Polarization rotation in second harmonic reflection from an isotropic layer of a chiral Troger base

Abstract

Second harmonic reflection (SHR) from molecules deposited on a surface has proved to be a sensitive probe of the molecular chirality. We focus here on ORD-SHR' which inimicks the usual linear technique of Optical Rotation Dispersion: the second harmonic polarization is analyzed for a p-polarized fundamental beam. Considering an isotropic film of a pure enantiomer, any rotation of the plane of polarization of the second harmonic beam compared to the p direction is a signature of the chirality of the molecules. It is a consequence of the breakdown of the symmetry of the experiment due to the lack of symmetry of the individual molecules. However, this polarization rotation in SHG does not appear for all the chiral molecules and is strongly dependent on the microscopic mechanism of the optical activity. The nonlinear optical activity indeed originates either from dipolar electric effects if the chirality arises from an excitonic coupling, or from dipolar magnetic or quadrupolar electric effects if the molecule exhibits a one electron chirality. We have shown that ORD-SHG was effective only in the former case, for an excitonic coupling chirality. Our point here is to measure the dispersion of the polarization rotation in SHG around the first electronic transition and to reproduce it by a microscopic model of the inolecular chirality. For that purpose, we specially selected the acridinesubstituted Troger base displayed on figure 1. The delocalized electrons along the acridine structures allow to observe SHR and the two acridines form the two noncolinear oscillators responsible for the excitonic coupling. Furthermore, this molecule is able to fix on a silica substrate in a well-defined way owing to its amine terminal groups. SHR experiments were performed with a 82 MHz Titanium-sapphire laser tunable in the 700-1000 nm range, which covers the absorption structure of the Troger base, once-frequency-doubled by the SHR process. We measured very large rotation angles, increasing continuously froin 10' to 70' when spanning the absorption structure. We performed all the experiments with both enantiomers and always observed opposite results, which proves unambiguously that the optical activity we measured is due to the molecular chirality. We then calculated the second order hyperpolarizability of this molecule introducing nonlinearities in the Kuhn model (see figure 1). This calculation was simplified by use of the electric dipolar approximation as the magnetic dipolar contributions are negligible for an excitonic coupling chirality. After orientational averaging over an isotropic surface, we determined analytical expressions for the chiral and achiral components of the second order susceptibility: where D, = w2- w2 -2iyw corresponds to the detuning between the laser at frequency w and the absorption peak at WO (widthy). IC is the coupling strength between the two oscillators and can be estimated from geometrical considerations. p characterises the oscillator anharmonicity responsible for the nonlinear response. The rotation angle is then calculated as the arctan of the ratio chirallachiral. It shows a very good agreement with our experimental results: it reproduces qualitatively the overall shape of the dispersion curve and quantitatively the magnitude of the rotation angles we measured.