Theoretical study on diradical characters and nonlinear optical properties of 1,3-diradical compounds

Area of Science:

• Computational Chemistry
• Materials Science
• Quantum Chemistry

Background:

• Open-shell 1,3-diradical compounds exhibit unique electronic structures with weak or intermediate π-single-bonding characters.
• Understanding the relationship between diradical character and nonlinear optical (NLO) properties is crucial for designing novel materials.

Purpose of the Study:

• To investigate the correlation between diradical character (y) and third-order NLO properties in 1,3-diradical compounds.
• To clarify the effects of 2,2-substituents on the structure-property relationships of 1,3-diphenylcyclopentane-1,3-diyl derivatives.

Main Methods:

• Utilized broken-symmetry density functional theory (DFT) to calculate diradical character and NLO properties.
• Analyzed the effects of electron-withdrawing substituents (OH, F) at the C2 position and asymmetric donor/acceptor substitutions on phenyl rings.

Main Results:

• The parent 1,3-diphenylcyclopentane-1,3-diyl is nearly a pure diradical (y ≈ 1).
• Electron-withdrawing substituents decrease diradical character but significantly enhance static second hyperpolarizabilities (γ) by factors of ~4.5 (OH) and ~6.4 (F).
• Asymmetric donor/acceptor substitutions on phenyl rings increase γ without significantly impacting diradical character.

Conclusions:

• A strong correlation exists between π-bonding character (diradical character) and third-order NLO properties in real 1,3-diradical systems.
• NLO measurements can serve as a novel probe for the unique chemical bonding nature in localized diradical compounds.