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

The synthesis and photophysical characterization of two different series of electron donor-acceptor dyads containing Ru(II) and Os(II) bis-terpyridines (M(tpy)(2)(2+)) were prepared and studied in order to compare the oft-studied Ru(tpy)(2)(2+) chromophore with the less studied Os(tpy)(2)(2+) 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 approximately 2.0 x 10(11) s(-1) (7) and 1.3 x 10(10) 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 approximately 6.7 x 10(10) s(-1) (7) and 1.2 x 10(10) s(-1) (8). The analogous Ru(II) complexes underwent charge separation with rate constants of 7.6 x 10(10) s(-1) (5) and approximately 2.3 x 10(10) s(-1) (6), while charge recombination in these complexes occurred with rate constants of approximately 2.1 x 10(10) s(-1) (5) and approximately 5.3 x 10(10) s(-1) (6). Electron transfer in the pyromellitimide-containing complexes occurred only for Os(II)-PI (10), which exhibited significantly slower electron transfer (approximately 4.3 x 10(6) s(-1)) and charge recombination (approximately 7.7 x 10(6) 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.