Enhanced SHG efficiencies in V-Shaped ferrocene appended pyrimidinium salt using salt methodology with theoretical insights

Abstract

The enhanced second-harmonic generation (SHG) effect was observed in the V-shaped ferrocene appended methylated pyrimidinium salt (2) than in the corresponding pyrimidine chromophore (1). Although the pyrimidine chromophore (1) was crystallized in a centrosymmetric monoclinic (P2/c) system, it still exhibits pseudo SHG efficiencies due to the synergic effect from the deviation of the axillary donor pendant methoxy phenyl and the ferrocene moiety from the plane and noncovalent interactions (C-H···π ), which results reduce the antiparallel alignment in the solid state. Electrochemical studies showed one-electron transfer from ferrocene to the ferrocenium ion (Fe2+⇌Fe3+), and we also observed a reduction peak at 0.42 V for the oxidation of iodide to iodine in the chromophore 2. The solvatochromic studies revealed that chromophore 1 showed positive solvatochromism (99 nm) due to a high excited-state dipole moment. In contrast, chromophore 2 showed only 19 nm, indicating a higher ground-state dipole moment than the excited state because of the methyl pyrimidinium ion. As expected, the ferrocene-appended pyrimidinium salt (2) shows 2.8-fold enhanced SHG efficiency than (1) arising from the formation of non-centrosymmetric environment in the bulk, which is achieved by adopting the “salt methodology” technique, by conversion of pyrimidine to pyrimidinium ion, creates parallel alignment by steric effect and dipolar repulsions It further increases the electron-withdrawing nature, consequently enhancing NLO effect. Moreover, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were performed at the B3LYP/6-31+G** level, with theoretical results aligning well with experimental findings.