Fluorosolvatochromism of Platinum Supramolecular Coordination Complexes: Stepwise Synthesis and Photophysical Properties of Organometallic Tetranuclear Pt(II) Squares

Abstract

The stepwise synthesis and characterization of three new mixed-ligand organometallic tetranuclear platinum squares were achieved. All of the complexes were constituted by the conjunction of two (2,2′-bpy)Pt-terph-Pt(2,2′-bpy) (terph = p-terphenyl) fragments linked by a variety of N^N ligands (4,4′-bipyridine (4,4′-bpy), 1,4-di(pyridin-4-yl)benzene (dpbz), and 4,4′-di(pyridin-4-yl)-1,1′-biphenyl (dpbph)), which occupied the fourth coordination site of each metal center, giving rise to square-shaped molecules of the general formula [Pt₂(2,2′-bpy)₂(terph)(N^N)]₂. Consequently, the tetranuclear complexes, {[Pt(2,2′-bpy)]₄(μ-terph)₂(μ-4,4′-bpy)₂}{PF₆}₄ (7), {[Pt(2,2′-bpy)]₄(μ-terph)₂(μ-dpbz)₂}{PF₆}₄ (8), and {[Pt(2,2′-bpy)]₄(μ-terph)₂(μ-dpbph)₂}{PF₆}₄ (9) were constructed. The photophysical properties of these complexes were studied both in the solid state and in various solvents, revealing fluorosolvatochromism.

1. Introduction

Platinum Supramolecular Coordination Complexes (SCCs) have emerged as attractive subjects of research, garnering significant attention owing to their various properties. They exhibit catalytic specificity in the formation of C-X bonds through reductive elimination [1,2]. Beyond their catalytic potential, platinum squares (a subclass of SCCs with a rectangular shape) have demonstrated noteworthy biological attributes [3] and have been investigated for their promising anticancer properties [4]. Their photophysical properties as dual emitters [5] and fluorescence probes have also been studied.

In recent times, several innovative strategies have been reported for the synthesis of platinum squares, highlighting the dynamic nature of ongoing research in this field. The synthetic strategy commonly involves the one-pot self-assembling of supramolecular units, starting from cis-mononuclear building blocks [6,7,8,9]. An equal amount of N^N ligands, such as 4,4′-bpy, is then added, resulting in the formation of a symmetric tetranuclear complex.

An alternative synthetic approach involves initially synthesizing one side of the square by forming a binuclear bridged platinum complex. Subsequently, two equivalents of another bridged ligand (BL) coordinate monodentataly to the metal center, resulting in the formation of a “pi” shaped complex. Finally, one more equivalent of the initial binuclear complex completes the fourth side of the square by coordinating the platinum centers to the ends of the BL (Figure 1). This method is known as a stepwise synthesis approach. It is worth noting that the presence of bulky auxiliary ligands on platinum centers, such as bisphosphines, hinders the self-rotation within the binuclear complex, resulting in mixtures of syn- and anti-conformers. Such mixtures impede the formation of platinum square complexes [10]. Overall, stepwise synthesis offers advantages by enabling the coordination of different types of bridged ligands in the same compound.

In 1990, M. Fujita et al. [11] documented the discovery of tetranuclear square complexes with the synthesis of [Pd(en)(4,4′-bpy)]₄(NO₃)₈, prepared through a one-step reaction between the mononuclear complex Pd(en)(NO₃)₂ and the ligand 4,4′-bpy. The synthesis of the same complex via a solventless reaction between the same reactants [12] was also reported, suggesting the formation of linear intermediates. Comparative studies revealed that the analogous Pt(II) tetranuclear complex exhibits greater stability than its Pd(II) counterpart, attributed to the higher kinetic inertia of the Pt(II) ion [13]. This Pt(II) complex received significant attention due to its cytotoxic properties against HL-60 cells [14] and its role as a specific G-quadruplex binder [15]. A few years later, Garci et al. reported an alternative synthetic approach for this type of complex utilizing mechanochemical ball milling, forming products in high yields [16].

To date, several Pd(II) tetranuclear complexes have been reported, utilizing various auxiliary ligands such as ethylenediamine [17], 2,2′-bpy and its derivatives [18], and chelated phosphines [19]. Furthermore, tetranuclear rhomboidal Pt(II) complexes have been reported to exhibit aggregation-induced emission properties [20,21]. Also, tunable emission properties were achieved in similar rhomboidal complexes by modifications to the building ligands [22]. The reaction of 2,9-bis[trans-Pt(PEt₃)₂NO₃]₂(μ-phenanthrene) with rigid dianionic carboxylate ligands affords rhomboid tetranuclear and triangular hexanuclear Pt(II) complexes, depending on the nature of the dicarboxylate ligand [23].

We synthesized and characterized three new mixed-ligand organometallic tetranuclear platinum squares using the stepwise approach. These complexes share a common bridging ligand, p-terphenyl (terph), while the other ligands increase in size: 4,4′-bipyridine (4,4′-bpy), 1,4-di(pyridin-4-yl)benzene (dpbz), and 4,4′-di(pyridin-4-yl)-1,1′-biphenyl (dpbph) (Scheme 1). To complete the platinum coordination sphere, we employed 2,2′-bpy as an auxiliary ligand. Thus, the tetranuclear complexes, {[Pt(2,2′-bpy)]₄(μ-terph)₂(μ-4,4′-bpy)₂}{PF₆}₄ (7), {[Pt(2,2′-bpy)]₄(μ-terph)₂(μ-dpbz)₂}{PF₆}₄ (8), and {[Pt(2,2′-bpy)]₄(μ-terph)₂(μ-dpbph)₂}{PF₆}₄ (9) were formed. Our objective was to investigate the photophysical properties of these platinum squares across different solvents.

2. Results and Discussion

2.1. Synthesis and Characterization

The synthesis of the complex [Pt(COD)Cl]₂(μ-terph) (COD = 1,5-cyclooctadiene) was successfully achieved by modifying the method originally reported for the complex [Pt(COD)Cl]₂(μ-bph) [1]. This synthetic procedure (Scheme 2) involved three steps. Firstly, a bromine–lithium exchange reaction was conducted at −78 °C in tetrahydrofuran. Subsequently, 4,4″-bis(trimethylstanyl)-p-terphenyl was synthesized in situ after the addition of trimethyltin chloride. Finally, another transmetallation reaction between the arylstannane and the platinum complex Pt(COD)Cl₂ resulted in the formation of the binuclear complex (1). This binuclear complex served as the starting material for subsequent reactions. In the following step, the coordinated COD in complex (1) was replaced by 2,2′-bpy, forming the complex [Pt(2,2′-bpy)Cl]₂(μ-terph).

The orientation of the replaceable ligand on the metal center plays a crucial role in facilitating the following reactions. The anti-conformer impedes the continuation of the reactions towards the square complex, whereas the syn-conformer promotes their formation. Several factors influence this equilibrium, including the steric hindrance by the auxiliary ligands and the rigidity of the bridging ligand [10].

The ¹H NMR spectrum of complex (1) (Figure S1) in CD₂Cl₂ showed only one set of signals due to its high symmetry. Additionally, it is worth noting that both isomers syn- and anti-exhibited precisely the same spectrum. In the ¹H NMR spectrum of (2), the signals of the coordinated 1,5-COD disappeared, indicating that the 1,5-COD was replaced by the ligand 2,2′-bpy in each Pt center (Figure S2). This reaction was accompanied by a noticeable change in the color of the suspended solid from white to bright yellow. Furthermore, the spectrum indicated that the terph exhibited high symmetry, containing two doublets and one singlet peak, all shifted downfield relative to complex (1). In contrast, 2,2′-bpy lacked symmetry, exhibiting more than the expected four signals. The presence of double signals attributed to the H6 and H6′, with Δδ = 0.86 ppm, suggested a distinct environment for the two pyridine rings of 2,2′-bpy (Figure S3). In the ROESY spectrum of (2) (Figure S4), a cross-peak between H6′ (8.80 ppm) and Ha (7.53 ppm) of terph suggested the proximity of H6′ to the aromatic ring of terph, resulting in a pronounced shielding effect. The signal of H6 appeared significantly downfield, at 9.66 ppm, indicating a strong interaction with the coordinated Cl. This difference was attributed to the orientation of the H6 and H6′ of 2,2′-bpy towards the bridged terph ligand. Also, a significant difference of 0.45 ppm was observed in the H5 and H5′ signals.

Additional evidence supporting the formation of (2) was derived from the HR-ESI-MS (Figure S5), where a distinct cluster peak was observed at m/z = 1043.1487. This peak was assigned to the single charged cation [C₄₀H₃₄N₄ClSO¹⁹⁵Pt₂]⁺, which could be formulated as {[Pt₂(2,2′-bpy)₂(DMSO)Cl](μ-terph)}⁺. The presence of the DMSO was justified by the preparation of the sample for HR-ESI-MS, where 5 μL of DMSO was added to improve the solubility.

The coordination of the N^N ligands to the platinum center without removing the Cl atoms was not successful. To address this issue, two equivalents of AgNO₃ were added (Scheme 3), and the mixture was subjected to sonication using a 750 W equipment. Following the removal of the precipitated AgCl, the complex {[Pt(2,2′-bpy)(CH₃CN)]₂(μ-terph)}²⁺, (3) was isolated. The HR-ESI-MS (Figure S6) and NMR spectra confirmed the proposed formula (Figure 2 and Figure S7).

Subsequently, the formation of “pi”-shaped complexes (4)–(6) was achieved by adding an excess of the required BLs (4,4′-bpy, dpbz, dpbph) to act monodentately. This was confirmed by the reduction of the BLs symmetry reflected in the signals of the ¹HNMR spectra. Specifically, in ¹HNMR of complex (4), four signals corresponding to 4,4′-bpy rings were observed (Figure 2). The coordinated pyridine ring showed significant downfield shifts for H6 (+0.60 ppm) and H5 (+0.29 ppm), while the proton signals of the non-coordinated ring shifted slightly downfield. Moreover, upfield shifts were observed for the three signals of terph by 0.3 ppm, probably due to a difference in bond strength between the Pt-Cl and Pt-py. On the other hand, a dramatic upfield shift of 2,2′-bpyH6 by 1.68 ppm was observed, indicating a strong shielding effect. This large shift may be interpreted by the change in the electron density of H6 due to the transition from the Pt-Cl^…H6 interaction in (2) to the proximity of H6 to the 4,4′-bpy coordinated pyridine ring in (4). The other protons of 2,2′-bpy remained almost unaffected after the coordination of 4,4′-bpy.

In the ROESY spectrum of (4) (Figure 3), cross-peaks between the H6(2,2′-bpy) → H2/6 (4,4′-bpy), H6′(2,2′-bpy) → Ha(terph), and Ha(terph) → H2/6(4,4′-bpy) confirmed the proposed “pi” shape for this complex. Similar results were observed for all the “pi”-shaped complexes (4)–(6).

In the ¹HNMR spectrum of complex (7), the signals that were assigned to the free pyridine rings of 4,4′-bpy were not observed anymore due to their coordination on the Pt(II) centers of the added complex (3). Also, complex (7) was highly symmetric, resulting in only one set of signals for each ligand (Figure S8). Similar results were also observed for complexes (8) and (9). At the ROESY spectra of tetranuclear complexes (7)–(9) (Figure 4 and Figure S9), cross-peaks between 2,2′-bpyH6′ → BLsH2/6, 2,2′-bpyH6 → terphHa, and BLsH2/6 → terphHa were observed. Considering the above constraints on the spatial placing of the ligands (2,2′-bpy, terph, BLs), we concluded that the tetranuclear platinum complexes adopted a rectangular structure.

The formation and stability of complexes (7)–(9) were confirmed through HR-ESI mass spectra. Within all spectra, three distinct cluster peaks were observed, corresponding to multicharged cations generated through the successive release of [PF₆]⁻ ions from the original complex. Consequently, the following cations were assigned: {[M]-4[PF₆]}⁴⁺, {[M]-3[PF₆]}³⁺, and {[M]-2[PF₆]}²⁺, with {[M]-4[PF₆]}⁴⁺ being the most predominant among them. The single charged cation {[M]-[PF₆]}⁺ was not observed. The isotopic patterns of these cations closely aligned with the theoretically simulated ones, as illustrated in Figure 5.

2.2. Photophysical Studies

2.2.1. Absorption and Emission Spectra of Complexes (7)–(9)

Table 1. Photophysical data of complexes (7)–(9).

Complex	UV/Vis Absorbance λmax [nm], (ε × 104 [M−1cm−1])		Excitation		Emission		QY%
	Solution (CHCl3)	Solid	λexc		λem
			Solution (CHCl3)	Solid	Solution (CHCl3)	Solid	Solution (CHCl3)	Solid
(7)	268 (15.30), 281 (12.2), 328 (5.2)	242, 313, 420	365	400	427	574	1.7%	4%
(8)	276 (12.2), 345 (2.1)	242, 315, 418	365	400	433, 615	597	5%	2%
(9)	278 (11.1)	242, 313, 422	365	400	431	578	1%	1%

summarizes the photophysical data of complexes (7)–(9), while Figure 6 illustrates their absorption and emission spectra.

The UV–vis spectra of complexes (7)–(9) in the solid state were similar, while their solution spectra exhibited significant differences from each other. In the solid state, the complexes showed three distinct absorbance bands, two in the UV region and one in the visible region. The absorption bands at 240 nm and 310 nm were attributed to n→π* or π→π* transitions of the aromatic ligands 4,4′-bpy, dpbz, and dpbph. Additionally, the third band observed in the visible region was associated with the metal-to-ligand charge transfer, [(5d)Pt→π*(L)] [24]. However, their absorption spectra in CHCl₃ exhibited remarkable differences. Complex (9) displayed an intense band at 278 nm assigned to intra-ligand transitions arising from their aromatic ring system [25]. In complex (8), this band shifted slightly to 276 nm, while, in complex (7), it was split to 268 nm and 281 nm, probably due to the differences in the absorption maxima of the containing ligands. A very weak band at 328 nm and 345 nm for complexes (7) and (8), respectively, was attributed to the [(5d)Pt→π*(L)] transition [24], while it was not observed for complex (9) due to its very weak intensity. Similar results have been reported for rhomboidal structures [22], where bands with weak intensity in the range of 420–480 nm were attributed to metal-to-ligand charge transfer [26].

Upon excitation of complexes (7)–(9) in the solid state with λ_exc = 400 nm, emission spectra were obtained, showing a single broad band in the visible region. In complexes (7) and (9), this band was observed at about 580 nm, while, in complex (8), at about 600 nm, each was characterized by low quantum yields. When excited in a dilute solution of CHCl₃ at 365 nm, only complex (8) demonstrated a strong emission band in the orange part of the spectrum (λ_em = 615 nm) accompanied by a weak band at 433 nm. In contrast, complexes (7) and (9) exhibited only a single weak band each at 430 nm. Moreover, complex (8) demonstrated the highest quantum yield (5%) in CHCl₃ compared to complexes (7) and (9). This complex stood out for its emission at λ_em = 615 nm, a feature not yet reported in the literature for this type of tetranuclear platinum square, which typically emits at wavelengths less than 600 nm [20,27].

Furthermore, complex (8) could be described as a dual-emitter (615 and 430 nm), a feature that set it apart from the other two complexes (7) and (9). In general, the dual-emitting effect, arising from distinct excited states [25], could find application in the production of white light [28]. Also, it is worth noting that while some of the literature highlights platinum squares as exceptional emitters, our compounds exhibited lower quantum yields, likely due to the rotational dynamics of phenyl rings leading to fluorescence quenching effects [8,29].

2.2.2. Solvent Effect on Emission Spectra of Complexes (7)–(9)

The emission spectra of complexes (7)–(9) were recorded in various solvents (Figure 7 and

Table 2. λ_em of complexes (7)–(9) in solvents with different polarities. DMF: N,N′-dimethylformamide, CH₃CN: acetonitrile, MeOH: methanol, EtOH: ethanol EA: ethyl acetate, THF tetrahydrofuran, DE: diethyl ether.

Complex	Acetone	CH2Cl2	CH3CN	CHCl3	DE	DMF	DMSO	EA	EtOH	MeOH	THF
(7)	429	437	426	427	429 451 613	429	430	431	432	464	436
	541	584									460
(8)	671	436	677	433	427	427	421	606	429	432	429
		626		616	605	680	690		610	632	614
(9)	433	428	472	431	429	421	421	427	457	467	421
	644	642	661		449			616			569

), with different polarities to explore their emission profiles. Upon excitation of their solutions at 365 nm, notable distinctions were observed. Specifically, complexes (7) and (9) in acetone and CH₂Cl₂ exhibited two distinct emission bands: an emission in the blue region and another one in the green (7) or red (9) region. Also, complex (9) exhibited these bands in acetonitrile and ethyl acetate. However, in the other solvents, they showed only a single band in the blue region (420–440 nm), with the exception of the diethyl ether. In diethyl ether, both complexes displayed a shoulder at 450 nm, with complex (7) additionally exhibiting a third band at 613 nm. These variations in emission spectra across different solvents highlight the solvent-dependent photophysical behavior of these complexes [21].

Regarding complex (8), notable inconsistencies were evident upon changes in solvents. Specifically, an orange-region band was observed at 605 nm in diethyl ether, while, in DMSO, it shifted closer to the near infra-red region, at 700 nm. In most solvents, excluding acetone, acetonitrile, and ethyl acetate, the complex exhibited two distinct emission bands: one in the blue region and another one in the red-to-orange region. In acetone, acetonitrile, and ethyl acetate, only one orange-red band was observed.

In conclusion, complexes (7)–(9) demonstrated varying shifts in the emission maxima of their orange-red band across different solvents, a phenomenon commonly referred to as fluorosolvatochromism. However, this magnitude of solvent-dependent variations was not observed for the blue-region band.

3. Experiments

3.1. Materials and Methods

All solvents were of analytical grade and were used without further purification. Tetrahydrofuran (THF) was distilled from sodium benzophenone ketyl, toluene was distilled from CaH₂, and they were stored over molecular sieves. 4,4′-bipyridine, 2,2′-bipyridine, and 1,4-di(pyridin-4-yl)benzene were purchased from Fluorochem, while 4,4′-di(pyridin-4-yl)-1,1′-biphenyl was purchased from TCI. Pt(COD)Cl₂, Sn(CH₃)₃Cl, and n-BuLi (2.5M in hexane) were purchased from Sigma. 4,4″-diiodo-p-terphenyl was synthesized according to the literature [30]. Also, 4,4″-bis(trimethylstannyl)-p-terphenyl and [Pt(COD)Cl]₂(μ-terph) were synthesized following literature methods [1].

¹H NMR spectra for all the compounds were recorded in a Bruker Avance NEO spectrometer operating at 500.13 MHz or in a Bruker Avance II spectrometer operating at 400.13 MHz. The spectra were processed by Topspin 4.2 (Bruker Analytik GmbH, Ettlingen, Germany). Two-dimensional COSY, TOCSY, and ROESY spectra were obtained following standard Bruker procedures. High-resolution electrospray ionization mass spectra (HR-ESI-MS) were recorded using a Thermo Scientific LTQ Orbitrap XL™ system. UV–vis spectra of the complexes were recorded on an Agilent Cary 60 UV–vis spectrophotometer with a xenon source lamp. Sonication of the reaction between [Pt(bpy)Cl]₂(μ-terph) and AgNO₃ in CH₃CN was accomplished using a Sonics & Materials 750 W instrument (Sonics & Materials, Inc., Leicestershire, UK).

3.2. Fluorescence Emission Studies

Emission studies were conducted using a Jasco FP-8300 fluorometer (Jasco, Tokyo, Japan), which was equipped with a xenon lamp and an integrated sphere for the solid samples. The determination of the relative quantum yield for solutions employed the formula Qs = Qr(Ar/As)(Es/Er)(ns/nr)². ‘A’ denotes the absorbance of the solutions, ‘E’ denotes the integrated fluorescence intensity of the emission spectrum, and ‘n’ denotes the refractive index of the solvents. Subscripts ‘r’ and ‘s’ indicate the reference and sample, respectively. A water solution of [Ru(bpy)₃]Cl₂ served as the reference standard (Qr = 0.04). The calculation of the relative quantum yield for the complexes in the solid state followed the equation Q = S₂/(S₀ − S₁), where ‘Q’ denotes the quantum yield of the solid state of the complexes, ‘S₂’ signifies the integrated emission intensity of the sample, and ‘S₀’ and ‘S₁’ refer to the excitation intensities of the standard and the sample, respectively.

3.3. Synthesis of Complexes (2)–(9)

[Pt(2,2′-bpy)Cl]₂(μ-terph) (2): 56.1 mg of 2,2′-bpy (0.78 mmol) was added to 90 mL of CH₂Cl₂ containing 60 mg (0.059 mmol) of the complex [Pt(COD)Cl]₂(μ-terph). The suspension was stirred for about 24 h at 30 °C. After 24 h, the suspension turned into a bright yellow solution. The solvent was evaporated and the solid was washed with hexane and diethyl ether. Yield: 85%. ¹H NMR: (500 MHz, 298K, CD₂Cl₂, δ in ppm): H6: 9.66 (d, 2H, ³J = 5.1); H6′: 8.80 (d, 2H, ³J = 5.6); H44′: 8.20 (d, 4H, ³J = 7.6); H3′: 8.17 (t, 2H, ³J = 6.0); H3: 8.10 (t, 2H, ³J = 4.8); H5: 7.79 (t, 2H, ³J = 5.1); Hcc’: 7.77 (s,4H); Haa′: 7.53 (d, 4H, ³J = 8.1); AHbb′: 7.46 (d, 4H, ³J = 8.3); H5′: 7.41 (t, 2H, ³J = 7.0). HR-ESI-MS (5mL CH₂Cl₂ + 20 μL of DMSO), positive (m/z): found 1043.1487, calc. 1043.1411 for [C₄₀H₃₄N₄ClSO¹⁹⁵Pt₂]⁺, assignable to the cation {[Pt₂(2,2′-bpy)₂(DMSO)Cl](μ-terph)}⁺.

{[Pt(2,2′-bpy)(CH₃CN)]₂(μ-terph)}(NO₃)₂ (3): 11.2 mg of AgNO₃ (0.066 mmol) was added to a suspension containing 34 mg (0.033 mmol) of complex (2) in 20 mL acetonitrile. The mixture was sonicated for 30 min in 375 W (1/2″ probe) at 55 °C. After sonication, the mixture was filtered and the solvent was evaporated. Yield: 99%. ¹H NMR: (500 MHz, 298 K, DMSO-d₆, δ in ppm): H6: 9.65 (d, 2H, ³J = 4.8); H33′: 8.82 (d, 4H, ³J = 8.1); H4: 8.51 (t, 2H, ³J = 8.0); H4′: 8.44 (t, 2H, ³J = 7.8); H5: 8.03 (t, 2H, ³J = 6.6); Hcc′: 7.86 (s, 4H) H6′: 7.75 (d, 2H, ³J = 6.4); H5′: 7.72 (t, 2H, ³J = 5.9); Haa′: 7.64 (d, 4H, ³J = 8.3); Hbb′: 7.61 (d, 4H, ³J = 8.5). HR-ESI-MS (CH₃CN), positive (m/z): found 506.1066, calc. 506.1058 for [C₄₂H₃₄N₆¹⁹⁵Pt₂]²⁺, assignable to the cation {[Pt(2,2′-bpy)(CH₃CN)]₂(μ-terph)}²⁺.

{[Pt(2,2′-bpy)(4,4′-bpy)]₂(μ-terph)}{PF₆}₂ (4): 46.3 mg (0.29 mmol) of 4,4′-bpy was added to 50 mL acetonitrile containing 30 mg (0.029 mmol) of complex (3) {[Pt(2,2′-bpy)(CH₃CN)](μ-terph)}{NO₃}₂ and 10.7 mg (0.058) mmol of KPF_6. The mixture was stirred for 18 h at room temperature. After the evaporation of the solvent, the orange solid was washed with H₂O and diethyl ether. Yield: 60%. ¹H NMR: (500 MHz, 298 K, DMSO-d₆, δ in ppm): H26(4,4′-bpy): 9.21 (d, 4H, ³J = 6.9); H3′: 8.80 (d, 2H, ³J = 8.4); H2′6′(4,4′-bpy): 8.78 (d, 4H, ³J = 6.2); H3: 8.76 (d, 2H, ³J = 8.0); H44′: 8.46 (t, 4H, ³J = 7.6); H6′: 8.35 (d, 2H, ³J = 5.9); H35(4,4′-bpy): 8.14 (d, 4H, ³J = 5.9); H6: 7.97 (d, 2H, ³J = 6.0); H3′5′(4,4′-bpy): 7.94 (d, 4H, ³J = 5.9); H5: 7.82 (t, 2H, ³J = 6.2); H5′: 7.74 (t, 2H, ³J = 6.7); Hcc′: 7.69 (s,4H); Haa′: 7.55 (d, 4H, ³J = 8.2); Hbb′: 7.43 (d, 4H, ³J = 8.2).

{[Pt(2,2′-bpy)(dpbz)]₂(μ-terph)}{PF₆}₂ (5): 67.3 mg (0.29 mmol) of dpbz was added to 40 mL acetonitrile containing 30 mg (0.029 mmol) of complex (2) {[Pt(2,2-bpy)(CH₃CN)](μ-terph)}{NO₃}₂ and 10.7 mg (0.058) mmol of KPF_6. The mixture was stirred for 22 h at room temperature. After the evaporation of the solvent, the orange solid was washed with acetone and diethyl ether. Yield: 75%. ¹H NMR: (500 MHz, 298 K, DMSO-d₆, δ in ppm): H26(dpbz): 9.06 (d, 4H, ³J = 6.4); H3′: 8.71 (d, 2H, ³J = 7.5); H2′6′(dpbz): 8.68 (d, 4H, ³J = 6.0); H3: 8.66 (d, 2H, ³J = 8.6); H44′: 8.49 (t, 4H, ³J = 7.9); H6′: 8.35 (d, 2H, ³J = 5.9); H35(dpbz): 8.21 (d, 4H, ³J = 6.6); Haa′bb′(dpbz): 8.28 (s,8H); H6: 8.21 (d, 2H, ³J = 6.1); H3′5′(dpbz): 8.15 (d, 4H, ³J = 6.8); H5: 8.03 (t, 2H, ³J = 7.1); H5′: 7.98 (t, 2H, ³J = 7.2); Hcc′: 7.69 (s,4H); Haa′: 7.58 (d, 4H, ³J =7.5); AHbb′: 7.46 (d, 4H, ³J = 7.87).

{[Pt(2,2′-bpy)(dpbph)]₂(μ-terph)}{PF₆}₂ (6): 90.5 mg (0.29 mmol) of dpbph was added to 70 mL acetonitrile containing 30 mg (0.029 mmol) of complex (2) {[Pt(2,2-bpy)(CH₃CN)](μ-terph)}{NO₃}₂ and 10.7 mg (0.058) mmol of KPF_6. The mixture was stirred for 24 h at room temperature. After the evaporation of the solvent, the orange solid was washed with THF and diethyl ether. Yield: 55%. ¹H NMR: (500 MHz, 298 K, DMSO-d₆, δ in ppm): H26(dpbph): 8.98 (d, 4H, ³J = 7.2); H2′6′(dpbph): 8.89 (d, 4H, ³J = 6.8); H3′: 8.88 (d, 2H, ³J = 7.6); H3: 8.87 (d, 2H, ³J = 7.1); H44′: 8.43 (t, 4H, ³J = 7.2); H6′: 8.27 (d, 2H, ³J = 6.1); H35(dpbph): 8.01 (d, 4H, ³J = 6.9); H6: 7.97 (d, 2H, ³J = 6.9); Haa′bb′cc’(dpbph): 7.92 (dd, 8H, ³J = 6.5); H5: 7.82 (t, 2H, ³J = 6.2); H5′: 7.75 (t, 2H, ³J = 7.2); Hcc′: 7.65 (s, 8H); AHaa′: 7.49 (d, 4H, ³J = 8.4); AHbb′: 7.35 (d, 4H, ³J = 8.2).

The tetranuclear Pt(II) complexes (7)–(9) were synthesized similarly. In a typical experiment, 1 eq. of complex (3) was added to 20–30 mL acetonitrile containing 1 eq. of the binuclear complexes (4)–(6)_. The mixture was heated for 20–25 h at 60 °C. After 19 h, the reaction mixture was evaporated to dryness under reduced pressure and the orange solid was washed several times with dichloromethane and diethyl ether.

{[Pt(2,2′-bpy)]₄(μ-terph)₂(μ-(4,4′-bpy)₂}{PF₆}₄ (7): 3.4 mg (0.020 mmol) of 4,4′-bpy was added to 20 mL acetonitrile containing 32 mg (0.020 mmol) of complex (4). Yield: 55%. ¹HNMR: (500 MHz, 298 K, DMSO-d₆, δ in ppm): H26(4,4′-bpy): 9.30 (d, 8H, ³J = 6.5); H3′: 8.82 (d, 4H, ³J = 8.4); H3: 8.78 (d, 4H, ³J = 8.5); H44′: 8.48 (tt, 8H, ³J = 7.6); H6′: 8.37 (d, 4H, ³J = 6.0); H35(4,4′-bpy): 8.26 (d, 8H, ³J = 6.9); H6: 7.88 (d, 4H, ³J = 4.9); H5: 7.81 (t, 4H, ³J = 7.1); H5′: 7.74 (t, 4H, ³J = 6.7); Hcc′: 7.63 (s, 8H); Haa′: 7.54 (d, 8H, ³J = 8.1); Hbb′: 7.39 (d, 8H, ³J = 8.3). HR-ESI-MS, positive (m/z): found 543.3665, calc. 543.3646 for [C₉₆H₇₂N₁₂¹⁹⁵Pt₄]⁴⁺; found 772.8104, calc. 772.8078 for [C₉₆H₇₂N₁₂PF₆¹⁹⁵Pt₄]³⁺; found 1231.6974, calc. 1231.6940 for [C₉₆H₇₂N₁₂P₂F₁₂¹⁹⁵Pt₄]²⁺.

{[Pt^II(2,2′-bpy)]₄(μ-terph)₂(μ-dpbz)₂}{PF₆}₄ (8): 3.2 mg (0.013 mmol) of dpbz was added to 20 mL acetonitrile containing 5 mg (0.013 mmol) of complex (5). Yield: 68%. ¹HNMR: (500 MHz, 298K, DMSO-d₆, δ in ppm): H26(dpbz): 9.14 (d, 8H, ³J = 6.7); H3: 8.81 (d, 4H, ³J = 8.3); H3′: 8.77 (d, 4H, ³J = 8.6); H44′: 8.47 (tt, 8H, ³J = 7.0); H6′: 8.37 (d, 4H, ³J = 5.7); Haa′bb′(dpbz): 8.16 (s, 8H); H35(dpbz): 8.13 (s, 4H); H6: 7.99 (d, 8H, ³J = 5.6); H5: 7.81 (t, 4H, ³J = 6.7); H5′: 7.74 (t, 4H, ³J = 6.8); Hcc′: 7.67 (s, 8H); Haa′: 7.56 (d, 8H, ³J = 8.1); Hbb′: 7.43 (d, 8H, ³J = 8.4). HR-ESI-MS, positive (m/z): found 581.3825, calc. 581.3803 for [C₁₀₈H₈₀N₁₂¹⁹⁵Pt₄]⁴⁺; found 823.4971, calc. 823.4953 for [C₁₀₈H₈₀N₁₂PF₆¹⁹⁵Pt₄]³⁺.

{[Pt^II(2,2′-bpy)]₄(μ-terph)₂(μ-dpbph)₂}{PF₆}₄ (9): 5.1 mg (0.016 mmol) of dpbph was added to 30 mL acetonitrile containing 31 mg (0.016mmol) of complex (6). Yield: 50%. ¹HNMR: (500 MHz, 298 K, DMSO-d₆, δ in ppm): H26(dpbph): 9.10 (d, 8H, ³J = 6.1); H3: 8.81 (d, 4H, ³J = 8.2); H3′: 8.77 (d, 4H, ³J = 8.4); H44′: 8.47 (t, 4H, ³J = 8.4); H6′: 8.38 (d, 4H, ³J = 5.8); H35(dpbph): 8.09 (dd, 8H, ³J = 6.8); Haa’bb’(dpbph): 8.08 (dd, 8H, ³J = 7.3); H6: 8.02 (d, 4H, ³J = 5.0); H5: 7.83 (t, 4H, ³J = 7.5); H5′: 7.75 (t, 4H, ³J = 6.3); Hcc′: 7.70 (s, 8H); Haa′: 7.56 (d, 8H, ³J = 8.1); Hbb′: 7.45 (d, 8H, ³J = 8.2). HR-ESI-MS, positive (m/z): found 619.3976, calc. 619.3959 for [C₁₂₀H₈₈N₁₂¹⁹⁵Pt₄]⁴⁺; found 874.1847, calc. 874.1828 for [C₁₂₀H₈₈N₁₂PF₆¹⁹⁵Pt₄]³⁺.

4. Conclusions

In conclusion, the tetranuclear complexes {[Pt(2,2′-bpy)]₄(μ-terph)₂(μ-4,4′-bpy)₂}{PF₆}₄ (7), {[Pt(2,2′-bpy)]₄(μ-terph)₂(μ-dpbz)₂}{PF₆}₄ (8), and {[Pt(2,2′-bpy)]₄(μ-terph)₂(μ-dpbph)₂}{PF₆}₄ (9) were synthesized through a stepwise method and characterized by NMR and HR-ESI mass spectrometry. The photophysical properties of these complexes were investigated both in the solid state and in various solvents. All complexes, (7)–(9), displayed solvent-dependent photophysical behavior, altering their emissions based on the solvent, a phenomenon known as fluorosolvatochromism. Complex (8) demonstrated a pronounced dependence of the emission λ_max on the polarity [31] of several solvents used, DMSO ≈ DMF ≈ CH₃CN > Acetone > CH₂Cl₂ > CHCl₃ > EA ≈ DE, following the general trend of increasing λ_max by the increase in solvent polarity.