How Dimerization Through a Spiro Junction Modifies the Nonlinear Optical Properties of a Push–Pull Organic Dye: Insights from Theory and Hyper-Rayleigh Scattering

The second-order nonlinear optical properties of a push-pull monomer, as well as of its dimer composed of two monomeric units connected through a non-conjugated spiro junction, are investigated by means of Hyper-Rayleigh Scattering and quantum chemical calculations. It is shown that dimerization with an orthogonal geometry enhances the first hyperpolarizability by ~20%, while it decreases the dipolar character of the HRS response. Theoretical calculations correlate well with experimental measurements and allow an in-depth rationalization of the impact of the spiro linkage onto the NLO properties, through a detailed analysis of the relevant electronic excited states.