New internal-charge-transfer second-order nonlinear optical chromophores based on the donor ferrocenylpyrazole moiety

A series of new N-arylated ferrocenepyrazole structures, carrying different donor or acceptor substituents in the para position of the aryl ring, has been synthesized by the Chan-Lam cross-coupling reaction. The nonplanar geometric molecular structure of some of these chromophores together with their crystal packing was determined by X-ray diffraction, and the HOMO and LUMO energy levels were evaluated by electrochemical and optical measurements and by density functional theoretical (DFT) calculations. By the investigation of solvent effects and time-dependent DFT (TD-DFT) calculations, the intense electronic absorption band around 270-310 nm was confirmed to be an internal-charge-transfer (ICT) band, showing a significant red shift by increasing the electron withdrawing properties of the substituent on the para position of the aryl ring. TD-DFT calculations and electric field induced second harmonic generation (EFISH) measurements of the quadratic hyperpolarizability have shown that also the second-order nonlinear optical (NLO) response of these new ICT chromophores can be tuned by changing the nature of the substituent. Both theoretical μβ and β and experimental EFISH μβ1907 and β1907 values are significant, with a quite satisfactory correlation of the general trend of theoretical μβ and EFISH μβ1907 values. The highest value of EFISH μβ1907 (410 × 10-48 esu) was measured for the chromophore carrying the strong electron withdrawing NO2 group