Intramolecular boron-locking strategy induced remarkable first hyperpolarizability: role of torsion angles between donor and acceptor units

Abstract

In conventional strategies to design donor–acceptor (D–A) organic molecules with a large electronic contribution to the first hyperpolarizability (β), the effects of the torsion angles (θ₁ and θ₂) between donor and acceptor moieties are barely considered. To address this issue, in this work, a promising and novel intramolecular boron-locking strategy combined with the different locking groups of different acceptors to control θ₁ and θ₂, has been proposed to make D–A organic molecules with large β values. Intriguingly, reducing the torsion angles will make the β value of the pyridiny thiophene triphenylamine unit (Py-Th-TPA) dramatically increase up to 94%, which is mainly ascribed to the smaller θ₁ and θ₂ leading to lower excited energy of the crucial excited state, and enhanced charge transfer (CT) from TPA to Py-Th moieties, and finally greatly increase the donor and acceptor part contributions to β. Correlation between the difference, |θ₁ − θ₂| and β, provides a large coefficient of determination, R² = 0.78, which demonstrates that |θ₁ − θ₂| can be regarded as a potential descriptor for designing nonlinear optics (NLO) materials with D–A architecture. Clearly, we uncovered that θ₁ and θ₂ play a crucial role in the performance of NLO materials with D–A fragments.