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 (λmax ∼ 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 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.