A water-soluble poly(phenylene ethynylene) with pendant phosphonate groups. Synthesis, photophysics, and layer-by-layer self-assembled films

This paper reports the synthesis, optical, and thin-film-forming properties of a new water-soluble poly(phenylene ethynylene) that features phosphonate solubilizing groups. The new polymer, PPE-PO3-, is prepared by a synthetic route that involves initial preparation of a neutral analogue polymer in which the phosphonate groups are in the form of dibutyl esters. The neutral polymer is converted to the water-soluble form by cleavage of the phosphonate ester groups using (CH3)(3)SiBr. PPE-PO3- absorbs in the blue of the visible region and features a moderately intense fluorescence. The absorption and fluorescence of the polymer vary strongly with pH, an effect which is believed to arise because the polymer exists in a strongly aggregated form at low pH and in a relatively unaggregated state at high pH. Ultrathin multilayer films of the PPE-PO3- are deposited by the layer-by-layer (LbL) method using either the cationic polyelectrolyte poly(diallyldimethylamine) or Zr(IV) as deposition "partners". Absorption spectroscopy indicates that the average bilayer thickness of the LbL films increases as the pH of the deposition solution decreases. This effect is believed to be due in part to the fact that the R-PO3- groups are partially neutralized at low pH; however, aggregation of the polymer in solution is also believed to contribute to the increase in the amount of polymer deposited at low pH. The fluorescence of PPE-PO3- is quenched very strongly in solution by cationic electron and energy acceptors, with Stern-Volmer quenching constants of approximate to10(7) M-1. Finally, an electrolumineseent device was fabricated using a 10 bilayer film consisting of PPE-PO3-/Zr(IV). The device turns on between 5 and 6 V and exhibits a yellow-orange emission.