Bonding, Luminescence, Metallophilicity in Linear Au3 and Au2Ag

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Bonding, Luminescence, Metallophilicity in Linear Au3 and Au2Ag Chains Stabilized by Rigid Diphosphanyl NHC Ligands Pengfei Ai,† Matteo Mauro,‡,# Christophe Gourlaouen,§ Serena Carrara,‡ Luisa De Cola,‡ Yeny Tobon,∥ Umberto Giovanella,⊥ Chiara Botta,⊥ Andreas A. Danopoulos,*,†,# and Pierre Braunstein*,† †

Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Université de Strasbourg, 4 rue Blaise Pascal, 67081 Strasbourg, France ‡ Laboratoire de Chimie et des Biomatériaux Supramoléculaires, Institut de Science et d’Ingénierie Supramoléculaires (I.S.I.S.), Université de Strasbourg, 8 allée Gaspard Monge, 67000 Strasbourg, France § Laboratoire de Chimie Quantique, Institut de Chimie (UMR 7177 CNRS), Université de Strasbourg, 1 rue Blaise Pascal, 67008 Strasbourg, France ∥ Laboratoire de Spectrochimie Infrarouge et Raman (LASIR) (UMR CNRS 8516), Université de Lille1, Sciences et Technologies, Bât. C5, 59655 Villeneuve d’Ascq Cedex, France ⊥ ISMAC−CNR, Via Corti 12, 20133 Milano, Italy # Institute for Advanced Study (USIAS), Université de Strasbourg, 4 rue Blaise Pascal, 67081 Strasbourg Cedex, France S Supporting Information *

ABSTRACT: The heterofunctional and rigid ligand N,N′diphosphanyl-imidazol-2-ylidene (PCNHCP; P = P(t-Bu) 2), through its phosphorus and two N-heterocyclic carbene (NHC) donors, stabilizes trinuclear chain complexes, with either Au3 or AgAu2 cores, and dinuclear Au2 complexes. The two oppositely situated PCNHCP (L) ligands that “sandwich” the metal chain can support linear and rigid structures, as found in the known tricationic Au(I) complex [Au3(μ3-PCNHCP,κP,κCNHC,κP)2](OTf)3 (OTf = CF3SO3; [Au3L2](OTf)3; Chem. Commun. 2014, 50, 103− 105) now also obtained by transmetalation from [Ag3(μ3PCNHCP,κP,κCNHC,κP)2](OTf)3 ([Ag3L2](OTf)3), or in the mixed-metal tricationic [Au2 Ag(μ3-PC NHCP,κP,κC NHC,κP)2 ](OTf)3 ([Au2AgL2](OTf)3). The latter was obtained stepwise by the addition of AgOTf to the digold(I) complex [Au2(μ2PCNHCP,κP,κCNHC)2](OTf)2 ([Au2L2](OTf)2). The latter contains two dangling P donors and displays fluxional behavior in solution, and the Au···Au separation of 2.8320(6) Å in the solid state is consistent with metallophilic interactions. In the solvento complex [Au3Cl2(tht)(μ3-PCNHCP,κP,κCNHC,κP)](OTf)·MeCN ([Au3Cl2(tht)L](OTf)·MeCN), which contains only one L and one tht ligand (tht = tetrahydrothiophene), the metal chain is bent (148.94(2)°), and the longer Au···Au separation (2.9710(4) Å) is in line with relaxation of the rigidity due to a more “open” structure. Similar features were observed in [Au3Cl2(SMe2) L](OTf)·2MeCN. A detailed study of the emission properties of [Au3L2](OTf)3, [Au3Cl2(tht)L](OTf)·MeCN, [Au2L2](OTf)2, and [Au2AgL2](OTf)3 was performed by means of steady state and time-resolved photophysical techniques. The complex [Au3L2](OTf)3 displays a bright (photoluminescence quantum yield = 80%) and narrow emission band centered at 446 nm with a relatively small Stokes’ shift and long-lived excited-state lifetime on the microsecond timescale, both in solution and in the solid state. In line with the very narrow emission profile centered in the violet-blue region, fabrication of organic light-emitting devices (OLEDs) comprising the [Au3L2](OTf)3 complex demonstrated its usefulness as a deep-blue emitter in solution-processed OLEDs. Electrochemical and Raman spectroscopic studies were also performed on [Au3L2](OTf)3. Experimental results were rationalized by means of Wave-Function Theory (WFT) and Density Functional Theory (DFT). MP2 calculations gave a satisfactory description of the structures of the cationic complexes [Au3L2]3+ and [Au2L2]2+ and pointed to Au···Au interactions having an electrostatic component owing to the dissimilar charge distribution in the chain caused by the heterofunctional ligand. The nature of the emitting states and their geometric distortions relative to the ground states in [Au3L2]3+ and [Au2L2]2+ was studied by DFT, revealing contraction of the Au···Au distances and coordination geometry changes by association of the dangling P donor, respectively.

Received: May 4, 2016

© XXXX American Chemical Society

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DOI: 10.1021/acs.inorgchem.6b01095 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

N-Heterocyclic carbene (NHC) donors are electronically related to trialkylphosphines and were first studied in association with luminescent Au(I) centers in benzimidazole2-ylidene complexes by Lin and co-workers.19 Since then, many NHC Au(I) complexes have been prepared, which display interesting optical properties.1f,j,6e,7c,12 The “common” NHC ligand (i.e., N,N′-dialkyl- or N,N′-diaryl-imidazol-2-ylidene), considered to be a good σ-donor, is able to engage in strong M−CNHC bonding providing stability, while as a consequence of its weak π-accepting ability, π* orbitals located on NHC moieties lie at higher energy with respect to other, widely employed chromophoric ligands. These features led to brightly emissive excited states in higher energy regions, in particular falling into the blue to ultraviolet (UV) region of the spectrum, which is an important feature in view of possible application in light-emitting devices.7c,12 Recently, bulky N-phosphanyl- or N,N′-diphosphanylfunctionalized NHC ligands have attracted attention as tunable scaffolds, providing rigidity thanks to the neighboring donors (NHC and Nimid-PR2) separated by rather short CN and P− N bonds.20

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INTRODUCTION Complexes of the coinage metals with d10 electronic configuration have been widely investigated because of the growing interest in their underlying structural and catalytic chemistry and their diverse properties, in particular the often intriguing and remarkable photophysical properties.1 Au(I) complexes occupy a special position in this research endeavor, since strong spin−orbit coupling results in the formation of triplet excited states via efficient intersystem crossing processes.1c,d,2,3 Aurophilic interactions of dispersion/correlation origin have a crucial impact on the efficiency and energy of luminescence.1e,4 They operate intermolecularly and/or intramolecularly (in polynuclear complexes) and can in principle be modulated by the geometry of the metal core and additional chemical and physical means, e.g., ligand environment, (counter)ions, solvent, temperature, and pressure.5−7 Intramolecular Au···Au contacts can be favored by bridging ligands, as found in various multinuclear Au(I) complexes with polyfunctional ligands. The stereoelectronic characteristics and the relative arrangement of the donors, as well as the ligand backbone rigidity, contribute to the strength of the aurophilic interactions, as judged by the intermetallic distance, and the Au coordination geometry. Consequently, tuning of these parameters at the molecular or supramolecular level has opened the way to Au(I) complexes with photocatalytic properties,8 stimuli-responsive photoemission,1c,d,f,3 and potential applications in the fields of materials, sensors,6d lightemitting devices,9 and biomedicine.10 The application of computational techniques on well performing functional complexes contributes to a better understanding of the detailed mechanism(s) of the excitation and emission processes, including the nature and the energetics of subtle geometrical and structural changes that occur upon excitation, the nature of the lowest-lying exited state, and the importance and nature of metal···metal interactions and metal cooperation.2b,11 Despite the complexity and intricacy of the parameters involved, derivatives displaying photoluminescence quantum yield (PLQY) approaching in some cases unity2b,7c,12 and tunable emission color have been described. Few theoretical studies have been reported on complexes in which Au(I) cations are kept in close contact by the ligand architecture. In particular, Pyykkö and co-workers and Schwerdtfeger and co-workers1e,4 have highlighted the role of aurophilic interactions on the stability of these species. Mendizabal et al.13 showed that a correlated WFT approach (MP2 calculations) was the most suitable method to correctly describe the structure, although DFT methods were perfectly adequate to study the optical properties of a triangle of Au(I) cations. More recently, studies have evidenced that in chains of Au cations, the lowest excited states were centered on the metal chain and are at the origin of the emissive properties.1k,2b,14 Polynuclear complexes with polydentate phosphines,1b,15 such as the vapochromic complex [Au2(dppe)2]2+ (dppe = bis(diphenylphosphino)ethane)16 and the linear trinuclear systems [Au3(dpmp)2]3+ and [Au3(dmmp)2]3+ (dpmp = bis(diphenylphosphinomethyl)phenylphosphine, dmmp = bis(dimethylphosphinomethyl)methylphosphine),17 bear relevance to the present studies. A correlation between the emission properties and the metal−metal separation in linear Aunbis(polyphosphine) complexes (n = 3, 4) has been recently reported.18

The heterofunctional and rigid N,N′-diphosphanyl-imidazol2-ylidene ligand (PCNHCP)(L) (where P = P(t-Bu)2) was found to be thermally stable enough toward migration of the P(t-Bu)2 from Nimid to CNHC and to be ideally suited for the stabilization of linear, trinuclear chain complexes,1n,20c,21 amenable to finetuning of the σ donation (P vs. CNHC vs P) and conducive to potentially interesting physical properties.1j,n The design idea based on L was typically exemplified by the linear trinuclear complex [Au3(μ3-PCNHCP,κP,κCNHC,κP)2](OTf)3 ([Au3L2](OTf)3), which displays short Au···Au separations (2.7584(2) Å, cf. 2.884 Å in bulk Au).20c This system provides a unique opportunity to correlate the structure of closely spaced, electron-rich Au(I) centers with the emergence of physical and chemical properties, to study Au···Au vibrations by Raman spectroscopy and establish possible functions of metal synergism. Related P,CNHC,P donor-type ligands with a flexible ethylene spacer between the donor atoms can also support diand trinuclear gold complexes, but the N,N′-diphosphanylfunctionalized NHC imposes shorter intermetallic separation (in the range 2.70−2.80 Å).2f Interestingly, the rigid, methylene-spaced polyphosphine ligands PRPRPR (where R = Ph, Me, Cy) also support Au3 chains in the related complexes Au 3 -(PPP) 2 , which feature intermetallic separation of 3.0093(8), 2.972(1), and 2.9323(8) Å, respectively,2b,17 but the larger ligand flexibility may lead to deviations from Au3 linearity (R = Me, Ph) and occasionally to complexes of higher nuclearity.18 We report here a transmetalation route to the known [Au3L2](OTf)3, and to a related new, trinuclear gold complex [Au3Cl2(tht)(μ3-PCNHCP,κP,κCNHC,κP)]OTf ([Au3Cl2(tht) L]OTf containing a single supporting L ligand, which results in longer Au···Au separations and a less rigid, more open structure. The novel dinuclear gold complex [Au2(μ2PCNHCP,κP,κCNHC)2](OTf)2 ([Au2L2](OTf)2), with dangling P donors, and the heterotrinuclear complex [Au2Ag(μ3PCNHCP,κP,κCNHC,κP)2](OTf)3 ([Au2AgL2](OTf)3) derived from it are also described. B


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Inorganic Chemistry Scheme 1. Transmetallation of [Ag3L2](OTf)3 with [AuCl(tht)]a

a

Reagents and conditions: (i) [AuCl(tht)] ([Ag3L2](OTf)3/[AuCl(tht)] 1:3.7 molar ratio), MeCN, room temperature, under exclusion of light.

The structure of [Au3Cl2(tht)L]OTf features a Au3 array bridged by one PCNHCP (L) ligand in a κP,κC,κP bonding mode, the coordination sphere of the terminal Au atoms being completed by a chloride and that of the central Au by a tht ligand. The coordination geometry of the three Au centers is virtually linear (P1−Au1−Cl1 172.76(9)° and C1−Au2−S1 178.5(3)°). By comparison with the structure of [Au3L2](OTf)3,20c the Au1−Au2−Au1′ angle of 148.94(2)° highlights the importance of a second PCNHCP ligand in imposing Au3 linearity in the former. Deviation from linearity is accompanied by a significant elongation of the intermetallic distances (cf. 2.9710(4) Å and 2.7584(2) Å in [Au3Cl2(tht)L]OTf and [Au 3 L 2 ](OTf) 3 , respectively). Finally, the Au−C NHC (2.004(13) Å), Au−P (2.235(2) Å), and Au−Cl (2.285(3) Å) distances are in the expected ranges.2d,f,23 When [AuCl(SMe2)] was used as a Au(I) precursor, the chain complex [Au3Cl2(SMe2)(μ3-PCNHCP,κP,κCNHC,κP)]OTf ([Au3Cl2(SMe2)L]OTf) was obtained, which has structural features analogous to those of [Au3Cl2(tht)L]OTf, with the SMe2 ligand replacing tht (Figure S1 of the Supporting Information). The consequences of different metrical data in the two trinuclear complexes [Au3Cl2(tht)L]OTf and [Au3L2](OTf)3 on their photophysical properties will be discussed below (see Photophysical Characterization section). Further attempts to expand the scope of the Au coordination chemistry with L were focused on the synthesis of suitable building blocks for assembling polynuclear structures. A starting point was the synthesis of the dinuclear complex [Au2(μ2PCNHCP,κP,κCNHC)2](OTf)2 ([Au2L2](OTf)2) which could be anticipated to react with other metal precursors to form trinuclear species. The reaction of the imidazolium salt PCHP, precursor to L, with [Au{N(SiMe3)2}(PPh3)] in the presence of a small amount of L (ca. 0.17 mol equiv) in THF afforded the desired dinuclear species. The choice of the Au silylamide precursor was guided by its solubility and the presence of the strong internal base, namely -N(SiMe3)2, the conjugate acid of which is not detrimental to the P−N bonds that are sensitive to protic reagents (Scheme 2). The structure of [Au2L2](OTf)2 was established by an X-ray diffraction analysis (Figure 2, see below) which confirmed the dinuclear motif and the μ2PCNHCP,κP,κCNHC bonding mode of the ligand. However, the 31P{1H} NMR spectrum in CD2Cl2 or CD3CN solutions (two sets of singlets at δ 134.1/134.8, 110.7/110.6 ppm and δ 132.0/132.7, 119.9/119.7 ppm, in a 1:5 ratio, respectively) indicates the presence of two, presumably isomeric species, whereas in the solid-state (CP-MAS 31P NMR spectroscopy), only two singlets (δ 130.8, 115.8 ppm) were observed (see Experimental Section and Figure 3). Furthermore, a variable-temperature 31P{1H}-NMR spectroscopic study of [Au2L2](OTf)2 dissolved in precooled (−78 °C) CD2Cl2 gave rise to two singlets at δ 131.0 and 117.8 ppm,

High yield and selective syntheses of heterometallic complexes and clusters remain of considerable interest in view of the often unique structural features, chemical and physical properties associated with the presence of different metals in tunable proportions in molecules of variable nuclearities.1g,18,22 All the complexes discussed in the present work have been fully characterized and their photophysical properties and Raman spectra investigated, compared, and rationalized on the basis of computational studies.

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RESULTS AND DISCUSSION Synthesis and Characterization of the Complexes. Initial attempts were focused on optimizing the selectivity and yields of the synthesis of [Au3L2](OTf)3. Transmetalation from the known trinuclear silver(I) complex [Ag 3 (μ 3 PCNHC P,κP,κCNHC ,κP)2 ](OTf)3 ([Ag 3L2](OTf)3 ) 20c with [AuCl(tht)] (tht = tetrahydrothiophene) in acetonitrile afforded [Au3L2](OTf)3 in high yield (80%) together with a new, air stable Au(I) complex, [Au 3 Cl 2 (tht)(μ 3 PCNHCP,κP,κCNHC,κP)]OTf ([Au3Cl2(tht)L]OTf), as a byproduct (Scheme 1). The two complexes were easily separated and purified by washing the solid mixture with a small amount of CH2Cl2 (see Experimental Section). The multinuclear NMR signature of [Au3Cl2(tht)L]OTf in CD3CN included 1H NMR peaks at δ 3.33 ppm (m, 4H) and 1.87 (m, 4H) assignable to one tht molecule and at 1.53 ppm (d, 3JHP = 17.9 Hz) for the tBu groups and the 31P{1H} NMR singlet at δ 133.7 ppm, indicating equivalence of the two coordinated phosphine groups. The complex [Au3Cl2(tht)L]OTf was crystallized from ether/acetonitrile and its molecular structure is shown in Figure 1.

Figure 1. Structure of the cation in [Au3Cl2(tht)L](OTf)·MeCN. H atoms, the Me groups of the t-Bu substituents, the triflate anion, and one molecule of MeCN are omitted for clarity; ellipsoids are at the 30% probability level. Selected bond lengths (Å) and angles [deg]: Au2−C1 2.004(13), Au1−P1 2.235(2), Au1−Cl1 2.285(3), Au1−Au2 2.9710(4), Au2−S1 2.318(4); P1−Au1−Cl1 172.76(9), C1−Au2−S1 178.5(3), Au1−Au2−Au1′ 148.94(2). C


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Inorganic Chemistry

Scheme 2. Reagents and Conditions: (i) [Au{N(SiMe3)2}(PPh3)] and L (PCHP/L/[Au{N(SiMe3)2}(PPh3)] = 7.3:1:6 molar ratio), THF, 3 days, room temperature; (ii) [Au(tht)2](CF3SO3) ([Au(tht)2](CF3SO3)/[Au2L2](OTf)2 = 1:1, MeCN, room temperature; (iii) [AgOTf] ([AgOTf]/[Au2L2](OTf)2 = 1:1), MeCN, room temperature

In the solid state, [Au2L2](OTf)2 has a centrosymmetric (C2) structure, with each L ligand bridging two Au centers via P,CNHC coordination, the second P donor group remaining dangling. The Au centers adopt a linear coordination geometry (ignoring the adjacent Au center; Figure 2). This bonding situation is similar to that observed in related digold(I) complexes containing a bidentate P,CNHC ligand.20b,c,24 The orientation of the lone pair of the uncoordinated P atoms, similar to that of the coordinated P, appears well suited for the “capture” of an additional metal center. The Au−Au separation of 2.8320(6) Å is longer than in [Au3L2](OTf)3 (2.7584(2) Å). We assign this solid-state structure to the isomer A observed in solution. Since the dangling phosphine groups of [Au2L2](OTf)2 can serve to bind an additional metal atom, cf. the stepwise formation of [Au3L2](OTf)3, it is conceivable that heterotrinuclear complexes could also become accessible in an analogous way by a proper choice of metal precursors. Thus, the reaction of [Au2L2](OTf)2 with AgOTf in MeCN afforded the mixed-metal chain complex [Au2Ag(μ3PC NHC P,κP,κC NHC ,κP) 2 ](OTf) 3 ([Au 2 AgL 2 ](OTf) 3 ). Its 31 1 P{ H} NMR spectrum is very diagnostic: one singlet at δ 137.2 ppm corresponds to the P atoms coordinated to Au and the two doublets centered at δ 121.7 ppm (JP‑109Ag = 542.9 Hz, JP‑107Ag = 469.5 Hz) to the P atoms coordinated to Ag. Structural elucidation of [Au2AgL2](OTf)3 by X-ray diffraction clearly established the presence of two adjacent homoleptic Au atoms (P−Au−P and C−Au−C) in the metal chain; however, a fully occupied Ag site could not be satisfactorily refined. Since the problem persisted after numerous recrystallizations, all of which gave crystalline samples with identical spectroscopic signatures, we ascribe the refinement difficulties of the third metal center to a crystallographic artifact or packing disorder in the structure.25 A detailed discussion of the metrical data in this complex is thus not possible (see Figure S2 of the Supporting Information). In all samples, the presence of the P−Ag−P subunit was unambiguously evidenced by 31P{1H} NMR spectroscopy. The photoluminescence properties of the airstable complex [Au2AgL2](OTf)3 are discussed below.

Figure 2. Structure of the cationic complex in [Au2L2](OTf)2. H atoms, the Me groups of the t-Bu substituents and the triflate anions are omitted for clarity. Ellipsoids are depicted at the 30% probability. Selected bond lengths (Å) and angles [°]: Au1−C1 2.038(8), Au1− P1′ 2.287(2), Au1−Au1′ 2.8320(6), C1−Au1−P1′ 176.2(2), C1− N1−P1 124.1(5), C1−N2−P2 118.1(5).

corresponding to the values of the chemical shifts observed in the solid state (A in Scheme 3) that persisted between −60 and −20 °C (Figure 3). When the temperature was raised, a second species appeared (B in Scheme 3), with an initial molar ratio B/A of 1:25, which increased to 1:9 at −10 °C and finally reached a constant value of 1:5 at 10 °C. Recooling the sample to −60 °C did not modify this ratio. The chemical shifts assigned to B support the presence of both dangling and coordinated P donors, rendering B a likely isomer of A. Furthermore, B appears to be thermodynamically more stable than A (Scheme 3) and provides a favorably preorganized disposition of the dangling P atoms to capture another metal. This hypothesis would nicely explain the outcome of the reaction of [Au2L2](OTf)2 with [Au(tht)2]OTf, giving [Au3L2](OTf)3: the presence of [Au(tht)2]OTf would shift the equilibrium between A and B in favor of B, which is immediately trapped to give [Au3L2](OTf)3. Simple tetranuclear addition products of [Au(tht)2]OTf to A were conceivable (see the structure of [Au3Cl2(tht) L](OTf)·MeCN), but have not been observed, even in the presence of two equivalents of [Au(tht)2]OTf. D


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Inorganic Chemistry

Figure 3. Stack plot of 31P{1H} NMR spectra of [Au2L2](OTf)2 in CD2Cl2 at −40 °C (green), −20 °C (purple), −10 °C (blue), 10 °C (red), 25 °C (cyan). CP-MAS solid-state NMR spectrum (black, bottom).

Scheme 3. Interconversion of the Presumed Regioisomers of [Au2L2](OTf)2 As Discussed in the Text: A (Heteroleptic Au Centers, C2), B (Homoleptic Au Centers, C2v Symmetry)

Photophysical Characterization. The photophysical properties of [Au3L2](OTf)3, [Au3Cl2(tht)L]OTf, [Au2L2](OTf)2, and [Au2AgL2](OTf)3 were investigated using steadystate and time-resolved techniques. The electronic absorption spectra of the complexes in dilute MeCN or CH2Cl2 are depicted in Figure 4, and the corresponding data are listed in Table 1. The electronic absorption spectrum of [Au3L2](OTf)3 in MeCN is characterized by narrow, intense featureless bands in the ultraviolet region at λmax = 345 and 270 nm (ε = 5.21 × 104 and 1.49 × 104 M−1 cm−1, respectively). The former can be attributed to the spin- and optically allowed singlet-manifold 1 [5dσ* → 6pσ] transition with strong metal-centered (MC) character. Ideally, such an optical excitation process can be seen as corresponding to the promotion of electron density from the antibonding dz2 to the bonding 6s/6pz orbital, where z represents the Au···Au···Au axis, resulting in increased metal···metal interaction (see also Computational Studies). The higher energy absorption band corresponds to the singletmanifold transition involving the NHC units, with admixed intraligand (1IL) and ligand (phosphine) to metal−ligand (Au− NHC) charge transfer (LPMLNHC′CT). Such assignments were made by comparison with related Au(I) complexes,8d,12 also

Figure 4. Electronic absorption spectra of [Au3L2](OTf)3 (black), [Au2L2](OTf)2 (red), and [Au2AgL2](OTf)3 (green) in MeCN and [Au3Cl2(tht)L]OTf (blue) in CH2Cl2 (solid traces). Normalized emission profiles (dashed traces) for [Au3L2](OTf)3 in MeCN at room temperature (black) and at 77 K frozen matrix (gray) upon excitation at λexc = 340 nm.

bearing optically transparent phosphine ligands.2b It is noteworthy that an intense band at lower energy (ca. 350 nm) is absent in complexes [Au2L2](OTf)2 and [Au3Cl2(tht) L]OTf, the absorption spectra of which are characterized by slightly more intense 1IL bands at λmax = 292 (ε = 2.37 × 104 M−1 cm−1) and 286 nm (ε = 1.78 × 104 M−1 cm−1), respectively. Moreover, a dilute sample of [Au2AgL2](OTf)3 in MeCN displays a relatively intense band in the region 315−247 nm (ε ≈ 1.3−1.6 × 104 M−1 cm−1) attributable to a combination of spin-allowed 1ILCT and 1MLCT excitation processes. To further investigate the nature of the absorption transitions of [Au3L2](OTf)3, solvent effects were studied, and the E


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Inorganic Chemistry

Table 1. Photophysical Data for [Au3L2](OTf)3, [Au2L2](OTf)2 and [Au2AgL2](OTf)3 in MeCN and for [Au3Cl2(tht)L]OTf, in CH2Cl2 under Dilute Conditions at Room Temperature and in Frozen Matrix at 77 K room temperature compound [Au3L2](OTf)3

[Au2L2](OTf)2

[Au2AgL2](OTf)3

[Au3Cl2(tht)L]OTf

λabs (ε) [nm, ( × 104 M−1 cm−1)]

λem [nm]

345 (5.21) 270 (1.49) 225 (2.14) 292 (2.37) 246sh (1.63) 225 (2.79) 315 (1.36) 288 (1.53) 247sh (1.56) 229sh (2.14)

446

286 (1.78)

719

τa [μs]

PLQYa (%)

0.51

11

τb [μs]

77 K matrix λem [nm]

PLQYb (%)

3.9

82

4.0d 6.9 (64%)e 1.6 (36%)

446, 560

442

τ [μs]

443c

4.2c

527c

16.6c

415, 515

1.8 (65%) 4.3 (35%)f 8.7 (71%)g 2.1 (29%)

5.9 (66%) 0.85 (34%)

Air-equilibrated. bDegassed. cIn MeCN. dλem = 447 nm. eλem = 520 nm. fRecorded at λem = 410 nm. gRecorded at λem = 550 nm; “sh” denotes a shoulder. a

photophysical properties recorded in MeCN were compared with those in CH2Cl2 and DMF solutions. The results are displayed in Figure 5 and listed in Table 2. As seen in Figure 5,

Table 2. Photophysical Data Obtained from the Study of Solvent Effects on Dilute Samples of [Au3L2](OTf)3 in CH2Cl2 and DMF at Room Temperaturea air-equilibrated

MeCN

CH2Cl2

DMF

λabs (ε) [nm, ( × 104 M−1 cm−1)]

λem [nm]

345 (5.21) 270 (1.49) 225 (2.14) 344, (4.61) 278 (1.22) 270 (sh, 1.13) 345 (1.32)

446

degassed

PLQY (%)

τ [μs]

PLQY (%)

0.51

11

3.9

82

445

1.1

27

3.0

74

445

0.193 (17%) 0.061 (11%) 0.009 (72%)

5

2.0

35

τ [μs]

a

Data in MeCN are also included for comparison. The samples were excited at λexc = 350 nm.

Figure 5. Electronic absorption and normalized emission spectra for [Au3L2](OTf)3 in MeCN (black), CH2Cl2 (red), and DMF (blue) recorded at room temperature upon excitation at λexc = 350 nm.

Upon photoexcitation in the region 290−300 nm at room temperature, dilute samples of [Au2L2](OTf)2 clearly showed an irradiation time-dependent emission spectrum, as displayed in Figure S3 (see Supporting Information). By comparison of the excitation spectra with the electronic absorption profile, it appears that upon irradiation of [Au2L2](OTf)2 at λexc = 290 nm, the photochemical product is most likely the [Au3L2](OTf)3 derivative, as inferred by the close resemblance of the excitation spectra recorded at λem = 450−470 nm and the corresponding absorption spectrum of [Au3L2](OTf)3 (Figure 4 and Figure S3 of the Supporting Information). Also, sequential emission spectra upon photoirradiation of a solution of [Au2L2](OTf)2 clearly showed the presence of an isosbestic point at about λem = 480 nm, supporting the idea that the rise of the chemical species associated with the emission band at higher energy is a direct consequence of the photochemical degradation of the parent dinuclear complex. Nonetheless, such a photodegradation process became slower when the sample was degassed by freeze−pump−thaw (see Figure S3 of the Supporting Information).

the lowest-energy absorption band did not show any modulation of the absorption maximum upon variation of the solvent polarity, ruling out any sizable charge transfer (CT) nature in the excitation process and supporting the assignment of a mainly metal-centered (MC) transition. However, a net decrease of the extinction coefficient was observed when DMFa more polar solvent with better solvation ability for ionic compoundswas used. This finding may indicate a partial involvement of the triflate counterion in the coordination sphere around the trinuclear complex, which influences the electronic ground-state properties of the latter, and/or an involvement of solvent molecules with high donor number in the coordination sphere of the complex.2b A slight modulation of the absorption maximum at higher energy was found on going from the more polar MeCN to the less polar CH2Cl2 (λabs, max = 270 vs 278 nm, respectively), confirming a partial CT nature of the transition involved. F


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Inorganic Chemistry

[Au2L2](OTf)2 and [Au3Cl2(tht)L]OTf (see Figure S6 of the Supporting Information). Finally, the photoluminescence properties in the solid state as neat samples were investigated, and the corresponding data are shown in Figure 6 and listed in Table 3. While

Emission spectra of [Au2AgL2](OTf)3 in dilute MeCN solution are displayed in Figure S4 (Supporting Information). The sample showed an emission maximum centered at 442 nm during the first recorded spectrum, but a steady decrease of the emission intensity was observed with a concomitant appearance of a second band at longer wavelength, attributable to a photodecomposition product during the experiment. In dilute degassed CH2Cl2 solution and upon irradiation at λexc = 290 nm, [Au3Cl2(tht)L]OTf displayed a PLQY of 0.8% with a broad and featureless emission profile centered at 719 nm. The excitation spectrum overlaps with the electronic absorption profile, and the luminescence decays with biexponential kinetics of τ1 = 0.85 μs (34%) and τ2 = 5.9 μs (66%) (Table 1 and Figure S3 of the Supporting Information). Such emission could be assigned to a Cl-metal-to-ligand charge transfer (3XLCT) radiative process, and even though an exciplex nature of such low-energy lying radiative transition cannot be ruled out at the present stage, there is no evidence for it in the excitation spectrum. It is noteworthy that upon irradiation in the lower energy absorption band at λexc = 300−350 nm, dilute samples of [Au3L2](OTf)3 were photostable and displayed a wavelengthindependent highly intense emission spectrum with narrow profile and λem = 446 nm (see Figure 4), with a Stokes shift as small as 6560 cm−1. The excitation spectrum recorded at such an emission band nicely corresponds to the absorption profile displayed in Figure S5 (see Supporting Information). This complex is stable in solution and under light exposure. Going from air-equilibrated to degassed conditions, the [Au3L2](OTf)3 samples did not show any change in the emission profile, yet, a strong increase of the PLQY was observed with values going from 11% to as high as 82%. The increase of PLQY was mirrored by a concomitant prolongation of the excited state lifetime from 0.51 to 3.9 μs, respectively, which strongly supports the triplet-manifold nature of the emitting excited state. Noticeably, the recorded chromaticity based on the 1931 Commission Internationale de l’Eclairage coordinates, CIE(x,y), is x = 0.157 and y = 0.024, corresponding to a saturated blue-violet emission (i.e., x ≈ 0.15, y < 0.08). To further investigate the nature of the radiative process involved in such strong emission, the photoluminescence properties were investigated as a function of solvent polarity (Table 2 and Figure 5). The independence of the emission energy with increasing solvent polarity as well as the dependence of the excited state lifetime and PLQY on the presence of quenching dioxygen support a triplet-manifold MC character of the emitting state, the nature of which can be described as 3 [5dσ*6pσ], similarly to what has been reported for closely related Au(I) complexes.2b,8d On going from a fluid solution of [Au3L2](OTf)3 at room temperature to a glassy matrix in frozen MeCN at 77 K, a bright emission appeared that is only slightly hypsochromically shifted (152 cm−1) with respect to the profile recorded at room temperature (see Figure 4, gray trace and Figure S5 of the Supporting Information), pointing toward a negligible charge transfer character of the emitting excited state. The excitation spectrum recorded upon monitoring such an emission band nicely corresponds to the electronic absorption spectrum at room temperature, indicating that the origin of the photoluminescence process is the same in both cases. As observed at room temperature, [Au2L2](OTf)2, [Au2AgL2](OTf)3, and [Au3Cl2(tht)L]OTf also showed in a frozen matrix a photochemical degradation process that was even faster for

Figure 6. Emission spectra of [Au3L2](OTf)3 (black trace), [Au3Cl2(tht)L]OTf (blue trace), and [Au2AgL2](OTf)3 (red trace) in the solid state as neat powder upon excitation at λexc = 300 nm.

Table 3. Photophysical Data for Complexes [Au3L2](OTf)3, [Au3Cl2(tht)L]OTf, and [Au2AgL2](OTf)3 in the Solid State As Neat Powder [Au3L2](OTf)3 [Au3Cl2(tht)L]OTf [Au2AgL2](OTf)3 a

λem [nm]

τ [μs]

PLQY (%)

442 665 404

2.7 −a 0.83 (42%) 1.8 (58%)

80 −a 38

Emission too weak to be recorded.

[Au2L2](OTf)2 is not photostable, even in the solid state as neat powder (data not shown), [Au3L2](OTf)3 and [Au2AgL2](OTf)3 display a very strong emission band centered at λem = 442 and 404 nm, respectively. The emission maximum and the photoluminescence profile of [Au3L2](OTf)3 are very similar to the spectra recorded in MeCN solution, and the PLQY retains the high value of 80%. On the basis of such similarities, we attribute such a radiative process to an emission with mainly 3 [5dσ*6pσ] character. Complex [Au3Cl2(tht)L]OTf exhibits a rather weak emission centered at 665 nm with most likely a 3 XLCT nature, similarly to that observed in solution at room temperature (see Table 3). Electroluminescence. To evaluate the solid-state electroluminescence (EL) properties of the bright emissive complex [Au3L2](OTf)3, OLEDs were fabricated by using a solutionbased process by means of a spin-coating technique. The active electroluminescent layer was obtained by blending [Au3L2](OTf)3 either in polyvinylcarbazole (PVK) or in tris(4carbazoyl-9-ylphenyl)amine (TCTA) hosts with doping concentrations of 10−20 wt %. Small molecules used as additives to balance charge transport within the emitting layer, i.e., 2,2′(1,3-phenylene)bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole] (OXD-7) and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-Hbenzimidazole) (TPBI) were added to either PVK or TCTA blends, respectively. With both hosts, a blue electroluminesG


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Inorganic Chemistry cence at around λEL = 447 nm was observed with only a slight bathochromic shift upon increasing complex concentration (see Figure 7). The EL spectral shape is broader with respect to

are displayed in Figure 8. When dissolved in MeCN containing 0.1 M TBAPF6, [Au3L2](OTf)3 does not show any anodic

Figure 8. Cyclic voltammograms recorded of 1 mM solution of [Au3L2](OTf)3 in MeCN and 0.1 M TBAPF6 as the supporting electrolytes. GC was the working electrode. Scan rate, 0.05−1 V s−1. Figure 7. EL spectra of devices ITO/PEDOT:PSS/PVK:OXD7: [Au3L2](OTf)3/Ba/Al at 10 wt % (red line) and 20 wt % (blue line) complex concentrations and ITO/PEDOT:PSS/PVK/TCTA:TPBi: [Au3L2](OTf)3/LiF/Al (black line) at 10% complex concentration. Voltage bias, 9 V. Inset, representative current density−luminance− voltage characteristic of the ITO/PEDOT:PSS/PVK:OXD7:[Au3L2](OTf)3/Ba/Al at 10 wt % complex doping concentration.

oxidation peaks in the electrochemical window, but three welldefined, irreversible cathodic processes peaking at −1.58, −2.02, and −2.43 V versus the Fc|Fc+ were recorded that are attributable to reduction events. Minor changes in the potential values of the cathodic waves were observed when NaOTf was employed as the supporting electrolyte, and the corresponding data are listed in Table S2 and shown in Figure S8 of the Supporting Information. Assuming that the donor characteristics of the CNHC in PCNHCP are comparable with other common NHCs such as IPr, IMes, etc., one would have expected that the internal Au atom, coordinated by two CNHC donors, would bear a higher electron density than the two external Au centers, making the latter easier to reduce. However, according to computational data (see below) and consistent with the (surprising) failure to chemically oxidize [Au3L2](OTf)3 with, e.g., iodine, the less electron rich Au atom appears to be the central one, presumably because of the stronger π-accepting properties of NHC vs phosphine donors (see below),26 and this has served as a basis for the assignment of the observed redox processes. Furthermore, the 77Se NMR chemical shift of the corresponding NHC-Se adduct of PCNHCP was found to be consistent with a significant π-acidity of the NHC donor in PCNHCP.26a For these reasons, possible reduction events that are expected to take place at the different metal centers are tentatively shown below, remembering that in the ground state the lateral Au centers are equivalent by symmetry:

photoluminescence, with CIE(x,y) chromaticity coordinates (0.19; 0.17). However, this broadening is not related to contributions from the hosts since the EL spectra of the PVK:OXD-7 and TCTA:TPBI peaked at λEL = 410 and 490 nm, respectively (see Figure S7 in the Supporting Information), while the devices of the two blends show essentially the same EL spectrum. The measured external quantum efficiency (EQE) of about 0.1% is rather low for all the devices analyzed; however this a proof-of-principle experiment to show that [Au3L2](OTf)3 can be used as a deep-blue emitter in OLEDs fabricated by simple and low-cost wet methods. Electrochemical Studies. In order to gain more insight into the electronic properties of [Au3L2](OTf)3, in particular with respect to the electron density at the chemically different Au centers, its electrochemical characteristics were probed by cyclic voltammetry (CV) and differential pulsed voltammetry (DPV) in acetonitrile, using two different supporting electrolytes, namely TBAPF6 and NaOTf. The reduction potentials, E°red, associated with Au reduction are listed in Table 4, and the corresponding cyclic voltammograms recorded by using TBAPF6 as a supporting electrolyte

Au(I) − Au(I) − Au(I) + e−

Table 4. Electrochemical Data of Complex [Au3L2](OTf)3 in a Solution of 0.1 M TBAPF6 in MeCN, Using Glassy Carbon (GC) As an Working Electrode scan rate [mV s−1] 50 100 200 500 1000

→ Au(I) − Au(0) − Au(I), E1° Au(I) − Au(0) − Au(I) + e−

Ered ° ,a TBAPF6 [V] −1.53, −1.54, −1.55, −1.57, −1.58,

− − − − −

1.95, 1.95, 1.96, 1.99, 2.02,

− − − − −

(1)

→ Au(0) − Au(0) − Au(I), E2°

2.50 2.43 2.41 2.41 2.43

(2)

Au(0) − Au(0) − Au(I) + e− → Au(0) − Au(0) − Au(0), E3°

(3)

The difference between E°1 , E°2 , and E°3 is related to the interaction between the metal centers: the higher the electronic communication, the greater the separation between their

a

Potential referenced to the couple Fc|Fc+. All the electrochemical processes were irreversible. H


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Inorganic Chemistry

Figure 9. Differential pulsed voltammograms of 1 mM solution of [Au3L2](OTf)3 in MeCN using (a) 0.1 M TBAPF6 and (b) NaOTf as a supporting electrolyte and GC as a working electrode. Pulse height is 0.025 V. of [Au3L2](OTf)3 and [Au2L2](OTf)2 and rationalize their photophysical and electrochemical behavior, calculations on the cationic complexes (the anions were not considered) were performed by means of wave function theory (WFT) and density functional theory (DFT; see details in the Supporting Information). The main geometric parameters of the optimization at various levels of calculation are given in Tables 5 and 6 (see Supporting Information for details on the

electrochemical potentials. Indeed, a weak interaction is expected to cause splitting of the processes into closely separated peaks as already reported.27 To better identify the different electrochemical processes involved, differential pulsed voltammetry (DPV) was performed by employing two different supporting electrolytes, and the corresponding data are displayed in Figure 9. The anion of the electrolyte plays a role in the stabilization of the product resulting from each reduction step. The anion OTf− could initially interact with the Au atoms, in an analogous manner that was established in the solid state structure of the Cu(I) and Ag(I) analogs of [Au3L2](OTf)3,20c but upon reduction, this could be no longer the case, and a decreased coordination ability and stabilization result in an easier third reduction step of the system. The gap between the second and third reduction steps increases from 198 mV to 525 mV when going from OTf− to PF6− (Figure 10), suggesting a stabilizing

Table 5. Important Interatomic Distances (Å) in the Computed Gas Phase Structure of [Au3L2]3+a method

P−AuL

AuL−AuC

AuC−C1

BLYP BLYP+D BP86+D B3LYP B3LYP+D MP2 Experimental

2.407 2.382 2.350 2.392 2.374 2.315 2.315

2.880 2.881 2.828 2.877 2.880 2.762 2.758

2.076 2.068 2.046 2.064 2.055 2.022 2.068

a

The ending + D denotes the inclusion of dispersion corrections in the DFT calculations. See Scheme 4 for the atom numbering.

Table 6. Important Interatomic Distances (Å) in the Computed Gas Phase Structure of [Au2L2]2+a method

P1−Au

Au−Au

Au−C1

Au−P2

B3LYP B3LYP+D MP2 Experimental

2.357 2.334 2.295 2.286

2.939 2.939 2.818 2.830

2.083 2.078 2.023 2.057

3.451 3.390 3.291 3.298

a

The ending + D denotes the inclusion of dispersion corrections in DFT calculations. See Scheme 4 for atom numbering.

Figure 10. Differential pulsed voltammograms of 1 mM solution of [Au3L2](OTf)3 in MeCN, and 0.1 M TBAPF6 (blue trace) or NaOTf (red trace) as supporting electrolytes. GC was working electrode. Pulse height is 0.025 V.

complete structures); the atom numbering adopted in this section is shown in Scheme 4. With all the DFT functionals tested, the Au−Au and Au−P distances are overestimated for both complexes, even with the inclusion of dispersion corrections. On the contrary, the MP2 calculations are in very good agreement with the experimental data, despite a slight underestimation of the Au−C1 bond distances. The better agreement of MP2 calculations with the experimental data illustrates the role played by static correlation effects on the Au···Au interaction. In contrast, the weak influence of dispersion corrections on the geometric parameters suggests that the short distance between the Au centers does not have its origin in such interactions. In an attempt to pin down the nature of the interactions operating between the Au atoms, we studied the electronic structure of [Au3L2] 3+ by means of a NBO analysis and computed the weak interactions through a NCI analysis (Figure 11).28b The detailed charges and electronic populations are given in Tables S3 and S4. Ligand

effect of PF6− upon reduction of the complex. Furthermore, NaOTf decreases the electrochemical window of MeCN, in particular on the cathodic side, because it is not resistant to the reduction.

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COMPUTATIONAL STUDIES

A. Computed Structures in the Gas Phase. Early calculations by the single excitation configuration interaction (CIS) method on the absorption and emission spectra of a binuclear gold(I) complex in acetonitrile and in the solid state have shown that the weak aurophilic interactions that exist in the ground state are greatly enhanced in the excited state.28a In order to gain insight into the electronic structures I


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Inorganic Chemistry Scheme 4. Atom Numbering for [Au2L2]2+ (left) and [Au3L2]

a

3+

(right) Used in the Calculationsa

For the Au atoms, the subscripts L and C refer to lateral and central, respectively. From the picture described above involving the charges at the AuL and AuC cations, it is clear that the charge distribution around AuC is anisotropic and most likely the source of the Au···Au interaction, identified by the NCI analysis: thus, there is an attractive electrostatic interaction between the Au atoms despite their positive charge (Figure 11). A very similar picture has emerged for the computed structures of [Au2L2]2+. Relevant data are given in Table 6. For this complex too, DFT overestimates the bond distances, even with inclusion of dispersion corrections, while MP2 gives very accurate geometrical parameters. B. Absorption Spectroscopy in Solution. The computed electronic absorption spectrum of [Au3L2]3+ is depicted in Figure 12. In the region between 250 and 500 nm, the computed peaks at 326

Figure 11. NCI analysis of the weak interactions in the [Au3L2]3+ complex. The green surfaces indicate the presence of van der Waals attractive dispersion interactions, the blue areas the presence of attractive electrostatic interactions, and the red surfaces, which are barely visible, correspond to steric repulsions. coordination is accompanied by a strong charge transfer from the ligands to the Au atoms. All three Au atoms remain positive but with partial charges of +0.28 for AuL and +0.43 for AuC. The Au valence orbitals are strongly perturbed by the presence of the ligand, resulting in depopulation of the 5d orbitals (especially for AuC) and strong population of the 6s orbital, in order to decrease electron repulsion between the ligand lone pairs and the Au d orbitals. It should be noticed that not all the 5d orbitals are equally affected, but only those pointing toward the ligands, viz., a population drop of the 5dx2−y2 (1.87e− for AuL and 1.75e− for AuC) and to a lesser extent the 5dxy orbital (1.95e−). Interestingly, back-donation from the AuC toward the NHC is also evidenced: whereas the dxz is almost full for AuL (1.97e−), it is slightly depleted in AuC (1.92e−). This is also supported by the significant population increase in the C 2s and C 2p orbitals of C1, respectively, upon complexation (Table S3). Furthermore, the cause of this electron population redistribution being the coordinated AuC was further demonstrated by studying the model where AuC is removed from the trinuclear structure. This resulted in a vanishing of the electronic population of C 2p, leaving the electronic populations of the nitrogen atoms unchanged. Finally, the three Au cations in the chain do not directly communicate, as manifested by the stability of the charges and electronic population on each of them in models where one of their corresponding neighbors has been removed.

Figure 12. Theoretical absorption spectra of [Au3L2]3+ (in red) and [Au2L2]2+ (in black). and 273 nm correspond to the experimental ones at 345 and 270 nm (Table 1); both originate from two pure electronic transitions. Additionally, an electronic transition computed at 355 nm shows negligible oscillator strength. There are no other lower energy transitions (singlets with wavelength longer than 355 nm). The experimental peak at 225 nm corresponds to the theoretical one centered at 226 nm, which arises from the merging of many (more than 15) transitions localized between 215 and 249 nm, dominated by two at 216 and 231 nm. The three singlet transitions (S1 = 355 nm, S2 = 326 nm, and S3 = 273 nm) possess a metal-centered (MC) component associated with metal-to-ligand charge transfer (MLCT) or ligand-to-metal charge transfer (LMCT); their nature is detailed in Figure 13. In both S1 and S2, the electron leaves a Au σ* orbital generated by the antibonding combination of the 5dx2−y2 of the three Au cations. In S1, the orbital J


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Inorganic Chemistry

phosphorus to a bonding π orbital between the Au and the carbon of the NHC in analogy to the S1; DS3 is analogous to S2. However, DS2 is the only transition with no MC character and is a mixed MLCT and ligand centered (LC) state. As with [Au3L2](OTf)3, the experimental highest energy band of [Au2L2](OTf)2 at 225 nm arises from the contribution of several electronic transitions from 217 to 238 nm with a major component at 228 nm. Two electronic transitions contribute to the latter: one consists of an electron transfer from the π system of the NHCs to a Au−C π bonding orbital; the second corresponds to an electron transfer from the lone pair of the dangling phosphorus to a bonding P−Au−Au−P σ orbital. The experimental shoulder at 246 nm is found in the computed spectrum at 249 nm. It corresponds to an intense absorption due to a pure electronic transition, an electron transfer from the π system of the NHC to a Au−Au bonding π orbital. In summary, the theoretical absorption spectra of [Au3L2]3+ and [Au2L2]2+ possess striking similarities with respect to the electronic transitions which arise from orbitals of the same type. A major difference though is the important role of the lone pairs of the dangling phosphorus donors in the electronic transitions of [Au2L2]2+. C. Emission Properties. Since the theoretical absorption spectra of [Au3L2]3+ have two well separated singlet states of different symmetry, we next optimized the lowest triplet states in these symmetries. It was found that they are of the same nature as the lowest singlets (Figure 15). The main geometric parameters and emission wavelengths associated with these triplet states are given in Table 7.

Figure 13. Electron density difference maps between the ground state and the excited state for S1 at 355 nm (left), S2 at 326 nm (middle), and S3 at 273 nm (right). The areas in red correspond to electronic depletion and in green to electronic enrichment. The transitions composing the absorption band at 226 nm are of two different types: either similar to S1 with an excitation from one of the deeper Au orbitals instead of the σ* or consisting of the reduction of the Au chain by electron transfer, mostly from the phosphorus lone pairs. populated is π bonding between the AuC and the NHC; in S2, it is mainly a Au σ orbital generated by the bonding combination of the empty 6pz orbitals of the three Au cations. S3 is different, as it mostly consists of a π*−π transition localized on the AuC and the NHCs. The presence of the NHC donor has a strong effect on the nature of the lowest excited state in S1 and on the transitions of higher energy. The NHC π system is involved through a bonding orbital, which consists of a mixing between an antibonding π* orbital of the NHC and the empty 6p orbital of the Au cations. In contrast to S1, in chains previously reported in the literature and supported by phosphorus only donors, the lowest transitions consisted of almost pure metalcentered states.14 The almost pure MC state of S2 is consistent with the explanation put forward for the absence of solvatochromism in [Au3L2](OTf)3. The energy of the lowest computed transition S1 (3.49 eV, 355 nm) and the associated large HOMO−LUMO gap may account for the absence of an oxidation peak in the window of potentials explored electrochemically (section 4). Furthermore, in the ground state geometry, S1 and S2 are separated by 0.3 eV (6.9 kcal·mol−1), suggesting that upon the first reduction, the electron density tends to localize on the AuC and the NHC rather than on the Au chain. The latter point was further confirmed by optimizing the two [Au3L2]2+ doublet states obtained from the reduction of S1 and S2 (see structures in the Supporting Information). The structure with the electron localized on AuC and NHC is 3.2 kcal·mol−1 more stable than that with the electron delocalized along the Au chain (see spin density of Figure S10, left and right, respectively). The experimental absorption spectrum of [Au2L2](OTf)2 (Table 1) is hypsochromically shifted compared to that of [Au3L2](OTf)3. The theoretical absorption spectrum of [Au2L2]2+ exhibits two peaks at 295 (referred to as DS3) and 225 nm (see Figure 12), in perfect agreement with the experimental values (292 and 225 nm). However, the lowest electronic transitions are two singlets at 335 (DS1) and 308 (DS2) nm, with no (DS1) or small (DS2) absorption intensities. The peak at 295 nm (DS3) is generated by only one transition, the nature of which is detailed in Figure 14. The nature of the three transitions (DS1)(DS3) of [Au2L2]2+ is similar to that of the (S1)-(S3) of [Au3L2]3+ with one major difference: the involvement of the dangling phosphorus atom in DS1, which consists of an excitation of an electron from a σ antibonding orbital between the Au atoms and the dangling

Figure 15. Spin density map of the optimized triplet T1 (left) and T2 (right) in [Au3L2]3+.

Table 7. Important Interatomic Distances (Å) of the Two Triplets T1 and T2 in [Au3L2]3+ in Solution and the Emission Wavelengths (in nm) Computed by ΔSCF and TD-DFT λem (ΔSCF) λem (TD-DFT) AuL−P AuC−C1 AuL−AuC

fundamental

T1

T2

2.368 2.048 2.865

421 442 2.369 2.032 2.740

409 420 2.345 2.058 2.711

The two triplet states are almost degenerate; T2 (triplet counterpart of S2) is only 0.5 kcal·mol−1 more stable than T1 (triplet counterpart of S1). Such a small difference is not significant at our level of calculation, and we cannot state which of the two triplets is the emitting one. However, the emission wavelength of T1 (Table 7) is in very good agreement with the experimental value (λem = 446 nm, Table 1). Upon geometry optimization, a contraction of the AuC···AuL distances was observed in both triplet states compared to the ground state geometries. This is due to the depopulation of the AuC−AuL antibond, and, for T2, the population of a σ bonding orbital leading to a greater contraction compared to T1. Simultaneously, we observed in T1 a small contraction of the AuC−C1 distance due to the population of the π AuC−C1 bonding orbitals. In contrast, this distance increased in T2 compared to the ground state. Finally, the Au−P distance is not affected in T1 but is slightly reduced in T2.

Figure 14. Electron density difference maps between the ground and the excited state for DS1 (left), DS2 (middle), and DS3 (right). The areas in red correspond to electronic depletion and areas in green to electronic enrichment. K


Article

Inorganic Chemistry As for the absorption spectra discussed above, the presence of a dangling phosphorus atom has a great influence on the emission properties of [Au2L2](OTf)2. Here too, only the two lowest triplet states referred to as DT1 and DT2 were optimized; their characteristics are reported in Table 8.

Table 8. Important Interatomic Distances (Å) of the Two Lowest Triplets DT1 and DT2 in [Au2L2]2+ in Solution and the Emission Wavelengths (in nm) Computed by ΔSCF and TD-DFT λem (ΔSCF) λem (TD-DFT) Au−P1 Au−C1 Au−Au Au−P2

fundamental

DT1

DT2

2.334 2.078 2.939 3.390

532 559 2.352 2.035 2.774 2.880

500 520 2.371 2.045 2.832 2.861

Figure 17. Raman spectra using 473, 532, and 633 nm excitation lines of a solid sample of [Au3L2](OTf)3 at room temperature, in the 50− 700 cm−1 region.

The nature of the triplet states is detailed in Figure 16. DT1 is the triplet counterpart of the DS1 transition. The situation is less clear for

molecules containing phosphine, imidazole, and Au−Au moieties.29 In addition, quantum chemical calculations were also performed to help the interpretation of the Raman spectrum, in particular the Au− Au stretching vibration. Whereas both symmetric and asymmetric Au− Au−Au stretching modes are expected in vibrational spectroscopy, only the Au−Au−Au symmetric stretching mode is Raman-active due to the symmetry of the complex (C2h; asymmetric motion is only IR active) with a fundamental mode predicted near 83 cm−1. A Raman band centered at 91 cm−1 was strongly enhanced when the blue excitation was used. This vibration can be attributed to the Au−Au− Au symmetric stretching mode of the [Au3L2](OTf)3 complex on the basis of theoretical calculations and the Au−Au stretching vibrations of similar compounds.29b,c,e This resonance-enhanced Raman band implies that the excited-state distortion is localized at the Au−Au bond, in agreement with the assignment of the 344 nm absorption band of [Au3L2](OTf)3 to a metal-centered transition. The main experimental Raman bands with their proposed assignment are summarized in Table 9.

Figure 16. Spin density map of the optimized triplet DT1 (left) and DT2 (right) in [Au2L2]2+. DT2. The contribution of the Au atom in the transition is smaller, with less spin density localized on the Au atom. Energetically, there is no ambiguity: the two triplets are well separated; the DT1 state is more stable by 7.5 kcal mol−1 compared to DT2. Furthermore, they are subject to greater geometry distortions. Again, in the excited states, we observed a contraction of the Au−Au and, to a lesser extent, of the Au−C1 distances and an elongation of the Au−P1 distance. However, the striking point that emerged is the behavior of the dangling phosphorus. Upon excited state geometry optimization, the Au−P2 distance shortened by as much as 0.5 Å, tending to a square planar ligand field around the Au atom. This change in the Au coordination sphere may explain the greater stabilization of the excited state compared to [Au3L2]3+. It is relevant here to recall the chelating− bridging behavior of the PCNHCP ligands in the dipalladium(I) complex [Pd2(μ2-P,CNHCP,κP,κCNHC,κP)2](OTf)2 where each Pd is chelated by the CNHC and P donors of one ligand and bound to the other P donor from the other ligand.21 In [Au2L2]2+, the absorption is hypsochromically shifted, whereas the emission is bathochromically shifted. The presence of a free donor in the vicinity of the partially oxidized Au cations stabilizes the excited state. The computed emission wavelength (Table 8) is in very good agreement with the experimental value (560 nm, Table 1). Raman Spectra of [Au3L2](OTf)3. The Raman spectra of powder samples of [Au3L2](OTf)3 were recorded by using the 473, 532, and 633 nm excitations (Figure 17). The spectra evolved with the excitation wavelengths due to the preresonance condition associated with either the S1 (MC/MLCT) or the S2 (MC) transitions (absorption maximum around 344 nm), these transitions calculated in acetonitrile being too close to allow an unambiguous assignment. Consequently, several Raman features were observed to enhance when the blue excitation was used. However, the largest enhancement was observed for the band centered at 91 cm−1, with a 5-fold intensity increase upon excitation at λ = 473 nm. The vibrational spectrum was interpreted according to previous analyses of the vibrations of

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CONCLUSION A comprehensive investigation of the structural and diverse spectroscopic properties of chain complexes with trinuclear Au3 and AgAu2 or dinuclear Au2 metal cores stabilized by heterofunctional PCNHCP (L) ligands has been carried out Table 9. Selected Experimental Raman Wavenumbers and Symmetry of [Au3L2](OTf)3 along with Proposed Assignments

a

L

assignmentsa

symmetry

experimental wavenumbers (cm−1)

ν(CC)ring νs(CAuC) + νs(NCN) νring‑breathing νs(CN)ring νs(CC) νas(C−P−C) νs(C−P−C) ν(NP) δ(CPC) δ(CCP) νas(PAuP) δ(CPN) νs(AuAuAu)

Ag Ag Ag Ag Ag, Bg Bg Ag Ag Ag Ag Bg Ag Ag

1562 1316 1206 1111 800 621 602 550 482 234 129 114 91

νs, symmetric stretching; νas, asymmetric stretching; δ, deformation. DOI: 10.1021/acs.inorgchem.6b01095 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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and supported by calculations performed through WFT and DFT theories. MP2 calculations gave the most satisfactory results to describe the complex structures, although a DFT approach allowed an analysis of the spectroscopic properties with a high level of accuracy. The complex [Au2L2](OTf)2, which contains two dangling phosphorus donors, was shown to be an excellent precursor to AgAu2 metal chains, and this rational synthetic approach could certainly be successfully extended in the future. All the polynuclear complexes presented in this work have metal centers with a d10 electronic configuration and display short or very short Au−Au separations. In the linear chain complex [Au3L2](OTf)3, there are two types of homoleptic Au(I) centers: the unique AuC with two CNHC donors and the two equivalent AuL’s bonded to phosphine donors. Although it has been established that aurophilic interactions play an important role in the photophysical properties of the metal complexes in which they occur, the analysis of the computational results from the complex [Au3L2]3+ points to the importance of the ligand bonded to the cations. Indeed, the difference in the charge transfer between the gold atoms due to their different ligand field tempers the repulsive Coulombic forces and introduces some attractive forces between the more reduced AuC atom and the more oxidized AuL atoms. The complex [Au3L2](OTf)3 displayed excellent photophysical properties, with a narrow photoluminescence profile in solution, peaking at about λem = 445 nm and a PLQY as high as 80%. It is noteworthy that such interesting features were retained in the solid state, paving the way to exploring its use as luminescent material in optoelectronic applications. A proof-of-concept OLED was fabricated by means of low-cost and easy wet deposition processing, which demonstrated that [Au3L2](OTf)3 is of potential interest as a deep-blue emitter in solution-processed OLEDs. Electrochemical and Raman spectroscopy studies were also performed on [Au3L2](OTf)3 and the enhanced Raman band around 91 cm−1 in the preresonance Raman spectrum of [Au3L2](OTf)3 strongly supports the occurrence of metal− metal interactions. From the theoretical studies, it was concluded that electrostatic interactions contribute to the short distance between the gold centers and that the presence of the NHC donor groups modifies the nature of the lowest excited states in both the absorption and emission spectra of [Au3L2](OTf)3. Somewhat unexpectedly, it was also found that the stabilization of the excited state of the dinuclear complex [Au2L2](OTf)2 results from additional coordination of the dangling phosphorus to the gold atoms, which deeply modifies the response of the complex to light excitation through the introduction of an extended π system, which may interact with the gold core, or a free donor able to bind the excited gold cations. Thus, despite an absorption at higher energy, the emission of [Au2L2](OTf)2 occurs at lower energy than in [Au3L2](OTf)3. Possible applications of this unusual feature could be considered for the design of nanoswitches. Potentially interesting design principles of photoluminescent Au-containing molecules may emerge from these observations, e.g., by promoting anisotropic charge distribution through ligand design or introducing substituent effects and tailoring by the choice of dangling donors. The potential of the NHC donor in emissive molecules is again clearly highlighted, and a detailed understanding of its role may lead to the development of better performing functional molecules.

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EXPERIMENTAL SECTION

Synthesis and Characterization. 1. General Methods. All manipulations involving organometallics were performed under argon using standard Schlenk techniques. Solvents were dried using standard methods and distilled under nitrogen prior to use or passed through columns of activated alumina and subsequently purged with nitrogen or argon. 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded at 298 K, unless otherwise specified, on a Bruker Avance 400, 500, or 600 spectrometer and referenced to the residual solvent resonance (1H and 13C) or external 85% H3PO4 in D2O(31P). Elemental analyses were performed by the “Service de microanalyses,” Université de Strasbourg. The imidazolium salt PCHP,20c the free carbene PCNHCP (L),20c and the complexes [Ag3L2](OTf)3,20c [AuCl(tht)],30 and [Au(tht)2](O3SCF3)31 were prepared according to the literature. 2. Synthesis of the Trinuclear Au(I) Complexes [Au3L2](OTf)3 and [Au3Cl2(tht)L]OTf. To a mixture of [Ag3L2](OTf)3 (0.080 g, 0.054 mmol) and [AuCl(tht)] (0.064 g, 0.200 mmol) was added MeCN (6 mL) at room temperature under protection from light and stirring was maintained overnight. A small excess of [AuCl(tht)] was used to ensure completion of the reaction. The reaction was monitored by 31 1 P{ H} NMR spectroscopy until the product peak (at δ 141) became dominant. After filtration through a Celite pad, the filtrate was evaporated to dryness, then the solid was washed with THF (10 mL) and dried under a vacuum to give a solid mixture of the known [Au3L2](OTf)320c and the new [Au3Cl2(tht)L]OTf. Pure [Au3L2](OTf)3 could be easily obtained by washing the solid with CH2Cl2 (1 mL) and drying under a vacuum, affording a white powder (0.075 g, 80%). The characterization data agree with our previous report.20c In the above synthesis, the CH2Cl2 solution obtained after filtration was left standing overnight in the air for the solvent to evaporate, and the resulting solid was washed quickly with 1 mL MeCN: [Au3Cl2(tht) L]OTf was obtained as a white powder (0.010 g, 15%). X-ray quality crystals of [Au3Cl2(tht)L](OTf)·MeCN were obtained by slow diffusion of ether into a MeCN solution. Analysis Found (Calcd for C24H46Au3Cl2F3N2O3P2S2) (%): C, 23.25 (22.96), H, 3.72 (3.69), N, 2.19 (2.23). 1H NMR (400 MHz, CD2Cl2): δ 8.39 (s, 2H, im-H), 3.64 (m, 4H, THT), 2.13 (m, 4H, THT), 1.53 (d, 36H, 3JHP = 17.8 Hz, C(CH3)3). 1H NMR (400 MHz, CD3CN): δ 7.78 (s, 2H, im-H), 3.33 (m, 4H, THT), 1.87 (m, 4H, THT), 1.53 (d, 36H, 3JHP = 17.9 Hz, C(CH3)3). 13C{1H} NMR (125 MHz, CD2Cl2): δ 193.1 (t, 2JCP = 35.1 Hz, NCN), 128.6 (im-C), 121.3 (q, 1JCF = 319.6 Hz, CF3), 42.3 (THT), 40.5 (d, 2JCP = 16.3 Hz, C(CH3)3), 31.0 (THT), 29.2 (d, 3JCP = 7.4 Hz, C(CH3)3). 31P{1H} NMR (162 MHz, CD2Cl2): δ 132.2 (s). 31 1 P{ H} NMR (162 MHz, CD3CN): δ 133.7 (s). 3. Synthesis of the Dinuclear Au(I) Complex [Au2L2](OTf)2. To a mixture containing the imidazolium salt PCHP (0.104 g, 0.205 mmol), free carbene L (0.010 g, 0.028 mmol), and [Au{N(SiMe3)2}(PPh3)] (0.104 g, 0.168 mmol) was added 5 mL of THF at room temperature: the additional small amount of L was used to prevent the formation of the trinuclear complex [Au3L2](OTf)3. After stirring was maintained for 3 days and the supernatant decanted, the residue was dried under a vacuum to give a white powder of [Au2L2](OTf)2 (0.070 g, 59%). Xray quality crystals were obtained by slow diffusion of ether into a CH2Cl2 solution. Analysis Found (Calcd for C40H76Au2F6N4O6P4S2) (%): C, 34.19 (33.83), H, 5.45 (5.17), N, 3.99 (3.80). 1H NMR (400 MHz, CD3CN): δ 8.01−7.99 (overlap. d, 2H, im-H), 7.87 (br, 2H, imH), 1.51 (d, 36H, 3JHP = 17.6 Hz, C(CH3)3), 1.28 (d, 36H, 3JHP = 13.2 Hz, C(CH3)3). 31P{1H} NMR (162 MHz, CD3CN): δ 134.8 (s), 132.7 (s), 119.7 (s), 110.6 (s). 1H NMR (400 MHz, CD2Cl2): δ 8.18 (br, 2H, im-H), 8.11 (d, 3JHP = 1.5 Hz) + 8.08 (d, 3JHP = 2.2 Hz) (2H, imH), 1.55 (d, 36H, 3JHP = 17.7 Hz, C(CH3)3), 1.29 (d, 36H, 3JHP = 13.2 Hz, C(CH3)3). 1H NMR (400 MHz, CD2Cl2, dissolved at −40 °C): δ 8.15 (s, 2H, im-H), 8.07 (s, 2H, im-H), 1.53 (d, 36H, 3JHP = 17.7 Hz, C(CH3)3), 1.28 (d, 36H, 3JHP = 13.2 Hz, C(CH3)3). 13C{1H} NMR (125 MHz, CD2Cl2): δ 199.8 (dddd, 2JC‑trans P = 116.3 Hz, 2JCP = 76.0 Hz, 2JCP = 31.9 Hz, 5+6JCP = 3.4 Hz, NCN), 129.0 (im-C), 128.2 + 128.0 (im-C), 122.4 (q, 1JCF = 320.3 Hz, CF3), 39.8 (d, 2JCP = 15.2 Hz, C(CH3)3), 36.0 (d, 2JCP = 31.1 Hz, C(CH3)3), 29.1 (d, 3JCP = 7.9 Hz, M


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Inorganic Chemistry C(CH3)3), 28.9 (d, 3JCP = 15.8 Hz, C(CH3)3). 31P{1H} NMR (162 MHz, CD2Cl2): δ 134.1 (s), 132.0 (s), 119.9 (s), 110.7 (s). 31P{1H} NMR (162 MHz, CD2Cl2, dissolved at −40 °C): δ 131.0 (s), 117.8 (s). 19F{1H} NMR (564 MHz, CD2Cl2): δ −79.9 (s). CP-MAS 31P NMR (202 MHz): δ 130.8 (s), 115.8 (s). 4. Synthesis of the Heterotrinuclear Complex [Au2AgL2](OTf)3. To a solution of [Au2L2](OTf)2 (0.035 g, 0.025 mmol) in MeCN (5 mL) was added AgOTf (0.007 g, 0.027 mmol) at room temperature. After the solution was stirred for 2 h, the volatiles were removed under reduced pressure, and the residue was washed with THF (5 mL) and dried under a vacuum to give [Au2AgL2](OTf)3 as a white powder (0.037 g, 89%). Analysis Found (Calcd for C41H76Au2AgF9N4O9P4S3) (%): C, 29.40 (29.63), H, 4.56 (4.61), N, 3.25 (3.37). 1H NMR (400 MHz, CD3CN): δ 8.13 (d, 2H, 3JHP = 2.3 Hz, im-H), 8.01 (d, 2H, 3JHP = 2.1 Hz, im-H), 1.55 (t, 36H, 3JHP = 9.3 Hz, C(CH3)3), 1.49 (d, 36H, 3 JHP = 8.5 Hz, C(CH3)3). 13C{1H} NMR (125 MHz, CD2Cl2): δ 195.0 (t, 2JCP = 14.2 Hz, NCN), 130.0 (im-C), 129.7 (im-C), 122.4 (q, 1JCF = 320.3 Hz, CF3), 41.6 (d, 2JCP = 6.0 Hz, C(CH3)3), 39.4 (C(CH3)3), 29.8−29.7 (C(CH3)3 and C(CH3)3). 31P{1H} NMR (162 MHz, CD3CN): δ 137.2 (s), two doublets centered at 121.7 (JP‑109Ag = 542.9 Hz, JP‑107Ag = 469.5 Hz). X-ray Crystallography. A summary of the crystal data and data collection and refinement parameters for the structures of [Au3Cl2(tht)L](OTf)·MeCN, [Au3Cl2(SMe2)L](OTf)·2MeCN, and [Au2L2)](OTf)2 are given in Table S1. X-ray diffraction data collection was carried out on a Bruker APEX II DUO Kappa-CCD diffractometer equipped with an Oxford Cryosystem liquid N2 device, using Mo Kα radiation (λ = 0.71073 Å). The crystal-detector distance was 38 mm. The cell parameters were determined (APEX2 software)32 from reflections taken from three sets of 12 frames, each at 10 s exposure. The structure was solved by direct methods using the program SHELXS-97.33 The refinement and all further calculations were carried out using SHELXL-97.34 The H atoms were included in calculated positions and treated as riding atoms using SHELXL default parameters. The non-H atoms were refined anisotropically, using weighted full-matrix least-squares on F2. A semiempirical absorption correction was applied using SADABS in APEX2.32 The crystallographic information files (CIF) have been deposited with the CSD, 12 Union Road, Cambridge, CB2 1EZ, U.K. and can be obtained on request free of charge, by quoting the publication citation and deposition numbers 1439290−1439292. Photophysical Characterizations. Room and Low (77 K) Temperature Measurements. Absorption spectra were measured on a double-beam Shimadzu UV-3600 UV−vis−NIR spectrophotometer and baseline corrected. Steady-state emission spectra at both room temperature in organic solvent and 77 K in a 2-MeTHF glassy matrix were recorded on a HORIBA Jobin-Yvon IBH FL-322 Fluorolog 3 spectrometer equipped with a 450 W xenon arc lamp as the excitation source, double-grating excitation and emission monochromators (2.1 nm mm−1 of dispersion; 1200 grooves mm−1) and a TBX-04 singlephoton-counting as the detector. Emission and excitation spectra were corrected for source intensity (lamp and grating) and emission spectral response (detector and grating) by standard correction curves. Timeresolved measurements were performed using the time-correlated single-photon-counting (TCSPC) option on Fluorolog 3. NanoLEDs (402 nm; fwhm

Bonding, Luminescence, Metallophilicity in Linear Au3 and Au2Ag

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