Nonlinear optical activity in dipolar organic-lanthanide complexes

A series of fifteen dipolar polymeric lanthanide complexes of trans-cinnamic acid, [Ln(C₉H₇O₂) ₃]_n, which crystallize in the non-centrosymmetric space groups R3c and P2₁, have been synthesized and the crystal structures of 14 of these have been determined. From the crystallographic data, two different coordination scenarios for these complexes have been found: Type I, CN = 9, Ln = La, Ce, Nd, Sm, Eu, Gd and Tb, and Type II, CN = 7, Ln = Y, Dy, Ho, Er, Tm, Yb and Lu. The linear and nonlinear photophysical properties of these complexes have been studied using various laser excitation wavelengths. The low temperature photoluminescence spectrum shows a distortion from the C₃ site symmetry of the Eu³⁺ complex. Strong second harmonic generation (SHG) has been observed for both Types I and II in the solid state. The normalized SHG intensities of the members of the entire series of lanthanide complexes have been compared and the solution state hyperpolarizabilities have also been measured for the study of the effects of 4f-electrons on nonlinear photophysical properties. These SHG parameters show maximum values near the center of the lanthanide series, as opposed to previously reported fairly linear or independent variations.