Synthesis and High Solid-State Fluorescence of Cyclic Silole Derivatives

Cyclotetrasiloxanes <b>1</b>–<b>3</b> 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 <b>1</b> and <b>3</b> and at 491 nm for AEE-active tetraphenylsilole compound <b>2</b>. However, <b>1</b>–<b>3</b> have high solid-state quantum yields (Φ_fl = 0.65–0.78) with fluorescence maxima at 377–390 nm for compounds <b>1</b> and <b>3</b> and at 517 nm for tetraphenylsilole- and silafluorene-containing compound <b>2</b>. Packing analysis of <b>1</b>–<b>3</b> in the crystal structure and MO and excited-state calculations were performed to explore possible fluorescence mechanisms in these compounds.