Photoinduced electron-transfer within osmium(II) and ruthenium(II) bis-terpyridine donor acceptor dyads

Abstract

The synthesis and photophysical characterization of two different series of electron donor–acceptor dyads containing Ru(II) and Os(II) bis-terpyridines (M(tpy)₂²⁺) were prepared and studied in order to compare the oft-studied Ru(tpy)₂²⁺ chromophore with the less studied Os(tpy)₂²⁺ chromophore. The first series of dyads incorporates a benzoquinone (BQ) group as the electron acceptor, whereas the second contains a substituted pyromellitimide (PI) group as the electron acceptor. Steady-state emission experiments indicated efficient quenching of the ³MLCT emission of the electronically excited Os(II)-BQ complexes (7–8) compared to both model complexes (3–4) and the Os(II)-PI complex 10. Femtosecond pump–probe absorption experiments on 7–8 yielded ultrafast electron transfer rate constants (k_ET) of ∼2.0 × 10¹¹ s⁻¹ (7) and 1.3 × 10¹⁰ s⁻¹ (8) that were in good agreement with the low emission quantum yield results. Charge-recombination (k_CR) in these complexes was also quite rapid, with rate constants of ∼6.7 × 10¹⁰ s⁻¹ (7) and 1.2 × 10¹⁰ s⁻¹ (8). The analogous Ru(II) complexes underwent charge separation with rate constants of 7.6 × 10¹⁰ s⁻¹ (5) and ∼2.3 × 10¹⁰ s⁻¹ (6), while charge recombination in these complexes occurred with rate constants of ∼2.1 × 10¹⁰ s⁻¹ (5) and ∼5.3 × 10¹⁰ s⁻¹ (6). Electron transfer in the pyromellitimide-containing complexes occurred only for Os(II)-PI (10), which exhibited significantly slower electron transfer (∼4.3× 10⁶ s⁻¹) and charge recombination (∼7.7 × 10⁶ s⁻¹) rate constants. The nearly thermoneutral free energy of electron transfer and short excited state lifetime in the case of Ru(II)-PI (9) presumably prevents electron transfer in this compound.