EXCITED-STATE DYNAMICS OF ONE-DIMENSIONAL AND 2-DIMENSIONAL SIGMA-CONJUGATED SILICON FRAME POLYMERS - DRAMATIC EFFECTS OF BRANCHING IN A SERIES OF HEXYLSILYNE-BRANCHED POLY(HEXYLMETHYLSILYLENE) COPOLYMERS

Time-resolved single photon counting techniques were used to compare and contrast the luminescence behavior of linear poly(hexylmethylsilylene), poly(hexylsilyne), and three copolymers incorporating nominally 1%, 5%, and 25% hexylsilyne branch points. While low levels of branching cause relatively small changes in the electronic absorption spectrum, the effect on the luminescence behavior is striking. The 340-nm sigma-sigma* excitonic emission dominating the spectrum of the linear polymer drops off sharply at 1-2% branching and is accompanied by the growth of a broad visible band possessing nonexponential decay behavior. The data suggest that adventitious branching may be responsible for the presence of similar bands often observed in the emission spectra of samples assumed to be completely linear. Higher degrees of branch incorporation result in the loss of the excitonic emission altogether with a further red shift of the visible band appearing directly analogous to the intense broad visible emissions observed for poly(alkylsilyne) network polymers. It is shown that this longer wavelength emission is suggestive of "traplike" relaxation phenomena and may be successfully described in terms of an electron-hole pair recombination model often applied to bandtail emission in semiconductors.