Probing the solvation structure and dynamics in ionic liquids by time-resolved infrared spectroscopy of 4-(dimethylamino)benzonitrile.

In this work we demonstrate the use of the push-pull model system 4-(dimethylamino)benzonitrile (DMABN) as a convenient molecular probe to investigate the local solvation structure and dynamics by means of time-resolved infrared spectroscopy (TRIR). The photochemical features associated with this system provide several advantages due to the high charge separation between the ground and charge transfer states involving the characteristic nitrile bond, and an excited state lifetime that is long enough to observe the slow solvation dynamics in organic solvents and ionic liquids. The conversion from a locally excited state to an intramolecular charge transfer state (LE-ICT) in ionic liquids shows similar kinetic lifetimes in comparison to organic solvents.

Electronic and spectroscopic properties of avobenzone derivatives attached to ditungsten quadruple bonds.

From the reactions between W2(T(i)PB)4, where T(i)PB is 2,4,6-triisopropylbenzoate, and 2 equiv of acid, 4-formylbenzoic acid, HBzald, 4-(3-oxo-3-phenylpropanoyl)benzoic acid, HAvo, or 4-(2,2-difluoro-6-phenyl-2H-1λ(3),3,2λ(4)-dioxaborinin-4-yl)benzoic acid, HAvoBF2, three new compounds W2(T(i)PB)2(Bzald)2, I, W2(T(i)PB)2(Avo)2, II, and W2(T(i)PB)2(AvoBF2)2, III, have been prepared. As solid compounds I and II are blue while compound III is green. Characterization of these compounds has been carried out by means of (1)H NMR, MALDI-TOF MS, steady-state absorption and emission spectroscopies, and femtosecond and nanosecond transient absorption and time-resolved infrared spectroscopies.

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)PB)4, where T(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-2H-1λ(3),3,2λ(4)-dioxaborinin-4-yl)benzoic acid, HAvoBF2, the compounds Mo2(T(i)PB)2(Bzald)2, I; Mo2(T(i)PB)2(Avo)2, II; and Mo2(T(i)PB)2(AvoBF2)2, III, have been isolated. Compounds I and II are red, and compound III is blue. The new compounds have been characterized by (1)H NMR, MALDI-TOF MS, steady-state absorption and emission spectroscopies, and femtosecond and nanosecond time-resolved transient absorption and infrared spectroscopies.

Photophysical properties of cis-Mo2 quadruply bonded complexes and observation of photoinduced electron transfer to titanium dioxide.

The compounds cis-Mo2(DAniF)2(L)2 have been prepared, where DAniF = (N,N')-p-dianisyl formamidinate and L = thienyl-2-carboxylate (Th), 2,2'-bithienyl-5-carboxylate (BTh), and 2,2':5',5″-terthienyl-5-carboxylate (TTh). The compounds have been characterized by proton nuclear magnetic resonance ((1)H NMR), ultraviolet-visible (UV-vis) absorption and emission, differential pulse voltammetry, and time-resolved transient absorption and infrared (IR) spectroscopy. An X-ray crystal structure was obtained for the thienyl complex.

Coordination of N,N-chelated Re(CO)3Cl units across a Mo2 quadruple bond: synthesis, characterization, and photophysical properties of a Re-Mo2-Re triad and its component pieces.

2-(2-Pyridyl)-4-methylthiazole carboxylic acid (PMT-H) and rhenium tricarbonyl chloride react to form the red crystalline compound fac-Re(PMT-H)(CO)3Cl, I, which is an analog of the well-known Re(bpy)(CO)3Cl molecule, where bpy is 2,2'-bipyridine. The acids PMT-H (2 equiv) and Re(PMT-H)(CO)3Cl (2 equiv) also react with Mo2(T(i)PB)4 (T(i)PB = 2,4,6-triisopropylbenzoate) in toluene to give the red compound trans-Mo2(T(i)PB)2(PMT)2, II, and the royal blue compound trans-Mo2(T(i)PB)2[(PMT)Re(CO)3Cl]2, III, respectively. The X-ray and spectroscopic characterization of I confirms its close relationship with Re(bpy)(CO)3Cl, as does the spectroscopic characterization of compounds II and III as analogs of other compounds of the form trans-M2(TiPB)2L2, where L is a π-acceptor ligand.

Photoinduced charge transfer involving a MoMo quadruply bonded complex to a perylene diimide.

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.

Electronic structure and excited-state dynamics of the molecular triads: trans-M2(T(i)PB)2[O2CC6H5-η6-Cr(CO)3]2, where M = Mo or W, and T(i)PB = 2,4,6-triisopropylbenzoate.

From the reactions between M(2)(T(i)PB)(4) and HO(2)CC(6)H(5)-η(6)-Cr(CO)(3) (2 equiv), the title compounds trans-M(2)(T(i)PB)(2)[O(2)CC(6)H(5)-η(6)-Cr(CO)(3)](2), where M = Mo or W, and T(i)PB = 2,4,6-triisopropylbenzoate have been prepared and characterized. Compound I (M = Mo) was characterized by a single crystal X-ray structural determination which revealed a centrosymmetric MoMo quadruply bonded molecule. Compound I is red and the tungsten complex II is blue as a result of intense metal-to-ligand charge transfer (MLCT), which is principally M(2)δ to benzoate π* with some chromium t(2g) participation, according to calculations employing density functional theory.

Photophysical properties of MM quadruply bonded complexes supported by carboxylate ligands, MM = Mo2, MoW, or W2.

While chemists have extensively studied the photophysical properties of d(6), d(8), and d(10) transition metal complexes, their early transition metal counterparts have received less attention. Quadruply bonded complexes of molybdenum and tungsten supported by carboxylate ligands have intense metal-to-ligand charge transfer (MLCT) absorptions that arise from the electronic coupling of the metal-metal (MM) δ orbital with the CO(2) π-system. This coupling may in turn be linked to an extended π-conjugated organic functional group.

Photophysical properties of MM quadruply bonded complexes (M = Mo, W) supported by carboxylate ligands: charge delocalization and dynamics in S1 and T1 states.

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(*).

Molecular and electronic structures and photophysical properties of quadruply bonded dimetal complexes (M = Mo or W) supported by trans-arylethynylcarboxylate ligands where aryl = p-tolyl or 9-anthrancenyl.

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).

Furan- and selenophene-2-carboxylato derivatives of dimolybdenum and ditungsten (M[quadruple bond]M): a comparison of their chemical and photophysical properties.

From the reactions between M(2)(T(i)PB)(4), where T(i)PB = 2,4,6-triisopropylbenzoate and two equivalents each of 2-furan carboxylic acid, FuCO(2)H, and 2-selenophene carboxylic acid, SpCO(2)H in toluene, the new compounds trans-M(2)(T(i)PB)(2)(O(2)CFu)(2) (1a M = Mo, 2a M = W) and trans-M(2)(T(i)PB)(2)(O(2)CSp)(2) (1b M = Mo, 2b M = W) were formed. These new compounds have been characterized by (1)H NMR, steady-state UV-Vis-NIR absorption and emission spectroscopy, cyclic and differential pulse voltammetry, and fs and ns transient absorption spectroscopy. The compound Mo(2)(T(i)PB)(2)(O(2)CSp)(2) (1b) has been characterized by single crystal X-ray crystallography.

Detection of the singlet and triplet MM δδ* states in quadruply bonded dimetal tetracarboxylates (M = Mo, W) by time-resolved infrared spectroscopy.

The compounds M(2)(O(2)C(t)Bu)(4) and M(2)(O(2)CC(6)H(5))(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 ((1)MLCT) band. In the region from 1500 to 1600 cm(-1), a long-lived excited state (>2 ns) has been detected for the compounds M(2)(O(2)C(t)Bu)(4) and Mo(2)(O(2)C-C(6)H(5))(4) with an IR absorption at ~1540 cm(-1) assignable to the asymmetric CO(2) stretch, ν(as)(CO(2)), of the triplet metal-metal δ-δ star ((3)MM δδ*) state. The fs-TRIR spectra of W(2)(O(2)C-C(6)H(5))(4) are notably different and are assigned to decay of the MLCT states.

A multidisciplinary approach to probing enthalpy-entropy compensation and the interfacial mobility model.

In recent years, interfacial mobility has gained popularity as a model with which to rationalize both affinity in ligand binding and the often observed phenomenon of enthalpy-entropy compensation. While protein contraction and reduced mobility, as demonstrated by computational and NMR techniques respectively, have been correlated to entropies of binding for a variety of systems, to our knowledge, Raman difference spectroscopy has never been included in these analyses. Here, nonresonance Raman difference spectroscopy, isothermal titration calorimetry, and X-ray crystallography were utilized to correlate protein contraction, as demonstrated by an increase in protein interior packing and decreased residual protein movement, with trends of enthalpy-entropy compensation.

Electron delocalization in the S1 and T1 metal-to-ligand charge transfer states of trans-substituted metal quadruply bonded complexes.

The singlet S(1) and triplet T(1) photoexcited states of the compounds containing MM quadruple bonds trans-M(2)(T(i)PB)(2)(O(2)CC(6)H(4)-4-CN)(2), where T(i)PB = 2,4,6-triisopropylbenzoate and M = Mo (I) or M = W (I(')), and trans-M(2)(O(2)CMe)(2)((N[(i) Pr ])(2)CC ≡ CC(6)H(5))(2), where M = Mo (II) and M = W (II(')), have been investigated by a variety of spectroscopic techniques including femtosecond time-resolved infrared spectroscopy. The singlet states are shown to be delocalized metal-to-ligand charge transfer (MLCT) states for I and I(') but localized for II and II(') involving the cyanobenzoate or amidinate ligands, respectively. The triplet states are MoMoδδ* for both I and II but delocalized (3)MLCT for I(') and localized (3)MLCT for II(').

Photophysical studies of trans-bis(phenylethynyldiisopropylamidinato)bis(acetato)dimetal complexes involving MM quadruple bonds where M = Mo or W.

The title compounds trans-M(2)(O(2)CMe)(2)[C((i)PrN)(2)C≡C-Ph](2), I (M = Mo) and II (M = W), show electronic absorptions in the visible region of the spectrum assignable to (1)MLCT [M(2)δ to phenylethynylamidinate π*]. These compounds show dual emission from S(1) and T(1) states. For both I and II, S(1) is (1)MLCT, but for I the T(1) state is shown to be MMδδ* while for II T(1) is (3)MLCT.

Concerning the photophysical properties of Re2(4+) and Re2(6+) carboxylate compounds.

The preparation and structure of Re(2)(dppm)(2)(O(2)CC(6)H(4)-p-NO(2))(2)Cl(2), where dppm = Ph(2)PCH(2)PPh(2), is reported together with its photophysical properties (absorption, steady state emission, fs- and ns-transient absorption spectroscopy) and electrochemistry. These data are compared with photophysical studies on the previously reported Re(2)(dppm)(2)(O(2)CCH(3))(2)Cl(2). The preparation of the complex Re(2)(O(2)CC(6)H(4)-p-NO(2))(4)Cl(2) is also reported together with its photophysical properties which allows for a comparison of the electronic structures and photophysical states of Re(2)(4+) and Re(2)(6+) containing complexes having MM configurations σ(2)π(4)δ(2)δ(*2) and σ(2)π(4)δ(2), respectively.

Molecular, electronic structure and spectroscopic properties of MM quadruply bonded units supported by trans-6-carboethoxy-2-carboxylatoazulene ligands.

The reaction between M(2)(TiPB)(4) (M = Mo, W) where TiPB = 2,4,6-triisopropylbenzoate and 6-carboethoxy-2-azulenecarboxylic acid (2 equiv.) in toluene leads to the formation of complexes M(2)(TiPB)(2)(6-carboethoxy-2-azulenecarboxylate)(2). Compound (M = Mo) is blue and compound (M = W) is green.

2-Thienylcarboxylato and 2-thienylthiocarboxylato ligands bonded to MM quadruple bonds (M = Mo or W): a comparison of ground state, spectroscopic and photoexcited state properties.

The compounds M(2)(TiPB)(2)(OSC-2-Th)(2) have been prepared from the reactions between M(2)(TiPB)(4) and Th-2-COSH (2 equiv) in toluene solution, where M = Mo (Mo(2)ThCOS) or W (W(2)ThCOS), 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 M(2)(O(2)CH)(2)(OSC-2-Th)(2) indicate that the highest occupied molecular orbital (HOMO) is principally M(2)delta and the lowest unoccupied molecular orbital (LUMO) is thienylthiocarboxylate pi* but with significant metal-sulfur mixing.

Sexithiophenes mediated by MM quadruple bonds: MM = Mo2, MoW, and W2.

The reactions between MM(TiPB)(4), where TiPB = 2,4,6-triisopropylbenzoate and MM = MoW and W(2), 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 = Mo(2). The compounds have been characterized by elemental analysis, (1)H 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.

Quadruply bonded dimetal units supported by 2,4,6-triisopropylbenzoates MM(TiPB)(4) (MM = Mo(2), MoW, and W(2)): preparation and photophysical properties.

The preparation and characterization (elemental analysis, (1)H NMR, and cyclic voltammetry) of the new compounds MM(TiPB)(4), where MM = MoW and W(2) and TiPB = 2,4,6-triisopropylbenzoate, are reported. Together with Mo(2)(TiPB)(4), previously reported by Cotton et al. (Inorg.

Ultrafast carbene-carbene isomerization.

The photochemistry of two isomeric aryl diazo ketones was investigated by fs time-resolved UV-vis and IR spectroscopies. Both diazo ketone excited states decompose in less than 300 fs by multiple pathways. One pathway involves concerted Wolff rearrangement and nitrogen extrusion, most likely in the syn rotomer.

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.

Study of concerted and sequential photochemical Wolff Rearrangement by femtosecond UV-vis and IR spectroscopy.

Photoinduced Wolff rearrangements were studied by femtosecond time-resolved UV-vis and IR transient absorption spectroscopy. For BpCN2COCH3 in acetonitrile the IR data indicate the presence of at least two mechanisms of ketene formation. The first process is fast proceeding in either 1BpCN2COCH3*, or in a hot carbene, or in both species, while the second is slow proceeding through the intermediacy of a relaxed carbene.

The dynamics of carbene solvation: an ultrafast study of p-biphenylyltrifluoromethylcarbene.

Ultrafast photolysis (lambda(ex) = 308 nm) of p-biphenylyltrifluoromethyl diazomethane (BpCN2CF3) releases singlet p-biphenylyltrifluoromethylcarbene (BpCCF3) which absorbs strongly at 385 nm in cyclohexane, immediately after the 300 fs laser pulse. The initial absorption maximum shifts to longer wavelengths in coordinating solvents (nitrile, ether, and alcohol). In low viscosity coordinating solvents, the initial absorption maximum further red shifts between 2 and 10 ps after the laser pulse.

Ultrafast study of 9-diazofluorene: direct observation of the first two singlet states of fluorenylidene.

Ultrafast photolysis of 9-diazofluorene (DAF) produces a broadly absorbing transient within the instrument time resolution (300 fs), which is assigned to an excited state of the diazo compound. The diazo excited state fragments to form fluorenylidene (Fl) in both its lowest energy singlet state (1Fl, 405-430 nm, depending on the solvent) and a higher energy singlet state (370 nm, 1Fl*). The excited singlet carbene has a lifetime of 20.