Emission and Photochemical Reactivity in Fluorescent Arene Complexes of Cyclopentadienylruthenium(II). Requirements for an Emissive CpRu(arene)⁺ Complex

The syntheses and characterization of five new compounds that contain the CpRu⁺ (Cp = η⁵-C₅H₅) moiety bound to a highly fluorescent arene ligand are reported. Nonemissive complexes of the highly emissive laser dyes coumarin-7 (3-(2′-Benzimidazolyl)-7-(N, N-diethylamino)coumarìn) and coumarin-30 (3-(2′-A, -methylbenzimidazolyl)-7-(N,N-diethylamino)coumarin) in which the CpRu⁺ group binds the dye benzimidazolyl arene ring were prepared and are compared to the structurally analogous, emissive complex of coumarin-6. Isomeric complexes of 9-phenylanthracene in which either the end anthracene ring or the substituent phenyl group is bound by the CpRu⁺ moiety were prepared for comparison to the structurally analogous complexes of rubrene (5, 6, 11, 12-tetraphenylnaphthacene). Room-temperature emission from fluid solutions of these [CpRu(η⁶-arene)] ⁺ complexes is only observed when the metal complex exhibits its lowest energy singlet absorption band at λ_max ≥ 480 nm. Reaction and emission quantum yields for the CpRu⁺ complexes of coumarin-6, rubrene, and 9-phenylanthracene are reported and used to elucidate the electronic and structural requirements for emission from these compounds.