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Article

Attachment of Luminescent Neutral “Pt(pq)(C≡CtBu)” Units to Di and Tri N-Donor Connecting Ligands: Solution Behavior and Photophysical Properties

by
Elena Lalinde
1,*,
M. Teresa Moreno
1,*,
Santiago Ruiz
1 and
Sergio Sánchez
2
1
Departamento de Química-Centro de Síntesis Química de La Rioja, (CISQ), Universidad de La Rioja, 26006, Logroño, Spain
2
School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK
*
Authors to whom correspondence should be addressed.
Inorganics 2014, 2(4), 565-590; https://doi.org/10.3390/inorganics2040565
Submission received: 16 September 2014 / Revised: 9 October 2014 / Accepted: 9 October 2014 / Published: 31 October 2014
(This article belongs to the Special Issue Organoplatinum Complexes)
Browse Figures
Versions Notes

Abstract

:
Binuclear derivatives [{Pt(pq)(C≡CtBu)}2(μ-L)] (1a–5a), containing a series of dinitrogen linker ligands and the trinuclear [{Pt(pq)(C≡CtBu)}3(μ-L)] (6a) [L = μ-1,3,5-tris(pyridine-4-ylethynyl)benzene], formed by bridge-splitting reactions with [Pt(pq)(μ-κCα:η2-C≡CtBu)]2 (Pt-1), are reported. The complexes are characterized by a combination of 1H NMR spectroscopy, mass spectrometry and X-ray crystallography (2a and 4a). 1H NMR proves the existence of a dynamic equilibrium in solution between the diplatinum complexes (species a), the corresponding mononuclear complex with terminal N-donor ligands (species b), the starting material (Pt-1) and the free ligand (L). The effects of concentration, temperature and solvent properties on the equilibrium have been studied. The optical properties of these systems have been investigated by UV-visible absorption and emission spectroscopies in solid state and in solution, and the nature of the transitions and the excited state analyzed by theoretical calculations on 2a.

1. Introduction

The interest of alkynyl platinum(II) complexes stems from their rich structural diversity [1,2,3,4], their interesting chemical reactivity [1] and more recently from their increasing potential in material science [5,6,7,8,9,10,11,12,13,14,15,16,17]. In this field, light-emitting complexes have attracted a great deal of attention owing to their use in optoelectronic devices, chemosensors, photovoltaic cells and photocatalysis.
By using the triple bond of the alkynyl ligand, a good number of di- and triplatinum complexes in which the Pt(II) centers are stabilized by double bridging alkynyl systems with different conformations have been reported [18,19,20,21,22,23,24,25,26,27,28,29]. However, the reactivity of these systems has been scarcely explored. It has been reported that binuclear Pt derivatives such as [trans-Pt(μ-κCα:η2-C≡CR)(C6F5)L]2 or cycloplatinate complexes [Pt(C^P)(μ-κCα:η2-C≡CR)]2 undergo bridge-splitting with classical donor ligands to give mononuclear σ-alkynyl Pt(II) complexes [18,23,24,27].
On the other hand, since the discovery in 1990 by Fujita of the first example of a rationally designed metallacycle, the molecular square [{Pd(en)(μ-4,4'-bpy)}4](NO3)8, prepared by self-assembly of [Pd(en)(ONO2)2] (en = ethylenediamine) with the linear linker 4,4'-bipyridine (4,4'-bpy) [30], there has been increasing interest in using linear, angular, triangular or flexible linkers with bidentate or tridentate binding nitrogen units to produce fascinating metal-mediated molecular architectures, including many macrocycles and cages. Several reviews have thoroughly covered this amazing field in the last years [31,32,33,34,35,36,37,38,39]. The inherent stability and rather low reactivity of the Pt(II) in contrast with other metals [i.e., Pd(II)] [40,41] has been the main attraction for the use of Pt(II) units for metallamacrocycles [42]. Furthermore, the attractive photophysical properties of Pt(II) complexes, such as low-energy, tunability and long-lived excited states, have prompted their incorporation into metallacycles [43,44,45,46,47]. The combination of the luminescent Pt(II) units in the corners with luminophore N-groups that can act as linkers can be exploited in highly emissive metallacycles [48]. Comparatively, platinum binuclear derivatives bridged by dinitrogen donor ligands are less represented [49,50,51,52,53,54,55,56], and particularly those with cycloplatinated fragments are scarce [49,50,51,52,53].
In this context, we have recently reported the synthesis and photophysical properties of the series [Pt(pq)(μ-κCα:η2-C≡CR)]2 (Hpq = phenylquinoline; R = tBu, Tol, C6H4OMe-3, C6H4CF3-4) [57], which represent the first examples of cyclometalated double alkynyl bridging complexes in which the photoluminescent properties have been studied. The photophysical properties are clearly influenced by the substituents on the alkynyl ligands in response to the variation of the Pt…Pt distance. Thus, whereas the ter-butyl derivative with a short Pt…Pt distance displays an emission originated from a mixed metal-metal-alkynyl to pq 3[(MM + L')LCT] excited state, in the aryl derivatives with a longer Pt…Pt distance, the emission arises from a 3L'LCT excited state with a small or negligible contribution of 3MMLCT character.
In this work, we study the reactivity of the ter-butyl derivative [Pt(pq)(μ-κCα:η2-C≡CR)]2 (Pt-1) towards bidentate and tridentate nitrogen linkers, assuming that both, the “Pt(pq)(C≡CtBu)” fragment and the linkers, are luminophores. This study has allowed us to synthesize a series of platinum(II) binuclear derivatives [{Pt(pq)(C≡CtBu)}2(μ-L)] bridged by N-N-donor ligands [L = pyrazine (pyz) 1a, 4,4'-bipyridine (bpy) 2a, 1,2-bis(bipyridyl)ethane (bpa) 3a, trans-1,2-bis(4-pyridyl)ethylene (bpe) 4a and 1,2-bis(4-pyridyl)acetylene (bpac) 5a], and the trinuclear branched derivative [{Pt(pq)(C≡CtBu)}3(μ-L)] (6a) containing the rigid conjugated pyridyl alkynyl ligand [μ-1,3,5-tris(pyridine-4-ylethynyl)benzene]. A combination of crystallography (2a and 4a), NMR, IR and mass spectrometry provides a complete picture of the equilibrium established in solution between derivatives with N-donor bridging ligands (1a–5a), mononuclear complexes with terminal N-ligands [Pt(pq)(C≡CtBu)(L-κN)] (1b–5b) and the precursor (Pt-1). Furthermore, we discuss the photophysical properties of these complexes in solid state and in solution and the theoretical calculations of 2a.

2. Results and Discussion

2.1. Synthesis and Characterization

As illustrated in Scheme 1, the synthesis of the new diplatinum derivatives [{Pt(pq)(C≡CtBu)}2(μ-L)] (L = pyz 1a, bpy 2a, bpa 3a, bpe 4a, bpac 5a) was achieved by bridged-cleavage reaction in CH2Cl2 of [Pt(pq)(μ-κCα:η2-C≡CtBu)]2 (Pt-1) with the corresponding dinitrogen donor ligand. Complexes 2a, 3a and 5a were obtained as orange solids in moderate (2a) or high (3a, 5a) yields by reaction of the starting material with one equivalent of the ligand, whereas for 1a and 4a two equivalents of ligand were added to obtain the complexes pure in solid state. A particularly reliable indicator of the final reaction is the lack of bridging υ(C≡C) band in the IR spectra of the final solid (see below). On the other hand, the branched trinuclear platinum complex [{Pt(pq)(C≡CtBu)}3(μ-tpab)] (6a) was obtained using the same strategy but with a Pt-1:ligand molar ratio of 3:2.
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Scheme 1. Synthesis of the derivatives 1a–6a.
Scheme 1. Synthesis of the derivatives 1a–6a.
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A combination of crystallography (2a and 4a), IR spectra, mass spectrometry and elemental analysis supports the formulation proposed in the Scheme 1. 1H NMR spectroscopy completes the picture in solution. The IR spectra of these complexes show one υ(C≡C) intense absorption (2114–2118 cm−1) in the typical range for terminal σ-coordinated alkynyl ligands, thus confirming the cleavage of the alkynyl bridging (μ-C≡CtBu)2 system. Furthermore, complexes 5a and 6a show one additional band at higher frequency (2223 5a, 2212 cm−1 6a), assigned to the υ(C≡C) stretch of the inner ethynyl entity of the 4-pyridylacetylene groups in the bpac and the tpab ligands. Because of the weak coordinative bonds, ESI is used as a soft ionization method. Analysis of the complexes in CH2Cl2 in the positive ion mode (exact mass) gave the corresponding molecular peak for each complex and the fragmentation peak [Pt(pq)(C≡CtBu)L]+ or [Pt2(pq)2(C≡CtBu)2L]+ (6a) by loss of the fragment [Pt(pq)(C≡CtBu)] (Figure 1). The cleavage occurs at the Pt-N covalent bond, indicating that this is the weakest link in this series of derivatives. In these complexes, there are common peaks associated with the starting material: the intact doubly σ-π alkynyl bridging molecular ion ([{Pt(pq)(C≡CtBu)}2]+, m/z 961), the loss of an alkynyl group ([Pt2(pq)2(C≡CtBu)]+, m/z 879) and the cleavage ([Pt(pq)(C≡CtBu)]+, m/z 481). In good qualitative agreement with the observations in the NMR spectra in solution (see below), these peaks are very intense in the mass spectra.
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Figure 1. Experimental isotope pattern and predicted isotopic distribution (green) in the electrospray mass spectrum of 6a, showing the peak [Pt3(pq)3(C≡CtBu)3(tpab)]+.
Figure 1. Experimental isotope pattern and predicted isotopic distribution (green) in the electrospray mass spectrum of 6a, showing the peak [Pt3(pq)3(C≡CtBu)3(tpab)]+.
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Single crystals suitable for X-ray analysis were grown by slow diffusing of n-hexane into a CHCl3 solution at −30 °C of 2a and 4a, respectively. The structures of [{Pt(pq)(μ-C≡CtBu)}2(μ-4,4'-bpy)]·2CHCl3·C6H14 (2a·2CHCl3·C6H14) and [{Pt(pq)(μ-C≡CtBu)}2(μ-bpe)]·4CHCl3 (4a·4CHCl3) were confirmed by X-ray crystallography (Figure 2, Figure 3 and Figure S1; Table 1 and Table S1). The asymmetric unit of 2a contains only one molecule, whereas in 4a the asymmetric unit is formed by one independent molecule together with two half molecules, which are completed by application of the symmetry elements. The conformation and metrical parameters of the three molecules are comparable, therefore, only the data for one of them are included in Table 1. The complexes are binuclear with a 4,4'-bpy (2a) or bpe (4a) group bridging the Pt(II) centers, which complete their distorted square planar environment with a bidentate pq ligand and one terminal alkynyl group. In both complexes, the N-pyridine bridging group is trans to the cyclometalated carbon, thus confirming that the reactions takes place with retention of the geometry in the precursor, and the Pt units adopt an anti-configuration.
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Figure 2. View of the molecular structure of the complex [{Pt(pq)(C≡CtBu)}2 (μ-4,4'-bpy)]·2CHCl3·C6H14 (2a·2CHCl3·C6H14).
Figure 2. View of the molecular structure of the complex [{Pt(pq)(C≡CtBu)}2 (μ-4,4'-bpy)]·2CHCl3·C6H14 (2a·2CHCl3·C6H14).
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Inorganics 02 00565 g003 1024
Figure 3. View of the independent molecule in the asymmetric unit in the molecular structure of the complex [{Pt(pq)(C≡CtBu)}2(μ-bpe)]·4CHCl3 (4a·4CHCl3).
Figure 3. View of the independent molecule in the asymmetric unit in the molecular structure of the complex [{Pt(pq)(C≡CtBu)}2(μ-bpe)]·4CHCl3 (4a·4CHCl3).
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In 2a, the two pyridyl rings form a dihedral angle between them of 16.45° (2a), whereas in the bpe-complex (4a) is of 24.0° in the complete molecule. However, due to symmetry, both rings are coplanar in the other molecules of 4a. The bpy and bpe groups form dihedral angles with each platinum coordination plane of 67.47°, 82.18° (2a) and 60.36–80.41° (4a), respectively. These complexes show a greater variability in the angles than that observed for the related binuclear complexes with the tridentate cyclometalating pip2NCN− ligand [{Pt(pip2NCN)}2(μ-NN)]2+ [NN = bpy 86°, bpe 85.2°, pip2NCNH = 1,3-bis(piperidylmethyl)benzene)] [49,50], but the Pt-N (bpy or bpe) distances [2.113(4), 2.125(4) Å 2a, 2.114(7)–2.145(7) Å 4a] are comparable. The details of the central C=C fragment in the three molecules of 4a [range C-C 1.240(19)–1.334(17) Å] and those of the terminal alkynyl units in both complexes are not unusual (see Table 1 and Table S1). The phenylquinolyl (pq) ligand is fluttered, forming dihedral angles of 13.33°, 13.97° (2a) and 19.82°, 9.97°, 20.02°, 10.04° (4a) with the Pt coordination planes, which is consistent with other complexes containing the “Pt(pq)” metallacycle [57].
Table
Table 1. Selected bond lengths and angles for [{Pt(pq)(C≡CtBu)}2(μ-4,4'-bpy)]·2CHCl3·C6H14 (2a·2CHCl3·C6H14) and [{Pt(pq)(C≡CtBu)}2(μ-bpe)]·4CHCl3 (4a·4CHCl3) (independent molecule).
Table 1. Selected bond lengths and angles for [{Pt(pq)(C≡CtBu)}2(μ-4,4'-bpy)]·2CHCl3·C6H14 (2a·2CHCl3·C6H14) and [{Pt(pq)(C≡CtBu)}2(μ-bpe)]·4CHCl3 (4a·4CHCl3) (independent molecule).
2a·2CHCl3·hexane4a·4CHCl3
Distances (Å)
Pt(1)-C(1)1.994(5)Pt(1)-C(1)1.972(9)
Pt(1)-N(1)2.115(4)Pt(1)-N(1)2.096(7)
Pt(1)-C(16)1.967(5)Pt(1)-C(16)1.949(9)
Pt(1)-N(2)2.113(4)Pt(1)-N(2)2.130(7)
C(16)-C(17)1.197(7)C(16)-C(17)1.209(12)
Pt(2)-C(32)1.980(5)Pt(2)-C(34)1.979(8)
Pt(2)-N(3)2.127(4)Pt(2)-N(3)2.136(7)
Pt(2)-C(47)1.961(5)Pt(2)-C(49)1.956(9)
Pt(2)-N(4)2.125(4)Pt(2)-N(4)2.145(7)
C(47)-C(48)1.209(7)C(49)-C(50)1.195(12)
--C(27)-C(28)1.290(9)
Angles (°)
N(1)-Pt(1)-C(1)80.9(2)N(1)-Pt(1)-C(1)80.3(3)
C(16)-Pt(1)-C(1)91.7(2)C(16)-Pt(1)-C(1)92.1(4)
C(16)-Pt(1)-N(2)88.50(19)C(16)-Pt(1)-N(2)87.8(3)
N(1)-Pt(1)-N(2)98.56(16)N(1)-Pt(1)-N(2)99.4(3)
Pt(1)-C(16)-C(17)175.2(5)Pt(1)-C(16)-C(17)176.3(9)
C(16)-C(17)-C(18)176.6(8)C(16)-C(17)-C(18)175.4(11)
N(3)-Pt(2)-C(32)80.6(2)N(3)-Pt(2)-C(34)80.9(3)
C(47)-Pt(2)-C(32)95.8(2)C(49)-Pt(2)-C(34)93.0(4)
C(47)-Pt(2)-N(4)81.36(19)C(49)-Pt(2)-N(4)83.9(3)
N(3)-Pt(2)-N(4)101.96(16)N(3)-Pt(2)-N(4)102.0(3)
Pt(2)-C(47)-C(48)167.6(5)Pt(2)-C(49)-C(50)174.7(9)
C(47)-C(48)-C(49)172.7(6)C(49)-C(50)-C(51)174.6(11)
C(27)-C(28)-C(31)126.5(14)C(24)-C(27)-C(28)121.4(14)
The crystal packing of 2a shows that the molecules are arranged forming dimers through moderate intermolecular π···π (pq···pq) interactions (3.271, 3.336 Å, Figure S2a), which interacts through secondary interactions with the crystallization solvents (CHCl3, n-hexane) and with other dimers (Hpy···πC≡C/Cpq) (Figures S2b). However, the supramolecular structure of 4a (Figure S3) does not show π…π interactions.
The 1H NMR spectra of 1a–6a in CDCl3 at room temperature, immediately after dissolving the corresponding solid brute or the crystalline material are consistent with the presence of four different molecules in solution. The resonances are associated with the presence of the new diplatinum complex (species a), with the starting material (Pt-1), the free ligand and, also the corresponding mononuclear complex with the dinucleating ligand acting as monodentate terminal group [Pt(pq)(C≡CtBu)(L-κN)], (hereafter denoted as species b). Complete experimental data obtained for all systems are summarized in the Experimental Section (labelling is shown in Scheme 1). As illustration, and for clarity, we only discuss the 4,4'-bpy complex (2a). The 1H NMR spectra of the microcrystalline 2a complex (B), the starting material Pt-1 (C) and a mixture Pt-1:4,4'-bpy in 1:4 molar ratio (A) are presented in Figure 4. It should be noted that the coordination of the pyridine N atoms to the Pt center is supported by the well-known coordination-induced shifts of the α-Hpy protons to downfield in relation to the free ligand (δ 8.75), which has been ascribed to the loss of electron density upon pyridine ring coordination. As seen in Figure 4B, only one signal appears at δ 9.00 (d), which is assigned to the bridging species 2a, whereas the two expected different resonances located at 8.96 (d), 8.79 (d) correspond to the terminal species 2b. This later signal (δ 8.79) lies close to that of the free bpy (δ 8.75), being therefore ascribed to the two α-Hpy protons of the uncoordinated pyridine ring in 2b. A particularly reliable indicator of the presence of starting material (Pt-1) is the signal H8 of the pq ligand, very deshielded (δ 9.75) in relation to the others. Fortunately, in all systems under study, the pyridine protons (pyrazine for 1a) and the H8pq signal of the Pt-1 are sufficiently separated from the other signals, so they can be used to establish the approximate ratios from their integrations. Due to remarkable overlapping (or even coincidence for pq signals) an accurate assignment for the rest of signals to individual complexes is not possible. These spectra are consistent with partial dissociation of the N-bridging ligand at room temperature in CDCl3, which could be driven by the trans labilizing effect of the C-cyclometalate atom on the N-donor ligand and the stability of the σ/π-C≡CtBu bridging system in the precursor Pt-1. A reasonable equilibrium (slow on the NMR scale) between the commented species is proposed in Scheme 2.
From a comparison of the analysis of the 1H NMR spectra of these five bimetallic assemblies, we conclude that the experimental approximate ratio determined for the four species depend on the N-donor ligand: (a:Pt-1:b:N-N) ≈ 1:13.1:6.2:8.9 pyz, 1:0.8:1.3:0.2 bpy, 1:0.4:0.9:0.1 bpa, 1:0.5:0.9:0.1 bpe, 1:0.9:1.4:0.2 bpac). The higher proportion of the bimetallic species (a) in solution was found with the more flexible and donor ligands (bpa, bpe), whereas the lowest was with the short and rigid pyrazine ligand being the order: bpa ≈ bpe > bpac ≈ bpy >> pyz. For complex 6, signals due to coordinated and free α-H pyridine protons are also observed together with that of the H8pq proton of Pt-1. However, in this system the possible occurrence of stepwise decoordination of the cyclometalating Pt units cannot be excluded.
Equilibria can be influenced significantly by changing concentration, temperature or solvent. Therefore, an examination of these parameters is important to get a more complete picture of the complexes under study. As representative example, we discuss in detail only the results of the 4,4'-bpy system (2). The influence of the concentration was confirmed by recording the 1H NMR spectra of 2a at different concentrations in CDCl3. As shown in Figure 5, dilution of a solution from a 6 × 10−3 to 1 × 10−3 M causes a progressive shift of the equilibria (i) and (ii) to the right, increasing the presence of Pt-1, 2b and free 4,4'-bpy with concomitant decreasing of 2a. By contrast, upon lowering the temperature to 218 K (Figure S4) the concentration of 2a increases, whereas those of Pt-1, 2b and free bpy decrease (2a:Pt-1:2b:N-N ratio, 298 K ≈ 1:0.8:1.3:0.2 to 218 K 1:0.4:0.8:0.1 for a solution 3 × 10−3 M in CDCl3). These results clearly confirm that the three complexes and the free ligand are involved in a dynamic equilibrium. By using CD3COCD3 as solvent for 2a the final ratio found was a:Pt-1:b:N-N ≈ 1:2.2:2.4:1.2 (Figure S5). Therefore, in this solvent not only the equilibria (i and ii) are shifted to the right in more extension to that observed in CDCl3, but also the formation of Pt-1 and bpy (equilibrium i) was favored in relation to 2b (ii). As was expected, the bimetallic (a)/mononuclear (b) ratio was also influenced by the concentration of the dinucleating N-N ligand. Thus, the 1H NMR spectra recorded for solutions formed by a mixing of Pt-1/L (L2–L5) 1:4 in CDCl3 (established by UV-vis, see below) show mainly the signals associated to the mononuclear species (b), together with the free ligand in excess and small amount of the bimetallic species (a) (a:Pt-1:b:N-N ratio ≈ 1:0:8:14, bpy system, Figure 4A. No signals associated with the starting material (Pt-1) are observed, indicating that the equilibria drawn in Scheme 2 are essentially shifted in counterclockwise in the presence of excess ligand. In the case of the pyrazine system, a large excess of ligand is required to eliminate completely the presence of the precursor (Pt-1:pyz ≈ 1:20), what is in good agreement with the greater amount of starting material observed when the solid 1a is dissolved. It is worth noting that from these solutions only the binuclear complexes 1a–5a and mixtures 1a–5a /1b–5b could be isolated. Despite many attempts, we never got crystals out of any of the mononuclear complexes. However, the proton spectra obtained under these conditions (ratio Pt-1:N-N 1:4 for ligands L2–L5 or 1:20 for L1) have allowed us to carry out a reasonable assignment of the signals of the mononuclear complexes 1b–5b (2D 1H-1H spectra). As the resonances of the starting material, the free ligands and the mononuclear complexes 1b–5b were known, it has been also possible to identify and to assign with some certainty some characteristic signals observed for the solids 1a–5a in CDCl3 solution (see Experimental Section).
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Figure 4. 1H NMR spectra (CDCl3, 400 MHz, 298 K, aromatic region) of: (A) Aliquot of a reaction mixture Pt-1/4,4'-bpy in a 1:4 molar ratio; (B) Microcrystalline sample of [{Pt(pq)(C≡CtBu)}2(μ-4,4'-bpy)] (2a); (C) [Pt(pq)(μ−κCα:η2-C≡CtBu)]2 (Pt-1).
Figure 4. 1H NMR spectra (CDCl3, 400 MHz, 298 K, aromatic region) of: (A) Aliquot of a reaction mixture Pt-1/4,4'-bpy in a 1:4 molar ratio; (B) Microcrystalline sample of [{Pt(pq)(C≡CtBu)}2(μ-4,4'-bpy)] (2a); (C) [Pt(pq)(μ−κCα:η2-C≡CtBu)]2 (Pt-1).
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Inorganics 02 00565 g012 1024
Scheme 2. Equilibrium proposed between the species a, b, Pt-1 and the free ligand in systems 1–5.
Scheme 2. Equilibrium proposed between the species a, b, Pt-1 and the free ligand in systems 1–5.
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Inorganics 02 00565 g005 1024
Figure 5. 1H NMR spectra at 298 K of 2a at different concentrations (mol/L). Selected resonances: Pt-1 (*), 2a (O), 2b (∆), free bpy (□).
Figure 5. 1H NMR spectra at 298 K of 2a at different concentrations (mol/L). Selected resonances: Pt-1 (*), 2a (O), 2b (∆), free bpy (□).
Inorganics 02 00565 g005

2.2. Photophysical Properties

To facilitate comparison, all absorption and emission spectral data are summarized in Table 2 and Table 3.
Table
Table 2. Absorption data for complexes 1a–6a (solid state, diffuse reflectance) and for mixtures Pt-1:L (1:4) (L2–L6) and 1:14 (L1) at 298 K (5 × 10−5 M CH2Cl2 solutions, see text).
Table 2. Absorption data for complexes 1a–6a (solid state, diffuse reflectance) and for mixtures Pt-1:L (1:4) (L2–L6) and 1:14 (L1) at 298 K (5 × 10−5 M CH2Cl2 solutions, see text).
Compoundsλabs/nm (103 ε M−1 cm−1)
1242 (65.3), 276 (62.4), 304 (55.9), 353 (40.1), 408 (20.6) CH2Cl2
300, 330, 365, 390, 430, 530 solid
2243 (63.8), 270 (59.6), 341 (17.7), 354 (18.7), 410 (13.3) CH2Cl2
305, 330, 355, 400, 500, 530 solid
3218 (67.3), 257 (65.1), 299 (42.5), 337 (20.6), 355 (20.2), 413 (17.7) CH2Cl2
300, 320, 350, 415, 525 solid
4242 (63.3), 276 (63.9), 314 (58.3), 355 (29.9), 412 (20.7) CH2Cl2
305, 345, 400, 425, 505, 535 solid
5243 (63.3), 278 (62.2), 325(60.4), 355 (33.2), 410 (20.8) CH2Cl2
305, 320, 360, 390, 420, 540 solid
6245 (80.1), 289 (79.0), 308(71.7), 329sh (43.9), 355 (35), 410 (6.0) CH2Cl2
310, 360, 420, 545 tail to 630 solid

2.2.1. Absorption Spectroscopy

In the solid state, the diffuse reflectance of the polymetallic assemblies are characterized by a low energy and distinctive feature in the range 500–540 nm (with shoulder in 2a and 4a), which is absent in the precursor (Figure S6). According to TD-DFT in gas phase for 2a (see below) this band is assigned to charge transfer from the Pt(pq)(C≡CtBu) units to the central N-N linker 1[(M + L + L')L''CT].
As mentioned above, the 1H NMR spectra of all complexes 1a–6a in CDCl3 solution are consistent with partial dissociation of the bridging ligand, establishing an equilibrium of the bimetallic complex (a) with starting material (Pt-1), the monometallic species (b) and the free ligand according to Scheme 2. Therefore, the obtained spectra are examined taking into account this behavior. The spectra of freshly prepared CH2Cl2 solutions of solid 1a–6a show high energy features (240–330 nm) due to the intraligand transitions (pq, C≡CR and N-N-donor ligand). As is shown in Figure 6A for the 4,4'-bpy system, the intensity of these high energy bands exceeds that of the starting material (Pt-1), as expected for the occurrence of overlapping pyridyl ligand-centered transitions in this region (see Table S2 for absorption of the free ligands). The moderately structured band at 355 nm (1–5) coincides with that observed in the starting material Pt-1, being attributed to 1IL (pq) charge transfer. However, the low-energy absorption (408–413 nm) appears remarkably blue-shifted in relation to the lowest manifold in the precursor, supporting cleavage of the double-alkynyl bridging system. In accordance with the NMR spectra commented before, the progressive addition of the corresponding N-N-donor ligand essentially causes the disappearance of Pt-1. By way of illustration, Figure 6B shows the spectra of the precursor (Pt-1), together with the changes observed upon successive addition of 4,4'-bpy. As it is observed, the maximum of the band is shifted to 410 nm with only 1 equiv. of ligand, but the band shows a long tail in the region where Pt-1 still absorbs, thus confirming its presence. Upon addition of ca. 4 equiv. of ligand, the red-side of band decreases considerably, in accordance with the essentially disappearance of Pt-1. The band changes relatively little with additional equivalents of ligand, though upon addition of more ligand (6–30 equiv.), a small decreasing of the tail is still observed. We attribute, tentatively, this latter change to a complete disappearance of bimetallic species (a) in solution, leaving mononuclear b complexes as the predominant metallic components. The fact that the stepwise addition of ligand takes place keeping the low energy maximum at 410 nm (with minor changes in the tail) suggests that the absorption profiles and electronic structures of bimetallic (species a) and mononuclear complexes (species b) are likely rather similar. A similar behavior has been previously observed in related systems [49]. In the case of the pyz-system, the low energy absorption band shows a gradual change and we determine a relation of ca. 1:14 as the point where the precursor essentially disappears, what is also in agreement with a greater dissociation of the N-ligand in the assembly. As is shown in Figure 6C, the ancillary N-N ditopic ligand has little influence in the low energy manifold. On the basis of previous spectroscopic investigations in phenylquinolyl and alkynyl platinum complexes [57], the low energy absorption band is tentatively ascribed to admixture of platina/alkynyl to cyclometalate (pq) charge transfer [d(Pt)/C≡C→π*(pq)] 1[(M + L')LCT]. This assignment is in agreement with the slight blue shift observed for the less electron donating pyrazine ligand (408 nm) and the slightly red shift seen for the most electron donating 1,2-bis(4-pyridyl)ethane (bpa, 413 nm). However, due to the low lying nature of some of the π* diimine ligands, contribution from platina-alkynyl to N-donor ligand charge transfer 1[Pt(C≡C)→π*(N-donor)] could be also plausible. This contribution is apparent in the bpac system (5), which displays enhanced absorption in the low energy tail (line rose).

2.2.2. Emission Spectroscopy

Qualitatively, the emissions of these complexes are much more intense in all media than those observed for the starting materials (Table 3 and Table S3). The emission profiles are excitation-wavelength independent, indicating that aggregates are not responsible for the observed spectra. The emission spectra of 1a–6a in solid state at room and at 77 K are shown in Figure 7. At room temperature, the bands are unstructured and maximize in the range 590–615 nm, whereas at low temperature the profiles become structured and slightly blue shifted. The decays for these solids were adequately modeled by a single exponential function (τ 0.3–11.4 μs 298 K; 7.6–39.2 μs 77 K) in the range of microseconds, revealing their triplet parentage. In the bpe-bridged binuclear compound 4a, the highly structured emission profile at low temperature, with peak maxima at 588, 648 and 702 nm, the observed vibronic spacing (close to that observed for the free ligand), and also the long lifetime (39.2 μs) are consistent with a predominantly bpe-centered 3IL 3(ππ)* excited state. However, the emission profiles of 1a (pyz) and 2a (bpy) are similar (590 nm 298 K; 572, 610 77 K 1a, 574, 612 nm 77 K 2a) and compares to those seen for typical phenylquinolyl platinum complexes (i.e., [Pt(pq){H2B(pz)2}] λem = 580, 610 nm) [58], what is consistent with emission from a 3MLCT excited state likely mixed with alkynyl to pq charge transfer contribution (3MLCT/3L'LCT). For the remaining complexes (3a, 5a and 6a), the low temperature profiles are also similar to those of 1a and 2a, but the maxima are slightly red shifted in the 6a(tpac) > 3a(bpa) > 5a(bpac), pointing to some contribution of the central N-linker ligand.
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Figure 6. (A) Absorption spectra of 2a, [Pt(pq)(μ-κCα:η2-C≡CtBu)]2 (Pt-1) and free 4,4'-bpy in CH2Cl2; (B) Normalized absorption spectra in CH2Cl2 of [Pt(pq)(μ-κCα:η2-C≡CtBu)]2 (Pt-1) and successive additions of 1, 2, 4, 6, 8 and 30 equiv. of 4,4'-bpy; (C) Low energy region of the absorption spectra of solids 1a–6a and Pt-1 in CH2Cl2.
Figure 6. (A) Absorption spectra of 2a, [Pt(pq)(μ-κCα:η2-C≡CtBu)]2 (Pt-1) and free 4,4'-bpy in CH2Cl2; (B) Normalized absorption spectra in CH2Cl2 of [Pt(pq)(μ-κCα:η2-C≡CtBu)]2 (Pt-1) and successive additions of 1, 2, 4, 6, 8 and 30 equiv. of 4,4'-bpy; (C) Low energy region of the absorption spectra of solids 1a–6a and Pt-1 in CH2Cl2.
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Figure 7. Normalized emission spectra of complexes 1a–6a in solid state (A) at 298 K; (B) at 77 K (λexc 400 nm).
Figure 7. Normalized emission spectra of complexes 1a–6a in solid state (A) at 298 K; (B) at 77 K (λexc 400 nm).
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Table
Table 3. Photophysical data for complexes 1a–6a (solid state) and for mixtures Pt-1:L (1:4) (CH2Cl2, 5 × 10−5 M).
Table 3. Photophysical data for complexes 1a–6a (solid state) and for mixtures Pt-1:L (1:4) (CH2Cl2, 5 × 10−5 M).
CompoundMedium (T ª/K)λem/nm (λexc/nm)τ/μsϕ(%)
1Solid (298)590maxa (365–550)8.013.8
Solid (77)572max, 610, 660sh (365–540)14.0-
5 × 10−5 M (298)595maxa (350–420)--
5 × 10−5 M (77)570max, 610, 660sh (365–450)--
2Solid (298)590 a (365–530)9.913.1
Solid (77)574max, 612, 650(365–530)12.2-
5 × 10−5 M (298)590 a (365–410)--
5 × 10−5 M (77)570max, 625, 660sh (365–430)--
3Solid (RT)610a (365–540)9.99.4
Solid (77)588max, 625 (365–550)7.7-
5 × 10−5 M (RT)595 (365–420)--
5 × 10−5 M (77)570max, 610, 660sh (365–460)--
4Solid (298)605 a (365–540)10.44.9
Solid (77)588max, 648, 702(365–540)39.2-
5 × 10−5 M (77) b 578max, 620, 650sh (365–440)13.8-
5Solid (298)596 a (365–540)11.46.8
Solid (77)596 a (365–540)15.2-
5 × 10−5 M (298)595 (365–420)9.2-
5 × 10−5 M (77)576max, 613, 660sh (365–440)14.4-
6Solid (298)615 a (365–500)0.31.1
Solid (77)598 a (365–480)7.6-
5 × 10−5 M (298)595 (365–415)--
5 × 10−5 M (77)570max, 612, 670sh (365–480)--
a Tail to 800 nm; b Non emissive at 298 K.
Due to the occurrence of the dissociation process commented above, the study of the emissions in solution was carried out using CH2Cl2 solutions with a Pt-1:L proportion of 1:4 (data are listed in Table 3). Under these conditions, the predominant species in solution is the mononuclear complex (for 2b–5b) or mixtures with the corresponding binuclear complex in the case of systems with pyrazine and the trinucleating 1,3,5-tris(pyridyl)acetylene ligand (1 and 6). As has been noted before, both species afford similar low energy absorption features. Not unexpectedly, the bpe complex (4b) is not emissive in fluid, probably due to easy nonradiative relaxation by forming a twisted triplet state (3p) [49,59,60,61]. The remaining complexes display a rather similar intense broad emission centered around 595 nm with negligible influence of the N-donor auxiliary ligand, suggesting a similar emissive state (Figure 8A, Table 3). Upon cooling at 77 K, the emission shifts remarkably to higher energies exhibiting structured profiles (Figure 8B) with minimal variations in λmax (range 570–576 nm). At 77 K, the bpe complex 4b is also emissive (line orange) exhibiting similar structured profile with a peak maximum at 578 nm, pointing to a similar emissive state. In fact, lifetime measurements for two representative complexes with ligands bpe and bpac in glass state are also similar (see Table 3). The emission is mainly attributed to mononuclear complexes and it is ascribed to admixture of 3MLCT and alkynyl to pq charge transfer (3MLCT/3L'LCT). Further support is obtained from the excitation spectra in fluid solution, which resemble the corresponding absorption spectra in these conditions. Identical profiles but with reduced intensity are obtained from solution of binuclear 1a–3a and 5a solids (or by using Pt-1/L 1:1 molar ratio) likely due to similar luminescence response of the species a and b (both present in solution), which are clearly more emissive than the starting material. As illustration, the different spectra obtained for the Pt-1/bpy system in different molar ratio are shown in Figure S7. Interestingly, in contrast to the nonemissive behavior of the mononuclear complex 4b, a diluted solution (5 × 10−5 M) of the bpe binuclear complex 4a displays an unstructured band located at 600 nm upon excitation at 420 nm, which is related to the presence of the more rigid 4a in solution. In glass, the band is only slightly structured and blue shift (565 max, 600 sh nm).
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Figure 8. Emission spectra of mixtures (see text) of Pt-1:L (1:4 ratio) in CH2Cl2 (5 × 10−5 M) at (A) 298 K and (B) 77 K.
Figure 8. Emission spectra of mixtures (see text) of Pt-1:L (1:4 ratio) in CH2Cl2 (5 × 10−5 M) at (A) 298 K and (B) 77 K.
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2.3. Theoretical Calculations

To shed some light, TD-DFT and DFT calculations have been carried out for the species 2a. The optimization in the ground state agrees well with the experimental structure (see Table S4 for details), the most remarkable difference being the lengthening of the Pt-N(pyridyl) distances. The distribution of the frontier molecular orbitals in the ground state and the corresponding partial molecular orbital composition (percentages), together with selected low-lying transitions in vacuum and in CH2Cl2 solution and the Cartesian coordinates are provided in Tables S5–S8. Some selected orbitals are shown in Figure 9. The HOMO and HOMO-1 have similar contribution from each one of the Pt and alkynyl units (i.e., HOMO Pt (32%) and C≡CtBu (61%) on fragment 2; HOMO−1 Pt (35%) and C≡CtBu (57%) on fragment 1), whereas the HOMO−2 and HOMO−3 are of similar energy and located on the unit 2 Pt(pq)(C≡CtBu). The HOMO−4 is centered on fragment 1 [Pt (30%), pq (55%) and C≡CtBu (13%)]. The LUMO is mainly centered on the bipyridine ligand (94%) but the two following low lying virtual orbitals LUMO+1 and LUMO+2 are, however, localized on the low lying pq goups (LUMO+1 93%; LUMO+2 93%). The lowest energy absorption calculated in phase gas at 494 nm (Table S6) compares to that seen in the experimental solid reflectance spectrum at 530 nm. This band arises mainly from the HOMO−4 to LUMO transition and can be described to charge transfer from the Pt(pq)(C≡CtBu) units of the fragment 1 to the central N-N linker 1[(M + L + L')L''CT]. The two following excitations calculated around 482 nm are of more complex configuration with significant charge transfer from HOMO and HOMO−1 to LUMO+1 and LUMO+2. These absorptions are mainly ascribed to platinum-alkynyl to cyclometalated 1[(M + L')LCT] and could be correlated with the experimental feature located at 500 nm.
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Figure 9. Molecular orbital plots for the computed S0 state of complex 2a (Pt(2) left; Pt(1) right).
Figure 9. Molecular orbital plots for the computed S0 state of complex 2a (Pt(2) left; Pt(1) right).
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By taking into consideration the solvent (CH2Cl2), there is an obvious blue shift in the lowest singlet excitations in agreement with the nature of its charge transfer (Table S7). Interestingly, the transition which involves charge transfer to the central bpy linker (HOMO−4 to LUMO) now has higher energy (S4, calculated at 419 nm). The three lower energy singlets (S1, S2 and S3) have similar energy values (435, 431 and 428 nm) and are mainly composed of excitations from HOMO−3 to HOMO→LUMO to LUMO+2. Therefore, the experimental band located in solution at 410 nm could be ascribed as an admixture of platinum-alkynyl-pq to pq charge transfer 1[(M + L + L')LCT] with contribution to the central bpy linker 1[(M + L + L')L''CT] (L = pq, L' = C≡CtBu, L'' = bpy).
To clarify the emission character of 2a, its triplet state geometry in gas phase was optimized (Table S9). The calculated emission as the energy difference between S0 and T1 states (584 nm) is in accordance with the experimental value (590 nm). The excitation takes place with clear changes in the frontier orbitals respect to the ground state. The SOMO−1 is now located on the pq (58%), Pt (26%) and the alkynyl ligand (16%) on fragment 1, whereas the SOMO is mainly centered in the pq(1) (92%) (Figure 10). In agreement, the localization of the spin density lies on one of the pq ligands and the platinum/alkynyl group of one fragment of the molecule (Figure S8). Thus, the emission has a mixed platinum/alkynyl to phenylquinolyl charge transfer character 3[(M + L')LCT] with some minor 3IL(pq) contribution, supporting the negligible influence of the N-donor linker.
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Figure 10. Molecular orbital plots for the computed T1 state of complex 2a.
Figure 10. Molecular orbital plots for the computed T1 state of complex 2a.
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3. Experimental Section

General Comments. All reactions were carried out under an atmosphere of dry argon, using standard Schlenk techniques. Solvents were obtained from a solvent purification system (M-BRAUN MB SPS-800, MBRAUN, Garching, Germany). NMR spectra were recorded at 293 K on Bruker ARX 300 or ARX 400 spectrometers (Madison, WI, USA). Chemical shifts are reported in ppm relative to external standards (SiMe4) and all coupling constants are given in Hz. The NMR spectral assignments of the phenylquinolyl ligands (Hpq) follow the numbering scheme shown in Scheme 1. IR spectra were obtained on a Nicolet Nexus FT-IR Spectrometer (Thermo Scientific, Waltham, UK), using KBr pellets. Elemental analyses were carried out with a Carlo Erba EA1110 CHNS-O microanalyzer (Carlo Erba, Rodano, Italy). Mass spectra were recorded on a HP-5989B mass spectrometer (Hewlett Packard, East Lyme, CT, USA) using the ES techniques (exact mass). The optical absorption spectra were recorded using a Hewlett-Packard 8453 (solution) spectrophotometer (Hewlett Packard, East Lyme, CT, USA) in the visible and near-UV ranges. Diffuse reflectance UV-vis (DRUV) data of pressed powder diluted with SiO2 were recorded on a Shimadzu (UV-3600 spectrophotometer with a Harrick Praying Mantis accessory, Harrick Scientific Products, New York, NY, USA) and recalculated following the Kubelka-Munk function. Emission and excitation spectra were obtained on a Jobin-Yvon Horiba Fluorolog 3-11 Tau-3 spectrofluorimeter (Horiba, Kyoto, Japan), with the lifetimes measured in phosphorimeter mode (6a in solid state at 298 K was measured using a Data station HUB-B with a nanoLED controller DAS6). Quantum yields in solid state were measured upon excitation at ~400 nm using a F-3018 Integrating Sphere mounted on a Fluorolog 3-11 Tau-3 spectrofluorimeter (Horiba, Kyoto, Japan). The starting material [Pt(pq)(μ-κCα:η2-C≡CtBu)]2 (Pt-1) [57] and the ligands di(4-pyridyl)acetylene [62] and 1,3,5-tris(pyridine-4-ylethynyl)benzene [63] were prepared according to the reported procedure.
Preparation of [{Pt(pq)(C≡CtBu)}2(μ-pyz)] (1a). A solution of [Pt(pq)(μ-κCα:η2-C≡CtBu)]2 (0.100 g, 0.104 mmol) in CH2Cl2 (20 mL) was treated with pyrazine (pyz) (0.017 g, 0.208 mmol) and the mixture was stirred for 3h. Evaporation to small volume (2 mL) afforded 1a as an orange solid, which was filtered and washed with n-hexane (5 mL) (0.089 g, 82%). Anal. Calcd for C46H42N4Pt2 (1041.01): C, 53.07; H, 4.07; N, 5.38. Found: C, 53.32; H, 4.11; N, 5.12. ESI (+): m/z (%) 481 (77) [Pt(pq)(C≡CtBu)]+, 879 (41) [Pt2(pq)2(C≡CtBu)]+, 961 (100) [{Pt(pq)(C≡CtBu)}2]+, 1041 (7) [M]+. IR (KBr) (cm−1): υ(C≡C) 2110 (m). 1H NMR (δ, 300.13 MHz, CDCl3) (1a:Pt-1:1b:free ligand, ~1:13.1:6.2:8.9): 9.75 (d, JH-H = 8.6, H8pq Pt-1), 8.90–8.84 (m, Hαpyz 1a, Hαpyz 1b), 8.63 (s, Hαpyz 1b), 8.60 (s, Hαpyz free), 8.42 (d, JH-H = 7.4, H12pq 1a, 1b), 8.30 (d, JH-H = 7.5, H12pq Pt-1, H3/4pq 1a, 1b), 8.27 (d, JH-H = 8.9, H3/4pq Pt-1); 7.96 (d, JH-H = 8.0, H3/4pq 1a, 1b), 7.89 (d, JH-H = 8.7, H3/4pq Pt-1), 7.80–7.72 (m, H8/5pq 1a, 1b; H7pq, H5pq, H9pq Pt-1), 7.65 (d, JH-H = 6.5, H9pq 1a, 1b), 7.51 (t, JH-H = 7.1, H6pq Pt-1), 7.49–7.29 (m, H8/5pq, H7/6pq 1a, 1b; H11pq Pt-1), 7.34 (t, JH-H = 8.1, H11pq Pt-1), 7.23–7.12 (m, H11pq, H10pq, H7/6pq 1a, 1b; H10pq Pt-1); 1.28 (s, HCH3 1a, 1b), 0.96 (s, HCH3 Pt-1).
Data of the Pt-1:pyz, 1:20: (1a:Pt-1:1b:free ligand, ~1:0:7:82). 1H NMR (δ, 400.17 MHz, CDCl3): 8.83 (s, Hαpyz 1a); 8.81 (s, Hαpyz 1b), 8.59 (s, Hαpyz 1b, Hαpyz free), 8.38 (d, JH-H = 7.9, H12pq 1a, 1b), 8.29 (d, JH-H = 8.5, H3/4pq 1a, 1b), 7.94 (d, JH-H = 8.7, H3/4pq 1a, 1b), 7.80 (d, JH-H = 7.9, H8/5pq 1a, 1b), 7.63 (d, JH-H = 7.8, H9pq 1a, 1b), 7.39–7.35 (m, H8/5pq, H7/6pq 1a, 1b), 7.20–7.09 (m, H11pq, H7/6pq, H10pq 1a, 1b), 1.27 (s, HCH3 1a, 1b).
Preparation of [{Pt(pq)(C≡CtBu)}2(μ-bpy)] (2a). A solution of [Pt(pq)(μ-κCα:η2-C≡CtBu)]2 (0.100 g, 0.104 mmol) in CH2Cl2 (20 mL) was treated with 4,4'-bipyridine (0.016 g, 0.104 mmol) and the obtained solution was stirred for 2 h. Evaporation to small volume (2 mL) and treating with n-hexane (10 mL) afforded 2a as an orange microcrystalline solid (0.073 g, 63%). Anal. Calcd for C52H46N4Pt2 (1117.14): C, 55.91; H, 4.15; N, 5.02. Found: 55.74; H, 3.93; N, 4.86. ESI (+): m/z (%) 481 (100) [Pt(pq)(C≡CtBu)]+, 637 (6) [Pt(pq)(C≡CtBu)(bpy)]+, 879 (10) [Pt2(pq)2(C≡CtBu)]+, 961 (77) [{Pt(pq)(C≡CtBu)}2]+, 1117 (3) [M]+. IR (KBr) (cm−1): υ(C≡C) 2117 (m). 1H NMR (δ, 400.17 MHz, CDCl3) (2a:Pt-1:2b:free ligand, ~1:0.8:1.3:0.2): 9.75 (d, JH-H = 8.6, H8pq Pt-1), 9.00 (d, JH-H = 5.9, Hαbpy 2a), 8.96 (d, JH-H = 5.8, Hαbpy 2b), 8.79 (d, JH-H = 5.3, Hαbpy 2b), 8.75 (d, JH-H = 5.1, Hαbpy free), 8.44 (d, JH-H = 7.4, JPt-H = 67.6, H12pq 2a, 2b), 8.30 (d, JH-H = 8.4, H12pq Pt-1, H3/4pq 2a, 2b), 8.27 (d, JH-H = 9.8, H3/4pq Pt-1), 7.96 (d, JH-H = 8.8, H3/4pq 2a, 2b), 7.88 (d, JH-H = 8.6, H3/4pq Pt-1), 7.79 (m, H8/5pq 2a, 2b; H7pq, H5pq Pt-1), 7.73 (d, JH-H = 7.9, H9pq Pt-1), 7.65 (d, JH-H = 7.7, H9pq 2a, 2b), 7.61–7.54 (m, Hβbpy 2a, Hβbpy 2b, Hβbpy free), 7.54 (m, H8/5pq 2, H6pq Pt-1), 7.36 (t, JH-H = 7.8, H7/6pq 2a, 2b), 7.34 (t, JH-H = 8.1, H11pq Pt-1), 7.20 (m, H11pq 2a, 2b, H10pq Pt-1), 7.12 (m, H7/6pq, H10pq 2a, 2b), 1.30 (s, HCH3 2a, 2b), 0.96 (s, HCH3 Pt-1).
At 218 K, 3 × 10−3 M (Pt-1:2a:2b:free ligand, ~1:0.4:0.8:0.1).
1H NMR (δ, 400.17 MHz, CD3COCD3, 3 × 10−3 M): (2a:Pt-1:2b:free ligand, ~1:2.2:2.4:1.2).
Data of the Pt-1:bpy, 1:4: (2a:Pt-1:2b:free ligand, ~1:0:8:14). 1H NMR (δ, 400.17 MHz, CDCl3): 9.00 (d, JH-H = 6.0, Hαbpy 2a), 8.94 (d, JH-H = 5.8, Hαbpy 2b), 8.78 (d, JH-H = 5.3, Hαbpy 2b), 8.74 (d, JH-H = 4.9, Hαbpy free), 8.44 (d, JH-H = 7.5, JPt-H = 67.6, 1H, H12pq 2a, 2b), 8.28 (d, JH-H = 8.6, H3/4pq 2a, 2b), 7.94 (d, JH-H = 8.6, H3/4pq 2a, 2b), 7.78 (d, JH-H = 7.8, H8/5pq 2a, 2b), 7.64 (d, JH-H = 7.5, H9pq 2a, 2b), 7.58 (d, JH-H = 5.7, Hβbpy 2b), 7.55 (d, JH-H = 5.6, Hβbpy 2b), 7.53 (d, JH-H = 5.1, Hβbpy free), 7.51 (d, JH-H = 7.7, H8/5pq 2a, 2b), 7.32 (t, JH-H = 7.5, H7/6pq 2a, 2b), 7.18 (t, JH-H = 7.1, H11pq 2a, 2b), 7.10 (t, JH-H = 7.1, H7/6/10pq 2a, 2b), 7.08 (t, JH-H = 7.6, H7/6/10pq 2a, 2b), 1.28 (s, HCH3 2a, 2b).
Preparation of [{Pt(pq)(C≡CtBu)}2(μ-bpa)] (3a). This compound was prepared as an orange microcrystalline solid (0.100 g, 85%) in a similar way to 2a, starting from [Pt(pq)(μ-κCα:η2-C≡CtBu)]2 (0.100 g, 0.104 mmol) and bis(4-pyridine)ethane (0.018 g, 0.104 mmol), but treating with Et2O (10 mL). Anal. Calcd for C54H50N4Pt (1141.33): C, 56.64; H, 4.40; N, 4.89. Found: 56.43; H, 4.31; N, 5.10. ESI (+): m/z (%) 481 (72) [Pt(pq)(C≡CtBu)]+, 662 (4) [Pt(pq)(C≡CtBu)(bpa)]+, 879 (50) [Pt2(pq)2(C≡CtBu)]+, 961 (100) [{Pt(pq)(C≡CtBu)}2]+, 1145 (16) [M]+. IR (KBr) (cm−1): υ(C≡C) 2119 (m). 1H NMR (δ, 400.17 MHz, CDCl3) (3a:Pt-1:3b:free ligand, ~1:0.4:0.9:0.1): 9.75 (d, JH-H = 8.6, H8pq Pt-1), 8.74 (d, JH-H = 5.7, Hαpy 3a), 8.69 (d, JH-H = 6.1, Hαpy 3b), 8.52 (d, JH-H = 5.4, Hαpy 3b), 8.50 (d, JH-H = 5.4, 1H, Hαpy free), 8.42 (d, JH-H = 7.5, JPt-H = 61.6, H12pq 3a, 3b), 8.30 (d, JH-H = 7.6, H12pq Pt-1), 8.27 (d, JH-H = 8.7, H3/4pq 3a, 3b, H3/4pq Pt-1), 7.94 (d, JH-H = 8.7, H3/4pq 3a, 3b), 7.88 (d, JH-H = 8.7, H3/4pq Pt-1), 7.80–7.73 (m, H8/5pq 3a, 3b; H5pq, H7pq, H9pq Pt-1), 7.64 (d, JH-H = 7.4, H9pq 3a, 3b), 7.53–7.45 (m, H8/5pq 3a, 3b, H6pq Pt-1), 7.35–7.29 (m, H7/6pq 3a, 3b, H11pq Pt-1), 7.22–7.16 (m, H11pq 3a, 3b, H1°°pq Pt-1), 7.11 (d, JH-H = 5.4, Hβpy 3a, Hβpy3b), 7.08–7.04 (m, Hβpy 3b, Hβpy free), 7.00 (d, JH-H = 7.8, H7/6pq , H1°°pq 3a, 3b), 3.03 (s, HCH2 3a), 3.00 (s, HCH2 3b), 2.98 (s, HCH2 free), 1.28 (s, HCH3 3a, 3b), 0.96 (s, HCH3 Pt-1).
Data of the Pt-1:bpa, 1:4: (3a:Pt-1:3b:free ligand, ~1:0:6:9). 1H NMR (δ, 400.17 MHz, CDCl3): 8.74 (d, JH-H = 6.3, Hαpy 3a), 8.69 (d, JH-H = 6.3, Hαpy 3b), 8.52 (d, JH-H = 5.4, Hαpy 3b), 8.50 (d, JH-H = 5.4, 1H, Hαpy free), 8.42 (d, JH-H = 7.4, JPt-H = 63.5, H12pq 3a, 3b), 8.26 (d, JH-H = 8.7, H3/4pq 3a, 3b), 7.93 (d, JH-H = 8.8, H3/4pq 3a, 3b), 7.77 (d, JH-H = 7.9, H8/5pq 3a, 3b), 7.63 (d, JH-H = 7.3, H9pq 3a, 3b), 7.46 (d, JH-H = 8.9, H8/5pq 3a, 3b), 7.33 (d, JH-H = 7.5, H7/6pq 3a, 3b), 7.17 (d, JH-H = 7.1, H11pq 3a, 3b), 7.11 (d, JH-H = 7.0, Hβpy 3a, Hβpy 3b), 7.07 (d, JH-H = 4.1, Hβpy 3b, Hβpy free); 7.00 (d, JH-H = 7.7, H7/6pq, H10q 3a, 3b), 3.03 (s, HCH2 3a), 3.00 (s, HCH2 3b), 2.98 (s, HCH2 free), 1.27 (s, HCH3 3a, 3b).
Preparation of [{Pt(pq)(C≡CtBu)}2(μ-bpe)] (4a). This compound was prepared as an orange solid (0.070 g, 59%) in a similar way to 2a, starting from [Pt(pq)(μ-κCα:η2-C≡CtBu)]2 (0.100 g, 0.104 mmol) and 1,2-di(4-pyridyl)ethylene (0.019 g, 0.104 mmol), precipitating in the reaction media. Anal. Calcd for C54H48N4Pt2 (1143.18): C, 56.74; H, 4.23; N, 4.90. Found: C, 56.59; H, 4.23; N, 4.87. ESI (+): m/z (%) 481 (100) [Pt(pq)(C≡CtBu)]+, 663 (6) [Pt(pq)(C≡CtBu)(bpe)]+, 879 (8) [Pt2(pq)2(C≡CtBu)]+, 961 (90) [{Pt(pq)(C≡CtBu)}2]+, 1143 (3) [M]+. IR (KBr) (cm−1): υ(C≡C) 2115 (s). 1H NMR (δ, 400.17 MHz, CDCl3) (4a:Pt-1:4b:free ligand, ~1:0.5:0.9:0.1): 9.75 (d, JH-H = 8.5, H8pq Pt-1), 8.85 (d, JH-H = 5.4, Hαpy 4a), 8.82 (d, JH-H = 4.9, Hαpy 4b), 8.66 (d, JH-H = 4.9, 1H, Hαpy 4b), 8.64 (d, JH-H = 4.7, 1H, Hαpy free), 8.43 (d, JH-H = 7.8, JPt-H = 61.4, 1H, H12pq 4a, 4b), 8.31–8.26 (m, H3/4pq 4a, 4b; H12pq, H3/4pq Pt-1), 7.96 (d, JH-H = 8.6, H3/4pq 4a, 4b), 7.89 (d, JH-H = 8.4, H3/4pq Pt-1), 7.79 (d, JH-H = 7.9, H8/5pq 4a, 4b; H7pq, H5pq Pt-1), 7.73 (d, JH-H = 8.0, H9pq Pt-1), 7.65 (d, JH-H = 7.4, H9pq 4a, 4b), 7.55–7.49 (m, H8/5pq 4a, 4b, H6pq Pt-1), 7.42–7.37 (m, Hβpy 4a, Hβpy 4b, Hβpy free), 7.35–7.30 (m, H7/6pq 4a, 4b, H11pq Pt-1), 7.26–7.19 (m, HC=CH 4a, HC=CH 4b, HC=CH free, H10pq Pt-1), 7.11 (d, JH-H = 7.0, H7/6pq, H1°°pq 4a, 4b), 1.29 (s, HCH3 4a, 4b), 0.96 (s, HCH3 Pt-1).
Data of the Pt-1:bpe, 1:4: (4a:Pt-1:4b:free ligand, ~1:0:5:8). 1H NMR (υ, 400.17 MHz, CDCl3): 8.83 (d, JH-H = 5.9, Hαpy 4a), 8.81 (d, JH-H = 6.0, Hαpy 4b), 8.65 (d, JH-H = 5.6, Hαpy 4b), 8.63 (d, JH-H = 5.4, Hαpy free), 8.44 (d, JH-H = 7.3, JPt-H = 62.0, H12pq 4a, 4b), 8.28 (d, JH-H = 8.7, H3/4pq 4a, 4b), 7.95 (d, JH-H = 8.7, H3/4pq 4a, 4b), 7.78 (d, JH-H = 7.8, H8/5pq 4a, 4b), 7.64 (d, JH-H = 7.6, H9pq 4a, 4b), 7.53 (d, JH-H = 8.7, H8/5pq 4a, 4b), 7.40 (d, JH-H = 5.5, Hβpy 4b), 7.39 (d, JH-H = 5.5, Hβpy free), 7.34 (t, JH-H = 7.6, H7/6pq 4a, 4b), 7.32 (s, HC=C 4a), 7.29 (s, HC=C 4b), 7.23 (s, HC=C 4b), 7.21 (s, HC=C free), 7.18 (t, JH-H = 8.3, H11pq 4a, 4b), 7.11 (t, JH-H = 7.5, H7/6pq, H10pq 4a, 4b), 1.29 (s, HCH3 4a, 4b).
Preparation of [{Pt(pq)(C≡CtBu)}2(μ-bpac)] (5a). This compound was prepared as an orange microcrystalline solid (0.085 g, 72%) in a similar way to 1a, starting from [Pt(pq)(μ-κCα:η2-C≡CtBu)]2 (0.100 g, 0.104 mmol) and di(4-pyridyl)acetylene (0.038 g, 0.208 mmol), but it was stirred for 12 h. Anal. Calcd for C54H46N4Pt2 (1141.16): C, 56.84; H, 4.06; N, 4.91. Found: C, 56.47; H, 4.08; N, 4.78. ESI (+): m/z (%) 481 (100) [Pt(pq)(C≡CtBu)]+, 663 (6) [Pt(pq)(C≡CtBu)(bpac)]+, 879 (55) [Pt2(pq)2(C≡CtBu)]+, 961 (86) [{Pt(pq)(C≡CtBu)}2]+, 1141 (3) [M]+. IR (KBr) (cm−1): υ(C≡C)(internal C≡C) 2223 (w), υ(C≡C) 2118 (s). 1H NMR (δ, 300.13 MHz, CDCl3) (5a:Pt-1:5b:free ligand, ~1:0.9:1.4:0.2): 9.75 (d, JH-H = 8.6, H8pq Pt-1), 8.89 (d, JH-H = 5.6, Hαpy 5a), 8.86 (d, JH-H = 5.6, Hαpy 5b), 8.68 (d, JH-H = 4.5, Hαpy 5b), 8.66 (d, JH-H = 6.0, Hαpy free), 8.41 (d, JH-H = 7.0, JPt-H = 62.3, H12pq 5a, 5b), 8.31–8.26 (m, H12pq, H3/4pq Pt-1, H3/4pq 5a, 5b), 7.96 (d, JH-H = 8.6, H3/4pq 5a, 5b), 7.89 (d, JH-H = 8.6, H3/4pq Pt-1), 7.80–7.77 (m, H8/5pq 5a, 5b; H7pq, H5pq Pt-1), 7.73 (d, JH-H = 7.9, H9pq Pt-1), 7.65 (d, JH-H = 7.2, H9pq 5a, 5b), 7.53–7.48 (m, H8/5pq 5a, 5b, H6pq Pt-1), 7.41 (m, Hβpy 5a, Hβpy 5b, Hβpy free), 7.39–7.32 (m, H7/6pq 5a, 5b, H11pq Pt-1), 7.19–7.09 (m, H7/6pq, H11pq, H10pq 5a, 5b; H10pq Pt-1), 1.29 (s, HCH3 5a, 5b), 0.96 (s, HCH3 Pt-1).
Data of the Pt-1:bpac, 1:4: (5a:Pt-1:5b:free ligand, ~1:0:4:7). 1H NMR (δ, 400.17 MHz, CDCl3): 8.90 (d, JH-H = 5.9, Hαpy 5a), 8.87 (d, JH-H = 6.4, Hαpy 5b), 8.68 (d, JH-H = 5.8, Hαpy 5b), 8.66 (d, JH-H = 5.8, Hαpy free), 8.44 (d, JH-H = 6.1, H12pq 5a, 5b), 8.29 (d, JH-H = 8.2, H3/4pq 5a, 5b), 7.95 (d, JH-H = 8.8, H3/4pq 5a, 5b), 7.80 (d, JH-H = 8.0, H8/5pq 5a, 5b), 7.65 (d, JH-H = 7.7, H9pq 5a, 5b), 7.50 (d, JH-H = 9.7, H8/5pq 5a, 5b), 7.42 (d, JH-H = 7.2, Hβpy 5a, 5b), 7.41 (d, JH-H = 5.9, Hβpy 5b, free), 7.36 (t, JH-H = 7.0, H7/6pq 5a, 5b), 7.21-7.07 (m, H7/6pq, H10pq, H11pq 5a, 5b), 1.30 (s, HCH3 5a, 5b).
Preparation of [{Pt(pq)(C≡CtBu)}3(μ-tpab)] (6a). A solution of [Pt(pq)(μ-κCα:η2-C≡CtBu)]2 (0.150 g, 0.156 mmol) in CH2Cl2 (20 mL) was treated with 1,3,5-tris(pyridine-4-ylethynyl)benzene (0.040 g, 0.104 mmol) (molar ratio 3:2) and the orange solution was stirred for 4 h. Evaporation to small volume (2 mL) and treating with n-hexane (5 mL) gave 6a as an orange microcrystalline solid (0.152 g, 80%). Anal. Calcd for C90H72N6Pt3 (1822.87): C, 59.30; H, 3.98; N, 4.61. Found: C, 59.19; H, 3.92; N, 4.40. ESI (+): m/z (%) 879 (51) [Pt2(pq)2(C≡CtBu)]+, 961 (100) [{Pt(pq)(C≡CtBu)}2]+, 1342 (21) [Pt2(pq)2(C≡CtBu)2L]+, 1660 (3) [M-2C≡CtBu]+, 1823 (2) [M]+. IR (KBr) (cm−1): υ(C≡C)(internal C≡C) 2212 (m), υ(C≡C) 2115 (m). 1H NMR (δ, 300.13 MHz, CDCl3): 9.75 (d, JH-H = 8.6, H8pq Pt-1), 8.85 (m, Hαpy 6a, Hαpy 6b), 8.65 (m, Hαpy 6b, Hαpy free), 8.42 (d, JH-H = 8.3, H12pq 6a, 6b, 6c), 8.30 (d, JH-H = 7.3, H12pq Pt-1, H3/4pq 6a, 6b, 6c), 8.27 (d, JH-H = 7.9, H3/4pq Pt-1), 7.96 (d, JH-H = 8.3, H3/4pq 6a, 6b, 6c), 7.89 (d, JH-H = 8.8, H3/4pq Pt-1), 7.80–7.72 (m, H8/5pq 6a, 6b, 6c; H7pq, H5pq, H9pq Pt-1, Hβpy 6a, C-HC6H3 6a, C-HC6H3 6b, C-HC6H3 free), 7.66-7.62 (m, H9pq 6, Hβpy 6b,), 7.53–7.49 (m, H8/5pq 6a, 6b, 6c, H6pq Pt-1), 7.39–7.42 (m, Hβpy free, H7/6pq 6a, 6b, 6c, H11pq Pt-1), 7.19–7.10 (m, H7/6pq, H11pq, H10pq 6a, 6b, 6c; H10pq Pt-1), 1.30 (s, HCH3 6a, 6b, 6c), 0.96 (s, HCH3 Pt-1).
Data of the Pt-1:tpab, 1:4. 1H NMR (δ, 400.17 MHz, CDCl3): 8.85 (d, JH-H = 4.6, Hαpy 6a), 8.81 (d, JH-H = 4.8, Hαpy 6b), 8.64 (d, JH-H = 4.2, Hα2py 6b, Hαpy free), 8.42 (d, JH-H = 7.4, H12pq 6a, 6b, 6c), 8.29 (d, JH-H = 8.6, H3/4pq 6a, 6b, 6c), 7.95 (d, JH-H = 8.6, H3/4pq 6a, 6b, 6c), 7.82–7.79 (m, JH-H = 8.0, H8/5pq 6a, 6b, 6c, Hβpy 6a), 7.78 (s, C-HC6H3 6a), 7.76 (s, C-HC6H3 6b), 7.75 (s, C-HC6H3 free), 7.64 (d, JH-H = 7.9, H9pq 6a, 6b, 6c), 7.60 (d, JH-H = 5.4, Hβpy 6b), 7.51 (d, JH-H = 8.9, H8/5pq 6a, 6b, 6c), 7.40–7.37 (m, Hβpy free, H7/6pq 6a, 6b, 6c), 7.20–7.09 (m, H7/6pq, H10pq, H11pq 6a, 6b, 6c), 1.30 (s, HCH3 6a, 6b, 6c).
X-ray Crystallography: Details of the structural analyses for all complexes are summarized in Table S10. Orange (2, 4) crystals were obtained by slow diffusion at room temperature of n-hexane into solutions of the complexes in CHCl3. In all the cases, graphite-monochromatic Mo-Kα radiation was used, X-ray intensity data were collected with a NONIUS-κCCD area-detector diffractometer (CAMCOR, Oregon, USA) and images processed using the DENZO (Academic Press: New York, NY, USA) and SCALEPACK suite of programs [64], carrying out the absorption correction at this point. The structures were solved by Direct and Patterson Methods using SIR2004 [65], and refined by full-matrix least squares on F2 with SHELXL-97 [66]. All non-hydrogen atoms were assigned anisotropic displacement parameters. All the hydrogen atoms were constrained to idealized geometries fixing isotropic displacement parameters 1.2 times the Uiso value of their attached carbon. Finally, the structures show some residual peaks greater than 1 eA−3 in the vicinity of the platinum atoms, but with no chemical meaning. CCDC 1024492-1024493 contain the supplementary crystallographic data for compounds 2a·2CHCl3·C6H14 and 4a·4CHCl3 [67].
Computational Details for Theoretical Calculations. DFT and TD-DFT calculations were performed on complex 2a with Gaussian 03 revision E.01 [68]. Geometries in the S0 ground state and T1 excited state were optimized using the restricted B3LYP (S0) or unrestricted U-B3LYP (T1) Becke’s three-parameter functional combined with Lee-Yang-Parr’s correlation functional [69,70,71]. The basis set used for the platinum centers was the LanL2DZ effective core potential [72] and 6-31G(d,p) for the ligand atoms. The solvent effect of the dichloromethane in the TD-DFT calculation was taken into consideration by the polarizable continuum model (PCM) [73] using CPCM [74].

4. Conclusions

In this study, we present a series of different bidentate N-donor ligands of different lengths and flexibility and one N-tridentate ligand, which undergo bridge-splitting reactions with [Pt(pq)(μ-κCα:η2-C≡CtBu)]2 (Pt-1) to form binuclear [{Pt(pq)(C≡CtBu)}2(μ-L)] (1a–5a) and trinuclear [{Pt(pq)(C≡CtBu)}3(μ-L)] (6a) derivatives. The structures of 2a and 4a have been confirmed by X-ray crystallography. Probably due to the trans labializing effect of the C-cyclometalating atom and the high stability of Pt-1, these complexes rearrange in solution giving rise to a dynamic equilibrium between the diplatinum complexes (1a–5a), the mononuclear species [Pt(pq)(C≡CtBu)(L-κN)] (1b–5b), (Pt-1) and the free ligand (L), as confirmed by 1H NMR experiments. The equilibrium is affected by the concentration, temperature and solvent polarity. The higher proportion of the bimetallic species (a) in solution was found with the more flexible and donor ligands (bpa, bpe), in concentrated solutions, at low temperatures and with less polar solvents.
TD-DFT calculations on 2a allow to assign the low-energy absorption band in solid state to Pt(pq)(C≡CtBu) to N-linker charge transfer (500–540 nm) 1[(M + L + L')L''CT], whereas in CH2Cl2 solution the low energy band (408–413 nm) is ascribed mainly to admixture of platina/alkynyl/pq to cyclometalate (pq) charge transfer 1[d(Pt)/C≡C/pq→π*(pq)] 1[(M + L + L')LCT] with contribution to the N-donor ligand 1[d(Pt)/C≡C/pq→π*(N-donor)]. These assemblies show a stronger luminescence than the starting material (Pt-1). The emission properties of the bpe-dimer 4a in solid state are consistent with a predominantly bpe-centered 3IL 3(ππ*) excited state. However, complexes 1a and 2a exhibit emission from an admixture platina/alkynyl to cyclometalated (pq) charge transfer 3[(M + L')LCT], as supported by DFT calculations on 2a, and in the remaining complexes (3a–5a) some additional contribution of the central N-linker can be invoked. For these complexes, experimental data (in solid state) and theoretical calculations (in gas phase for 2a) suggest that the excitation of the molecule seems to introduce a remarkable change in the nature of the HOMO-LUMO/SOMO-SOMO-1, increasing the energy of the π* diimine-based orbitals above the cyclometalated-based orbitals. The emission in solution of the binuclear species (a) (predominant in mixtures Pt-1:L 1:1) and the mononuclear species; (b) (predominant in mixtures Pt-1:L 1:4) is rather similar, suggesting a similar excited state for both types of species, attributed in all complexes to 3[(M + L')LCT].

Acknowledgments

This work was supported by the Spanish MICINN (Projects CTQ2008-06669-C02-02/BQU and CTQ2013-45518-P and a grant for Santiago Ruiz). The authors thank CESGA for computer support.

Author Contributions

Elena Lalinde and M. Teresa Moreno: responsible for research publication (methodology, data analysis and preparation of the manuscript). Santiago Ruiz: experimental work and crystallographic studies. Sergio Sánchez: theoretical calculations.

Conflicts of Interest

The authors declare no conflict of interest.

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MDPI and ACS Style

Lalinde, E.; Moreno, M.T.; Ruiz, S.; Sánchez, S. Attachment of Luminescent Neutral “Pt(pq)(C≡CtBu)” Units to Di and Tri N-Donor Connecting Ligands: Solution Behavior and Photophysical Properties. Inorganics 2014, 2, 565-590. https://doi.org/10.3390/inorganics2040565

AMA Style

Lalinde E, Moreno MT, Ruiz S, Sánchez S. Attachment of Luminescent Neutral “Pt(pq)(C≡CtBu)” Units to Di and Tri N-Donor Connecting Ligands: Solution Behavior and Photophysical Properties. Inorganics. 2014; 2(4):565-590. https://doi.org/10.3390/inorganics2040565

Chicago/Turabian Style

Lalinde, Elena, M. Teresa Moreno, Santiago Ruiz, and Sergio Sánchez. 2014. "Attachment of Luminescent Neutral “Pt(pq)(C≡CtBu)” Units to Di and Tri N-Donor Connecting Ligands: Solution Behavior and Photophysical Properties" Inorganics 2, no. 4: 565-590. https://doi.org/10.3390/inorganics2040565

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