Hyperbranched poly(phenylenesilolene)s: Synthesis, thermal stability, electronic conjugation, optical power limiting, and cooling-enhanced light emission

Silole-containing hyperbranched polyphenylenes (1) are synthesized, which exhibit high thermal stability, extended electronic conjugation, excellent optical power limiting performance, and novel cooling-enhanced photoluminescence. The homopolycyclotrimerization of 1,1-diethynyl-2,3,4,5-tetraphenylsilole (2) and its copolycyclotrimerizations with 1-octyne catalyzed by TaCl₅-Ph₄Sn proceed smoothly at room temperature and produce completely soluble polymers in high yields (up to ∼85%). The molecular structures of 1 are characterized by spectroscopic analyses. The thermal stability of 1 is evaluated by thermogravimetric analyses, which detect virtually no weight losses when the polymers are heated to ∼300°C. The hyperbranched polyphenylenes are electronically conjugated, as suggested by their strong absorption in the visible spectral region (λ_max ∼520 nm). Because of this extended electronic conjugation, polymers 1 are nonlinear optically active and strongly attenuate the optical power of intense laser pulses, whose optical limiting performances are superior to that of C₆₀, the best-known optical linear. The photoluminescence of the polymers is dramatically enhanced by cooling their solutions to low temperatures. This unique phenomenon of cooling-enhanced emission is probably caused by the restricted intramolecular rotations of the phenyl rings upon the axes of the single bonds linked to the silole cores at the cryogenic temperatures.