Fluorescence studies of poly(p-phenyleneethynylene)s. The effect of anthracene substitution

The poly(p-phenyleethynylene) molecules, PI-PIII, synthesized by a palladium-catalyzed cross-coupling reaction of diiodobenzene derivatives and derivatives of 1,4-diethynylbenzene, are highly luminescent materials. The polymers are soluble by virtue of the -OC₁₆H₃₃ groups introduced on the aromatic rings and by controlling their molecular weight. The fluorescent quantum yields are between 0.35 and 0.40 depending on the material. The excited-state lifetimes of the polymers are 1 - 2 ns, slightly shorter than that of the model compound, 1,4-diphenylethynyl-2,5-dibutoxybenzene, which has a lifetime of 3 ns. Incorporation of anthracene, coupled at the 9,10-positions by using 9,10-bibromoanthracene, into the polymer backbone decreases the quantum yield to between 0.05 and 0.27 depending on the anthracene content. In addition, low-energy electronic transitions and longer wavelength emission bands associated with the anthracene group are produced. The polymers harvest optical energy and transfer it to the anthracene resulting in emission from this chromophore. In the case where only terminal anthracene units, introduced by using 9-bromo-10-phenylethynylanthracene, are present, the process is very efficient with >95% of the energy being transferred to the end groups.