Global aromaticity at the nanoscale

The 1H and 19F NMR spectra of cP6·T6F Q+ in oxidation states Q = 0, 2, 4 and 6 provide strong evidence for global diatropic and paratropic ring currents.

“The 1 H and 19 F NMR spectra of cP6·T6F Q + in oxidation states Q = 0, 2, 4 and 6 (Figure 3) provides strong evidence for global diatropic and paratropic ring currents. 1,2”

The NICS plots for cP6 Q+ (Q=0,2,4,6) calculated using LC-ωhPBE (ω=0.1) reproduce the shielding effects observed in the NMR spectra.

“These plots reproduce the shielding effects observed in the NMR spectra (Figure 3).”

In the 2+ oxidation state (Q=2), cP6·T6F 2+ exhibits a global diatropic ring current.

“Q = 2, cP6·T6F 2+ (82 π; aromatic): The 1 H NMR spectrum is broad, but the 19 F resonance is shielded compared with unbound T6F , indicating a global diatropic ring current. The presence of a diatropic ring current is supported by the splitting between THSin and THSout, which were unambiguously assigned through observation of heteronuclear NOEs from THSin to the CF3 group.”

In the 6+ oxidation state (Q=6), cP6·T6F 6+ exhibits a global diatropic ring current.

“Q = 6, cP6·T6F 6+ (78 π; aromatic): The 1 H NMR spectrum of the hexacation exhibits smaller shifts than the other oxidation states, but the oin/oout and THSin/THSout splittings indicate a global diatropic ring current, as confirmed by the 19 F NMR signal.”

In the 4+ oxidation state (Q=4), cP6·T6F 4+ exhibits a large global paratropic ring current.

“Q = 4, cP6·T6F 4+ (80 π; antiaromatic): The , and template signals at 17-25 ppm indicate a large global paratropic ring current. This is confirmed by the wide splitting between THSin and THSout and by the shift of the 19 F NMR signal.”

The direction of the ring current in cP6·T6F Q+ can be switched between aromatic and anti-aromatic by changing the number of π-electrons by oxidation.

“The spectra in Figure 3 show that the direction of the ring current can be switched between aromatic and anti-aromatic by changing the number of π-electrons ( Q = 2) by oxidation.”

The NICS plot for cP6 4+ has a low symmetry (C2 rather than D6h) reflecting the distortion expected in an antiaromatic ring.

“The NICS plot for cP6 4+ has a low symmetry ( C 2 rather than D 6h) reflecting the distortion expected in an antiaromatic ring (analogous to the D 2h geometry of cyclobutadiene). 51”

In the neutral state (Q=0), cP6·T6F has no global ring current.

“Q = 0, cP6·T6F (84 π; non-aromatic): In the neutral state, each individual porphyrin has a normal aromatic ring current, as attested by the shielded and protons of the template, but there is no global ring current; the ortho -aryl protons oin and oout are almost coincident, as are the external and internal trihexylsilyl signals (THSin and THSout).”

Cyclic porphyrin oligomers (cPN) exhibit global ring currents after oxidation, reduction or optical excitation, but not in the neutral ground state.

“The largest aromatic rings yet investigated are cyclic porphyrin oligomers, which exhibit global ring currents after oxidation, reduction or optical excitation, but not in the neutral ground state.”

The ring currents in porphyrin nanorings are well reproduced by density functional theory (DFT), although the choice of DFT functional often turns out to be critical.

“The ring currents in these systems are well reproduced by density functional theory (DFT), although the choice of DFT functional often turns out to be critical.”

LC-ωhPBE (ω = 0.1) generally gives NICS values that are consistent with experimental NMR shifts.

“We found that LC-ωhPBE (ω = 0.1) generally gives NICS values that are consistent with experimental NMR shifts. 2,4,36”

The largest macrocycle yet tested for redox-promoted global aromaticity is cP12.

“The largest macrocycle yet to have been tested for redox-promoted global aromaticity is cP12 . 2”

No global ring current is detected in cP12·(T6F)2 Q+ in any oxidation state.

“In keeping with this simple picture, no global ring current is detected in cP12·(T6F)2 Q + in any oxidation state. 2”

cP12·(T6eF)2 exhibits global diatropic ring currents in the 6+ and 10+ oxidation states and a global paratropic current in the 8+ oxidation state, in agreement with Hückel's rule.

“1 H, 13 C and 19 F NMR spectroscopy reveal global diatropic ring currents in the 6+ and 10+ oxidation states and a global paratropic current in the 8+ oxidation states, in agreement with Hückel's rule; cP12·(T6eF)2 Q + : Q = 6, 162 π aromatic; Q = 8: 160 π antiaromatic; Q = 10: 158 π aromatic.”