Non-centrosymmetric crystals of new Nbenzylideneaniline derivatives as potential materials for non-linear optics

摘要

Three new N-benzylideneaniline derivatives [p-nitrobenzylidene-p-phenylamineaniline (I), 2,4-dinitrobenzylidene-p-phenylamineaniline (II) and pdinitrobenzylidene- p-diethylamineaniline (III)] containing electron–push–pull groups have been prepared. They present a planar N-benzylideneaniline core and neighbouring functional atoms, which are related through an efficient intramolecular charge transfer (CT). Two of the derivatives crystallize in noncentrosymmetric space groups, a necessary condition for non-linear optical (NLO) responses. The NLO properties were calculated for the molecular conformations determined by single-crystal X-ray diffraction as well as for the four molecules packed into each corresponding unit cell, using a quantumchemical method at the cam-B3LYP/NLO-V level of theory. As expected from antiparallel face-to-face stacking through centrosymmetry, the main NLO descriptors – namely, the first hyperpolarizability (β_(tot)) and its projection on the dipole moment direction (β_(vec)) – are almost zero for the tetramer of derivative III. Interestingly, the calculated first hyperpolarizability decreases in the noncentrosymmetric unit-cell content of derivative II when compared to its single molecule, which may be related to its molecular pillaring, similar to that observed in derivative III. On the other hand, a desirable magnification of the NLO properties was found for packed units of derivative I, which may be a consequence of its parallel face-to-tail stacking with the CT vectors of all molecules pointing in the same direction. Moreover, the CT vector of compound I makes an angle of θ = 33.6° with its crystal polar axis, resulting in a higher-order parameter (cos~3θ = 0.6) compared with the other derivatives. This is in line with the higher macroscopic second-order NLO response predicted for derivative I, β_(tot) = 120.4 × 10~(-30) e.s.u.