Syntheses, Optical Properties, and Theoretical Investigation of Silafluorenes and Spirobisilafluorenes Bearing Electron-Donating Aminostyryl Arms around a Silafluorene Core

π‐Extended silafluorenes and spirobisilafluorenes bearing electron‐donating aminostyryl substituents at the 2,7‐ or 3,6‐positions were synthesized by a Horner–Wadsworth–Emmons reaction. The electronic influence of spirocyclic structure and substitution mode of the aminostyryl substituents was investigated by UV/Vis spectroscopy, which indicated the existence of a spiroconjugation effect in the 3,6‐substituted spirobisilafluorene. They exhibited moderate to strong fluorescence emission, and the fluorescence properties were compatible with the UV/Vis absorption characteristics, except for the 3,6‐substituted spirobisilafluorene, which showed relatively large enhancement of fluorescence quantum yield and Stokes shift. The influence of the spirocyclic structure and substitution mode on the photophysical properties of the silicon compounds was investigated by DFT calculations. Spiro‐ versus π conjugation: Silafluorenes and spirobisilafluorenes (see picture) bearing aminostyryl groups at the 2,7‐ or 3,6‐positions were synthesized. Their UV/Vis data reveal the influence of σ(SiC)*–π* hyperconjugation and spiroconjugation on their photophysical properties. Their fluorescence properties are compatible with UV/Vis absorption data, except for the 3,6‐substituted spirobisilafluorene, for which DFT calculations on a model compound revealed the presence of spiroconjugation.