Determination of geometry of 1:1 molecular complexes in solution by hyper-Rayleigh scattering

Abstract

Based on the theoretical finding that the geometry of 1:1 complexes formed between two molecules containing planar aromatic rings, governs the relative contribution of the molecular first hyperpolarizability tensor elements to the total hyperpolarizability (β), we have demonstrated how the geometry of such complexes in solution can be described. Hyper-Rayleigh scattering (HRS) technique has been used to measure the second harmonic (SH) scattered light from the complexes in a polarization resolved manner along two mutually perpendicular directions (X and Z) in the laboratory frame (XY being the plane of linear and circular polarization and Z being the direction of propagation of the polarized incident light beam). The macroscopic depolarization ratios, D and D' obtained from the polarization resolved HRS measurements are analyzed to obtain the geometry of two 1:1 electron donor-acceptor complexes between p-xylene and tetrachloro-p-benzoquinone, and durene and dichlorodicyano-p-benzoquinone. The typical values of the two depolarization ratios, D and D', for a planar molecule with C_2v symmetry are 0.2 and 0.33, respectively. The D and D' values obtained for the complexes are higher at ~0.24 and ~0.4, respectively. This implies a deviation of the complex geometry from the C_2v symmetry that is expected from a perfect cofacial or T-shape structures. Perhaps, a twisted V shape with tilt and twist between the two molecular planes would explain the geometry of the complexes.