Probing Interligand Electron Transfer in the 1 MLCT S 1 Excited State of trans-Mo 2 L 2 L' 2 Compounds: A Comparative Study of Auxiliary Ligands and Solvents

The interligand charge dynamics of the lowest singlet metal-to-ligand charge-transfer states ( MLCT S states) of a series of quadruply bonded trans-Mo (NN) (O C-X) paddlewheel compounds are investigated, where NN is a π-accepting phenylpropiolamidinate ligand and O C-X (X = Me, Bu, T PB, or CF ) is an auxiliary carboxylate ligand. The compounds show strong light absorption in the visible region due to MLCT transitions from the Mo center to the NN ligands. The transferred electron density was followed by femtosecond time-resolved infrared (fs-TRIR) spectroscopy with vibrational reporters such as the ethynyl groups on the NN ligands. The observed fs-TRIR spectra show that these compounds have asymmetric MLCT S excited states where the transferred electron mainly resides on a single NN ligand. The presence of interligand electron transfer (ILET) is suggested to explain the shape of the ν(C≡C) bands and the influence of auxiliary ligands and solvents on the interligand electronic coupling. The ILET in the MLCT S state is shown to be sensitive to the functional groups on the auxiliary ligands while being less responsive to changes in solvents.