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

Abstract

The emission properties and external stimuli-induced emission changes of novel mononuclear platinum(II) complexes [Pt^II(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 (λ_em: 617–623 nm) and green (λ_em: 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; λ_em: 663 nm) and an orange-emitting yellow anhydrate crystal of 3 (3O; λ_em: 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 (λ_em: 550 nm) owing to the restriction of inter-complex contacts after excitation. Exposing 1R to MeOH vapor rendered yellow-emitting yellow anhydrate crystals (1Y; λ_em: 558 nm), with the color remaining almost unchanged. Heating 1R gave orange-emitting orange anhydrate crystals (1O; λ_em: 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; λ_em: 531 nm) was reversibly converted to an orange-emitting orange anhydrate powder (2O; λ_em: 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.