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

Abstract

Two new series of dirhodium(II,II) complexes cis-[Rh₂(μ-DTolF)₂(L)₂][BF₄]₂ and cis-[Rh₂(μ-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 ³MC (MC = metal-centered) arising from the Rh₂(π*) → Rh₂(σ*) transition. In the case of cis-[Rh₂(μ-DTolF)₂(dppn)₂]²⁺ and cis-[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 μs, 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(¹LLCT), 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 ¹LLCT 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(II) complexes.