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 (2) and octa(1-pyrenyl)cyclotetrasiloxane (3) were successfully prepared from di(1-pyrenyl)silanediol (1) 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 2 were oriented outward, and no pi-pi stacking of adjacent pyrenyl groups was observed. However, all pairs of adjacent pyrenyl groups at 1-and 3-positions in 3 are oriented in the same direction and were pi-pi stacked with respect to each other. The UV-vis spectra of 2 and 3 in organic solvents showed a slight broadening of the peaks, as observed for typical pyrene derivatives. Interestingly, the fluorescence spectra of 2 showed small monomer and strong excimer emissions; however, those of 3 showed only a strong excimer emission in all solvents. Partially pyrenylated cyclotri-and tetrasiloxanes (compounds 4 and 5) showed solvent-dependent monomer and excimer spectra as observed for di(1-pyrenyl)silane derivatives, implying that the excimer emissions of 2 and 3 arise from mainly geminal and vicinal pyrenyl groups, respectively.

Automated summary

Cyclosiloxanes directly bearing polyaromatic groups on silicon atoms have been investigated. It was found that in cyclotrisiloxane, the pyrenyl groups are oriented outward without pi-pi stacking between adjacent pyrenyl groups, while in cyclotetrasiloxane, the adjacent pyrenyl groups are oriented in the same direction and pi-pi stacked with each other. Moreover, the fluorescence spectra of cyclotrisiloxane showed both monomer and excimer emissions, while those of cyclotetrasiloxane only showed excimer emission.