Luminescence of mononuclear Pt(II) complexes with glycolate: external stimuli-induced excimer emission changes to oligomer emissions

The emission properties and external stimuli-induced emission changes of novel mononuclear platinum(II) complexes [PtII(4R2bpy)(gl)] (R = H (1), Me (2), tBu (3); 4R2bpy: 4,4′-disubstituted-2,2′-bipyridine; Hgl−: glycolate) were investigated. In solution, all complexes showed orange (λem: 617–623 nm) and green (λem: 488–495 nm) emissions at room temperature and 77 K, respectively, and the emission change is caused by a rigid chromic effect. X-ray crystallography of a red-emitting yellow trihydrate crystal of 1 (1R; λem: 663 nm) and an orange-emitting yellow anhydrate crystal of 3 (3O; λem: 560 nm) revealed the presence of weak π–π interactions between 4R2bpy moieties. When 1R was cooled to 153 K, the emission gradually turned to yellow (λem: 550 nm) owing to the restriction of inter-complex contacts after excitation. Exposing 1R to MeOH vapor rendered yellow-emitting yellow anhydrate crystals (1Y; λem: 558 nm), with the color remaining almost unchanged. Heating 1R gave orange-emitting orange anhydrate crystals (1O; λem: 597 nm). Consequently, the dehydration processes turn an excimer emission of 1R into two types of oligomer emissions (1Y and 1O). A green-emitting yellow 3.5 hydrate crystalline powder of 2 (2G; λem: 531 nm) was reversibly converted to an orange-emitting orange anhydrate powder (2O; λem: 564 nm) upon heating and exposure to H2O vapor, and the behavior is derived from switching between the monomer and an oligomer states. The crystal 3O exhibited an oligomer emission without responsiveness to external stimuli.