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

Abstract

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.