Photophysical study of tolylterpyridine complexes. Intramolecular electron transfer in an osmium(II) dyad

Abstract

Spectroscopic and photophysical properties of 4′-p-tolyl-2,2′ : 6′,2″-terpyridine (tterpy) complexes of Fe^II, Ru^II and Os^II and of a dyad [Os^II(tterpy)(dmbipy-pterpy)]⁴⁺{dmbipy-pterpy = 1-methyl-1′-[4-(2,2′:6′,2″-terpyridine-4-yl)benzyl]-4,4′-bipyridinediium} are described. The luminescence of the osmium(II) complex is the most intense of the three whereas that of the other complexes is hardly or not detectable. The luminescence quantum yields of the osmium(II) and ruthenium(II) complexes (Φ_E= 0.009 and 0.000 01 respectively in ethanol) are much lower than those of the corresponding 2,2′-bipyridine (bipy) complexes (Φ_E= 0.006 and 0.078 respectively in ethanol). This may be explained by the less-rigid structure of terpy complexes rather than by deactivation via a low-lying d state. Excited-state spectra and lifetimes of the complexes of Ru^II and Os^II have been determined by laser flash spectroscopy. Transient absorptions are assigned to triplet metal-to-ligand charge transfer states with appreciable singlet character. The presence of absorption bands in the visible region is interpreted in terms of localization of the excited electron on one of the two equivalent ligands. The excited-state lifetimes of the complexes of Ru^II and Os^II measured in ethanol at room temperature are 4.5 and 245 ns respectively. The long-lived *[Os(tterpy)₂]²⁺ reacts with an electron acceptor such as 1,1-dimethyl-4,4′-bipyridinium by electron transfer (k_q= 9.2 × 10⁸ dm³ mol^–1 s^–1 in water). Comparison of the properties of the dyad with those of [Os(tterpy)₂]²⁺ shows in particular that the steady-state and transient emissions of the dyad chromophore are quenched by the viologen moiety. The quenching most probably involves an intramolecular electron transfer in a preferential direction over a long distance (11.8 Å) and leads to a charge-separated state of lifetime 720 ps. Such properties make terpy dyads excellent candidates for the design of molecular photodiodes.