How Rigid Are Anthranilamide Molecular Electrets?

Abstract

As important as molecular electrets are for electronic materials and devices, conformational fluctuations strongly impact their macrodipoles and intrinsic properties. Herein, we employ molecular dynamics (MD) simulations with the polarizable charge equilibrium (PQEq) method to investigate the persistence length (L_P) of molecular electrets composed of anthranilamide (Aa) residues. The PQEq-MD dissipates the accepted static notions about Aa macromolecules, and L_P represents the shortest Aa rigid segments. The classical model with a single L_P value does not describe these oligomers. Introducing multiple L_P values for the same macromolecule follows the observed trends and discerns the enhanced rigidity in their middle sections from the reduced stiffness at their terminal regions. Furthermore, L_P distinctly depends on solvent polarity. The Aa oligomers maintain extended conformations in nonpolar solvents with L_P exceeding 4 nm, while in polar media, increased conformational fluctuations reduce L_P to about 2 nm. These characteristics set key guidelines about the utility of Aa conjugates for charge-transfer systems within organic electronics and energy engineering.

Figure 1

Structure of Aa oligomers comprising Box residues. (a) Chemical structure of Box trimer showing the repeating unit spanning 4.2 Å. (b) Conformations from MD for Aaa-Box_n–2-Aaa oligomers (n = 10, 20, and 40) in Tol, illustrating the flexibility of the macromolecules.

Figure 2

Depiction of a segment of a macromolecule (between the i^th and the k^th residue) as a flexible system with a contour length of . The angle θ_i,j depicts the direction that a vector makes with a reference vector . The vector, connecting the head of and the tail of , represents the end-to-end distance when i = 1, i.e., the first residue of the polymer, and k = n, i.e., the last residue of a polymer composed of n units. The length of each vector, , is the same, designated with L_B. The differences between the lengths of the shown arrows originate from the projection of the three-dimensional arrangement of these vectors in the plane of their two-dimensional representation of the figure.

Figure 3

Visualization of the variability of the end-to-end distance, h, of Aa oligomers with 10 to 40 residues across different solvent environments. (a) A single residue of Aa indicating the numbering of the carbon atoms and attachments of the N- and C-terminal amides. The dihedral angles, ϕ and φ, quantify the rotation around the bonds between the amides and the aromatic ring responsible for the conformational variations along the backbones of the electret oligomers. (b) Chemical structure illustrating the end-to-end span, h, across the oligomer. (c) The Kuhn length, β_K, of oligomers with number of residues, n = 10, 20 and 40, in different solvent environments, i.e., Tol, DCM, and MeCN. (d-l) Variation of h and its averages through the 1 ns MD simulations for the three trials of the oligomers with (d,g,j) 10, (e,h,k) 20 and (f,i,l) 40 residues (d-f) in MeCN, (g-i) in DCM and (j-l) in Tol.

Figure 4

Correlation decays, i.e., ⟨cosθ_i,j⟩ vs |j–i| (the circular markers), derived from the 1 ns MD simulations of the Aa oligomers (Figure 3d-l), along with monoexponentially data fits (eq 3a) (solid lines). The data-fit residuals are displayed on the top of each graph. Decays of ⟨cosθ_i,j⟩ for: (a) Tol, the data fit yields L_P = 5.3 residues; (b) DCM, the data fit yields L_P = 3.2 residues; and (c) MeCN, the data fit yields L_P = 3.2 residues.

Figure 5

⟨cosθ_i,j⟩ correlation decays and the of Aa oligomers for solvents with different polarity. (a) Representative correlation decays for Tol, DCM and MeCN, along with the triexponential fits (eq 4) and the corresponding residuals from these SCF analysis. The θ_i,j values are extracted from the MD simulation for the electret surrounded by explicitly introduced Tol, DCM and MeCN solvents, and the corresponding cos θ_i,j are averaged of all the segmented frames from the 1 ns MD simulations. (b) The L_P of the Aa electrets obtained from the triexponential SCF fits and assigned to L_P⁽⁰⁾, L_P^(T) and L_P^(K), based on the patterns from the GF analysis (see Supporting Information), for the Aa oligomers with n = 10, 20, and 40 residues. The amplitude averages of the L_P from the triexponential fits, provides a single value for the L_P considering the contributions from all the components.