Effects of a semiflexible linker on the mechanochromic photoluminescence of bis(Pt-salen) complex

We report the crystal structures and the photophysical properties of a dinuclear Pt complex (Pt₂L²) with bis(salen) and 2-methylenepropane-1,3-diyl as the ligand and linker, respectively; the mononuclear analogue (PtL¹) and the corresponding Ni and Pd complexes were also examined. Solution-state measurements indicate that the two salen units in the dinuclear complexes are nearly independent and the linker exerts only a minimal influence. Solid-state ultraviolet-visible spectra of the Pt complexes show a weak absorption band at 520 nm, which is absent in the solution-state spectra. From crystal structure analysis and polarization spectra, the weak band is attributed to the stacked molecular structure of the complex. In response to mechanical stimuli, Pt₂L² microcrystals and smeared thin films show photoemission of various colors (mechanochromic photoluminescence), whereas PtL¹ does not show any substantial emission. We attribute the changes in emission color to the semiflexible linker, which contributes to the fragility of molecular packing in the crystals under shear stress.