Effect of a steric spacer on chromophoric interactions of ruthenium complexes containing covalently bound anthracene

Abstract

Abstract To study interchromophore interactions in a ruthenium complex linked covalently to an anthryl group, we have synthesized 4-(9-anthrylethyl),4′-methyl-2,2′-bipyridine (dmb-an) and complexed it with ruthenium in two asymmetric complexes, [Ru(L) 2 (dmb-an)] 2+ , where L4,4′-dimethyl, 2,2′-bipyridine (dmb) or 4,4′-di- tert -butyl, 2,2′-bipyridine (dtb). In both complexes, the two independent chromophores rapidly undergo singlet—singlet and triplet—triplet energy transfer which nearly quantitatively quenches luminescence: Φ F =3.5 × 10 −4 from the anthryl group and Φ −5 from the metal—ligand charge transfer state of the complex. Both complexes also readily undergo photoinduced oxygenation of the anthryl group. The anthryl group is charge transfer quenched by methyl viologen, with a cage escape yield that differs substantially for the two complexes. When Ldmb, the cage escape yield is 0.26, in common with other ruthenium tris-imines and much less than free anthracene. However, when Ldtb, the cage escape yield rises to 0.55±0.05. We interpret the improved yield as arising from the bulky tert -butyl substituents, which prevent the anthryl moiety from close approach to the metal center and reduce metal-mediated enhancement of reverse charge transfer.