Mechanochromism in the luminescence of novel cyclometalated platinum(ii) complexes with α-aminocarboxylates†
Six novel phosphorescent cyclometalated platinum(II) complexes with α-aminocarboxylato ligands, [PtII(ppy)L] (ppy = 2-phenylpyridinato, L− = Gly, Ala, Leu, Ile, Phe, Sar), were synthesized, and the structures were determined by X-ray crystallography. [PtII(ppy)L] (L− = Gly, Ala) in crystals are in monomeric structures and stack through π–π interactions to form columns, and the features of the luminescence are similar to each other and those in solution, suggesting little influence of the π–π interactions on the luminescence. [PtII(ppy)Leu] has a dimeric structure through the Pt–Pt interaction. [PtII(ppy)Sar] showed two pseudo-polymorphs, one of which contains both monomeric and dimeric forms, while the other consists of only dimeric units. Intra-dimer π–π interactions were observed in both the dimeric units. [PtII(ppy)L] (L− = Leu, Ile, Phe) in the solid state displayed different spectral patterns of luminescence from those in solution, suggesting that the dimeric structures through the Pt–Pt interaction in the solid state are dissociated into the corresponding monomeric ones in solution. The complexes except [PtII(ppy)Phe] in the solid state exhibited reversible luminescent mechanochromism in response to the mechanical grinding and treatment with a few drops of solvent. These phenomena are induced by the change in the energy level of the triplet state due to the change in the extent of intermolecular interactions, which appeared due to the crystalline–amorphous phase conversion.