Article pubs.acs.org/IC
Luminescent Iridium(III) Cyclometalated Complexes with 1,2,3Triazole “Click” Ligands Timothy U. Connell,†,‡ Jonathan M. White,†,‡ Trevor A. Smith,† and Paul S. Donnelly*,†,‡ †
School of Chemistry and ‡Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Melbourne, Victoria 3010, Australia S Supporting Information *
ABSTRACT: A series of cyclometalated iridium(III) complexes with either 4-(2-pyridyl)-1,2,3-triazole or 1-(2-picolyl)-1,2,3-triazole ancillary ligands to give complexes with either 5- or 6-membered chelate rings were synthesized and characterized by a combination of X-ray crystallography, electron spin ionization−high-resolution mass spectroscopy (ESI-HRMS), and nuclear magnetic resonance (NMR) spectroscopy. The electronic properties of the complexes were probed using absorption and emission spectroscopy, as well as cyclic voltammetry. The relative stability of the complexes formed from each ligand class was measured, and their excited-state properties were compared. The emissive properties are, with the exception of complexes that contain a nitroaromatic substituent, insensitive to functionalization of the ancillary pyridyl-1,2,3-triazole ligand but tuning of the emission maxima was possible by modification of the cyclometalating ligands. It is possible to prepare a wide range of optimally substituted pyridyl-1,2,3-triazoles using copper Cu(I)-catalyzed azide alkyne cycloaddition, which is a commonly used “click” reaction, and this family of ligands represent an useful alternative to bipyridine ligands for the preparation of luminescent iridium(III) complexes.
■
INTRODUCTION Octahedral organometallic iridium(III) complexes such as [IrIII(ppy)3], where the cyclometalating ligand 2-phenylpyridine (ppy) or its derivatives act as bidentate anionic ligands, exhibit long-lived phosphorescence at room temperature.1−4 The strong spin−orbit coupling induced by the heavy iridium(III) ion promotes efficient intersystem crossing from the singlet state to the triplet state. An attractive property of iridium(III) luminescent complexes for application in emerging technologies is the ability to tune the emissive properties across the visible spectrum through control of the ligand architecture. Synthetically, this entails substitution of the cyclometalating ligands or the replacement of one cyclometalating ligand with a different (ancillary) ligand to form either neutral or positively charged heteroleptic complexes. Substituted 1,2,3-triazoles formed via the Cu(I)-catalyzed azide−alkyne cycloaddition (CuAAC), which is a commonly used “click” reaction, form metal complexes with a variety of coordination modes.5,6 The reaction of 2-ethynylpyridine and an organic azide yields 4-(2-pyridyl)-1,2,3-triazole and bidentate “diimine-like” ligands capable of forming 5-membered chelate rings that are the structural analogue of the classical 2,2′bipyridine. In comparison to the synthesis of singly functionalized 2,2′-bipyridine ligands, the broad scope and functional group tolerance of the CuAAC reaction allows for the easy formation of bidentate unsymmetrical ligands in good yields and in a single synthetic step. An alternative CuAAC approach is the synthesis of 1-(2-picolyl)-1,2,3-triazole bidentate ligands, formed © 2016 American Chemical Society
via the reaction of 2-picolylazide with para-substituted benzyl azides. These derivatives form 6-membered chelate rings, following metal coordination. Increased popularity in the CuAAC reaction to produce 1,2,3triazoles has led to several recent reports of iridium(III) complexes with 1,2,3-triazoles introduced into either the cyclometalating or ancillary ligands.7−20 Our interest in the synthesis of this family of compounds was partially stimulated by their considerable potential in electrochemiluminescence (ECL) applications.17,20−24 It is of particular significance that 1,2,3trizole-containing ancillary ligands induce a large hypsochromic shift in emission, compared to complexes prepared with 2,2′bipyridine, which enables multiple color detection when combined with [Ru(bpy)3]2+, which is a commonly used ECL emitter.25 The modular synthetic approach of “click”-derived ligands, combined with the tunable emission and unique photophysics of iridium(III) emitters, is also of interest for the development of luminescent probes for imaging proteins in live cells.26 In this manuscript, we present a systematic study of luminescent cyclometalated iridium(III) complexes that contain either 5- or 6-membered chelate rings with 4-(2-pyridyl)-1,2,3triazole or 1-(2-picolyl)-1,2,3-triazole based ligands, respectively. To date, iridium(III) complexes with 1-(2-picolyl)-1,2,3-triazole have not been reported and their properties are compared to the Received: November 11, 2015 Published: March 3, 2016 2776
DOI: 10.1021/acs.inorgchem.5b02607 Inorg. Chem. 2016, 55, 2776−2790
Article
Inorganic Chemistry Scheme 1. Synthesis of CuAAC-Derived Ancillary Ligands
complexes containing 4-(2-pyridyl)-1,2,3-triazole ligands. Changes to the emission and electrochemical properties of the iridium(III) complexes caused by altering the substituents on the 1,2,3-triazole ligands are presented.
Scheme 2. Synthesis of Cyclometalated Iridium(III) Complexes with 1,2,3-Triazole-Containing Ancillary Ligands
■
SYNTHESIS Triazole-containing ancillary ligands were synthesized using modified CuAAC reaction conditions (see Scheme 1). The organic azides benzyl azide and 2-picolylazide were isolated prior to their use in CuAAC reactions, while the nitro- and methoxysubstituted derivatives were prepared in situ and immediately reacted with the corresponding alkyne.27 An organic solution of each L1x ligand (where x = a−c) was washed with a basic aqueous solution containing ethylenediamine tetraacetic acid (H2Na2EDTA, pH 8) to remove the copper catalyst. The products were purified by recrystallization and obtained in good yield. The iridium(III) complexes containing either L1x and L2x ancillary ligands were synthesized by mixing the ancillary ligand with the appropriate chlorido-bridged iridium(III) dimer, [Ir(C^N)2(μ-Cl)]2 where C^N = ppy, 2-(2,4- difluorophenyl)pyridine (dfppy) or 1-phenylisoquinoline (piq), in a mixture of dichloromethane and methanol at ambient temperature for 16 h, during which the reactants fully dissolved (see Scheme 2). The anions of the crude complexes were substituted via anion methathesis with sodium tetrafluoroborate and the complexes were isolated in good yields, following subsequent recrystallization. The metal complexes were analyzed by electrospray ionization−high-resolution mass spectrometry (ESI-HRMS) and spectra of the L1x complexes showed a single monocationic species with the characteristic iridium isotopic pattern. In contrast, the spectra of complexes with L2x ligands showed the parent peak (for [Ir(ppy)2L2a]+ m/z 737.202), along with many other species, including large amounts of the iridium(III) cyclometalated core ([Ir(ppy)2]+ m/z 501.095), the protonated ancillary ligand [L2a + H]+ (m/z 237.113), and even the solvated complex [Ir(ppy)2(CH3CN)]+ (m/z 542.122). The relative intensity of these peaks varied with the fragmentor voltage.
retained.1,28,29 The coordination of the L1x ligands to the iridium(III) core formed a five-membered planar metallacycle with a N3−Ir−N4 bite angle between 76.11(6)° and 76.8(2)° (see Table 2), similar to other reported crystal structures of iridium(III) complexes with 4-(2-pyridyl)-1,2,3-triazole ancillary ligands.12,15,16,18 The L2x ancillary ligands formed six-membered chelate rings when coordinated to the iridium(III) atom (Figure 2) and exhibited larger bite angles compared to the complexes with 4-(2pyridyl)-1,2,3-triazole (L1x) ligands. For example, bite angles were between 84.75(8) ° for [Ir(ppy)2L2c]BF4 and 86.1(4) ° for [Ir(ppy)2L2a]BPh4. The L2x ancillary ligands coordinated in a bent fashion, adopting a ‘boat-like’ conformation with fold angles approximately 112.0(2) ° (C27−C28−N4 in [Ir(ppy)2L2c]BF4). The Ir−Npyridyl bond length in the ancillary ligands is longer than the Ir−Ntriazole bond. Bond lengths of Ir−N3 ranged from 2.162(2) Å in [Ir(dfppy)2L1a]BF4 to 2.206(2) Å in [Ir(ppy)2L2c]BF4 Previous reports suggest that the N5 atom (according to atom labeling used in Figures 1 and 2) of the
■
X-RAY CRYSTALLOGRAPHY Single crystals suitable for X-ray crystallography were obtained for several of the complexes (see Table 1, Figure 1, and Figure 2), as either the tetrafluoroborate, hexafluorophosphate, or tetraphenylborate salt. We previously reported the single-crystal X-ray structures of [Ir(ppy)2L1a]Cl and [Ir(dfppy)2L1a]BF4 as part of earlier work describing their ECL properties and have included their structures here for the benefit of this systematic investigation.21 All complexes display a distorted octahedral C2N4 geometry around the iridium(III) atom with the two coordinated Npyridyl atoms trans to each other and the two organometallic coordinated C atoms cis to each other, confirming that the geometry of the iridium(III) precursor was 2777
DOI: 10.1021/acs.inorgchem.5b02607 Inorg. Chem. 2016, 55, 2776−2790
Article
Inorganic Chemistry Table 1. X-ray Crystallographic Dataa [Ir(ppy)2L1b]PF6 formula molecular weight, MW color and habit crystal size system space group temp, T unit-cell parameters a b c α β γ U Z calc density, D wavelength absorption coefficient F(000) reflections measured independent reflections R [I > 2σ(I)] wR(F2) (all data) a
[Ir(piq)2L1a]BF4
[Ir(ppy)2L2a]BPh4· CH2Cl2
[Ir(dfppy)2L2a]BF4· 3CHCl3
[Ir(ppy)2L2c]BF4· CH2Cl2
[Ir(piq)2L2a]PF6· 1.5CH2Cl2
C36H27N7O2IrPF6 926.81
C44H32N6IrBF4 923.76
C61H50N6IrBCl2 1140.98
C39H27N6IrF8BCl9 1253.72
C38H32N6OIrBF4Cl2 938.60
C45.5H35N6IrPF6Cl3
yellow block 0.44 mm × 0.39 mm × 0.18 mm monoclinic P21/c 130.0(1) K
red slab 0.55 mm × 0.28 mm × 0.08 mm monoclinic P21/n 130.0(1) K
yellow needle 0.47 mm × 0.04 mm × 0.02 mm monoclinic P21/c 130.0(1) K
pale yellow prism 0.166 mm × 0.0754 mm × 0.0428 mm triclinic P1̅ 130.0(1) K
yellow block 0.1845 mm × 0.1140 mm × 0.0584 mm monoclinic P21/c 130.0(1) K
orange-red needle 0.5115 mm × 0.0684 mm × 0.0351 mm monoclinic P21/c 130.0(1) K
12.7547(2) Å 19.0611(2) Å 14.2521(2) Å 90° 99.2370(10)° 90° 3420.02(8) Å3 4 1.800 g cm−3 0.7107 Å 4.029 mm−1
9.62320(10) Å 17.0053(2) Å 22.3048(3) Å 90° 90.591(1)° 90° 3649.88(8) Å3 4 1.681 g cm−3 0.7107 Å 3.721 mm−1
9.6124(6) Å 12.6634(13) Å 41.7159(3) Å 90° 93.(430)° 90° 5068.6(7) Å3 4 1.495 g cm−3 1.5418 Å 6.430 mm−1
12.7660(6) Å 14.2197(9) Å 14.6535(8) Å 66.207(6)° 83.125(4)° 66.905(5)° 2236.5(2) Å3 2 1.862 g cm−3 1.5418 Å 11.370 mm−1
13.70380(10) Å 15.24220(10) Å 17.1576(2) Å 90° 91.0140(1)° 90° 3583.25(5) Å3 4 1.750 g cm−3 1.54184 Å 9.119 mm−1
14.3335(2) Å 14.05720(10) Å 21.5094(3) Å 90° 105.6790(10)° 90° 4172.64 Å3 4 1.766 g cm−3 1.54184 Å 8.929 mm−1
1816 55780
1824 44482
2296 24897
1220 12559
1848 26882
2188 30894
13647
16998
7515
7972
7489
8735
0.0303 0.0654
0.0289 0.0623
0.0779 0.2035
0.0467 0.1094
0.0221 0.0586
0.0363 0.1028
Legend: ppy, 2-phenylpyridine; piq, 1-phenylisoquinoline; and dfppy, 2-(2,4-difluorophenyl)pyridine.
between the C30−C33 atoms of the ancillary L2c ligand (Figure 3). Similar π−π interactions between the C3−C6 atoms of the 2(2,4-difluorophenyl)-pyridinato cyclometalating ligands were observed in the structure of [Ir(dfppy)2L2a]BF4 (see Figure 3).
coordinated triazole heterocycle is a weaker sigma-donor than the N4 atom and metal complexes that coordinate through the N5 atom (e.g., L2x ancillary ligand) are less stable than complexes featuring coordination through the N4 nitrogen (e.g., L1x ligands).31−34 It was therefore surprising to find that Ir−Ntriazole bond lengths found in the L2x series of complexes were not significantly longer than the similar bond in the L1x series of complexes (Table 2). Both the longest and the shortest Ir− Ntriazole bond lengths were observed in complexes with L2x ancillary ligands; Ir−N5 2.116(4) Å in [Ir(dfppy)2L2a]BF4 and 2.156(4) Å in [Ir(ppy)2L2c]BF4. The Ir−Ntriazole bond length in the remaining L2x containing complexes and all the Ir−N4 bond lengths in the L1x complexes were between these extremes. Rhenium(I) tricarbonyl complexes with 4-(2-pyridyl)-1,2,3triazoles and 1-(2-picolyl)-1,2,3-triazoles as bidentate ligands also showed no trend in the Re−Ntriazole bond length based on chelation mode.31−34 Close contact interactions between the cation and anion were observed in most solved structures and often occurred between the anion and ancillary ligand, particularly the hydrogen atom of the triazole heterocycle. This was most evident in [Ir(ppy)2L1b]PF6 with multiple close contacts between the ancillary ligand, L1b and the hexafluorophosphate anion (Figure S1 in the Supporting Information). The only compound with no close contact interactions was [Ir(ppy)2L2c]BF4. Each complex crystallized as a racemic mixture of the Δ and Λ isomers. The packing arrangement of the enantiomers in the crystal lattice of [Ir(ppy)2L2c]BF4 exhibited π−π interactions
■
CHARACTERIZATION BY NUCLEAR MAGNETIC RESONANCE AND AN ASSESSMENT OF THE RELATIVE STABILITY OF THE COMPLEXES All complexes were characterized using a combination of onedimensional (1D) and two-dimensional (2D) NMR experiments. Integration of all signals in each 1H NMR spectra and the number of signals in each 13C NMR spectra matched the respective number of H and C atoms in each metal complex. Long-range C−F coupling was observed for many signals in the two complexes with fluorine substituents: [Ir(dfppy)2L1a]BF4 and [Ir(dfppy)2L2a]BF4. Following coordination of the ancillary ligand, most signals undergo a downfield shift; this shift was most pronounced for the H atom of the triazole heterocycle (Figure 4). In the series of complexes with the L2x ligands, the methylene carbon is part of the metallacycle formed, which prevents free rotation of the C−C bonds and the now-diasterotopic hydrogen atoms in [Ir(ppy)2L2a]BF4 are viewed as an AB doublet (2JH−H = 15.9 Hz). The 1H NMR spectra of cations [Ir(ppy)2L1x/2x]+ were aniondependent (particularly, the H atom of the triazole heterocycle). The complex [Ir(piq)2L2a]BF4 showed limited solubility in deuterated chloroform so spectra were also acquired in deuterated acetonitrile. Interestingly, the 1H NMR spectrum 2778
DOI: 10.1021/acs.inorgchem.5b02607 Inorg. Chem. 2016, 55, 2776−2790
Article
Inorganic Chemistry
Figure 1. ORTEP30 representations (40% thermal ellipsoids) of (A) [Ir(ppy)2L1a]+, (B) [Ir(ppy)2L1b]+, (C) [Ir(dfppy)2L1a]+, and (D) [Ir(piq)2L1a]+. Counter ions, hydrogen atoms, and solvent molecules were omitted for the sake of clarity. Structures shown in panels (A) and (C) have been reported previously.21
(1LC) in the UV region (λmax = 252 nm in [Ir(ppy)2L1a]BF4) and weaker bands attributed to both spin-allowed and spin-forbidden metal-to-ligand charge transfer transitions (1MLCT and 3 MLCT) at wavelengths of λmax > 350 nm (Figure 6, Table 3). The energy of the MLCT absorptions was affected by altering the cyclometalating ligands; the absorption maxima of the 2-(2,4difluorophenyl)pyridinato-containing complexes exhibited a hypsochromic shift (∼20 nm), relative to the 2-phenylpyridinato derivatives, and the 1-phenylisoquinolinato ligands induced a large bathochromic shift of both the 1MLCT and 3MLCT absorption bands. Different substitution of the benzyl group in the L 1x ligands showed no significant change in the corresponding absorption spectra. Absorption profiles of the L2x containing complexes were also very similar with the exception of [Ir(ppy)2L2b]BF4, which had a more prominent shoulder at λmax = 300 nm (see Figure 7 and Table 4). All of the complexes are luminescent at room temperature with the exception of the nitroaromatic containing complexes [Ir(ppy)2L1b]BF4 and [Ir(ppy)2L2b]BF4. The emission spectra all show resolved vibronic structure, consistent with other reported iridium(III) complexes of 1,2,3-triazole ligands, due to mixing of the 3LC and 3MLCT excited states.12,13,15,35,36 Substitution of the 2-phenylpyridinato cyclometalating ligands with 2-(2,4-difluorophenyl)pyridinato or 1-phenylisoquinolinato resulted in a respective hypsochromic or bathochromic shift in emission consistent with the relative position of the MLCT
changed over 12 hours as the complex partially dissociated (final concentrations of 3:2 complex/dissociation products were determined by integration of the methylene protons in the L2a ligand) to the free ligand and what we suspected was the acetonitrile solvated complex, [Ir(piq)2(CH3CN)2]BF4. Subsequent study of the other L2a complexes showed similar levels of dissociation while comparison of the 1H NMR spectra of [Ir(ppy)2L2a]BF4 after 12 h and [Ir(ppy)2(CH3CN)2]BF4 confirmed the speculated dissociation product. The complexes also showed similar relative stabilities when analyzed by high-performance liquid chromatography (HPLC) (see Figure 5), relative to chromatograms of the free ligands and the iridium(III) precursor [Ir(ppy)2(μ-Cl)]2 (most likely, the solvated complex [Ir(ppy)2(CH3CN)2]+ when dissolved in the mobile phase). The complex [Ir(ppy)2L1a]BF4 eluted as a single peak; the slight difference between the retention time of the absorption and emission spectrum was due to the configuration of the detectors. In contrast, analysis of [Ir(ppy)2L2a]BF4 under the same conditions gave three peaks in the ultraviolet (UV) chromatogram, the complex (Rt = 19.33 min) again dissociating to give the ligand L2a (Rt = 10.12 min) and the cyclometalated iridium(III) acetonitrile solvate complex (Rt =16.17 min). As expected, only the [Ir(ppy)2L2a]+ showed any emission at λem = 480 nm. Characterization by Electronic Spectroscopy. All the complexes featured strong π−π* ligand centered transitions 2779
DOI: 10.1021/acs.inorgchem.5b02607 Inorg. Chem. 2016, 55, 2776−2790
Article
Inorganic Chemistry
Figure 2. ORTEP30 representations (40% thermal ellipsoids) of (A) [Ir(ppy)2L2a]+, (B) [Ir(ppy)2L2c]+, (C) [Ir(dfppy)2L2a]+, and (D) [Ir(piq)2L2a]+. Counter ions, hydrogen atoms, and solvent molecules were omitted for the sake of clarity.
quantum yield was reported to correlate with an increase in the flexibility of 2-(2′-pyridyl)indole compounds.31,38,39 The complex [Ir(piq)2L2a]BF4 was the exception to this trend; while the relative quantum yield was approximately half that of [Ir(piq)2L1a]BF4, Φem = 0.09 compared with Φem = 0.22 respectively, the measured emission lifetimes were very similar, τ = 3.6 μs compared with τ = 3.2 μs and there was no significant increase in the calculated nonradiative rate constant (knr 2.2 and 2.8 × 105 s−1 respectively). The two complexes with nitroaromatic-containing ancillary ligands, [Ir(ppy)2L1b]BF4 and [Ir(ppy)2L2b]BF4, exhibit different photophysical properties. The structure of the emission spectra for [Ir(ppy)2L1b]BF4 was similar to the other L1x complexes, although less-resolved. The relative quantum yield and measured emission lifetime were several orders of magnitude lower than the other L1x complexes. The complex [Ir(ppy)2L2b]BF4 was essentially nonemissive in solution at room temperature, a weak featureless band was observed (λem = 513 nm) with a quantum yield below Φem < 0.001. These differences are consistent with electrochemical measurements (vide inf ra).
Table 2. Selected bond angles and distances
a
compound
bite angle (deg)
Ir−Npyridyl bond length (Å)
Ir−Ntriazole bond length (Å)
[Ir(ppy)2L1a]Cla [Ir(ppy)2L1b]PF6 [Ir(dfppy)2L1a]BF4a [Ir(piq)2L1a]BF4 [Ir(ppy)2L2a]BPh4 [Ir(ppy)2L2c]BF4 [Ir(dfppy)2L2a]BF4
76.8(2) 76.23(9) 76.34(9) 76.11(6) 86.1(4) 84.75(8) 86.0(2)
2.172(5) 2.174(3) 2.162(2) 2.169(2) 2.18(1) 2.206(2) 2.193(7)
2.142(5) 2.129(2) 2.137(2) 2.130(2) 2.12(1) 2.156(2) 2.116(4)
Structures were previously reported in ref 21.
absorption bands. Emission quantum yields and lifetimes were measured in deoxygenated acetonitrile, quantum yields were measured relative to known standards.37 The complexes with L1x ancillary ligands had quantum yields between Φem = 0.2−0.3 and emission lifetimes in the microsecond range. The quantum yields and emission lifetimes of complexes containing L2x ancillary ligands were lower by an order of magnitude. The calculated radiative (Φem/τ) and nonradiative ((1-Φem)/τ) rate constants show that the poorer photophysical properties of the L2x containing complexes result from an increase in nonradiative paths of decay (most likely promoted by the increased flexibility of the L2x ligands relative to their L1x isomers); the knr values are an order of magnitude greater than the L1x containing complexes, despite having a similar range of kr values (Tables 3 and 4). Rhenium(I) tricarbonyl complexes with L1x and L2x type ligands show a similar trend in electronic spectra and a decrease in
■
ELECTROCHEMISTRY The electrochemistry of the complexes was investigated by cyclic voltammetry (see Table 5 and Figure 8). The complexes exhibited two main features, formally assigned as either IrIV/IrIII metal oxidation at positive potential or various ligand reductions at negative potential. The metal oxidation process at positive potentials was quasi-reversible, the cathodic current on the 2780
DOI: 10.1021/acs.inorgchem.5b02607 Inorg. Chem. 2016, 55, 2776−2790
Article
Inorganic Chemistry
Figure 3. ORTEP30 representation of (A) π−π interactions between the C30−C33 atoms of the ancillary L2c ligand in [Ir(ppy)2L2c]BF4 and (B) π−π interactions between the C3−C6 atoms of the 2-(2,4-difluorophenyl)-pyridinato cyclometalating ligands in [Ir(dfppy)2L2a]BF4.
Figure 4. Partial 1H NMR spectra (500 MHz, CDCl3) of the ligand L1a, [Ir(ppy)2L1a]BF4, L2a, and [Ir(ppy)2L2a]BF4. Residual solvent is marked with an asterisk.
reverse sweep increased and peak separation (ΔEp) decreased with an increase in the scan rate (Figure 9). The IrIV/IrIII process in the [Ir(ppy)2L2x]BF4 series of complexes was slightly higher in potential (E°′ > 0.04−0.07 V) than the [Ir(ppy)2L1x]BF4 derivatives; for example, for [Ir(ppy)2L2a]BF4, E°′ = 1.32 V, compared to E°′ = 1.27 V for [Ir(ppy)2L1a]BF4 (Table 5). A comparison between iridium(III) complexes with 4-(2-pyridyl)1,2,3-triazole ancillary ligands and previously reported 4-(2pyridyl)-1,2,4,-triazole ligands (where the ligand is deprotonated
and a better sigma donor) revealed a higher oxidation potential in the positively charged complexes.40−42 The reduced sigma donation makes the metal ion formally more positive, which increases the difficulty of oxidation leading to a higher measured potential.14,41 The N5 nitrogen of the coordinated triazole heterocycle, as in the complex [Ir(ppy)2L2a]BF4, is thought to be less electron-rich than the N4 position.31−34 Excluding the complexes with an aromatic nitro substituent, the L1x series of complexes also showed quasi-reversible 2781
DOI: 10.1021/acs.inorgchem.5b02607 Inorg. Chem. 2016, 55, 2776−2790
Article
Inorganic Chemistry
couple on the reverse sweep indicative of a chemical reaction, following the irreversible cathodic processes. Fluorination of the 2-phenylpyridinato ligands in [Ir(dfppy)2L1a]BF4 and [Ir(dfppy)2L2a]BF4 caused a small increase in the reductive wave (+0.06 V, relative to the corresponding 2phenylpyridinato complexes) and a much larger increase of the oxidative wave (+0.33 and 0.32 V, respectively) (see Figure 8). The greater increase of the oxidation potential is thought to occur because the cyclometalated phenyl rings (where the fluorine substituents are located) provide a substantial contribution to the metal-centered highest occupied molecular orbital (HOMO). Fluorination of the cyclometalating ligands causes a stabilization of the HOMO, which, in turn, increases the oxidation potential. In contrast, the lowest unoccupied molecular orbital (LUMO) is mainly centered on the pyridine rings of these ligands and therefore the reductive processes are less affected by fluorination.17 Extending the conjugation of the cyclometalating ligands in [Ir(piq)2L1a]BF4 saw little change in the oxidation, relative to [Ir(ppy)2L1a]BF4 (−0.02 V) and two reductive couples (Figure 8A). These two waves at E°′ = −1.58 and −1.80 V were both reversible one-electron processes. The first reductive wave in [Ir(piq)2L2a]BF4 was measured as E°′ = −1.53 V and quasi-reversible (cathodic current on the reverse sweep increased and peak separation decreased as the scan rate was increased). The first reductive process in both 1-phenylisoquinolinato complexes was formally assigned as the reduction of a cyclometalating ligand.45,46 The ancillary ligand does not contribute to the LUMO in either [Ir(piq)2L1a]BF4 or [Ir(piq)2L2a]BF4 and may explain the improved emission quantum yield and lifetime of [Ir(piq)2L2a]BF4, relative to the other L2x-containing complexes. The electrochemical data provides insight into the energies of the frontier orbitals. The separation between the observed oxidation and reduction potentials is related to the observed optical transitions, and correlates especially well for MLCT transitions in metal polypyridyl complexes.43,45,47,48 The HOMO and LUMO energy for the iridium(III) complexes studied was calculated (see Table 6) from the electrochemical data, according to the equation
Figure 5. RP-HPLC chromatograms detailing the stability of [Ir(ppy)2L1a]BF4 (left) and [Ir(ppy)2L2a]BF4 (right). Absorbance (λ = 254 nm), luminescence (λex = 337 nm, λem = 480 nm). Samples were loaded onto a reverse-phase C18 column, 0−100% acetonitrile in water (0.1% CF3CO2H) over 30 min.
Figure 6. Absorbance (top) and normalized emission (bottom) spectra of iridium(III) complexes containing L1x ancillary ligands in acetonitrile. Absorbance was measured using 20 μM solutions; emission was measured using 5 μM deoxygenated solutions irradiated at λex = 337 or 380 nm.
onset E HOMO(LUMO) = − (E onset − E Fc ) − 4.80 eV
where Eonset and Eonset are, respectively, the onset reduction Fc potential of the sample and of ferrocene, while the value −4.80 eV is the HOMO energy level of ferrocene under vacuum.49,50 The complexes [Ir(ppy)2 L1a]BF4, [Ir(ppy) 2L2a]BF 4, [Ir(ppy)2L1c]BF4, and [Ir(ppy)2L2c]BF4 all have calculated ΔE HOMO−LUMO values between 3.04 eV and 3.13 eV, corresponding to a transition energy at λcalc ≈ 400 nm. While this is slightly lower in energy than the transition observed experimentally (λmax = 379 nm), it remains consistent across these complexes, which all share the same emission profile. Correlation of the calculated transitions with measured 1MLCT absorption bands showed similar trends for [Ir(dfppy)2L1a]BF4 (λcalc = 373 nm, λmax = 358 nm) and especially [Ir(piq)2L1a]BF4 (λcalc = 438 nm, λmax = 434 nm), as well as their L2a derivatives. The calculated molecular orbital (MO) separation of the complexes containing nitroaromatic functional groups, [Ir(ppy)2L1b]BF4 and [Ir(ppy)2L2b]BF4, did not correlate well with the observed MLCT absorption bands but this is attributed to the reduction of the nitro group that is not involved in the optical transition. Similar results were observed in rhenium(I) tricarbonyl complexes of 4-(2-pyridyl)-1,2,3-triazole ligands.38
processes at negative potentials attributed to ligand-based processes. On the other hand, the L2x series of complexes showed completely irreversible reductive waves (Figure 8). The cathodic peak potential (Epc) of these complexes closely matched those of the corresponding L1x analogue; for example, for [Ir(ppy)2L1c]BF4 and [Ir(ppy)2L2c]BF4, the Epc was −1.81 and −1.82 V, respectively. For the complexes containing 2phenylpyridinato cyclometalating ligands, the first reductive wave was assigned to the pyridyl ring of the 4-(2-pyridyl)-1,2,3triazole ancillary ligand.13,17,43 The reduction of the cyclometalating ligands was not observed in this solvent system. The complexes featuring nitroaromatic functional groups, [Ir(ppy)2L1b]BF4 and [Ir(ppy)2L2b]BF4 show different electrochemical behavior. Both exhibit a reversible one-electron process (ΔEp = 0.067 V, compared to ΔEp = 0.066 V for Fc+/Fc) at E°′ = −1.03 and −1.02 V, respectively, assigned to the reduction of the nitro group to a stable radical anion (see Figure S2 in the Supporting Information).44 This was followed by a series of irreversible processes that affected the reversibility of the first 2782
DOI: 10.1021/acs.inorgchem.5b02607 Inorg. Chem. 2016, 55, 2776−2790
Article
Inorganic Chemistry Table 3. Summary of Photophysical Data for 4-(2-pyridyl)-1,2,3-triazole Containing Iridium(III) Complexes Absorbance λmax (nm) [Ir(ppy)2L1a]BF4
[Ir(ppy)2L1b]BF4
[Ir(ppy)2L1c]BF4
[Ir(dfppy)2L1a]BF4
[Ir(piq)2L1a]BF4
Emission
ε (M−1 cm−1)
252
44400
379
4490
465
151
254 379 465
55100 4930 151
252
51600
379
5030
465
200
245
51100
358
4960
445
159
290 334(sh) 352(sh)
43300 18300 17100
374(sh)
11100
434 520 540
6610 342 200
kr (× 105 s−1)
knr (× 105 s−1)
λem (nm)
Φem
τ (μs)
478 506
0.30
2.2
491
0.006
0.004
478 506
0.31
2.2
1.4
3.1
453 481
0.19
2.2
0.86
3.7
592 632
0.22
3.6
0.6
2.2
1.4
15
3.2
2500
The iridium(III)−nitrogen bond lengths were similar when the metal coordinated through either the N4 or N5 position of the triazole heterocycle. The triazole complexes with 6-membered chelate rings, formed by coordination of the L2x ligands, are less stable than the L1x complexes. The [Ir(ppy)2L1x]BF4 complexes showed better emission quantum yields and longer emission lifetimes and were more stable to ligand substitution in solution. Substitution of the triazole ligand did not affect the emission maxima; however, both hypsochromic and bathochromic tuning of emission was possible by modifying the cyclometalating ligands. The presence of a nitroaromatic group lead to a marked decrease in emission, and this was rationalized by changes to the properties of the excited state, as measured by cyclic voltammetry. The scope and versatility of the CuAAC reaction means it is relatively straightforward to synthesize substituted pyridine-1,2,3-triazole ligands for use as ancillary ligands in the formation of cyclometalated organometallic iridium(III) complexes. The resulting complexes have excellent emissive properties that have potential application in electrochemiluminescence, optical devices, photocatalysis, and cellular imaging.
■
Figure 7. Absorbance (top) and normalized emission (bottom) spectra of iridium(III) complexes containing L2x ancillary ligands in acetonitrile. Absorbance was measured using 20 μM solutions; emission was measured using 5 μM deoxygenated solutions irradiated at λex = 337 or 380 nm. The emission band of [Ir(ppy)2L2b]BF4 was very weak (Φem < 0.001) and was therefore not included.
EXPERIMENTAL SECTION
Reagents and solvents were purchased from various commercial sources and used without further purification, unless otherwise stated. The commercial iridium(III) precursor [Ir(dfppy)2(μ-Cl)]2 was provided by colleagues from Deakin University (Waurn Ponds, Victoria, Australia). Microanalyses for C, H, and N were carried out by the Campbell Microanalytical Laboratory (Dunedin, New Zealand). NMR spectra were acquired on a Model FT-NMR 500 spectrometer (Varian, Palo Alto, CA, USA). 1H NMR spectra were acquired at 500 MHz, {1H}13C NMR spectra were acquired at 125.7 MHz, and 19F NMR was acquired at 470 MHz. All NMR spectra were recorded at 298 K. Chemical shifts
■
CONCLUSIONS The synthesis and characterization of a series of iridium(III) complexes with 1,2,3-triazole positional isomers that coordinate to form either a 5- or 6-membered chelate ring are presented. 2783
DOI: 10.1021/acs.inorgchem.5b02607 Inorg. Chem. 2016, 55, 2776−2790
Article
Inorganic Chemistry Table 4. Summary of Photophysical Data for 1-(2-picolyl)-1,2,3-triazole Containing Iridium(III) Complexes Absorbance λmax (nm) [Ir(ppy)2L2a]BF4
[Ir(ppy)2L2b]BF4
[Ir(ppy)2L2c]BF4
[Ir(dfppy)2L2a]BF4
[Ir(piq)2L2a]BF4
Emission
ε (M−1 cm−1)
252
43800
375
4260
465
130
264 295 375 465
42600 30300 7150 260
257
53400
375
4710
465
140
246
52200
357
5360
445
280
290 340(sh) 352(sh)
41100 18400 18300
375(sh)
10900
430 520 550
6990 240 100
λem (nm)
Φem
τ (μs)
kr (× 105 s−1)
knr (× 105 s−1)
478 506
0.021
0.20
1.0
49
513
