The Effect of Intrinsic Rigidity on the Optical Properties of Ppv Derivatives

Light scattering and optical spectroscopy experiments were performed on [2-methoxy, 5-(2’-ethyl-hexyloxy-p-phenylene-vinylene] (MEH-PPV), poIy[2butoxy, 5-(2’-ethyl-hexyloxy-p-phenylene-vinylene] (BEH-PPV), and poIy[2dicholestanoxy-p-phenylene-vinylene] (BCHA-PPV) in solution with p-xylene to determine the effects of side chain size on the structure and optical properties of PPV. We find that increasing the size of the solubilizing side chains increases the intrinsic persistence length of the PPV backbone and this change in rigidity has dramatic effects on the photoluminescence of PPV. Quantum yield determination relative to Rhodamine 6G shows the luminescence efficiency of the rigid BCHA-PPV is 0.66 k 0.05 and that the yield decreases to 0.20 f 0.05 for MEH-PPV, the most coiled derivative. Excitation profiles show an increase in non-radiative decay at high energies when the backbone is more flexible. There is currently much interest in PPV because of its application in luminescent d e ~ i c e s l ~ . Like many conjugated polymers, PPV is insoluble in its pure form due to a high dielectric mismatch with conventional solvents. This intractability has been addressed by chemically attaching side chains to the polymer backbone. Side chains provide an organic layer that surrounds the polymer backbone, much like a surfactant, and make the polymer more compatible with organic solvents. While many conjugated polymers are processed in the derivatized form, very little is understood about the effects of functionalization on the structural, optical, and electronic properties of these polymers. Both experimental and theoretical studies have shown that the steric interactions of the side chains induce significant rigidity of the polymer b a c k b ~ n e ~ ? ~ . The effect of macroscopic disorder on the optical properties of PPV and PPV derivatives have been studied e~ tens ive ly~ .~ . With techniques such as tensile drawing and gel processing, these experiments have shown that increased orientation and improved structural order produces a narrowing of both the absorption and emission spectra along with a reduction in apparent Stokes shift. These results have been interpreted as an increase in effective conjugation length, Leff as a result of increased chain order. Still to be understood is how the effective conjugation length is derived from intrinsic polymer features. 507 D ow nl oa de d by [ N ew Y or k U ni ve rs ity ] at 1 3: 25 2 4 A pr il 20 13 508 C.L. GETTINGER ET AL. In this paper we report a comparative study of the photoluminescence properties of three PPV derivatives: MEH-PPV, BEHPPV, and BCHA-PPV (figure 1). Each has been derivatized with an increasingly larger side group in order to solubilize the PPV. With static and dynamic light scattering measurements we have determined the molecular weight Mw, radius of gyration RG, and hydrodynamic radius RH of the polymers. From these data we calculate a persistence length 1 for each polymer and find that increasing the size of the side chain increases 1 of the PPV backbone. Excitation profiles P BEH-PPV