Preparation and Physicochemical Properties of [6]Helicenes Fluorinated at Terminal Rings.

The first racemization-stable helicene derivatives fluorinated at terminal rings, 1,2,3,4-tetrafluoro[6]helicene (6) and 1,2,3,4,13,14,15,16-octafluoro[6]helicene (15), were synthesized via the Wittig reaction followed by oxidative photocyclization in an overall yield of 41% of 6 and 76% of 15. The changed electronic structure in fluorinated helicenes was reflected in a slight shift of UV-vis absorption, fluorescence excitation, and emission spectra maxima when compared to unsubstituted [6]helicene. Cyclic voltammetry revealed a moderate decrease in the HOMO-LUMO gap with increasing fluorination.

2-Bromo[6]helicene as a Key Intermediate for [6]Helicene Functionalization.

The synthesis of 2-bromo[6]helicene was revised and improved up to 51% yield. Its reactivity was thoroughly investigated, and a library of 17 different carbon, boron, nitrogen, phosphorus, oxygen and sulfur substituted derivatives was prepared. The racemization barrier for 2-bromo[6]helicene was determined, and the usage of enantiomers in the synthesis of optically pure helicenes was rationalized.

p-Doping of graphene in hybrid materials with 3,10-diazapicenium dications.

,'-Didodecyl-substituted 3,10-diazapicenium salts featuring bromide and hexafluorophosphate counterions have been designed as novel dopants to realize individualized graphene sheets in a series of cutting edge experiments and to intrinsically stabilize them p-doping. Importantly, electrochemical studies revealed two consecutive irreversible one-electron reductions of the ,'-didodecyl-substituted 3,10-diazapicenium salts to yield the corresponding radical cation and neutral quinoidal species. Formation of both species was accompanied by characteristic changes in the absorption spectra.