Mixed alkyl- and alkoxy-substituted poly[(phenylene ethynylene)-alt-(phenylene vinylene)] hybrid polymers:: Synthesis and photophysical properties

We report the synthesis, characterisation and photophysical properties of mixed alkyl- and alkoxy-substituted poly[(phenylene ethynylene)-alt-(phenylene vinylene)] hybrid polymers with the general constitutional unit: (-Ph-C=C-Ph-C=-C-Ph-CH=CH-Ph-CH=CH-)(n) (7). The polymers were obtained through the Horner-Wadsworth-Emmons olefination reaction of alkyl-substituted lumino-phoric dialdehydes, 5a-b, and alkoxy-substituted bisphosphonates. Contrary to their alkoxy-substituted congeners, 10a-b, which exhibit solution fluorescence quantum yields close to unity, dialdehydes (5a-b) fluoresce poorly at room temperature, but show phosphorescence at 77 K. High molecular weight polymers were obtained showing poor solubility compared to solely alkoxy-substituted polymers (11) of the same backbone Constitutional units. The presence of alkyl side groups in 7 leads to a 15 nm blue shift of their absorption spectra relative to 11. Both types of polymers show however identical emission spectra with only a slight blue shift (3-4 nm) of the emission peak of 7 relative to 11. This clearly proves that the chromophore system responsible for the emission in both types of compounds is located around the lower-energy, alkoxy-substituted phenylene-vinylene portion of the polymers as confirmed by fluorescence kinetics investigations.