Bithiophenesilane Dendrimers:  Synthesis and Thermal and Optical Properties

For the first time the synthesis and thermal and optical properties of a number of first-generation bithiophenesilane dendrimers with alkyl end groups, namely, tris{5‘-[methylbis(5‘-hexyl-2,2‘-bithienyl-5-yl)silyl]-2,2‘-bithienyl-5-yl}methylsilane ( 1 ), tris{5‘-[methylbis(5-hexyl-2-thienyl)silyl]-2,2‘-bithienyl-5-yl}methylsilane ( 2 ), and tris{5‘-[methylbis(5-ethyl-2-thienyl)silyl]-2,2‘-bithienyl-5-yl}methylsilane ( 3 ), are described. The influence of the dendritic architecture on the thermal and optical properties was investigated and compared to those of the linear and branched bithiophenesilanes. The results obtained showed a significant increase of the fluorescence quantum yield Q with an increasing number of bithiophenesilane fragments being a constituent part of the dendritic molecule. It reaches 30% for dendrimer 1 with nine such fragments as compared to Q = 6% for 5,5‘-bis(trimethylsilyl)-2,2‘-bithiophene and Q = 1.8% for 5-hexyl-2,2‘-bithiophene. This effect was explained by the impact of the specific dendritic molecular structure on the fluorescence efficiency. It is noteworthy that the emission wavelength was maintained at the same violet-blue region (380−385 nm) for all the compounds under consideration. Tuning the terminal alkyl chains of the dendrimers allowed creating stable amorphous materials, in contrast to the dendrimer without alkyl chains, tris{5‘-[methylbis(2-thienyl)silyl]-2,2‘-bithienyl-5-yl}methylsilane ( 4 ), or model bithiophenesilanes, all of which tend to crystallize under different conditions. Bearing in mind the high thermal stability and excellent solubility of the bithiophenesilane dendrimers, these materials present a new promising class of violet-blue emitters for photonics, organic electronics, and other applications.