Ring-current aromaticity in triplet states of 4n π electron monocycles

Abstract

The ipsocentric approach to orbital contributions to current density predicts that a triplet state of a 4n p-electron H€ u uckel monocycle should be aromatic, in the sense of supporting a diatropic ring current, and that this current should be carried by just four of the p electrons (three for n ¼ 1). The magnetic criterion of aromaticity is thus in agreement with BairdÕs classic energy-based prediction of aromatic stabilisation for these species.

Reversal of Hückel (anti)aromaticity in the lowest triplet states of hexaphyrins and spectroscopic evidence for Baird's rule

“…This is ascribed to the inherent instability of most antiaromatic molecules, which can induce distortions to nonaromatic (or less antiaromatic) forms 32,33,41,42 . In summary, the spectroscopic features of Hückel aromatic and antiaromatic hexaphyrins in their respective T 1 states, when combined with calculations of standard aromaticity indices, provide experimental evidence for the reversal of Hückel (anti)aromaticity as compared to the corresponding ground states. Moreover, both sets of findings, namely experimental and theoretical, are in good agreement with previous theoretical analyses that predicted a reversal of (anti)aromaticity in T 1 states of such simple annulenes as benzene, cyclobutadiene and cyclooctatetraene 5,[8][9][10][11][12][13][14][15][16][17][18][19][20] . On the basis of the present findings, we suggest that the ability to manipulate (anti)aromaticity through selection of a desired electronic state, ground-state singlet or excited-state triplet, will provide a new tool for modulating the chemical features and reactivity of highly conjugated systems.…”

“…This suggestion, put forward in 1972 on the basis of perturbation molecular orbital theory, predicted that the Hückel (anti)aromaticity observed for ground-state systems would become reversed in the lowest triplet state. In other words, annulenes with [4n]π-electrons would exhibit aromatic character, while [4n + 2]π-electron systems should show antiaromatic character in direct contrast to what is seen in the corresponding ground states 5 . Baird's pioneering prediction [5][6][7] has been supported by an enormous body of theoretical work, including studies based on aromatic indices such as nucleus-independent chemical shifts (NICS), the aromatic stabilization energy (ASE) and the harmonic oscillator model of aromaticity (HOMA) [7][8][9][10][11][12][13][14][15][16][17][18][19][20] . There have also been numerous experimental observations in relation to excited-state aromaticity, particularly by Wan and colleagues, for example, by studying stable planar [4n]π-annulenes in their lowest triplet (T 1 ) and singlet excited (S 1 ) states 6,[21][22][23][24] . Furthermore, Ottosson and colleagues observed the aromatic chameleon spectroscopic features of fulvene based on sensitization, in accord with their previous theoretical model 7,25,26 .…”

Double aromaticity and ring currents in open-shell systems

“…Qualitative prediction of ring current properties of [n]annulenes and related species, both closed [21,24] and open-shell [4,25] is straightforwardly achieved by using symmetry selection rules in conjunction with the Frost-Musulin diagram [26]. A general model for open-shell states of all-carbon rings can be constructed by adapting the Nested Frost-Musulin diagrams that already provided a convenient method of predicting electron occupancy and currents within the p out and p in subsystems of the closed-shell members of this family [10].…”

Triplet‐State Aromaticity of 4nπ‐Electron Monocycles: Analysis of Bifurcation in the π Contribution to the Electron Localization Function

“…[1] Later high-level quantum chemical calculations by Gogonea, Schleyer, and Schreiner supported Baird's theory, as demonstrated by planarization, bond-length equalization, aromatic stabilization energies (ASE), negative nucleus-independent chemical shifts (NICS), downfield 1 H NMR shifts, and significant diamagnetic orbital susceptibilities. [2] Fowler et al used the sense of the ring current in open-shell states of 4np-electron monocycles as evidence of their aromatic character, [3] and the identification of planar triplet ground states of C 5 H 5 + and C 5 Cl 5 + gives experimental support for Baird's analysis. [4,5] Still, applications of Baird's theory for the rationalization of triplet-state properties of conjugated molecules are scarce.…”