Fig. 1.

[4n]Annulene derivatives with and without Baird aromaticity upon photoexcitation. a, b, Molecular structures and ORTEP drawings (50% ellipsoid probability) of ^Th4 COT _Saddle (a) and ^Th6 CDH _Screw (b), which have 8π-electron and 12π-electron annulene cores (red colored), respectively. c, d, Schematic illustrations of the energy barriers for the ring inversion events of ^Th4 COT _Saddle (c) and ^Th6 CDH _Screw (d), where colored arrows represent the movement directions of the thiophene rings of the same color. ^Th4 COT _Saddle and ^Th6 CDH _Screw invert through planar and nonplanar transition states, respectively, as shown in the square brackets. Black and pink-colored solid curves represent energy barriers in the ground and photoexcited states, respectively. Upon photoexcitation, the activation enthalpy for the ring inversion (∆H ^‡ _inv) of ^Th4 COT _Saddle is lowered by 21–22 kcal mol^–1, but that of ^Th6 CDH _Screw remains unchanged