Aromatic and antiaromatic ring currents in a molecular nanoring.

Area of Science:

• Supramolecular Chemistry
• Physical Organic Chemistry
• Quantum Mechanics

Background:

• Aromatic and antiaromatic molecules exhibit distinct electron delocalization and ring currents (diatropic and paratropic, respectively).
• Persistent currents in mesoscopic rings share similarities with molecular ring currents but are studied in different size regimes.
• The relationship between molecular aromaticity and quantum coherence in mesoscopic rings remains poorly understood.

Purpose of the Study:

• To investigate ring currents in molecules of intermediate size, bridging the gap between molecular and mesoscopic systems.
• To explore the connection between aromaticity and quantum-coherence effects in macrocyclic systems.
• To demonstrate the tunability of aromatic/antiaromatic behavior through oxidation state manipulation.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy was employed to probe electronic properties.
• Density Functional Theory (DFT) calculations were used to model and understand the electronic structure.
• Investigation focused on a six-porphyrin nanoring template complex with a 2.4 nm diameter.

Main Results:

• The six-porphyrin nanoring exhibited antiaromatic behavior (80 π electrons) in its 4+ oxidation state.
• The same nanoring displayed aromatic behavior (78 π electrons) in its 6+ oxidation state.
• The antiaromatic state showed significant paramagnetic susceptibility without unpaired electrons.

Conclusions:

• Adjusting the oxidation state of a macrocycle can promote global ring currents by suppressing local component currents.
• This study demonstrates aromaticity and antiaromaticity in a 7.5 nm circumference molecule at room temperature.
• Quantum coherence can persist in surprisingly large molecular frameworks, expanding the scope of aromaticity studies.