Ultrafast transient absorption spectra in the deep to near UV range (212-384 nm) were measured for the [Cu(II)(MeOH)(5)Cl](+) complexes in methanol following 255-nm excitation of the complex into the ligand-to-metal charge-transfer excited state. The electronically excited complex undergoes sub-200 fs radiationless decay, predominantly via back electron transfer, to the hot electronic ground state followed by fast vibrational relaxation on a 0.4-4 ps time scale. A minor photochemical channel is Cu-Cl bond dissociation, leading to the reduction of copper(II) to copper(I) and the formation of MeOH·Cl charge-transfer complexes. The depletion of ground-state [Cu(II)(MeOH)(5)Cl](+) perturbs the equilibrium between several forms of copper(II) complexes present in solution. Complete re-equilibration between [Cu(II)(MeOH)(5)Cl](+) and [Cu(II)(MeOH)(4)Cl(2)] is established on a 10-500 ps time scale, slower than methanol diffusion, suggesting that the involved ligand exchange mechanism is dissociative.
Oxidative damage to purine nucleic acid bases proceeds through quinoidal intermediates derived from their corresponding 8-oxo-7,8-dihydropurine bases. Oxidation studies of 8-oxo-7,8-dihyroadenosine and 8-oxo-7,8-dihydroinosine indicate that these quinoidal species can produce stable cross-links with a wide variety of nucleophiles in the 2-positions of the purines. An azide precursor for the adenosine iminoquinone has been synthesized and applied in ultrafast transient absorption spectroscopic studies. Thus, the adenosine iminoquinone can be observed directly, and its susceptibility to nucleophilic attack with various nucleophiles as well as the stability of the resulting cross-linked species have been evaluated. Finally, these observations indicate that this azide might be a very useful photoaffinity labeling agent, because the reactive intermediate, adenosine iminoquinone, is such a good mimic for the universal purine base adenosine.
Copper(II) complexes are extremely labile with typical ligand exchange rate constants on the order of 10 6 −10 9 M −1 s −1 . As a result, it is often difficult to identify the actual formation mechanism of these complexes. In this work, using UV−vis transient absorption when probing in a broad time range (20 ps to 8 μs) in conjunction with DFT/TDDFT calculations, we studied the dynamics and underlying reaction mechanisms of the formation of extremely labile copper(II) CuCl 4 2− chloro complexes from copper(II) CuCl 3 − trichloro complexes and chloride ions. These two species, produced via photochemical dissociation of CuCl 4 2− upon 420 nm excitation into the ligand-to-metal-charge-transfer electronic state, are found to recombine into parent complexes with bimolecular rate constants of (9.0 ± 0.1) × 10 7 and (5.3 ± 0.4) × 10 8 M −1 s −1 in acetonitrile and dichloromethane, respectively. In dichloromethane, recombination occurs via a simple one-step addition. In acetonitrile, where [CuCl 3 ]− reacts with the solvent to form a [CuCl 3 CH 3 CN] − complex in less than 20 ps, recombination takes place via ligand exchange described by the associative interchange mechanism that involves a [CuCl 4 CH 3 CN] 2− intermediate. In both solvents, the recombination reaction is potential energy controlled.
Graphical abstractHighlights Pyrenedihydrodioxins(PDHDs) are very effective DNA binding agents. Pyrenequinone is released photochemicallyfrom PDHDs. Pyrenequinone cleaves DNA ribose backbones and damages bases in a localized fashion Pyrenequinone initiates its photochemical release from PDHD autocatalytically. Electron capture by the coordinated action of two pyridinium rings is described. ABSTRACTPyrenedihydrodioxins (PDHD) comprise effective DNA intercalation agents that are masked ortho-quinones, which can be released by near ultraviolet or visible irradiation. We have studied the binding and photoreactions of chiral dipyridiniumPDHDs with herring sperm DNA and an 11-mer duplex DNA containing all 10 basepair steps. Binding affinities to herring sperm DNA were determined for purified enantiomers (K b = 1.6 ±0.15 ×10 5 and 2.3 ± 0.2 ×10 5 M -1 ). UVmelting experiments using the 11-mer DNA revealed significant stabilization of duplex DNA, 2 (T m = 11.5° and T m = 15.3° C). Both enantiomers linearized (double-strand cleavage) supercoiled X174 plasmid DNA with high efficiency. PDHDs have specificity to cleave and/or damage DNA duplexes at Gs and have preferable binding to GG DNA sites.A full range of transient spectroscopy from the ultrafast femtosecond to the microsecond domains has been applied in the study of this system. These studies have revealed a novel mechanism for quinone release via the pyrene radical cation and the entrapment of the released electron by the coordinated action of the two pyridinium rings. These same studies have shown that the released pyrenequinone can photochemically initiate the further release of pyrenequinone. Thus, this reaction is autocatalytic and can be initiated with visible light.
Carbocyclic geminal dibromides are important intermediates in synthesis of complex heterocyclic molecules and natural products. a Ultrafast time-resolved techniques can help to obtain information about the electronic structure of the intermediates which appears in the chemistry and photochemistry of these molecules. Also, relaxation dynamics and reactivity of these intermediates in different solvents can be characterized. This information can be used for the design of different compounds with desired properties. In the current work, 1,1-dibromocycloalkanes with 3-, 4-, and 5-member rings were synthesized using previously described procedures. b c d For all three samples, ultrafast time-resolved absorption experiments with UV-excitation were performed in acetonitrile and methylcyclohexane solvents. It was shown that excitation of dibromocycloalkane solutions with 250 nm short (40 fs) pulses forms excited state absorption (ESA) of parent molecules in the spectral range from 360 to 760 nm. Within 800 fs, ESA decays with beginning of formation product bands. Next within 50 ps, these broad bands reach their maximum intensity and form well-defined broad peaks centered at 550, 600 and 400 nm for 3-, 4-, and 5-member rings respectively. These product species are long-lived and begin to decay at 1 ns. Obtained results suggest the formation of isomer products (CH 2 )n − C − Br − Br (n=1-3), similar to the isomeric species (HBrCBr-Br) observed in isomerization of bromoform. e f a
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