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

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 (COTSaddle)-C-Th4 and (CDHScrew)-C-Th6, 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 (CDHScrew)-C-Th6, which inverts through a nonplanar transition state, the inversion of (COTSaddle)-C-Th4, progressing through a planar transition state, was remarkably accelerated upon photoexcitation. As predicted by Baird's theory, the planar conformation of (COTSaddle)-C-Th4 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).