Emission behaviors of unsymmetrical 1,3-diaryl-β-diketones: A model perfectly disclosing the effect of molecular conformation on luminescence of organic solids

A series of unsymmetrical 1,3-diaryl-beta-diketones 1-6 displaying molecular conformation-dependent fluorescence quantum yields have been synthesized. Crystals with planar molecular conformation such as 1, 2, 3 and 4 are highly fluorescent (phi(f): 39-53%), and the one holding slightly twisted conformation (5) is moderately luminescent (phi(f) = 17%), while crystal 6 possessing heavily bent structure is completely nonluminous (phi(f) similar to 0). The distinct fluorescence efficiencies are ascribed to their different molecular conformations, since all the crystals hold the same crystal system, space group and crystal packing structures. Additionally, the fluorescent crystals 1-5 display low threshold amplified spontaneous emission (ASE) with small full widths at half-maximum (FWHM: 3-7 nm), indicating their potential as candidates for organic crystal lasing devices.