Preparation and characterization of nanofibrous perylene-diimide - Polyelectrolyte composite thin films

Composite nanofibrous thin films of a cationic, water-soluble perylene diimide and oppositely charged polyelectrolyte are prepared by sequential deposition from separate aqueous solutions of the two precursors. These materials may find future applications as semiconducting wires in organic electronics and photovoltaics. A new asymmetrically substituted perylene diimide (designated C11OPDI+) incorporating a hydrophobic ether tail is employed in their synthesis. Poly(acrylate) is used as the polyelectrolyte. Solution-phase and thin-film spectroscopic data show the composites form by binding and aggregation of C11OPDI+ to the polyelectrolyte. Tapping-mode AFM data show that the resulting nanofibers are tens of micrometers in length and are highly curved. Cross-sectional fiber size is shown to depend on the number of deposition cycles. Polarization-dependent fluorescence microscopy indicates the C11OPDI+ chromophores align perpendicular to the local long axis of the nanofibers. The C11OPDI+ molecules are concluded to form tail-to-tail parallel d-stacked structures that run along the fiber axis and are sandwiched between polyelectrolyte regions. In comparison to alternative methods, nanofiber formation is shown to be greatly enhanced when the composite is prepared by sequential deposition. A mechanism for enhanced fiber formation involving slow growth and solvent annealing of the composites is proposed.