Electrochemistry and Electrogenerated Chemiluminescence of Quinoxaline Derivatives

We report the electrochemistry, spectroscopy, and electrogenerated chemiluminescence (ECL) of donor-acceptor compounds containing a quinoxaline derivative as the central core coupled with fluorene (CFPQ, CFPP, and MFPQ) and triphenylamine moieties (MAPQ). Cyclic voltammetry of all four derivatives show reversible reduction waves (assigned to the formation of the radical anion) localized in the quinoxaline moiety (the acceptor group), whereas oxidation waves show behavior that depends upon the nature of the donor group. With a pair of strong donor groups, triphenylamine, (MAPQ), exhibits a two electron oxidation wave due to the two noninteracting donor moieties. The fluorene derivatives (CFPQ, CFPP, and MFPQ) show less reversible oxidation waves at scan rates of 100 mV/s, but CFPP and CFPQ become reversible at higher scan rates. In their absorbance and emission spectra, all of the compounds, except MFPQ, exhibit large Stokes shifts, which are related to the rearrangement of the excited-state and also to solvent effects. ECL was observed for MAPQ, CFPQ, and CFPP in MeCN/benzene solutions that generally agree with the photoluminescence spectra, suggesting that the energy of annihilation is sufficient to form singlet excited states (the S-route). The ECL of MAPQ was strong enough to be seen with the naked eye. MFPQ did not produce ECL, probably because of the instability of the radical cation.