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 π–π 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.