Spectacular luminescent behaviour of tandem terpyridyl platinum(II) acetylide complexes attributed to solvent effect on ordering of excited states, “ion-pair” formation and molecular conformations

Abstract

A series of oligomeric tandem terpyridyl platinum(II) complexes, namely [(^tBu₃tpy)Pt(C≡Ctpy)PtCl](OTf)₂ (3), [(^tBu₃tpy)Pt(C≡Ctpy)PtC≡C^tBu](OTf)₂ (4), [(^tBu₃tpy)Pt(C≡Ctpy)PtC≡Ctpy](OTf)₂ (5), and [(^tBu₃tpy)Pt(C≡Ctpy)Pt(C≡Ctpy)PtCl](OTf)₃ (6), were prepared and their spectroscopic properties and self-aggregating behaviour were examined. In particular, complex 4 exhibits unusually higher emission quantum yield in CH₂Cl₂ (ϕ = 0.43) than that in CH₃CN (ϕ < 0.1), which is attributed to the formation of a “contact ion pair” in chlorinated solvents, such as CHCl₃, CH₂Cl₂ and C₆H₅Cl. DFT calculations revealed that both intersystem crossing (ISC) and radiative decay of this complex are less effective in CH₃CN than in CH₂Cl₂, thus accounting for the low emission quantum yield in CH₃CN.

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Luminescent platinum(II) complexes are distinctly different from octahedral d⁶ metal complexes of Ru(II) and Ir(III) in that the square planar geometry of Pt(II) complexes allows inner-sphere type metal–metal and metal–ligand interactions, which can profoundly impact the properties of electronic excited states. In this regard, platinum(II) complexes bearing π-conjugated N- and C-donor ligands were found to display intriguing photo-physical and luminescent properties^1–8 that can be harnessed for practical applications in material science and luminescent sensory devices. A particular class of complexes possess a [(tpy)PtC≡CAr]⁺ (tpy = 2,2′:6′,2′′-terpyridine; Ar = aryl group) motif which displays interesting aggregation behaviour as a consequence of intermolecular Pt^II⋯Pt^II and π⋯π interactions. The intermolecular Pt^II⋯Pt^II interactions have led to unique solvatochromism,^5a,5c,9 temperature-dependent phosphorescence¹⁰ and formation of self-assembled gel¹¹ and nanowires.¹²

In this work, the assembly of oligomeric Pt(II) complexes, [(^tBu₃tpy)Pt(C≡Ctpy)PtCl](OTf)₂ (3), [(^tBu₃tpy)Pt(C≡Ctpy)PtC≡C^tBu](OTf)₂ (4), [(^tBu₃tpy)Pt(C≡Ctpy)PtC≡Ctpy](OTf)₂ (5), and [(^tBu₃tpy)Pt(C≡Ctpy)Pt(C≡Ctpy)PtCl](OTf)₃ (6), all having {(tpy)PtC≡C} moieties tethered by a C≡C group, were prepared and their spectroscopic/photophysical properties were examined (Scheme 1).‡ Complex 2, used for the synthesis of 3–6, was first reported by Ziessel and co-workers.^17,18 Notably, both high emission quantum yield (0.43) and long emission lifetime (11 μs) of [(^tBu₃tpy)Pt(C≡Ctpy)PtC≡C^tBu](OTf)₂ (4) in CH₂Cl₂ solution at room temperature are unique among the [(tpy)Pt(C≡CR)]⁺ complexes reported in the literature. This complex is weakly emissive in CH₃CN solution, and its emission intensity in CH₃CN is enhanced by adding chlorinated organic solvents (such as CHCl₃, CH₂Cl₂, 1,2-Cl₂C₂H₄, and C₆H₅Cl).

Scheme 1 Platinum(II) complexes 1–6 studied in this work and the labelling scheme of aryl protons of the terpyridyl ligands with m = 1, 2 standing for the 1st and 2nd sets of acetylide terpyridyl, respectively.

Based on ¹H NMR and emission measurements, the luminophore cations of 4 were found to aggregate in CH₃CN solution and such aggregation is suppressed in chlorinated solvents, which is attributed to the formation of a “contact ion pair”. The DFT calculations suggested a spectacular solvent effect on the ordering of excited states: the T₂ is only 173 cm⁻¹ above S₁ in CH₂Cl₂, while ΔE(T₃ − S₁) = 426 cm⁻¹ in CH₃CN, and thus intersystem crossing (ISC) is more efficient in CH₂Cl₂. In addition, there is a low-lying triplet excited state, T₂, in CH₃CN which is only ∼500 cm⁻¹ below the T₃ state. Fast internal conversion from T₃ to T₂ state is thus possible. However, the Pt centre in this T₂ state is different from that in the S₁ state, rendering the direct spin–orbit coupling between T₂ and S₁ states ineffective. As such, harnessing the triplet excited state in CH₃CN is inefficient, thus further contributing to its low emission quantum yield.

The method of Sonogashira¹³ was adopted for the synthesis of complexes 1, 2, 4, and 5, involving cross-coupling reactions of [(^tBu₃tpy)PtCl]⁺ with terminal acetylides, 4′-ethynyl-2,2′:6′,2′′-terpyridine (HC≡Ctpy) or HC≡C^tBu in the presence of diisopropylamine/copper iodide in CH₂Cl₂ or CH₂Cl₂–CH₃CN. Refluxing [(COD)PtCl₂] with 2 or 5 in H₂O/acetone (1/4, v/v) gave 3 and 6 respectively. Results of ¹H NMR, ESI-MS, FAB-MS studies and elemental analyses are consistent with the formulation of 1–6.

The labelling scheme for the aryl protons of 1–6 is shown in Scheme 1; the signals in the aromatic region (7.0–9.5 ppm) were assigned using ¹H–¹H COSY. The ¹H NMR spectra of 3–6 are poorly resolved with broad peaks at ambient temperature and vary with temperature. As 4 has a better solubility than 3, 5 and 6 in CD₃CN, ¹H NMR study of 4 in CD₃CN was performed over a larger temperature range (253 to 323 K; Fig. 1a) and at various complex concentrations (5.0 × 10⁻³ to 3.4 × 10⁻⁴ mol dm⁻³, Fig. 1b). Except for H³ and H^a1, all aryl proton signals shift downfield (0.11–0.30 ppm) as the temperature increases from 253 K to 323 K. At temperatures ≤ 298 K, the ¹H NMR spectrum of 4 is poorly resolved with broad signals in the aromatic region. When the concentration of 4 is decreased from 5.0 × 10⁻³ to 3.4 × 10⁻⁴ mol dm⁻³, there is a gradual downfield shift of all aryl proton signals.

Fig. 1 (a) Variable temperature ¹H NMR spectra (500 MHz) of 4 in CD₃CN solution (∼6 × 10⁻³ mol dm⁻³; 253–323 K). (b) ¹H NMR spectra (400 MHz, 298 K) of 4 in CD₃CN solutions at various concentrations: 3.4 × 10⁻⁴ (top); 2.5 × 10⁻³ (middle); 5.0 × 10⁻³ (bottom) mol dm⁻³.

The ¹H NMR spectra of 3 in DMF-d₇ solution recorded at temperature from 299 to 393 K reveal a downfield shift of the aryl proton signals with increasing temperature (Fig. 2). The protons with the largest shift are those in close proximity to the platinum atom (H^5′, H^6′, H^d1 and H^e1, 0.52–0.81 ppm, from 299 to 393 K). The chemical shifts of H³, H^3′, H^a1, H^b1 and H^c1 are less sensitive to temperature. This kind of behavior has similarly been observed in the ¹H NMR spectrum of 5 in DMF-d₇ solution at temperature from 300 to 393 K (Fig. S5).

Fig. 2 ¹H NMR spectra (500 MHz) of 3 in DMF-d₇ solution at temperature 273–393 K.

Broadening of the aryl proton signals at room temperature could originate from aggregation of the complexes in solutions. The aggregation behaviour of 3–6 as revealed by their ¹H NMR spectra at temperature above 298 K is reminiscent of [(4-ⁿBu–C₆H₄C≡C)Pt(tpy–R–tpy)Pt(C≡C–C₆H₄–ⁿBu-4)](PF₆)₂ (R = O(CH₂CH₂O)₃, O(CH₂CH₂O)₄ or O(CH₂)₁₀O) reported by Yam and coworkers, who ascribed the line broadening and upfield shifting of the ¹H NMR signals to self-assembly driven by intramolecular Pt⋯Pt and π⋯π stacking interactions.¹⁰ Here, we attribute the broad aryl proton signals of 3–6 at ambient temperature to intermolecular aggregation. Intramolecular Pt⋯Pt interaction would not be possible due to the short and rigid C≡C linkage in complexes 3–6; the decrease of rotational freedom of the aryl rings could only be accounted for by intermolecular Pt⋯Pt and/or π⋯π interactions. These intermolecular interactions increase the relaxation time of the proton signals which become chemically indistinguishable at ambient temperature. With an increase in temperature, aggregation is less favorable¹⁰ and the aryl proton signals of 3–6 become well-resolved. The downfield shifts of the aryl proton signals of 3–6 with increasing temperature are attributed to de-shielding of the aryl protons accompanied by disruption of the π⋯π stacking interactions. Consistent with the presence of molecular aggregation, the ¹H NMR signals of 4 shift downfield with decrease in complex concentration since molecular aggregation is ruptured.

The absorption bands (Fig. S7) of 1–6 in CH₃CN solutions at λ_max < 400 nm are similar to the intraligand transitions (π→π*) of the tpy and acetylide ligands.^3–7 With reference to previous studies on [(tpy)PtR]⁺ (R = acetylide),¹⁶ the low energy absorption peak(s) of the alkyl acetylide complexes 1 and 4 at λ_max = 414 and 462 nm respectively are assigned to mixed metal-to-ligand (MLCT) and ligand-to-ligand charge-transfer (LLCT) transitions, ¹[π(Pt–C≡C)→π*(tpy)]. The absorption bands of the aryl acetylide complexes 2 and 5 at λ_max = 405 and 459 nm respectively, on the other hand, are assigned to ¹LLCT (¹[π(C≡CAr)→π*(tpy)]) transitions. For 3 and 6, with chloride directly attached to the platinum ion, their peak maxima at λ_max = 423–426 nm are of ¹MLCT character (¹[dπ(Pt)→π*(tpy)]). Similarly, in CH₃CN solution, the emission bands (Fig. S7) of 1 and 4 at λ_max = 537 and 570 nm respectively are attributed to mixed excited states with ³MLCT/³LLCT character. The emissions of 3 and 6 (λ_max = 544–546 nm) are attributed to ³MLCT excited states. For 2 and 5, the emission maxima at 519 and 549 nm in CH₃CN are assigned to ³LLCT transitions.¹⁶ The emission spectrum of 5 is solvent dependent with λ_max = 560 nm in CH₂Cl₂ and λ_max = 550 and 671 nm in CH₃CN (Fig. S7). This reveals that excimeric interaction (λ_max = 671 nm) is more pronounced in CH₃CN than in CH₂Cl₂ solutions. We propose that in CH₃CN solutions, the [(^tBu₃tpy)Pt(C≡Ctpy)PtC≡Ctpy]²⁺ and TfO⁻ ions are well separated and stacking interactions between {(tpy)Pt} moieties become more favourable. Interestingly, the intensity of the excimeric emission at λ_max = 671 nm is more enhanced for 6, revealing that the {Pt(tpy)} moiety facilitates excimeric emission arising from π⋯π interactions between the neighbouring terpyridyl ligands.

Among the complexes [(^tBu₃tpy)Pt((C≡Ctpy)Pt)_n−1L]ⁿ⁺, we observed that the emission quantum yield of [(^tBu₃tpy)Pt(C≡Ctpy)PtC≡C^tBu](OTf)₂ (4) is solvent-dependent. This complex is strongly emissive in chlorinated solvents such as CHCl₃ (ϕ = 0.39), CH₂Cl₂ (ϕ = 0.43) and 1,2-C₂H₄Cl₂ (ϕ = 0.33); however it is weakly emissive/non-emissive in CH₃CN, acetone, DMF, CH₃OH and THF (Fig. 3).

Fig. 3 Left: UV-vis spectra of complex 4 in various organic solvents (concentration = 1 × 10⁻⁵ mol dm⁻³). Right: Table to show the emission data of complex 4 in various organic solvents.

The UV-vis absorption and emission spectral changes of complex 4 in CH₃CN mixed with CH₂Cl₂ at different compositions are depicted in Fig. 4. Upon increasing the CH₂Cl₂ composition while maintaining the complex concentration as constant (concentration = 2 × 10⁻⁵ mol dm⁻³), the absorption band at 438 nm (100% CH₃CN) gradually shifts to 447 nm (100% CH₂Cl₂). At the same time, the emission quantum yield gradually increases from ϕ = 0.03 (100% CH₃CN) to ϕ = 0.43 (100% CH₂Cl₂) and the emission maximum of 569 nm in 100% CH₃CN is slightly shifted to 575 nm in 100% CH₂Cl₂.

Fig. 4 Upper: Photographs of complex 4 (concentration = 2 × 10⁻⁵ mol dm⁻³) in CH₃CN mixed with 0–100% CH₂Cl₂ (a) under ambient light (in air), (b) under UV light irradiation (350 nm; in air), (c) under UV light irradiation (350 nm; in degassed environment). Bottom: Emission spectra of complex 4 (concentration = 2 × 10⁻⁵ mol dm⁻³) in CH₃CN mixed with 0–100% CH₂Cl₂ (Inset: plots of intensity at 570 nm and emission quantum yield versus % of CH₂Cl₂ in CH₃CN).

Complex 4 is slightly soluble in chlorobenzene. Similar UV-vis absorption and emission spectral changes are observed upon mixing CH₃CN solution of 4 with chlorobenzene at different compositions (0–90%; Fig. S8).We found that 4 in 10% CH₃CN/90% chlorobenzene mixture shows an emission maximum at 575 nm with high emission quantum yield of 0.29. A series of organic solvents were added to CH₃CN solution of 4 in order to examine the solvent effect on the emission intensity. The emission intensity of 4 in CH₃CN (2 × 10⁻⁵ mol dm⁻³; 2 mL) is dramatically enhanced upon mixing with chlorinated organic solvents (1 mL), such as CH₂Cl₂, CHCl₃, 1,2-Cl₂C₂H₄, 1,1-Cl₂C₂H₄, 1,1,2-Cl₃C₂H₃, cis/trans-1,2-Cl₂C₂CH₂, ClC₆H₅, 1,2-Cl₂C₆H₄, 1,3-Cl₂C₆H₄ or 1,3,4-Cl₃C₆H₃. However, no detectable change of the emission intensity was found upon addition of the following solvents: CHBr₃, CH₃I, 1,2-Br₂C₂H₄, C₆H₆, CH₃CN, CH₃OH, C₂H₅OH, CH₃COOC₂H₅, (CH₃)₂C=O, C₂H₅OC₂H₅, DMF, H₂O, 1,4-dioxane and THF to an CH₃CN solution of 4.

In the literature, it has been reported that intermolecular or nanoscopic aggregation would induce enhancement in emission intensity but with little effect on the emission energy.¹⁴ In this work, no nanosized particles were observed by either dynamic light scattering (DLS) or transmission electron microscopy (TEM) measurements in pure CH₂Cl₂ and CHCl₃ solutions of 4 or in CH₃CN/ClC₆H₅ (1 : 9), CH₃CN–CHCl₃ (1 : 9) solution mixtures of 4, revealing that the emission enhancement is different from the systems reported in the literature.¹⁴ We propose that for this tandem terpyridyl platinum(II) acetylide complex [(^tBu₃tpy)Pt(C≡Ctpy)PtC≡C^tBu](OTf)₂ (4), solvent can affect (1) the formation of “ion pairs”¹⁵ and the molecular aggregation behavior and (2) the ordering of excited states. These two factors could contribute to the spectacular phenomenon of switching-on luminescence upon varying the solvent, and are discussed in the following sections.

(1) Formation of “ion pairs” and molecular aggregation behavior

Since the solubility of a complex is mainly dependent on the solvent polarity, solvents of high polarity (dielectric constant ε_r > 20) dissolve ionic compounds to give solutions which are called “electrolytic solvents”. Solvents of low polarity (dielectric constant ε_r < 20) which dissolve ionic compounds poorly are termed “non-electrolytic solvents”.¹⁵ When the charged complex 4 is dissolved in chlorinated solvents such as CH₂Cl₂ and CHCl₃ having low dielectric constants (ε_r < 10: for CH₂Cl₂ε_r = 9.1; for CHCl₃ε_r = 4.8), formation of free complex cations becomes less favourable and “contact ion pairs” may result. Thus the intramolecular motion between two {(tpy)PtC≡C} moieties of complex 4 would be restricted by the counterion (Fig. 5). This will diminish the interactions between solvent molecules and the luminophore cation or two TfO⁻ anions (Fig. 5).

Fig. 5 Upper: Formation of “contact ion pair” in chlorinated solvents (ε_r < 10). Bottom: Formation of “solvent-separated ion pair” in CH₃CN (ε_r = 37.5).

In solvents of intermediate dielectric constants (10 < ε_r < 40; such as CH₃CN, ε_r = 37.5), ion pairs with solvent molecules surrounding the luminophore cation and two TfO⁻ anions can be described as “solvent-separated ion pairs” (Fig. 5). In CH₃CN, the π-conjugated luminophore cations of 4 would aggregate due to well isolated ions in “solvent-separated ion pairs”. By using a scanning electron microscope (SEM), slow evaporation of an CH₃CN solution (∼ 2 × 10⁻⁴ mol dm⁻³) of complex 4 was found to give “donut” shaped submicron superstructures with diameters of 200–400 nm and with the average inner diameter of 60 nm. However an amorphous solid was obtained upon evaporation of CH₂Cl₂ solution (∼ 2 × 10⁻⁴ mol dm⁻³) of complex 4 (Fig. 6). This is attributed to the sterically bulky tert-butyl groups attached on both the terpyridine and acetylene ligands, which prohibit the head-to-head or head-to-tail stacking of the aryl units in CH₃CN solution. We propose that the “donut” shaped nanostructure of complex 4 comes from the self-assembly of cations of 4 having “pinwheel” type aggregation with respect to each other to maximize π⋯π stacking and/or Pt⋯Pt interaction.

Fig. 6 Upper: SEM images of complex 4 prepared from slow evaporation of CH₃CN (left) and CH₂Cl₂ solution (right). Bottom: Emission spectra of solid 4 prepared from slow evaporation of CH₃CN and CH₂Cl₂ (Inset: Photograph of different solid 4 under 350 nm UV light irradiation).

To further account for the different aggregation behaviour in various solvents, we performed a series of ¹H NMR and emission measurements in CD₃CN and CDCl₃/CD₃CN mixtures at various volumic fractions of CDCl₃. All of these experiments were carried out at controlled temperatures (298 K or 323 K) and a constant complex concentration (Fig. 7).

Fig. 7 (a) Emission spectra of complex 4 in different compositions of CDCl₃ (10% to 90%) in CD₃CN mixture at 298 K. (b) Normalized emission spectra of complex 4 in CD₃CN (concentration = 6.0 × 10⁻⁴ mol dm⁻³ and 1.8 × 10⁻³ mol dm⁻³) at 298 K and 323 K, respectively. (c) ¹H NMR spectra at 298 K. (d) ¹H NMR spectra at 323 K in CD₃CN, CDCl₃ and different compositions of CDCl₃ (10% to 90%) in CD₃CN mixture.

At 298 K and at complex concentrations of 6.0 × 10⁻⁴ mol dm⁻³ and 1.8 × 10⁻³ mol dm⁻³ in CD₃CN, the ¹H NMR spectra are poorly resolved with broad signals in the aromatic region (Fig. 7c). Under these conditions, the emission spectra of these solutions reveal a weakly emissive band at λ_max = 670 nm consistent with the proposition that cations of 4 aggregate in CD₃CN solution (Fig. 7b) leading to broadening of ¹H NMR signals. However, upon increasing the temperature to 323 K, the ¹H NMR signals of 4 at 6.0 × 10⁻⁴ mol dm⁻³ become better-resolved (Fig. 7d) and the emission maximum is blue shifted to 577 nm though the emission intensity is still low. This suggests that the fast intermolecular motion of the CD₃CN molecules and cations of 4 at higher temperature (323 K) would partially suppress the aggregation. On the other hand, the ¹H NMR spectrum of 4 at 1.8 × 10⁻³ mol dm⁻³ exhibits broadened signals and the corresponding emission spectrum (Fig. 7b) shows a broad emission band with λ_max from 577 nm to 670 nm revealing that the aggregation of cations of 4 in CD₃CN still prevails at this complex concentration and temperature (323 K).

The ¹H NMR signals of 4 become well-resolved and its emission intensity at 577 nm is enhanced, as the composition of CDCl₃ increases from 10% to 90% in CDCl₃/CD₃CN mixture (6.0 × 10⁻⁴ mol dm⁻³) at both 298 K and 323 K (Fig. 7c and 7d). We suggest as the composition of CDCl₃ increases the concentration of “contact ion pairs” and the degree of aggregation are suppressed. Subsequent restriction of intramolecular motion between two {(tpy)PtC≡C} moieties would enhance the radiative decay leading to an increase in emission intensity of 4 at 577 nm.

This explanation can be used to account for the strongly emissive properties of complex 4 (λ_max = 576 nm; ϕ = 0.43) in CH₂Cl₂. The “contact ion pairs” of 4 in CH₂Cl₂ render the luminophore cations well-isolated from each other, thus disfavouring the intermolecular and intramolecular interactions accounting for high emission quantum yield. Indeed, this luminescent enhancement property has only been found in chlorinated solvents (except CCl₄). Apart from CH₃CN, this property has not been observed in solvents having dielectric constants in the range 10 < ε_r < 40 (such as DMF, DMSO, pyridine, CH₃NO₂, acetone, CH₃OH, CH₃CH₂OH, etc.) and the solvents with low ε_r values such as Et₂O, THF, dioxane, etc.

(2) Excited state ordering in various solvents

DFT calculations were performed on the model complex [((CH₃)₃tpy)Pt(C≡Ctpy)PtC≡CCH₃]²⁺ (4′), in which the ^tBu groups were replaced by CH₃ groups to provide similar electronic effect but the computations were less demanding (Figure 8).

Fig. 8 Model complex [((CH₃)₃tpy)Pt(C≡Ctpy)PtC≡CCH₃]²⁺ (4′) for DFT calculations.

In order to have phosphorescence occur after electron excitation, intersystem crossing (ISC) has to be effective to move the complex from an excited singlet state to a triplet excited state via spin–orbit coupling (SOC). Scheme 2 presents the ordering of electronic excited states of 4′ in CH₂Cl₂ and CH₃CN at S₀ optimized geometry.

Scheme 2 Excited state orderings of [((CH₃)₃tpy)Pt(C≡Ctpy)PtC≡CCH₃]²⁺ at S₀ optimized geometry.

In order to have efficient ISC, the two coupling singlet and triplet excited states have to be close in energy and the d orbitals for the coupling excited states have to be different. In CH₂Cl₂, T₂ is only 173 cm⁻¹ above S₁, and thus ISC is effective. However, in CH₃CN, T₃, which is of the same parentage as T₂ in CH₂Cl₂, is 426 cm⁻¹ above S₁ and ISC is less effective compared with that in CH₂Cl₂. Moreover, there is a low-lying triplet excited state (T₂) that is only 511 cm⁻¹ below T₃ and internal conversion (IC) from T₃ to T₂ should be rapid. This T₂ excited state, which is derived from the d orbitals of Pt(A) (Pt coordinated to the tpy rings with CH₃ substituents), has to couple with the high-lying singlet excited state (> 4500 cm⁻¹) for phosphorescence to occur. Thus, triplet emission quantum yield is low in CH₃CN.

In conclusion, complexes 3–6 having tandem two to three {(tpy)Pt} units exhibit interesting aggregation properties that are absent for the monomeric complexes 1 and 2. Among the complexes studied in this work, [(^tBu₃tpy)Pt(C≡Ctpy)PtC≡C^tBu](OTf)₂ (4) has the highest emission quantum yield (0.43) and the longest lifetime (11 μs) in CH₂Cl₂ solution. Complex 4 forms “contact ion pairs” in chlorinated solvents with low dielectric constant, but “solvent-separated ion pairs” in CH₃CN with high dielectric constant. Based on the results of ¹H NMR and emission experiments, we suggest that intermolecular aggregation between cations of 4 would prevail in CH₃CN. As the composition of chlorinated solvent in CH₃CN increases or in pure chlorinated solvent, the concentration of “contact ion pairs” of complex 4 would increase. Subsequently, suppression of intermolecular aggregation between cations of 4 and restriction of intramolecular motion would enhance the emission intensity at 577 nm by enhancing radiative decay. DFT calculations revealed that the ISC and radiative decay are less effective in CH₃CN than in CH₂Cl₂, thus contributing to low emission quantum yield in CH₃CN. The spectacular solvent effect on the luminescent behaviour of complex 4 can be accounted for by the “ion pair” formation, aggregation behaviour, structural conformation and excited state ordering in which all these can be attributed to the planar coordination geometry of platinum(II).

Acknowledgements

We gratefully acknowledge financial support from the Innovation and Technology Commission of the HKSAR Government (ITF/016/08NP), the University Grants Committee of the Hong Kong SAR (AoE/P-03/08), Research Grants Council of Hong Kong SAR (HKU 7008/09P), National Natural Science Foundation of China/Research Grants Council Joint Research Scheme [N_HKU 752/08] and The Chinese Academy of Sciences-Croucher Foundation Funding Scheme For Joint Laboratories. Dr S. C. F. Kui acknowledges the small project funding (200807176030) and the post-doctoral fellow scholarship from The University of Hong Kong.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental and computational details. See DOI: 10.1039/c0sc00427h

‡ General procedures. The solvents used for synthesis were analytical grade. [(^tBu₃tpy)PtC≡C^tBu](OTf) (1),¹⁶[(^tBu₃tpy)PtC≡Ctpy](OTf) (2),¹⁷ [(COD)PtCl₂] (COD = 1,5-cyclooctadiene), HC≡Ctpy (4′-ethynyl-2,2′:6′,2′′-terpyridine)¹⁸ were synthesized according to literature methods. Fast atom bombardment (FAB) mass spectra were obtained on a Finnigan Mat 95 mass spectrometer using 3-nitrobenzyl alcohol as matrix. Electrospray Ionization (ESI) mass spectra were obtained on a Finnigan LCQ quadrupole ion trap mass spectrometer (samples were dissolved in HPLC grade acetonitrile). ¹H NMR spectra were recorded on a Bruker AVANCE 600, DRX 400 and 500 FT-NMR spectrometers. The ¹H NMR chemical shifts were referenced to those of the residual proton/carbon atoms of CD₃CN and DMF-d₇. Peak assignments were based on ¹H–¹H, and NOESY 2-D NMR experiments. UV-visible absorption spectra were obtained on a Hewlett-Packard 8453 diode array spectrophotometer. Emission spectra were measured on a Spex Fluorolog-3 spectrofluorometer. Elemental analyses were performed by the Institute of Chemistry at the Chinese Academy of Science, Beijing. TEM were performed on Philips Tecnai G2 20 S-TWIN with an accelerating voltage of 200 kV. The TEM images were taken by a Gatan MultiScan Camera Model 794. The SEM images were taken on a LEO 1530 FEG scanning electron microscope operating at 5.0 kV. TEM samples were prepared by depositing a few drops of MeCN solution or MeCN–Et₂O 1 : 1 solution mixture of the platinum complexes on Formvar® coated copper grids. Excess solvent was removed by a piece of filter paper and this was followed by drying the sample in a vacuum desiccator. SEM samples were prepared by dropping a drop of MeCN solution or MeCN–Et₂O 1 : 1 solution mixture of the platinum complexes onto silicon wafers onto the SEM specimen mounts. All the samples for SEM measurements were sputtered with gold thin film (20 s, < 2 nm thickness).[(^tBu₃tpy)Pt(C≡Ctpy)PtCl](OTf)₂(3). [(COD)PtCl₂] (5 mg, 0.04 mmol) and 2 (36 mg, 0.036 mmol) were dissolved in a mixture of H₂O/acetone (1/4, v/v, 25 mL). The solution was heated at 100 °C for 12 h to give a bright orange solution. Upon removal of solvent, the orange solid was dissolved in warm CH₃CN (10 mL) and the solution was filtered into an aqueous solution of NaOTf (34 mg, 2 mL). A reddish orange precipitate was collected on a sintered-glass funnel and air-dried. Yield: 35 mg, 70%. Anal. Calcd for C₄₆H₄₅N₆ClO₆S₂F₆Pt₂·2H₂O: C, 38.97; H, 3.48; N, 5.93. Found: C, 38.90; H, 3.44; N, 5.93. ¹H NMR (500 MHz, DMF-d₇): δ 1.49 (s, 18H, ^tBu), 1.56 (s, 9H, ^tBu), 7.38 (br, 2H, H^5′), 7.54 (br, 2H, H^d1), 8.28 (br, s, 2H, H^e1), 8.41 (t, 2H, J_HH = 7.4 Hz, H^c1), 8.48 (br, s, 2H, H^6′), 8.61 (s, 2H, H³), 8.70 (d, 2H, J_HH = 6.4 Hz, H^b1), 8.80 (s, 2H, H^3′), 8.87 (s, 2H, H^a1). FAB-MS (+ve NBA matrix): m/z 1233 [3 − (OTf)]⁺, 1083 [3 − 2(OTf)]⁺; ESI-MS (MeCN): m/z 1233 [3 − (OTf)]⁺, 542 [3 − 2(OTf)]²⁺. IR (Nujol): v_max (C≡C) 2097; 2122 cm⁻¹.[(^tBu₃tpy)Pt(C≡Ctpy)PtC≡C^tBu](OTf)₂(4). Ligand HC≡C^tBu (2.3 mg, 0.028 mmol) was added to a solution of 3 (35 mg, 0.025 mmol), CuI (1 mg, 0.005 mmol), and diisopropylamine (0.5 mL) in CH₂Cl₂–CH₃CN (1/1, v/v, 20 mL). The resultant solution, after stirring for 12 h at room temperature under an argon atmosphere, was evaporated to dryness in vacuo. The crude product was purified by chromatography on an alumina (neutral) column with (CH₂Cl₂–MeCN) (1/1, v/v) as eluent. Orange needles were obtained by vapor diffusion of diethyl ether into an acetonitrile solution. Yield: 20 mg, 56%. Anal. Calcd for C₅₂H₅₄N₆O₆S₂F₆Pt₂·2H₂O: C, 42.68; H, 4.00; N, 5.74. Found: C, 42.61; H, 3.81; N, 6.10. ¹H NMR (500 MHz, CD₃CN): δ 1.10 (s, 9H, C≡C^tBu), 1.47 (s, 18H, ^tBu), 1.58 (s, 9H, ^tBu), 7.23 (br, s, 2H, H^d1), 7.39 (d, 2H, J_HH = 4.2 Hz, H^5′), 7.93 (t, 2H, J_HH = 7.6 Hz, H^c1), 8.13 (s, 2H, H³), 8.21 (br, s, 4H, H^3′ and H^b1), 8.30 (s, 2H, J_HH = 5.5 Hz, H^e1), 8.32 (s, 2H, H^a1), 8.49 (d, 2H, J_HH = 7.1 Hz, H^6′). FAB-MS (+ve NBA matrix): m/z 1278 [4 − (OTf)]⁺, 1129 [4 − 2(OTf)]⁺; ESI-MS (MeCN): m/z 1278 [4 − (OTf)]⁺, 564 [4 − 2(OTf)]²⁺. IR (Nujol): v_max (C≡C) 2093; 2124 cm⁻¹.[(^tBu₃tpy)Pt(C≡Ctpy)PtC≡Ctpy](OTf)₂(5). Ligand HC≡Ctpy (11 mg, 0.039 mmol) was added to a solution of 3 (35 mg, 0.025 mmol), CuI (1 mg, 0.005 mmol), and diisopropylamine (0.5 mL) in CH₂Cl₂–CH₃CN (1/1, v/v, 20 mL). After stirring for 12 h at room temperature under an argon atmosphere, the solution was evaporated to dryness in vacuo, and the solid was dissolved in a minimum amount of acetonitrile. The solution was filtered through Celite and the filtrate was stored at 4 °C for 3 days. The light brown precipitate was washed with CH₂Cl₂ (5 mL) to afford a brownish orange solid. Yield: 30 mg, 52%. Anal. Calcd for C₆₃H₅₅N₉O₆S₂F₆Pt₂·2H₂O: C, 46.18; H, 3.63; N, 7.81. Found: C, 45.78; H, 3.53; N, 7.86. ¹H NMR (500 MHz, 353 K, DMF-d₇): δ 1.16 (s, 9H, ^tBu), 1.51 (s, 18H, ^tBu), 7.46 (br, s, 2H, H^d2), 7.69–7.72 (m, 4H, H^5′ and H^d1), 7.99 (s, 2H, H^c2), 8.30 (br, s, 2H, H^a2), 8.36 (t, 2H, J_HH = 7.4 Hz, H^c1), 8.59–8.61 (m, 6H, H³, H^a1 and H^b2), 8.72 (br, s, 6H, H^3′, H^b1 and H^e2), 8.78–8.85 (m, 4H, H^6′ and H^e1). FAB-MS (+ve NBA matrix): m/z 1454 [5 − (OTf)]⁺, 1304 [5 − 2(OTf)]⁺; ESI-MS (MeCN): m/z 1454 [5 − (OTf)]⁺, 652 [5 − 2(OTf)]²⁺. IR (Nujol): v_max (C≡C) 2095; 2124 cm⁻¹.[(^tBu₃tpy)Pt(C≡Ctpy)Pt(C≡Ctpy)PtCl](OTf)₃(6): [(COD)PtCl₂] (15 mg, 0.04 mmol) and 6 (50 mg, 0.030 mmol) were dissolved in H₂O/acetone (1/4, v/v, 25 mL). The solution was heated at 100 °C for 24 h to give a red solution, which was filtered into an aqueous solution of NaOTf (50 mg, 2 mL). The brownish orange precipitate was collected and air-dried. Yield: 32 mg, 52%. Anal. Calcd for C₆₄H₅₅N₉ClO₉S₃F₉Pt₃: C, 38.78; H, 2.80; N, 6.36. Found: C, 38.74; H, 3.10; N, 6.20. ¹H NMR (500 MHz, 353 K, DMF-d₇): δ 1.67 (s, 18H, ^tBu), 1.71 (s, 9H, ^tBu), 7.97–7.99 (br, 2H, H^5′), 8.09–8.11 (br, fused with DMF-d₇ peak, 2H, H^d2), 8.18–8.21 (br, fused with DMF-d₇ peak, 2H, H^d1), 8.66 (t, J = 8.0 Hz, 2H, H^c2), 8.86–8.88 (m, 4H, H³ and H^b2), 8.90–8.95 (m, 8 H, H^3′, H^a1, H^b1 and H^c1), 9.01 (s, 2H, H^a2), 9.06 (m, 2H, H^e2), 9.14 (d, J = 6.1 Hz, 2H, H^e1), 9.24 (m, 2H, H^b1). FAB-MS (+ve NBA matrix): m/z 1834 [6 − (OTf)]⁺, 1685 [6 − 2(OTf)]⁺, 1534 [6 − 3(OTf)]⁺; ESI-MS (MeCN): m/z 1833 [6 − (OTf)]⁺. IR (Nujol): v_max (C≡C) 2097; 2124 cm⁻¹.

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