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This review focuses on photocyclization reactions involving alkenes and arenes. Photochemistry opens up synthetic opportunities difficult for thermal methods, using light as a versatile tool to convert stable ground-state molecules into their reactive excited counterparts. This difference can be particularly striking for aromatic molecules, which, according to Baird’s rule, transform from highly stable entities into their antiaromatic “evil twins”. We highlight classical reactions, such as the photocyclization of stilbenes, to show how alkenes and aromatic rings can undergo intramolecular cyclizations to form complex structures. When possible, we explain how antiaromaticity develops in excited states and how this can expand synthetic possibilities. The review also examines how factors such as oxidants, substituents, and reaction conditions influence product selectivity, providing useful insights for improving reaction outcomes and demonstrating how photochemical methods can drive the development of new synthetic strategies. 

Geminal hyperconjugation is a key electronic effect that modulates bond distances, barrier widths, and thermochemical driving forces in heavy-atom tunneling reactions involving opening and closing three-membered rings. 

Aromaticity and antiaromaticity are pivotal concepts in chemistry, with significant implications for molecular properties and reactivity. 

Cation tuning is a simple yet powerful strategy to modulate the reactivity of polymerization catalysts but the design rules to achieve maximum cation effects are not well understood. In the present work, it was demonstrated that inserting a methylene spacer between a nickel phenoxyimine complex and an M-polyethylene glycol (PEG) (where M = Li+, Na+, K+, or Cs+) unit led up to >70-fold increase in ethylene polymerization activity and 6-fold higher polymer molecular weight relative to that of the first-generation catalysts. It is hypothesized that these effects are due to the exclusive formation of 1:1 over 2:1 nick-el:alkali species and closer proximity of the M-PEG moiety to the nickel center. These results suggest that the successful creation of cation-responsive catalysts requires an understanding of the cation binding stoichiometry as well as the structural and electronic changes associated with its host-guest interactions. 

Abstract Whether tetra‐tert‐butyl‐s‐indacene is a symmetricD_2hstructure or a bond‐alternatingC_2hstructure remains a standing puzzle. Close agreement between experimental and computed proton chemical shifts based on minima structures optimized at the M06‐2X, ωB97X‐D, and M11 levels confirm a bond‐localizedC_2hsymmetry, which is consistent with the expected strong antiaromaticity of TtB‐s‐indacene. 

The self-trapping nano-loop structures of [1]rotaxanes exhibited multiple Förster resonance energy transfer (FRET) OFF/ON patternsviadual and sequential locking/unlocking upon UV exposure.