Chemiluminescence of a Firefly Luciferin Analogue Reveals that Formation of the Key Intermediate Responsible for Excited State Generation Occurs on a Fully Concerted Step.

The chemiluminescence (CL) reaction of eight different 2-(4-hydroxyphenyl)-4,5-dihydrothiazole-4-carboxylate esters with an organic superbase and oxygen was investigated through a kinetic and computational study. These esters are all analogues to the luciferin substrate involved in efficient firefly bioluminescence. The kinetic data obtained from CL emission and light absorption assays were used in the context of linear free energy relationships (LFER); we obtained the Hammett reaction constant ρ = +1.62 ± 0.09 and the Brønsted constant ßlg = -0.39 ± 0.04. These observations from LFER, together with activation parameters obtained from Arrhenius plots, suggest that the formation of the high-energy intermediate (HEI) 1,2-dioxetanone occurs via a concerted mechanism during the rate-determining step of the reaction. Calculations performed using density functional theory support a late transition state for HEI formation within the reaction mechanism pathway, which was described considering geometric parameters, Wiberg bond indices from natural bond order analysis, and the atomic charges derived from the electrostatic potential.

Evidence for the Formation of 1,2-Dioxetane as a High-Energy Intermediate and Possible Chemiexcitation Pathways in the Chemiluminescence of Lophine Peroxides.

A kinetic study of the chemiluminescent (CL) reaction mechanism of lophine-derived hydroperoxides and silylperoxides induced by a base and fluoride, respectively, provided evidence for the formation of a 1,2-dioxetane as a high-energy intermediate (HEI) of this CL transformation. This was postulated using a linear Hammett relationship, consistent with the formation of negative charge on the transition state of HEI generation (ρ > 1). The decomposition of this HEI leads to chemiexcitation with overall low singlet excited state formation quantum yield (ΦS from 1.1 to 14.5 × 10-5 E mol-1); nonetheless, ΦS = 1.20 × 10-3 E mol-1 was observed with both peroxides substituted with bromine. The use of electron-donating substituents increases chemiexcitation efficiency, while it also reduces the rate for both formation and decomposition of the HEI. Different possible pathways for HEI decomposition and chemiexcitation are discussed in light of literature data from the perspective of the substituent effect. This system could be explored in the future for analytical and labeling purposes or for biological oxidation through chemiexcitation.

The decomposition of triphenylimidazole-para-acetate follows specific base catalysis and can be conveniently followed by fluorescence.

The kinetics of the decomposition reaction of 4-(4,5-diphenyl-1H-imidazol-2-yl)phenyl acetate (1) in basic alcoholic media was investigated, using a simple fluorescence (FL) spectrophotometric procedure. The process was conveniently studied using FL, since the triphenylimidazole-derived ester 1 and its reaction products (the corresponding phenol 2 and phenolate 2- ) are all highly fluorescent (ΦFL  > 37%). By carefully selecting excitation and emission wavelengths, observed rate constants k1 in the order of 10-3 to 10-2  s-1 were obtained from either reactant consumption (λex  = 300 nm, λem  = 400 nm) or product formation (λex  = 350 nm, λem  = 475 nm); these were shown to be kinetically equivalent. Intensity-decay time profiles also gave a residual FL intensity parameter, shown to be associated to the distribution of produced species 2 and 2- , according to the basicity of the medium. Studying the reaction in both methanol (MeOH) and isopropanol (iPrOH), upon addition of HO- , provided evidence that the solvent's conjugate base is the active nucleophilic species. When different bases were used (tBuO- , HO- , DBU and TEA), bimolecular rate constants kbim ranging from 4.5 to 6.5 L mol-1  s-1 were obtained, which proved to be non-dependent on the base pKaH , suggesting specific base catalysis for the decomposition of 1 in alcoholic media.