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 (Φ_fl = 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 (Φ_fl = 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.