Photoinduced intramolecular tryptophan oxidation and excited-state behavior of [Re(L-AA)(CO)_3(α-diimine)]~+ (L = pyridine or imidazole, AA = tryptophan, tyrosine, phenylalanine)

摘要

ReI 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 ~3MLCT excited state*[Re~(II)(L-AA)(CO)_3(N,N~(?-))] ~+. 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~(?+))(CO)_3(N,N ~(?-))]~+. The ET occurs with a 8-40 ns lifetime depending on L, N,N, and the solvent. The CS state is characterized by ν(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~(?+). 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~(?-)→ Trp~(?+) back-ET the rates of which are comparable to those of the forward ET, 30-60 ns. This study independently demonstrates that Trp can act as an electron-hopping intermediate in photodriven ET systems based on Re-labeled proteins and supramolecules. Photoinduced ET in Trp-containing Re complexes also can be used to generate Trp~(?+) and investigate its spectral properties and reactivity.