Theoretical study of new acceptor and donor molecules based on polycyclic aromatic hydrocarbons

Abstract

Functionalized polycyclic aromatic hydrocarbons (PAHs) are an interesting class of molecules in which the electronic state of the graphene-like hydrocarbon part is tuned by the functional group. Searching for new types of donor and acceptor molecules, a set of new PAHs has recently been investigated experimentally using ultraviolet photoelectron spectroscopy (UPS). In this work, the electronic structure of the PAHs is studied theoretically with the help of B3LYP hybrid density functionals. Using the ΔSCF method, electron binding energies have been determined which affirm, specify and complement the UPS data. Symmetry properties of molecular orbitals are analyzed for a categorization and an estimate of the related signal strength. While σ-like orbitals are difficult to detect in UPS spectra of condensed film, calculation provides a detailed insight into the hidden parts of the electronic structure of donor and acceptor molecules. In addition, a diffuse basis set (6-311++G**) was used to calculate electron affinity and LUMO eigenvalues. The calculated electron affinity (EA) provides a classification of the donor/acceptor properties of the studied molecules. Coronene-hexaone shows a high EA, comparable to TCNQ, which is a well-known classical acceptor. Calculated HOMO–LUMO gaps using the related eigenvalues have a good agreement with the experimental lowest excitation energies. TD-DFT also accurately predicts the measured optical gap.

Introduction

Charge transfer salts have become of particular interest during the past two decades, in which both the interest in electronics on the molecular scale as well as the methods of chemical synthesis have strongly increased. Typically, charge transfer (CT) complexes, consisting of π-electron donors (D) and acceptors (A), show one or several properties like metallic conductivity, superconductivity, and magnetism that are relevant for nanoelectronic applications [1], [2]. Many of the organic conductors and superconductors are based on the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) donor and various acceptors [2], [3]. Furthermore, poly cyano derivatives have mainly been used as acceptors like tetracyanoquinodimethane (TCNQ) [4]. In this article, a detailed theoretical study of the electronic properties of new PAHs is presented, which makes it possible to identify the experimental results and complements the picture of experimentally determined orbital states.

Flat aromatic molecules such as polycyclic aromatic hydrocarbons (PAHs), have attracted attention because of their extended π-electron systems [5], [6]. Larger PAH molecules called nanographene have been synthesized in the bottom-up approach with desired size, structure, and symmetry [7]. Garito and Heeger concluded that the extension of the π system would lead to a lowering of intramolecular Coulomb repulsion in the anions of the acceptors, resulting in more stable radical anions and in highly conducting CT complexes [8]. The electronic structure of PAHs can be tuned via the type and number of substitutions around the periphery. This way the functionalized PAHs may serve as a prototype for the creation of a new class of molecules with tailored chemical and electronic properties. However, although the electronic structure of different donor and acceptor molecules have been widely studied, little attention has been paid by theory and experiment to illustrate the electronic structure and the effect of functional groups on donors and acceptors based on PAHs.

In this study we calculate the electronic structure of new acceptors and donors based on coronene and pyrene molecules (see Fig. 1). A hybrid density functional (B3LYP) method is used to calculate the ionization potentials (IPs). The results are compared with ultraviolet photoelectron spectroscopy (UPS) measurements [9]. This comparison makes it possible to assign the measured signals to specific molecular orbitals, providing a self consistent validation of the experimental results. Furthermore, the calculations provide the IPs of orbitals that could not be measured, experimentally.

Unfortunately, gas phase measurements were not possible in the experiment because of the small amount of material available (a few milligrams). Instead, UPS measurements had been performed on PAH multilayers on a Au surface. The molecules are preferentially planar to the surface while measurements are made in the perpendicular direction; therefore, σ-type orbitals could not be measured in practice. In the theoretical study all orbitals can be investigated so that the respective IPs become accessible. In general, the IPs of PAHs adsorbed on a metal surface are shifted by a constant energy value with respect to the gas phase results. The discrepancy stems from the photo-hole screening induced by the image charge in the metal [9]. A similar effect occurs in multilayer films in which the screening is induced by the neighboring molecules. The energy shift between the theoretical results corresponding to isolated molecules and the measurements on the condensed multilayers can be taken into account by adjusting the HOMO levels of both systems. Applying this energy shift to all IPs must provide a good correspondence between the experimentally measurable orbitals and the theoretical results, which serves as a useful consistency check of both measurement and calculation. For the given set of molecules the energy shift is very small so that the IP results for isolated molecules and those in the multilayer can be compared directly.

Coronene has a high symmetry (D_6h), a small band gap and the right size for processing techniques. Therefore, coronene and its derivatives are promising for applications like molecular electronics optoelectronics or sensors [10], [11], [12], [13]. Kato et al. [14] also predicted superconductivity of coronene in a charge transfer configuration. In this article, properties of hexamethoxycoronene (HMC) and coronene-hexaone (CHO) are studied. Furthermore, calculations of tetramethoxypyrene (TMP) and pyrene-tetraone (PTO) have been performed and compared with experimental results. These poly aromatic molecules have a different symmetry and structure in comparison with coronene based molecules, which makes a comparison of these two families instructive. Calculation of tetracyanoquinodimethane (TCNQ), a well-known classical acceptor, is used as a benchmark for the calculations of the acceptor molecules. All investigated molecules are shown in Fig. 1. Electronic structures of the donor and acceptor molecules are theoretically studied based on the B3LYP/6-31G(d) level of theory and compared with the ultraviolet photoelectron spectroscopy (UPS) measurements. Electron binding energies or ionization potentials of molecules are calculated with the delta self consistent field (ΔSCF) method using B3LYP.