On the viability of achieving chiral separation through the optical manipulation of molecules

Abstract

Several different optical methods have recently been proposed for the potential separation of chiral molecules according to their intrinsic handedness. Applying fundamental symmetry and electrodynamical principles provides a perspective that casts doubt over the viability of some of the more extravagant claims. However there is a genuine basis for achieving chiral separation by using circularly polarized light to deliver chirally sensitive optical forces. The mechanism comes into play when molecules (or nanoscale particles) are optically trapped in a laser beam by forward Rayleigh scattering, as a result of trapping forces that depend on positioning within the beam profile. In such a setup, chiral molecules experience subtle additional forces associated with a combination of electric and magnetic transition dipoles; when circularly polarized light is used for the trapping, a discriminatory response can be identified that has the capacity to separate left-and righthanded molecular isomers. Here, clear differences can be observed between the behavior of isotropic liquids and poled solutions or liquid crystals. Detailed analysis provides an objective basis to assess new prospects for the recognition and differentiation of molecules with opposite chiral form, identifying and paving the way for future commercial applications.

Discriminatory optical force for chiral molecules

“…Unfortunately, the form reported by Bradshaw and Andrews for ∆W , as seen above, is incorrect and their neglect of electric-quadrupole contributions cannot be justified. Consequently, the results seen in equations ( 10), ( 13), ( 15), ( 16), ( 17), ( 18) and ( 19) of [12]; (4), ( 5), ( 7) and ( 8) of [13]; (10), ( 11), ( 12), ( 13), ( 17), ( 19) and ( 21) of [14] and ( 1) and ( 2) of [16] are in error, as they are not uniquely defined: their forms change with changes in the location chosen for the origin of the molecular multipole expansion. An energy shift and associated force cannot take values that depend on the manner in which we choose to calculate them.…”

“…Bradshaw and Andrews have expressed their view that dispersive optical forces derive from "forward Rayleigh scattering" [12,13,14,15,16]. Specifically, that these forces arise as the gradients of energy shifts, the forms of which follow from second-order perturbation theory:…”

“…However, from a fundamental photonic perspective, such a system cannot relate to a chiral force (or a discriminatory energy) since the initial and final states are not identical [30]. This is readily demonstrated: since two beams are required for the proposed mechanism, and the process is elastic (no overall excitation of relaxation of molecular electronic state occurs) then any contributory mechanism has to involve a scattering event in which a photon is annihilated from one beam and a photon created into the other Since the initial and final radiation states of such a process are not identical, energy shifts cannot arise, and it becomes evident that there are no grounds for discriminatory forces to arise" [13] (see also [12]). That this line of reasoning is flawed is obvious from the fact that it does not give rise to the experimentally verified dipole optical force in an optical lattice, as discussed above.…”

“…We write in response to a number of recent publications authored by Bradshaw and Andrews [12,13,14,15,16] on what is, essentially, our discriminatory optical force for chiral molecules. Our work [1,2], as well as the other founding contributions [8, 9, 10] described above, is barely acknowledged by Bradshaw and Andrews except, that is, to make claims against it: in particular that helicity fringes do not give rise to the effects predicted by us [1,2].…”

Controlled Enantioselective Orientation of Chiral Molecules with an Optical Centrifuge

“…Chirality is a ubiquitous natural phenomenon met in various areas of science and technology ranging from physics of fundamental forces and practical aspects of drug design to studies on the origin of life and biomolecular homochirality [1]. Differentiation and separation of enantiomers in a mixture is an important problem, involving measurements of enantiomeric excess, handedness of a given compound, and devising techniques for manipulating chiral molecules [2][3][4][5][6][7][8][9][10][11][12][13][14][15].…”