Binuclear Complexes and Extended Chains Featuring Pt^II-Tl^I Bonds: Influence of the Pyridine-2-Thiolate and Cyclometalated Ligands on the Self-Assembly and Luminescent Behavior

Platinum solvate complexes [Pt(C₆F₅)(C^N)(S)] [C^N = phenylpyridinyl (ppy), S = dimethyl sulfoxide (DMSO) (A); C^N = benzoquinolinyl (bzq), S = CH₃COCH₃ (B)] react with [Tl(Spy)] (Spy = 2-pyridinethiolate) to afford binuclear [{Pt(C₆F₅)(C^N)}Tl(Spy)] [C^N = ppy (1) and bzq (2)] species containing a Pt-Tl bonding interaction, supported by a μ-Spy-N,S bridging ligand, as confirmed by X-ray diffraction. However, the related reactions with [Tl(SpyCF₃-5)] [SpyCF₃-5 = 5-(trifluoromethyl)-2-pyridinethiolate] give neutral extended chains [{Pt(C₆F₅)(C^N)}Tl(SpyCF₃-5)]_n [C^N = ppy (3) and bzq (4)]. 3 features a zigzag -Pt-Tl···S-Pt- chain, generated by Pt-Tl and Tl···S bonds, with the SpyCF₃ acting as a μ-N:κ²S bridging ligand, whereas 4 displays an unsupported ···Tl-Pt···Tl-Pt··· backbone (angle of ca. 158.7°). The lowest-energy absorption bands in the UV-vis spectra in CH₂Cl₂, associated with ¹L′LCT transitions with minor ¹LC/¹MLCT (L′ = Spy or SpyCF₃-5; L = C^N) character, are similar for all complexes 1-4, demonstrating that for 3 and 4 the chains break down in solution to yield similar bimetallic Pt-Tl units. For 2, two different forms, 2-o (orange) and 2-y (yellow), exhibiting different colors and emissions were found depending on the isolation conditions. Slow crystallization favors formation of the thermodynamically more stable yellow form (2-y), which exhibits a high-energy (HE) structured emission band, whereas fast crystallization gives rise to the orange form (2-o), with a remarkably lower energy structureless emission. Complexes 1 and 3 exhibit dual luminescence in the solid state at 298 K: an unstructured low-energy band associated with ³ππ∗ excimeric emission due to π···π (C^N) interactions and a more structured HE band, assigned, with support of density functional theory calculations, to an intraligand ³LC (C^N) excited state mixed with some ligand (SPy)/platinum-to-ligand (C^N)³[(L′ + M)LCT] charge transfer. Chain 4 only shows a HE band at 298 K, attributed to a ³L′LCT (SpyCF₃ → bzq) excited state mixed with a minor ³MLCT/³MM′CT (M = Pt; M′ = Tl) contribution. At 77 K, the ππ∗-stacking emission is predominant in all complexes, except in the form 2-y. Interestingly, 2-4 exhibit reversible mechanochromic color and luminescence changes, with remarkable red shift and increased quantum yields, and upon exposure to solvents, they are restored to their original color and emission. On the basis of powder X-ray diffraction studies, a plausible mechanism of the mechanochromic processes is proposed, involving reversible crystalline-to-amorphous phase transitions.