Picosecond forward electron transfer and nanosecond back electron transfer in an azacrown-substituted [(bpy)Re(CO)3(L)]+ complex: direct observation by time-resolved UV-visible absorption spectroscopy

Intramolecular electron transfer in the excited state of a [(bpy)ReI(CO)3(L)]+ complex (bpy = 2,2‘-bipyridine), in which L contains a pendant azacrown ether that acts as an electron donor (L = N-[4-(4,7,10,13-tetraoxa-1-azacyclopentadecyl)benzoyl]-4-aminopyridine), has been studied directly using picosecond and nanosecond time-resolved UV−visible absorption spectroscopy. Picosecond studies show that the metal-to-ligand charge-transfer (MLCT) state produced on excitation, [(bpy•-)ReII(CO)3(L)]+, undergoes forward electron transfer with a rate constant of kFET = 2.0 × 109 s-1 to generate a ligand-to-ligand charge-transfer (LLCT) state, [(bpy•-)ReI(CO)3(L•+)]+, in which the metal has been reduced back to Re(I) and charge separation has been effected between the bipyridine and azacrown ligands. Nanosecond studies show that the LLCT state returns to the ground state by back electron transfer from the bipyridine to azacrown ligand, with a rate constant of kBET = 5.3 × 107 s-1. Studies of complexes in which the azacrown complex is protonated, or is absent, demonstrate that intramolecular electron transfer to form the LLCT state does not occur in these cases. Forward electron transfer in the azacrown complex takes place on the picosecond time scale: it is weakly exoergonic and occurs in the Marcus normal region, with electronic coupling between the azacrown ligand and the rhenium metal center of ca. 100 cm-1. Back electron transfer takes place on the nanosecond time scale: it is strongly exoergonic and occurs in the Marcus inverted region, with much weaker electronic coupling between the bipyridine and azacrown ligands. The rapid formation of a long-lived charge-separated state indicates that this molecule has a suitable design for a photochemical device.