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

Michihisa Ueda, Kjell Jorner, Young Mo Sung, Tadashi Mori, Qi Xiao, Dongho Kim (), Henrik Ottosson (), Takuzo Aida () and Yoshimitsu Itoh ()
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Michihisa Ueda: The University of Tokyo
Kjell Jorner: Uppsala University
Young Mo Sung: Yonsei University
Tadashi Mori: Osaka University
Qi Xiao: The University of Tokyo
Dongho Kim: Yonsei University
Henrik Ottosson: Uppsala University
Takuzo Aida: The University of Tokyo
Yoshimitsu Itoh: The University of Tokyo

Nature Communications, 2017, vol. 8, issue 1, 1-9

Abstract: 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.

Date: 2017
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DOI: 10.1038/s41467-017-00382-1

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