Structural, Optical, and Electrochemical Properties of Three-Dimensional Push–Pull Corannulenes

Electrochemically active corannulene derivatives with various numbers of electron-donating 4-(N,N-dimethylamino)­phenylethynyl (1–4) or electron-withdrawing cyanobutadienyl peripheral substitutents (5–8) were prepared. The latter derivatives resulted from formal [2 + 2] cycloaddition of cyanoolefins to 1–4 followed by retro-electrocyclization. Conformational properties were examined by variable-temperature NMR and X-ray diffraction and opto-electronic properties by electronic absorption/emission spectra and electrochemical measurements; these analyses were corroborated by dispersion-corrected density functional calculations at the level of B97-D/def2-TZVPP. In CH2Cl2, 1–4 exhibit intramolecular charge-transfer (ICT) absorptions at 350–550 nm and green (λem ∼ 540 nm) or orange (600 nm) fluorescence with high quantum yields (56–98%) and are more readily reduced than corannulene by up to 490 mV. The variation of optical gap and redox potentials of 1–4 does not correlate with the number of substituents. Cyanobutadienyl corannulenes 5–8 show red-shifted ICT absorptions with end-absorptions approaching 800 nm. Intersubstituent interactions lead to distortions of the corannulene core and lower the molecular symmetry. NMR, X-ray, and computational studies on 5 and 8 with one cyanobutadienyl substituent suggested the formation of intermolecular corannulene dimers. Bowl-inversion barriers around ΔG⧧ = 10–11 kcal/mol were determined for these two molecules.