The effect of the conformation on the quadratic nonlinear optical response of metal carbonyl based chromophores with one-dimensional charge transfer capabilities: a computational investigation

The synthesis and the quadratic nonlinear optical (NLO) properties of (4-(4'-dimethylaminophenyl)pyridine)chromium and tungsten pentacarbonyl are reported. Experimentally, the chromophores exhibit molecular hyperpolarizabilities (beta) equal to 15.5 and 21.7 x 10(-30) cm(5) esu(-1), respectively at 1.064 mu m, which arise from a charge transfer capability along the molecular C-2 symmetry axis, in relation to the pi-conjugated structure of the ligand. At a more fundamental level, these molecules have been investigated as possible synthetic targets to illustrate the ZINDO prediction that, in chromophores with one-dimensional charge transfer capabilities a specific set of substituents could be found to switch the beta direction, on passing from a ground state geometry with co-planar aromatic rings to a conformation in which the rings are perpendicular. Due to the C-2 symmetry requirement, this situation implies that beta is necessarily vanishing for a specific conformation. The possibilities to observe experimentally and to use this intriguing effect in a perspective of a molecular switch are critically evaluated.