SPECTROSCOPIC STUDY OF ELECTRON-TRANSFER IN A TRIFUNCTIONAL LYSINE WITH ANTHRAQUINONE AS THE ELECTRON-ACCEPTOR

The transient properties of the redox-active amino acid dyads [Anq-Lys(RuIIb2m)2+-OCH3](PF6)2 and [Boc-Lys-(RuIIb2m)2+-NH-prPTZ](PF6)2 and the trifunctionalized amino acid [Anq-Lys(RuIIb2m)2+-NH-prPTZ](PF6)2, where Anq is 9, 10-anthraquinone-2-carbonyl, Lys is L-lysine, b is 2,2'-bipyridine, m is 4'-methyl-2,2'-bipyridine-4-carbonyl, and prPTZ is 3-(10H-phenothiazine-10)propyl, were examined in CH3CN after nanosecond laser excitation. This series of redox-active assemblies was prepared by attaching derivatives of the ruthenium tris(bipyridyl) chromophore, the electron donor phenothiazine, and/or the electron acceptor anthraquinone to L-lysine with amide bonds. Emission from the chromophore was efficiently quenched (>95%) by the attached donors or acceptors in all three cases. Irradiation of [Anq-Lys(RuIIb2m)2+-NH-prPTZ] with 420-nm, 4-ns laser pulses resulted in net electron transfer from prPTZ to Anq, mediated by the metal-to-ligand charge-transfer (MLCT) excited state of the ruthenium chromophore, as observed by nanosecond transient absorption and time-resolved resonance Raman spectroscopies. The resulting redox-separated state, [(Anq.-)-Lys(RuIIb2m)2+-NH-(prPTZ.+)], at 1.54 eV, was formed with a quantum efficiency of 26% at its maximum appearance and persisted for 174 ns in CH3CN at 25-degrees-C.