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

Binuclear derivatives [{Pt(pq)(C≡CBu)}2(μ-L)] (1a–5a), containing a series of dinitrogen linker ligands and the trinuclear [{Pt(pq)(C≡CBu)}3(μ-L)] (6a) [L = μ-1,3,5-tris(pyridine-4-ylethynyl)benzene], formed by bridge-splitting reactions with [Pt(pq)(μ-κC:η-C≡CBu)]2 (Pt-1), are reported. The complexes are characterized by a combination of H NMR spectroscopy, mass spectrometry and X-ray crystallography (2a and 4a). H 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.

In this context, we have recently reported the synthesis and photophysical properties of the series [Pt(pq)(μ-κC α :η 2 -C≡CR)]2 (Hpq = phenylquinoline; R = t Bu, 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 3 L'LCT excited state with a small or negligible contribution of 3 MMLCT character.

Synthesis and Characterization
As illustrated in Scheme 1, the synthesis of the new diplatinum derivatives [{Pt(pq)(C≡C t Bu)}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-κC α :η 2 -C≡C t Bu)]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≡C t Bu)}3(μ-tpab)] (6a) was obtained using the same strategy but with a Pt-1:ligand molar ratio of 3:2.

Scheme 1. Synthesis of the derivatives 1a-6a.
A combination of crystallography (2a and 4a), IR spectra, mass spectrometry and elemental analysis supports the formulation proposed in the Scheme 1. 1 H 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≡C t Bu)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≡C t Bu)L] + or [Pt2(pq)2(C≡C t Bu)2L] + (6a) by loss of the fragment [Pt(pq)(C≡C t Bu)] (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- alkynyl bridging molecular ion ([{Pt(pq)(C≡C t Bu)}2] + , m/z 961), the loss of an alkynyl group ([Pt2(pq)2(C≡C t Bu)] + , m/z 879) and the cleavage ([Pt(pq)(C≡C t Bu)] + , 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.  1 and 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.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  17) Å] and those of the terminal alkynyl units in both complexes are not unusual (see Tables 1 and 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].S2a), which interacts through secondary interactions with the crystallization solvents (CHCl3, n-hexane) and with other dimers (Hpy•••C≡C/Cpq) (Figures S2b).However, the supramolecular structure of 4a (Figure S3) does not show  …  interactions.
The 1 H 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≡C t Bu)(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 1 H 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 H 8 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 H 8 pq 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≡C t Bu 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 1 H 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 H 8 pq 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 1 H 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 1 H NMR spectra recorded for solutions formed by a mixing of Pt-1/L (L 2 -L 5 ) 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 L 2 -L 5 or 1:20 for L 1 ) have allowed us to carry out a reasonable assignment of the signals of the mononuclear complexes 1b-5b (2D 1 H-1 H 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).

Photophysical Properties
To facilitate comparison, all absorption and emission spectral data are summarized in Tables 2 and 3.

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≡C t Bu) units to the central As mentioned above, the 1 H 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 1 IL (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*(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 * diimine ligands, contribution from platina-alkynyl to N-donor ligand charge transfer 1 [Pt(C≡C)*(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).

Emission Spectroscopy
Qualitatively, the emissions of these complexes are much more intense in all media than those observed for the starting materials (Tables 3 and 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 ( 0.3-11.4μs 298 K; 7.6-39.2μs 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 3 IL 3 ()* 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}] em = 580, 610 nm) [58], what is consistent with emission from a 3 MLCT excited state likely mixed with alkynyl to pq charge transfer contribution ( 3 MLCT/ 3 L'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.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 ( 3 p) [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 max (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 3 MLCT and alkynyl to pq charge transfer ( 3 MLCT/ 3 L'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).

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≡C t Bu (61%) on fragment 2; HOMO−1 Pt (35%) and C≡C t Bu (57%) on fragment 1), whereas the HOMO−2 and HOMO−3 are of similar energy and located on the unit 2 Pt(pq)(C≡C t Bu).The HOMO−4 is centered on fragment 1 [Pt (30%), pq (55%) and C≡C t Bu (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≡C t Bu) 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 platinumalkynyl to cyclometalated 1 [(M + L')LCT] and could be correlated with the experimental feature located at 500 nm.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≡C t Bu, 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 3 IL(pq) contribution, supporting the negligible influence of the N-donor linker.

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-κC α :η 2 -C≡C t Bu)]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.

Figure 10 .
Figure 10.Molecular orbital plots for the computed T1 state of complex 2a.
a Tail to 800 nm; b Non emissive at 298 K.