Abstracts

Linear free energy relationships (LFER) have long been used as tools for the determination of reaction mechanisms in physical organic chemistry, particularly to characterize the transition state of key steps within a given transformation. In this context, chemiluminescence (CL) reactions have benefited from LFER such as the Hammett (ρ), Yukawa-Tsuno (ρ, r), and Brønsted (β_lg) plots to gain valuable insights regarding the identity of in situ generated intermediates. For example, it has been observed that the formation of the 1,2-dioxetanone acting as the high-energy intermediate (HEI) of a CL system analog to the firefly luciferin substrate occurs on a fully concerted step (ρ = +1.62, r = 0.22, β_lg = –0.39 _{in ACN}, Mello et al., J. Org. Chem. 2024, 89, 345). The generation of the cyclic peroxidic dimer of CO₂ (namely, 1,2-dioxetanedione) as the HEI of the peroxyoxalate transformation (PO-CL) has also been confirmed by LFER (ρ = +2.2, β_lg = –1.1 _{in water}, Silva et al., J. Org. Chem. 2021, 86, 11434). Indeed, the PO-CL system has been investigated under the scope of Hammett plots on various occasions (Alves et al., Photochem. Photobiol. Sci. 2015, 14, 320; Maruyama et al., 2013, 252, 222; Koike et al., Chem. Commun. 2003, 794; Silva et al., Luminescence 2002, 17, 362). Important aspects of the complex reaction sequence involved in the CL oxidation of lophine were also unveiled by Hammett plots (White and Harding, Photochem. Photobiol. 1965, 4, 1129; Philbrook and Maxwell, Tetrahedron Lett. 1964, 5, 1111), particularly to sustain the hypothesis that a 1,2-dioxetane is involved as HEI (ρ = +1.5, Boaro et al., J. Org. Chem. 2021, 86, 6633). The occurrence of an intramolecular electron transfer in the thermal CL decomposition of acridinium-substituted 1,2-dioxetanes was also proposed based on substituent effects (ρ = +1.3, Ciscato et al., J. Org. Chem. 2010, 75, 6574). This presentation will focus on these and other specific examples of CL transformations, giving a comprehensive view of how the use of LFER led to a better understanding of CL mechanisms while also suggesting possible pathways of “enlightening” with these tools.