Experimental and Theoretical Evidence for Aromatic Stabilization Energy in Large Macrocycles

Abstract

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 π-electrons. Here we demonstrate that ASE can be detected experimentally in π-conjugated porphyrin nanorings with Hückel circuits of 76-108 π-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 (DFT) calculations reproduce the results for both types of experiment, and predict ASEs in the range 1-16 kJ mol -1 . The experimental ASE in porphyrin nanorings are compared with an experimental ASE of [18]annulene of approximately 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 around 37 kJ mol -1 in [18]annulene. This work contributes towards fundamental understanding of aromaticity in large macrocycles.

How Aromatic Are Molecular Nanorings? The Case of a Six‐Porphyrin Nanoring**

“…Hence, if the corresponding experimental values are only −1.87 ppm and −2.83 ppm, respectively, this is clearly suggesting that c‐P6⋅T6 6+ is either very weakly aromatic or non‐aromatic, as the CAM‐B3LYP results indicate. This result is further reinforced by a recent finding [57] that the experimental aromatic stabilization energy of c‐P6⋅T6 6+ is ca. 1.2 kcal mol −1 , which is relatively small for an aromatic molecule.…”

How Aromatic Are Molecular Nanorings? The Case of a Six‐Porphyrin Nanoring**

“…These results bring about two important conclusions.F irst of all, it reinforces the idea that CAM-B3LY P(or M06-2X) are more adequate methods to obtain the geometry of potential aromatic molecules than B3LYP,w hich incurs large delocalization errors.Second, through various aromaticity probes,we have shown that the B3LYP geometry of c-P6 6+ corresponds to aq uite aromatic molecule with d o' Àd o = À7.23 ppm, and Dd a = À12.26 ppm. Hence,ifthe corresponding experimental values are only À1.87 ppm and À2.83 ppm, respectively,this is clearly suggesting that c-P6•T6 6+ is either very weakly aromatic or non-aromatic,a st he CAM-B3LYP results indicate.This result is further reinforced by arecent finding [57] that the experimental aromatic stabilization energy of c-P6•T6 6+ is ca. 1.2 kcal mol À1 ,w hich is relatively small for an aromatic molecule.…”

How Globally Aromatic Are Six-Porphyrin Nanorings?

“…[5] Global (anti)aromaticity was also confirmed by magnetic susceptibility measurements, [2] and by analysis of oxidation potentials and rates of conformational exchange. [7] Global ring currents were not detected in the ground states of neutral nanorings, but variation in the fluorescence behavior of neutral c-PN[bN] cyclic oligomers as a function of ring size, N, indicate that the neutral singlet excited states exhibit global (anti)aromaticity. [4] In a recent Research Article, [9] Casademont-Reig, Guerrero-Avilés, Ramos-Cordoba, Torrent-Sucarrat, and Matito compared our experimental 1 H NMR chemical shifts for c-P6[b6] Q (Q = 0, +4, +6 and +12) with those that they calculated using two different functionals, B3LYP and CAM-B3LYP.…”

“…Recently, we published experimental evidence for global aromaticity and antiaromaticity in porphyrinbased nanorings with Hückel circuits of up to 162 π-electrons. [2][3][4][5][6][7][8] These macrocycles are dramatically larger than previouslyreported aromatic rings, and we have suggested that studies on such large macrocycles could shed light on the similarity between aromatic ring currents and persistent currents in nonmolecular quantum rings. [8] They may also help elucidate a link between aromatic ring currents and the currents that flow through molecular wires driven by a voltage bias.…”