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 (λ_em ∼ 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.