Solid-emissive fluorophores constructed by a non-planar heteropolycyclic structure with bulky substituents: synthesis and X-ray crystal structures

Abstract

Novel solid-emissive indeno[1,2-b]benzo[4,5-e]pyran-11-one-type fluorophores 3a–3c having non-planar structures with sterically hindered substituents (R = butyl, phenyl, and thienyl) have been designed and conveniently synthesized. The fluorescence quantum yields of 3a–3c in 1,4-dioxane were 3a (Φ = 0.053) ≫ 3b (Φ = 0.013) > 3c (Φ = 0.003). On the other hand, the solid-state fluorescence quantum yields of the fluorophores were 3a (Φ = 0.39) > 3b (Φ = 0.15) > 3c (Φ = 0.06). To elucidate the large differences in the quantum yields in solution and in the solid state and among the fluorophores 3a–3c, we performed time-resolved fluorescence spectroscopic measurements, semi-empirical molecular orbital calculations (AM1 and INDO/S), and X-ray crystallographic analyses of 3a–3c. The comparison of the values of the radiative and non-radiative rate constants determined by the time-resolved spectroscopic measurements in solution and in the crystalline state supported that non-radiative decay is reduced by restriction of the rotation of the phenyl and thienyl rings in the solid state. In addition, the X-ray crystal structures demonstrated that, in 3a and 3b, the non-planar structure with sterically hindered substituents prevents the fluorophores from forming short π–π contacts and produces strong solid-state fluorescence. On the other hand, in the crystal of 3c, the formation of continuous intermolecular CH⋯S bonding between neighboring fluorophores was found to increase short π–π contacts and reduce the fluorescence intensity.