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

Abstract

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.

7-Cyano-15-methoxy[6]helicene through a photochemical pathway and investigation of its chiroptical and photophysical properties

“…Thus, no other regioisomer like an S-shaped compound was identified nor isolated from the reaction mixture [41][42][43]. Photodimers such as 4 have been observed during the photocyclization step [44] but not in this study. The whole photocyclodehydrogenation process proceeded via isomerization of (E )-2 to (Z )-2 followed by an intramolecular electrocyclization leading to trans-dihydro [6]helicenes that are oxidized into the [6]helicene.…”

Helicenes Fused with Hexafluorocyclopentene (HFCP): Synthesis, Structure, and Properties

“…The [6]‐ and [4]‐helicene moieties in 13 have the same helical chirality, as shown in Figure a, sharing a peripheral benzene ring. The structures of the two helicene moieties in 13 are similar to those of the parent [6]helicene and [4]helicene,[5e], [14a] comparing the distances between the peripheral benzene rings in the helicene skeletons (Figure a–c). In the packing diagram (Figure d), a substantial number of intermolecular CH ··· F short contacts were observed.…”

“…Recently, helicenes bearing fluorine substituents have been reported by several groups (Figureà) . Fluorine‐substitution of PAHs has been extensively investigated in order to improve the properties of such materials, including solubility, oxidation stability, electron‐acceptability, n‐type carrier mobility, and crystallinity .…”

Oxidative Photocyclization of Aromatic Schiff Bases in Synthesis of Phenanthridines and Other Aza-PAHs

“…A sequential photocyclization of diimines could lead to the formation of helical diaza-PAHs, similarly to the widespread photochemical synthesis of helicenes [ 41 , 42 ] which often utilizes a double cyclization of two double stilbene-like bonds. For this purpose, diimines 9a , b were synthesized from bis-triflate 8a or dibromoarene 8b , respectively, by coupling with benzophenone imine ( Scheme 7 ).…”