Intra- and intermolecular electronic relaxation of the second excited singlet and the lowest excited triplet states of 1,3-dimethyl-4-thiouracil in solution

Intramolecular processes of deactivation of 1,3-dimethyl-4-thiouracil (DMTU) from the second excited singlet (S2) (pi, pi*) and the lowest excited triplet (T1) (pi, pi*) states have been studied using perfluoro-1,3-dimethylcyclohexane (PFDMCH) as a solvent. The spectral and photophysical (PP) properties of DMTU in CCl4, hexane and water have also been described. For the first time, the fluorescence from S2 state DMTU has been observed. The picosecond lifetime of DMTU in the S2 state (tau(S2)) in PFDMCH has been proposed to be determined by a very fast intramolecular reversible process of hydrogen abstraction from the ortho methyl group by the thiocarbonyl group. The shortening of tau(S2) in CCl4 is interpreted to be caused by the intermolecular interactions between DMTU (S2) and the solvent. Results of the phosphorescence decay as a function of DMTU concentration were analyzed using the Stern-Volmer formalism, which enabled determination of the intrinsic lifetime of the T1 state (tau0(T1)) and rate constants of self-quenching (k(sq)). The lifetimes, tau0(T1), of DMTU in PFDMCH and CCl4 are much longer than the values hitherto obtained in more reactive solvents. The PP properties of DMTU both in the S2 and T1 states have been shown to be determined by the thiocarbonyl group.     

Quantum-chemical study of the structure of the acetyl fluoride molecule in the ground and lowest excited singlet and triplet electronic states

The structure of the conformationally flexible acetyl fluoride molecule (CH3CFO and CD3CFO) in the ground (S0) and lowest excited triplet (T1) and singlet (S1) electronic states was calculated by different quantum-chemical methods (RHF, UHF, MP2, CASSCF). The equilibrium geometric parameters and harmonic vibrational frequencies of the molecules in these electronic states were estimated. The calculations demonstrated that the electronic excitation causes considerable conformational changes involving the rotation of the CH3(CD3) top and a substantial deviation of the CCFO carbonyl fragment from planarity. For large-amplitude vibrations, namely, for the torsional vibration in the S0 state and the torsional and inversion (nonplanar carbonyl fragment) vibrations in the T1 and S1 states, the quantum-mechanical problems were solved in one-dimensional (1D) and two-dimensional (2D) approximations. The results of calculations are in good agreement with experimental data.     

Synthesis and characterization of one- to three-dimensional compounds composed of paradodecatungstate-B cluster and transition metals as linkers

Three new extended frameworks built from paratungstate and transition metals have been synthesized and characterized. In the compound Na₈[{Cd (H₂O)₂}(H₂W₁₂O₄₂)]·32H₂O (1), two neighboring paratungstate-B ions [H₂W₁₂O₄₂]¹⁰⁻ are linked by [Cd(H₂O)₂]²⁺ units, leading to the formation of infinite one-dimensional (1D) anion chain [{Cd(H₂O)₂}(H₂W₁₂O₄₂)]_n⁸ⁿ⁻. The anion [{Co(H₂O)₃}{Co(H₂O)₄}(H₂W₁₂O₄₂)]_n⁶ⁿ⁻ of the compound Na₆[{Co(H₂O)₃}{Co(H₂O)₄}(H₂W₁₂O₄₂)]·29H₂O (2) shows a layer-like (2D) structure in which paratungstate-B units are linked by CoO₆ octahedra, while the anion [{Co(H₂O)₃}₃(H₂W₁₂O₄₂)]_n⁴ⁿ⁻ of the compound (H₃O⁺)₃[{Na(H₂O)₄}{Co(H₂O)₄}₃(H₂W₁₂O₄₂)]·24.5H₂O (3) is a three-dimensional (3D) anionic polymer that consists of paratungstate-B units linked by CoO₆ octahedra. Compound 3 can reversibly adsorb and desorb water molecules leading to the color reversibly change from pink to violet. The preliminary magnetic measurement and electrochemical properties of compounds are performed. The crystal structure of unexpected product Na₄[NiW₆O₂₄H₆]·13H₂O (4) is described here for the rare report of crystal structure information on the Anderson-type polyoxotungstate which has nickel as a heteroatom.     

A study of interaction in the lowest singlet and triplet states of H2

The potential energy curves of the ground state and the first excited state of H₂ are examined in terms of the electronic force acting on each nucleus. The results reveal the detailed course of events that occur when two hydrogen atoms with parallel and antiparallel electron spins approach one another from a large internuclear separation.     

Syntheses and structures of mononuclear and binuclear transition metal complexes (Cu,Zn, Ni) with (salicylideneglycine and imidazole)

Four transition metal (Cu(II), Zn(II) and Ni(II)) complexes with a Schiff-base ligand (salicylideneglycine) have been synthesized. All complexes have been characterized by elemental analysis, IR spectra and UV-vis spectroscopy. Single-crystal analyses were performed with (C₉H₇NO₃)Cu(C₃H₄N₂) (1), (C₉H₇NO₃)Zn(C₃H₄N₂)₂ (2), (C₉H₇NO₃)₂Ni₂(C₃H₄N₂)₄ (3) and (C₉H₇NO₃)Ni(C₃H₄N₂)₂(C₄H₅N₂O) · CH₃OH · 0.5H₂O (4) and fluorescence spectra and thermogravimetric analyses were also carried out. Structural analyses show that 1, 2 and 4 have similar coordinated modes with the tridentate amino-Schiff-base ligand, but differ from the binuclear nickel complex 3. The tridentate amino-Schiff-base ligand contains aliphatic nitrogen, phenoxy, and carboxylic oxygen as three donor atoms. In addition, inter- and intra-molecular hydrogen bonds are also discussed.     

Theoretical study of the structure of the CClF2NO and CCl2FNO molecules in the ground and lowest excited singlet and triplet electronic states

The potential energy surfaces of the nitroso compounds CClF₂NO and CCl₂FNO in the ground and lowest excited singlet and triplet electronic states were studied by various ab initio methods (including multiconfigurational methods). The equilibrium geometric parameters, vibrational frequencies, internal rotation potential functions, and rotational contours of bands in the S₁ S₀ vibronic spectrum of the CClF₂NO molecule were calculated. For the molecules under consideration, the quantum-mechanical problem on torsional motion was solved. The results of calculations are, on the whole, in good agreement with experiment.     

Synthesis,structure, and biological evaluation of three Cu(II) and Ni(II) (E)-3-(3,4-dimethoxyphenyl)acrylate complexes with organic diamines as potential urease inhibitors

Three new complexes (1–3) have been synthesized and characterized by X-ray single crystal determination and evaluated for inhibitory activity on jack bean urease. All the complexes contained a new cinnamic acid derivative as the ligand (C₁₁H₁₂O₄), (E)-3-(3,4-dimethoxyphenyl)acrylic acid, and crystallized in monoclinic C2/c space group. Complex 1 (C₁₁H₁₁O₄)₄(C₃N₂H₈)₂Cu₂ (C₃N₂H₈?=?1,2-diaminopropane) was obtained with a?=?20.488(2), b?=?19.596(2), c?=?15.2500(13), β?=?93.502(2)°, V?=?6111.2(10)?ų, Z?=?4, R ₁ ?=?0.0616, and wR ₂ ?=?0.2059. Complex 2 (C₁₁H₁₁O₄)₄(C₃N₂H₈)₂Cu₂ (C₃N₂H₈=1,3-diaminopropane) was obtained with a?=?20.2494(12), b?=?19.5732(12), c?=?14.8940(8), β?=?96.884(2)°, V?=?5860.6(6)?ų, Z?=?4, R ₁ ?=?0.0409, and wR ₂ ?=?0.1107. Complex 3 (C₁₁H₁₁O₄)₂(C₂N₂H₆)₂Ni₂·H₂O (C₂N₂H₆?=?ethylenediamine) was obtained with a?=?28.359(2), b?=?6.5422(5), c?=?16.8587(14), β?=?101.359(2)°, V?=?3066.5(4)?ų, Z?=?4, R ₁ ?=?0.0422, and wR ₂ ?=?0.1190. It was found that copper(II) complexes 1 [IC₅₀?=?4.71?μM] and 2 [IC₅₀?=?3.15?μM] showed strong inhibitory activity against jack bean urease compared with acetohydroxamic acid [IC₅₀?=?10.01?μM] as a positive reference. Unfortunately, 3 exhibited no inhibitory activity.     

Exploring 9‐arylcarbazole moiety as the building block for the synthesis of photoluminescent group 10–12 heavy metal diynes and polyynes with high‐energy triplet states

A new series of organic‐soluble and thermally stable group 10 platinum(II) polyyne polymers functionalized with 9‐arylcarbazole moiety trans‐[? Pt(PBu₃)₂C?CRC?C? ]_n (R = 9‐arylcarbazole‐3,6‐diyl; aryl = phenyl, p‐methylphenyl, p‐fluorophenyl) were prepared in good yields by Hagihara's dehydrohalogenative polymerization of trans‐[PtCl₂(PBu₃)₂] with HC?CRC?CH under ambient conditions. The regiochemical structures of the polymers were characterized by multinuclear NMR spectroscopy. We discuss the optical spectroscopy of these polymetallaynes and compare the results with their bimetallic molecular model complexes trans‐[Pt(Ph)(PEt₃)₂C?CRC?CPt(Ph)(PEt₃)₂] as well as its group 11 gold(I) and group 12 mercury(II) congeners [(PPh₃)AuC?CRC?CAu(PPh₃)] and [MeHgC?CRC?CHgMe]. The structural properties of several model complexes were studied by X‐ray crystallography. The influence of the heavy metal atom and the 9‐aryl substituent of carbazole on the phosphorescence behavior and the spatial distribution of the lowest singlet (S₁) and triplet (T₁) excitons in these metalated alkynyl systems are comprehensively elucidated. The present work indicates that the efficiency of organic triplet emissions harnessed through the heavy‐atom effect of group 10–12 transition metals in the main chain generally follows the order Pt > Au > Hg but the optical properties of the materials are relatively insensitive to the nature of the 9‐aryl group on the carbazolyl ring. All of these metallaynyl‐carbazole materials with high‐energy T₁ states of 2.68 eV or higher show high phosphorescence efficiencies at low temperatures. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5588–5607, 2006     

Photoinduced omega-bond dissociation in the higher excited singlet (S2) and lowest triplet (T1) states of a benzophenone derivative in solution

Photochemical properties of photoinduced omega-bond dissociation in p-benzoylbenzyl phenyl sulfide (BBPS) in solution were investigated by time-resolved EPR and laser flash photolysis techniques. BBPS was shown to undergo photoinduced omega-bond cleavage to yield the p-benzoylbenzyl radical (BBR) and phenyl thiyl radical (PTR) at room temperature. The quantum yield (phi(rad)) for the radical formation was found to depend on the excitation wavelength, i.e., on the excitation to the excited singlet states, S2 and S1 of BBPS; phi(rad)(S2) = 0.65 and phi(rad)(S1) = 1.0. Based on the CIDEP data, these radicals were found to be produced via the triplet state independent of excitation wavelength. By using triplet sensitization of xanthone, the efficiency (alpha(rad)) of the C-S bond fission in the lowest triplet state (T1) of BBPS was determined to be unity. The agreement between phi(rad)(S1) and alpha(rad) values indicates that the C-S bond dissociation occurs in the T1 state via the S1 state due to a fast intersystem crossing from the S1 to the T1 state. In contrast, the wavelength dependence of the radical yields was interpreted in terms of the C-S bond cleavage in the S2 state competing with internal conversion from the S2 to the S1 state. The smaller value of phi(rad)(S2) than that of phi(rad)(S1) was proposed to originate from the geminate recombination of singlet radical pairs produced by the bond dissociation via the S2 state. Considering the electronic character of the excited and dissociative states in BBPS showed a schematic energy diagram for the omega-bond dissociation of BBPS.     

Ab initio study of the structure of 2,2-difluoroethanal in the ground and lowest excited triplet electronic states

The molecular structure of 2,2-difluoroethanal (DFE) in the ground (S₀) and lowest excited triplet (T_i) electronic states was investigated byab initio quantum-chemical methods. In the S₀ state, the DFE molecule exists as the only stablecis conformer. The T_i↓S₀ electronic excitation is accompanied by the rotation of the top and the deviation of the carbonyl fragment from planarity. For the DFE molecule in the T_i state, six minima corresponding to three pairs of enantiomers were found on the potential energy surface. Based on this potential energy surface, the problems on torsion and inversion nuclear motions were solved in the one- and two-dimensional approximations, and the interaction between these motions was revealed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 989–995, June, 2000.     

Transmetallation reactions of [Sn(R)2(Ph2PC6H4-2-S)2] with metal complexes of the Group 10: Stereoselective synthesis of cis-[M(Ph2PC6H4-2-S)2] (M=Ni, Pd, Pt)

The complexes cis-[M(Ph₂PC₆H₄-2-S)₂] M=Ni, Pd, Pt were stereoselectively synthesized by transmetallation reactions of [M(Cl)₂(NCC₆H₅)₂] M=Pd, Pt or NiCl₂·6H₂O with [Sn(R)₂(Ph₂PC₆H₄-2-S)₂] R=Ph, ⁿBu or ^tBu. The conformation of the Pd and Pt derivatives being unequivocally confirmed by single crystal X-ray diffraction studies showing both metal centers to be into a slightly distorted square planar environment, the main distortion being due to the steric hindrance caused by the aromatic rings in the phosphine moiety.     

Bivalent ability for charge transfer in process of hydrogen interaction with surfaces of transition metals

The ability for bivalent charge transfer (CT) during hydrogen adsorption on transition metal surfaces and in the course of transition metal hydride formation is discussed. The change of the dipole moment of hydrogen adatoms, caused by transition from a strongly bound adsorption state to a weakly bound state, is demonstrated. The possibility of the CT reversion between the hydrogen adsorbate and the transition metal adsorbent, caused by a change of the surface structure, is described. The CT within the adsorbate, corresponding to the distinguished steps of the process of transition metal hydride formation, is shown.     

Thermal relaxation in excited electronic states of d and d metal complexes

Photophysical kinetic results have played an important role in assessing excited state relaxation pathways in transition metal complexes. The applicability of a kinetic analysis is critically dependent on the quality of the individual decay rates, the temperature range examined, and the model used to extract the activation parameters. The extensive literature describing the temperature dependence of excited state depopulation in d³ and d⁶ complexes permits an evaluation of both the power and limitations of kinetic arguments in assessing the mechanism of excited state relaxation.     

Interaction of Naproxen with transition metals: synthesis,characterization, anti-inflammatory activity and kinetic studies

Coordination complexes of transition metals (Co, Ni, Cu, Fe, Cr, Ru, Ir, Mn, and Zn) with 6-methoxy-α-methyl-2-naphthalene acetic acid (Naproxen) and triphenylphosphine have been synthesized and characterized by conductance, elemental analysis, UV-Vis, AAS, and FT-IR spectroscopy. The elemental analyses data reveal the presence of 1:1 (metal : ligand) stoichiometry and the IR data suggest that naproxen functions as a bidentate ligand in coordination with transition metals. The anti-inflammatory assays of these complexes have significant effect.     

Tandem transmetallation and oxidative addition reactions of [Sn(R)2(Ph2PC6H4-2-S)2] with transition metal complexes of the Group 9

The reactions of [Sn(Ph)₂(Ph₂PC₆H₄-2-S)₂] with trans -[M(Cl)(CO)(PPh₃)₂] M=Ir, Rh afford the complexes [Rh(Ph₂PC₆H₄-2-S)₂(SnClPh₂)] (1) and [Ir(CO)(Ph₂PC₆H₄-2-S)₂(SnClPh₂)] (2) as final products of two processes, a transmetallation reaction and an oxidative addition process. The crystal structures of both complexes have been determined, showing the rhodium compound to be into a slightly distorted square base pyramidal geometry, while that of the iridium derivative can be described as a distorted octahedron.     

Exploring the lowest energy triplet potential energy surface for cyclic C4H4 isomers with the complete basis set ab initio method. Is the transformation of triafulvene into cyclobutadiene possible in their excited states?

High level computational studies were performed with the aim being to explore the possibility of converting the experimentally available triafulvene into the hard-to-detect cyclobutadiene. The method is based on the simple approach used to excite triafulvene into the triplet state, and then through various reaction channels, come to the aromatic triplet cyclobutadiene. Triplet cyclobutadiene is only a few kcal/mol higher in energy than singlet cyclobutadiene and should be easily relaxed into the latter. Several reaction pathways that include only a concerted mechanism, as well as reaction pathways that include the radical formation–recombination were also explored. Some possible approaches for experimentally obtaining the singlet or triplet cyclobutadiene were suggested.     

N，N＇－二（2－羟基苄烯）－2－羟基苯甲基二胺及N，N＇－二（2－羟基苄基）－2－羟基苯甲基二胺与过渡金属配合物的合成和表征

在四氢呋喃中合成了Ｓｃｈｉｆｆ碱配体Ｎ，Ｎ＇－二（２－羟基苄烯）－２－羟基苯甲基二胺（ＳＢ）与二价过渡金属镍、铜、锌的三核或双核配合物；用钠汞齐还原ＳＢ得到了其氢化物Ｎ，Ｎ＇－二（２－羟基苄基）－２－羟基苯甲基二胺（ＨＳＢ），并在乙醇中合成了它与铜的双核配合物。对这些配合物进行了元素分析和热分析，并测定了电导、红外光谱和紫外光谱，确定了分子式，讨论了可能的配位结构。     

Synthesis,crystal structures,thermal properties,and DNA-binding studies of transition metal complexes with imidazole ligands

A new series of complexes of transition metal (Cu, Zn, Ni) perchlorate with imidazole have been synthesized and characterized by elemental analysis, infrared (IR), UV-Vis spectroscopy, and single-crystal X-ray diffraction. Based on elemental and spectral data, the complexes are M(C₃H₄N₂) _x (ClO₄)₂ (M?=?Cu, Zn, x?=?4; M?=?Ni, x?=?6; C₃H₄N₂?=?imidazole). The crystal structures of Cu(C₃H₄N₂)₄(ClO₄)₂ (1) and Zn(C₃H₄N₂)₄(ClO₄)₂ (2) show metals surrounded by four nitrogens of imidazole, while the nickel complex Ni(C₃H₄N₂)₆(ClO₄)₂ (3) has six nitrogens of imidazole. Intra- and inter-molecular hydrogen bonds exist between hydrogen of imidazole and oxygen of perchlorate. The thermal stabilities of 1, 2, and 3 at different heating rates (β?=?5°C?min^?1, 10°C?min^?1, and 15°C?min^?1) show that all the complexes exhibit two thermal decomposition stages; the sequence of thermal stability is 2?>?1?>?3. 1, 2, 3, and imidazole display DNA binding ability, ascertained by UV-Vis titration.     

Coordination properties of 1-allyl-2-methylimidazole relative to some metal ions in the solid state and aqueous solution

Cobalt(II) and copper(II) complexes with 1-allyl-2-methylimidazole (L), of general formula [ML₂(NO₃)₂], have been prepared in the solid state. The compounds were characterised by structural, spectroscopic and magnetic measurements. The metal ions in both six coordinate complexes are surrounded by two nitrogen atoms of the imidazole rings and four oxygen atoms of the chelating nitrato group (the CoN₂O₄ and CuN₂O₄ chromophores). The structure of both chromophores is described by a very distorted tetragonal bipyramid. The formation of successive complexes of the azole with Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ and Cd²⁺ in aqueous solution was followed potentiometrically. An irregularity in the K_n constants of successive Co²⁺ and Zn²⁺ complexes suggests a change in the coordination sphere of the central ions from octahedral to tetrahedral. With the Co²⁺–1-allyl-2-methylimidazole system, the change has been proven by inspection of the visible absorption spectra.