Open-Shell Character Dependences of the Second Hyperpolarizability in Two-Dimensional Tetraradicaloids

Area of Science:

• Computational chemistry
• Materials science
• Quantum mechanics

Background:

• Second hyperpolarizability (γ) is crucial for nonlinear optical (NLO) materials.
• Understanding the relationship between open-shell character and γ is key for designing advanced NLO systems.
• Conventional global diradical characters offer limited insight into local electronic structures.

Purpose of the Study:

• To investigate the dependence of second hyperpolarizability (γ) on open-shell character in tetraradicaloid models.
• To introduce and utilize local diradical characters (y_intra and y_inter) for a deeper understanding of electronic structure.
• To explore new strategies for enhancing γ in multiradical systems.

Main Methods:

• Numerically exact solutions of the extended Hubbard model for tetraradicaloid models.
• Definition and application of local diradical characters (y_intra, y_inter) and comparison with global ones (y₀, y₁).
• Verification using ab initio quantum chemical calculations on realistic tertraradical models (BDTDA dimer, disilene dimer).

Main Results:

• Two distinct mechanisms for enhancing γ components (γ_intra, γ_inter) based on local diradical character values.
• Enhancement of a single γ component at intermediate local diradical character values.
• Further enhancement of both γ components when local diradical characters are similar and small (≤∼0.3).

Conclusions:

• Local diradical characters provide a more comprehensive understanding of electronic structure and its effect on γ.
• The study identifies specific conditions for significant enhancement of second hyperpolarizability in multiradical systems.
• These findings pioneer a new class of 2D multiradical NLO systems with potentially superior performance.