Synthesis, Light Emission, Nanoaggregation, and Restricted Intramolecular Rotation of 1,1-Substituted 2,3,4,5-Tetraphenylsiloles

A series of ten 2,3,4,5-tetraphenylsiloles with different 1,1-substituents [XYSi(CPh)4] were
prepared, and three of these, i.e., 1,1,2,3,4,5-hexaphenylsilole [X = Y = Ph (3)], 1-ethynyl-1,2,3,4,5-pentaphenylsilole [X = Ph, Y = C⋮CH (15)], and 1,1-bis(phenylethynyl)-2,3,4,5-tetraphenylsilole [X = Y = C⋮CPh (18)], were characterized crystallographically. The ground-
and excited-states of the siloles were influenced by the inductive effect of the 1,1-substituents:  with an increase in their electronegativity, the absorption and emission spectra
of the siloles bathochromically shifted. A simple and reliable TLC-based method was
developed for measurement of the solid-state luminescence spectra of the siloles. When
molecularly dissolved in common solvents at room temperature, all the siloles were practically
nonemissive (“off”). When poor solvents were added, the silole molecules clustered into
nanoaggregates, which turned the emission “on” and boosted the photoluminescence quantum
yields by up to 2 orders of magnitude (aggregation-induced emission). The silole emission
could also be greatly enhanced by increasing the viscosity and decreasing the temperature
of the silole solutions. The solution thickening and cooling experiments suggest that the
aggregation-induced emission is caused by the restricted intramolecular rotations of the
peripheral aromatic rings upon the axes of the single bonds linked to the central silole cores.