Excited‐state aromaticity and antiaromaticity special issue

The application of aromaticity and antiaromaticity to reaction mechanisms

“…In contrast, Baird's rule states that species with 4n + 2 π-electrons are antiaromatic in the lowest triplet state and those with 4n π-electrons are aromatic [8] . Excited-state aromaticity could become a cornerstone for photochemistry but is less developed [ 9 , 10 ]. Currently, excited-state (anti)aromaticity can be used to explain some observations of photophysical and photochemical behaviors [11] , [12] , [13] .…”

Polycyclic Heteroaromatic π‐Linkers Provide Dithienylethene Switches with Favorable Thermal and Photochemical Properties for Solar‐Energy Storage

“…Regarding photochemical reactivity, it has already been mentioned that the onset of ESAA in the benzene π‐linker of T‐ 1o promotes the photocyclization of this switch [13,15] . Moreover, as further support for this result, it has also been shown that the manifestation of excited‐state aromaticity (ESA) [28–30] in the cyclobutadiene π‐linker of a dithienylcyclobutadiene switch (system T‐ 2o in Scheme 1) completely inhibits the photocyclization of this compound [15] . However, besides ESA, one other factor that could exert a negative influence on the photochemical reactivity of T‐ 2o is the larger ring strain in cyclobutadiene as compared to benzene.…”

“…In these switches, the two thiophenes are typically bridged by an aromatic π-linker, such as benzene, [13,15] tetrafluorobenzene, [12] naphthalimide, [5] pyrazine, [10] quinoxaline [10] or helicene. [10] In our own recent work, we have reported both experimental and computational evidence that a T-type dithienylethene harboring a benzene π-linker is able to undergo photocyclization in a fast and efficient manner, thanks to the onset of excited-state antiaromaticity (ESAA) [28][29][30] in the benzene moiety upon light absorption by the ring-open T-1o form (see Scheme 1) of the switch. [13,15] During the course of the reaction, the π-linker loses this ESAA to finally become nonaromatic in the ground state of the ring-closed T-1c form (see Scheme 1).…”