posted on 2019-03-23, 00:00authored byXuefeng Lu, Tullimilli Y. Gopalakrishna, Yi Han, Yong Ni, Ya Zou, Jishan Wu
The
synthesis of carbon nanobelts (CNBs) with well-defined size
and structure remains a challenging topic in nanocarbon chemistry
and materials science. Herein, we report the synthesis, physical characterization,
and supramolecular properties of two fully conjugated bowl-shaped
CNBs (CNB1/CNB2), in which six/eight cyclopenta-rings
are fused onto a macrocycle containing three/four alternately linked
2,7-pyrenyl and 2,7-phenanthryl units. The existence of five-membered
rings results in a bowl-shaped geometry, as revealed by X-ray crystallographic
analysis and density functional theory calculations. Both molecules
contain an alternate aromatic phenanthrene-co-quinoidal
pyrene structure to satisfy Clar’s aromatic sextet rule. The
smaller size CNB1 has a deeper bowl depth (d = 4.997 Å) than CNB2 (d = 3.682
Å) and cannot undergo bowl-to-bowl (BTB) inversion below 373
K in toluene. However, the larger size CNB2 shows a smaller
BTB inversion barrier (∼12 kcal/mol) at the coalescent temperature
(248 K), which was estimated by variable-temperature NMR measurements.
Both compounds exhibit a small energy gap and amphoteric redox behavior
with multiple redox waves. The dications of CBN1 and CBN2, and the tetracation of CBN2, are experimentally
accessible by chemical oxidation with NO·SbF6, all
displaying unusual open-shell singlet diradical character with a small
singlet–triplet energy gap (ΔES–T = −2.71 kcal/mol for CBN12+, −2.50 kcal/mol for CBN22+, and −2.00 kcal/mol for CBN24+). The dications are globally aromatic while the tetracations
are globally antiaromatic according to NMR measurements and theoretical
calculations (anisotropy of the induced current density, nucleus independent
chemical shift, and 2D isochemical shielding surface). The small bowl-shaped CNB1 demonstrates selective encapsulation of fullerene C70 over C60, with a large association constant (Ka = 8.066 × 105 M–1 in toluene). However, the larger size CNB2 does not
exhibit any encapsulation with both C60 and C70.