Triplet state homoaromaticity: concept, computational validation and experimental relevance

Abstract

Conjugation through space can give rise to aromaticity in the lowest excited triplet state, with impact for photochemistry.

Impact of Excited-State Antiaromaticity Relief in a Fundamental Benzene Photoreaction Leading to Substituted Bicyclo[3.1.0]hexenes

“… 4 Further support for this conclusion is found through computations ( Figure 2 ). Although the benzenium ion has a clear T 1 homoaromatic structure, 51 decay from this C s -symmetric structure to the S 0 state leads back to the C 2 v -symmetric benzenium ion 2 and not to the bicyclo[3.1.0]hexenium cation 3 . Also, there is no stationary point in the T 1 state that corresponds to a bicyclo[3.1.0]hexenium ion (the vertically excited T 1 -state structure of 3 is a non-stationary point 58.9 kcal/mol above the S 0 state).…”

Exciton Self-Trapping in sp2 Carbon Nanostructures Induced by Edge Ether Groups

“…Thereby, all the edge-oxidized C54 ether geometries in the S1 state are globally nonplanar, except for C54–Op7 (see Figure S7). This structural planarization is associated with the fact that a planar configuration at the S1 state geometry favors the enlargement of the π-conjugation network, which provides stabilization to the excited state. ,, In comparison, this local planarization was not found for the center-oxidized C54 ethers (see Figure S9 for distance changes of the carbon–carbon atoms of the ether moiety after excited state relaxation); as can be expected, the cyclic ether rings embedded by the outmost edge sp 2 carbon network are significantly restricted to achieving a planar configuration during excited-state relaxation. In fact, the relaxed S1 geometry of each center-oxidized C54 ether only shows a slight structural difference from the ground-state geometry, mainly involving slight distance changes in regard to the C–C bonds.…”

Magnetically induced ring currents in metallocenothiaporphyrins

“…[46][47][48] Analogously, the triplet states of (С 5 Н 5 ) 2 FeP and H 2 -(С 5 Н 5 ) 2 FeP are expected to be aromatic and antiaromatic, respectively, according to Baird's aromaticity rule. [48][49][50] The same aromaticity rules can be applied to the isoelectronic (С 5 Н 5 ) 2 CoP + and H 2 -(С 5 Н 5 ) 2 CoP + cations as well as to the corresponding compounds containing the 4d metals: Reversed aromaticity rules are expected for the corresponding Ni compounds with two electrons more than the Fe containing compounds. The Cr and Mo containing compounds with two valence electrons less than [Fe]-cenothiaporphyrin and the valence isoelectronic Mn + and Tc + cations are also expected to have reverse aromaticity compared to the corresponding [Fe]cenothiaporphyrin.…”

“…The antiaromatic singlet states become an aromatic triplet state and vice versa, following the Baird rule for triplet-state aromaticity. 49,50,52 The MIRC strengths are summarized in Table 1. Previously calculated nucleus-independent magnetic shielding functions and experimental 1 H NMR chemical shifts also suggested that the singlet state of (С 5 Н 5 ) 2 FeP is antiaromatic and the singlet state of H 2 -(С 5 Н 5 ) 2 FeP is aromatic.…”