Ultrafast kinetics of supramolecules with a Ru(II)- or Os(II)-polypyridyl light absorber, cis-Rh(III)Cl2-polypyridyl electron collector, and 2,3-bis(2-pyridyl)pyrazine bridge

A series of photoexcited mixed-metal supramolecules are shown to undergo electron transfer from a metal-to-ligand charge transfer state (MLCT) to generate a metal-to-metal charge transfer state (MMCT) in ∼7ps. Unlike the MLCT/MMCT free energy difference, the MMCT/ground state energy varies in the series, thus relaxation times span ∼20ps to ∼30ns. [Display omitted] The femtosecond transient absorption spectra (fsTA) and excited state kinetics for a series of six structurally related mixed-metal polypyridyl supramolecules are reported. Each complex consists of one or two light absorbers (LA) with Ru(II) or Os(II) centers attached to a Rh(III)-centered electron collector (EC) by an aromatic bridging ligand (BL). The resulting bimetallic and trimetallic complexes have LA-BL-EC and LA-BL-EC-BL-LA architectures, respectively. Excitation at 470nm light populates metal-to-bridging ligand charge transfer states (MLCT), showing a transient absorption band near 380nm due to π→π∗ transitions of a bridging ligand-localized radical anion and a transient bleach around 525nm resulting from formal oxidation of the LA metal in the excited state. Loss of the ligand localized radical signal during the first 10ps reflects conversion of the excited state population from an MLCT state into metal-to-metal (i.e. M(dπ)-to-Rh(dσ∗)) charge transfer states (MMCT). Each complex shares a similar ultrafast component, indicating that the kinetics governing MLCT→MMCT population transfer do not depend on the nature of the LA. Return to the ground state, however, is strongly LA dependent and controlled by the free-energy difference between the MMCT state and ground state, as well as an associated large reorganization energy.