Selective elimination in dialkoxy-PPV precursors yielding polymers with isolated tetraalkoxy-stilbene units

In the polymerization of 2,5-diethoxy-1,4-phenylenedimethylene-bis(tetrahydrothiophenium chloride) leading to the 2,5-diethoxy-poly(phenylenevinylene) (2,5-diethoxy-PPV) precursor we have varied the NaOH concentration from equimolar to a five-fold excess. Elimination to the conjugated form is observed during polymerization by using concentrations higher than equimolar. Suprisingly, this elimination is very selective, yielding tetraethoxy-stilbene units only. Almost no delocalization over more than two benzene rings, i.e. hexaethoxy-distyrylbenzene units, is observed in polymers with as much as 35% elimination (determined by 1H NMR spectroscopy). This selectivity is explained by the preference of an E2-elimination yielding products with the highest gain in resonance energy at every step: tetraalkoxy-stilbenes. Absorption and emission spectroscopies reveal that significant energy transfer from benzene to stilbene units occurs and that a blue photoluminescence with λmax = 415 nm is present.