Article pubs.acs.org/Organometallics
Tuning the Energy Emission from Violet to Yellow with Bidentate Phosphine Gold(III) Complexes Vanesa Fernández-Moreira,† Jéssica Cámara,† Ekaterina S. Smirnova,†,§ Igor O. Koshevoy,‡ Antonio Laguna,† Sergey P. Tunik,§ M. Carmen Blanco,*,†,⊥ and M. Concepción Gimeno*,† †
Departamento de Química Inorgánica, Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), Universidad de Zaragoza-CSIC, Pedro Cerbuna 12, 50009 Zaragoza, Spain ‡ Department of Chemistry, University of Eastern Finland, Joensuu, 80101, Finland § St. Petersburg State University, Universitetskaya nab., 7/9, 199034 St. Petersburg, Russia ⊥ Centro Universitario de la Defensa, Academia General Militar, Ctra. de Huesca s/n, 50090 Zaragoza, Spain S Supporting Information *
ABSTRACT: The synthesis and characterization of luminescent gold(III) compounds, obtained by coordination of the metal center to different phosphines, is described. To avoid deactivation of luminescence by the presence of low-energy d−d ligand field states in the gold(III) center, the ligands bonded to the metallic center have been carefully chosen, among which we used bidentate phosphines with different numbers of phenylene or alkynyl-phenylene spacers and pentafluorophenyl groups. The reaction of [Au(C6F5)3(tht)] (tht = tetrahydrothiophene) with the corresponding diphosp h i n e s g a v e t h e c o m p le x es [ { Au ( C 6 F 5 ) 3 } 2 ( 1 , 4 PPh2(C6H4)nPPh2)] (n = 1−3) and [{Au(C6F5)3}2(PPh2CC(C6H4)nCCPPh2)] (n = 0−2). The study of their optical behavior reveals emission color variations from violet to yellow for the compounds containing the phosphines with one, two, and three phenylene spacers, respectively, and much more fine-tuning, from deep blue to brilliant blue for those intercalating alkynyl and phenylene spacers. Four of the new complexes were also characterized by X-ray diffraction crystallography, showing supramolecular structures formed through hydrogen bonding.
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platinum(II) compounds.3 Many previous reports suggest the presence of low-energy d−d ligand field states as a possible cause, because they could quench the luminescence (i.e., provide a nonradiative deactivation of the excited state) via thermal vibration or by energy transfer.4 Moreover, at this point it is worth mentioning experimental problems in the preparation of gold(III) complexes because of the possible reductive elimination reactions to give gold(I) species. In order to increase stability of the gold(III) species, pentafluorophenyl ligands can be introduced to protect the gold center, thus providing high stability of the target products.5 Consequently, it is possible to conclude that the photophysics of gold(III) derivatives is far less studied than the analogous d8 metal complexes. The first example of a luminescent gold(III) complex was described in 1993 by Yam and co-workers, who reported a series of emissive gold(III) diimine compounds of the type ̂ ̂ [AuR2(LL)]ClO4 (R = Mes, CH2SiMe3; LL = 2,2′-bpy, phen, dpphen), suggesting that the emission could be assigned either to the metal-perturbed IL, MLCT, or LLCT transitions or to a
INTRODUCTION Gold chemistry has undergone a profuse development in the last years, mostly because of the capacity to form aurophilic interactions, which usually provide interesting optical properties mainly in gold(I) complexes.1 Simultaneously, it has been revealed that it is possible to improve the efficiency of organic light-emitting devices by the introduction of a metal atom such as gold, taking advantage of the heavy-atom effect by generation of excited triplet states. This allows for harvesting singlet and triplet excitons; then both of them are able to decay radiatively, hence improving the internal quantum efficiency of the device compared to standard OLEDs, where only the singlet state is contributing to emission of light.2 Developing bright deep blue phosphorescent emitters is still a challenge nowadays. Therefore, there is an increasing demand for reaching a similar efficiency to that of green and red PHOLED emitters, which are already in use in Samsung and LG displays. Despite many gold(I) complexes being luminescent, the gold(III) compounds scarcely show efficient emission, mainly at low temperature, in the solid state or in a glassy matrix. There are only a few examples of the gold(III) complexes that show luminescence in solution, despite gold(III) being isoelectronic and isostructural to the well-known emissive © XXXX American Chemical Society
Received: February 16, 2016
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DOI: 10.1021/acs.organomet.6b00135 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Scheme 1. Synthesis of Complexes 1−6a
a Conditions: (i) 1/2 PPh2C6H4PPh2, (ii) 1/2 PPh2(C6H4)2PPh2, (iii) 1/2 PPh2(C6H4)3PPh2, (iv) 1/2 PPh2CC−CCPPh2, (v) 1/2 PPh2C CC6H4CCPPh2, (vi) 1/2 PPh2CC(C6H4)2CCPPh2.
ligands. A fine-tuning of the energy emission is easily achieved within the same ligands, with the stepwise variation of the number of spacers in the diphosphine, resulting in emissions ranging from violet, to blue, to yellow on going from one, to two, to three phenylene spacers in one case and from deep blue to bright blue intercalating alkynyl and phenylene spacers in the second case.
mixture of them.6 Later, it was shown that the introduction of strong field σ-donor ligands makes the metal center electronrich and enhances luminescence of the corresponding complexes, because of the higher population of emissive excited states and lower rate of their nonradiative deactivation because of the shift of the d−d states to higher energy. The best results have been obtained by the use of cyclometalated ligands or alkyne precursors, which provide luminescent cyclometalated gold(III) complexes of the C^N^C type, where possible π-interactions are responsible for intense emission.3a,7,8 Alternatively, for the C^N^N complexes9 the photoredox and luminescence properties at 77 K were assigned to the metalperturbed IL transitions. Strongly σ-donating ligands such as Nheterocyclic carbenes (NHCs) have also been employed to give cyclometalated compounds with interesting optical characteristics.10 The use of alkynyl ligands, which are also strong σdonors, is another strategy to prepare emissive gold(III) species, which has been successfully employed by Yam or Venkatesan and co-workers.3a,c,11 Phosphines represent another group of ligands that have been extensively used in gold(I) chemistry to obtain luminescent species,12 and they also could favor the luminescence in Au(III) complexes. Surprisingly, there are no examples of emissive gold(III) derivatives with phosphine ligands, and the reason could be that phosphines tend to promote reduction of the gold(III) center to the gold(I) oxidation state. With this idea in mind, we started the project aimed at developing the synthesis of luminescent gold(III) derivatives with rigid polydentate phosphine ligands, which were successfully used in preparation of luminescent gold(I) complexes, and whose emissions were based neither on the presence of aurophilic interactions nor on the charge transfer onto ancillary ligands.13 Herein we report on the synthesis and characterization of several gold(III) derivatives with the bidentate rigid phosphines containing phenylene or alkynyl-phenylene moieties as spacer groups. These compounds show a remarkable luminescent behavior (both in solution and in solid state), and as far as we are aware, they are the first examples bearing diphosphine
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RESULTS AND DISCUSSION
Synthesis and Characterization of the Complexes. With the aim of preparing luminescent gold(III) complexes based on rigid diphosphine ligands we have chosen the [Au(C6F5)3(tht)] derivative as a suitable precursor, in which the pentafluorophenyl groups stabilize the high oxidation state of the metal center and tetrahydrothiophene can be easily substituted by other ligands.14 The selected diphosphines contain different numbers of phenylene or dialkynyl-phenylene spacers, as shown in Scheme 1. The target compounds 1−6 were obtained in high yields (80−88%) as white solids by reaction of the gold substrate with the corresponding phosphine in a 2:1 molar ratio. They are soluble in halogenated solvents and non-soluble in hexane or diethyl ether. The IR spectra of 1−6 display similar spectroscopic patterns with the typical bands of the pentafluorophenyl groups coordinated to gold(III) at ∼792 (m) and ∼965 (s) cm−1.15 In the spectra of 4−6 an additional absorption band was found around 2173 cm−1, which can be easily assigned to the triple bond vibration in the phenylene-alkynyl spacers. The 31P{1H} NMR spectra of 1−6 show a single unresolved multiplet at 17.4 (1, 3), 17.3 (2), 0.0 (4), −3.8 (5), and −4.1 (6) ppm, which correspond to the equivalent phosphorus atoms found in each of the complexes, magnetically coupled with the fluorine nuclei of the C6F5 ligands. A considerable high-field shift demonstrated by signals of 4−6 may be attributed to a higher shielding of the phosphorus atoms by the alkynyl-containing spacers. The coordination of the C6F5 groups to gold centers is also detected by the 19F NMR spectra (see Experimental Section), which show a typical pattern of the Au(C6F5)3 moiety with two different sets of resonances (2:1 ratio) generated by two pentafluorophenyl groups located in cis- and trans-position B
DOI: 10.1021/acs.organomet.6b00135 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Figure 1. Molecular structure of complex 3 showing the atom-labeling scheme. Selected bond lengths (Å) and angles (deg): Au1−C28 2.052(4), Au1−C34 2.065(4), Au1−C22 2.076(4), Au1−P1 2.3786(10), P1−C1 1.813(4), P1−C16 1.813(4), P1−C10 1.815(4); C28−Au1−C34 87.31(15), C28−Au1−C22 88.90(15), C34−Au1−C22 175.91(15), C28−Au1−P1 175.39(11), C34−Au1−P1 90.14(11), C22−Au1−P1 93.75(11), C1−P1− Au1 110.88(13), C16−P1−Au1 112.23(13), C10−P1−Au1 113.64(13).
relative to the phosphine ligand. The mass spectra (MSMALDI+) of 1−6 do not show the molecular peaks (neither in the ESI+ spectra), as the pentafluorophenyl group is easily lost in the ionization process, and several fragments assigned to the protonated phosphine ligands together with molecular fragments of the complexes under study are observed. Crystal Structure Determinations. The crystal structures of complexes 3, 4, 5, and 6 were established by X-ray diffraction. The molecular structure of 3 is shown in Figure 1. The complex crystallizes in the triclinic space group P1̅ , and the molecule is disposed in the symmetry center. The central phenylene spacer is disordered over the symmetry center in two positions. The gold center has a square planar geometry with all the angles close to 90°. The Au−C distances range from 2.052(4) to 2.076(4) Å, and the Au−P bond length is 2.3786(10) Å, which are typical values for the gold(III) center bonded to carbon and phosphorus atoms. The structure of complex 4 is shown in Figure 2. The molecule crystallizes in the monoclinic space group P21/n with two independent molecules; consequently only half of each molecule constitutes the asymmetric unit. The gold(III) center has a square planar geometry with very regular angles, close to 90°. The Au−C distances lie in the range 2.054(7)−2.070(7) Å and are very similar to those found in complex 3. Similarly to complex 3, the shortest Au−C bond in 4, 2.070(7) Å, is trans to the phosphine ligand, and the Au1−P1 bond, 2.3561(11) Å, is shorter compared to that in 3. The C47−C48 distance (1.196(19) Å) and the C48−C48#1 bond (# = 1−x, −y+1, −z; 1.372(14) Å) display elongation and contraction compared to regular triple and single C−C bonds, respectively, which indicates a certain degree of delocalization within the conjugated alkyne chain. The structure of 5 is shown in Figure 3. The molecule crystallizes in the monoclinic P21/n space group, and only half of the molecule is in the asymmetric unit. The gold(III) center has a square planar geometry with the Au−C and Au−P bond distances similar to those found in complexes 3 and 4. However, in contrast to complex 3, the longest Au−C bond in 5, 2.071(5) Å, is trans to the phosphine ligand and the Au1−P1 bond (2.3541(13) Å) is the shortest compared to the corresponding values in compounds 3 and 4. This is probably due to the more basic nature of the phosphine with the alkynylphenylene spacers, which results in a stronger trans-effect of the
Figure 2. Molecular structure of complex 4 showing the atom-labeling scheme. Selected bond lengths (Å) and angles (deg): Au1−C11 2.054(7), Au1−C1 2.059(6), Au1−C21 2.070(7), Au1−P1 2.3561(17), P1−C47 1.758(8), C47−C48 1.196(19), C48−C48#1 (# = 1−x, −y+1, −z) 1.372(14); C11−Au1−C1 88.7(3), C11−Au1− C21 88.5(3), C1−Au1−C21 176.4(3), C11−Au1−P1 179.2(2), C1− Au1−P1 90.29(18), C21−Au1−P1 91.56(18).
phosphine in 5. The C−C distance in the alkynyl units equals 1.189(7) Å, which is in agreement with the presence of a triple bond in the spacer. The solid-state crystal packing of 5 displays several short intermolecular contacts, which can be classified as H−F hydrogen bonds, mainly between the hydrogen atoms of the phenylene spacer and the fluorine atoms of the C6F5 groups. These contacts force the formation of a supramolecular structure as shown in Figure 4 with slightly bent molecules of 5 to form an infinite 3D network. The structure of complex 6 has also been established by Xray diffraction analysis, and the molecule is shown in Figure 5. Analogously to its congeners, the gold(III) centers in 6 have a square planar geometry and the Au−C and Au−P bonds are similar to those found in the related phosphine alkynyl complexes. Absorption and Emission Studies. The absorption and emission spectra of complexes 1−6 in the solid state and in dichloromethane solution have been measured, both at room C
DOI: 10.1021/acs.organomet.6b00135 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Figure 3. Molecular structure of complex 5 with selected atomic numbering scheme. Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg): Au1−C21 2.059(5), Au1−C11 2.070(4), Au1−C1 2.071(5), Au1−P1 2.3541(13), P1−C47 1.747(5), P1−C31 1.801(5), P1−C41 1.805(5), C47−C48 1.189(7); C21−Au1−C11 176.77(19), C21−Au1−C1 89.01(17), C11−Au1−C1 89.79(17), C21−Au1−P1 89.96(13), C11−Au1−P1 91.30(13), C1−Au1−P1 178.44(13).
Figure 4. Supramolecular structure of 5 formed by the H···F contacts, which are around 2.422 Å for F5···H51, 2.506 Å for F6···H50, or 2.502 Å for F12···H50.
Figure 5. Molecular structure of complex 6 with selected atomic numbering scheme. Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg): Au1−C11 2.049(3), Au1−C21 2.061(3), Au1−C1 2.066(3), Au1−P1 2.3520(10), P1−C47 1.742(3), Au2−C111 2.060(3), Au2−C101 2.062(3), Au2−C91 2.071(3), Au2−P2 2.3529(11), P2−C62 1.735(3), C47−C48 1.196(4), C61−C62 1.198(4); C11−Au1−C21 90.48(12), C21−Au1−C1 89.01(17), C11−Au1−C1 89.79(17), C21−Au1−P1 89.96(13), C11−Au1−P1 91.30(13), C1−Au1−P1 178.44(13).
temperature and at 77 K. The DRUV spectra for complexes 1− 3 showed absorptions with the maxima close to those in the free phosphine ligands, which is red-shifted as the number of
phenylene spacers in the phosphine increases (Table 1). This type of absorption in this energy range have been attributed to a σ → π* transition originating from the promotion of an D
DOI: 10.1021/acs.organomet.6b00135 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Table 1. Photophysical Properties of 1−6 in the Solid State 298 K DRUV, λmax, nm
a
1 2 3
283 291 314
4 5
313 300
6
340
excitation, λmax, nm
emission, λmax, nm
77 K τobs, μs
QY film
272, 321 316,365 365
468, 494a 381, 397sh, 424ash 518, 555, 598sh
166 0.04 911
1.4
>350 350
