Connecting Solution-Phase to Single-Molecule Properties of Ni(Salophen)

Area of Science:

• Surface Science and Molecular Electronics
• Coordination Chemistry
• Physical Chemistry

Background:

• Understanding the electronic properties of metal complexes like Ni(salophen) is crucial for molecular electronics.
• Previous studies often faced challenges in correlating single-molecule behavior with bulk properties or solution-phase measurements.

Purpose of the Study:

• To investigate the correlation between Ni(salophen) structure and properties in single-molecule versus bulk measurements.
• To compare measurements performed under ultra-high vacuum (UHV) conditions versus those in solution.
• To establish a foundation for accurately calculating single-molecule electronic properties of Ni(salophen).

Main Methods:

• Formation of a self-assembled monolayer (SAM) of Ni(salophen) on Au(111) under ultra-high vacuum (UHV) conditions.
• Characterization of molecular structure using Scanning Tunneling Microscopy (STM).
• Determination of Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) levels via elastic tunneling spectroscopy (ETS) at the single-molecule level.
• Confirmation of electronic properties using Ultraviolet Photoelectron Spectroscopy (UPS) and Cyclic Voltammetry (CV) for monolayer quantities.

Main Results:

• STM revealed that Ni(salophen) forms a SAM with a molecular structure identical to X-ray crystallographic data.
• Single-molecule ETS, supported by UPS and CV, determined HOMO-LUMO gap (3.28 eV) and (HOMO-1)-HOMO gap (0.36 eV).
• The study establishes a strong correlation between single-molecule and bulk properties, and UHV versus solution-phase measurements.

Conclusions:

• Microscopy-based surface experiments and gas-phase calculations can provide valuable insights into metal(salen) complex properties.
• The findings offer a new basis for selecting computational methods (hybrid functionals) to accurately predict single-molecule electronic levels.
• This work bridges the gap between surface science techniques and traditional solution-based characterization for metal complexes.