Revealing Parallel Inter- and Intra-ligand Charge Transfer Dynamics in [Ru(L)2(dppz)]2+ Molecular Lightswitch with N K-edge X-ray Absorption Spectroscopy.

Abstract

In photoactive metal complexes the localization of photoexcited charges dictates the site of chemical reactivity, but few studies measure the charge redistribution in these systems with spatial precision. Herein, we track the inter- and intra-ligand charge transfer processes that underpin light-driven charge separation in the well-studied "molecular lightswitch" [Ru(bpy)2dppz]2+ (aqueous [RutheniumII(2,2'-bipyridine)2(dipyrido[3,2-a:2',3'-c]phenazine)]2+[Cl-]2) by probing the electronic structure of ligand nitrogen atoms in real-time using ultrafast x-ray absorption spectroscopy and first principles calculations. We confirm the localization of excited electron density on the phenazine N atoms of dppz and we newly identify two parallel electron transfer pathways to populate this state.  Sub-70 fs electron transfer to the phenazine portion of dppz is observed and attributed to intra-ligand electron transfer following Ru-to-dppz metal-to-ligand charge transfer (MLCT) excitation. This fast charge transfer was not reported in prior ultrafast studies. The slower (ca. 2 ps) charge transfer reported extensively in time-resolved optical absorption and emission studies is reassigned here to inter-ligand electron "hopping" between nearly isoenergetic ligand moieties following Ru-to-bpy MLCT excitation. The results demonstrate much faster charge separation than previously identified in this well-studied system, highlighting how extended azaacene ligand motifs promote the competitive charge transfer processes needed to drive light-driven electron transfer chemistry.