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

Abstract

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 (β) equal to 15.5 and 21.7 × 10⁻³⁰ cm⁵ esu⁻¹, respectively at 1.064 μm, which arise from a charge transfer capability along the molecular C₂ symmetry axis, in relation to the π-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 β 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 C₂ symmetry requirement, this situation implies that β 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.