Probing the Molecular Interactions at the Conducting Polyaniline Nanomaterial Surface via a Pyrene Fluorophore

Abstract

We report a unique fluorophore cumulant dopant approach for tracing the molecular interaction at the polyaniline nanomaterial surface. Emulsion polymerization of aniline + pyrene sulfonic acid (PSA) produced well-defined fluorescent polyaniline nanofibres of 250 nm diameters with lengths up to 1-3 -M. Post-polymerization doping of a polyaniline emerldine base with PSA produced fluorescent agglomerated nanospheres of 500-700 nm in diameter. Dynamic light scattering (DLS) analysis of the aniline + PSA complex confirmed the existence of ?25 nm micellar aggregates in the emulsion which templates for the polyaniline nanofibers during the chemical oxidation process. The compositions of the dopant and polymer in the nanomaterials were determined by absorbance spectroscopy. The plots of incorporation of PSA in the nanomaterials versus feed revealed that the dopants incorporated a five times higher amount in the emulsion route compared to that of post-doping process. Wide angle X-ray diffraction (WXRD) analysis showed that the nanofibers were produced as highly ordered and crystalline through the strong dopant-polyaniline interactions, whereas the post-doped samples were found to be amorphous. Upon photo excitation, the nanofibers and nanospheres emitted blue light in the 370?425 nm region with respect to PSA chromophores. The photoluminescence intensity and the quantum yields of the nanospheres were found to be much higher compared to that of the nanofibers. The reason for the difference in the luminescent behaviors of the nanomaterials was attributed to the variation in the self-organization of polymer?dopant. In the well-ordered nanofibers, large amounts of PSA chromophores were confined in the nonluminescent polyaniline matrix: as a consequence, only few active chromophores at the nanofiber surface are available for photoluminescence. In the post-polymerization doping, the chromophores primarily bind at the surface of the nanospheres which resulted in the enhancement of their fluorescence characteristics. Time-resolved fluorescence decay measurements showed double exponential decay with respect to a very fast decay (-1) followed by a long-lived slow decay (?2). The nanospheres were found to have almost three times higher ?1 = 9.8 ns compared to that of nanofibers ?1 = 3.50 ns. Determination of radiative and nonradiative decay rate constants revealed that nanofibers have much smaller K-values compared to that of nanospheres. It is fairly evident from the present investigation that, though polyaniline nanofibers are highly crystalline and produce solid state ordered materials, they possessed weak luminescent characteristics because of the restricted motion of the dopant molecules which are trapped inside the solid matrix.