Versatile pyrazine derivatives with robust nonlinear optical (NLO) performance: symmetrical design and theoretical exploration
摘要
The work uses quantum chemistry to study symmetrically substituted pyrazine-based derivatives Pyr-1 to Pyr-8 that follow the donor-π-donor (D-π-D) or acceptor-π-acceptor (A-π-A) pattern aimed at better performance in nonlinear optics (NLO). The properties were studied using DFT and TD-DFT methods at the M06-2X/6-311G** level. All these derivatives exhibit high planarity and extended conjugation, validated by low MPP and SDP values, which facilitate charge transfer within the molecule. Using the electrophilicity and nucleophilicity indices, it is found that Pyr-5 and Pyr-8 are strong nucleophiles, whereas Pyr-1 and Pyr-2 exhibit a strong electron-accepting tendency. Analysis of the ELF, MEP, and EDD maps confirms that there is efficient delocalization of electrons and current flow within the molecule. DOS and FMO analysis show the HOMO-LUMO gap in Pyr-6 and Pyr-8 is smaller than that in Pyr-7, allowing for more efficient transfer of electrons. TDM results suggest that highly planar compounds exhibit strong electron-hole correlations and closely spaced transitions. In addition, NBO analysis reveals that, among all the studied compounds, Pyr-1 shows the strongest LP → π* and π → π* interactions and is the most energetically stable. This is attributed to its dipyrrolopyrrole moieties and greater conjugation. The third-order hyperpolarizability of Pyr-8 is 463.1 × 10− 36 esu, approximately 64 times greater than that of p-nitroaniline (7.2 × 10− 36 esu), due to its planar dipyrrolopyrrole termini and extended conjugation. According to TD-DFT, the material’s enhanced NLO behavior is supported by its low excitation energy, strong oscillator strength, and high transition dipole moments. The results from this study offer practical advice for making effective third-order NLO materials for use in optoelectronics.