Influence of the conjugation pattern on the photophysical properties of alkoxy-substituted PE/PV hybrid polymers

Alkoxy-substituted phenylene-ethynylene (PE)/phenylene-vinylene (PV) hybrid polymers of general constitutional structure (-Ar-CdropC-Ar-CHdropCH-)(n) (2), (-Ar-CdropC-Ar-CHdropCH-ArCdropC-)(n) (3), and (-Ar-Cdrop-C-Ar-Cdrop-C-Ar-CHdropCH-Ar-CHdropCH-)(n) (4) have been synthesized and characterized. Comparisons of their photophysical properties in solution as well as in solid state (photoconductivity, absorption, emission, and electroluminescence) with those of their corresponding alkoxy-substituted poly(phenylene-ethynylene) (1) and poly(phenylene-vinylene) (5) have been carried out. Large octadecyloxy groups were selected as side chains in order to reduce the effect of pi-pi stacking interaction on the properties of the polymers in the solid state. This resulted in easy detection of photoconductivity and higher fluorescence quantum yields in solid state. The hybrid conjugation pattern in 4 is more favorable for photoconductivity than that in 2. A red shift of the absorption and emission spectra in solution is observed from 1 over 2b, 3, and 4 to 5. LED devices of ITO/PEDOT/polymer/Ca configuration have been fabricated with all compounds, except 2a. The phenylvinylene side groups in 3 not only give rise to a red shift of the solid-state photoluminescence spectrum and electroluminescence spectrum (relative to 2b) but also bring about a decrease of the turn-on voltage and improve the electroluminescence efficiency for more than 100 times.