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)-(C 6 H 4 )-(C 6 H 3 )-R {R = H ( 1 ), OCH 3 ( 2 ), CF 3 ( 3 )}] and D-D’-π-A ( 4 – 6 ), [Fc-(OCH 3 -C 6 H 4 )-C = CH=CN-(C 6 H 4 )-(C 6 H 3 )-R {R = H ( 4 ), OCH 3 ( 5 ), CF 3 ( 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.