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 similar to85%). 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 similar to300 degreesC. The hyperbranched polyphenylenes are electronically conjugated, as suggested by their strong absorption in the visible spectral region (lambda(max) similar to 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 C-60, the best-known optical limiter. 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.