Molecular design and third-order nonlinear optical properties of D-A type pyrazine derivatives

In this work, three series of 12 novel donor-acceptor (D-A) pyrazine derivatives (B/S/J series) were designed and synthesized via a molecular engineering strategy. The B series was constructed based on the fundamental framework; the S series introduced triphenylamine with stronger electron-donating ability on the basis of the B series to enhance intramolecular charge transfer (ICT); while the J series modified the molecular structure by incorporating methoxybenzene, altering the electron cloud distribution and optimizing the molecular architecture. The systematic investigation revealed that structural tuning significantly influences their third-order nonlinear optical (NLO) properties. Both experimental and theoretical studies demonstrated that the synergistic effect between triphenylamine donors and nitrobenzene acceptors effectively enhances ICT. Specifically, compound S-series exhibits exceptional third-order nonlinear polarizability (S-2, χ_I^((3) ) = 35.18×10⁻¹³ esu) and second-order hyperpolarizability (S-4, γ = 7.93×10⁻³³ esu) in solution state. Through solid-state doping optimization (Polymethyl methacrylate (PMMA) composite films), the NLO performance was enhanced by three orders of magnitude (β = 27.84×10⁻⁹ m/W), providing a new paradigm for designing high-stability organic NLO materials. In this study, UV-Vis absorption spectroscopy and fluorescence spectroscopy were employed to investigate the ICT process; cyclic voltammetry was used to explore the electrochemical behavior; the Z-scan technique was applied to study the third-order NLO properties of the solutions at 532 nm; and density functional theory (DFT) calculations were utilized for theoretical verification, revealing the relationship between molecular structures and properties from multiple perspectives.