Structural Analysis and Electronic Properties of Negatively Charged TCNQ: 2D Networks of (TCNQ)₂Mn Assembled on Cu(100)

A compound two-dimensional monolayer mixing Mn atoms and 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules was synthesized by supramolecular assembly on a Cu(100) surface under ultrahigh-vacuum conditions. The interactions in the Mn(TCNQ)₂ network and in the full system are analyzed from a molecular orbital perspective and in the light of scanning tunneling microscopy (STM) imaging and simulations. Structural, electronic, and magnetic properties are studied in detail using density functional theory (DFT) calculations. In the absence of Cu and depending on the theoretical method used, the TCNQ species can be formally described as either dianions TCNQ²⁻ interacting with Mn⁴⁺ cations according to ligand field theory (using GGA calculations) or, alternatively, as radical monoanions interacting with Mn²⁺ cations in a high spin state (using GGA+U calculations). In the complete system including a Cu substrate, whatever theoretical method is used, TCNQs appear as dianions interacting with both Mn²⁺ cations in a high spin state (d⁵) and charged copper surface atoms.