Highly Emissive, Water-Repellent, Soft Materials: Hydrophobic Wrapping and Fluorescent Plasticizing of Conjugated Polyelectrolyte via Electrostatic Self-Assembly

Sulfonated poly(diphenylacetylene) (SPDPA) is used as an anionic conjugated polyelectrolyte to examine stoichiometric electrostatic self-assembly with homologous cationic surfactants (octadecyl)_X(methyl)_Y ammonium bromides (O_XM_YABs) having different numbers of long hydrophobic tails. The SPDPA–O_XM_YAB complexes formed show significantly increased water contact angle and enhanced fluorescence (FL) emissions compared with the pristine SPDPA. The complexes exist in a gum state at room temperature owing to the plasticizer effect of the hydrophobic tails, hence they are very soft and highly stretchable. The hydrophobicity, softness, and FL quantum efficiency of the SPDPA–O_XM_YAB complexes increase as the number of hydrophobic tails increases. SPDPA adsorbs uniformly onto filter papers to produce fluorescent papers. The SPDPA-adsorbed papers have many unique applications, including FL image writing, fingerprinting, stamping, and inkjet printing using the surfactant solutions as an ink to reveal high-resolution FL images. In particular, multideposit inkjet-printing using SPDPA and O_XM_YAB solutions as inks produces water-resistant, embedded figures in paper currency.