Theoretical Study on the Correlation between Open-Shell Electronic Structures and Third-Order Nonlinear Optical...

Area of Science:

• Materials Science
• Theoretical Chemistry
• Quantum Chemistry

Background:

• One-dimensional (1D) stacked chains of π-radicals are of interest for their unique electronic structures.
• Understanding the relationship between open-shell electronic structure and nonlinear optical (NLO) properties is crucial for designing advanced materials.

Purpose of the Study:

• To theoretically investigate the correlation between open-shell electronic structure and third-order NLO properties in 1D stacked π-radical chains.
• To establish design guidelines for high-performance open-shell molecular NLO materials.

Main Methods:

• Employed finite N-mer models of methyl or phenalenyl radicals with varying stacking distances.
• Evaluated diradical characters (y_av, y_SD) and extrapolated to the N → ∞ limit.
• Utilized the spin-unrestricted long-range-corrected (LC-)UBLYP method for calculations.

Main Results:

• Diradical character indices (y-based) correlate with the second hyperpolarizability per dimer (γ∞).
• Significant enhancement of γ∞ and the polymer/dimer ratio (γ∞/γ(N=2)) observed at small stacking distances and small stacking distance alternation (SDA).
• The LC-UBLYP method accurately reproduced the trend of γ∞.

Conclusions:

• Small stacking distances and SDA are key factors for enhancing NLO properties in 1D π-radical chains.
• The study provides insights for designing high-performance open-shell molecular NLO materials.