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

Dalton Transactions Pub Date: 2009-07-06 DOI: 10.1039/B903130H

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 (kET) 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 (kCR) 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.

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