Chirality

New Optical Probes of Chiral Molecules

Conventional optical activity phenomena, such as optical rotation and circular dichroism, are based on the interference of induced oscillating electric- and magnetic (and electric-quadrupole) moments, and arise from a differential response to left and right circularly polarized light. Circularly polarized light is clearly chiral, and so it is remarkable that the action of three linearly polarized electric fields can also be a chiral probe in a homogeneous and isotropic liquid. This is indeed the case, when two optical fields of different frequency interact coherently to generate light at their sum- or difference frequency [1-3]. SFG is in general a purely electric-dipolar nonlinear-optical process and the signal photons from an optically active liquid are only generated, if the molecules are chiral.

In the study of SFG from liquids, we have observed a new electro-optic effect, i.e. the influence of a static electric field on the intensity of SFG. The effect arises when a static electric field is applied to coherent sum-frequency generation in an optically active liquid. The static field does not change the phase matching conditions of the sum-frequency process, but it gives rise to an electric-field induced contribution to the signal. The beat between chirality-sensitive SFG (a second-order process) and achiral electric-field induced sum-frequency generation (a third order process) yields a contribution to the intensity that is linear in the static electric field and that changes sign with the enantiomer. The effect can therefore be used to determine the absolute sign of the isotropic part of the sum-frequency hyperpolarizability and hence the handedness of chiral molecules in solution [4].

[1] P. Fischer, D.S. Wiersma, R. Righini, B. Champagne, and A.D. Buckingham, Three-wave mixing in chiral liquids, Phys. Rev. Letters, 85 (2000), 4253-4256.
[2] P. Fischer, K. Beckwitt, F.W. Wise, and A.C.Albrecht, The chiral specificity of sum-frequency generation in solutions, Chem. Phys. Letters, 352 (2002), 463-468.
[3] P. Fischer, F.W. Wise, and A.C. Albrecht, Chiral and achiral contributions to sum-frequency generation from optically active solutions of binaphthol, J. Phys. Chem. A, 107 (2003), 8232-8238.
[4] P. Fischer, A.D. Buckingham, K. Beckwitt, D.S. Wiersma, and F.W. Wise, A new electro-optic effect: Sum-frequency generation from optically active liquids in the presence of a dc electric field, Phys. Rev. Letters, 91 (2003), 173901.
For a recent review article, see: P. Fischer and F. Hache, “Nonlinear optical spectroscopy of chiral molecules”, Chirality, 17 (2005), 421.