Photophysical properties of heteroaromatic ring-fused(di)benzosiloles

Benzosiloles fused to heterocycles such as thiophene,benzothiophene,and benzofuran,and indole-and benzosilole-fused dibenzosiloles were prepared by palladium-catalyzed intramolecular coupling of the corresponding 2-(arylsilyl)aryl triflates in good to high yields.Molecular and crystal structures of 5,7-dihydro-5,5,7,7-tetrakis(1-methylethyl)bis[1]benzosilolo-[2,3-b:3',2'-d]thiophene,6-methyl-12,12-diisopropyl-12H-indololo[3,2-b][1]silafluorene,and 5,5,11,11-tetraisopropyl-5,11H-benzosi lolo[3,2-c]silafluorene were determined by X-ray diffraction analysis.The UV absorption spectra of the(di)benzosilole derivatives in cyclohexane red-shifted when compared to 1,1-diisopropyldibenzosilole,indicating that replacing a benzene ring of dibenzosilole by the heterocycles as well as fusion of indole and benzosilole moieties onto dibenzosilole narrowed the HOMO-LUMO gaps of the π-conjugation system.The thiophene-fused benzosiloles were faintly fluorescent in solution and in the solid state,whereas the dibenzosiloles exhibited luminescence with moderate and high quantum yields in cyclohexane and in microcrystals,respectively.In other words,aggregation-induced emission was observed for the dibenzosiloles.Notably,5,5,11,11-tetraisopropyl-5,11H-benzosilolo[3,2-c]silafluorene in microcrystals exhibited violet fluorescence(λmax = 396 nm) with a quantum yield of 0.70.Density functional theory(DFT) calculations of the prepared(di)benzosiloles were also performed.