The compounds M2(O2CtBu)4 and M2(O2CC6H5)4, where M = Mo or W, have been examined by femtosecond time-resolved IR (fs-TRIR) spectroscopy in tetrahydrofuran with excitation into the singlet metal-to-ligand charge-transfer (1MLCT) band. In the region from 1500 to 1600 cm–1, a long-lived excited state (>2 ns) has been detected for the compounds M2(O2CtBu)4 and Mo2(O2C–C6H5)4 with an IR absorption at ∼1540 cm–1 assignable to the asymmetric CO2 stretch, νas(CO2), of the triplet metal–metal δ−δ star (3MM δδ*) state. The fs-TRIR spectra of W2(O2C–C6H5)4 are notably different and are assigned to decay of the MLCT states. In 3MM δδ*, the removal of an electron from the δ orbital reduces MM δ to CO2 π* back-bonding and causes a shift of νas(CO2) to higher energy by ∼30–60 cm–1, depending on the metal. TRIR spectroscopy also provides evidence for M2(O2CtBu)4, where M = Mo or W, having MM δδ* S1 states with νas(CO2) distinct from those of the 3MM δδ* states.

The preparation and characterization (elemental analysis, 1H NMR, and cyclic voltammetry) of the new compounds MM(TiPB)4, where MM = MoW and W2 and TiPB = 2,4,6-triisopropylbenzoate, are reported. Together with Mo2(TiPB)4, previously reported by Cotton et al. (Inorg. Chem. 2002, 41, 1639), the new compounds have been studied by electronic absorption, steady-state emission, and transient absorption spectroscopy (femtosecond and nanosecond). The compounds show strong absorptions in the visible region of the spectrum that are assigned to MMδ to arylcarboxylate π* transitions, 1MLCT. Each compound also shows luminescence from two excited states, assigned as the 1MLCT and 3MMδδ* states. The energy of the emission from the 1MLCT state follows the energy ordering MM = Mo2 > MoW > W2, but the emission from the 3MMδδ* state follows the inverse order: MM = W2 > MoW > Mo2. Evidence is presented to support the view that the lower energy emission in each case arises from the 3MMδδ* state. Lifetimes of the 1MLCT states in these systems are ∼0.4−6 ps, whereas phosphorescence is dependent on the MM center: Mo2 ∼ 40 μs, MoW ∼ 30 μs, and W2 ∼ 1 μs.

The reactions between MM(TiPB)4, where TiPB = 2,4,6-triisopropylbenzoate and MM = MoW and W2, and the (2,2′:5′,2′′-terthiophene)-5-carboxylic acid, TThH (2 equiv) leads to the formation of new compounds trans-MM(TiPB)2(TTh)2, II and III, respectively, as well as to the previously reported compound I, when MM = Mo2. The compounds have been characterized by elemental analysis, 1H NMR spectroscopy, electronic absorption, and emission spectroscopies together with cyclic voltammetry and differential pulse voltammetry. Calculations on the model compounds I′, II′, and III′, where formate ligands substitute for TiPB, have been carried out employing density functional theory (DFT) and time-dependent DFT. These complexes display intense 1MLCT absorptions (MMδ to thienyl carboxylate) and have oxidations and reductions that are metal (MMδ) and thienyl ligand based, respectively. All compounds show emission in the near-IR region. At low temperature the NIR emission from I and II shows clear evidence of vibronic features due to υ(MM) ∼ 350−390 cm−1, and all compounds show evidence of a vibronic feature due to υ(CO2) ∼ 1200 cm−1. Transient absorption spectroscopy reveals relatively short-lived S1 states, τ ∼ 10 ps, and longer lived T1 states: τ ∼ 72 μs for I, ∼ 160 ns for II, and ∼ 90 ns for III. The chemistry described here reveals the remarkable influence of MMδ to TTh π electronic coupling on the optoelectronic properties of the thienyl chains.

The compounds M2(TiPB)2(OSC-2-Th)2 have been prepared from the reactions between M2(TiPB)4 and Th-2-COSH (2 equiv) in toluene solution, where M = Mo (Mo2ThCOS) or W (W2ThCOS), TiPB = 2,4,6-triisopropylbenzoate and Th = thienyl. The molybdenum and tungsten compounds are pink and blue, air-sensitive, ether soluble solids that show M+ ions in the mass spectrometer and metal and ligand based reversible oxidation and reduction waves, respectively, by cyclic voltammetry. Electronic structure calculations on the model compounds M2(O2CH)2(OSC-2-Th)2 indicate that the highest occupied molecular orbital (HOMO) is principally M2δ and the lowest unoccupied molecular orbital (LUMO) is thienylthiocarboxylate π* but with significant metal−sulfur mixing. The intense visible absorptions arise from 1MLCT, M2δ to thienylthiocarboxylate. The photoexcited states of these molecules have been studied by transient absorption spectroscopy and steady state emission. These properties are compared with those of previously reported thienylcarboxylate compounds, M2(TiPB)2(O2C-2-Th)2, where M = Mo (Mo2ThCO2) or W (W2ThCO2).

A series of metal−metal quadruply bonded compounds [(tBuCO2)3M2]2(μ-TT) where TT = thienothiophenedicarboxylate and M = Mo, 1A, and M = W, 1B and [(tBuCO2)3M2]2(μ-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(μ-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 ∼10 ps is dominated by intersystem crossing (ISC), leading to T1 states that in turn possess long lifetimes, ∼70 µs (M = Mo) or 3 µs (M = W). These properties are contrasted with the photophysical properties of conjugated organic systems incorporating metal ions of the later transition elements.