Femtosecond Study of Dimolybdenum Paddlewheel Compounds with Amide/Thioamide Ligands: Symmetry, Electronic Structure, and Charge Distribution in the ¹MLCT S₁ State

Four photophysically interesting dimolybdenum paddlewheel compounds are synthesized and characterized: <b>I</b> and <b>II</b> contain amide ligand (<i>N</i>,3-diphenyl-2-propynamide), and <b>III</b> and <b>IV</b> contain thioamide ligand (<i>N</i>,3-diphenyl-2-propynethioamide). <b>I</b> and <b>III</b> are <i>trans</i>-Mo₂L₂(O₂C-T^<i>i</i>PB)₂-type compounds, and <b>II</b> and <b>IV</b> are Mo₂L₄-type compounds, where O₂C-T^<i>i</i>PB is 2,4,6-triisopropylbenzoate. <b>I</b>–<b>IV</b> display strong light absorption due to metal to ligand charge transfer (MLCT) transitions from molybdenum to the amide/thioamide ligands. Charge transfer dynamics in the MLCT excited states of <b>I</b>–<b>IV</b> have been examined using femtosecond transient absorption (fs-TA) spectroscopy and femtosecond time-resolved infrared (fs-TRIR) spectroscopy. The asymmetric amide/thioamide ligands show two forms of regioarrangements in the paddlewheel compounds. Analyses of the ν­(CC) bands in the fs-TRIR spectra of <b>I</b> and <b>II</b> show similar electron density distribution over ligands in their ¹MLCT S₁ states where only two amide ligands are involved and the transferred electron is mainly localized on one of them. The fs-TRIR spectra of <b>III</b> and <b>IV</b>, however, show different charge distribution patterns where the transferred electron is fully delocalized over two thioamide ligands in <b>III</b> and partially delocalized in <b>IV</b>. Fast interligand electron transfer (ILET) was recognized as the explanation for the various charge distribution patterns, and ILET was shown to be influenced by both the ligands and the ligand arrangements

Synthesis, Structure, and Photophysical Properties of Mo₂(NN)₄ and Mo₂(NN)₂(T^<i>i</i>PB)₂, Where NN = <i>N</i>,<i>N</i>′‑Diphenylphenylpropiolamidinate and T^<i>i</i>PB = 2,4,6-Triisopropylbenzoate

Two dimolybdenum compounds featuring amidinate ligands with a CC bond, Mo₂(NN)₄ (<b>I</b>), where NN = <i>N</i>,<i>N</i>′-diphenylphenylpropiolamidinate, and <i>trans</i>-Mo₂(NN)₂(T^<i>i</i>PB)₂ (<b>II</b>), where T^<i>i</i>PB = 2,4,6-triisopropylbenzoate, have been prepared and structurally characterized by single-crystal X-ray crystallography. Together with Mo₂(DAniF)₄ (<b>III</b>), where DAniF = N,N′-bis­(<i>p</i>-anisyl)­formamidinate, all three compounds have been studied with steady-state UV–vis, IR, and time-resolved spectroscopy methods. <b>I</b> and <b>II</b> display intense metal to ligand charge transfer (MLCT). Singlet state (S₁) lifetimes of <b>I</b>–<b>III</b> are determined to be 0.7, 19.1, and 2.0 ps, respectively. All three compounds have long-lived triplet state (T₁) lifetimes around 100 μs. In femtosecond time-resolved infrared (fs-TRIR) experiments, one ν­(CC) band is observed at the S₁ state for <b>I</b> but two for <b>II</b>, which indicate different patterns of charge distribution. The electron would have to be localized on one NN ligand in <b>I</b> and partially delocalized over two NN ligands in <b>II</b> to account for the observations. The result is a standard showcase of excited-state mixed valence in coordination compounds

Femtosecond Study of Dimolybdenum Paddlewheel Compounds with Amide/Thioamide Ligands: Symmetry, Electronic Structure, and Charge Distribution in the ¹MLCT S₁ State

Four photophysically interesting dimolybdenum paddlewheel compounds are synthesized and characterized: <b>I</b> and <b>II</b> contain amide ligand (<i>N</i>,3-diphenyl-2-propynamide), and <b>III</b> and <b>IV</b> contain thioamide ligand (<i>N</i>,3-diphenyl-2-propynethioamide). <b>I</b> and <b>III</b> are <i>trans</i>-Mo₂L₂(O₂C-T^<i>i</i>PB)₂-type compounds, and <b>II</b> and <b>IV</b> are Mo₂L₄-type compounds, where O₂C-T^<i>i</i>PB is 2,4,6-triisopropylbenzoate. <b>I</b>–<b>IV</b> display strong light absorption due to metal to ligand charge transfer (MLCT) transitions from molybdenum to the amide/thioamide ligands. Charge transfer dynamics in the MLCT excited states of <b>I</b>–<b>IV</b> have been examined using femtosecond transient absorption (fs-TA) spectroscopy and femtosecond time-resolved infrared (fs-TRIR) spectroscopy. The asymmetric amide/thioamide ligands show two forms of regioarrangements in the paddlewheel compounds. Analyses of the ν­(CC) bands in the fs-TRIR spectra of <b>I</b> and <b>II</b> show similar electron density distribution over ligands in their ¹MLCT S₁ states where only two amide ligands are involved and the transferred electron is mainly localized on one of them. The fs-TRIR spectra of <b>III</b> and <b>IV</b>, however, show different charge distribution patterns where the transferred electron is fully delocalized over two thioamide ligands in <b>III</b> and partially delocalized in <b>IV</b>. Fast interligand electron transfer (ILET) was recognized as the explanation for the various charge distribution patterns, and ILET was shown to be influenced by both the ligands and the ligand arrangements

Femtosecond Study of Dimolybdenum Paddlewheel Compounds with Amide/Thioamide Ligands: Symmetry, Electronic Structure, and Charge Distribution in the ¹MLCT S₁ State

Four photophysically interesting dimolybdenum paddlewheel compounds are synthesized and characterized: <b>I</b> and <b>II</b> contain amide ligand (<i>N</i>,3-diphenyl-2-propynamide), and <b>III</b> and <b>IV</b> contain thioamide ligand (<i>N</i>,3-diphenyl-2-propynethioamide). <b>I</b> and <b>III</b> are <i>trans</i>-Mo₂L₂(O₂C-T^<i>i</i>PB)₂-type compounds, and <b>II</b> and <b>IV</b> are Mo₂L₄-type compounds, where O₂C-T^<i>i</i>PB is 2,4,6-triisopropylbenzoate. <b>I</b>–<b>IV</b> display strong light absorption due to metal to ligand charge transfer (MLCT) transitions from molybdenum to the amide/thioamide ligands. Charge transfer dynamics in the MLCT excited states of <b>I</b>–<b>IV</b> have been examined using femtosecond transient absorption (fs-TA) spectroscopy and femtosecond time-resolved infrared (fs-TRIR) spectroscopy. The asymmetric amide/thioamide ligands show two forms of regioarrangements in the paddlewheel compounds. Analyses of the ν­(CC) bands in the fs-TRIR spectra of <b>I</b> and <b>II</b> show similar electron density distribution over ligands in their ¹MLCT S₁ states where only two amide ligands are involved and the transferred electron is mainly localized on one of them. The fs-TRIR spectra of <b>III</b> and <b>IV</b>, however, show different charge distribution patterns where the transferred electron is fully delocalized over two thioamide ligands in <b>III</b> and partially delocalized in <b>IV</b>. Fast interligand electron transfer (ILET) was recognized as the explanation for the various charge distribution patterns, and ILET was shown to be influenced by both the ligands and the ligand arrangements

Synthesis, Structure, and Photophysical Properties of Mo₂(NN)₄ and Mo₂(NN)₂(T^<i>i</i>PB)₂, Where NN = <i>N</i>,<i>N</i>′‑Diphenylphenylpropiolamidinate and T^<i>i</i>PB = 2,4,6-Triisopropylbenzoate

Two dimolybdenum compounds featuring amidinate ligands with a CC bond, Mo₂(NN)₄ (<b>I</b>), where NN = <i>N</i>,<i>N</i>′-diphenylphenylpropiolamidinate, and <i>trans</i>-Mo₂(NN)₂(T^<i>i</i>PB)₂ (<b>II</b>), where T^<i>i</i>PB = 2,4,6-triisopropylbenzoate, have been prepared and structurally characterized by single-crystal X-ray crystallography. Together with Mo₂(DAniF)₄ (<b>III</b>), where DAniF = N,N′-bis­(<i>p</i>-anisyl)­formamidinate, all three compounds have been studied with steady-state UV–vis, IR, and time-resolved spectroscopy methods. <b>I</b> and <b>II</b> display intense metal to ligand charge transfer (MLCT). Singlet state (S₁) lifetimes of <b>I</b>–<b>III</b> are determined to be 0.7, 19.1, and 2.0 ps, respectively. All three compounds have long-lived triplet state (T₁) lifetimes around 100 μs. In femtosecond time-resolved infrared (fs-TRIR) experiments, one ν­(CC) band is observed at the S₁ state for <b>I</b> but two for <b>II</b>, which indicate different patterns of charge distribution. The electron would have to be localized on one NN ligand in <b>I</b> and partially delocalized over two NN ligands in <b>II</b> to account for the observations. The result is a standard showcase of excited-state mixed valence in coordination compounds

Electronic and Spectroscopic Properties of Avobenzone Derivatives Attached to Mo₂ Quadruple Bonds: Suppression of the Photochemical Enol-to-Keto Transformation

From the reactions between Mo₂(T^<i>i</i>PB)₄, where T^<i>i</i>PB is 2,4,6-triisopropylbenzoate, and 2 equiv of the acids 4-formylbenzoic acid, HBzald; 4-(3-oxo-3-phenylpropanoyl)­benzoic acid, HAvo; and 4-(2,2-difluoro-6-phenyl-2<i>H</i>-1λ³,3,2λ⁴-dioxaborinin-4-yl)­benzoic acid, HAvoBF₂, the compounds Mo₂(T^<i>i</i>PB)₂(Bzald)₂, <b>I</b>; Mo₂(T^<i>i</i>PB)₂(Avo)₂, <b>II</b>; and Mo₂(T^<i>i</i>PB)₂(AvoBF₂)₂, <b>III</b>, have been isolated. Compounds <b>I</b> and <b>II</b> are red, and compound <b>III</b> is blue. The new compounds have been characterized by ¹H NMR, MALDI-TOF MS, steady-state absorption and emission spectroscopies, and femtosecond and nanosecond time-resolved transient absorption and infrared spectroscopies. Electronic structure calculations employing density functional theory and time-dependent density functional theory have been carried out to aid in the interpretation of these data. These compounds have strong metal-to-ligand charge transfer, MLCT, and transitions in the visible region of their spectra, and these comprise the S₁ states having lifetimes ∼5–15 ps. The triplet states are Mo₂δδ* with lifetimes in the microseconds. The spectroscopic properties of <b>I</b> and <b>II</b> are similar, whereas the planarity of the ligand in <b>III</b> greatly lowers the energy of the MLCT and enhances the intensity of the time-resolved spectra. The Mo₂ unit shifts the ground state equilibrium entirely to the enol form and quenches the degradation pathways of the avobenzone moiety

Electronic and Spectroscopic Properties of Avobenzone Derivatives Attached to Mo₂ Quadruple Bonds: Suppression of the Photochemical Enol-to-Keto Transformation

From the reactions between Mo₂(T^<i>i</i>PB)₄, where T^<i>i</i>PB is 2,4,6-triisopropylbenzoate, and 2 equiv of the acids 4-formylbenzoic acid, HBzald; 4-(3-oxo-3-phenylpropanoyl)­benzoic acid, HAvo; and 4-(2,2-difluoro-6-phenyl-2<i>H</i>-1λ³,3,2λ⁴-dioxaborinin-4-yl)­benzoic acid, HAvoBF₂, the compounds Mo₂(T^<i>i</i>PB)₂(Bzald)₂, <b>I</b>; Mo₂(T^<i>i</i>PB)₂(Avo)₂, <b>II</b>; and Mo₂(T^<i>i</i>PB)₂(AvoBF₂)₂, <b>III</b>, have been isolated. Compounds <b>I</b> and <b>II</b> are red, and compound <b>III</b> is blue. The new compounds have been characterized by ¹H NMR, MALDI-TOF MS, steady-state absorption and emission spectroscopies, and femtosecond and nanosecond time-resolved transient absorption and infrared spectroscopies. Electronic structure calculations employing density functional theory and time-dependent density functional theory have been carried out to aid in the interpretation of these data. These compounds have strong metal-to-ligand charge transfer, MLCT, and transitions in the visible region of their spectra, and these comprise the S₁ states having lifetimes ∼5–15 ps. The triplet states are Mo₂δδ* with lifetimes in the microseconds. The spectroscopic properties of <b>I</b> and <b>II</b> are similar, whereas the planarity of the ligand in <b>III</b> greatly lowers the energy of the MLCT and enhances the intensity of the time-resolved spectra. The Mo₂ unit shifts the ground state equilibrium entirely to the enol form and quenches the degradation pathways of the avobenzone moiety

Probing Interligand Electron Transfer in the ¹MLCT S₁ Excited State of <i>trans</i>-Mo₂L₂L′₂ Compounds: A Comparative Study of Auxiliary Ligands and Solvents

The interligand charge dynamics of the lowest singlet metal-to-ligand charge-transfer states (¹MLCT S₁ states) of a series of quadruply bonded <i>trans</i>-Mo₂(NN)₂(O₂C–X)₂ paddlewheel compounds are investigated, where NN is a π-accepting phenylpropiolamidinate ligand and O₂C–X (X = Me, ^<i>t</i>Bu, T^<i>i</i>PB, or CF₃) is an auxiliary carboxylate ligand. The compounds show strong light absorption in the visible region due to MLCT transitions from the Mo₂ 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 ¹MLCT S₁ 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 ¹MLCT S₁ state is shown to be sensitive to the functional groups on the auxiliary ligands while being less responsive to changes in solvents