Electronic structure of twisted and planar rubrene molecules: a density functional study

Area of Science:

• Materials Science
• Solid State Physics
• Computational Chemistry

Background:

• Rubrene thin films are crucial for organic electronic devices.
• Understanding molecular conformation (flat vs. twisted) is key to optimizing film properties.
• X-ray absorption spectroscopy (XAS) provides insights into electronic structure.

Purpose of the Study:

• To calculate and compare X-ray absorption spectra (XAS) for flat and twisted rubrene molecules using DFT.
• To validate simulated XAS with experimental C K-edge near-edge X-ray absorption fine structure (NEXAFS) data.
• To elucidate the relationship between molecular conformation, electronic structure, and conductivity in rubrene thin films.

Main Methods:

• Density Functional Theory (DFT) calculations for XAS, Density of States (DOS), and frontier molecular orbitals (HOMO/LUMO).
• Comparison of simulated XAS with experimental NEXAFS data for rubrene thin films of varying thicknesses.
• Analysis of electron density distribution and HOMO-LUMO gap variations between flat and twisted rubrene.

Main Results:

• Simulated XAS spectra accurately reproduce experimental NEXAFS data within ±0.3 eV.
• NEXAFS spectra of rubrene thin films are a combination of contributions from flat and twisted molecules.
• Twisted rubrene molecules show a unique spectral peak and altered contributions from backbone/wings compared to flat molecules.
• Twisted molecules exhibit a lower HOMO-LUMO gap and redistributed electron density, leading to reduced conductivity despite the smaller gap.

Conclusions:

• The conformation of rubrene molecules significantly impacts their electronic and spectral properties.
• DFT simulations combined with NEXAFS provide a powerful tool for characterizing rubrene thin films.
• Understanding conformational effects is essential for designing high-performance rubrene-based organic electronic devices.