Second-Harmonic Generation within the P2₁2₁2₁ Space Group, in a Series of Chiral (Salicylaldiminato)tin Schiff Base Complexes

A series of 10 (2−11) new chiral (salicylaldiminato)tin Schiff base complexes obtained from the Schiff base condensation of 4-(diethylamino)salicylaldehyde and various amino acids, in the presence of diphenyltin oxide, is reported together with the parent achiral (1) derivative. Compounds 2−7 crystallize in the P2₁2₁2₁ orthorhombic space group, and their quadratic nonlinear optical properties are investigated. At the molecular level, the derivatives possess similar electronic spectra (λ_max ≈ 395 nm), and hence molecular hyperpolarizabilities (β), in relation to the same π-conjugated “push−pull” electronic structure. In the solid state, they exhibit efficiencies in second-harmonic generation (SHG) up to 8 times that of urea. The SHG intensities of chromophores 2−6 appear largely correlated with a simple “degree of chirality” (d_χ) parameter, defined from the molecular geometries. The intriguing issue of a possible quantification of chirality by means of SHG measurement is addressed.