Beyond Spin Models in Orbitally Degenerate Open-Shell Nanographenes

Area of Science:

• Materials Science
• Quantum Chemistry
• Condensed Matter Physics

Background:

• Open-shell nanographenes traditionally focus on spin as the sole low-energy degree of freedom.
• Existing models often overlook the interplay between spin and orbital dynamics in these systems.

Purpose of the Study:

• To investigate low-energy excitations in nanographenes beyond the spin-only paradigm.
• To explore the role of orbital degeneracy in coupled spin-orbital dynamics.
• To analyze nitrogen-doped triangulenes as a model system.

Main Methods:

• Density-functional theory (DFT) calculations.
• Hubbard model simulations, including multiconfigurational and random-phase approximations.
• Analysis of orbital degeneracy in non-benzenoid nanographenes.

Main Results:

• Demonstrated that certain nanographenes host low-energy excitations with strongly coupled spin and orbital degrees of freedom.
• Identified orbital degeneracy, arising from non-benzenoid structures, as crucial for these coupled dynamics.
• Revealed a rich interplay between orbital and spin degrees of freedom in nitrogen-doped triangulenes.

Conclusions:

• The spin-only paradigm for open-shell nanographenes is insufficient.
• Coupled spin-orbital physics is essential for understanding specific nanographene systems.
• This work necessitates a revised theoretical framework for nanographene research.