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. The specific rotation of tetrafluoro[6]­helicene 6 enantiomers was found to be approximately 25% lower than
that of unsubstituted [6]­helicene. The theoretical study of the racemization
barrier suggested a reasonable shift toward higher energy with increasing
fluorination. The increasing fluorination also significantly affected
the intermolecular interactions in the crystal lattice. The observed
CH···F interactions led to the formation of 1D-molecular
chains in the crystal structures of both fluorinated helicenes.