Electrochemistry of 6-Nitro-1′,3′,3′-trimethylspiro[2H-1-benzopyran-2,2′-indoline] associated with zeolite Y and MCM-41 silicates.: Light-driven site-selective electrocatalytic effect on N,N,N′,N′-tetramethylbenzidine oxidation

The electrochemical response of open/closed forms of 6-nitro-1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'indoline] (SP) associated with zeolite Y (SP@Y) and MCM-41 silicates with or without framework Al (SP@AlMCM and SP@MCM, respectively) is described upon immersion of zeolite-modified polymer film electrodes into MeCN using LiClO4, Et4NClO4, and Bu4NPF6 as supporting electrolytes. These samples show a reverse photochemical behavior, the open form undergoing ring closure to the closed one upon illumination with visible light (lambda > 450 nm). Oxidation processes at +1.10 and +0.80 V vs AgCl (3 M NaCl)/Ag, corresponding to open and closed SP, respectively, exhibit a significant reversibility, suggesting that post-electron-transfer reactions resulting in the apparently irreversible voltammetry observed in solution are highly hindered here. Reduction of all boundary-associated SP forms occurs in two reversible one-electron steps involving the nitrobenzene group. Upon irradiation, SP@Y shows a remarkable site-selective electrocatalytic effect on the oxidation of N,N,N,N'-tetramethylbenzidine (TMB) in Et4NClO4 and LiClO4 electrolytes. Rotating disk voltammetry indicates that the rate-determining step can be attributed to the formation of a surface adduct between TMB and closed SP associated with the zeolite boundary. The value of the second-order rate constant of this process is 2.2 x 10(5) L/(mol s) in 0.10 M Et4NClO4/MeCN.