Segmented poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] via xanthate and dithiocarbamate precursors: a comparative study of thermal eliminations

Abstract

Segmented poly[2-methoxy-5-(2-ethylhexloxy)-1,4-phenylene vinylene] (MEH-PPV)-x's, which contain conjugated segments of varying lengths that are interspersed by nonconjugated units along the polymer backbone, were synthesized by selective thermal elimination of precursors containing controlled amounts (x) of a thermally labile group, namely, xanthate or dithiocarbamate (DTC). These precursors were in turn synthesized by competitive nucleophilic substitution of the Wessling polyelectrolyte with varying molar fractions of the respective nucleophiles - potassium ethyl xanthate or sodium diethyl dithiocarbamate. Methanol, used as the reaction medium, also served to introduce the second thermally less labile nucleophilic substituent. This approach for the preparation of segmented MEH-PPV-x is superior to the previous approach that used acetate as the thermally labile group, because it offers greater control over the composition despite a simpler synthetic procedure. Detailed studies of the thermal-elimination kinetics of the three precursors, namely, acetate, xanthate, and DTC, both in solution and in thin films, were carried by in situ monitoring of their ultraviolet-visible spectra. These studies revealed that the rates of elimination followed the order, DTC > xanthate > acetate. The activation energies for the elimination were, however, not widely different (ca. 30 kcal/mol), suggesting that the rates primarily reflected differences in the pre-exponential factor. After elimination, the segmented MEH-PPV-x samples exhibited the expected redshift in their absorption and fluorescence spectra with an increasing molar fraction (x) of eliminated segments, which was accompanied by a drastic reduction in the fluorescence quantum yields.