Synthesis, crystal structures, and second-order nonlinear optical properties of new chiral ferrocenyl materials

A series of chiral enantiomerically pure analogues of (E)-(2-(4-nitrophenyl)ethenyl)ferrocene (2), substituted by R (3, R = Me; 4, R = CH2OH; 5, R = SiMe3), in the 2-position on the cyclopentadienyl ring, were synthesized. Measurements by electric-field-induced second-harmonic generation (EFISH) and calculations by INDOS/CI-SOS show that the different chromophores have closely related molecular NLO responses. However, the crystal packings for the different compounds, observed by X-ray diffraction on a monocrystal, are completely different. In any case, the chirality of the constituting molecules avoids the centrosymmetry of the crystals and therefore preserves the NLO efficiency in the solid state. In the case of 3, the crystal packing is close to the centrosymmetry and so the bulk NLO efficiency is low (6 times that of urea, lambda = 1.907 mu m) but not nil, as is the case for 2. For 4, the crystal packing is much better for NLO (efficiency 20 times that of urea, lambda = 1.907 mu m) and is almost optimized for 5 (efficiency 100 times that of urea, lambda = 1.907 mu m). The correlation between molecular and bulk NLO responses was studied using the model proposed by Zyss.