Novel blue photoluminescent copolymers containing bipyridine and organosilicon

Two novel bipyridine- and silicon-containing PPV-based copolymers (P1 and P2) having the same structure but different molecular weights were synthesized by the Wittig reaction between an organosilicon monomer and two different bipyridine monomers. 5,5′-bis(diethyl phosphinatylmethyl)-2,2′-bipyridine and 5,5′-bis(triphenylphosphoniomethyl)-2,2′-bipyridine dibromide were used as the bipyridine reagents. The former gave copolymer P1 with a higher molecular weight (M_n = 9299) than the latter (M_n = 5051), which was responsible for the poorer solubility in common organic solvents. Thermal analysis (TGA) shows that P2 starts to decompose at ca. 340 °C in nitrogen. A glass transition temperature (T_g) of 88.6 °C was observed by DSC analysis. UV-vis spectra show that there is no obvious change between the polymer in solution and as films for P2 while 6 nm of red-shift was noted for P1 in the film state. PL spectra reveal that the polymers as films emit at much longer wavelength than that in CHCl₃ solution. The bathochromic effect for P1 is larger than that for P2, which results in green emission as films. The redox behavior of the two polymers was investigated by cyclic voltammetry (C-V), which demonstrates irreversible n-doping and p-doping processes. The band gaps of the polymers were estimated from both the optical method and electrochemical measurements.