Electrogenerated chemiluminescence 73: acid–base properties, electrochemistry, and electrogenerated chemiluminescence of neutral red in acetonitrile

We characterized the acid–base properties, electrochemistry, and electrogenerated chemiluminescence (ECL) of neutral red in acetonitrile (MeCN). To determine the acid–base properties, the basic form (NR) of neutral red was prepared and titrated with anhydrous HClO4, yielding pKa = 6.5 in MeCN. NR showed three irreversible electrochemical oxidations and a one-electron reversible reduction. However, the acid form (NRH+) was reduced at more positive potentials than NR in a two-electron transfer reaction. Benzoyl peroxide (BPO) was used as a co-reactant to generate ECL on reduction of NR because it produces a strong oxidizing agent ( C 6 H 5 CO 2 ) on reduction. A high concentration of BPO quenched ECL. NRH+ did not produce ECL because its reduction was by the transfer of two electrons and the energy available in the ECL reaction was not sufficient to generate an excited state. However, the NR ECL showed a maximum intensity at 610 nm, which corresponds to the λmax of NRH+ fluorescence rather than that of NR. The ECL intensity of NR depended strongly on the concentration of NR, BPO, and trifluoroacetic acid (TFA). The integrated ECL intensity of NR, compared with that of Ru ( bpy ) 3 2 + , yielded a relative ECL intensity ratio, INR/IRu(II) of ∼0.05.