Two-State Model of Antiaromaticity:  The Low Lying Singlet States

Abstract

Aromatic and antiaromatic compounds are resonance hybrids of two cyclic covalent Kekulé structures. In both, two combinations can be formed, an in-phase and an out-of-phase one. In aromatic compounds having an odd number of conjugated double bonds, the in-phase combination is the ground state and the out-ofphase one is an excited state. In antiaromatic compounds, having an even number of conjugated electron pairs, the situation is reversed; the ground state is formed by the out-of-phase combination. This causes the ground state of these molecules to be a non-totally symmetric one, which in turn means that it has a biradical character. Moreover, the out-of-phase combination is necessarily unstable, being a transition state between the two bond-alternating Kekulé structures. By comparison to noncyclic biradicals such as perpendicular olefins, the antiaromatic cyclic structures are strongly stabilized, reducing the activation barrier from around 50-60 kcal/mol to around 3-5 kcal/mol. Therefore, the bond-alternating structures are easily interconverted at ambient temperatures and in the process acquire biradical character, making them highly reactive and difficult to synthesize. The in-phase combination of the two Kekulé structures is a strongly stabilized totally symmetric excited state which has a similar geometry to that of the ground transition state.

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 .…”

Cyclobutadiene: The Antiaromatic Paradigm?

“…However, it is difficult to quantitate this repulsion, and the pronounced alternation in bond length in CBD may also be attributed to strong vibronic coupling between the ground electronic state and the first excited singlet state. [23] Two corollaries follow from the above considerations: a) "strain" is a multifaceted property which is composed of several nonadditive effects that were invented by chemists to understand this property, and it cannot be reduced to angular deformation and/or steric repulsion; b) it is therefore not a priori clear why and how this property changes on going from cyclobutane to cyclobutene to CBD. It is unclear whether s antiaromaticity is the same for puckered (as in cyclobutane) and planar (as in CBD) four-membered rings and how it depends on the size of the ring.…”

Spin Switching in Molecular Quantum Cellular Automata Based on Mixed-Valence Tetrameric Units

“…1a, in which the axes X and Y are directed along the diagonals of the square, while in majority of papers these axes are chosen to be directed along the sides of the square (see, for example, refs. 73,74 ). For this reason the labels in the present paper can differ from those in the cited papers but this, of course, does not influence the results.…”