Binuclear Complexes and Extended Chains Featuring PtII-TlI Bonds: Influence of the Pyridine-2-Thiolate and Cyclometalated Ligands on the Self-Assembly and Luminescent Behavior

Laburpena

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. © 2016 American Chemical Society.