Experimental and Theoretical Evidence for Aromatic Stabilization Energy in Large Macrocycles

Enhanced thermodynamic stability is a fundamental characteristic of aromatic molecules, yet most previous studies of aromatic stabilization energy (ASE) have been limited to small rings with up to 18 pi-electrons. Here we demonstrate that ASE can be detected experimentally in pi-conjugated porphyrin nanorings with Hiickel circuits of 76-108 pi-electrons. This conclusion is supported by analyzing redox potentials to calculate the energy change for isodesmic reactions that convert an aromatic ring to an antiaromatic ring or vice versa. It is also supported by analyzing the energy barriers to conformational equilibria that disrupt aromaticity in the transition state. Both types of experiment indicate that cationic porphyrin nanorings display ASEs of 1-5 kJ mol(-1). Density functional theory calculations reproduce the results for both types of experiment and predict ASEs in the range of 1-16 kJ mol(-1). The experimental ASEs in porphyrin nanorings are compared with an experimental ASE of [18]annulene of similar to 11 kJ mol(-1), deduced from analysis of the energy barriers to conformational equilibria in [16], [18], and [20]annulene. Calculated energies of isodesmic reactions give an ASE of similar to 37 kJ mol(-1) in [18]annulene. This work contributes to a fundamental understanding of aromaticity in large macrocycles.

Automated summary

The study demonstrates the detection of ASE in pi-conjugated porphyrin nanorings experimentally, with DFT calculations predicting a similar range of ASE. Comparison with [18]annulene suggests similar ASE in porphyrin nanorings, contributing to the fundamental understanding of aromaticity in large macrocycles.