Abstract
At the nanoscale, it has been rather troublesome to properly explore the properties associated with electronic systems exhibiting a radical nature using traditional electronic structure methods. Graphene nanoflakes, which are graphene nanostructures of different shapes and sizes, are typical examples. Recently, TAO-DFT (i.e., thermally-assisted-occupation density functional theory) has been formulated to tackle such challenging problems. As a result, we adopt TAO-DFT to explore the electronic properties associated with diamond-shaped graphene nanoflakes with n = 2–15 benzenoid rings fused together at each side, designated as n-pyrenes (as they could be expanded from pyrene). For all the n values considered, n-pyrenes are ground-state singlets. With increasing the size of n-pyrene, the singlet-triplet energy gap, vertical ionization potential, and fundamental gap monotonically decrease, while the vertical electron affinity and symmetrized von Neumann entropy (which is a quantitative measure of radical nature) monotonically increase. When n increases, there is a smooth transition from the nonradical character of the smaller n-pyrenes to the increasing polyradical nature of the larger n-pyrenes. Furthermore, the latter is shown to be related to the increasing concentration of active orbitals on the zigzag edges of the larger n-pyrenes.
Highlights
Graphene, a wonder material consisting of a hexagonal arrangement of carbon atoms in a plane, has revolutionized science and technology over the past few decades [1,2]
In several recent investigations [5,37,39,40,42,45], the occupation numbers of orbitals from TAO-DFT have been shown to be close to the occupation numbers of natural orbitals obtained with the variational two-electron reduced-density-matrix-driven complete-active-space self-consistent-field (CASSCF) (v2RDM-CASSCF) method, which is an accurate MR electronic structure method, leading to a qualitatively similar tendency in describing the radical nature associated with various PAHs
Here we continue employing TAO-DFT to explore the electronic properties associated with n-pyrenes with n = 2–15, which could pave the way for their potential applications in electronics and optoelectronics
Summary
A wonder material consisting of a hexagonal arrangement of carbon atoms in a plane, has revolutionized science and technology over the past few decades [1,2]. In several recent investigations [5,37,39,40,42,45], the occupation numbers of orbitals from TAO-DFT have been shown to be close to the occupation numbers of natural orbitals obtained with the variational two-electron reduced-density-matrix-driven CASSCF (v2RDM-CASSCF) method, which is an accurate MR electronic structure method, leading to a qualitatively similar tendency in describing the radical nature associated with various PAHs. In particular, TAO-DFT has been employed to explore the electronic properties associated with zigzag GNFs of different shapes and sizes, including zigzag GNRs (i.e., rectangular GNFs) [5], hexagon-shaped GNFs [12], and triangle-shaped GNFs [15] in recent years.
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