Chimerical pyrene-based dibenzo[7]helicene rac-1 and 2H-pyran[7]helicene (M,R,R)-(-)-2, in which two pyrene subunits are fused to the [7]helicene/[7]heterohelicene scaffold, were synthesised by means of Ni(0) - or Co(I) -mediated [2+2+2] cycloisomerisation of dipyrenyl-acetylene-derived triynes. Pyrene-based dibenzo[7]helicene 1 was obtained in enantioenriched form by enantioselective cycloisomerisation under Ni(0) /QUINAP catalysis (57 % ee) or in enantiopure form by racemate resolution by liquid chromatography on a chiral column. 1,3-Allylic-type strain-controlled diastereoselective cycloisomerisation was employed in the synthesis of enantiopure (M,R,R)-(-)-2. Physicochemical properties of 1 and 2 encompassing the helicity assignment, stability to racemisation, X-ray crystal structure, UV/Vis, experimental/calculated electronic circular dichroism and fluorescence spectra were studied. Accordingly, comparison of the X-ray crystal structure of (M,R,R)-(-)-2 with calculated structures (DFT: B3LYP/cc-pVDZ, B97D/cc-pVDZ) indicated that its helical backbone is slightly over-flattened owing to intramolecular dispersion forces between tert-butylated pyrene subunits. Both 1 and 2 are fluorescent (with quantum yields in dichloromethane of ΦF =0.10 and 0.17, respectively) and are suggested to form intramolecular excimer states upon excitation, which are remarkably stabilised and exhibit large Stokes shifts (296 and 203 nm, respectively).

Department of Chemistry Memorial University St John's NL A1B 3X7 709 864 3702

Department of Inorganic Chemistry Faculty of Science Charles University Prague Hlavova 2030 8 128 43 Prague 2

Institute of Organic Chemistry and Biochemistry Academy of Sciences of the Czech Republic Flemingovo nám 2 166 10 Prague 6 220 183 133

References provided by Crossref.org

Stereocontrolled Synthesis of Chiral Helicene-Indenido ansa- and Half-Sandwich Metal Complexes and Their Use in Catalysis

Helical Bilayer Nanographenes: Impact of the Helicene Length on the Structural, Electrochemical, Photophysical, and Chiroptical Properties

Dihydrogen contacts observed by through-space indirect NMR coupling