Energetics of Baird aromaticity supported by inversion of photoexcited chiral [4n]annulene derivatives

Abstract

For the concept of aromaticity, energetic quantification is crucial. However, this has been elusive for excited-state (Baird) aromaticity. Here we report our serendipitous discovery of two nonplanar thiophene-fused chiral [4n]annulenes Th4 COT Saddle and Th6 CDH Screw, which by computational analysis turned out to be a pair of molecules suitable for energetic quantification of Baird aromaticity. Their enantiomers were separable chromatographically but racemized thermally, enabling investigation of the ring inversion kinetics. In contrast to Th6 CDH Screw, which inverts through a nonplanar transition state, the inversion of Th4 COT Saddle, progressing through a planar transition state, was remarkably accelerated upon photoexcitation. As predicted by Baird’s theory, the planar conformation of Th4 COT Saddle is stabilized in the photoexcited state, thereby enabling lower activation enthalpy than that in the ground state. The lowering of the activation enthalpy, i.e., the energetic impact of excited-state aromaticity, was quantified experimentally to be as high as 21–22 kcal mol–1.

Electron‐Deficient Bipyrrole Boomerangs: Bright Fluorophores Obtained via Double C−H Bond Activation

“…The use of 1 equiv of silver(I) carbonate and 10 mol % of palladium(II) acetate produced cNDA1 EE in near‐quantitative yield. Ag 2 CO 3 could be replaced with AgOAc, Ag 2 O, or AgF, whereas AgNO 3 and Cu(OAc) 2 were found to be ineffective. 2,3‐Dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) and percarboxylic acids provided moderate yields of cNDA1 EE when used as stoichiometric oxidants.…”

Is Excited‐State Aromaticity a Driving Force for Planarization of Dibenzannelated 8π‐Electron Heterocycles?

“…Quantum chemical calculations tell that COT in the S 1 and T 1 states exhibits planar D 8h structures and magnetic properties typical of high degree of aromaticity . Several experimental observations related to large structural changes in the excited states, when compared to the S 0 state, have been reported for COT and a number of COT derivatives . With regard to dibenzannelated heterocycles, dibenz[ b,f ]oxepin displays a large Stokes’ shift and well‐defined vibrational fine structure in the fluorescence spectrum, evidences that a change from a V‐shaped to a planar conformation occurs in the S 1 state (Figure ), and similar findings were made for dibenz[ b,f] thiepin.…”

Unraveling Excited-Singlet-State Aromaticity via Vibrational Analysis

“…[9][10][11][12][13] Particularly, understanding the role of excited-state aromaticity in the structural changes and the stabilization and destabilization of molecules upon irradiation enables mechanistic elucidation of organic photoreactions and expansion of synthetic protocols. [14][15][16] Excited-state aromaticity was first described by Baird. He proposed that the Hü ckel aromatic and anti-aromatic characteristics are reversed in the lowest pp* triplet (T 1 ) state (known as Baird's rule).…”