Pulsed Photolysis Research Articles

Free Base and Metal Complexes of 5,15-Diaza-10,20-dimesitylporphyrins: Synthesis, Structures, Optical and Electrochemical Properties, and Aromaticities

The synthesis, structures, optical and electrochemical properties, and aromaticity of a series of 5,15-diaza-10,20-dimesitylporphyrins (M-DAP; M = Pb, H(2), Ni, Pd, Pt, Zn; mesityl = 2,4,6-trimethylphenyl) are reported. Treatment of mesityl-substituted bis(5,5'-dibromodipyrrin) with sodium azide in the presence of lead(II) acetylacetonate afforded Pb-DAP, which was quantitatively converted to H(2)-DAP by acidolysis. The free base H(2)-DAP reacted with palladium(II), platinum(II), and zinc(II) salts to give Pd-DAP, Pt-DAP, and Zn-DAP, respectively. The crystal structures, optical and electrochemical properties, and aromaticities of these β-unsubstituted M-DAPs were comprehensively investigated by X-ray crystallography, UV-vis absorption/fluorescence spectroscopy, nanosecond flash photolysis, cyclic and differential pulse voltammetry, NMR spectroscopy, and density functional theory calculations. The obtained data show that replacement of the 5- and 15-methine carbons with nitrogen atoms alters the intrinsic properties of the porphyrin 18π system as follows: (i) the coordination spheres at the N(4) core become contracted while keeping high planarity; (ii) the Q bands are red shifted and largely intensified; (iii) the electron-accepting ability is enhanced, whereas the electron-donating ability is reduced; (iv) the radiative decay rates from the S(1) state are enhanced; and (v) the aromaticity of the 18π circuit is slightly reduced in terms of both geometric and magnetic criteria. These optical and electrochemical properties of M-DAPs stem from their characteristic frontier orbitals; two HOMOs and two LUMOs are nondegenerate as a result of the incorporation of the electronegative nitrogen atoms at the two meso positions. In addition, the group 10 metals incorporated at the core finely tune the fundamental properties of DAP π systems through inductive effects as well as dπ-pπ antibonding orbital interactions; the HOMO-LUMO gaps of the group 10 metal complexes increase in the order Ni-DAP < Pd-DAP < Pt-DAP.

Absolute Rate Coefficient and Mechanism of Gas Phase Reaction of Ketenyl Radical and SO2

The kinetics of the gas phase reaction of the ketenyl radical with SO(2) was investigated over the temperature range 296-568 K using a laser-photofragment/laser-induced fluorescence technique (LP/LIF). The reactor pressure was 10 Torr N(2) or He. Pulsed photolysis of ketene (CH(2)CO) at 193 nm was used as the source of HCCO radicals. The rate coefficient for the title reaction was determined to be described by k(T) = (1.05 ± 0.33) × 10(-12) exp[(690 ± 98)K/T] cm(3) s(-1) molecule(-1) (2σ error). We applied the coupled cluster and density functional theory to explore the mechanism of the title reaction. The dominant reaction pathway begins with a barrierless association of the C of the CH group of HCCO and the O atom of SO(2).

Laboratory Kinetic and Mechanistic Studies on the OH-Initiated Oxidation of Acetone in Aqueous Solution

The OH-initiated oxidation of acetone in aqueous solution is investigated because of its potential implications in atmospheric chemistry. The UV-spectrum of the transient acetonylperoxy radical was measured. Two characteristic absorption bands of the acetonylperoxy radical spectrum are found in the 220-400 nm wavelength region. The rate constant for the recombination reaction of the acetonylperoxy radical was determined as a function of temperature for the first time in aqueous solution with k(rec,298K) = (7.3 ± 1.3) × 10(8) M(-1) s(-1), E(A) = 4.5 ± 3.3 kJ mol(-1), and A = (4.7 ± 2.7) × 10(9) M(-1) s(-1). Furthermore, kinetic investigations of the OH-initiated oxidation of methylglyoxal and pyruvic acid were performed with the following results: for methylglyoxal, k(second) = (6.2 ± 0.2) × 10(8) M(-1) s(-1), E(A) = 12 ± 2 kJ mol(-1), and A = (7.8 ± 0.2) × 10(9) M(-1) s(-1); for pyruvic acid (pH = 0), k(second) = (3.2 ± 0.6) × 10(8) M(-1) s(-1), E(A) = 15 ± 5 kJ mol(-1), and A= (1.1 ± 0.1) × 10(11) M(-1) s(-1); for pyruvate (pH = 6), k(second) = (7.1 ± 2.4) × 10(8) M(-1) s(-1), E(A) = 25 ± 19 kJ mol(-1), and A = (1.5 ± 0.4) × 10(13) M(-1) s(-1). Quantitative product studies were done as a function of the number of laser photolysis pulses for acetone and its oxidation products methylglyoxal, hydroxyacetone, pyruvic acid, acetic acid, and oxalic acid. After the recombination reaction of acetonylperoxy radicals, there are two possible decomposition reactions where the primary products methylglyoxal and hydroxyacetone are formed. From product analysis after a single photolysis laser shot, the ratio of the main product-forming reactions was determined as (A) 30% and (B) 56% for the methylglyoxal formation via channel A to yield two molecules of methylglyoxal and channel B to yield one molecule of methylglyoxal and one molecule of hydroxyacetone. The remaining product can be ascribed to channel C, the radical-retaining channel forming alkoxy radicals with a yield of 14%. Pyruvic acid and acetic acid were found to be the major intermediates estimated with concentrations in the same order of magnitude and a similar time profile, indicating that acetic acid is also a possible oxidation product of methylglyoxal.

Formation and relaxation dynamics of iso-CH2Cl–I in cryogenic matrices

Photolysis of chloroiodomethane (CH(2)ClI) in cryogenic matrices followed by recombination of the nascent radical pair produces an isomer (CH(2)Cl-I) that features a halogen-halogen (Cl-I) bond. Using ultrafast laser pulses, it is possible to follow the formation of this isomer by transient electronic absorption in low-temperature matrices of N(2), CH(4), and Ar. Frequency-domain measurements provide vibrational and electronic spectra, and electronic structure calculations give the structures of the isomers and the minimum energy path that connects them. The ultrafast experiments cleave the C-I bond with a 267-nm photolysis pulse and probe the formation of the isomer at wavelengths between 435 nm and 510 nm. The longest wavelengths preferentially interrogate vibrationally excited molecules, and their transient absorption shows that the highly vibrationally excited isomer appears within 1 to 2 ps, depending on the matrix, likely reflecting the loss of 2000 cm(-1) or more of energy in a strong, inelastic collision of the fragments with the matrix. The subsequent relaxation of the vibrationally excited isomer occurs in 20 to 40 ps, a time that is comparable to those observed for halomethane molecules and their isomers in liquids and in supercritical CO(2). These observations suggest that the formation and initial relaxation of the isomer in dense media do not depend strongly on the identity of the surroundings.

Measurement of Absolute Absorption Cross Sections for Nitrous Acid (HONO) in the Near-Infrared Region by the Continuous Wave Cavity Ring-Down Spectroscopy (cw-CRDS) Technique Coupled to Laser Photolysis

Absolute absorption cross sections for selected lines of the OH stretch overtone 2ν(1) of the cis-isomer of nitrous acid HONO have been measured in the range 6623.6-6645.6 cm(-1) using the continuous wave cavity ring-down spectroscopy (cw-CRDS) technique. HONO has been generated by two different, complementary methods: in the first method, HONO has been produced by pulsed photolysis of H(2)O(2)/NO mixture at 248 nm, and in the second method HONO has been produced in a continuous manner by flowing humidified N(2) over 5.2 M HCl and 0.5 M NaNO(2) solutions. Laser photolysis synchronized with the cw-CRDS technique has been used to measure the absorption spectrum of HONO produced in the first method, and a simple cw-CRDS technique has been used in the second method. The first method, very time-consuming, allows for an absolute calibration of the absorption spectrum by comparison with the well-known HO(2) absorption cross section, while the second method is much faster and leads to a better signal-to-noise ratio. The strongest line in this wavelength range has been found at 6642.51 cm(-1) with σ = (5.8 ± 2.2) × 10(-21) cm(2).

Theoretical and experimental investigation of the C 2H + SO 2 reaction over the range T = 295–800 K

Absolute rate constants for the gas-phase reaction C 2H + SO 2 are experimentally determined over the temperature range 295–800 K. C 2H radicals are generated by pulsed 193-nm photolysis of C 2H 2 in the presence of SO 2 and He or N 2 buffer gas. The temperature dependence of the rate constants is established as k SO 2 ( T) = (0.86 ± 0.08) T 3.80±0.13 exp[−(1222 ± 79)/ T] cm 3 s −1. The rate constants are moderately high at these temperatures and show negative temperature dependence. CCSD(T)/6-311++G(3df,2p) calculations of the potential energy surface confirm experimental findings and show that the reaction products are HCCO + SO.

Photolysis of 6-ethoxy-2,2,4-trimethyl-8-nitro-1,2-dihydroquinoline. Unusual dependence on the irradiation wavelength

The spectral and kinetic parameters of transient species generated in the irradiation of 6-ethoxy-2,2,4-trimethyl-8-nitro-1,2-dihydroquinoline were examined by stationary and pulse photolysis in the solvents: heptane, acetonitrile, methanol, and ethanol. Upon excitation of the long-wavelength absorption band (λex > 450 nm), a reversible photochemical reaction was revealed, and the spectral and kinetic parameters of three transient species observed in the photolysis were characterized (λmax = 390, 400, and 420 nm (acetonitrile), k = 97, 500, and 2000 s−1, respectively). The absorption spectra and the rate constants of the decay of transient species are almost independent of the medium polarity and the presence of oxygen in the system. The excited state generated during irradiation to the short-wavelength absorption band (290 < λex < 350 nm) is inactive in the photochemical reaction and deactivates without the formation of transient species. The mechanism of the reversible photochemical reaction is suggested, which involves the opening of the heterocycle N-C bond upon photoexcitation of the long-wavelength absorption band and the thermal back reaction.

Direct Photoreduction and Ketone-Sensitized Reduction of Nitrospirobenzopyranindolines by Aliphatic Amines

The photoreduction of 6-nitrospiro[2H-1-benzopyran-2,2'-indoline] (N1) and two derivatives (N2 and N3) by diethylamine or triethylamine (TEA) in solution was studied by pulsed and steady-state photolysis. The quantum yield of coloration of the ring-closed Sp form, due to photoinduced ring opening, decreases in acetonitrile with increasing the TEA concentration. The main reason is reaction of TEA with the triplet-excited open merocyanine form. Quenching of this triplet state by amines is rather inefficient for N1-N3; the rate constant for triplet quenching by TEA is k(6) = (2-3) × 10(6) M(-1) s(-1). The secondary transient with an absorption maximum at 420 nm is ascribed to the radical anion. This and the corresponding α-aminoethyl radical subsequently undergo slow termination reactions, yielding a relatively stable product with a maximum at 420-450 nm, which is attributed to a ring-opened dihydromerocyanine (MH(-)). The mechanisms of the two subsequent reduction reactions are discussed. Using acetone as sensitizer the same dihydroproduct was obtained with the Sp form as acceptor, indicating a reaction sequence from photogenerated radicals via a ring-opened radical to MH(-)/MH(2). The effect of TEA concentration on the direct and ketone-sensitized reduction mechanisms was analyzed. Photoreduction by amines, due to competing triplet quenching, is strongly decreased on admission of oxygen.

Photochemistry of Dithiocarbamate Cu(II) Complex in CCl4

Laser pulse photolysis was used to study the nature and reactions of intermediates in the photochemistry of the flat dithiocarbamate complex Cu(Et(2)dtc)(2) in CCl(4). A nanosecond laser pulse (355 nm) is shown to induce intermediate absorption bands of bivalent copper complex whose coordination sphere contains a dithiocarbamate radical Et(2)dtc(•) and a chloride ion at the axial position ([(Et(2)dtc)Cu(Et(2)dtc(•))Cl(a)]). At room temperature during some microseconds after the laser pulse, this intermediate interacts with the initial complex to form presumably a dimer [Cu(2)(Et(2)dtc)(3)(Et(2)dtc(•))Cl]. The latter vanishes in the second-order reaction. Analysis of kinetic and spectral features gives the arguments for the formation of a cluster [Cu(2)(Et(2)dtc)(3)Cl-tds-Cu(2)(Et(2)dtc)(3)Cl], which produces a new absorption band at 345 nm. The cluster decomposes in ∼5 ms into final products, a binuclear complex [Cu(2)(Et(2)dtc)(3)Cl] and tetraethylthiuramdisulfide (Et(4)tds).

Hetero-oligomerization of Neuronal Glutamate Transporters

Excitatory amino acid transporters (EAATs) mediate the uptake of glutamate into neuronal and glial cells of the mammalian central nervous system. Two transporters expressed primarily in glia, EAAT1 and EAAT2, are crucial for glutamate homeostasis in the adult mammalian brain. Three neuronal transporters (EAAT3, EAAT4, and EAAT5) appear to have additional functions in regulating and processing cellular excitability. EAATs are assembled as trimers, and the existence of multiple isoforms raises the question of whether certain isoforms can form hetero-oligomers. Co-expression and pulldown experiments of various glutamate transporters showed that EAAT3 and EAAT4, but neither EAAT1 and EAAT2, nor EAAT2 and EAAT3 are capable of co-assembling into heterotrimers. To study the functional consequences of hetero-oligomerization, we co-expressed EAAT3 and the serine-dependent mutant R501C EAAT4 in HEK293 cells and Xenopus laevis oocytes and studied glutamate/serine transport and anion conduction using electrophysiological methods. Individual subunits transport glutamate independently of each other. Apparent substrate affinities are not affected by hetero-oligomerization. However, polarized localization in Madin-Darby canine kidney cells was different for homo- and hetero-oligomers. EAAT3 inserts exclusively into apical membranes of Madin-Darby canine kidney cells when expressed alone. Co-expression with EAAT4 results in additional appearance of basolateral EAAT3. Our results demonstrate the existence of heterotrimeric glutamate transporters and provide novel information about the physiological impact of EAAT oligomerization.

I2 dissociation by O2(a1Δ) generated from the reaction O(1D)+N2O

Abstract A new kinetic scheme for the dissociation of I 2 by O 2 ( a 1 Δ) has been proposed by Azyazov et al. [7] . The reactions initiated by UV photolysis of N 2 O/I 2 mixtures have been used to probe details of this dissociation model, and provide additional validation. Pulsed photolysis of both N 2 O and I 2 , was used to generate I( 2 P 1/2 ), I( 2 P 3/2 ), and O 2 ( a 1 Δ). Emission spectroscopy and laser induced fluorescence techniques were used to follow the time evolutions of I( 2 P 1/2 ) and I 2 . Suppression of the chain propagation stage of I 2 dissociation has been observed for the first time. Computational modeling indicated that the observed kinetics are consistent with the revised dissociation model.

ChemInform Abstract: Conclusive Evidence for Site-Specific C-H Bond Cleavage Resulting from 248 nm Photolysis of the Ethyl Radical.

Experiments involving two photolysis lasers and one probe laser demonstrate that 248 nm excimer laser radiation will induce C–H bond cleavage preferentially at the β position in the ethyl radical. To facilitate carbon site labeling, selectively deuterated chloroethanes (ClCH2CD3 and ClCD2CH3) are used as precursor compounds. Two‐photon ionization via resonance with the Lyman‐α transition is used to detect H (or D) atoms. An initial 193 nm photolysis pulse serves to cleave the C–Cl bond in ClCH2CH3, while a second pulse at 248 nm dramatically enhances H‐atom production. Experiments on ClCH2CD3 and ClCD2CH3 clearly show that this enhancement occurs preferentially through carbon–hydrogen bond cleavage at the β carbon site. It is apparent that 248 nm photon absorption by the ethyl radical is an important step in the overall mechanism.

ChemInform Abstract: pH-Induced Effects on the Photophysics of Dipyridyl Ketones

The photophysics of dipyridyl ketones (DPKs) has been investigated in aqueous solutions as a function of pH using pulsed and steady-state emission spectroscopy and nanosecond laser flash photolysis. All six DPK isomers were found to emit phosphorescence in aqueous frozen solutions at 77 K, but only three of them exhibited phosphorescence emissions and showed triplet transient absorption at room temperature. From both phosphorescence and triplet–triplet absorption titrations, they revealed proton-transfer reactions in the excited state occurring at different pH/H0 values than the ground-state acid–base equilibrations. Comparing results from different techniques gave evidence that the carbonyl oxygen is involved in the triplet state protonation (pK*≈ 3.5–6) for the isomers where it conjugates with the ring nitrogen atom, while proton addition prevalently occurs on the nitrogen for the unconjugated meta–meta isomer. A second protolytic equilibrium (pK*≈–2 to –1.6) implies formation of nitrogen diprotonated ions for all the isomers. The presence of two positive charges in the molecules prevents further protonation at the carbonyl group in the ground states as well as in the excited states.

Effect of the solvent composition in methanol-pentane and methanol-acetonitrile mixtures on the quantum yield of proton transfer and reactions of transient species in the photolysis of 2,2,4,6-tetramethyl-1,2-dihydroquinoline

The photolysis of 2,2,4,6-tetramethyl-1,2-dihydroquinoline in binary MeOH-C5H12 and MeOH-MeCN mixtures was studied in dependence on the solvent composition by steady state and pulse photolysis. The photoinduced proton transfer from the N-H bond to the C(3) carbon atom of the heterocycle does not occur up to the methanol concentration of 0.25 (1 vol %) and 2.5 mol l−1 (10 vol %) in the MeOH-C5H12 and MeOH-MeCN mixtures, respectively. The trends in the increase in the relative quantum yield of the proton transfer reaction (Qmix/QMeOH) and the decay rate constant for 2,2,4,6-tetramethyl-2,3-dihydroquinoline, the product of the proton transfer reaction, in dependence on the composition differ significantly for these binary mixtures. The results are interpreted in terms of peculiarities in the aggregation of methanol and the distribution of the DHQ molecules and transient species in the mixtures.

Photocyclization of 2,6-dichlorodiphenylamines in solution

The reactions of diclofenac, meclofenamic acid and 2,6-dichlorodiphenylamine were studied by pulsed and steady-state photolysis. The primary photoprocess of diclofenac is ring closure, the quantum yield of cyclization in dichloromethane and aqueous solution is Φ cyc = 0.03 and 0.2, respectively. The results of the two related dichlorodiphenylamines are similar in the respect that the products are the corresponding 1-chlorocarbazoles and Φ cyc is small in organic solvents and largest in aqueous solution.

Towards a quantitative study of the VUV photolysis of methane: preliminary experiment on trichloromethane

Photolysis of methane in Titan's stratosphere is the starting point of gas phase carbon chemistry. Quantitative studies of methane photolytic products are of utmost importance for Titan atmosphere models. With this aim, two experimental strategies are presented in this article. Preliminary results demonstrate the possibility of using CRDS absorption coupled with pulsed photolysis on the example of a halogenated derivative of methane: Trichloromethane (CHCl3 ).

Effect of the method of production of TiO2/CdS nanohetero film structures on the effectiveness of photoinduced charge separation

The properties of TiO2/CdS nanohetero structures produced by the chemical and photocatalytic deposition of CdS nanoparticles on the surface of nanocrystalline films of TiO2 under the conditions of laser pulse photolysis were studied. It was shown that in the case of photocatalytically formed TiO2/CdS nanocomposites the spatial separation of the photogenerated charges between the components of the hetero structure, leading to the formation of intermediates of the photochemical transformations (TiIII centers in the TiO2 nanoparticles and S– radical-anions in the CdS nanoparticles), is an order of magnitude more effective.

Reactivity of arylnitroso oxides to triphenylphosphine

The kinetics of reactions between phenylnitroso oxide, 4-CH3O-, 4-CH3-, or 4-Br-phenylnitroso oxide and triphenylphosphine in acetonitrile at 295 ± 2 K were studied using pulsed photolysis. Only trans-nitroso oxides enter this reaction. The rate constants of the reaction increase with increasing electron-acceptor properties of the substituent in the aromatic ring of nitroso oxide; they are on the order of 105 to 106 l mol−1 s−1. The extinction coefficient for trans-4-methylphenylnitroso oxide at 420 nm was estimated at 3.9 × 103 l mol−1 cm−1.

Effect of the nature of inert solvent on the decay kinetics of the carbocation generated in the photolysis of 1,2-dihydroquinolines in methanol

The dependence of the quantum yield and the decay rate constant for the carbocation generated in the photolysis of 6-ethoxy-1,2,2,4-tetramethyl-1,2-dihydroquinoline (6-EtO-DHQ) in methanol on the solvent composition was studied in the mixtures of methanol with isopropanol, acetonitrile, and pentane by pulse photolysis. The addition of these solvents decreases the yield of the carbocation and differently affects the kinetic parameters of its decay. The carbocation decay in the mixtures MeOH-i-PrOH and MeOH-C5H12 is described by the pseudo-first order equation (k 1), with the dependence of k 1 having a maximum at 50 vol % of MeOH in the MeOH-i-PrOH mixtures, and k 1 increasing with a decrease in the MeOH concentration in the MeOH-C5H12 mixtures. In the MeOH-MeCN mixtures, the value of k 1 decreases with a decrease in the MeOH concentration, and, at the concentration of MeOH lower than 50 vol %, the contribution of the second-order reaction (k 2) is observed. The activation energies and preexponential factors were determined in the MeOH-C5H12 mixtures of different compositions, and it was shown that E act practically did not depend on the solvent composition and were close to E act for other carbocations obtained in MeOH. The increase in k 1 with a decrease in the MeOH concentration is caused by an increase in the preexponential factor. The results were discussed on the basis of the reaction mechanism involving the competing reactions of the carbocation combination with two nucleophilic particles, the MeOH molecule and the MeO− anion. The composition of the mixture and the nature of the inert solvent affect strongly the course of these reactions.

Unusually Slow Photodissociation of CO from (η6-C6H6)Cr(CO)3 (M = Cr or Mo): A Time-Resolved Infrared, Matrix Isolation, and DFT Investigation

The photochemistry of (η6-C6H6)M(CO)3 (M = Cr or Mo) is described. Photolysis with λexc. > 300 nm of (η6-C6H6)Cr(CO)3 in low-temperature matrixes containing CO produced the CO-loss product, while lower energy photolysis (λexc. > 400 nm) produced Cr(CO)6. Pulsed photolysis (λexc. = 400 nm) of (η6-C6H6)Cr(CO)3 in n-heptane solution at room temperature produced an excited-state species (1966 and 1888 cm−1) that decays over 150 ps to (η6-C6H6)Cr(CO)2(n-heptane) (70%) and (η6-C6H6)Cr(CO)3 (30%). Pulsed photolysis (λexc. = 266 nm) of (η6-C6H6)Cr(CO)3 in n-heptane produced bands assigned to (η6-C6H6)Cr(CO)2(n-heptane) (1930 and 1870 cm−1) within 1 ps. These bands increase with a rate identical to the rate of decay of the excited-state species and the rate of recovery of (η6-C6H6)Cr(CO)3. Photolysis of (η6-C6H6)Mo(CO)3 at 400 nm produced an excited-state species (1996 and 1898 cm−1) and traces of (η6-C6H6)Mo(CO)2(n-heptane) within 1 ps. For the chromium system CO-loss can occur following excitation at both 400 and 266 nm via an avoided crossing of a MACT (metal-to-arene charge transfer) and MCCT/LF (metal-to-carbonyl charge transfer/ligand field) states. This leads to an unusually slow CO-loss following excitation with 400 nm light. Rapid CO-loss is observed following 266 nm excitation because of direct population of the MCCT/LF state. The quantum yield for CO-loss in the chromium system decreases with increasing excitation energy because of the competing population of a high-energy unreactive MACT state. For the molydenum system CO-loss is a minor process for 400 nm excitation, and an unreactive MACT state is evident from the TRIR spectra. A higher quantum yield for CO-loss is observed following 266 nm excitation through both direct population of the MCCT/LF state and production of a vibrationally excited reactive MACT state. This results in the quantum yield for CO-loss increasing with increasing excitation energy.