A PHOTOLUMINESCENCE STUDY OF POLY(PHENYLENE VINYLENE) DERIVATIVES - THE EFFECT OF INTRINSIC PERSISTENCE LENGTH

We report the results of light scattering, absorption, excitation, and emission spectroscopy of three polyphenylene vinylene (PPV) derivativ es; poly[2-methoxy, 5-(2'-ethyl-hexyloxy-p-phenylene- vinylene] (MEH-P PV), poly[2-butoxy, 5-(2'-ethyl-hexyloxy-p-phenylene-vinylene] (BEH-PP V), and poly[2-dicholestanoxy-p-phenylene-vinylene] (BCHA-PPV) in solu tion with p-xylene. We find that increasing the size of the solubilizi ng side chains increases the intrinsic persistence length of the polyp henylene vinylene backbone and that this change in stiffness has drama tic effects on the photoluminescence of polyphenylene vinylene. We hav e determined the luminescence quantum efficiencies of the polyphenylen e vinylene derivatives relative to a known standard, Rhodamine 6G, and find that the photoluminescence can be greatly enhanced by increasing the intrinsic stiffness of the polymer backbone. The stiffest polymer , poly[2-dicholestanoxy-p-phenylene-vinylene] (BCHA-PPV), has a quantu m efficiency of 0.66+/-0.05. The quantum efficiency decreases to 0.22/-0.05 for poly[2-butoxy, 5-(2'-ethyl-hexyloxy-p-phenylene-vinylen (BE H-PPV) and 0.20+/-0.05 for poly[2-methoxy, 5-(2'-ethyl-hexyloxy-p-phen ylene-vinylen (MEH-PPV), the most coiled derivative. Excitation profil es of the three derivatives also show an increase in nonradiative deca y at high energies when the polymer assumes a more coiled conformation . Thus, the quantum yields are dependent on pump energy.