Solvent Assisted Tuning of Morphology of a Peptide-Perylenediimide Conjugate: Helical Fibers to Nano-Rings and their Differential Semiconductivity

Abstract

Understanding the regulatory factors of self-assembly processes is a necessity in order to modulate the nano-structures and their properties. Here, the self-assembly mechanism of a peptide-perylenediimide (P-1) conjugate in mixed solvent systems of THF/water is studied and the semiconducting properties are correlated with the morphology. In THF, right handed helical fibers are formed while in 10% THF-water, the morphology changes to nano-rings along with a switch in the helicity to left-handed orientation. Experimental results combined with DFT calculations reveal the critical role of thermodynamic and kinetic factors to control these differential self-assembly processes. In THF, P-1 forms right handed helical fibers in a kinetically controlled fashion. In case of 10% THF-water, the initial nucleation of the aggregate is controlled kinetically. Due to differential solubility of the molecule in these two solvents, elongation of the nuclei into fibers is restricted after a critical length leading to the formation of nano-rings which is governed by the thermodynamics. The helical fibers show superior semi-conducting property to the nano-rings as confirmed by conducting-AFM and conventional I-V characteristics.

Figure 1

(A) Molecular structure of P-1; FESEM images of P-1 (10 μM) nanostructures obtained from THF/water mixed solvents with different volume ratios of (B) THF; (C) 10% THF and (D) 50% THF. All measurements were carried out after 72 h of mixing.

Figure 2

(A) Absorption spectra and (B) emission spectra of P-1 (10 µM) in different volume ratio of THF/water under ambient condition. Inset of (A) Absorption ratio of the 0–0 and 0–1 transition at different THF content. All measurements were carried out after 72 h of mixing.

Figure 3

CD spectra of P-1 (10 µM) in different volume ratio of THF/water under ambient condition. All measurements were carried out after 72 h of mixing.

Figure 4

Time dependent (A–C) absorption and (D–F) CD spectra of P-1 (10 μM) in THF, 50% THF and 10% THF mixed solvent systems respectively.

Figure 5

FESEM images of P-1 (10 μM) nanostructures obtained from 10% THF and 50% THF mixed solvents at different time intervals.

Figure 6

Temperature dependent absorption (A–C) and CD (D–F) spectra of 10 μM solutions of P-1 in different THF-water compositions.

Figure 7

(A) ¹H NMR (aromatic region) spectra and (B) IR (carbonyl region) of P-1 (10 μM) from different THF-water compositions.

Figure 8

(A) Optimized geometry of the monomer from DFT calculations, (B) Left handed stacking arrangement of P-1 molecule from DFTB calculations.

Figure 9

Schematic presentation of the thermodynamic and kinetic control of the self-assembly process of P-1 in different solvent composition to show the formation mechanism for helical nano-fibers and nano-rings.

Figure 10

(A–D) C-AFM analyses of thin film of P-1 (10 μM) prepared from THF (A,B) and 10% THF (C,D). (A and C) 3D topography of the nanofibers and nano-rings respectively; (B and D) I-V characteristics of nanofiber and nano-rings obtained from C-AFM. (E) Current-voltage characteristics of P-1. The thin films of P-1 have been deposited from different solvents as the legends indicate.