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) derivatives; poly[Zmethoxy, 5-(2’-ethyl-hexyloxy-p-phenylenevinylene] (MEH-PPV), poly[Zbutoxy, 5-(2’-ethyl-hexyloxy-p-phenylene-vinylene] (BEH-PPV), and poly[2-dicholestanoxy-p-phenylene-vinylene] (BCHA-PPV) in solution with p-xylene. We find that increasing the size of the solubilizing side chains increases the intrinsic persistence length of the polyphenylene vinylene backbone and that this change in stiffness has dramatic effects on the photoluminescence of polyphenylene vinylene. We have determined the luminescence quantum efficiencies of the polyphenylene 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 quantum efficiency of 0.66+0.05. The quantum efficiency decreases to 0.2250.05 for poly[Zbutoxy, 5-(2’-ethyl-hexyloxy-p-phenylene-vinylene] (BEH-PPV) and 0.20t0.05 for poly[2-methoxy, 5-(2’-ethyl-hexyloxy-p-phenylene-vinylene] (MEH-PPV), the most coiled derivative. Excitation profiles of the three derivatives also show an increase in nonradiative decay at high energies when the polymer assumes a more coiled comformation. Thus, the quantum yields are dependent on pump energy.