Preparations and photophysical properties of fused and nonfused thienyl bridged MM (M = Mo or W) quadruply bonded complexes.

A series of metal-metal quadruply bonded compounds [(tBuCO2)3M2]2(mu-TT) where TT = thienothiophenedicarboxylate and M = Mo, 1A, and M = W, 1B and [(tBuCO2)3M2]2(mu-DTT) where DTT = dithienothiophenedicarboxylate and M = Mo, 2A, and M = W, 2B, has been prepared and characterized by elemental analysis, ESI- and MALDI-TOF mass spectrometry and 1H NMR spectroscopy. Their photophysical properties have also been investigated by steady-state absorption as well as transient absorption and emission spectroscopy. The optimized structures and the predicted low energy electronic transitions were obtained by DFT and time-dependent DFT calculations, respectively, on model compounds. These results, in combination with the respective properties of the compounds [(tBuCO2)3M2]2(mu-BTh) (BTh = 2,5'-bithienyldicarboxylate, M = Mo, 3A, and M = W, 3B), allow us to make a comprehensive comparison of the fused (compounds 1A, 1B, 2A, and 2B) and the nonfused thienyl (compounds 3A and 3B) dicarboxylate bridged compounds of molybdenum and tungsten. The electrochemical studies show singly oxidized radical cations that are valence trapped on the EPR time-scale and are classified as Class 1 (M = Mo) or Class 2 (M = W) on the Robin and Day scale for mixed valence compounds. The new compounds exhibit intense metal to bridge ligand charge transfer absorption bands in the far visible and near IR (NIR) region. Both molybdenum and tungsten complexes show dual emission, but for molybdenum, the phosphorescence is dominant while for tungsten the emission is primarily fluorescence. Femtosecond transient absorption spectroscopy shows that the relaxation dynamics of the S1 states which have lifetimes of approximately 10 ps is dominated by intersystem crossing (ISC), leading to T1 states that in turn possess long lifetimes, approximately 70 micros (M = Mo) or 3 micros (M = W). These properties are contrasted with the photophysical properties of conjugated organic systems incorporating metal ions of the later transition elements.