Synthesis and Characterization of Cyclotri- and Tetrasiloxanes with Pyrenyl Groups.

Cyclosiloxanes directly bearing polyaromatic groups on silicon atoms have scarcely been reported. Herein, hexa(1-pyrenyl)cyclotrisiloxane () and octa(1-pyrenyl)cyclotetrasiloxane () were successfully prepared from di(1-pyrenyl)silanediol () in the presence of a weak base such as tetraethylammonium acetate and triethylamine in MeCN. The structure of the cyclosiloxanes bearing multiple pyrenyl groups in the solid and solution states was evaluated by NMR, X-ray crystallography, and density functional theory (DFT) calculations. All pyrenyl groups of were oriented outward, and no π-π stacking of adjacent pyrenyl groups was observed. However, all pairs of adjacent pyrenyl groups at 1- and 3-positions in are oriented in the same direction and were π-π stacked with respect to each other. The UV-vis spectra of and in organic solvents showed a slight broadening of the peaks, as observed for typical pyrene derivatives. Interestingly, the fluorescence spectra of showed small monomer and strong excimer emissions; however, those of showed only a strong excimer emission in all solvents. Partially pyrenylated cyclotri- and tetrasiloxanes (compounds and ) showed solvent-dependent monomer and excimer spectra as observed for di(1-pyrenyl)silane derivatives, implying that the excimer emissions of and arise from mainly geminal and vicinal pyrenyl groups, respectively.