Intimate electronic coupling in cationic homodimeric iridium(iii) complexes

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Intimate electronic coupling in cationic homodimeric iridium(III) complexes†

Ahmed M. Soliman, Daniel Fortin, Pierre D. Harvey* and Eli Zysman-Colman*

Received 17th April 2012, Accepted 28th May 2012DOI: 10.1039/c2dt30836c

Herein, we report two new cationic iridium(III) homodinuclear structures linked through a diyne moiety atthe 5-position of the bipyridyl ligand (1,4-di(2,2′-bipyridin-5-yl)buta-1,3-diyne) and compare these tomononuclear model systems bearing a 5-ethynyl-2,2′-bipyridine ligand. Low energy bands observed inthe absorption spectra point to charge-transfer transitions for all four complexes, with these bandsred-shifted in the case of the two dinuclear complexes. Electrochemical studies show metal-centredoxidation and ligand-centred first reduction potentials. In the case of the dimer bearing 2-phenylpyridine(ppyH) cyclometallating ligands, cyclic voltammetry (CV) measurements reveal two one-electronoxidation waves and a corresponding reduction in the HOMO–LUMO gap (ΔEred-ox) compared to amononuclear system, pointing to a significant electronic coupling between the two iridium(III) metals.The room temperature emission spectrum of this dimer is also bathochromically shifted, corroborating theCV data. In the case of the iridium dimer bearing 2-(2,4-difluorophenyl)-5-methylpyridine (dFMeppy)ligands, only a single one-electron oxidation wave is observed, but with the expected smaller ΔEred-ox

value, compared to its mononuclear counterpart. The emission spectra at room temperature aregenerally broad and featureless with only modest quantum efficiencies (ΦPL = 1.4–8.4%) in2-methyltetrahydrofuran (2-MeTHF) solution. All complexes emit at 77 K with lifetimes on the order of4 μs. A combined density functional theory (DFT) and time-dependent DFT (TDDFT) study reveals thatthe emission process is best described as a mixed metal-to-ligand/ligand-to-ligand charge transfer(MLCT/LLCT).

Introduction

Both neutral and cationic iridium(III) complexes of the form(C^N)2Ir(L^X), where C^N is a cyclometallating ligand such asthe anion of 2-phenylpyridine (ppy) and L^X is either ananionic or neutral bidentate ligand such as acetylacetone (acac)or 2,2′-bipyridine (bpy), have each been the object of intensestudy over the past decade.1 The tremendous interest in thesetwo classes of complexes stems from the improved photophysi-cal properties compared to complexes of other metals.1b,c,2

Notably, the emission energy of iridium(III) complexes can bereadily tuned through synthetic modification of the ligands,which is made more facile when the complex is heteroleptic, thephotoluminescent and electroluminescent quantum efficienciesare very high, approaching unity for fac-Ir(ppy)3 and its

derivatives. Moreover, the iridium(III) complexes are thermallystable and relatively chemically inert. In particular, cationiciridium complexes have been incorporated into a wide variety ofapplications from light-emitting electrochemical cells (LEECs)for use in large-scale flat panel lighting,3 to chemosensors,4 tobiolabelling agents,5 to solar fuels6 and as photosensitizers fororganic transformations.7 Most of these studies have concen-trated on mononuclear complexes.

Taking into account the rich number of examples of polymersthat include iridium(III),8 there is in fact a paucity of examples ofoligohomonuclear iridium complexes, of which only a smallsubset include charged iridium centers (Chart 1). The scarcity ofliterature examples stems in part from the inefficient synthesesand nontrivial purifications that were the object of several earlyinvestigations. These oligonuclear complexes are promisingcandidates for opticoelectronic devices and solar energy conver-sion.9 To the best of our knowledge, Haasnoot, Balzani andco-workers were the first to explore homodinuclear iridium com-plexes.10 As part of a more exhaustive study, they explored aneutral di-iridium complex, 1, with the metals bridged through a3,5-bis(pyridin-2-y1)-1,2,4-triazole. Cyclic voltammetry (CV)studies indicate that the two Ir centers are electronically coupledthrough the HOMO with irreversible oxidation waves at 1.13and 1.30 V vs. SCE in acetonitrile (ACN). Similarly, Tsuboyamaand co-workers reported a dinuclear complex with iridiumcenters bridged through a 1,4-bis(pyridin-2-yl)benzene

†Electronic supplementary information (ESI) available: Absorption,emission and excitation spectra for each complex. Half-wave CV curvesfor 10–13. Full computational output, including figures of calculatedabsorption spectra, tables of 100 lowest energy transitions calculated byTDDFT and tables of calculated emission energies and isodensitysurface plots for selected orbitals. 1H NMR and 13C NMR spectra for10–13. See DOI: 10.1039/c2dt30836c

Département de Chimie, Université de Sherbrooke and the CentreQuébécois sur les Matériaux Fonctionnels, Sherbrooke, QC, Canada,J1K 2R1. E-mail: pierre.harvey@usherbrooke.ca,eli.zysman-colman@usherbrooke.ca; Fax: +819-821-8017

9382 | Dalton Trans., 2012, 41, 9382–9393 This journal is © The Royal Society of Chemistry 2012

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unit (2).11 They observed a large red-shift in the 298 K emissionspectrum (λem 665 nm) compared to fac-Ir(ppy)3 with a photo-luminescent quantum efficiency, ΦPL, one order of magnitudelower at 4%. Bruce and co-workers recently reported on a seriesof green-yellow, highly luminescent (ΦPL ca. 50% in DCM)dinuclear metallomesogens bearing a tetraacetylethane bridgingligand.12 Lee, Do and co-workers elongated the bridging ligand,employing a 1,3-bis(3-phenyl-3-oxopropanoyl)benzene, towardsthe study of dinuclear complex 3.13 The resulting photophysicalbehavior pointed to an electronic decoupling between the twometals while the first oxidation was found to occur through a2-electron process. By contrast, even though they employed asimilar spacer geometry, placing the iridiums 7.47 Å apart,Calborean, Mazzanti and co-workers14 demonstrated the 298 Kemission profile for neutral piconilate-derived bis(iridium) com-plexes 4a–4b to be slightly red-shifted and with ΦPL and τereduced compared to mononuclear systems. DFT calculations

demonstrated that for each of 4a and 4b, the LUMO spans thebridging 2,2′:6′,2′′-terpyridine-6,6′′-dicarboxylic acid, implyinga significant electronic communication between metal centers.The shortest bridging group reported to date is a cyanide ligand,which promoted the formation of a highly luminescent tetra-nuclear assembly, [{Ir(ppy)2(CN)}4], with photophysical andelectrochemical properties reminiscent of the mononuclearcyanide complex.15

Neve, Campagna and co-workers investigated two cationichomodinuclear iridium complexes with metals linked through adiarylester-type moiety.16 In their two examples (5a and 5b),cyclic voltammetry studies demonstrated a two-electron oxi-dation process at ca. 1.18 V vs. SCE, an indication that the twoiridium centers are electronically decoupled, while the intensityof the metal-to-ligand charge transfer (1MLCT) absorption bandswere found to be additive compared to mononuclear analogs andthe existence of photoluminescent energy transfer could not be

Chart 1 Reference homodinuclear iridium complexes.

This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 9382–9393 | 9383

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conclusively assigned from an analysis of their luminescentproperties. De Cola and co-workers investigated two homodinuc-lear systems (6 and 7a) wherein the two iridium centers werelinked through an oligo(para-n-phenylene) spacer (n = 3, 4).17

As in the previous study, absorption bands were found to betwice as intense in 6 and 7a compared to a mononuclear com-pound while their luminescence and transient absorption profilesindicate weak electronic coupling between metal units. Replace-ment of the ppy ligands with 2-(2,4-difluorophenyl)pyridines(dFppy) (7b) results in an expected blue-shift in emission from606 to 546 nm.18 Biexponential decay kinetics was observed atboth 77 and 298 K, attributed to an additional contribution fromthe photoluminescentally active bridge.

Barbieri, Collin, Sauvage and co-workers19 described a cat-ionic bimetallic complex (8), employing a 3,8-dipyridyl-4,7-phe-nanthroline bridging ligand, wherein they were able to isolateeach of the three diastereomers through the use of a chiral coun-terion. The photophysical behaviour for the three diasteromerswas found to be identical with λem at 654 nm in ACN solutionwith a 295 K ΦPL and τe, of ca. 0.6% and 46 ns, respectively,implying conjugation throughout the ancillary ligand and elec-tronic coupling between the two metals. Bolink, Ortí and co-workers recently investigated the incorporation of dinuclearassemblies (9a–9b) for LEEC applications.20 Owing to thenature of the bridging unit, the two metal centers are electroni-cally decoupled with emission found to be very weak due to thenon-innocence of the bridge. Oddly, the electrochemical gap forthe two are not manifestly dissimilar, implying that the first oxi-dation is not localized about the iridium, unlike most cationiciridium complexes, a conclusion supported by DFT calculations,which place the HOMO on the conjugated linker. Two-layerLEEC devices operating at 3 V were found to be poorly lumine-scent (10 cd m−2) with external quantum efficiencies (EQE) onthe order of 0.13%.

Recently, Chandrasekhar, Bera et al. isolated a series of cat-ionic dinuclear and trinuclear assemblies.21 Photophysicalcharacterization for a trinuclear assembly bridged through twonaphthyridine–triazole ligands reveals a somewhat electronicallycoupled set of iridium metals, evidenced by the red emission,bathochromically shifted compared to a mononuclear modelsystem.21a The same group more recently reported a series ofbridged dinuclear iridium complexes (bridging ligands including1,2-bis(4-pyridyl)ethane (bpa), 1,3-bis(4-pyridyl)propane (bpp),trans-1,2-bis(4-pyridyl)-ethylene, (bpe) N,N′-bis(2-pyridyl)-methylene-hydrazine, (abp) N,N′-(bis(2-pyridyl)formylidene)-ethane-1,2-diamine, (bpfd).22 Solely in the cases of the dimer

incorporating conjugated ligand motifs (bpe or abp) was someelectronic coupling observed by CV studies.

Quici and co-workers,23 Whittle and Williams24 and Barigelletti,Collin and co-workers25 have each investigated dinuclearbis(terdentate) iridium systems. In the former case, the bindingmotif about each Ir was identical and spanned through a m-ter-phenyl unit with clear indication that the two Ir metals do notcommunicate with each other. In the latter two cases, severaldimetallic assemblies were investigated with oligo(para-n-phe-nylene) bridging units (n = 2, 3). Similar to that observed byDe Cola and co-workers,18 the p-phenylene spacer was found tobe photophysically non-innocent. When the two iridiumspossess the same coordination environment, markedly reducedelectronic communication was observed.24a,25 However, whentwo iridiums possessed different coordination environments andoxidation states, CV and PL studies implied a substansive inter-action between the two Ir metals with weak emission at607 nm.24b Not surprisingly, when the bridging p-phenylene isremoved, electronic coupling is substantially reinforced.25

From this survey of the polyhomonuclear iridium complexliterature, with but two exceptions (e.g., 4a, 4b)14,24b assembliesexhibiting hybrid photophysical behavior with electronicallycoupled iridiums are limited to examples with polytopic hetero-cyclic ancillary ligands. Herein, we report two structurallyrelated examples of homodinuclear complexes (11 and 13)possessing hybrid photophysical and electrochemical behavior(Chart 2). Their properties are contrasted with respective mono-nuclear model systems (10 and 12). A combined density func-tional theory (DFT) and time-dependent DFT (TDDFT) studypoints to conjugation through the ancillary ligand, affecting theLUMO energy and promoting a significant red-shift in the emis-sion spectrum.

Experimental section

Synthesis

General procedures. Commercial chemicals were used as sup-plied. All experiments were carried out with freshly distilledanhydrous solvents obtained from a Pure SolvTM solvent purifi-cation system from Innovative Technologies except wherespecifically mentioned. Triethylamine (Et3N), N,N-diisopropyl-amine (i-Pr2NH) were distilled over CaH2 under a nitrogenatmosphere. CuI,26 [(ppy)2Ir-μ-Cl]2,

27 [(diFMeppy)2Ir-μ-Cl]2,28

2-(2,4-difluorophenyl)-5-methylpyridine,29 5-bromo-2-iodo-pyridine,5 5-bromo-2,2′-bipyridine,30 5-trimethylsilylethynyl-

Chart 2 Complexes in study.

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2,2′-bipyridine,30 and 5-ethynyl-2,2′-bipyridine,30 were purifiedor prepared following literature procedures. Complexes 10 and12 were formed following respective dimer cleavage with5-ethynyl-2,2′-bipyridine, following the procedure of Non-oyama.27 All reagents wherein the synthesis is not explicitlydescribed were purchased and used without further purification.Flash column chromatography was performed using silica gel(Silia-P from Silicycle, 60 Å, 40–63 μm). Analytical thin layerchromatography (TLC) was performed with silica plates withaluminum backings (250 μm with indicator F-254). Compoundswere visualized under UV light. 1H and 13C NMR spectra wererecorded on a Bruker Avance spectrometer at 400 MHz and100 MHz, respectively, Bruker Avance spectrometer at 300 MHzand 75 MHz, respectively or a Varian INOVA spectrometer at600 MHz and 151 MHz, respectively. The following abbrevia-tions have been used for multiplicity assignments: “s” forsinglet, “d” for doublet, “t” for triplet and “m” for multiplet.Deuterated chloroform (CDCl3) or acetonitrile-d3 (CD3CN) wereused as solvents of record. Melting points (Mps) were recordedusing open-end capillaries on a Meltemp melting point apparatusand are uncorrected. GC-MS samples were performed on aShimadzu QP 2010 Plus equipped with a HP5-MS 30 m ×0.25 mm ID × 0.25 μm film thickness column. High resolutionmass spectra were recorded on either a VG Micromass ZAB-2For a Waters Synapt MS G1 (ESI-Q-TOF) at the Université deSherbrooke.

[Ir(ppy)2(5-ethynyl-2,2′-bipyridine)] hexafluorophosphate (10).Red solid (Yield: 76%). The characterization matched that foundfor our previous study.30

Bis[Ir(ppy)2(5-ethynyl-2,2′-bipyridine)] hexafluorophosphate (11).A dry flask was charged with 10 (76 mg, 0.09 mmol, 1 equiv.),CuCl (9.2 mg, 0.09 mmol, 1 equiv.) and pyridine (20 mL). Themixture was stirred under O2 for 12 h. The crude solution waspurified by flash chromatography using DCM on silica gel andthen with 10% MeOH/DCM on silica gel to yield 57 mg of redsolid (Yield: 76%). Rf: 0.44 (10% MeOH/DCM). Mp: 215 °C(dec). 1H NMR (400 MHz, CD3CN) δ (ppm): 8.52 (d, J =6.4 Hz, 4H), 8.20 (d, J = 8.6 Hz, 4H), 8.14 (t, J = 8.0 Hz, 6H),8.06 (d, J = 7.1 Hz, 6H), 7.99 (d, J = 5.2 Hz, 10H), 7.93–7.73(m, 4H), 7.69–7.50 (m, 4H), 7.17–6.99 (m, 2H), 6.93 (dd, J =17.0, 10.1 Hz, 2H), 6.34–6.17 (m, J = 18.0, 7.4 Hz,4H).13C NMR (151 MHz, CD3CN) δ (ppm): 172.55, 172.44,161.26, 160.00, 158.86, 156.16, 154.94, 154.81, 154.68, 154.56,149.42, 149.18, 147.41, 144.66, 143.90, 136.89, 136.64, 135.67,134.24, 130.82, 130.24, 130.17, 129.56, 128.87, 128.72, 128.00,127.97, 127.31, 125.33, 125.16, 84.10, 83.54. LR-MS(EI, 70 eV) (m/z): 679 (M2+); 501, 236, 164. HR-MS (EI,70 eV): Calculated (C72H46F8Ir2N8): 680.1557 (M2+); Found:680.1508 (M2+).

[Ir(diFMeppy)2(5-ethynyl-2,2′-bipyridine)] hexafluorophosphate(12). The dimeric complex [(diFMeppy)2Ir-μ-Cl]2 (130 mg,0.13 mmol, 0.45 equiv.) was dissolved in DCM (6 mL) andmethanol (6 mL) and 5-ethynyl-2,2′-bipyridine (53 mg,0.29 mmol, 1 equiv.) was added. The mixture was heated to60 °C over 18 h. The solution was cooled to RT and extractedwith water (3 × 50 mL), then washed with ether (3 × 50 mL) toremove unreacted bipyridine. To the aqueous solution wasslowly added a solution of NH4PF6 (10 mL, 10% w/w in H2O)under gentle stirring. The first drop caused the precipitation of an

orange solid. The suspension was conserved at 0 °C for 2 h andfiltered and the resulting solid was washed with cold water toyield 146 mg of an orange solid (Yield: 55%). Rf: 0.54 (10%MeOH/DCM). Mp: 190 °C (dec). 1H NMR (300 MHz, CDCl3)δ (ppm): δ 8.76 (d, J = 8.4 Hz, 2H), 8.22 (dd, J = 14.9, 7.3 Hz,4H), 7.92 (d, J = 2.2 Hz, 2H), 7.65 (d, J = 7.8 Hz, 2H),7.57–7.49 (m, 1H), 7.18 (d, J = 5.9 Hz, 2H), 6.56 (ddd,J = 11.7, 4.8, 2.3 Hz, 2H), 5.62 (ddd, J = 13.6, 8.3, 2.1 Hz, 2H),3.44 (s, 1H), 2.21 (s, 3H), 2.18 (s, 3H). 13C NMR (151 MHz,CDCl3) δ (ppm): δ 164.18 (dd, J = 13.2, 5.3 Hz), 162.48 (dd,J = 12.3, 6.3 Hz), 161.81 (dd, J = 12.6, 8.0 Hz), 161.50 (dd,J = 14.5, 7.0 Hz), 160.08 (dd, J = 12.6, 8.1 Hz), 154.87, 154.68,152.43 (d, J = 6.4 Hz), 152.24 (d, J = 6.6 Hz), 152.16, 149.94,147.91, 147.85, 143.18, 140.56, 140.20, 140.18, 139.99, 134.26,134.22, 128.69, 127.43, 126.55, 125.55, 123.93, 123.49, 123.38(d, J = 5.0 Hz), 123.26, 113.87 (dd, J = 17.4, 1.9 Hz),113.70 (dd, J = 17.7, 2.1 Hz), 99.32 (t, J = 26.5 Hz), 99.27 (t,J = 20.7 Hz), 85.94, 77.85, 18.34, 18.26. LR-MS (EI, 70 eV):781 (M+), 601. HR-MS (EI, 70 eV): Calculated (C36H24F4IrN4):781.1569. Found: 781.1563.

Bis[Ir(diFMeppy)2(5-ethynyl-2,2′-bipyridine)] hexafluorophos-phate (13). A dry flask was charged with 12 (35 mg,0.037 mmol, 1 equiv.), CuCl (4 mg, 0.04 mmol, 1 equiv.) andpyridine (10 ml). The mixture was stirred under O2 for 12 h. Thecrude solution was purified by flash chromatography using DCMon silica gel and then with 10% MeOH/DCM on silica gel toyield 19 mg of red solid (yield: 56%). Rf: 0.36 (10%MeOH/DCM). Mp: 240 °C (dec). 1H NMR (300 MHz, CD3CN)δ (ppm): δ 8.63 (s, 2H), 8.56 (d, J = 8.3 Hz, 4H), 8.32–8.12 (m,8H), 8.10–7.94 (m, 4H), 7.76 (d, J = 7.1 Hz, 4H), 7.63–7.49 (m,2H), 7.44 (s, 2H), 7.37 (d, J = 8.0 Hz, 2H), 6.69 (t, J = 9.9 Hz,4H), 5.71 (dd, J = 18.4, 8.8 Hz, 2H), 2.21 (s, 6H), 2.18 (s, 6H).13C NMR (75 MHz, CD3CN) δ (ppm): δ 169.15 (dd, J = 12.3,3.8 Hz), 167.46 (dd, J = 12.3, 3.8 Hz), 166.97 (d, J = 13.0 Hz),166.12 (dd, J = 33.2, 7.0 Hz), 165.25 (d, J = 12.7 Hz), 161.09,159.91, 159.19, 158.27 (d, J = 6.4 Hz), 156.44, 157.98 (d,J = 6.6 Hz), 154.97, 154.72, 154.44, 147.85, 145.54 (d, J =5.1 Hz), 145.01, 144.93, 141.19, 139.90, 134.37, 133.34 (d,J = 28.3 Hz), 131.17, 129.92, 129.08, 128.66, 128.51 (d, J =7.9 Hz), 128.35, 127.52, 119.25 (dd, J = 18.3, 2.0 Hz), 118.91(dd, J = 18.1, 2.4 Hz), 103.99 (t, J = 28.1 Hz), 84.16, 83.94,22.41, 22.38. LR-MS (EI, 70 eV) (m/z): 780 (M2+); 601.HR-MS (EI, 70 eV): Calculated (C72H46F8Ir2N8): 780.1494(M2+); Found: 780.1481 (M2+).

Photophysical characterization. All samples were prepared in2-methyltetrahydrofuran (2-MeTHF), which was distilled overCaH2 under nitrogen or in HPLC grade acetonitrile (ACN) forthe external reference, [Ru(bpy)3](PF6)2. Absorption spectrawere recorded at room temperature and at 77 K in a 1.0 cmcapped quartz cuvette and an NMR tube inserted into a liquidnitrogen filled quartz dewar, respectively, using a ShimadzuUV-1800 double beam spectrophotometer. Molar absorptivitydetermination was verified by linear least squares fit of valuesobtained from at least three independent solutions at varyingconcentrations with absorbances ranging from 0.01–2.6. Steady-state emission spectra were obtained by exciting at the lowestenergy absorption maxima using a Horiba Jobin Yvon Fluoro-log-3 spectrofluorometer equipped with double monochromatorsand a photomultiplier tube detector (Hamamatsu model R955).

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Emission quantum yields were determined using the opticallydilute method.31 A stock solution with absorbance of ca. 0.5 wasprepared and four dilutions were prepared with dilution factorsof 40, 20, 13.3 and 10 to obtain solutions with absorbances ofca. 0.013, 0.025, 0.038 and 0.05, respectively. The Beer–Lambert law was found to be linear at the concentrations of thesolutions. The emission spectra were measured after the sol-utions were rigorously degassed with solvent-saturated nitrogengas (N2) for 20 min prior to spectrum acquisition using septa-sealed quartz cells from Starna. For each sample, linearitybetween absorption and emission intensity was verified throughlinear regression analysis and additional measurements wereacquired until the Pearson regression factor (R2) for the linear fitof the data set surpassed 0.9. Individual relative quantum yieldvalues were calculated for each solution and the values reportedrepresent the gradient value. The equation Φs = Φr(Ar/As)(Is/Ir)-(ns/nr)

2 was used to calculate the relative quantum yield of eachof the sample, where Φr is the absolute quantum yield of thereference, n is the refractive index of the solvent, A is the absor-bance at the excitation wavelength, and I is the integrated areaunder the corrected emission curve. The subscripts s and r referto the sample and reference, respectively. A solution of[Ru(bpy)3](PF6)2 in ACN (Φr = 0.095) was used as the externalreference.32 The experimental uncertainty in the ΦPL is conserva-tively estimated to be 10%, though we have found that statisti-cally we can reproduce PLQYs (ΦPL) to 3% relative error.A pulsed NanoLED at 341 nm (pulse duration <1 ns; fwhm =14 nm), mounted directly on the sample chamber at 90° to theemission monochromator, was used to excite the samples andphotons were collected using a FluoroHub from Horiba JobinYvon single-photon-counting detector. The luminescence life-times were obtained using the commercially available HoribaJobin Yvon Decay Analysis Software version 6.4.1, softwareincluded within the spectrofluorimeter.

Electrochemical characterization. Cyclic voltammetry wereperformed on an Electrochemical Analyzer potentiostat model600D from CH Instruments. Solutions for cyclic voltammetrywere prepared in ACN and degassed with ACN-saturated nitro-gen bubbling for ca. 15 min prior to scanning. Tetra(n-butyl)-ammonium hexafluorophosphate (TBAPF6; ca. 0.1 M in ACN)was used as the supporting electrolyte. It was recrystallized twicefrom EtOH and dried under vacuum prior to use. A non-aqueousAg/Ag+ electrode (silver wire in a solution of 0.1 M AgNO3 inACN) was used as the pseudo-reference electrode; a glassy-carbon electrode was used for the working electrode and aPt electrode was used as the counter electrode. The redox poten-tials are reported relative to a saturated calomel (SCE) electrodewith a ferrocenium/ferrocene (Fc+/Fc) redox couple as aninternal reference (0.40 V vs. SCE).33

Computational methodology. Calculations were performed withGaussian 0934 at the Université de Sherbrooke with the Mam-mouth super computer supported by Calcul Québec. The DFT35

and TDDFT36 were calculated with the B3LYP37 method. The3-21G*38 basis set was used for C, H and N, and VDZ (valencedouble ζ) with SBKJC effective core potentials38a,39 for iridium.The predicted phosphorescence wavelengths were obtained byenergy differences between the triplet and singlet optimizedstates (ΔSCF method).40 The calculated absorption spectraand related MO contributions were obtained from the TD-DFT/

singlets output file and GaussSum 2.1.41 ATHF quantum mecha-nical continuum solvation model was employed.42

Results and discussion

Synthesis

Mononuclear complexes 10 and 12 were obtained in good yieldvia cleavage of the respective μ-dichloro iridium dimers inrefluxing 1 : 1 DCM–MeOH. The corresponding homodinuclearcomplexes 11 and 13 were also obtained in good yield throughGlaser43 coupling reactions with two equivalents of 10 and 12,respectively, in the presence of stoichiometric CuCl under anoxygen atmosphere. The aromatic peaks within the 1H NMRspectra of 11 and 13 were broadened compared to 10 and 12,respectively, owing to the presence of diasteromeric mixture ofisomers (ΛΛ, ΛΔ and ΔΔ, Fig. 1). The distinct 1H NMR signalsof the methyl groups at ca. 2.2 ppm in 12 and 13 illustrateclearly the formation of the diastereomeric mixtures. Owing tothe C1-symmetry of 12, the two methyls are diastereotopic andtwo singlets are observed. In 13, owing to the pseudo-C2 sym-metry present on the NMR time scale, the four methyl groups,which are now slightly more deshielded, are discernable as twosinglets. The photophysical and electrochemical characterizationwas conducted on the mixture as it had previously been shownthat these properties are invariant with respect to stereo-chemistry.14,19 All complexes were fully characterized by 1Hand 13C NMR, melting point determination and low- and high-resolution mass spectrometry.

Electrochemistry

Electrochemical measurements for 10–13 were obtained in ACNsolution with 0.1 M n-Bu4NPF6 as the supporting electrolyte andthe results are reported in Table 1 in V versus SCE with cyclicvoltammograms (CVs) shown in Fig. 2. Complexes 10–13 eachexhibit a reversible first metal-centred Ir(III) to Ir(IV) oxidationwave extending to the cyclometallating ligand.44 For 10, thiswave is centered at 1.27 V, the same as that measured for[(ppy)2Ir(bpy)]PF6.

44 There is a single reversible reduction waveat −1.21 V assigned as being localized on the bipyridyl ligand,corroborating results obtained in DCM solution by Castellanoand co-workers.45 With electron-withdrawing fluorines incorpo-rated onto the cyclometallating unit, 12, the oxidation wave isanodically shifted by 310 mV while the first reduction wave isanodically shifted to a lesser extent (80 mV). The result is anincrease in the HOMO–LUMO gap of 230 mV, corroboratingthe blue shift observed in the 298 K emission spectra (seebelow).

The electrochemical profile for dimer 11 is more complex.The first oxidation is reversible and, at 1.30 V, is assigned to theIr(III)/Ir(IV) redox couple of one of the Ir metals, analogous to theprocess observed for 10. A second reversible oxidation wave isobserved at 1.56 Vand is assigned to the oxidation of the secondIr metal. The presence of a second distinct oxidation wave is anindication that the metal moieties are in electronic communi-cation with each other.22,46 The first reduction wave is signifi-cantly anodically shifted at −0.95 V compared to 10, indicatinga substantial stabilization of the LUMO due to the increased

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conjugation through the bipyridyl backbone. Two other revers-ible reduction waves at −1.06 and −1.44 have been assigned totwo further reduction events on the ancillary ligand as Castellanohad previously determined that reduction processes on thecyclometallating ligand in 10 occur at much larger potentials(−1.85 V). The increase in conjugation in 11 results in an overallreduction in the HOMO–LUMO gap of 230 mV comparedto 10.

For dimer 13, unlike 11, only a single pseudo-reversible1-electron oxidation wave is observed, cathodically shifted by40 mV compared to 12. It is conceivable that the second oxi-dation wave is masked by the first or alternatively it is maskedby the solvent oxidation wave. The poor resolution between thefirst and second oxidation peaks of metal complexes linked via adiyne bridge is not unprecedented as was previously observedfor diferrocenylbutadiyne.46 Three reduction waves are observed

Fig. 1 1H NMR spectra for 10–13 in CDCl3 at 298 K.

Table 1 Redox data (V vs. SCE) obtained in ACN and DFT calculated HOMO and LUMO energies for 10–13

Complex E12,ox

(V vs. SCE) ΔEp (mV) ΔE (V vs. SCE) E12,red

(V vs. SCE) ΔEp (mV)

Calculated energy(eV)

ΔEHOMO–LUMO (eV)EHOMO ELUMO

10 1.27 60 2.48 −1.21 62 −5.97 −3.04 −2.9311 1.30 60 2.25 −0.95 56 −6.13 −3.68 −2.45

1.56 58 −1.06 58−1.44 58

12 1.58 63 2.71 −1.13 57 −6.28 −3.13 −3.1513 1.54 64 2.34 −0.80c — −6.44 −3.73 −2.71

−1.20 63−1.41 63

aMeasured in ACN (ca. 1.5 mM) with nBu4NPF6 (ca. 0.1 M) as the supporting electrolyte using a scan rate of 200 mV s−1. Potentials (V) arereported vs. an SCE standard electrode and were calibrated using an internal standard Fe/Fe+ redox couple (0.40 V in ACN); see ref. 33. Reversibleand pseudo-reversible processes (ipa/ipc > 0.9) are reported as E1

2= (Epa+Epc)/2 and result from one-electron processes. See text for discussion. bDFT

calculations at the B3LYP/3-21G* for heavy atoms except lr and B3LYP/SBKJC-VDZ for lr with a THF solvent continuum model. c Irreversibleprocess.

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at −0.80, −1.20 and −1.41 V. The first wave is irreversible. TheHOMO–LUMO gap is 2.34 V, smaller by 370 mV compared tothat of 12. It is unclear why the first reduction band in 13 shouldbe irreversible.

To summarize, the behavior observed in the electrochemicalgap for each of 10–13 along with the calculated HOMO–LUMOgap (see below) point to oxidations occurring on the iridium(s)and involving the cyclometallating ligand while reductions arelocalized on the bipyridyl ligand, with some contribution fromthe metal centers. The electrochemical gap increases with the

presence of electron-withdrawing fluorines and decreases in thehomodinuclear complexes, due to increased conjugationimparted by the presence of the diyne bridge (see Fig. S6† for acompilation of oxidation and reduction half-wave cyclicvoltammograms).

Photophysical characterization. Ground-state absorption

The normalized absorption spectra for 10–13 in 298 K air-saturated 2-MeTHF solution are shown in Fig. 3, with molarabsorptivities summarized in Table 2. All four complexes showan intense band at around 260 nm, ascribed to π–π* singlet-based ligand-centered (1LC) transitions on the cyclometallatingand ancillary ligands.40b In the region between 300 and 400 nmare a series of weaker bands, attributed to metal-to-ligand (bpy)charge transfer transitions (1MLCT). At lower energy are verylow intensity bands that are due to spin-forbidden 3MLCT tran-sitions, visible owing to the strong spin–orbit coupling ofiridium. The absorption profile for 12, is very similar to that of10, a complex for which we30 and others45 had previouslyreported its complete photophysical characterization. The pres-ence of the electron-withdrawing fluorine atoms imparts aca. 15 nm blue shift and a fourfold increase in molar absorptivity

Fig. 2 Cyclic voltammograms of 10–13 in ACN (0.1 M n-Bu4NPF6)obtained at 200 mV s−1. Multiple scans are shown to demonstrate theelectrochemical stability of the complexes.

Fig. 3 Normalized absorption spectra for 10–13 in 2-MeTHF at298 K.

Table 2 Photophysical properties of 10–13 obtained in 2-MeTHF

Complex

Absorbance 298 K (nm) λmax (nm)Stokesshifts (cm−1)

ΦPLb (%)

τ (μs) kr knr

[Molar absorptivities(×104 M−1 cm−1)] 77 Ka 298 K 77 K 298 K 77 K 298 K (×105 s−1) (×105 s−1)

10 265 [2.3]; 310 [1.0]; 325 [0.8];380 [0.2]; 450 [0.l]

536 623 3570 6170 8.5 4.12 0.16 5.3 57.2

11 270 [1.6]; 350 [0.8]; 360 [0.9];385 [0.7], 455 [0.1]

556 647 3990 6520 1.4 4.53 0.62 0.2 15.9

12 250 [5.2]; 265 [5.2]; 315 [3.1];330 [2.4]; 365 [0.8]; 425 [0.1]

490 [1.0]; 527 [0.9];568 [0.5]; 625 [0.2]

561 3120 5700 5.8 4.46 0.51 1.1 18.5

13 250 [6.5]; 315 [4.1]; 355 [3.5],385 [2.5], 455 [0.2]

560 [1.0]; 605 [0.8];665 [0.3]

558 4120 4060 2.4 4.58 0.72 0.3 13.6

a [Relative intensity of emission peak]. bMeasured at 298 K using [Ru(bpy)3](PF6)2 ΦPL = 9.5% in ACN as the external standard, see ref. 32.

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in the CT bands. Homodimeric complexes 11 and 13 possess1MLCT bands that are about 20–25 nm bathochromically shiftedcompared to their mononuclear counterparts. Such an observedred-shift is consistent with increased conjugation on the ancillaryligand.

Emission

The 298 K and 77 K emission profiles for 10–13 were recordedin 2-MeTHF solution and are shown in Fig. 4. The emissionprofiles for 10–13 at 298 K are broad and featureless with λmax

values spanning a range from 558 to 647 nm, attributed to amixed ligand-to-ligand and metal-to-ligand charge transfer(3LLCT–3MLCT) involving the ancillary ligand, commonlyobserved for cationic heteroleptic iridium complexes40b,47 andcorroborated by DFT calculations (see computational section).The emission profile for 12 is significantly blue-shifted(1770 cm−1) compared to 10 as the fluorine atoms are known tostabilize the HOMO of this class of iridium complexes.44 Bycontrast, a red-shift of 600 cm−1 for the emission λmax combinedwith broadened spectral features is observed in 11 compared to10 due to the increase in conjugation of the ancillary ligand.Such a red-shift in emission denotes the existence of electroniccommunication between the two metal subunits. Oddly, theemission profile for 13 was found to be essentially identical tothat for 12 at 298 K, which was not observed at 77 K. The pres-ence of a large rigidochromic shift of the emission spectra at77 K for 10–12 complexes indicates significant CT characterduring emission. Such a hypsochromic shift is expected in theglass state as reorganization of solvent molecules is inhibited inthe frozen medium. Surprisingly, there is an absence of a rigido-chromic shift at 77 K for 13, which, coupled with the fine struc-ture in the emission spectrum, would seem to point to greater LCcharacter for this complex. Indeed, the 77 K emission spectra forfluorinated complexes 12 and 13 exhibit significant fine structurewhile those of 10 and 11 remain featureless though their respect-ive emission envelopes have narrowed somewhat compared totheir RT emission spectra. Homodinuclear 11 and 13 exhibitemission spectra maxima that are red-shifted by 670 and2550 cm−1, respectively, from 10 and 12. Excitation spectra for

10–13 (Fig. S1–S4†) accurately reproduce the low energy CTfeatures found in the respective absorption spectra.

Photoluminescent quantum efficiencies (ΦPL) for 11 and 13were found to be moderately decreased compared to their mono-nuclear counterparts. Interestingly, unlike the comparison inphotophysical behavior between [(ppy)2Ir(bpy)](PF6) (λmax =602 nm, ΦPL = 9.3% in ACN) and [(dFMeppy)2Ir(bpy)](PF6)(λmax = 527 nm, ΦPL = 61.4% in ACN; dFMeppy = 2-(2,4-difluorophenyl)-5-methylpyridine),44 there was not the expectedobserved increase in ΦPL upon blue-shifting of the emissionspectrum in 10 vs. 12. This is due to a five-fold decrease in kr,which offsets the three-fold decrease in knr. The vibrations of theethynyl group thus seem to be an outlet for non-radiative decay.Emission intensity was found to decay monoexponentially atboth 77 and 298 K, with τe at 77 K on the order of 4 μs for eachof the four complexes. Lifetimes at 298 K increased modestlyfor homodinuclear complexes 11 and 13 (0.62 and 0.72 μs,respectively) compared to 10 and 12, respectively (0.16 and0.51 μs, respectively). Radiative decay constants, kr, were thusfound to decrease by an order of magnitude for the homodinuc-lear complexes, while knr values were found to decrease onlyslightly. Noteworthy, both ΦPL and τe were each found to be anorder of magnitude larger in 11 and 13 than observed for 8,resulting in kr values significantly lower than calculated for 8(1.3 × 105 s−1).19

Computational modeling

DFT and TDDFT calculations were undertaken in order toascertain and understand the origins of the electronic structuresand transitions of these systems. We have previously demon-strated that employing B3LYP/3-21G* on heavy atoms andB3LYP/SBKJC-DVZ on Ir provides accurate predictions forground and excited state properties in cationic iridium com-plexes.30,40b,44,48 In order to best emulate the experimental con-ditions, a quantum mechanical THF solvation continuum modelwas incorporated into our computational protocol.42 In order tounderstand the impact of dimerization, the calculated orbitalenergies for the 15 highest occupied and of the 15 lowest unoc-cupied molecular orbitals (MOs), together with contour plots forselected relevant MOs for 10–13 are shown in Fig. 5. For the

Fig. 4 Normalized emission spectra for 10–13 in 2-MeTHF at (a) 298 K and (b) 77 K.

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computations, 10-TMS and 12-TMS, which are trimethylsilyl-ethynyl versions of 10 and 12 were used as model systems for10 and 12, respectively. The TMS groups serve as spectroscopi-cally passive dummy fragment.

The HOMO (H) for each of the four complexes possesses sig-nificant contribution from both the πppy orbitals and the Ir dπorbitals, similar to that observed for other heteroleptic cationiciridium complexes. Homodinuclear 11 and 13 possess pseudo-degenerate H and H−1 orbitals. Generally, the electron densitydistribution for next 5 highest-energy occupied MOs, are alllocalized principally on the πppy orbitals with some contributionfrom the metal. The LUMO (L) for each of the four complexesis essentially exclusively π*bpy in nature, extending out onto theethynyl unit. For 11 and 13, the orbital picture for the L shows aconjugated system across both bpy units. The L+1 is signifi-cantly destabilized for all four complexes. For 10-TMS, the L+1is destabilized by 0.99 eV and is a mixed π*ppy and Ir dπ*system while for 12-TMS the destabilization is slightly larger at1.04 eV but the L+1 is better characterized as a mixed π*ppy,π*bpy and Ir dπ* system. The orbital symmetry for the L+1 orbi-tals for both 11 and 13 is similar to that of their respectiveLUMOs, whereby the main difference between the L and L+1 is

the implication of the other orthogonal π* orbitals of the ethynylunits. The L+1 orbital in each of 11 and 13 is destabilized by0.56 and 0.44 eV, respectively, compared to the LUMO. To sum-marize, the computations point to a series of low energy mixedLLCT/MLCT transitions, corroborating the assignments madefor the absorption spectra.

Addition of the two fluorines onto the cyclometallating ligandresults in a stabilization of the HOMO by 0.31 eV. The magni-tude of this stabilization is the same in both the mononuclear(10-TMS vs. 12-TMS) and homodinuclear systems (11 vs. 13).There is also a weak stabilization of the LUMO, indicating thatthere is some electronic coupling mediated through the metalcontribution present in both the HOMO and the LUMO. Themagnitude of this stabilization is 0.09 and 0.03 eV for the mono-and homodinuclear systems, respectively. The result of incorpor-ation of fluorine atoms is a net increase of the HOMO–LUMOgap of 0.22 and 0.28 eV, respectively for the mono- (12) andhomodinuclear (13) systems, which explains the net observedblue shift in the emission spectra (Fig. 5). The effect of increas-ing the conjugation along the bpy backbone in the homodinuc-lear complexes is a stabilization of both the HOMO and theLUMO. The LUMO stabilization calculated is significant at 0.64

Fig. 5 Calculated energy level scheme for the Kohn–Sham orbitals between HOMO−15 to LUMO+15 of 10-TMS, 11, 12-TMS and 13, includingcontour plots of selected most relevant MOs and the associated TDDFT calculated HOMO–LUMO energy gap (in eV). Selected MOs have beencolored to aide in the analysis. The HOMO and HOMO−1 for 11 and 13 are quasi-degenerate. Green indicates electron density located on both πppyand Ir dπ orbitals, red indicates electron density localized mainly on the π*bpy with minor contribution from the metal.

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and 0.59 eV, respectively, for 11 and 13 compared to 10-TMSand 12-TMS. Here again, the presence of electron density on theIr in both the HOMO and the LUMO also results also in a stabil-ization of the former at 0.16 eV for the homodinuclear systems.The HOMO–LUMO gap for the homodinuclear speciesdecreases by between 0.42–0.48 eV, corroborating the behaviorobserved in the 77 K emission spectra as well as the CV data.

TDDFT calculations of the 100 lowest energy vertical spin-allowed excitation transitions were performed to permit a com-parison of the experimental and calculated absorption spectra.Detailed assignments for these transitions are reported in TablesS5–S9† and they are overlayed over the experimentally obtainedabsorption spectra as vertical bars whose height denotes theirrespective oscillator strength (Fig. S5–S8†). A summary of themost important low energy transitions is shown in Table 3.For each of the four complexes under investigation, theHOMO → LUMO transition is essentially symmetry forbidden,with oscillator strengths (f ) computed to be smaller than 0.0002.The principal low energy features (λ > 350 nm) in the absorptionspectrum for 10 are modeled as a mixture of states (S3–S5) ofsimilar intensity (f ca. 0.047) in 10-TMS, exhibiting mainly amixed MLCT/LLCT character; the S4 state exhibits significantligand centered (LC) πppy → π*ppy character. Similarly for 12,the spectral region at energies below 350 nm is modeled in12-TMS as a mixture of states with dominant contributions fromH−1 → L (S3), H−4 → L (S5), H → L+1 (S6), all characterizingmixed MLCT/LLCT transitions. Low energy features in 11 and13 are red-shifted owing to the increased conjugation presentin the ancillary ligand. The low energy absorption bands

(λ > 450 nm) for 11 are characterized as emanating mainlyfrom a superposition of S3 (H → L+1), S5 (H−4 → L) and S7(H−2 → L) states while those for 13 are reproduced primarilythrough contributions from S3 (H−2 → L), S5 (H−4 → L). Thenature of each of these transitions is MLCT/LLCT. Those tran-sitions at higher energy and intensity (λ ca. 400 nm for 11 andca. 390 nm for 13) are characterized as possessing mixed LCbpy

and MLCT/LLCT character. Overall, though the computationsoverestimate the permissibility (f) of the mixed transitions between390–400 nm in both 11 and 13, the gross features of the experi-mental absorption spectra are reproduced by the computations.

The mixed MLCT/LLCT transition is also clearly operationalduring emission from the T1 state for 10–13 as evidenced by thespin density distribution calculations (Fig. 6), which correspondto a superposition of the electron density topologies of theHOMO and LUMO. The spin density maps for 11 and 13 illus-trate the increased conjugation across the entire ancillary ligand.Emission energies were calculated using the ΔSCF approach,which is the difference in total energy of the complex betweenthe T1 and S0 states, possessing a geometry optimized at thatrespective state.40b Emission energy values obtained for10-TMS, 11, 12-TMS and 13 are, respectively, 574, 650, 522 and565 nm. The predictions compare favorably to the 77 K emissionmaxima for mononuclear complexes 10 (λmax = 536 nm) and 12(λmax = 490 nm) as well as 13 (λmax = 560 nm). For 11, the pre-diction is red-shifted compared to the experimental λmax (λmax =556 nm). Notably, the predicted emission energy for the homo-dinuclear complexes matches quite closely the 298 K λem values(11, λmax = 647 nm; 13, λmax = 560 nm). A potential explanation

Table 3 Principal theoretical low energy electronic transitions (ΔE), with corresponding oscillator strengths (f), and assignments for 10-TMS, 11,12-TMS and 13a

Complex State

f Major contributions Assignment(nm) (eV)

10-TMS S1 550.7 2.25 0.0002 H → L (98%) MLCT/LLCTS3 415.3 2.99 0.0451 H−1 → L (92%) MLCT/LLCTS4 393.0 3.16 0.0489 H → L+1 (96%) LCppy/MLCTS5 390.1 3.18 0.0468 H−3 → L (87%) MLCT/LLCT

11 S1 634.1 1.96 0.0001 H → L (90%) MLCT/LLCTS2 633.6 1.96 0 H−1 → L (89%) MLCT/LLCTS3 480.3 2.58 0.0182 H → L+1 (53%) MLCT/LLCT

H−4 → L (25%) MLCT/LLCTS5 476.4 2.60 0.0335 H−4 → L (53%) MLCT/LLCT

H → L+1 (31%) MLCT/LLCTS7 468.9 2.64 0.0351 H−2 → L (83%) MLCT/LLCT

12-TMS S1 500.9 2.48 0.0001 H → L (98%) MLCT/LLCTS3 413.8 3.00 0.0202 H−1 → L (92%) MLCT/LLCTS5 374.5 3.31 0.0724 H−4 → L (93%) MLCT/LLCTS6

b 361.4 3.43 0.0503 H → L+1 (75%) MLCT/LLCTH → L+2 (21%) MLCT/LLCT

13 S1 560.4 2.21 0.0001 H → L (85%) MLCT/LLCTS2 560.0 2.21 0 H−1 → L (85%) MLCT/LLCTS3 464.2 2.67 0.0284 H−2 → L (67%) MLCT/LLCT

H−3 → L (23%) MLCT/LLCTS5 454.2 2.73 0.0256 H−4 → L (69%) MLCT/LLCT

H−6 → L (12%) MLCT/LLCT

aH = HOMO; L = LUMO. b The electron density is not delocalized onto the alkynyl unit.

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for the poor emission prediction for 11 is that in rigid mediathere is significant twisting about the Calkyne–Calkyne bond thusinhibiting conjugation throughout the bipyridyl backbone in 11and that the computations do not accurately reproduce this con-formation. Conversely, at 298 K rotation about the Calkyne–

Calkyne bond is rapid and the computed geometry matches moreaccurately the dynamic conformation present at this temperaturefor 11, resulting in an accurate predicted emission energy calcu-lation. The computations predict a much more modest red-shiftin emission for 13 compared to 12-TMS versus 11 compared to10-TMS.

The computations corroborate clearly the experimental datathat reveal that the two metal centers in 11 and 13 are in elec-tronic communication with each other, explaining the observedred-shift in the 298 K absorption spectra, the emission profile at77 K and the smaller electrochemical gap in the CV curves com-pared to mononuclear model systems; however, the photophysi-cal and electrochemical behavior for 13 was much harder todiscern given similar emission profiles at 298 K and lack of reso-lution by CV to detect a second distinct oxidation wave. Thisstudy attests to the power of DFT calculations to accuratelymodel the electronic properties of multimetallic species andaugers well for the in silico design of new photonic materials.

Conclusion

Herein, we reported the synthesis, photophysical and electro-chemical characterization as well as theoretical investigation oftwo homodinuclear cationic iridium(III) complexes and comparedthese to mononuclear model systems. The bridging diyne unitmediates an electronic coupling between the two metal subunitsand influences the electronic properties of the dimer complexes.Future studies will be directed to finding ligand motifs that main-tain the electronic communication between metal centers withoutunduly impacting quantum efficiencies.

Acknowledgements

This research was supported by the Natural Sciences and Engin-eering Research Council of Canada (NSERC), le Fonds

Québécois de la Recherche sur la Nature et les Technologies(FQRNT), the Centre d’Études des Matériaux Optiques et Photo-niques de l’Université de Sherbrooke (CEMOPUS), and theCentre Québécois sur les Matériaux Fonctionnels (CQMF).

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