Self-Assembly of a Functional Oligo(Aniline)-Based Amphiphile into Helical Conductive Nanowires

Abstract

A tetra(aniline)-based cationic amphiphile, TANI-NHC(O)C5H10N(CH3)3(+)Br(-) (TANI-PTAB) was synthesized, and its emeraldine base (EB) state was found to self-assemble into nanowires in aqueous solution. The observed self-assembly is described by an isodesmic model, as shown by temperature-dependent UV-vis investigations. Linear dichroism (LD) studies, combined with computational modeling using time-dependent density functional theory (TD-DFT), suggests that TANI-PTAB molecules are ordered in an antiparallel arrangement within nanowires, with the long axis of TANI-PTAB arranged perpendicular to the nanowire long axis. Addition of either S- or R- camphorsulfonic acid (CSA) to TANI-PTAB converted TANI to the emeraldine salt (ES), which retained the ability to form nanowires. Acid doping of TANI-PTAB had a profound effect on the nanowire morphology, as the CSA counterions' chirality translated into helical twisting of the nanowires, as observed by circular dichroism (CD). Finally, the electrical conductivity of CSA-doped helical nanowire thin films processed from aqueous solution was 2.7 mS cm(-1). The conductivity, control over self-assembled 1D structure and water-solubility demonstrate these materials' promise as processable and addressable functional materials for molecular electronics, redox-controlled materials and sensing.

Figure 1

(a) Molecular structure and DFT-optimized space-filling model of TANI-NHC(O)C₅H₁₀N(CH₃)₃⁺Br^– (TANI-PTAB). (b) Concentration dependent UV–vis spectra of TANI-PTAB in water (arrow indicates change on increasing concentration), inset: linear dependence of absorbance on concentration (values taken at λ_max = 580 nm). (c) Critical aggregation concentration for aqueous TANI-PTAB solution, measured by pyrene fluorescence, CAC = 1 × 10^–4 M.

Figure 2

(a) TEM (stained with 1% uranyl acetate) and (b) cryo-TEM (unstained) of EB TANI-PTAB forming bundles of nanofibers in aqueous solution (4 × 10^–3 M and 1 × 10^–3 M, respectively). (c) Histogram of the measured widths of nanofibers within bundles, (d) enlarged section of cryo-TEM image showing nanofiber bundles. Scale bars: 50 nm (a, b), 10 nm (d).

Figure 3

Temperature-dependent UV–vis for TANI-PTAB (1 × 10^–3 M, aqueous solution). Arrow indicates direction of change on increasing temperature. Inset: the mole fraction of aggregated molecules, α(T), as a function of temperature.

Figure 4

(a) Calculated orientation of the transition dipole moment relative to the EB TANI-PTAB molecular structure. (b) LD and UV–vis spectra of the HOMO–LUMO spectral region of TANI-PTAB (4 × 10^–3 M). (c) Birefringent optical textures observed under crossed polars for TANI-PTAB between glass plates (4 × 10^–3 M), scale bar: 100 μm. (d) Proposed alignment of TANI chromophores within nanofibers, with transition dipole moment (white arrow) overlaid, relative to fiber axis (black arrow).

Figure 5

UV–vis spectra showing the transition from doped ES TANI(CSA)₂-PTAB to dedoped EB TANI-PTAB with dilution. Arrows indicate change on diluting TANI(CSA)₂-PTAB (1 × 10^–4 M).

Figure 6

(a) TEM (stained with 1% uranyl acetate) and (b) cryo-TEM (unstained) of TANI(CSA)₂-PTAB (4 × 10^–3 M). (c) Histogram showing comparison of measured nanowire widths for EB and CSA-doped TANI-PTAB. Scale bars: 50 nm.

Figure 7

Exciton-coupled CD spectra of TANI-PTAB doped with both oppositely handed CSA enantiomers and racemic CSA to form TANI(CSA)₂-PTAB, with UV–vis of the same spectral region.

Figure 8

(a) Representative I–V curve for TANI(CSA)₂-PTAB spin-cast on glass substrates with prepatterned gold electrodes in bottom-contact collinear 4-point probe configuration (separation 250 μm). (b) AFM image of spin-cast TANI(CSA)₂-PTAB film (a scratch has been made in the film to expose the substrate and allow film thickness calculation).