Highly anisotropic helicene-based NLOphores for Two Photon Circular Dichroism (TPCD)

Light comprises oscillating electric and magnetic fields (or alternatively a stream of photons ) propagating through materials. With modern laser techniques it is possible to generate intense irradiation that promotes multi-photon events such as second harmonic generation (SHG: frequency doubling). Visible light interacts with matter by exciting electrons from a low energy state (non-bonding or pi-bonding orbitals) to higher energy states (or virtual excited states in the case of SHG). Conventional dyes used in nonlinear optics combine donor (electron-rich) and acceptor (electron deficient) end-groups with an unsaturated linker. We will demonstrate that when the linker is twisted into a helix, the mirror image forms will allow novel experiments on two-photon circular dichroism (TPCD), an effect that although predicted has never been observed. The class of molecules chosen for study in this project have electrons in helical conjugated pi-systems, and this is the source of their handedness ( anisotropy / chirality ). A simple analogy to illustrate this is that my left-hand feels different if introduced into a right hand glove. The project seeks to define the differences when left-hand light or spin encounters right-handed or left-handed materials.To succeed, we need materials where the left- and right-hand forms are very different ( highly anisotropic ). With a large difference, there is the best prospect of characterising the consequences of the difference. Helices are fundamental left- and right-handed structures that occur in Nature in many ways. Their simplicity is a source both of great beauty and unique potential in scientific research. At a molecular scale, helicenes and heterohelicenes exhibit the same chiral properties (handedness). The choice of heterohelicenes for this project ensures highly anisotropic structures while retaining and extending the general form of the heterocyclic component of the linker in the well-established thiophene-linked ATOP and PETCN classes of NLO-responsive dyes. The approach to be adopted is the design and synthesis of novel helical analogues of ATOP and PETCN , by using tetrathiohelicenes in place of the simple thiophene ring of the conventional dye. This extension of the pi system will itself have a significant effect on the NLO properties, but crucially the helicenyl-linked derivatives have chirality built into the pi system. This will be evaluated as the basis for the discovery of the novel photophysics properties.The project will use new methods of kinetic resolution (KR) of helicenes, and employ multiple KR in the synthesis of the target molecules in exclusively left- or right-handed forms, by a method that exploits the different rates of reaction of the left/right-handed intermediates at several stages during the synthesis. We seek to compare two (D)-(chiral-pi)-(A) designs - (i) an unsymmetrical substitution pattern with a single (D)-(chiral-pi)-(A) feature spanning a full turn of the helix; (ii) a novel example with two shorter (D)-(chiral-p)-(A) structures each occupying a half turn of the helix.Collaborative strategies for experimental and theoretical evaluation of these molecular designs are proposed.