N-Dopants Based on Dimers of Benzimidazoline Radicals: Structures and Mechanism of Redox Reactions

Dimers of 2-substituted N,N′-dimethylbenzimidazoline radicals, (2-Y-DMBI)₂ (Y=cyclohexyl (Cyc), ferrocenyl (Fc), ruthenocenyl (Rc)), have recently been reported as n-dopants for organic semiconductors. Here their structural and energetic characteristics are reported, along with the mechanisms by which they react with acceptors, A (PCBM, TIPS-pentacene), in solution. X-ray data and DFT calculations both indicate a longer C-C bond for (2-Cyc-DMBI)₂ than (2-Fc-DMBI)₂, yet DFT and ESR data show that the latter dissociates more readily due to stabilization of the radical by Fc. Depending on the energetics of dimer (D₂) dissociation and of D₂-to-A electron transfer, D₂ reacts with A to form D⁺ and A^- by either of two mechanisms, differing in whether the first step is endergonic dissociation or endergonic electron transfer. However, the D⁺/0.5D₂ redox potentials - the effective reducing strengths of the dimers - vary little within the series (ca. -1.9V vs. FeCp₂^+/0) (Cp=cyclopentadienyl) due to cancelation of trends in the D^+/0 potential and D₂ dissociation energy. The implications of these findings for use of these dimers as n-dopants, and for future dopant design, are discussed. Benzimidazoline dimers are effective n-dopants for organic electronics. Vis-NIR kinetics help elucidate the mechanisms by which they react with acceptors (A); these depend on both the dimer and A and can be rationalized using electrochemistry and dissociation energies (see scheme). X-ray diffraction shows central C-C bond lengths do not correlate with dissociation energy.