Limits in the second-order response of [M(H2imXdt) (H2imYdt)] neutral complexes (M=Ni, Pd, Pt; H2imXdt=monoanion of imidazolidine-2-chalcogenone-4,5-dithione; X=O, S, Se; Y=O, S, Se; X≠Y): a pure theoretical study based on TD-DFT approach and ZORA formalism

Static and dynamic first hyperpolarizabilities, associated with an external electric perturbation in the dipole approximation, have been calculated for the neutral [M(H2imXdt)(H2imYdt)] (M=Ni, Pd, Pt; H2imXdt=monoanion of imidazolidine-2-chalcogenone-4,5-dithione; X=O, S, Se; Y=O, S, Se; X≠Y) model complexes within the TD-DFT approach. These complexes have been never synthesized, but they are useful for studying the NLO activity of the skeleton of the centrosymmetric neutral [M(RR′timdt)2], synthesized and characterized, removing the inversion center without extensive change to the original molecule.The simple two-state model did not allow to explain our results, suggesting the importance of other excited states on the second order response. Furthermore the ZORA (zero order regular approximation) results show an increase of the beta tensors with respect to the non-relativistic ones, which is roughly proportional to the atomic number of the coordinated metal. These findings may indicate that transitions strongly involving the metal are relevant for the second order response. Although the studied complexes show a large first hyperpolarizability, in particular the Pt- complexes (the [Pt(H2imOdt)(H2imSdt)] complex shows <f>βvec(SHG)=34.8×10−30</f> esu at 0.50 eV), it is unable to compete with the organic materials extensively used for the non-linear optical applications. On the other hand, it is noteworthy that we studied pseudo-centrosymmetric chromophores. Their response is expected to be only a starting point and this study a suggestion for achieving better results by modifying atoms found to be strongly involved in transitions related to the response. [Copyright &y& Elsevier]