Anion Stabilization in Electrostatic Environments

Excess charge stabilization of molecules in metallic environments is of particular importance for fields such as molecular electronics and surface chemistry. We study the energetics of benzene and its anion between two metallic plates. We observe that orientational effects are important at small inter-plate separation. This leads to benzene oriented perpendicular to the gates being more stable than the parallel case due to induced dipole effects. We find that the benzene anion, known for being unstable in the gas-phase, is stabilized by the plates at zero bias and an inter-plate distance of 21 Å. We also observe the effect of benzene under a voltage bias generated by the plates; under a negative bias, the anion becomes destabilized. We use the electron localization function to analyze the changes in electron density due to the bias. These findings suggest that image effects such as those present in nanoscale devices, are able to stabilize excess charge and should be important to consider when modeling molecular transport junctions and charge-transfer effects. The stabilization of a transient electron in a molecular system by a complex nanoscale environment is relevant for the fields of molecular electronics and transport1–5 as well as to the understanding of molecular properties on surfaces such as in surface-enhanced Raman spectroscopy.6–13 A major challenge is that of understanding the interaction between extended metallic systems, such as surface metal gates or a nanoparticle, and an adsorbed molecule. From a theoretical standpoint, these two components are characterized by different lengthscales. Differing approaches for handling this intrinsically multiscale problem have been taken. For example, several authors have focused on the molecular system. These efforts include the PCM14–17 and COSMO methodologies,18,19 where the metal surfaces or nanoparticles are simulated as cavity surfaces discretized into finite elements. The PCM technique has provided valuable insight into surface-enhancement processes.20–23 Recently, we have also modeled nanoscale systems using finite element and basis set methods. Our approach includes partitioning the Green’s function into a contribution that is effectively the interaction of the particles in vacuum (obtaining the 1/r potential), and an additional term which is effectively due to the electrostatic environment (unpublished results). A parallel formulation has been developed by one of us for quantum dots.24–27 2 In this case, the system is determined by the finite-difference discretization of arbitrarily-shaped boundaries, where these lie at the edges of a calculation box. This treatment of gated systems has enabled the study of properties such as Coulomb blockade energetics,25 quantum-dots selfconsistent structures26 and more recently, Förster coupling in nanoparticle excitonic circuits.27 In this work we focus on molecular systems interacting with a nanoscale environment. As a representative example, we study the benzene anion. It is known that a single benzene molecule in the gas phase is unable to bind an excess electron as shown by a negative vertical electron affinity (VEA) of −1.15 eV.28,29 VEA is defined as the energy difference between the neutral species at its ground-state geometry and the anion species at the neutral geometry as shown in Eq. (1). We use atomic units throughout to represent all quantities, unless otherwise noted. VEA =Eneutral (Rneutral)−Eanion (Rneutral) (1) VDE =Eneutral (Ranion)−Eanion (Ranion) (2) Similar to the VEA, the vertical detachment energy (VDE), shown in Eq. (2) is defined as the energy difference between the neutral species at the anion’s ground-state geometry and the anion at that same geometry. The VEA and VDE represent lower and upper bounds, respectively, of the electron affinity when the nuclear configuration is not significantly different in the anionic and neutral geometry.30–32 Specifically, Mitsui et al.33 observed the formation of negatively-charged benzene clusters (C6H6) − n . In the gas phase, the vertical detachment energy (VDE) is between 0.47 and 0.56 eV for n = 53−124 with an estimated value of 0.84 eV when n is extrapolated to infinity. Why does a cluster of molecules bind an excess electron when a single molecule does not? One explanation is that the polarizability of the cluster is greater than the single molecule. An excess electron can therefore more readily induce a dipole in the cluster to which it can favorably interact. In a similar fashion, the polarizability of a local dielectric and conducting medium in a nanostructured system can also stabilize anionic states of molecules.