Chirality control symmetry breaking

The asymmetric polymerization in crystalline architectures provides an excellent environment to conduct the absolute asymmetric synthesis of polymers, and also provides an effident route for the ampHfication of chirality. Mirror-symmetry breaking might occur either through total asymmetric transformations, either in enantiomorphous crystals that have self-assembled from achiral molecules, or within racemic crystalline architectures which are delineated by chiral rims or surfaces when one of the chiral faces is blocked by an interface. The self-assembly of nonracemic mixtures into a mixture comprising eutectic compositions of a racemic compound and an enantiomorphous assembly, followed by asymmetric transformation, provides a series of thermodynamically controlled, alternative routes for the effident ampHfication of homochirality. 

Symmetry breaking associated with chiral phenomena is a theme that recurs across the sciences—from the intricacies of the electroweak interaction and nuclear decay [1-3] to the environmentally influenced dimorphic chiral structures of microscopic planktonic foraminifera [4, 5], and the genetically controlled preferential coiling direction seen in the shells of snail populations [6, 7]. 

Here, the final three terms are a Ginzburg-Landau expansion in powers of i j. The coefficient t varies as a function of temperature and other control variables. When it decreases below a critical threshold, the system undergoes a chiral symmetry-breaking transition at which i becomes nonzero. The membrane then generates effective chiral coefficients kHp = k n>i f and kLS = which favor membrane curvature and tilt modulations, respec-... 

M. Shapiro Our approach [M. Shapiro and P. Brumer, Controlled Photon Induced Symmetry Breaking Chiral Molecular Products from Achiral Precursors J. Chem. Phys. 95, 8658 (1991)], differs from that of Quack in that we show how to generate chirality (from achiral precursors). 

Although not immediately obvious, this control scenario relies entirely upon quantum interference effects. To see this note that in the absence of an e0 (0 pulse, excitation from D) or L) to level /), for example, occurs via one photon excitation with e, (0, i = 1, 2. In this case, as noted above, there is no chiral control. By contrast, with nonzero e0 (0 there is an additional (interfering) route to [ /), i.e., a two-photon route using e7 (t) excitation to level ),j i, followed by an e0 (0-induced transition from Ej) to Ei). The one- and two-photon routes interfere, thus causing symmetry breaking transitions. 

In this way, the above two methods are very effective to control ee and choose one of the chiral domains. It is noted that both methods use no chiral molecular species. Instead, CPL and twist cell geometry are used as chiral stimuli, which act as a symmetry-degeneracy-breaking field. This field triggers and accelerates a preferential formation of one of the two possible chiral conglomerates, which is fixed in a B4-like phase. The monochiral films obtained by achiral molecules open... 

---