Highly Efficient Blue Fluorescence from 3,2′-Silylene-Bridged 2-Phenylindoles in the Solid State

3,2'-Silylene-bridged 2-phenylindoles were prepared in high yields by palladium-catalyzed silicon-migrative intramolecular coupling of 2-(2-indolylsilyl)aryl triflates. The high thermal stability of the benzosilole-fused indoles was indicated by thermogravimetric analysis (TGA). The absorption spectra revealed that the absorption edges, namely, the HOMO-LUMO energy gaps, could be effectively tuned by substitution with functional groups such as chloro, cyano, methoxy, and phenyl at the meta-positions with respect to nitrogen and silicon, whereas there was no effective conjugation between the core and an aryl group on nitrogen. The changes in the HOMO-LUMO gaps, which were induced by variation of the functional groups, were well reproduced by density functional theory calculation. The indoles exhibited blue fluorescence with high-to-excellent quantum yields both in cyclohexane and in powder as well as in doped poly(methyl methacrylate) (PMMA) films. The emission maxima of each indole exhibited a red-shift, which followed the general order: cyclohexane -> PMMA film -> powder. X-ray analysis of the diisopropylsilylene-bridged parent indole indicated that there was no close contact like pi-pi stacking in the crystal, presumably because of the steric effect of the bulky silylene bridge. Examination of the photophysical properties of 3,2-diisopropylmethylene-bridged 2-phenylindole revealed that the quantum yields of the carbon analogue in the solid state were much lower than the corresponding silylene-bridged indole. Thus, the silylene bridge is found to be essential for obtaining high-to-excellent solid-state quantum efficiency using a molecular design based on a 2-phenylindole framework.