New functionalised heterohelicenes : syntheses and applications

Helicenes are a class of ortho-condensed aromatic or heteroaromatic non planar and helical-shaped molecules. This kind of conjugated system allows for possible use in various fields: such as a spacer in classical push-pull NLO materials, chiral ligands for catalysts and asymmetrical molecular recognition. In this thesis I have focused on the tetrathia[7]helicene ([7]TH) which, in principle, should be easily functionalised. The promising future of [7]TH has been hampered by some major drawbacks in its synthesis. Two of these are, firstly, the need to perform at least one photocyclisation in the reaction sequence, which usually limits the amount of processable helicene (also due to the insolubility of the olefinic precursor of the helicene) and, secondly, the multistep synthetic path affording a very low overall yield (10% from the starting materials). During the doctorate studies I have set up two new approaches for the synthesis of [7]TH which can be summerised as follow: 1)introducing trialkylsilyl groups (R = SiMe3, Si(i-Pr)3) in the thiophene rings, which lead to the soluble E alkenes precursors of the corresponding helicenes; 2) stereoselective synthesis of some substituted alkene in the Z configuration. These Z isomers show higher solubility than their trans counterparts, because of the different packing of the molecules in the solid state, and also because we could provide them with substituents on the double bond; moreover, they are easily cyclised due to their favourable configuration. By these two approaches I have synthesised and characterised several new helicenes and their NLO properties are currently under investigation. In collaboration with the CNR of Milan, I have also investigated the fluorescence properties of a series of molecular precursor of helicenes, and in particular, using a highly fluorescent silylated olefinic compound, it has been developed an OLED device with an external QY efficiency of 0.4%, and emission in the blue.