Chemiluminescence emission in Fenton reaction driven by 1,2-dihydroxybenzenes: Mechanistic approaches using 4-substituted ligands

Abstract

Fenton (F) and Fenton–like (FL) reactions can be amplified by dihydroxybenzenes (DHBs). These compounds chelate and reduce Fe(III), promoting the hydroxyl radical production (OH). The products or intermediaries of F and FL reactions driven by DHBs can produce chemiluminescence (CL) with different profiles, depending on the type of DHB involved. In this work, CL produced by F and FL systems driven by different -para substituted DHBs was measured and compared with the reactivity of each system and with the structural parameters of each DHB. CL emission was not related to the reactivity of each studied system but was favored by DHBs substituents with -NHR and –OH groups combined in the branching (NHR-DHBs). PLS multivariate regression models were constructed using computational parameters for each DHB, quinone (Q) and semiquinone (SQ) to find the influence of structural and electronic parameters over CL emission. Analysis showed that in NHR-DHBs, the higher CL exhibited could be explained by cycling ability of these compounds. In DHBs with an electron-donor group (EDG) the CL emission would depend only on the stability of the intermediary species generated by DHB and OH reaction. While DHBs with electron-withdrawing groups (EWG) showed that CL will increase depending on the stability of the intermediaries by resonance, and by the acidity of the hydroxyl protons of the ring. PLS-SQ showed that spin densities were strongly correlated with an increase in CL emission. DHBs with substituents that favor the delocalization of charge in the SQ to the ramification would enhance CL emission. Meanwhile, when the delocalization is promoted over the DHB-ring, these systems become more reactive, and the CL emission is disadvantaged by quinone formation.