Chirality for Enhanced Second Harmonic Generation Response in Ferrocene-Appended D−π–A Naphthalimides

Research on second harmonic generation (SHG) is emerging for chiral molecules as they inherently break inversion symmetry. In this report, we have compared the SHG efficiencies of newly synthesized ferrocene (donor)-appended racemic (rac-)/chiral (S-) D−π–A push–pull naphthalimide (acceptor) chromophores Fc-Ph-R (1 and 3) and Fc-CH═C(CN)-Ph-R (2 and 4), R = naphthalimide and spectroscopically characterized them. Single-crystal X-ray diffraction (XRD) analysis showed that 3 and 4 (chiral) crystallized in chiral space groups (P1 and P212121), giving enhanced SHG efficiencies at the bulk level (2.3 times higher than the reference KDP), as observed from the Kurtz–Perry powder technique, compared to 1 and 2 (racemic), which crystallized in centrosymmetric space groups (P1̅ and Pccn). Redox behavior and photophysical studies, including solvatochromism, were described. At the molecular level, chromophores 3 and 4 showed higher molecular hyperpolarizability (β) values, as observed from Density Functional Theory (DFT). The residual NLO response of racemic (rac-) chromophores is attributed to noncovalent interactions, whereas that of chiral (S-) chromophores shows substantial results from the synergistic effect of chirality (inherently lacking an inversion center) and noncovalent interactions. A high β value and SHG efficiency were observed in chromophore 4 because of its built-in chirality along with the additional cyanovinylene acceptor moiety in the D−π–A system.