Conformational, Structural, and Chiroptical Properties of an Overcrowded Triply Fused Carbo[7]helicene

• Published in: Journal of organic chemistry 2024-01, Vol.89 (1), p.498-504
• Main Authors: Artigas, Albert, Ferdi, Nawal, Rémond, Maxime, Rigoulet, Florian, Vanthuyne, Nicolas, Hagebaum-Reignier, Denis, Carissan, Yannick, Naubron, Jean-Valère, Giorgi, Michel, Favereau, Ludovic, Coquerel, Yoann
• Format: Article
• Language: English
• Subjects: Chemical Sciences; Organic chemistry
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/acs.joc.3c02239

Recently, the synthesis of the racemate of an overcrowded triply fused carbo[7]­helicene of formula C66H36 with three carbo[7]­helicenes fused within a central six-membered ring was described. This molecule was found to embed an extremely contorted central six-membered ring and two negative curvatures. We report herein the resolution of the corresponding enantiomers and their conformational, structural, photophysical, and chiroptical properties. The racemization of the triply fused carbo[7]­helicene was determined to proceed at a rate of k rac = 8.06 × 10–4 s–1 at 175 °C in ortho-dichlorobenzene, corresponding to a barrier to enantiomerization ΔG enant ‡ = 140.4 kJ·mol–1, a value significantly lower than for pristine carbo[7]­helicene. Interestingly, the crystalline structures of the racemic and enantiopure materials show some differences regarding the molecular geometry, with an increased negative curvature in the latter cases. This unusual curved delocalized π-conjugated system afforded notably green fluorescence at room temperature and far-red phosphorescence at low temperature. Finally, electronic circular dichroism and circularly polarized luminescence responses of the enantiopure compounds have been measured and showed very close absorption and emission dissymmetry factors, g abs and g lum, respectively, of ca. 2.6 × 10–3, indicating a similar chiral rigid geometry for both ground and excited states.