Y-shaped quinoxaline-based chiral and racemic ferrocene naphthalimides for enhanced second harmonic generation and circularly polarized luminescence.

Racemic (Fc-Qn-NI-Rac) and chiral (Fc-Qn-NI-S) derivatives of new Y-shaped ferrocene conjugated quinoxaline naphthalimides were successfully synthesized from a ferrocenyl quinoxaline precursor (Fc-Qn-Br) and spectroscopically characterized in second harmonic generation (SHG) and circularly polarized luminescence (CPL) studies. According to cyclic voltammetry, the redox behaviour showed a higher anodic shift of 0.52-0.53 V than that of free ferrocene (Fe2+ ⇌ Fe3+: 0.40 V), and also a new reduction peak was identified between -0.47 and -0.64 V assigned to the reduction of the electron-deficient naphthalimide unit to its radical anion. The chiral Fc-Qn-NI-S chromophore showed a substantial SHG response {2.3 times higher than that of the reference potassium dihydrogen phosphate (KDP)}, demonstrating the potential of introducing chirality into NLO materials which inherently lack an inversion centre. CPL analysis revealed a chiroptic emission dissymmetry factor of glum = +1.8 × 10-3 for chiral Fc-Qn-NI-S. At the molecular level, Fc-Qn-NI-S showed higher molecular first hyperpolarizability (β) values than the precursor Fc-Qn-Br, according to the Density Functional Theory (DFT) calculations. High β value and SHG efficiency were obtained for Fc-Qn-NI-S because of the synergistic effect of designing multi-dimensional Y-shaped quinoxaline and inbuilt chirality in the naphthalimide D-π-A system.