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From the reactions between Mo2(T(i)PB)4, where T(i)PB = 2,4,6-triisopropylbenzoate and two equivalents of the carboxylic acid LH (LH = 4-nitrobenzoic acid and 4'-nitro[1,1'-biphenyl]-4-carboxylic acid) the compounds trans-M2(T(i)PB)2L2 have been prepared: I (L = 4-nitrobenzoate and M = Mo), II (L = 4'-nitro-1,1'-biphenylcarboxylate and M = Mo) and III (L = 4-nitrobenzoate and M2 = MoW). The compounds have been characterized by (1)H NMR, UV-Vis and steady state emission spectroscopy, ns and fs transient absorption spectroscopy and cyclic voltammetry. These data are compared with predictions based on electronic structure calculations on model compounds where T(i)PB is substituted for formate. Together these data indicate stronger ground-state coupling of the Mo2δ and ligand π* systems in I relative to II but this order is reversed in the photo excited S1(1)MLCT state. Attempts to prepare the W2 containing analogs were unsuccessful.

Evidence, based on femtosecond transient absorption and time resolved infrared spectroscopy, is presented for photoinduced charge transfer from the Mo2δ orbital of the quadruply bonded molecule trans-Mo2(T(i)PB)2(BTh)2, where T(i)PB = 2,4,6-triisopropyl benzoate and BTh = 2,2'-bithienylcarboxylate, to di-n-octyl perylene diimide and di-n-hexylheptyl perylene diimide in thin films and solutions of the mixtures. The films show a long-lived charge separated state while slow back electron transfer, τBET ~ 500 ps, occurs in solution.

Quadruply bonded dinuclear metal complexes of molybdenum and tungsten have the MM configuration σ(2)π(4)δ(2) and a considerable degree of attention has been devoted to studies of the δ → δ(*) transition. For compounds of the type M(2)(O(2)CR)(4), the CO(2) π(*) orbitals introduce a M(2) δ to ligand π(*) transition, a (1)MLCT absorption which may be lower in energy than the δ → δ(*) and is more intense, thus obscuring the observation of the latter. When the R group is a conjugated organic system such as an aryl group, the (1)MLCT shifts to even lower energy and emission is seen from this S(1) state in addition to phosphorescence from the T(1) state which may be either (3)MLCT or (3)MMδδ(*). The latter typically occurs around 1100 nm with a lifetime that ranges from ~1 μs (M = W) to 100 μs (M = Mo). The S(1)(1)MLCT states have lifetimes of ~1-20 ps, allowing for fs and ns studies of the charge distribution/localization with time in both the S(1) and T(1) states, which is quite rare for transition metal coordination complexes. Of particular interest and focus have been complexes of the type trans-M(2)L(2)L'(2) where L and L' are carboxylate or amidinate groups for which only one set of ligands allows for expansive Lπ-M(2)δ-Lπ conjugation and has a low energy (1)MLCT. Compounds of this type have excited states that may be considered as mixed valence (MV) ions [L-M(2)(+)-L(-)] ↔ [L(-)-M(2)(+)-L] where the hole resides on the M(2) unit and the electron is either localized on one ligand, a class I or II MV ion, or is fully delocalized over both ligands, a class III ion in the Robin and Day scheme. Examples of these systems will be described along with the newly prepared complexes trans-M(2)(T(i)PB)(2)(O(2)CC≡C-9-anthracene)(2), M = Mo, W, that have the IR-active reporter groups CO(2) and C≡C.

From the reactions between M(2)(T(i)PB)(4), where T(i)PB = 2,4,6-triisopropylbenzoate and M = Mo or W, in toluene and each of the respective carboxylic acids (2 equiv) the quadruply MM bonded compounds trans-M(2)(T(i)PB)(2)(O(2)C-C≡C-Ar)(2) have been prepared where Ar = p-tolyl and M = Mo, , and M = W, , and Ar = 9-anthracenyl, where M = Mo, 2a, and M = W, 2b. Single crystal X-ray crystallographic studies of 1a and 2b confirmed the trans substitution pattern about the Mo(2)(4+) unit and the centrosymmetric molecules have structural features that indicate extensive Lπ-Mo(2)δ-Lπ conjugation involving the arylethynylcarboxylates. The compounds are intensely colored as a result of the HOMO → LUMO, metal δ-to-ligand π* charge transfer (1)MLCT transition: 1a (orange), 1b (red), 2a (blue) and 2b(green). The compounds 1a, 2a, 1b and 2b have been characterized by UV-Vis-NIR absorption and emission spectroscopy, cyclic voltammetry, femtosecond (fs) and nanosecond (ns) transient absorption spectroscopy and by fs time-resolved infrared spectroscopy in the region of ν(C≡C), ν(CO(2)) and from 1400-1000 cm(-1). Aided by density functional theory, (DFT) and time dependent DFT, the electronic structures of the ground state and the S(1) and T(1) states are described. The molybdenum compounds have short lived (1)MLCT states, 1a ~ 5.0 ps and 2a ~ 10.5 ps, that undergo intersystem crossing to long lived (3)MoMoδδ* states: 1a ~ 101 μs and 2a ~ 83 μs. The tungsten complexes show interesting time-resolved infrared spectra in the ν(C≡C) region when compared with their ground state. Compound 1b shows ν(C≡C) at 1975 cm(-1) for the (1)MLCT state which decays with τ ~ 0.7 ps to ν(C≡C) at 2000 cm(-1) for the (3)MLCT state. For 2b the (1)MLCT is characterized by ν(C≡C) at 2150 cm(-1), τ ~ 19 ps, and a very broad absorption with a maximum ~1970 cm(-1) which is proposed to arise from a low energy electronic transition. The (3)MLCT state for shows no evidence of ν(C≡C) and is suggested to have an electron localized principally on the anthracenyl portion of the ligand, a proposal that finds support from the nature of triplet transient absorption spectrum of 2b.

The preparation and characterization of the compounds trans-M(2)(T(i)PB)(2)(O(2)C-CH=CH-2-C(4)H(3)S)(2) where M = Mo or W and T(i)PB = 2,4,6-triisopropylbenzoate are reported. The optical spectra of the new compounds are compared with those of related trans-M(2)(T(i)PB)(2)L(2) compounds where L = O(2)C-C(6)H(4)-4-CN, O(2)C-α,α'-terthienyl (TTh), and O(2)C-4-C(6)H(4)N-B(C(6)F(5))(3), that show strong metal-to-ligand charge transfer bands because of M(2)δ to Lπ conjugation, and are notably temperature dependant due to the various conformations of the two trans-L groups. Upon cooling the spectral features sharpen as the planar geometry that optimizes M(2)δ-Lπ conjugation is favored. As the electronic coupling of the two trans-Lπ systems increases the (0,0) electronic transition gains intensity indicating a greater nesting of the ground state (S(0)) and excited state (S(1)) potential energy surfaces. These features are discussed in terms of the related electronic coupling of [M(2)]-[M(2)] complexes.