Molecular and macromolecular hetero[n]helicenes: synthesis and applications

Helicenes are polycyclic aromatic compounds formed by ortho-fused aromatic rings. The steric hindrance of the substituents and/or the terminal aryl rings forces the molecular backbone to assume a helical shape, typical of these intrinsically chiral systems. Therefore, the molecule can wind into two opposite directions: dextrorotatory for the (P)-enantiomer; or levorotatory for the (M)-enantiomer. This PhD dissertation is devoted to the synthesis of Thia-Bridged Triarylamine Hetero[4]Helicenes (TBTH[4]H): a peculiar example of [4]helicenes structurally reminiscent of parent triarylamines and phenothiazines. This similarity confers these systems remarkable red-ox properties and mono-electron oxidation to the corresponding radical cations has been reported. The introduction of thioether bridges in the parent substrates makes these molecules structural analogues of [4]helicenes, nevertheless, thanks to the long C–S bonds, their racemization energy barrier is roughly 17 kcal/mol higher than the corresponding [4]carbohelicenes, making instead, these thia-helicenes configurationally stable at room temperature. The synthesis of these compounds is achieved via an initial sulfenylation reaction on a triarylamine or a N-aryl phenothiazine using phthalimidesulfenyl chloride and a subsequent Lewis Acid-mediated (typically AlCl3) ring closure. This methodology tolerates the presence of various functional groups and in different positions, giving access of structurally diverse substrates. Their configurational stability allows HPLC resolution using an amylose-based chiral stationary phase, allowing to study their intriguing chiroptical properties. The main objectives and achievements of this PhD work regard: i) the design of a new chalcogen-mediated organocatalytic strategy for the synthesis of these compounds; ii) the optimization of an optical resolution strategy; iii) the study of the red-ox pathways occurring in TBTH[4]H; iv) synthesis and red-ox properties of TBTH[4]H homodimers; v) the incorporation of TBTH[4]H in norbornene-type polymers and the study of their red-ox properties; vi) assembly of nanostructured materials for spintronics applications. The last chapter (vii) is devoted to the research work carried out under the supervision of Prof. Scott Denmark (UIUC, USA), regarding chalcogen-mediated stereoselective synthesis.