Photoinduced Intramolecular Tryptophan Oxidation and Excited-State Behavior of [Re(L-AA)(CO)3(α-diimine)]+ (L = Pyridine or Imidazole, AA = Tryptophan, Tyrosine, Phenylalanine)

Re-I carbonyl-diimine complexes [Re(L-AA)(CO)(3)(N,N)](+) (N,N = bpy, phen) containing an aromatic amino acid (AA), phenylalanine (Phe), tyrosine (Tyr), or tryptophan (Trp), linked to Re by a pyridine-amido or imidazole-amido ligand L have been synthesized and their excited-state properties investigated by nanosecond time-resolved IR (TRIR) and emission spectroscopy. Near-UV optical excitation populates a Re-I(CO)(3)-> N,N (MLCT)-M-3 excited state *[Re-II(L-AA) (CO)(3)-(N,N center dot-)](+). Decay to the ground state (50-300 ns lifetime) is the only excited-state deactivation process observed in the case of Phe and Tyr complexes, whereas the Trp-containing species undergo a Trp(indole)->*Re-II electron transfer (ET) producing a charge-separated (CS) state, [Re-I(L-Trp(center dot+))(CO)(3)(N,N center dot-)](+). The ET occurs with a 8-40 ns lifetime depending on L, N,N, and the solvent. The CS state is characterized by v(CO) IR bands shifted to lower wavenumbers from their respective ground-state positions and two bands at 1278 and 1497 cm(-1) tentatively attributed to Trp(center dot+). The amido bridge is affected by both the MLCT excitation and the subsequent ET, manifested by the shifts and intensity changes of the amide-I IR band at about 1680 cm(-1). The CS state decays to the ground state by a N,N center dot--> Trp(center dot+) back-ET the rates of which are comparable to those of the forward ET, 30-60 ns. This study independently demonstrates that Tip can act as an electron-hopping intermediate in photodriven ET systems based on Re-labeled proteins and supramolecules. Photoinduced ET in Tip-containing Re complexes also can be used to generate Trp(center dot+) and investigate its spectral properties and reactivity.