Dependence of Electrochemical and Electrogenerated Chemiluminescence Properties on the Structure of BODIPY Dyes. Unusually Large Separation between Sequential Electron Transfers

Electrochemistry and electrogenerated chemiluminescence (ECL) of selected substituted BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes have been studied. The location and nature of substituents on positions 1-8 are important in predicting the behavior, and especially the stability, of the radical ions formed on electron transfer. Dyes with unsubstituted positions 2, 6, and 8 show a kinetic contribution to both oxidation and reduction. Dyes with only unsubstituted positions 2 and 6 and a substituted 8 position show chemically reversible reduction but irreversible oxidation. Unsubstituted positions 2 and 6 tend to show dimer formation on oxidation. Completely substituted dyes show nernstian oxidation and reduction. Oxidation and reduction studies of simple BODIPY dyes show an unusually large separation between the first and second reduction peaks and also the first and second oxidation peaks, of about 1.1 V, which is very different from that observed for polycyclic hydrocarbons and other heteroaromatic compounds, where the spacing is usually about 0.5 V. Electronic structure calculations confirmed this behavior, and this effect is attributed to a greater electronic energy required to withdraw or add a second electron and a lower relative solvation energy for the dianion or dication compared with those of the polycyclic hydrocarbons. ECL was generated for all compounds either by annihilation or by using a co-reactant.