In today’s episode of “Fun With Chirality”, we have a look at phenomenon that could be very useful, come out of the blue, and which the very authors who report it have no explanation for. This is from a new paper in Angewandte Chemie from a team in Germany (TU-München) who have been looking at the behavior of enantiomers when adsorbed onto a glass surface (in this case, plain old borosilicate BK7, nothing fancy).
To their no doubt great surprise, they found that shining circularly polarized laser light onto such a surface causes selective desorption of enantiomers. They were able to coat the glass with various thicknesses of a racemic mixture of BINOL (1,1′-bi-2-naphthol, a classic chiral ligand) by evaporating it in vacuo, and hit this with laser pulses out at 600 and 650nm (which is outside the absorption range of the compound). Measurements of optical activity taken by second harmonic generation circular dichroism (SHG-CD, a very sensitive technique) showed that the remaining film (at the irradiated spot) gradually became more optically active as repeated circularly polarized laser pulses hit it – and this activity was invariably in the same direction for each handedness of the light pulses. This happened at both wavelengths, but linearly polarized light did not show the effect at all. They have, they say, no choice but to conclude that the circularly polarized laser pulses are selectively desorbing one enantiomer over the other, and I certainly don’t see any other explanation for the data, either.
But how that’s happening is another question. Laser desorption is supposed to be pretty much a pure thermal process. The energy of the incident photons is coupled right into vibrational modes, and the molecule just shakes itself right off the surface. And while that’s surely a big part of the story, there must be some quantum mechanical component if circular polarization makes such a difference, and the authors sort of throw their hands up at this point. They have a model of the overall process (that is, desorption versus heating of the surrounding molecules in the film) if you stipulate such a QM contribution, but where that comes from has yet to be put on a sound basis.
Interestingly, this model fits the observed data the best if there is a rapid and quite selective clearance (90% ee) of one enantiomer from the upper layers of the film as the first laser pulses hit. That, the authors say, is probably an overestimate, and experiments with slightly enantioenriched films suggest a lower bound of 20% ee for the process, so it’s probably somewhere between those two (which admittedly doesn’t tell you much beyond what you could have guessed while standing next to the instrument with a puzzled expression!) But the fact that it happens at all is extraordinary – this could lead to completely new ways to analyze and perhaps even purify optically active compounds.
You’re probably wondering (I was) what happens when you made a film of enantiomerically pure BINOL and try zapping that. Unfortunately, the pure enantiomers crystallize out in structured optically active layers when you form films of them, as opposed to the far more amorphous-looking racemic films (an effect which had already been described in the literature). And this leads to very nonlinear and hard-to-reproduce anisotropy in the films – they have larger-scale structures in them which are optically active themselves, and the data go all over the place. No doubt there will be other odd effects that show up as this phenomenon is further explored, and there are a lot of experiment that immediately suggest themselves that I’m sure the authors are banging away on right now. But if there’s a method to separate and characterize enantiomers that doesn’t rely on chiral reagents but only on polarized laser light, a lot of us would like to hear more about it!