NIR-II Emission from Cyclometalated Dinuclear Pt(III) Complexes

Abstract

Half-lantern Pt(II) dinuclear complexes [{Pt(C^∧N_pz)(μ-S^∧N^R)}₂] (HC^∧N_pz = 1-naphthalen-2-yl-1H-pyrazole; R = H, HS^∧N: 2-mercaptopyrimidine 1; R = CF₃, HS^∧N^F: 4-(trifluoromethyl)-2-mercaptopyrimidine 2) were selectively obtained as single isomers with the C^∧N groups in an anti-arrangement and rather short metallophilic interactions (d_Pt–Pt = 2.8684(2) Å for 2). They reacted with haloforms in the air and sunlight to obtain the corresponding oxidized diplatinum(III) derivatives [{Pt(C^∧N_pz)(μ-S^∧N^R)X}₂] (X = Cl (1-Cl), Br (1-Br), I (1-I, 2-I)). The single-crystal X-ray structures exhibit Pt–Pt distances typical for the existence of a metal–metal bond, which evidence fairly well the influence of the axial ligand (X). The reactions of 1 and 2 with CHI₃ in the dark afforded mixtures of [IPt(C^∧N_pz)(μ-S^∧N)₂Pt(C^∧N_pz)CHI₂] and 1-I or 2-I, with the former being the major species under an Ar atmosphere, while the reactions of 1 with CHBr₃ and CHCl₃ need light to occur. These Pt₂(III,III) complexes display low-energy absorptions and emissions that strongly depend on the axial ligand. In the solid state, they show a broad NIR emission ranging from 985 to 1070 nm at RT that suffers a hypsochromic shift when cooling down to 77 K. The photoemissive behavior of the dinuclear Pt(II) and Pt(III) systems is disclosed with the aid of density functional theory calculations.

Short abstract

Half-lantern dinuclear Pt(II) complexes can be regarded as an effective springboard to obtain metal−metal bonded Pt₂(III,III) emitters in the NIR-II spectral window at room temperature.

Introduction

Near-infrared (NIR) light emitters are drawing increasing attention for their advanced technological applications,¹⁻⁴ in particular, those emitting between 1000 and 1700 nm, formally defined as the NIR-II window. This spectral region, also known as the second transparency or biological window, is typically characterized by a reduced absorption and low photon scattering, enabling a higher degree of penetration through biological tissues and enhancing imaging resolution.⁵ Thus, efficient and long-wavelength NIR light emitting materials with low toxicity and deep penetration are highly attractive for photodynamic therapy,⁶ biomedical sensing,⁷ and optical imaging.⁸ Yet, their design and synthesis are still a challenging endeavor.

Phosphorescent platinum(II) complexes with π-aromatic systems are an active area of research in this field due to the versatile photophysical properties. The square-planar structure gives rise to ground- and excited-state interactions involving self-aggregation and excimer formation, which result in red-to-NIR emissions, typically attributed to metal–metal to ligand charge transfer (MMLCT) [dσ*(Pt)₂ → π*(L)] or excimer ligand-to-ligand transitions.⁹ However, only a few examples with NIR luminescence close to or beyond 1000 nm at room temperature have been reported to date. Most of them are obtained by molecule stacking of diiminebis(σ-acetylide) Pt(II) derivatives¹⁰⁻¹² or self-assembled Pt(II) complexes with chelated ligands such as pyrazinyl pyrazolate or pyridyl pyrimidinate.¹³ However, minimal environmental stimuli such as mechanical pressure,^11,14 temperature,^11,15 or contact to chemical vapors^10−12,16 can unintentionally perturb the interactions between the stacked molecules and induce substantial changes in their photophysical properties. To avoid this, the use of 4-bond bridging groups as auxiliary ligands that provide a metal framework with rather short metal–metal distances is a suitable and well-known approach to achieve luminescent compounds with efficient low-lying emissions.¹⁷ In this regard, half-lantern platinum complexes with C^∧N cyclometalated ligands and strong intermetallic interactions (d_Pt–Pt ≤ 3 Å) are some of the most representative cases of ³MMLCT [dσ*(Pt)₂ → π*(C^∧N)] emitters. Among them, mercapto-¹⁸⁻³¹ or hydroxy-^23,32,33 substituted N-heterocycles have been typically used as 4-bond bridging ligands to prepare compounds with bright red emissions that, in some cases, have been successfully implemented in organic light-emitting diodes.^24−27,34 Recently, α-carboline³⁵ or 10H-pyrido[3,2-b][1,4]benzoxazine³⁶ has been used instead so as to push further the emission toward the deep red or NIR spectral region because the use of these rigid bridges confine close Pt···Pt contacts. Besides, comprehensive studies on these and also on related complexes have shown that changes on the chromophoric C^∧N ligands can push the emission to red as well. By these means, not only the lowest unoccupied molecular orbital (LUMO) energy level (π*(C^∧N)) is modified but also that of the highest occupied molecular orbital (HOMO) (dσ*(Pt)₂). The π-backdonation from the Pt center to the C^∧N aromatic system provokes a shortening of the Pt center distance, which leads to a reduced H–L energy gap and consequently a red-shifted emission.^{27,32,35−37}

As a result of the short Pt–Pt distances, these lantern-type complexes can experience two-center two-electron oxidations with halogens (X₂) or halocarbons (RX) to give metal–metal bonded Pt₂(III,III) complexes.^29,38−43 These dinuclear Pt(III) species have been considered as nonemissive because of their extremely short-lived dσ* excited states. There have been reported just four d⁷–d⁷ systems with emitting properties: [Pt₂(C^∧N)₂(N^∧S)₂Cl₂] (HS^∧N = 5-phenyl-1,3,4-oxadiazole-2-thiol; HC^∧N = 2,4-difluoro-phenylpyridine),⁴⁴ [Pt₂(HPO₄)₄X₂]^4– (X = Cl, Br),⁴⁵ and [Pt₂(μ-pop)₄X₂]^4– (pop= (HO₂P)₂O, X = Cl, Br, SCN, py)⁴⁶ in the red spectral region and with the latter only being emissive at low temperatures. In addition, compounds [Pt₂(μ–C₆H₃–5-R-2-AsPh₂)₄X₂] (R = Me, ⁱPr; X = Cl, Br, I)⁴⁷ displayed NIR emissions at room and low temperatures.

Throughout our investigations on luminescent half-lantern Pt(II) complexes, we reported the red light-emitting compounds anti-[{Pt(bzq)(μ-S^∧N)}₂] (Hbzq = benzo[h]quinoline, HS^∧N = 2-mercaptobenzothiazole,⁴¹ 2-mercaptobenzoxazolate⁴²) with photoluminescent quantum yield up to 90% in a solution of toluene. Nonetheless, the analogous compounds [{Pt(bzq)(μ-S^∧N)}₂] (HS^∧N: 2-mercaptopyrimidine,³⁹ 4-(trifluoromethyl)-2-mercaptopyrimidine⁴³) did not show luminescence in the visible region, nor did their Pt(III) derivatives. In this work, to investigate potential NIR emitters based on dinuclear platinum complexes, we have prepared the half-lantern compounds of Pt(II) [{Pt(C^∧N_pz)(μ-S^∧N^R)}₂] (HC∧N_pz = 1-naphthalen-2-yl-1H-pyrazole; R = H, HS^∧N: 2-mercaptopyrimidine 1; R = CF₃, HS^∧N^F: 4-(trifluoromethyl)-2-mercaptopyrimidine 2). Then, we have explored their redox chemistry by reacting them with haloforms, obtaining the two-electron-oxidized dinuclear Pt(III) complexes [{Pt(C^∧N_pz)(μ-S^∧N^R)X}₂] (X = Cl (1-Cl), Br (1-Br), I (1-I, 2-I)). X-ray diffraction studies, theoretical calculations, and photophysical investigations were carried out on the Pt₂(II,II) and Pt₂(III,III) complexes and their results compared to the benzoquinolinate derivatives.

Experimental Section

General Methods

Compound [Pt(C^∧N)Cl(NCMe)](A) was prepared accorded to the reported protocol.⁴⁸ 2-Mercaptopyrimidine (HS^∧N), 4-(trifluoromethyl)-2-mercaptopyrimidine (HS^∧N^F), NEt₃, AgClO₄, and BaSO₄ were used as purchased from Across Organics, TCI, Aldrich, and Alfa Aesar, respectively. IR spectra were recorded on a PerkinElmer Spectrum 100 FT-IR Spectrometer (ATR in the range 250–4000 cm^–1). Mass spectral analyses were performed with a Microflex MALDI-TOF Bruker or an Autoflex III MALDI-TOF Bruker instrument. C, H, and N analyses were carried out in a PerkinElmer 2400 CHNS analyzer. ¹H, ¹⁹F, ¹⁹⁵Pt{¹H} NMR spectra were recorded on a Bruker Avance 400 MHz instrument using the standard references: SiMe₄ for ¹H, CFCl₃ for ¹⁹F, and Na₂PtCl₆ in D₂O for ¹⁹⁵Pt. J is given in Hz and assignments are based on ¹H–¹H COSY experiments. Caution! Perchlorate salts of metal complexes with organic ligands are potentially explosive. Only small amounts of material should be prepared, and these should be handled with great care.

Preparation of [{Pt(C^∧N_pz)(μ-S^∧N)}₂] (1)

AgClO₄ (90.0 mg, 0.43 mmol) was added to a suspension of A (200.3 mg, 0.43 mmol) in acetonitrile (20 mL). After 7 h of reaction at room temperature in the dark, the mixture was filtered through Celite and washed with acetonitrile. The resultant solution was evaporated to dryness to give a pale-yellow residue, which was then reacted with 2-mercaptopyrimidine (48.4 mg, 0.43 mmol) in 20 mL of acetone/methanol (1/1) and NEt₃ (0.5 mL) at reflux for 1.5 h. After this time, the suspension was concentrated to ca. 2 mL. The precipitated was filtered, washed with methanol (2 × 3 mL), and dried to give 1 as a yellow solid. Yield: 152.6 mg, 71%. Anal. Calcd for C₃₄H₂₄N₈S₂Pt₂: C, 40.88; H, 2.42; N, 11.22; S, 6.42. Found: C, 40.42; H, 2.13; N, 10.94; S, 6.56. ¹H NMR data (400 MHz, CD₂Cl₂): 8.77 (dd, ³J_{H6′–H5′} = 5.7, ⁴J_{H6′–H4′} = 2.6, 1H, H₆′), 8.32 (dd, ³J_{H4′–H5′} = 4.5, ⁴J_{H4′–H6′} = 2.5, 1H, H₄′), 7.72 (s, ³J_Pt–H = 56.9, 1H, H_ortho), [7.58–7.44] (m, 2H, H_naph), [7.39–7.26] (m, 2H, H_naph), 7.04 (d, ³J_H–H = 1.8, 1H, H_pz), [6.86–6.77] (m, 2H, H_pz, H₅′), 6.54 (s, 1H, H_meta), 6.21 (t, ³J_H–H = 2.3, 1H, H_pz). ¹⁹⁵Pt{¹H} NMR (85.6 MHz, THF-d₈): δ = −3549.5 (s). MS (MALDI+): m/z 997.9 [{Pt(C^∧N)(μ-S^∧N)}₂]⁺.

Preparation of [{Pt(C^∧N_pz)(μ-S^∧N^F)}₂] (2)

Compound 2 was synthesized following the same procedure used for 1 but using A (250.4 mg, 0.54 mmol), AgClO₄ (112.3 mg, 0.54 mmol), and (4-trifluoromethyl)-2-mercaptopyrimidine (97.1 mg, 0.54 mmol). 2 was obtained as an orange solid. Yield: 200.5 mg, 65%. Anal. Calcd for C₃₆H₂₂N₈S₂F₆Pt₂: C, 38.10; H, 1.95; N, 9.87; S, 5.65. Found: C, 37.79; H, 1.89; N, 10.14; S, 6.06. ¹H NMR data (400 MHz, acetone-d₆): 9.10 (d, ³J_{H5′–H6′} = 5.8, 1H, H₆′), 7.73 (s, ³J_Pt–H = 54.8, 1H, H_ortho), 7.57 (d, ³J_H–H = 8.5, 1H, H_naph), 7.51 (d, ³J_H–H = 7.0, 1H, H_naph), 7.40 (d, ³J_{H5′–H6′} = 5.8, 1H, H₅′), [7.36–7.28] (m, 3H, 2 H_naph, H_pz), 7.24 (d, ³J_H–H = 2.3, 1H, H_pz), 6.99 (s, 1H, H_meta), 6.33 (t, ³J_H–H = 2.3, 1H, H_pz). ¹⁹F NMR (376.5 MHz, acetone-d₆): – 71.09 (s). ¹⁹⁵Pt{¹H} NMR (85.6 MHz, acetone-d₆): δ = −3559.2 (s). MS (MALDI+): m/z 1134.1 [{Pt(C^∧N)(μ-S^∧N^F)}₂]⁺

Preparation of [{Pt(C^∧N_pz)(μ-S^∧N)Cl}₂] (1-Cl)

A suspension of compound 1 (100.1 mg, 0.10 mmol) in CHCl₃ (85 mL) was left to react in the air, at room temperature, and in sunlight. After 7.5 h of reaction, the mixture was evaporated to dryness and treated with Et₂O (10 mL); the resulting precipitate was filtered, washed with Et₂O (10 mL), and dried to give 1-Cl as a yellow-orange solid. Yield: 77.2 mg, 72%. Anal. Calcd for C₃₄H₂₄Cl₂N₈S₂Pt₂: C, 38.17; H, 2.26; N, 10.47; S, 5.99. Found: C, 37.73; H, 2.19; N, 10.08; S, 5.57. ¹H NMR data (400 MHz, CD₂Cl₂): δ = 9.50 (dd, ³J_{H6′–H5′} = 5.8, ⁴J_{H6′–H4′} = 2.3, 1H, H₆′), 8.58 (dd, ³J_{H4′–H5′} = 4.6, ⁴J_{H4′–H6′} = 2.3, 1H, H₄′), [7.63–7.58] (m, 1H, H_naph), 7.53 (d, ³J_H–H = 2.2, 1H, H_pz), [7.48–7.38] (m, 3H, H_naph), 7.32 (s, ³J_Pt–H = 39.0, 1H, H_ortho), [7.05–6.99] (m, 2H, H₅′, H_pz), 6.58 (s, 1H, H_meta), 6.54 (t, ³J_H–H = 2.6, 1H, H_pz). ¹⁹⁵Pt{¹H} NMR (85.6 MHz, CD₂Cl₂): δ = −2368.1 (s). MS (MALDI+): m/z 1033.8 [{Pt(C^∧N)(μ-S^∧N)}₂Cl]⁺.

Preparation of [{Pt(C^∧N_pz)(μ-S^∧N)Br}₂] (1-Br)

CHBr₃ (25 μL, 0.28 mmol) was added to a suspension of 1 (70.8 mg, 0.071 mmol) in acetone (90 mL) in the air, at RT, and in sunlight. After 7 h of reaction, the suspension was concentrated to ca. 3 mL, treated with n-hexane (20 mL), and then filtered and washed to give 1-Br as a dark orange solid. Yield: 58.8 mg, 72%. Anal. Calcd for C₃₄H₂₄Br₂N₈S₂Pt₂: C, 35.24; H, 2.09; N, 9.67; S, 5.53. Found: C, 34.79; H, 1.96; N, 9.48; S, 5.47. ¹H NMR data (400 MHz, CD₂Cl₂): δ = 9.63 (dd, ³J_{H6′–H5′} = 5.9, ⁴J_{H6′–H4′} = 2.3, 1H, H₆′), 8.56 (dd, ³J_{H4′–H5′} = 4.5, ⁴J_{H4′–H6′} = 2.3, 1H, H₄′), [7.62–7.57] (m, 1H, H_naph), 7.55 (d, ³J_H–H = 2.0, 1H, H_pz), [7.46–7.39] (m, 3H, H_naph), 7.26 (s, ³J_Pt–H = 38.6, 1H, H_ortho), [7.04–6.99] (m, 2H, H₅′, H_pz), 6.57 (t, ³J_H–H = 2.6, 1H, H_pz), 6.51 (s, 1H, H_meta). ¹⁹⁵Pt{¹H} NMR (85.6 MHz, CD₂Cl₂): δ = −2516.1 (s). MS (MALDI+): m/z 1078.7 [{Pt(C^∧N)(μ-S^∧N)}₂Br]⁺.

Preparation of [{Pt(C^∧N_pz)(μ-S^∧N)I}₂] (1-I)

CHI₃ (103.0 mg, 0.26 mmol) was added to a suspension of 1 (65.1 mg, 0.065 mmol) in acetone (15 mL) in the air, at RT, and in sunlight. After 16 h of reaction, the suspension was concentrated to ca. 2 mL, and then 20 mL of n-hexane was added to give a red garnet solid that was filtered, washed, and dried. Yield: 66.0 mg, 81%. Anal. Calcd for C₃₄H₂₄I₂N₈S₂Pt₂: C, 32.60; H, 1.93; N, 8.95; S, 5.12. Found: C, 32.15; H, 2.10; N, 8.52; S, 4.90. ¹H NMR data (400 MHz, CD₂Cl₂): δ = 9.82 (dd, ³J_{H6′–H5′} = 5.8, ⁴J_{H6′–H4′} = 2.3, 1H, H₆′), 8.53 (dd, ³J_{H4′–H5′} = 4.4, ⁴J_{H4′–H6′} = 2.3, 1H, H₄′), [7.61–7.55] (m, 2H, H_naph, H_pz), [7.44–7.39] (m, 3H, H_naph), 7.18 (s, ³J_Pt–H = 40.7, 1H, H_ortho), 7.03 (d, ³J_H–H = 3.3, 1H, H_pz), 6.99 (dd, ³J_{H5′–H6′} = 5.8, ³J_{H5′–H4′} = 4.6, 1H, H₅′), 6.60 (t, ³J_H–H = 2.6, 1H, H_pz), 6.42 (s, 1H, H_meta). ¹⁹⁵Pt{¹H} NMR (85.6 MHz, CD₂Cl₂): δ = −2767.5 (s). MS (MALDI+): m/z 1124.7 [{Pt(C^∧N)(μ-S^∧N)}₂I]⁺.

Preparation of [{Pt(C^∧N_pz)(μ-S^∧N^F)I}₂] (2-I)

Compound 2-I was synthesized by the same procedure as that of 1-I but using 2 (70.2 mg, 0.062 mmol) and CHI₃ (97.3 mg, 0.25 mmol). 2-I was obtained as a purple solid. Yield: 66.8 mg, 78%. Anal. Calcd For C₃₆H₂₂F₆I₂N₈S₂Pt₂: C, 31.14; H, 1.60; N, 8.07; S, 4.62. Found: C, 30.79; H, 1.54; N, 7.96; S, 4.45. ¹H NMR data (400 MHz, acetone-d₆): δ = 10.14 (d, ³J_{H5′–H6′} = 6.0, 1H, H₆′), 7.83 (d, ³J_H–H = 2.3, 1H, H_pz), 7.66 (d, ³J_{H5′–H6′} = 6.0, 1H, H₅′), [7.64–7.58] (m, 2H, H_naph, H_pz), [7.53–7.47] (m, 1H, H_naph), [7.46–7.38] (m, 2H, H_naph), 7.24 (s, ³J_Pt–H = 40.7, 1H, H_ortho), 6.92 (s, 1H, H_meta), 6.73 (t, ³J_H–H = 2.6, 1H, H_pz). ¹⁹F NMR (376.5 MHz, acetone-d₆): – 70.91 (s). ¹⁹⁵Pt{¹H} NMR (85.6 MHz, acetone-d₆): δ = −2763.3 (s). MS (MALDI+): m/z 1261.1 [{Pt(C^∧N)(μ-S^∧N^F)I}₂]⁺.

Computational Methods

Density functional calculations were carried out on the ground (S₀) and triplet (T₁) state with the Gaussian 09 suite of programs, using the M06 hybrid density functional⁴⁹ together with Grimme’s D3 dispersion correction.⁵⁰ The ECP-60-mwb for Pt and ECP-46-mwb, for I, pseudopotentials⁵¹ were used, and the 6-31G(d)^52,53 basis sets were used for all other atoms. General geometry optimizations were performed without any symmetry restriction and in the gas phase. Frequency calculations were performed in order to determine the nature of the stationary points found in S₀ and T₁ (no imaginary frequencies for minima). The time-dependent density-functional (TD-DFT) calculations were also carried out in the gas phase. Mulliken population analysis was carried out as implemented in the Gaussian 09 package.⁵⁴ The ChemissianLab program package was used for analysis and graphic representation of molecular orbitals and for Mayer bond order (BO) analysis. Atomic coordinates for the optimized structures are included as a separate .xyz file.

Results and Discussion

Synthesis and Characterization of New Pt₂(II,II) and Pt₂(III,III) Complexes. Reactivity of Pt₂(II,II) Half-Lantern Compounds toward Haloforms

Compounds 1 and 2 were obtained following the synthetic pathway depicted in Scheme 1, which starts with the chlorine abstraction from compound [Pt(C^∧N_pz)Cl(NCMe)] (HC^∧N = 1-naphthalen-2-yl-1H-pyrazole, A) with AgClO₄ in acetonitrile (step a) followed by the elimination of AgCl and evaporation of the solvent. Afterward, the residue was treated with equimolecular amounts of HS^∧N^R and excess of NEt₃ in refluxing acetone/methanol for 1.5 h (step b). The workup of the reactions afforded the corresponding compounds [{Pt(C^∧N_pz)(μ-S^∧N^R)}₂] (R = H, HS^∧N: 2-mercaptopyrimidine 1; R = CF₃, HS^∧N^F: 4-(trifluoromethyl)-2-mercaptopyrimidine 2) as pure yellow and orange solids in good yields.

Scheme 1. Synthesis Route for Dinuclear Pt(II) and Pt(III) Complexes.

Numerical scheme for NMR characterization.

These compounds were fully characterized through different techniques (see the Experimental Section). The ¹H and ¹⁹F NMR spectra (Figures S1–S2) display the expected set of signals for a single symmetric isomer, the anti one, as confirmed by single-crystal X-ray diffraction of complex 2 (see Figure 1 and Table 1). As can be seen, complex [{Pt(C^∧N_pz)(μ-S^∧N^F)}₂] (2) is a neutral dinuclear species of Pt(II) formed by two fragments “Pt(C^∧N_pz)” doubly bridged by two 4-(trifluoromethyl)-2-mercaptopyrimidine ligands. They show an anti configuration and a Pt···Pt distance of 2.8684(2) Å, which is among the shortest reported for half-lantern diplatinum(II) complexes.^{20,24,26,28,30,41,55,56} Each Pt(II) center coordinates to the two donor atoms of the naphthyl pyrazole ligand (C, N), a N atom of one mercaptopyrimidine group (S^∧N^F), and a S atom of the other one. Each Pt(II) center shows a distorted square-planar environment, which is mainly due to the small bite angle of the C,N-cyclometalated ligand [81.1(13)° Pt(1), 80.9(1)° Pt(2)]. This isomer has a head-to-tail configuration of the two bridging S∧N^F groups with a 2-fold axis perpendicular to the midpoint of the Pt(II)···Pt(II) line. Regarding the C^∧N_pz groups, an anti-arrangement of these can be found within the complex.

Figure 1.

Molecular structure of 2. Thermal ellipsoids are drawn at their 50% probability level; solvent molecules and hydrogens are omitted for clarity.

Table 1. Selected Bond Lengths (Å) and Angles (deg) of Complex 2.

distances (Å)	Pt1	Pt2
Pt1–Pt2	2.8684 (2)
Pt–NC∧N	2.026 (3)	2.022 (3)
Pt–CC∧N	2.002 (4)	2.001 (4)
Pt–NN∧S	2.137 (3)	2.131 (3)
Pt–SN∧S′	2.2726 (9)	2.2787 (9)
Angles (deg)
NC∧N–Pt–CC∧N	81.10 (13)	80.95 (13)
CC∧N–Pt–SN∧S′	95.77 (11)	95.88 (10)
SN∧S′–Pt–NN∧S	92.69 (9)	92.34 (8)
NC∧N–Pt–NN∧S	91.03 (12)	91.35 (12)

Also, they display an offset stacking [torsion angle N(1)–Pt(1)–Pt(2)–N(5), 120.2(2)°] without close π–π intramolecular interactions, most likely to reduce repulsions in this kind of compounds.^28,55 The platinum coordination planes are not completely parallel to one another since the interplanar angle is 15.12(8)°. Nonetheless, the short Pt···Pt distance and the perpendicularity between the Pt–Pt line and both metal coordination planes [angles = 8.64(6)° Pt(1); 7.85(5)° Pt(2)] suggest a significant interaction of the 5d_z² orbitals of both platinum centers. Within the crystal packing, weak π–π intermolecular interactions between the C^∧N ligands (C–C distances between 3.34 and 3.40 Å) give rise to slipped π–π stacking of adjacent molecules (Figure S3).

As expected from the short Pt–Pt distance and as found in other lantern,⁵⁷ half-lantern,^29,39 or even pyrazolate^58,59 dinuclear Pt(II) complexes, compound 1 undergoes two-center two-electron oxidation upon treatment with haloforms CHX₃ (X = Cl, Br, and I) in the air and sunlight to give the corresponding dihalogenated diplatinum(III) complexes [{Pt(C^∧N_pz)(μ-S^∧N)X}₂] (X = Cl 1-Cl, Br 1-Br, I 1-I) as yellow-orange, orange, and red garnet solids, respectively, in high yield (see Scheme 1 path c). For comparison and reproducibility, compound 2-I was prepared by the reaction of 2 with CHI₃ in the air and sunlight. All compounds were fully characterized (see the Experimental Section and Figures 2, 3 and S4–S9 in the Supporting Information). Their ¹H NMR spectra are very similar and show small changes with respect to the corresponding starting complexes. See the downfield [0.7–1.1 ppm] and upfield [∼0.5 ppm] shifts of the signals corresponding to H₆′ and H_ortho to the Pt–C bonds. Additionally, the ³J_Pt–H_ortho values decrease in comparison with those found in their parent compounds due to the higher oxidation state of the Pt center. This was also evident from the significant downfield shift (ΔδPt: 782–1181 ppm) of their ¹⁹⁵Pt NMR signals from 1 and 2 (δ¹⁹⁵Pt ∼ −3550 ppm) (see Figures 2 and S8). Besides, the ¹⁹⁵Pt resonances appear more deshielded as the electronegativity of the axial ligand is greater (−2368.1 (1-Cl) vs −2767.5 ppm (1-I)), as in other reported Pt₂(III,III).⁵²

Figure 2.

¹⁹⁵Pt{¹H} NMR spectra of 1, 1-Cl, 1-Br, and 1-I.

Figure 3.

Molecular structure of 1-I (left) and 2-I (right). Thermal ellipsoids are drawn at their 50% probability level; solvent molecules and hydrogens are omitted for clarity.

The X-ray structures of 1-Cl, 1-I, and 2-I are depicted in Figures 3 and S9, and a selection of bond distances and angles is listed in Table 2. They confirmed the anti configuration of the molecule along with the retention of the half-lantern structure with respect to the starting complexes.

Table 2. Selected Bond Lengths (Å) and Angles (deg) of Complexes 1-Cl, 1-I, and 2-I.

distances (Å)	1-Cl	1-I	2-I (Pt1)	2-I (Pt2)
Pt–Pt	2.5898 (6)	2.6365 (5)	2.6491 (2)
Pt–NC∧N	2.062 (7)	2.061 (7)	2.048 (4)	2.053 (4)
Pt–CC∧N	2.009 (8)	2.029 (7)	2.019 (5)	2.022 (4)
Pt–NS∧N	2.139 (7)	2.168 (6)	2.172 (4)	2.166 (4)
Pt–SS∧N	2.307 (2)	2.292 (2)	2.2913 (11)	2.2932 (12)
Pt–X	2.4428 (19)	2.7518 (5)	2.7678 (3)	2.7632 (3)
Angles (deg)
NC∧N–Pt–CC∧N	79.9 (3)	80.6 (3)	80.40 (17)	80.48 (17)
NC∧N–Pt–NS∧N	93.6 (3)	90.8 (3)	95.17 (15)	93.11 (15)
CC∧N–Pt–SS∧N	97.7 (2)	95.2 (2)	94.67 (13)	95.79 (14)
NS∧N–Pt–SS∧N	88.87 (19)	93.37 (19)	89.80 (11)	90.62 (11)
CC∧N–Pt–X	88.8 (2)	85.5 (2)	85.99 (13)	86.46 (12)
NC∧N–Pt–X	88.30 (17)	87.58 (18)	89.49 (10)	87.16 (10)
NS∧N–Pt–X	90.96 (17)	94.71 (18)	94.92 (10)	93.44 (10)
SS∧N–Pt–X	89.04 (7)	87.69 (5)	87.23 (3)	88.87 (3)
Pt–Pt–X	175.00 (5)	174.92 (2)	175.476 (10)	176.968 (11)

Each Pt(III) center has a distorted octahedral environment with the axial positions occupied by a halogen atom (Cl or I) and the other Pt(III) center and with the X–Pt–Pt angles being close to 175°. The Pt–Pt distances (2.5898(6) Å 1-Cl, 2.6365(5) Å 1-I, 2.6491(2) Å 2-I) are shorter than that found for complex 2, indicating the existence of a single metallic bond between both Pt(III) centers. They also reflect the trans influence of the axial ligand X, with that of the chloride derivative being shorter than the iodide one.^41,42,47,59 All intermetallic distances fall in the low range of those observed for Pt₂(III,III) complexes^18,29,41,42 and they are even shorter than the bzq derivatives with the same bridging ligand [{Pt(bzq)(μ-S^∧N)X}₂] (X: Cl 2.6132(2) Å, I 2.6401(2) Å).³⁹ The two Pt coordination planes are almost parallel to each other with small interplanar angles [11.10(5)° 1-Cl, 9.32(7)° 1-I, 8.90(10)° 2-I] and Pt–Pt lines near perpendicular to the coordination planes (4.12(6)° for 2-I). Inspection of the crystal packing revealed some π–π interactions for 1-Cl between the C^∧N groups of adjacent molecules (d_C–C: 3.333 Å), stacking them into infinite 1D-chains (see Figure S9). This arrangement is also supported by H···Cl interactions (d_H–Cl: 2.821 Å, d_C–Cl: 3.699 Å) between the C^∧N and the axial Cl ligands from side-on molecules.

Concerning the reactivity of 1 and 2 toward haloforms, there are some particularities worth mentioning. In the first place, the reactions of 1 with CHCl₃ and CHBr₃ only proceed with sunlight. If the reaction is performed in the dark, overnight, we recover the starting material (see Figure S10 as an example of 1 with CHBr₃). Second, the reactions of 1 and 2 with CHI₃ can take place in the air and protected from light, but it gives mixtures of two species [IPt(C^∧N_pz)(μ-S^∧N)₂Pt(C^∧N_pz)CHI₂] and [{Pt(C^∧N_pz)(μ-S^∧N)I}₂] (1-I or 2-I) (see Scheme 2 and Figure S11). They were detected by NMR spectroscopy; the former one presents two sets of signals for the C^∧N_pz and the N^∧S ligands due to the nonequivalence of the two Pt moieties and a singlet with ¹⁹⁵Pt satellites corresponding to the CHI₂ fragment [δ_H: 4.40, ²J_Pt,H = 20.8 Hz (R = H); 4.42, ²J_Pt,H = 19.4 Hz (R = CF₃)]. Then, the simultaneous formation of both species, [IPt(C^∧N_pz)(μ-S^∧N)₂Pt(C^∧N_pz)CHI₂] and [{Pt(C^∧N_pz)(μ-S^∧N)I}₂], in the dark points to a radical mechanism for the thermal oxidation.⁶⁰

Scheme 2. Oxidation Reactions of 1 and 2 with CHI₃.

To confirm this, we carried out comparative ¹H NMR experiments of 1 with CHI₃ (1:4) in acetone-d₆, protected from light, under argon and oxygen atmospheres. As shown in Figure S12, after 15 min (t₀), the reaction was completed giving mixtures of 1-I and [IPt(C^∧N_pz)(μ-S^∧N)₂Pt(C^∧N_pz)CHI₂] with different ratios depending on the experimental conditions. Under an Ar atmosphere, the relation between compounds 1-I and [IPt(C^∧N_pz)(μ-S^∧N)₂Pt(C^∧N_pz)CHI₂] was 1:2.85; however in an O₂ atmosphere, it was 1:0.28. Also, we observed that the mixture 1-I and [IPt(C^∧N_pz)(μ-S^∧N)₂Pt(C^∧N_pz)CHI₂] remains unchanged for at least 24 h while the reaction was kept in the dark (see Figure S12). Nonetheless, if the reactions with CHI₃ are performed in a straightforward manner in the air and in sunlight, compounds 1 and 2 are completely reacted to yield the corresponding complexes: [{Pt(C^∧N_pz)(μ-S^∧N^R)I}₂] (R = H 1-I, CF₃2-I). Thus, the experimental results are closely related to our previous research, in which detailed mechanistic studies were performed for the activation of C–X bonds in haloforms by dinuclear Pt(II) complexes.^39,59 In short, these reactions proceed through a radical pathway (eqs 1–4) through the thermal or photochemical homolytic cleavage of the X–C bond in a [Pt···Pt···XCHX₂] adduct.

1

2

3

4

The concomitant formation of Pt₂X^• and ^•CHX₂ (R^•) radicals justifies the simultaneous formation of Pt₂X₂ and Pt₂RX, with O₂ acting as an efficient radical trap (R^•), increasing the ratio [{Pt(C^∧N)(μ-S^∧N)I}₂]:[IPt(C^∧N)(μ-S^∧N)₂Pt(C^∧N)CHI₂]. Also, in the presence of CHI₃, species [IPt(C^∧N)(μ-S^∧N)₂Pt(C^∧N)CHI₂] transform completely into [{Pt(C^∧N)(μ-S^∧N)I}₂] under irradiation with UV-light or sunlight. In this case, the reaction of 1 with CHI₃ is thermally initiated, while those with CHBr₃ and CHCl₃ need sunlight to occur. By contrast, the reactions of the analogous [{Pt(bzq)(μ-S^∧N)}₂] (HN^∧S = 2-mercaptopyrimidine)³⁹ with both CHI₃ and CHBr₃ proceeds in the dark, indicating the significance of the C^∧N group in this reactivity. In previous works,^39,59 we demonstrated that MMLCT species, with the HOMO being a dσ* orbital constructed from d_z² Pt atoms, in their ground or excited state, are those that trigger the C–X activation (see Scheme 3).

Scheme 3. Proposed Pathway for the First Step in the Conversion of Pt(II,II) into Pt(III,III) Complexes by Reaction with Haloforms.

Therefore, both the bond dissociation energies (increasing in the sequence C–I < C–Br < C–Cl) and the energy of the HOMO in the [Pt₂] complex are crucial in the first step of this reaction. Since the π-backdonation from the Pt center to the naph-pz system in 1 is expected to be minor in relation to the bzq one, due to a less extended aromatic system, a weaker overlapping of the d_z² Pt orbitals would be found in 1. To check this, we carried out DFT calculations on complexes 1 and [{Pt(bzq)(μ-S^∧N)}₂] (HN^∧S = 2-mercaptopyrimidine), finding a decrease of the σ*(d_z²–d_z²) HOMO energy in 1 (−5.10 eV) when compared to that of [{Pt(bzq)(μ-S^∧N)}₂] (−4.99 eV, see Figure S13) due to a smaller energy splitting of the bonding σ and antibonding σ* orbitals. Then, there is need for light to reach MMLCT excited species in 1 to promote the C–Br bond breaking in the first step of the radical process.

Photophysical and Computational Studies

Absorption Spectra and DFT Studies

The UV–vis absorption spectra of the Pt₂(II,II) and Pt₂(III,III) complexes were measured in solution and in the solid state (Figures 4 and S14 and Table 3). The Pt₂(II,II) complexes in the solid state show some weak low energy absorptions at ca. 415 nm, while those for the Pt₂(III,III) derivatives appear at higher wavelengths, λ > 450 nm (see Figure 4 left for 1 and 1-X), indicating a noticeable change in the frontier orbitals. In the latter, the absorptions are strongly dependent on the axial ligand X, with the energy maxima decreasing in the order Cl^– > Br^– > I^–. In the solution of CH₂Cl₂, we observe these same trends (see Figure S14 top). The S^∧N^F compounds (2 and 2-I) present a greater solubility than their S^∧N counterparts. Thus, they could be measured in different solvents (10^–4 M), showing a small negative solvatochromism, which is characteristic of charge transfer transitions (Figure S14 bottom). As reported in the literature, the low energy bands in the Pt₂(II,II) half-lantern complexes have been typically attributed to a ¹MMLCT [dσ*(Pt–Pt) → π*(C^∧N)].^{18−30,32,33,35,36} Those for metal–metal bond Pt₂(III,III) derivatives are originated from an admixture of axial ligand (X) to metal–metal charge transfer, (¹XMMCT) and ¹MC [dσ–dσ*] transitions^47,61 or from ¹LMMCT [π(S^∧N) → dσ*(Pt–Pt)] transitions.⁴⁴

Figure 4.

Left: normalized diffuse reflectance spectra in the solid state. Right: normalized absorption spectra in the solid state of 1, S₁ calculated transitions in the gas phase (bar) and molecular orbital plots (isovalue 0.03).

Table 3. Absorption Data in Solution (10^–4 M) and in the Solid State^a at 298 K.

compound	λ/nm (103 ε M–1 cm–1)
1	286 (45.9), 315sh (31.2), 357sh (10.9), 405(4.7) CH2Cl2
	250, 305, 364, 415 solid
2	313 (23.2), 345sh (10.6) THF
	318 (31.0), 362 (8.8), 370sh (7.9), 404 (4.7) CH2Cl2
	323 (28.9), 362 (9.5), 391sh (6.3) MeCN
	295, 420 tail to 525 solid
1-Cl	300sh (36.4), 345 (11.1), 371sh (7.0), 412 (4.5) CH2Cl2
	259, 316, 384, 454 tail to 625 solid
1-Br	327 (25.8), 344sh (20.7), 378 (12.8), 429 (8.8) CH2Cl2
	268, 317, 460 tail to 650 solid
1-I	312sh (37.1), 351 (18.4), 440sh (11.1), 501 (18.5) CH2Cl2
	262, 316, 353, 423, 514 tail to 700 solid
2-I	312sh (37.7), 372 (15.0), 510 (20.8) toluene
	318sh (33.6), 368 (17.1), 506 (22.9) CH2Cl2
	312sh (33.4), 370 (15.4), 504 (20.4) THF
	313sh (34.5), 365 (17.7), 500 (19.7) MeCN
	262, 307, 379, 423, 518 tail to 750 solid

^aDiffuse reflectance.

To help with the UV–vis assignments, DFT and TD-DFT calculations were performed on 1, 2, 1-Cl, and 1-I (see Tables S2, S3 and Figures 4, 5, S15). As shown in Figures 4 right and S15, the selected calculated absorptions (S₁ or S₂) fit rather well with the experimental ones. In both Pt₂(II,II) complexes, 1 and 2, the main contribution to the S₁ is the HOMO → LUMO transition, where the dσ* Pt–Pt orbital largely participates on the HOMO (∼75%) while the LUMO is completely centered on the S^∧N bridging ligand (∼95%), see Tables S2 and S3. Thus, the lowest energy absorption for both, 1 and 2, is attributed to a ¹MML′CT [dσ*(Pt–Pt) → π*(S^∧N)] excited state. This assignment is consistent with the red shift observed in the absorption bands in 2 with respect to 1. The electron-withdrawing group CF₃ in the S^∧N lowers the LUMO level, decreasing the energy bandgap.

Figure 5.

Normalized absorption spectra in the solid state, calculated transitions in the gas phase (gray bars), and molecular orbital plots (isoval. 0.03) for compounds 1-Cl (left) and 1-I (right).

When comparing the lowest energy absorptions of 1 and 2 (λ_abs: 405 and 404 nm for 1 and 2), in solution of CH₂Cl₂, with those of the analogous bzq derivatives, [{Pt(bzq)(μ-S^∧N^R)}₂] (λ_abs: 496 nm R = H³⁹ and 486 nm R = CF₃⁴³), a clear shift to higher energies is observed. As indicated before, we have carried out DFT calculations on complex [{Pt(bzq)(μ-S^∧N^R)}₂] (R = H, 2-mercaptopyrimidine)³⁹ and compared them with those of complex 1 (Figure S13). They showed that the HOMO in both complexes is constructed from dσ* orbitals but that of complex [{Pt(bzq)(μ-S^∧N)}₂] lays at a higher energy (−4.99 eV) than the one in 1 (−5.10 eV), indicating a better overlapping of the d_z² Pt orbitals in the former. However, the LUMO in complex [{Pt(bzq)(μ-S^∧N)}₂] (−1.585 eV) is mostly located on the bzq ligand, whereas in complex 1 (−1.25 eV), it is based on the S^∧N bridging ligand. These calculations confirmed that the greater aromatic system of the benzoquinolinate ligand with respect to the naphthyl pyrazolate sharply changes the nature of the LUMO and leads to major stabilization. Also, this allows a greater π-backdonation from the Pt center to the bzq system, provoking a shortening of the Pt···Pt distance. Both effects lead to a decrease of the HOMO–LUMO energy gap and consequently, a red shift of lowest energy absorption along with a change in its nature, ¹MML′CT [dσ*(Pt–Pt) → π*(S^∧N)] in 1 and 2, but ¹MMLCT[dσ*(Pt–Pt) → π*(C^∧N)] in the bzq derivatives. All this indicates the great influence of the C∧N ligand in the optical properties of half-lantern complexes.

In the Pt₂(III,III) complexes, we will focus on the S₁ transition [H-2 → LUMO (85%)] for 1-I and on the S₂ for 1-Cl, in view of the negligible oscillator strength (o.s.) of S₁, in this case. So, by analyzing the transitions involved in S₂, we can observe certain orbital mixing for 1-Cl: HOMO → LUMO (38%), H-5 → LUMO (31%) and H-2 → LUMO (18%). As illustrated in Figure 5 and according to the frontier orbital compositions (Table S2), the lowest energy absorption would be mainly attributed to mixed ¹LL′MMCT[π(C^∧N/S^∧N) → dσ*(Pt–Pt)]/¹LL′XT[π(C^∧N/S^∧N) → π*(X)] for 1-Cl but for 1-I to a ¹XMMCT[π(X) → dσ*(Pt–Pt)] transition.

Emission Spectra

The photoluminescent properties were examined under an argon atmosphere in the solid state (Table 4). Unlike analogous compounds [{Pt(bzq)(μ-S^∧N^R)}₂] (R = H,³⁹ CF₃⁴³), complex 1 shows at 298 K an emission at ca 600 nm (Φ_PL: 1.6%; τ: 0.1 μs) that becomes more structured and exhibits longer relaxation time when cooling down to 77 K (τ: 1.9 μs, see Figure S16). According to the decay times and the calculated spin density distribution for the T₁ state (see Figure S16 and Tables 4 and S4), the phosphorescent emission of 1 is attributed to a ³MML′CT [dσ*(Pt–Pt) → π*(S^∧N)] excited state. This is coherent with the shortening of the Pt–Pt distance from 2.964 Å in S₀ to 2.820 Å in T₁ due to the electron promotion from an antibonding orbital (dσ*Pt₂) upon excitation (Table S5). However, no emission was detected for complex 2 either at room or low temperatures. Taking into account the DFT calculations and the application of the Energy Gap Law to Pt(II) chromophores,⁶² the electron-withdrawing CF₃ group in the S^∧N ligands decreases the energy bandgap in 2, which can easily lead thermal deactivation processes. Besides, since the calculated spin density distribution for the T₁ state is located mostly on the S∧N ligand (Table S4), the CF₃ substituent can provide more vibrational and rotational motions, increasing nonradiative pathways.

Table 4. Photophysical Data in the Solid State.

comp	T [K]	λex [nm]	λem [nm] (τ [μs])
1	298	455	595 (0.1)
	77	455	572max, 614 (1.9)
1-Cl	298	500	985 (0.023)
	77	450	945 (3.222)
1-Br	298	500	985 (0.033)
	77	450	950 (3.042)
1-I	298	525	1070 (0.014)
	77	450	1055 (0.865)
2-I	298	—	—
	77	450	1050 (0.635)

Strikingly, powdery samples of the Pt₂(III,III) compounds (1-Cl, 1-Br, 1-I) show at room temperature NIR emissions with maxima ranging from 985 to 1070 nm that are strongly dependent on the axial ligand X (Figure 6). Upon cooling down to 77 K, these emissions experience a hypsochromic shift and undergo a 100-fold increase in their lifetime decays (Table 4). A similar increment was also observed in related complexes [Pt₂(HPO₄)₄X₂]^4– (X = Cl, Br).⁴⁵ Compound 2-I shows no emission in the solid state at room temperature but it does at 77 K with λ_max and τ analogous to those of 1-I (see Figure 6, inset). As observed in related iodo-complexes of Pt₂(III)⁴⁷ and in other Pt(IV)^63,64 compounds, these exhibit weak emissions or even no emissions at all due to the effect of the iodo ligand. By analyzing the population of the frontier orbitals (Table S2), we observed that the higher π-donation of the I implies a greater participation along with a minor contribution of the metal center with respect to the chloro-derivatives. Therefore, the spin–orbit coupling induced by the Pt heavy atom would be less effective in the iodo-complexes, leading to less emissive compounds. In addition, the CF₃ group may increase the nonradiative pathways in complex 2-I with regard to 1-I.

Figure 6.

Normalized emission spectra in the solid state at 298 K (—). Inset: normalized emission spectra at 77 K (---).

As stated before, the Pt₂(III,III) complexes have been typically reported as nonemissive with the exception of very few examples that emit in the red^38,44,45 and NIR⁴⁷ spectral regions. If we compare them with our dinuclear Pt(III) systems, we clearly observe that our emissions are pushed further in the NIR. See as an example the emission bands in the solid state at 298 K for 1-Cl and 1-I (985 and 1070 nm, respectively) compared to those of [Pt₂(μ-As^∧C)₄X₂] (X = Cl, 915 nm and X = I, nonemissive at rt).⁴⁷

With the aim of gathering more information about the origin of these emissions, we calculated the spin-density distributions in the optimized first excited states (T₁) (Figure S17 and Table S4). They are located along the X–Pt–Pt-X axis, essentially on the X ligand (0.778 1-Cl, 1.031 1-I) and the Pt center (0.857 1-Cl, 0.669 1-I). There are also minor contributions from the N^∧S (0.237 1-Cl, 0.184 1-I) and C^∧N (0.128 1-Cl, 0.116 1-I) groups. Thus, the emissions would be mainly attributed to ³XMMCT [σ(X) → dσ*(Pt–Pt)] excited states with a minor contribution of the S^∧N^R groups that become slight more important in the chloro derivatives. This assignment would be in close agreement with those reported for [Pt₂(μ-As^∧C)₄X₂] (X = Cl, Br, I), which were associated with an increasing admixture of XMMCT [σ(X) → dσ*(Pt–Pt)] with MC [dσ → dσ*] going from Cl-/Br-to the I-derivatives.⁴⁷ Also for further confirmation, we analyzed the optimized geometries of the T₁ states for 1-Cl and 1-I. They show a similar structure to that of the S₀ state but with a substantial elongation of the Pt–Pt distance from 2.675 Å (S₀) to 2.988 Å (T₁) for 1-Cl as an example (see Table S5 for 1-I). This would be consistent with a decrease in the Pt–Pt BO as a consequence of the electron promoted to the dσ*(Pt)₂ orbital in the excitation process. To evaluate this Pt–Pt bonding, Mayer BO analyses were carried out in the S₀ and T₁ states. As listed in Table S5, the BO of 1-Cl in the S₀ is 0.61, whereas in T₁, it is 0.27. Then, this Pt–Pt interaction is significantly weakened in the first excited state of both complexes, 1-Cl and 1-I, which would be supporting the ³XMMCT character of these emissions. Finally, if we compare the calculated spin density on the Pt center in complexes 1-I (0.669) and 1-Cl (0.857), a weaker spin–orbit effect induced by the Pt heavy atom would be expected for 1-I, which would decrease the kr. This is in accordance with our experimental results for the iodo-complexes 1-I and 2-I.

Conclusions

Half-lantern dinuclear Pt(II) complexes bearing naphthyl-pyrazole as the cyclometalated ligand have been prepared and oxidized with haloforms to give the corresponding diplatinum(III) derivatives with halogenides as axial capping ligands. The Pt₂(II,II) complex 1 shows a yellow emission at both RT and 77 K, while the oxidized Pt₂(III,III) counterparts display emission bands around 1000 nm that strongly depend on the nature of the axial ligand. The C^∧N-skeleton seems to be responsible for the encountered differences with respect to analogous half-lantern complexes with bzq as the C^∧N cyclometalated group. First is the reactivity of Pt₂(II,II) compounds toward haloforms. In this case, light is essential to initiate and complete the reaction of 1 with CHBr₃. Second, it affects the nature of the absorption and emission bands of Pt₂(II,II) compounds that are MML′CT [dσ*(Pt–Pt) → π*(S^∧N)] based. In the third place and finally, we have obtained Pt₂(III,III) compounds that exhibit long-wavelength NIR bands (∼1000 nm) at room temperature that are mainly due to ³XMMCT [σ(X) → dσ*(Pt–Pt)] excited states. These discrete molecules with a semirigid dinuclear structure held together by two bridging groups and a metal–metal bond constitute an approachable platform to achieve emitters in the NIR-II spectral region. By regulating the nature of the axial ligands as well as the cyclometalating and bridging groups, we can introduce structural modifications that certainly affect the photophysical properties. These results contribute significantly to opening out the possibilities for the highly desired NIR-II emitters.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.3c04314.

• Experimental appendix including photophysical and computational methods and X-ray crystallography details (2299209–2299212); NMR spectra and X-ray diffraction figures for structural characterization; theoretical calculations; and photophysical properties (PDF)

• Cartesian coordinates of the DFT-optimized structures for complexes 1, 2, 1-Cl, 1-I, and [{Pt(bzq)(μ-S∧N)₂}] (HS∧N = 2-mercaptopyrimidine) (XYZ)

The authors declare no competing financial interest.