Synthesis and photophysical properties of cyclosiloxanes with substituted naphthyl groups

Abstract

Cyclosiloxanes bearing 1-naphthyl and 5-substituted naphthyl groups have been prepared from the corresponding silanediols with bases such as triethylamine and 1,8-diazabicyclo[5.4.0]undec-7ene (DBU) in moderate yields. The selectivity for the synthesis of cyclotrisiloxanes [-SiAr2-O-]3 and cyclotetrasiloxanes [-SiAr2-O-]4 can be controlled by the substituents on the naphthyl rings and the strength of the base. A less reactive silanediol by introducing of electron-donating groups such as dimethylamino groups gave kinetically controlled cyclotrisiloxane. Di(1-naphthyl)silanediol was also found to give the corresponding cyclotrisiloxane with triethylamine. Meanwhile, reactive silanediol bearing electron-withdrawing groups afforded the corresponding thermodynamically stable cyclotetrasiloxane even in the presence of triethylamine. Strong base such as DBU promoted the rearrangement from cyclotrisiloxane to cyclotetrasiloxane by HPLC analysis, therefore the latter product was obtained as the main product. The photophysical properties of cyclotrisiloxanes 4b (Ar = 1-naphthyl) and 4c (Ar = 1-(5-N,N-dimethylaminonaphthlyl)), and cyclotetrasiloxanes 5a (Ar = 1-(5-cyanonaphthyl)) and 5b (Ar = 1-naphthyl) were carried out in different organic solvents. Cyclotrisiloxane 4b showed both monomer and excimer emission. Cyclotetrasiloxanes 5a and 5b showed characteristic excimer emission due to the fully overlapping naphthyl groups on the vicinal silicon atoms in the ground state from X-ray and theoretical calculations. Cyclotrisiloxane 4c showed broad emission and high quantum yield. The emission wavelength depended on the solvent polarity due to the ICT of the naphthyl groups. The solubility of the cyclosiloxanes was improved compared to previously reported cyclosiloxanes having 1-pyrenyl groups.