Effect of Oriented External Electric Fields on the Electronic Properties of Linear Acenes: A Thermally Assisted Occupation DFT Study

Abstract

Recently, oriented external electric fields (OEEFs) have earned much attention due to the possibility of tuning the properties of electronic systems. From a theoretical perspective, one can resort to electronic structure calculations to understand how the direction and strength of OEEFs affect the properties of electronic systems. However, for multi-reference (MR) systems, calculations employing the popular Kohn-Sham density functional theory with the traditional semilocal and hybrid exchange-correlation energy functionals can yield erroneous results. Owing to its decent compromise between accuracy and efficiency for MR systems at the nanoscale (i.e., MR nanosystems), in this study, thermally assisted occupation density functional theory (TAO-DFT) is adopted to explore the electronic properties of n-acenes (n = 2-10), containing n linearly fused benzene rings, in OEEFs, where the OEEFs of various electric field strengths are applied along the long axes of n-acenes. According to our TAO-DFT calculations, the ground states of n-acenes in OEEFs are singlets for all the cases examined. The effect of OEEFs is shown to be significant on the vertical ionization potentials and vertical electron affinities of ground-state n-acenes with odd-number fused benzene rings. Moreover, the MR character of ground-state n-acenes in OEEFs increases with the increase in the acene length and/or the electric field strength.

Keywords: OEEFs; TAO-DFT; acenes; electronic properties; multi-reference character.

Figure 1

Structure of 5-acene, containing five linearly fused benzene rings, in an OEEF, where the arrow indicates the OEEF direction.

Figure 2

The singlet–triplet energy gap of n-acene in an OEEF of the electric field strength F = 0.000, 0.001, 0.002, 0.003, 0.004, and 0.005 a.u., calculated using spin-unrestricted TAO-LDA.

Figure 3

Vertical ionization potential for the ground state of n-acene in an OEEF of the electric field strength F = 0.000, 0.001, 0.002, 0.003, 0.004, and 0.005 a.u., calculated using spin-unrestricted TAO-LDA.

Figure 4

Vertical electron affinity for the ground state of n-acene in an OEEF of the electric field strength F = 0.000, 0.001, 0.002, 0.003, 0.004, and 0.005 a.u., calculated using spin-unrestricted TAO-LDA.

Figure 5

Fundamental gap for the ground state of n-acene in an OEEF of the electric field strength F = 0.000, 0.001, 0.002, 0.003, 0.004, and 0.005 a.u., calculated using spin-unrestricted TAO-LDA.

Figure 6

Symmetrized von Neumann entropy for the ground state of n-acene in an OEEF of the electric field strength F = 0.000, 0.001, 0.002, 0.003, 0.004, and 0.005 a.u., calculated using spin-unrestricted TAO-LDA.

Figure 7

Occupation numbers of active orbitals for the ground state of n-acene in an OEEF of the electric field strength F = (a) 0.000, (b) 0.001, (c) 0.002, (d) 0.003, (e) 0.004, and (f) 0.005 a.u., calculated using spin-restricted TAO-LDA. Here, the HOMO/LUMO is denoted as the H/L for brevity.