Directional charge transfer and highly reducing and oxidizing excited states of new dirhodium(II,II) complexes: potential applications in solar energy conversion

摘要

Two new series of dirhodium(II,II) complexes c/s-[Rh2(u-DTolF)2(L)2l[BF4l2 and c/s-[Rh2(u-F-form)2(L)2l[BF4]2 were synthesized and fully characterized {DTolF = p-ditolylformamidinate, F-form = p-difluorobenzylformamidinate; L = the chelating diimine ligands dpq (dipyrido[3,2-f:2',3'-h)lquinoxaline), dppz (dipyrido[3,2-a:2',3'-clphenazine) and dppn (benzo[i]dipyrido[3,2-a:2',3'-h]quinoxaline). The complexes undergo facile oxidation and exhibit directed ligand-to-ligand charge transfer (LLCT) excited states upon excitation from the corresponding formamidinate to the diimine ligand. Time-resolved studies reveal that the LLCT states decay with lifetimes that range from 16 to 100 ps and generate a longer-lived excited state. For complexes with dpq and dppz ligands, the longer-lived excited state, with lifetimes that range from 40 to 100 ns, has been assigned as ~3MC (MC = metal-centered) arising from the Rh2(π} → Rh2(σ) transition. In the case of c/s-[Rh2(μ-DTolF)2(dppn)2]~(2+) and c/s-[Rh2(μ-F-form)2(dppn)2]~(2+), the dppn-centered ~3ππ excited state is observed from ~10 ps to ~2 ns, but following its decay, the ~3MC state of each complex is observed with lifetimes of 2.4 and 3.0 us, respectively. The long lifetimes observed for the dppn complexes is explained by a pre-equilibrium of the low-lying ~3ππ and ~3MC states. The excited state oxidation potentials, E_(ox)/LLCT), for the complexes are calculated to lie between-2.5 and-2.8 V vs. SCE and E_(ox)(~3MC)-----1.8 V vs. SCE. The H-LCT excited states of the complexes are also good oxidizing agents, with E_(red)(~1LLCT) ~ +1.3 V vs. SCE, making them significantly better oxidizing and reducing agents than commonly used Ru(u) complexes.