Experimental and Theoretical Evidence for Aromatic Stabilization Energy in Large Macrocycles

Area of Science:

• Organic Chemistry
• Physical Chemistry
• Supramolecular Chemistry

Background:

• Aromaticity, a key feature of cyclic molecules, confers enhanced thermodynamic stability.
• Previous studies on aromatic stabilization energy (ASE) were largely confined to small ring systems (up to 18 π-electrons).
• The experimental detection of ASE in larger π-conjugated systems remained a significant challenge.

Purpose of the Study:

• To experimentally detect and quantify aromatic stabilization energy (ASE) in large π-conjugated porphyrin nanorings.
• To investigate the extent of aromaticity in macrocycles exceeding the traditional 18 π-electron limit.
• To compare experimental findings with theoretical calculations and established aromatic systems like [18]annulene.

Main Methods:

• Analysis of redox potentials to calculate energy changes for isodesmic reactions (aromatic <-> antiaromatic).
• Measurement of energy barriers to conformational equilibria that disrupt aromaticity.
• Density functional theory (DFT) calculations to support experimental observations and predict ASE values.

Main Results:

• Experimental detection of ASE in cationic porphyrin nanorings with Hückel circuits of 76-108 π-electrons.
• Measured ASEs in porphyrin nanorings ranged from 1-5 kJ mol⁻¹.
• DFT calculations predicted ASEs in the range of 1-16 kJ mol⁻¹, consistent with experimental data.

Conclusions:

• Aromatic stabilization energy is experimentally verifiable in large π-conjugated porphyrin nanorings.
• This study extends the understanding of aromaticity to macrocycles with significantly more than 18 π-electrons.
• The findings contribute to a fundamental comprehension of aromaticity in extended π-systems and large macrocycles.