Supramolecular One-Dimensional n/p-Nanofibers

Abstract

Currently, there is a broad interest in the control over creating ordered electroactive nanostructures, in which electron donors and acceptors are organized at similar length scales. In this article, a simple and efficient procedure is reported en-route towards the construction of 1D arrays of crystalline pristine C60 and phenyl-C61-butyric acid methyl ester (PCBM) coated onto supramolecular fibers based on exTTF-pentapeptides. The resulting n/p-nanohybrids have been fully characterized by a variety of spectroscopic (FTIR, UV-Vis, circular dichroism, Raman and transient absorption), microscopic (AFM, TEM, and SEM), and powder diffraction (X-ray) techniques. Our experimental findings document the tendency of electroactive exTTF-fibers to induce the crystallization of C60 and PCBM, on one hand, and to afford 1D n/p-nanohybrids, on the other hand. Furthermore, photogenerated radical ion pairs, formed upon visible light irradiation of the n/p-nanohybrids, feature lifetimes on the range of 0.9-1.2 ns.

Figure 1. Chemical structures and cartoon showing the coating process of the exTTF fibers by C₆₀ and PCBM leading to the formation of 1D n/p nanohybrids (A).

The existence of C₆₀ domains in the 1D n/p-nanohybrids triggers the oriented growth of crystal nuclei of C₆₀/PCBM on top of them (B).

Figure 2. Ultraviolet-visible (UV-Vis) and circular dichroism (CD) spectra for exTTF-fiber and n/p-nanohybrids.

(A) UV-Vis (upper part) and CD (lower part) spectra of 1 (grey line, 1.3 × 10⁻³ M) and the mixture of 1:C₆₀ (black line, 1.3 × 10⁻³ M) in TCE following aging for 5 days. (B) UV-Vis (upper part) and CD (lower part) spectra of 1 (grey line, 1.3 × 10⁻³ M) and 1:PCBM (black line, 1.3 × 10⁻³ M) in TCE following aging for 5 days.

Figure 3. Powder X-ray diffraction (PXRD) for exTTF-fibers, n/p-nanohybrids and schematic illustration of the molecular packing for n/p-nanohybrids.

(A) PXRD pattern for 1 obtained from a dispersion of exTTF-fibers in a mixture TCE:toluene. (B) PXRD pattern for 1:C₆₀ nanohybrid. In the inset, “h_full” denotes the observed C₆₀ reflections in the 2θ region between 8 and 11. (C) Cartoon showing the molecular packing observed for the exTTF-fibers and for the 1:C₆₀ nanohybrid. The distances observed by PXRD experiments are remarked in the cartoon.

Figure 4. Differential absorption spectra for the n/p-nanohybrids.

(A) Differential absorption spectra (visible and near-infrared) obtained upon femtosecond flash photolysis (480 nm) of 1:C₆₀ in TCE with several time delays between 0.1 and 6750.0 ps at room temperature. (B) Time-absorption profiles of the spectra at 530 and 700 nm, monitoring the charge separation/charge recombination. (C) Differential absorption spectra (visible and near-infrared) obtained upon femtosecond flash photolysis (480 nm) of 1:PCBM in TCE with several time delays between 0 and 7000.0 ps at room temperature. (D) Time-absorption profiles of the spectra at 517 and 685 nm, monitoring the charge separation/charge recombination.

Figure 5. Atomic Force Microscopy (AFM) images and height profiles of the n/p-nanohybrids.

AFM images and the corresponding height profiles of the nanohybrids (A) 1:C₆₀ (2.6 × 10⁻⁴ M) and (B) 1:PCBM (2.6 × 10⁻⁴ M) showing the presence of nanohybrid fibers with an increase in height with respect to the initial exTTF-fibers. Arrows indicate C₆₀ or PCBM domains over the fibers.

Figure 6. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) for the n/p-nanohybrids.

(A–C) are TEM images of both nanohybrids (1:C₆₀, 1.5 × 10⁻⁴ M, (A,B) and (1:PCBM, 1.5 × 10⁻⁴ M, (C), arrows remark the presence of C₆₀ or PCBM domains along the fiber’s axis. Scale bars: 200 nm. (D–F) SEM images of both nanohybrids (1:C₆₀, 4.3 × 10⁻⁴ M, (D,E) and (1:PCBM, 4.3 × 10⁻⁴ M, F), arrows show the lengthening of C₆₀ and PCBM domains grown in the axial direction of the fibers.