Synthesis and fluorescence properties of 9,9-dimethylfluorene-diyl bridged molecular gyrotops: effects of slight fluorophore motion on fluorescence efficiency in the solid state

The fluorescence of organic molecules has been widely investigated in the development of functional materials. Clarifying the relationship between molecular motion and fluorescence quantum yield (FQY) can aid in developing intensity-controllable fluorophores. Herein, FQYs of 9,9-dimethylfluorene-diyl bridged disilabicyclo[n.n.n]alkanes were compared both in solution and in the solid state. Two macrocage derivatives, C18 (n = 18) and C22 (n = 22), were synthesized, and their structures were characterized using NMR spectroscopy and X-ray crystallography. The FQYs in solution were almost the same; however, the FQY in the solid state of large-cage C22 was smaller than that of small-cage C18. Using solid-state H-2 NMR, a shorter spin-lattice relaxation time was measured for H-2 nuclei in the deuterated fluorophore of C22-d(3) compared to C18-d(3), indicating that the slight motion of the deuterated fluorophore, such as libration, inside the cage causes more efficient relaxation. Thus, the lower FQY of C22 was attributed to the librational motion, which occurs on a time scale close to the fluorescence time scale. The results of this study are expected to contribute to the molecular design of environmentally responsive fluorescent materials.