TRANS-CHROMOPHORE-QUENCHER COMPLEXES BASED ON RUTHENIUM(II)

A series of chromophore-quencher complexes, trans-[Ru(bpy)(2)(py(a))(p y(b))](n+) (bpy = 2,2'-bipyridyl; py(a), py(b) = substituted pyridyl l igands; n = 2-4) have been prepared and characterized. These contain c ombinations of the electron donor PTZ in py-PTZ (py-PTZ = 10-(4-picoly l)phenothiazine) and the electron accepters MQ(+) (1-methyl-4,4'-bipyr idinium cation), py-AQ anthraquinon-2-yl)carbonyl)amino)methyl)pyridin e), and py-MPAA -pyridyl)-beta-(1-methylpyridinium-3-yl)acrylamide cat ion), and 4-ethylpyridine (4-Etpy). The asymmetrical complexes (py(a) not equal py(b)) are synthesized by using the precursor trans-[Ru(bpy) (2)(DMSO)(2)](CF3SO3)(2) . 0.5H(2)O, which is converted into trans-[Ru (bpy)(2)(py-PTZ)(DMSO)](CF3SO3)(2) . 0.5H(2)O (1) by reaction with py- PTZ in DMSO/acetone at room temperature. 1 is treated with LiCl in aqu eous DMF at 105 degrees C to prepare trans-[RuCl(bpy)(2)(py-PTZ)]CF3SO 3 . H2O (2). 2 is converted into the derivatives trans-[Ru(bpy)(2)(py- PTZ)(4-Etpy)](PF6)(2) . 0.5H(2)O (5), trans-[Ru(bpy)(2)(py-PTZ)(MQ(+)) ](PF6)(3) (7), trans-[Ru(bpy)(2)(py-PTZ)(py-AQ)](PF6)(2) . H2O (9), an d trans-[Ru(bpy)(2)(py-PTZ)(py-MPAA)](PF6)(3) . H2O (11) by chloride a bstraction at room temperature in the presence of 4-Etpy, [MQ](PF6), p y-AQ, and [py-MPAA](PF6), respectively. The salts trans-[Ru(bpy)(2)(4- Etpy)(MQ(+))](PF6)(3) . H2O (6), trans-[Ru(bpy)(2)(4-Etpy)(py-AQ)](PF6 )(2) (8), and trans-[Ru(bpy)(2)(4-Etpy)(py-MPAA)](PF6), (10) are synth esized by chloride abstraction from trans-[RuCl(bpy)(2)(4-Etpy)]PF6 . H2O in the presence of [MQ](PF6)(3) py-AQ, or [py-MPAA](PF6). The symm etrical complexes trans-[Ru(bpy)(2)(MQ(+))(2)](PF6)(4) . 3H(2)O (3) an d trans-[Ru(bpy)(2)(py-PTZ)(2)](PF6)(2) . H2O (4) are synthesized by r eaction of trans-[Ru(bpy)(2)(H2O)(2)](CF3SO3)(2) with an excess of the appropriate ligand in DMF at 100 degrees C. Visible light irradiation of these complexes at room temperature leads to photochemical decompo sition. At lower temperatures (<200 K), emission typical of Ru-III-bpy (.-) (and Ru-III-MQ(.)) MLCT states is observed. The appearance of a t hermally activated decay pathway above 150 K signals the presence of a crossing to a photochemically reactive ligand-field state. As a resul t, significant formation of redox-separated states by electron transfe r is not observed for the redox triads trans-[Ru(bpy)(2)(py-PTZ)(X)](n +) (X = py-AQ, n = 2; X = py-MPAA or MQ(+), II = 3). Nonetheless, the results obtained have important implications for the stereochemical de sign of Ru(II)-based chromophore-quencher complexes.