyl-kN)phenyl-kC]iridium (fac-tris(2-phenylpyridinato-C2,N)iridium(III), [Ir(ppy)3], 99 %), tris[3,5-difluoro-2-(2-pyridinylChem. Eur. J. 2014, 20, 3322 – 3332
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kN)phenyl-kC]iridium (tris[2-(4,6-difluorophenyl)pyridinato-C2,N]iridium(III), [Ir(df-ppy)3], 96 %), tripropylamine (TPA, 98 %) and tetrabutylammonium hexafluorophosphate (TBAPF6, 99.5 %, electrochemical grade) were purchased from Sigma–Aldrich (Australia). Tris[2-(1H-pyrazol-1-yl-kN2)phenyl-kC]iridium (tris(phenylpyrazole)iridium(III), [Ir(ppz)3], > 99 %) and fac-tris[(3-methyl-1H-imidazol-1-yl2(3 H)-ylidene)-1,2-phenylene]iridium (fac-tris(1-phenyl-3-methylimidazolin-2-ylidene-C,C(2)’)iridium(III), [Ir(pmi)3], > 99 %) were purchased from LumTech (Taiwan). The hexafluorophosphate salt of tris(2,2’-bipyridine-kN1,kN1’)ruthenium(2+) ([Ru(bpy)3]2 + ) was prepared from [Ru(bpy)3]Cl2·6 H2O (Strem Chemicals, USA). Acetonitrile was distilled over calcium hydride under grade 5 argon. Solutions were degassed with argon. Bis[2-(2-pyridinyl-kN)phenyl-kC][2-(3([Ir(ppy)2phenyl-1H-1,2,4-triazol-5-yl-kN1)pyridinato-kN]iridium (ptp)]) and bis[3,5-difluoro-2-(2-pyridinyl-kN)phenyl-kC][2-(3phenyl-1H-1,2,4-triazol-5-yl-kN1)pyridinato-kN]iridium ([Ir(df-ppy)2(ptp)]) were prepared as previously described.[8a,c] Bis[2-(2-pyridinylkN)phenyl-kC][2-[1-(phenylmethyl)-1H-1,2,3-triazol-4-yl-kN3]pyridine-kN]iridium(1+) hexafluorophosphate(1-) ([Ir(ppy)2(ptb)]PF6) and bis[3,5-difluoro-2-(2-pyridinyl-kN)phenyl-kC][2-[1-(phenylmethyl)-1H-1,2,3-triazol-4-yl-kN3]pyridine-kN]iridium(1+) hexafluorophosphate(1-) ([Ir(df-ppy)2(ptb)]PF6), were prepared by anion metathesis from the corresponding chloride and tetrafluoroborate salts, which were synthesised as described below.
Synthesis of [Ir(ppy)2(ptb)]Cl (4) [{Ir(ppy)2}2(m-Cl)2] (50 mg, 47 mmol) was added to a solution of 2-(1(benzyl)-1H-1,2,3-triazol-4-yl)pyridine (22 mg, 93 mmol) in 3:1 dichloromethane/methanol (3 mL). The suspension was stirred in the dark for 16 h at ambient temperature, during which time dissolution of the solids occurred. The yellow solution was evaporated to dryness then dissolved in acetonitrile (5 mL) and filtered. The resulting filtrate was concentrated and dried in vacuo to yield the product as a yellow solid (69 mg, 96 %). Numbering of carbons for NMR assignments is shown in Figure S18 in the Supporting Information. 1H NMR (CDCl3, 500 MHz): d = 5.68 (m, 2 H; C5-H2), 6.29 (d, J = 7.0 Hz, 1 H; C22-H), 6.31 (d, J = 7.48 Hz, 1 H; C33-H), 6.86 (td, J = 6.6, 1.2 Hz, 1 H; C21-H), 6.89 (td, J = 6.6, 1.2 Hz, 1 H; C32-H), 6.92 (m, 1 H; C14-H), 6.94 (m, 1 H; C25-H), 6.97 (m, 1 H; C20-H), 7.01 (td, J = 7.6, 0.8 Hz, 1 H; C31-H), 7.18 (t, J = 6.3 Hz, 1 H; C11-H), 7.29 (m, 3 H; C1-H, C2-H), 7.41 (m, 3 H; C3-H, C13-H), 7.60 (d, J = 5.7 H; 1 H; C24-H), 7.64 (m, 2 H; C19-H, C30-H), 7.73 (m, 3 H; C12-H, C15-H, C26-H), 7.88 (m, 2 H; C16-H, C27-H), 7.96 (td, J = 7.8, 1.2 Hz, 1 H; C10-H), 8.99 (d, J = 7.9 Hz, 1 H; C9-H), 10.69 ppm (s, 1 H; C6-H); 13C NMR (CDCl3, 125 MHz): d = 50.7 (C5), 119.4 (C16), 119.5 (C27), 122.3 (C20), 122.7 (C31), 122.9 (C14), 123.3 (C25), 124.4 (C19), 124.8 (C30), 124.9 (C9), 125.0 (C11), 128.8 (2C, C2), 128.9 (C1), 129.1 (2C, C3), 129.6 (C6), 130.1 (C21), 130.8 (C32), 131.9 (C33), 132.0 (C22), 134.0 (C4), 137.9 (C15), 138.0 (C26), 139.8 (C10), 143.7 (C18), 143.8 (C29), 146.5 (C23), 148.4 (C13), 148.9 (C7), 149.4 (C24), 149.6 (C12), 150.1 (C34), 150.2 (C8), 167.7 (C28), 168.5 ppm (C17); HRMS: m/z calcd for [C36H28N6Ir] + : 737.2005; found: 737.1994.
Synthesis of [Ir(df-ppy)2(ptb)]BF4 (7) This compound was synthesised by modification to a previously reported procedure.[11] [{Ir(df-ppy)2}2(m-Cl)2] (72 mg, 59 mmol) was added to a solution of 2-(1-(benzyl)-1H-1,2,3-triazol-4-yl)pyridine (28 mg, 118 mmol) in 3:1 dichloromethane/methanol (3 mL). The suspension was stirred in the dark for 16 h at ambient temperature, during which time dissolution of the solids occurred. The yellow solution was evaporated to dryness then dissolved in dichloromethane (10 mL) and a saturated methanol solution of NaBF4 (2 mL) was added. The turbid solution was stirred for 1 h
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Full Paper and the solvent removed. The solid was suspended in water and sonicated for 5 min then collected by filtration and washed with copious amounts of H2O then hexane. The crude material was purified by silica gel chromatography using a gradient of dichloromethane!10 % methanol in dichloromethane to yield product as a light yellow solid (74 mg, 70 %). 1H NMR (CDCl3, 500 MHz): d = 5.61 (m, 2 H; C5-H2), 5.67 (dd, J = 8.5, 2.3 Hz, 1 H; C22-H), 5.73 (dd, J = 8.3, 2.3 Hz, 1 H; C33-H), 6.55 (m, 2 H; C20-H, C31-H), 7.00 (ddd, J = 7.3, 6.0, 1.3 Hz, 1 H; C14-H), 7.07 (m, 1 H; C25-H), 7.33 (m, 4 H; C1H,C2-H, C11-H), 7.39 (m, 2 H; C3-H), 7.43 (dd, J = 5.7, 0.7 Hz, 1 H; C13H), 7.58 (d, J = 5.1 Hz, 1 H; C24-H), 7.77 (dd, J = 5.5, 1.0 Hz, 1 H; C12H), 7.81 (t, J = 7.9 Hz, 2 H; C15-H, C26-H), 8.02 (d, J = 8.0 Hz, 1 H; C10H), 8.30 (t, J = 9.8 Hz, 2 H; C16-H, C27-H), 8.43 (d, J = 8.0 Hz, 1 H; C9-H), 9.26 ppm (s, 1 H; C6-H); 13C NMR (CDCl3, 125 MHz): d = 56.2 (C5), 99.1 (t, J = 26.8 Hz, 1C, C20), 99.4 (t, J = 26.7 Hz, 1C, C31), 114.2 (dd, J = 17.6, 2.6 Hz, 1C, C33), 114.3 (dd, J = 17.9, 2.7 Hz, 1C, C22), 123.3 (C14), 123.6 (d, J = 20.1 Hz, 1 H; C16 or C27, cannot resolve), 123.7 (d, J = 19.8 Hz, 1 H; C16 or C27, cannot resolve), 123.9 (C25), 124.3 (C9), 126.6 (C11), 127.7 (C6), 127.8 (m, 1 H; C18), 127.9 (m, 1 H; C29), 129.0 (2C, C3), 129.4 (3C, C1, C2), 133.2 (C4), 139.0 (C15), 139.2 (C26), 140.5 (C10), 148.5 (C13), 148.6 (C7), 149.6 (C24), 149.7 (C8), 149.7 (C12), 153.4, 153.5 (C23, C24, cannot resolve), 161.2 (dd, J = 261.6, 12.6 Hz, 1C, C19), 161.5 (dd, J = 261.1, 12.7 Hz, 1C, C30), 163.3 (dd, J = 257.1, 12.4 Hz, 1C, C21), 163.7 (dd, J = 258.0, 12.3 Hz, 1C, C32), 164.1 (d, J = 7.0 Hz, 1C, C28), 165.0 ppm (d, J = 6.9 Hz, 1C, C17); 19F NMR (CDCl3, 471 MHz): d = 105.2 (m, 1 F), 106.1 (q, J = 8.29 Hz, 1 F), 108.2 (t, J = 11.48 Hz, 1 F), 109.2 (t, J = 11.22 Hz, 1 F), 151.5 (m, 3 F); HRMS: m/z calcd for [C36H24N6F4Ir] + : 809.1628; found: 809.1602.
Computational Infrastructure National Facility (NCI-NF) are acknowledged for substantial computing resources. Keywords: density functional calculations electrochemiluminescence · electrochemistry · iridium luminescence
X-ray crystallography Crystals of [Ir(df-ppy)2(ptb)]BF4 and [Ir(ppy)2(ptb)]Cl were mounted in low temperature oil then flash cooled. Intensity data were collected at 130 K (unless otherwise stated) on an X-ray diffractometer with CCD detector using MoKa (l = 0.71073 ) radiation. Data were reduced and corrected for absorption.[30] The structures were solved by direct methods and difference Fourier synthesis using the SHELX[31] suite of programs as implemented within the WINGX[32] software. Thermal ellipsoid plots were generated using the program ORTEP-3. CCDC-970288 ([Ir(df-ppy)2(ptb)]BF4) and 970287 ([Ir(ppy)2(ptb)]Cl) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Anion metathesis of iridium(III) complexes The relevant complex was dissolved in dichloromethane (10 mL) and a saturated methanol solution of NH4PF6 (1 mL) was added. The turbid solution was stirred for 1 h and the solvent removed. The solid was suspended in water and sonicated for 5 min then collected by filtration and washed with copious amounts of H2O then hexane. The crude material was recrystallised from ethanol and dried in vacuo.
Acknowledgements This work was funded by the Australian Research Council (FT100100646, LE120100213, DP1094179). The Victorian Partnership for Advanced Computing (VPAC) and the National Chem. Eur. J. 2014, 20, 3322 – 3332
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Received: November 18, 2013 Published online on March 3, 2014
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& Luminescence
Understanding Electrogenerated Chemiluminescence Efficiency in Blue-Shifted Iridium(III)-Complexes: An Experimental and Theoretical Study Gregory J. Barbante,[a] Egan H. Doeven,[a] Emily Kerr,[a] Timothy U. Connell,[b] Paul S. Donnelly,[b] Jonathan M. White,[b] Thais Lpes,[a, d] Sarah Laird,[c] David J. D. Wilson,[c] Peter J. Barnard,[c] Conor F. Hogan,*[c] and Paul S. Francis*[a]
Chem. Eur. J. 2014, 20, 3322 – 3332
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Full Paper Abstract: Compared to tris(2-phenylpyridine)iridium(III) ([Ir(ppy)3]), iridium(III) complexes containing difluorophenylpyridine (df-ppy) and/or an ancillary triazolylpyridine ligand [3-phenyl-1,2,4-triazol-5-ylpyridinato (ptp) or 1-benzyl-1,2,3triazol-4-ylpyridine (ptb)] exhibit considerable hypsochromic shifts (ca. 25–60 nm), due to the significant stabilising effect of these ligands on the HOMO energy, whilst having relatively little effect on the LUMO. Despite their lower photoluminescence quantum yields compared with [Ir(ppy)3] and [Ir(dfppy)3], the iridium(III) complexes containing triazolylpyridine ligands gave greater electrogenerated chemiluminescence (ECL) intensities (using tri-n-propylamine (TPA) as a co-reactant), which can in part be ascribed to the more energetically favourable reactions of the oxidised complex (M + ) with both TPA and its neutral radical oxidation product. The calculated iridium(III) complex LUMO energies were shown to
Introduction Electrogenerated chemiluminescence (ECL) is a form of luminescence produced by high-energy reactions between electrogenerated precursors,[1] in which the electronically excited states responsible for the emission of light can be generated through the annihilation between oxidised and reduced forms of the same species, or by using a sacrificial co-reactant. The application of a co-reactant ECL as a highly sensitive mode of detection has been predominantly based on the use of tris(2,2’-bipyridine)ruthenium(II) ([Ru(bpy)3]2 + ), and related polyimine–ruthenium(II) complexes, with characteristic orange/ red emissions (lmax ca. 590–700 nm).[1] Over the last decade, however, numerous researchers have begun to explore chemiluminescence and ECL reactions with cyclometalated iridium(III) complexes exhibiting a wide range of electrochemical properties and emission maxima that can be tuned through subtle changes in the structure of one or more ligands.[2] These [a] Dr. G. J. Barbante, Dr. E. H. Doeven, E. Kerr, T. Lpes, Prof. P. S. Francis Centre for Chemistry and Biotechnology, Faculty of Science Engineering and Built Environment, Deakin University Waurn Ponds, Victoria 3216 (Australia) Fax: (+ 61) 3-5227-2356 E-mail: [email protected] [b] T. U. Connell, Dr. P. S. Donnelly, Prof. J. M. White School of Chemistry and Bio21 Molecular Science and Biotechnology Institute, University of Melbourne Melbourne 3010 (Australia) [c] S. Laird, Dr. D. J. D. Wilson, Dr. P. J. Barnard, Dr. C. F. Hogan Department of Chemistry La Trobe Institute for Molecular Sciences, La Trobe University Victoria 3086 (Australia) E-mail: [email protected] [d] T. Lpes Present address: Department of Chemistry Federal University of Santa Maria Santa Maria, Rio Grande do Sul 97.105-900 (Brazil) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304500. Chem. Eur. J. 2014, 20, 3322 – 3332
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be a good predictor of the corresponding M + LUMO energies, and both HOMO and LUMO levels are related to ECL efficiency. The theoretical and experimental data together show that the best strategy for the design of efficient new blue-shifted electrochemiluminophores is to aim to stabilise the HOMO, while only moderately stabilising the LUMO, thereby increasing the energy gap but ensuring favourable thermodynamics and kinetics for the ECL reaction. Of the iridium(III) complexes examined, [Ir(df-ppy)2(ptb)] + was most attractive as a blue-emitter for ECL detection, featuring a large hypsochromic shift (lmax = 454 and 484 nm), superior co-reactant ECL intensity than the archetypal homoleptic green and blue emitters: [Ir(ppy)3] and [Ir(df-ppy)3] (by over 16-fold and threefold, respectively), and greater solubility in polar solvents.
complexes have created new possibilities for multiplexed ECL detection systems.[2d,g,r,s] However, in contrast to the vast range of orange/red-emitting metal complex electrochemiluminophores,[1b,c,g, 3] relatively few blue emitters are available, and the most effective design of blue-emitting complexes for ECL detection is yet to be fully elucidated. We have therefore used electrochemical, spectroscopic and computational techniques to explore a series of blue-emitting iridium(III) complexes that exhibit various potentially attractive structural attributes for conventional and multiplexed ECL detection. Derived from the extensive development of iridium-complexes for other photonic applications,[4] a common strategy to create luminophores with higher energy emission than the classic green-emitter tris(2-phenylpyridine)iridium(III) ([Ir(ppy)3]; lmax ca. 520 nm),[2d,e,s] is to incorporate electron-withdrawing fluorine substituents on the cyclometalating ring of ppy, which stabilise the HOMO level to a much greater extent than the LUMO.[2g,n,s] Numerous alternative ligands have also been explored to manipulate properties such as emission wavelength or ECL efficiency, to allow more facile preparation or to introduce functionality suitable for labelling or immobilisation.[2f–i,l,n,p,q,u,v, 5] These ligands have a variety of influences on the frontier molecular orbitals[4b, 6] and, therefore, the electrochemical and spectroscopic properties relevant to ECL detection. In an important recent example, Zanarini et al.[2n] reported several green and blue electrochemiluminophores based on cationic iridium complexes with two anionic ppy or difluorophenylpyridine (df-ppy) ligands and a neutral 1,2,3-triazol-4-ylpyridine ligand, functionalised at the 1-nitrogen position by using simple “click” chemistry.[7] Neutral iridium(III) complex analogues incorporating anionic 1,2,4-triazol-5-ylpyridinato ligands (functionalised at the 3-carbon position) have been developed as efficient high-energy emitters for photonic applications,[8] but have not yet been utilised for ECL. Herein, we directly compare the properties of four iridium(III) complexes containing ppy or df-ppy and substituted 1,2,3-triazol-4-ylpyridine or 1,2,4-triazol-5-ylpyridinato ligands (Figure 1), against
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Figure 1. Iridium(III) complexes.
the archetypal [Ru(bpy)3]2 + , [Ir(ppy)3], and [Ir(df-ppy)3] complexes, and two additional homoleptic species with nitrogen heterocyclic ligands: tris(1-phenyl-3-methylimidazolin-2-ylidene-C,C2’)iridium(III) ([Ir(pmi)3])[6] and tris(1-phenylpyrazolylN,C2’)iridium(III) ([Ir(ppz)3])[6, 9] that have found recent application in the emission and electron-blocking layers of light-emitting devices.[10]
with the heteronuclear single quantum coherence (gHSQC) and heteronuclear multiple bond correlation (gHMBC) spectra were used for the complete assignment of the hydrogen and carbon atoms of bicyclic phenylpyridine ligands. A comparison of the 13C spectra of each compound shows the resonances due to the carbon atoms bonded to fluorine in [Ir(df-ppy)2(ptb)]BF4 are shifted down-field when compared with the analogous carbon atoms in [Ir(ppy)2(ptb)]Cl; d = 163.3 and 161.2 ppm and d = 130.1 and 124.4 ppm, respectively (Figure S12 in the Supporting Information), due to the large inductive effect of the fluorine substituent. Conversely, the resonances assigned to the carbon atoms ortho to the fluorine atoms are shifted up-field; d = 114.4, 99.1 and 127.8 ppm compared with d = 132.0, 122.3 and 143.7 ppm in the non-fluorinated cyclometalating ligand, presumably due to mesomeric effects. The carbon atom bound to the iridium is meta relative to both fluorine atoms, and the resonance assigned to this atom is shifted down-field, d at about 153.5 ppm (cannot resolve between two signals) in [Ir(df-ppy)2(ptb)]BF4 compared with d = 146.6 ppm in [Ir(ppy)2(ptb)]Cl (Figure S12 in the Supporting Information). The meta position is typically influenced by inductive effects, which account for the down-field shift observed. The increased electron deficiency at this carbon has been proposed as intrinsic to the hypsochromic shift observed in iridium(III) complexes with fluorinated cyclometalating ligands.[7, 8] Both [Ir(ppy)2(ptb)]Cl and [Ir(df-ppy)2(ptb)]BF4 were characterised by single-crystal X-ray crystallography (Figure 2
Results and Discussion Preparation of iridium(III) complexes The homoleptic iridium(III) complexes were obtained from commercial sources and the [Ir(ppy)2(ptp)] and [Ir(df-ppy)2(ptp)] species were synthesised as previously described.[8a,c] However, the synthesis of [Ir(df-ppy)2(ptb)]PF6 involved modification of a previous procedure,[11] and here we also prepared the novel [Ir(ppy)2(ptb)]PF6 analogue. The synthesis and characterisation of these two species thus warrant some elaboration. The hexafluorophosphate salts of these complexes were prepared (by means of anion metathesis with NH4PF6) from the corresponding chloride and tetrafluoroborate salts. Synthesis of [Ir(ppy)2(ptb)]Cl and [Ir(df-ppy)2(ptb)]BF4 was based on the procedure of Mydlak and co-workers,[11] as described in detail in the Experimental Section. The complexes were characterised by 1H, 13 C homo- and heteronuclear experiments, as well as 19F NMR spectroscopy where relevant (Figures S1–S11 in the Supporting Information). The 1H and 19F NMR spectra of [Ir(df-ppy)2(ptb)]BF4 were consistent with those previously reported.[11] Integration of peaks in the 1H NMR spectrum showed the expected number of signals, but significant overlap made total assignment difficult. Correlation spectroscopy (gCOSY), along Chem. Eur. J. 2014, 20, 3322 – 3332
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Figure 2. An ORTEP representation of the complex cations a) [Ir(df-ppy)2(ptb)] + (7) and b) [Ir(ppy)2(ptb)] + (4) with thermal ellipsoids at the 40 % probability level. Hydrogen atoms and solvent molecules omitted for clarity.
and Table 1). The iridium cations in both complexes are in a distorted octahedral C2N4 environment and the Npyridyl atoms are in a meridional arrangement, which demonstrates the geometry of the starting material is retained at the temperatures used during synthesis. Bond distances between atoms that make up the coordination sphere are typical of other isolated iridium(III) complexes with triazole-containing ancillary ligands.[7, 8c, 11, 12] Spectroscopic properties The absorption spectra of the iridium(III) complexes typically show intense bands between 240 and 300 nm, which can be ascribed to spin-allowed p!p* ligand-centred (LC) transitions, and weaker bands above 300 nm that have been assigned to both allowed and spin-forbidden metal-to-ligand charge trans-
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Full Paper due to efficient (thermally activated) population of a non-radiative metal-centred (MC) ligand-field state involving rupture of a metal–ligand bond.[4c, 9b] Like [Ir(ppz)3], [Ir(pmi)3] has highenergy MLCT absorption bands, but the non-radiative excited state is destabilised by the strong bonding between the metal and phenylimidazole carbene ligand, resulting in relatively intense near-UV photoluminescence.[6] The overall emissions from these iridium(III) complexes, ranging from near-UV to green, are characterised in Figure 3. Absolute quantum yields of the phenylpyridine-based complexes were in reasonable agreement with recent reports[8a,c, 9b, 11, 14] and found to decrease in the order: [Ir(ppy/df-ppy)3] > [Ir(ppy/df-ppy)2(ptp)] > [Ir(ppy/ df-ppy)2(ptb)] + (Table 2). The quantum yield of [Ir(pmi)3] found here, however, was significantly higher than the previously reported value.[6]
Table 1. Crystallography data. [Ir(df-ppy)2(ptb)]BF4·2 CHCl3 [Ir(ppy)2(ptb)]Cl·2CH2Cl2 formula Mr color and habit crystal size [mm3] system space group T [K] a [] b [] c [] a [8] b [8] g [8] V [3] Z 1calcd [Mg m3] l [] m [mm1] F(000) reflns measured independent reflns R [I > 2s(I)] wR(F2) (all data)
C38H26N6IrCl6BF8 1134.36 pale yellow block 0.56 0.38 0.23 triclinic P1¯ 130.0(1) 10.8801(2) 12.9381(3) 14.1902(3) 83.380(2) 86.396(2) 88.246(2) 1979.73(7) 2 1.903 0.71070 3.855 1104.0 24 648 15 076 0.0376 0.0891
C38H32N6IrCl5 942.14 yellow plate 0.44 0.25 0.07 triclinic P1¯ 130.0(1) 13.0177(3) 13.5889(3) 13.9571(3) 98.458(2) 110.482(2) 117.030(2) 1916.61(8) 2 1.633 0.71070 3.868 928 35 374 8781 0.0333 0.0948
fer (MLCT) transitions (Table 2 and Figure S13 in the Supporting Information).[9a] Strong spin-orbit coupling of the metal centre promotes mixing of these charge-transfer transitions with the higher energy spin-allowed ligand-centred transitions.[13] The MLCT bands of [Ir(ppz)3] and [Ir(pmi)3] are significantly higher in energy than those of the ppy/df-ppy-based iridium(III) complexes, indicative of greater separation between the d and p* orbitals.[9a] With the exception of [Ir(ppz)3], each of the iridium(III) complexes exhibited intense photoluminescence in acetonitrile at room temperature (Table 2). Substitution of a ppy ligand of [Ir(ppy)3] with the 1,2,4- or 1,2,3-triazolylpyridine ligand resulted in more structured emission bands that were blue-shifted by approximately 22 and 34 nm. Replacement of the ppy ligands with df-ppy resulted in further blue-shifts of 29 and 24 nm (Figure S14 in the Supporting Information). [Ir(ppz)3] is essentially non-emissive at room temperature (Fp < 0.01),[9b]
Figure 3. CIE 1931 (2-degree observer) chromaticity characterisation of the luminescence of [Ru(bpy)3]2 + (1), [Ir(ppy)3] (2), [Ir(ppy)2(ptp)] (3), [Ir(ppy)2(ptb)] + (4), [Ir(df-ppy)3] (5), [Ir(df-ppy)2(ptp)] (6), [Ir(df-ppy)2(ptb)] + (7) and [Ir(pmi)3] (9). CIE color space representation generated with efg’s Computer Lab software (http://www.efg2.com/Lab).
Table 2. Selected spectroscopic (br = broad, s = shoulder) and electrochemical data for the iridium(III) complexes in acetonitrile: [Ir(ppy)3] (2), [Ir(ppy)2(ptp)] (3), [Ir(ppy)2(ptb)] + (4), [Ir(df-ppy)3] (5), [Ir(df-ppy)2(ptp)] (6), [Ir(df-ppy)2(ptb)] + (7), [Ir(ppz)3] (8) and [Ir(pmi)3] (9). E0’ vs. Fc0/ + [V] ox red 2 3 4 5 6 7 8 9
0.33 0.67,[i] 0.86 0.87 0.68 0.96,[i] 1.22 1.17 0.38 0.22
2.67 2.48 2.19 2.53 2.39 2.14[j] – –
Absorbance lmax [nm]
lmax[nm][a]
380(br), 280, 242, 204 350(br), 269 380(br), 256 350(br), 273, 235 350(br), 254, 205 367(br), 248, 205 322, 296, 260(s), 242, 210 300, 230(s), 209
527 492, 478, 495 463, 454, – 384,
Photoluminescence Fp (ref.)[b]
517 508 489 484, 510(s) 405, 423(s)
ECL
99 5 (90,[c] 98,[d] 89[e])[9b, 14a,b] 74 4 (45[f])[8a] 37 2 99 5 (98,[d] 97[e])[9b, 14b] 52 1 (37[g])[8c] 21 1 (22[h])[11] ( 0.43 V vs. Fc;[2s] Eq. (3)]. Secondly, the reduction potential of M should not be significantly more negative than that of the reductant derived from the coreactant; in this case the neutral radical, TPAC (2.09 V vs Fc).[2s] This ensures that the LUMO level of M + is below that of the reductant, so that excited state formation is thermodynamically feasible; moreover, this also allows for other ECL pathways to exist, such as that embodied in Equations (8) and (9). The data in Table 2 show that the complexes producing the three lowest ECL intensities, [Ir(pmi)3], [Ir(ppz)3] and [Ir(ppy)3], do not satisfy the first criterion. The radical TPAC + , however, is also generated at the electrode surface [Eq. (2)], enabling the relatively weak ECL from [Ir(ppy)3]. There is an evident correlation between the first oxidation potential and the ECL intensity among the iridium complexes with [Ir(df-ppy)2(ptb)] + being the brightest ECL emitter (despite having the lowest photoluminescence quantum yield). Unlike [Ru(bpy)3]2 + and the efficient red-emitting ECL iridium(III) complexes described by Kim et al.,[2h] each of the investigated iridium(III) complexes had reduction potentials beyond the TPAC donor, negating Equations (8) and (9) as a significant light-producing pathway. However, the LUMO energy levels calculated for the oxidised (M + ) complexes (Figure 7) are consistently lower in energy (by ca. 0.5 eV) compared with the parent complexes. This suggests that the pathway represented by Equation (5) should be efficient for the complexes containing ptb ligands and, to a lesser extent, those containing ptp ligands, with [Ir(ppy)3] as a borderline case. No efficient pathways are apparent for [Ir(ppz)3] and [Ir(pmi)3], for which no ECL was observed. The overall trend of increasing ECL intensity with less negative first reduction potential (and lower calculated LUMO level) can be understood in the context of the Marcus–Hush theory of electron transfer kinetics; that is, the rate of reaction in Equation (5) increases with increasing exergonicity because this reaction (leading to the excited state product) occurs in the so called “normal” region. On the other hand, the increase in ECL intensity with first oxidation potential (and lower HOMO level) is a consequence of the rate of the competing reaction leading to the ground state product [Eq. (6)] being progressively more inhibited deeper into the Marcus inverted region. As illustrated in Figure S17 in the Supporting Information, values of DG for Equations (5) and (6) computed using the DFT calculated HOMO and LUMO levels for M + show similar trends with ECL intensity.
Conclusion Of the iridium(III) complexes studied, [Ir(df-ppy)2(ptb)] + was most attractive as a blue-emitter for ECL detection, featuring a considerable hypsochromic shift (ca. 60 nm) and over 16-fold greater co-reactant ECL intensities than the parent [Ir(ppy)3] complex, good solubility in polar solvents, and convenient synChem. Eur. J. 2014, 20, 3322 – 3332
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thesis of functionalised analogues that would be suitable for immobilisation or ECL-labelling experiments.[19] As demonstrated in this study, the use of df-ppy and triazolylpyridine ligands provides considerable hypsochromic shifts compared to [Ir(ppy)3] due to the greater stabilising effect on the HOMO energy than the LUMO. Moreover, the co-reactant ECL intensities are increased by the more favourable reaction of the oxidised complex M + with both TPA and TPAC (indicated by the relative oxidation and reduction potentials in Table 2 and the M + LUMO energies in Figure 7). Overall it is clear that the best strategy when seeking to design efficient blue-shifted ECL emitters is to aim to significantly stabilise the HOMO, while moderately stabilising the LUMO, thereby increasing the energy gap but ensuring favourable thermodynamics and kinetics for the ECL reaction. The iridium(III) complex (M) LUMO energies were shown to be a good predictor of M + LUMO energies. However, although the LUMO levels and the HOMO levels were reasonably good predictors of the co-reactant ECL efficiency, these studies show that neither oxidation potentials, reduction potentials nor photophysical properties should be considered in isolation in the design of new ECL emitters.
Experimental Section General procedures Reagents and solvents were purchased from commercial sources and used without further purification unless otherwise stated. 1 H NMR spectra were recorded at 500 MHz, and 13C NMR spectra were collected at 125 MHz on a Varian FT NMR 500 spectrometer. All chemical shifts were referenced to residual solvent peaks and are quoted in ppm relative to TMS. ESI-MS spectra were recorded on an Agilent 6510 ESI-TOF LC/MS mass spectrometer.
Absorption and photoluminescence emission spectra UV-visible absorption spectra were collected using a Cary 300 Bio UV/Vis spectrophotometer (Varian Australia, Mulgrave, Vic., Australia) with 1 cm path length quartz cells. Photoluminescence spectra were collected with a Cary Eclipse Spectrofluorimeter (Varian Australia), using a 1 cm quartz cuvette (5 nm bandpass, 1 nm data interval, PMT voltage: 800 V). Emission spectra correction factors were established using an Optronic Laboratories spectral irradiance standard (model OL 245 m) with constant current source (model OL 65A). UV/Vis and photoluminescence spectra were recorded at 1 and 10 mm respectively in acetonitrile. Absolute quantum yield measurements and CIE data were obtained using a Horiba JY Nanolog 3 fluorescence spectrophotometer equipped with iHR-320 emission monocromator (100 g mm1 grating), 450 W xeon arc (1200 g mm1 grating) and NanoLED excitation sources, Fluorohub single-photon counting controller and TBX Picosecond photon detection module/Symphony II (1LS-256-OE) LN2 cooled CCD detectors. Absolute quantum yields and CIE data were collected using the xeon arc lamp as the excitation source at 330, 350 or 450 nm, 2.5 nm emission and excitation slits, a 150 mm QuantaPhi integrating sphere and the Symphony II LN2 cooled CCD detector. Quantum yields and CIE coordinates were calculated using the supplied Fluorescence (Horiba JY) software, and are the average of three replicates. Samples for quantum yield measurements were prepared in an oxygen free environment (glovebox) at concentrations between 1 and 5 mm using freshly purified (solvent press) dry/de-
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Full Paper gassed acetonitrile before being sealed in a quartz cuvette and rapidly transferred to the instrument for analysis.
Electrochemistry and electrogenerated chemiluminescence Experiments were performed using Autolab PGSTAT 101 (for cyclic voltammograms) and Autolab PGSTA12 (for chronoamperometrey and cyclic voltammograms with ECL detection) potentiostats (Metrohm Autolab B.V., Netherlands). The electrochemical cell consisted of a cylindrical glass cell with a quartz window base and Teflon cover with spill tray. The cell was encased in a custom-built lighttight faraday cage. A conventional three-electrode configuration, consisting of a glassy carbon 3 mm diameter working electrode, shrouded in Teflon (CH Instruments, Austin, TX, USA), reference electrode and gold wire counter electrode. Potentials were referenced to the ferrocene/ferrocenium couple measured in situ (1 mm) in each case. The presented CVs were not corrected for the capacitive (non-faradaic) current. ECL spectra were obtained using an Ocean Optics CCD, model QE65pro, interfaced with our electrochemical cell using an optic fibre (1.0 m, 1.0 mm core diameter), collimating lens, and custom cell holder, and the acquisition was triggered using a HR 4000 Break-Out box in conjunction with the potentiostat. The complexes were prepared at a concentration of 0.25 mm for voltammetric measurements, and 0.1 mm for ECL measurements in freshly distilled acetonitrile, with 0.1 m tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte, with a co-reactant concentration of 10 mm TPA for ECL experiments. Prior to each experiment, the working electrode was polished using 0.3 mm and then 0.05 mm alumina with water on a felt pad, sonicated in MilliQ water (1 min), rinsed in freshly distilled acetonitrile and dried with a stream of N2. The working electrode was then positioned at an appropriate distance (ca. 2 mm) from the bottom of the cell for detection of the ECL signal, and the solution was purged with grade 5 argon for 15 min prior to measurement. ECL spectra were recorded using a 45 s integration time, and single 40 s chronoamperometry pulse at 1.4 V with the exception of [Ir(ppy)3], for which 0.4 V was used due to the inhibition of ECL at high overpotentials.[2r,s] Spectra were integrated to determine the relative ECL intensities.
Computational methods Density functional theory (DFT) calculations were carried out within the Gaussian 09 suite of programs.[20] Ground state geometries were optimised in the absence of solvent with the mPW1PW91[21] functional in conjunction with the def2-SVP basis set and associated core potential.[22] The mPW1PW91 functional has previously been demonstrated to yield reliable results for such systems.[1g, 23] Geometries optimised with LANL2DZ and 6–31 + G(d) basis sets and effective core potentials were equivalent. Singlepoint energy calculations were carried out with the def2-TZVP basis set and core potential.[22] The polarisable continuum model (PCM)[24] self-consistent reaction field (SCRF) was used to model solvent effects at the gas-phase optimised geometries with a solvent of acetonitrile, consistent with the experimental system. Frontier MO energies were calculated using DFT MOs with mPW91PW91, PBE,[25] B3LYP,[26] BP86,[26a, 27] and wB97XD.[28] An SCF convergence criteria of 108 a.u. was employed throughout. Molecular orbital analysis was carried out with the AOMix program.[29]
Chemicals fac-Tris[2-(2-pyridinyl-kN)phenyl-kC]iridium (fac-tris(2-phenylpyridinato-C2,N)iridium(III), [Ir(ppy)3], 99 %), tris[3,5-difluoro-2-(2-pyridinylChem. Eur. J. 2014, 20, 3322 – 3332
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kN)phenyl-kC]iridium (tris[2-(4,6-difluorophenyl)pyridinato-C2,N]iridium(III), [Ir(df-ppy)3], 96 %), tripropylamine (TPA, 98 %) and tetrabutylammonium hexafluorophosphate (TBAPF6, 99.5 %, electrochemical grade) were purchased from Sigma–Aldrich (Australia). Tris[2-(1H-pyrazol-1-yl-kN2)phenyl-kC]iridium (tris(phenylpyrazole)iridium(III), [Ir(ppz)3], > 99 %) and fac-tris[(3-methyl-1H-imidazol-1-yl2(3 H)-ylidene)-1,2-phenylene]iridium (fac-tris(1-phenyl-3-methylimidazolin-2-ylidene-C,C(2)’)iridium(III), [Ir(pmi)3], > 99 %) were purchased from LumTech (Taiwan). The hexafluorophosphate salt of tris(2,2’-bipyridine-kN1,kN1’)ruthenium(2+) ([Ru(bpy)3]2 + ) was prepared from [Ru(bpy)3]Cl2·6 H2O (Strem Chemicals, USA). Acetonitrile was distilled over calcium hydride under grade 5 argon. Solutions were degassed with argon. Bis[2-(2-pyridinyl-kN)phenyl-kC][2-(3([Ir(ppy)2phenyl-1H-1,2,4-triazol-5-yl-kN1)pyridinato-kN]iridium (ptp)]) and bis[3,5-difluoro-2-(2-pyridinyl-kN)phenyl-kC][2-(3phenyl-1H-1,2,4-triazol-5-yl-kN1)pyridinato-kN]iridium ([Ir(df-ppy)2(ptp)]) were prepared as previously described.[8a,c] Bis[2-(2-pyridinylkN)phenyl-kC][2-[1-(phenylmethyl)-1H-1,2,3-triazol-4-yl-kN3]pyridine-kN]iridium(1+) hexafluorophosphate(1-) ([Ir(ppy)2(ptb)]PF6) and bis[3,5-difluoro-2-(2-pyridinyl-kN)phenyl-kC][2-[1-(phenylmethyl)-1H-1,2,3-triazol-4-yl-kN3]pyridine-kN]iridium(1+) hexafluorophosphate(1-) ([Ir(df-ppy)2(ptb)]PF6), were prepared by anion metathesis from the corresponding chloride and tetrafluoroborate salts, which were synthesised as described below.
Synthesis of [Ir(ppy)2(ptb)]Cl (4) [{Ir(ppy)2}2(m-Cl)2] (50 mg, 47 mmol) was added to a solution of 2-(1(benzyl)-1H-1,2,3-triazol-4-yl)pyridine (22 mg, 93 mmol) in 3:1 dichloromethane/methanol (3 mL). The suspension was stirred in the dark for 16 h at ambient temperature, during which time dissolution of the solids occurred. The yellow solution was evaporated to dryness then dissolved in acetonitrile (5 mL) and filtered. The resulting filtrate was concentrated and dried in vacuo to yield the product as a yellow solid (69 mg, 96 %). Numbering of carbons for NMR assignments is shown in Figure S18 in the Supporting Information. 1H NMR (CDCl3, 500 MHz): d = 5.68 (m, 2 H; C5-H2), 6.29 (d, J = 7.0 Hz, 1 H; C22-H), 6.31 (d, J = 7.48 Hz, 1 H; C33-H), 6.86 (td, J = 6.6, 1.2 Hz, 1 H; C21-H), 6.89 (td, J = 6.6, 1.2 Hz, 1 H; C32-H), 6.92 (m, 1 H; C14-H), 6.94 (m, 1 H; C25-H), 6.97 (m, 1 H; C20-H), 7.01 (td, J = 7.6, 0.8 Hz, 1 H; C31-H), 7.18 (t, J = 6.3 Hz, 1 H; C11-H), 7.29 (m, 3 H; C1-H, C2-H), 7.41 (m, 3 H; C3-H, C13-H), 7.60 (d, J = 5.7 H; 1 H; C24-H), 7.64 (m, 2 H; C19-H, C30-H), 7.73 (m, 3 H; C12-H, C15-H, C26-H), 7.88 (m, 2 H; C16-H, C27-H), 7.96 (td, J = 7.8, 1.2 Hz, 1 H; C10-H), 8.99 (d, J = 7.9 Hz, 1 H; C9-H), 10.69 ppm (s, 1 H; C6-H); 13C NMR (CDCl3, 125 MHz): d = 50.7 (C5), 119.4 (C16), 119.5 (C27), 122.3 (C20), 122.7 (C31), 122.9 (C14), 123.3 (C25), 124.4 (C19), 124.8 (C30), 124.9 (C9), 125.0 (C11), 128.8 (2C, C2), 128.9 (C1), 129.1 (2C, C3), 129.6 (C6), 130.1 (C21), 130.8 (C32), 131.9 (C33), 132.0 (C22), 134.0 (C4), 137.9 (C15), 138.0 (C26), 139.8 (C10), 143.7 (C18), 143.8 (C29), 146.5 (C23), 148.4 (C13), 148.9 (C7), 149.4 (C24), 149.6 (C12), 150.1 (C34), 150.2 (C8), 167.7 (C28), 168.5 ppm (C17); HRMS: m/z calcd for [C36H28N6Ir] + : 737.2005; found: 737.1994.
Synthesis of [Ir(df-ppy)2(ptb)]BF4 (7) This compound was synthesised by modification to a previously reported procedure.[11] [{Ir(df-ppy)2}2(m-Cl)2] (72 mg, 59 mmol) was added to a solution of 2-(1-(benzyl)-1H-1,2,3-triazol-4-yl)pyridine (28 mg, 118 mmol) in 3:1 dichloromethane/methanol (3 mL). The suspension was stirred in the dark for 16 h at ambient temperature, during which time dissolution of the solids occurred. The yellow solution was evaporated to dryness then dissolved in dichloromethane (10 mL) and a saturated methanol solution of NaBF4 (2 mL) was added. The turbid solution was stirred for 1 h
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Full Paper and the solvent removed. The solid was suspended in water and sonicated for 5 min then collected by filtration and washed with copious amounts of H2O then hexane. The crude material was purified by silica gel chromatography using a gradient of dichloromethane!10 % methanol in dichloromethane to yield product as a light yellow solid (74 mg, 70 %). 1H NMR (CDCl3, 500 MHz): d = 5.61 (m, 2 H; C5-H2), 5.67 (dd, J = 8.5, 2.3 Hz, 1 H; C22-H), 5.73 (dd, J = 8.3, 2.3 Hz, 1 H; C33-H), 6.55 (m, 2 H; C20-H, C31-H), 7.00 (ddd, J = 7.3, 6.0, 1.3 Hz, 1 H; C14-H), 7.07 (m, 1 H; C25-H), 7.33 (m, 4 H; C1H,C2-H, C11-H), 7.39 (m, 2 H; C3-H), 7.43 (dd, J = 5.7, 0.7 Hz, 1 H; C13H), 7.58 (d, J = 5.1 Hz, 1 H; C24-H), 7.77 (dd, J = 5.5, 1.0 Hz, 1 H; C12H), 7.81 (t, J = 7.9 Hz, 2 H; C15-H, C26-H), 8.02 (d, J = 8.0 Hz, 1 H; C10H), 8.30 (t, J = 9.8 Hz, 2 H; C16-H, C27-H), 8.43 (d, J = 8.0 Hz, 1 H; C9-H), 9.26 ppm (s, 1 H; C6-H); 13C NMR (CDCl3, 125 MHz): d = 56.2 (C5), 99.1 (t, J = 26.8 Hz, 1C, C20), 99.4 (t, J = 26.7 Hz, 1C, C31), 114.2 (dd, J = 17.6, 2.6 Hz, 1C, C33), 114.3 (dd, J = 17.9, 2.7 Hz, 1C, C22), 123.3 (C14), 123.6 (d, J = 20.1 Hz, 1 H; C16 or C27, cannot resolve), 123.7 (d, J = 19.8 Hz, 1 H; C16 or C27, cannot resolve), 123.9 (C25), 124.3 (C9), 126.6 (C11), 127.7 (C6), 127.8 (m, 1 H; C18), 127.9 (m, 1 H; C29), 129.0 (2C, C3), 129.4 (3C, C1, C2), 133.2 (C4), 139.0 (C15), 139.2 (C26), 140.5 (C10), 148.5 (C13), 148.6 (C7), 149.6 (C24), 149.7 (C8), 149.7 (C12), 153.4, 153.5 (C23, C24, cannot resolve), 161.2 (dd, J = 261.6, 12.6 Hz, 1C, C19), 161.5 (dd, J = 261.1, 12.7 Hz, 1C, C30), 163.3 (dd, J = 257.1, 12.4 Hz, 1C, C21), 163.7 (dd, J = 258.0, 12.3 Hz, 1C, C32), 164.1 (d, J = 7.0 Hz, 1C, C28), 165.0 ppm (d, J = 6.9 Hz, 1C, C17); 19F NMR (CDCl3, 471 MHz): d = 105.2 (m, 1 F), 106.1 (q, J = 8.29 Hz, 1 F), 108.2 (t, J = 11.48 Hz, 1 F), 109.2 (t, J = 11.22 Hz, 1 F), 151.5 (m, 3 F); HRMS: m/z calcd for [C36H24N6F4Ir] + : 809.1628; found: 809.1602.
Computational Infrastructure National Facility (NCI-NF) are acknowledged for substantial computing resources. Keywords: density functional calculations electrochemiluminescence · electrochemistry · iridium luminescence
X-ray crystallography Crystals of [Ir(df-ppy)2(ptb)]BF4 and [Ir(ppy)2(ptb)]Cl were mounted in low temperature oil then flash cooled. Intensity data were collected at 130 K (unless otherwise stated) on an X-ray diffractometer with CCD detector using MoKa (l = 0.71073 ) radiation. Data were reduced and corrected for absorption.[30] The structures were solved by direct methods and difference Fourier synthesis using the SHELX[31] suite of programs as implemented within the WINGX[32] software. Thermal ellipsoid plots were generated using the program ORTEP-3. CCDC-970288 ([Ir(df-ppy)2(ptb)]BF4) and 970287 ([Ir(ppy)2(ptb)]Cl) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Anion metathesis of iridium(III) complexes The relevant complex was dissolved in dichloromethane (10 mL) and a saturated methanol solution of NH4PF6 (1 mL) was added. The turbid solution was stirred for 1 h and the solvent removed. The solid was suspended in water and sonicated for 5 min then collected by filtration and washed with copious amounts of H2O then hexane. The crude material was recrystallised from ethanol and dried in vacuo.
Acknowledgements This work was funded by the Australian Research Council (FT100100646, LE120100213, DP1094179). The Victorian Partnership for Advanced Computing (VPAC) and the National Chem. Eur. J. 2014, 20, 3322 – 3332
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Received: November 18, 2013 Published online on March 3, 2014
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