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.