Synthesis and High Solid-State Fluorescence of Cyclic Silole Derivatives

Cyclotetrasiloxanes 1–3 containing
different silole-based fluorogenic units (silafluorene, 1,3-diphenyl-9-silafluorene,
tetraphenylsilole) were synthesized by cohydrolysis and condensation
reactions. Their optical properties in solution and as crystals were
studied. These compounds have low quantum yields in solution (Φfl = 0.01–0.18) with fluorescence maxima at 359–375
nm for silafluorene-containing compounds 1 and 3 and at 491 nm for AEE-active tetraphenylsilole compound 2. However, 1–3 have high
solid-state quantum yields (Φfl = 0.65–0.78)
with fluorescence maxima at 377–390 nm for compounds 1 and 3 and at 517 nm for tetraphenylsilole-
and silafluorene-containing compound 2. Packing analysis
of 1–3 in the crystal structure and
MO and excited-state calculations were performed to explore possible
fluorescence mechanisms in these compounds.