Molecular engineering of heteroleptic metal-dithiolene complexes with optimized second-order NLO response

The structure-property relationship for the class of square-planar d 8 metal heteroleptic dithiolene complexes, which exhibit negative solvatochromism and high molecular first hyperpolarizability, is discussed on the basis of experimental and theoretical results. The role of each ligand and of the metal in the electronic distribution, is highlighted with the view to design candidates for optimal 2nd NLO response. This class of complexes, bearing groups with different electron-withdrawing capability attached to the dithiolene core, behaves as intervalence compounds where ligands occur in formally different oxidation states, one reducing (dithiolate, prevailing contribution to the highest occupied molecular orbital (HOMO) at high energy, the donor) and the other oxidizing (dithione, prevailing contribution to the lowest unoccupied molecular orbital (LUMO) at low energy, the acceptor). The HOMO-LUMO transition, mediated by the d orbitals of coordinated metal, has thus ligand to ligand or mixed metal-ligand to ligand charge transfer character and is responsible for the 2nd order NLO activity.Joint experimental and theoretical results highlight the factors affecting the high negative second-order polarizability values, which are relatable to the high difference in dipole moments between excited and ground state enhanced by the electric field of the solvent, the large oscillator strength for the charge transfer transition, and the relatively low energy gap. Moreover, in this geometrically defined donor-metal-acceptor arrangement, steric factors affect the electronic distribution and thus the NLO properties. Therefore, torsion angle at the dithione ligand inversely correlates with the oscillator strength and the molecular first hyperpolarizability.The properties of a polymethylmetacrylate film incorporating a sample belonging to this class of complexes and showing a good second-harmonic generation, are also discussed to support the potential for applications in optoelectronics of this class of 2nd order NLO-chromophores.