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

作者：Li Zhanyong; Leed Nicholas A; Dickson Karn Nicole M; Dunbar Kim R^*; Turro Claudia

来源：Chemical Science, 2014, 5(2): 727-737.

DOI：10.1039/c3sc52366g

Two new series of dirhodium(II,II) complexes cis-[Rh-2(mu-DTolF)(2)(L)(2)][BF4](2) and cis-[Rh-2(mu-F-form)(2)(L)(2)][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]quinoxaline), dppz (dipyrido[3,2-a:2',3'-c]phenazine) 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 (MC)-M-3 (MC = metal-centered) arising from the Rh-2(pi*) -> Rh-2(sigma*) transition. In the case of cis-[Rh-2(mu-DTolF)(2)(dppn)(2)](2+) and cis-[Rh-2(mu-F-form)(2)(dppn)(2)](2+), the dppn-centered (3)pi pi* excited state is observed from similar to 10 ps to similar to 2 ns, but following its decay, the (MC)-M-3 state of each complex is observed with lifetimes of 2.4 and 3.0 mu s, respectively. The long lifetimes observed for the dppn complexes is explained by a pre-equilibrium of the low-lying (3)pi pi* and (MC)-M-3 states. The excited state oxidation potentials, E*(ox)((LLCT)-L-1), for the complexes are calculated to lie between -2.5 and -2.8 V vs. SCE and E*(ox)(3MC) similar to -1.8 V vs. SCE. The (LLCT)-L-1 excited states of the complexes are also good oxidizing agents, with E*(red)(1LLCT) similar to +1.3 V vs. SCE, making them significantly better oxidizing and reducing agents than commonly used Ru(II) complexes.

• 出版日期2014