Exploring the impact of substituents and π-conjugation on structural, optical and nonlinear optical studies in ferrocene-appended D-π-A and D-D’-π-A chromophores

A new series of push-pull chromophores (1–6) with ferrocene and methoxyphenyl donors and π-acceptor groups were synthesized and characterized. The chromophores (1–3) [Fc-C = C(CN)-(C6H4)-(C6H3)-R {R = H (1), OCH3 (2), CF3 (3)}] and D-D’-π-A (4–6), [Fc-(OCH3-C6H4)-C = CH=CN-(C6H4)-(C6H3)-R {R = H (4), OCH3 (5), CF3 (6)}] were studied for their photophysical and nonlinear optical (NLO) properties. Single-crystal X-ray diffraction studies confirmed the crystal structures of selected chromophores 1, 3, 4, and 6, showing various non-covalent interactions such as H-bonding and C-H⋅⋅⋅⋅⋅π interactions. The second-order nonlinear optical (NLO) response of chromophore 6 shows significantly enhanced second-harmonic generation (SHG) efficiency, approximately 2.9 times higher than that of standard potassium dihydrogen phosphate (KDP), due to the extended π-conjugation results in deviation of chromophores from planarity, which prevents antiparallel alignment in the bulk. To gain deeper insight into structure-property relationships, bond length alternation (BLA) values were evaluated based on BLA, correlating with NLO performance. Furthermore, density functional theory (DFT) calculations at the B3LYP/6–31 + G** level included both static and dynamic energy-dependent hyperpolarizabilities, showing good agreement with experimental optical and NLO results.