Laser Photolysis Studies Research Articles

Laser photolysis studies on Cu I complexes of thia-calix[3]pyridine. Phosphorescence from the intramolecular charge-transfer excited state

Copper(I) complexes of thia-calix[3]pyridine in dichloromethane exhibit phosphorescence from the MLCT triplet states at room temperature. The phosphorescence spectrum shifts to red on going from 300 to 180 K. The laser photolysis study carried out in the temperature range 300–180 K reveals that phosphorescence originates from the two triplet states, T H and T L, which are in thermal equilibrium with the energy difference of ca. 5 kcal mol −1.

The Aryl Cation – the Trapping and Characterisation of a Hyper-Reactive Species

The article traces the development of the epr and optical spectroscopic characterisation and progressive theoretical understanding of the aryl cation, starting from the initial cryogenic epr study of 2,5-dialkoxyl-4-morpholinophenyl cation, which defined unambiguously its triplet state character, through simple MO, ab initio and DFT calculations on substituted aryl cations, to picosecond laser photolysis studies in fluorinated solvents, noting especially the early and vital contribution of Martyn Symons.

Picosecond and femtosecond laser photolysis studies of a photochromic diarylethene derivative: multiphoton gated reaction

Cycloreversion (ring-opening) process of one of the photochromic diarylethene derivatives, 1,2-bis(2-methyl-3-benzothienyl)perfluorocyclopentene (BPFC), was investigated by means of picosecond and femtosecond laser photolysis methods. The drastic enhancement of the reaction yield was observed by the picosecond laser exposure. On the other hand, the cycloreversion reaction yield under femtosecond laser exposure was consistent with the steady-state light irradiation. The excitation intensity effect of the reaction profiles revealed that the successive multiphoton absorption process leading to higher excited states opened the efficient cycloreversion process.

Picosecond laser photolysis studies on photoinduced electron transfer processes in poly(1-vinylpyrene) in solutions

The primary processes of photoinduced electron transfer (ET) dynamics in poly(1-vinylpyrene) (PVPy) doped with electron acceptors were investigated by means of picosecond transient absorption measurement in solutions with different polarities. The simple scheme (Scheme 1) based on a one-dimensional random walk model that the cation radical continuously migrates along pendant aromatic moieties in the polymer chain interprets the hole transfer (HT) dynamics in the longer delay time after the excitation. However, in the early stage, the deviation observed due to the dimer cation formation, which acts as a trap site for the cation and a very slow HT process occurred as a result of a detrapping process. By integrating the present results with the previous investigation on other aromatic vinyl polymers, the role of the dimer cation on the hole migration process along the polymer chain were discussed.

Laser photolysis studies of endoergonic triplet energy transfer in solution by observing the carbon–sulfur bond cleavage of triplet-sensitized naphthylmethyl phenyl sulfide

A bimolecular rate constant ( k q) and an efficiency ( α TET) of an endoergonic triplet energy transfer from triplet α-naphthyl phenyl ketone (NPK) to α-naphthylmethyl phenyl sulfide (NMPS) in solution, where the energy difference (Δ E T) is 1.7 kcal mol −1 , have been determined by means of nanosecond laser flash photolysis. Base on the Arrhenius plots of the k q value for the NPK–NMPS system, an activation energy (Δ E a) for the endoergonic triplet energy transfer is found to be 2.6 kcal mol −1 , which is larger than the endothermic energy gap ( ΔE T =1.7 kcal mol −1 ). The endoergonic triplet energy transfer in solution is demonstrated to proceed with the aid of thermal activation for the first time.

Cl − Concentration Dependence of Photovoltage Generation by Halorhodopsin from Halobacterium salinarum

The photovoltage generation by halorhodopsin from Halobacterium salinarum (shR) was examined by adsorbing shR-containing membranes onto a thin polymer film. The photovoltage consisted of two major components: one with a sub-millisecond range time constant and the other with a millisecond range time constant with different amplitudes, as previously reported. These components exhibited different Cl − concentration dependencies (0.1–9 M). We found that the time constant for the fast component was relatively independent of the Cl − concentration, whereas the time constant for the slow component increased sigmoidally at higher Cl − concentrations. The fast and the slow processes were attributed to charge (Cl −) movements within the protein and related to Cl − ejection, respectively. The laser photolysis studies of shR-membrane suspensions revealed that they corresponded to the formation and the decay of the N intermediate. The photovoltage amplitude of the slow component exhibited a distorted bell-shaped Cl − concentration dependence, and the Cl − concentration dependence of its time constant suggested a weak and highly cooperative Cl −-binding site(s) on the cytoplasmic side (apparent K D of ∼5 M and Hill coefficient ≥5). The Cl − concentration dependence of the photovoltage amplitude and the time constant for the slow process suggested a competition between spontaneous relaxation and ion translocation. The time constant for the relaxation was estimated to be >100 ms.

Laser Photolysis Studies of Oxy- and Carbonylhemoglobin in Red Blood Cells. Effects of Cell Membrane on Reversible Binding of O2 and CO

Nanosecond laser photolysis studies of hemoglobin in red blood cells (RBCs) were performed to elucidate the effects of cell membrane on the oxygen uptake and release mechanism of RBCs. Oxy RBCs readily photodissociated O2 by the 355-nm laser pulse to give deoxy RBCs, which return to oxy RBCs at 1 atm air within ca. 50 μs after the pulse. The decay of deoxy RBCs is expressed as a sum of the two exponential functions of time, indicating that the oxygen-rebinding reaction is composed of the two processes. The rate constants, kf and ks, for the faster and slower processes of the oxygen rebinding, measured as a function of the oxygen concentration, revealed that (1) kf is independent of the oxygen concentration, [O2], and (2) ks asymptotically increases with an increase in [O2] to a limiting value. These kinetic results were markedly different from those obtained with the laser photolysis of oxyhemoglobin molecules in aqueous solutions. On the basis of the kinetic studies for oxygen rebinding, the cell membrane of RBCs is suggested to dominate the rates for oxygen uptake and release of hemoglobin in RBCs. For comparison with oxygen, the CO-binding reactions of deoxy RBCs and deoxyhemoglobin molecules in aqueous solutions were studied by laser flash photolysis. The decay profiles of deoxy RBCs produced by laser photolysis of carbonyl RBCs at various CO concentrations were identical with those of deoxyhemoglobin molecules in aqueous solutions. Thus, in contrast to O2, CO molecules are supposed to freely diffuse in and out of RBCs through the cell membrane.

Studies on reactivity of benzoyl and benzoylperoxy radicals produced by laser flash photolysis of dibenzoyldiazene in aerated solutions

Photolysis of dibenzoyldiazene gives benzoyl radicals. In aerated solutions, the benzoyl radicals react with oxygen to yield benzoylperoxy radicals. Spin trapping studies indicate that 5,5′dimethyl-1-pyrroline N-oxide reacts with the benzoylperoxy radicals to produce the adduct which exhibits ESR parameters, AN = 13.8 G and AHβ = 10.1 G. Laser photolysis studies reveal that the rate constants for the reaction between the benzoyl radical and oxygen are ca. 4 × 109 M-1 s-1 in toluene, acetone, and ethyl acetate. The benzoylperoxy radicals undergo one-electron oxidation of tetramethyl-p-phenylenediamine, TMPD, to give an ion pair. The ion pair has an absorption spectrum similar to that of the TMPD cation radical. The formation of the ion pair is detected by monitoring the absorbance change at 600 nm after laser pulsing. From the kinetic studies for the formation of the ion pair in the presence of olefins, the bimolecular rate constants for reactions between several olefins and the benzoylperoxy radical are determined. The electrophilic addition of the benzoylperoxy radicals to olefins is discussed in comparison with the addition reactions of thiyl radicals to olefins. The detection and determination of the dipole moments of both the benzoylperoxy radicals and the ion pair are carried out with the use of the time-resolved microwave dielectric absorption technique. The distance between the positive and negative ions in the ion pair is estimated as 0.20 nm.

Laser photolysis studies of the carbon–sulfur bond cleavage induced in the triplet exciplex of benzylnaphthyl sulfide and aromatic ketones

The photoinduced dissociation of the carbon–sulfur bond in benzyl-α-naphthyl sulfide (BNS) in acetonitrile has been studied by laser flash photolysis. Upon direct photoexcitation of BNS, C–S bond fission occurs in the excited singlet state, resulting in the formation of the α-naphthylthiyl and benzyl radicals with a quantum yield of 0.22. Triplet sensitization of BNS by xanthone and benzophenone causes the simultaneous formation of the lowest triplet state (T1) of BNS and the radicals. The efficiencies of the formations of the T1 state and the radicals are 0.53 and 0.41, respectively. When triplet BNS decays, further formation of the radicals is observed with an efficiency of 0.95. The decay rate of triplet BNS is enhanced non-linearly with increasing concentration of the ketone. The second cleavage can be interpreted by considering the formation of the triplet exciplex which consists of triplet BNS and ketone. The mechanism of the efficient C–S bond dissociation ia the triplet exciplex is discussed.

Photodissociation of Nitric Oxide from Nitrosyl Metalloporphyrins in Micellar Solutions

Laser-photolysis studies of nitrosyl metalloporphyrins (MP), (NO)FeIIHem (Hem = protoporphyrin IX), (NO)CoIIOEP (OEP = octaethylporphyrin), and (NO)MnIITPP (TPP = tetraphenylporphyrin), in aqueous ionic micellar solutions were carried out. The nitrosyl porphyrins in the micellar solutions readily photodissociate NO, leaving the MP in micelles: the quantum yields are 0.15 (±0.1) for (NO)FeIIHem, 0.55 (±0.05) for (NO)CoIIOEP, and 0.21 (±0.02) for (NO)MnIITPP. The MP thus produced recombine with NO to regenerate the parent nitrosyl porphyrins. The decay of MP in the absence of the excess NO follows second-order kinetics with the rate constant kMNO(second) (MNO = FeNO, CoNO, and MnNO). In the presence of excess NO, the decay of MP follows pseudo-first-order kinetics with the rate constant kM (M = Fe, Co, and Mn). The value of kM was measured as a function of [NO]. For FeIIHem and CoIIOEP, the plots of kCo vs [NO] and kFe vs [NO] gave straight lines. The slopes of the lines obtained with FeIIHem and CoIIOEP afford the bimolecular rate constants kFeNO(pseudo) and kCoNO(pseudo), respectively. It is found that kFeNO(pseudo) > kFeNO(second) and kCoNO(pseudo) > kCoNO(second). The differences between kMNO(pseudo) and kMNO(second) observed for FeIIHem and CoIIOEP are interpreted by assuming that (1) NO molecules in the micellar solutions are dissolved in both micelles and the aqueous phase and (2) NO molecules trapped in micelles hardly react with MP because of the electrostatic repulsion between ionic micelles. In the case of MnIITPP, the pseudo-first-order rate constant, kMn, is found to asymptotically increase with an increase in [NO] to a limiting value. The reaction mechanisms for the nitrosylation of MP in micellar solutions are discussed in detail on the basis of the kinetic studies.

Photooxidation of 2 ′-deoxyguanosine 5 ′-monophosphate (dGMP) by flavin adenine dinucleotide (FAD) via electron transfer: a laser photolysis study

The study of the photosensitization mechanism of dGMP by flavin adenine dinucleotide (FAD) as sensitizer was carried out by time-resolved 308 and 248 nm laser flash photolysis. Direct evidence of electron transfer from dGMP to either triplet state of FAD ( 3 FAD *) or oxidized FAD radical (FAD + /FAD(−H) ) was obtained. A comparison of nucleotide-free and -bound riboflavin suggests that the nucleotide environment does not markedly influence sensitizer reactivity.

Structure and photochemistry of dicyanocobalt(III) tetraphenylporphyrin. Photochromic reaction caused by photodissociation of axial ligand.

Chlorocobalt(III) tetraphenylporphyrin, (Cl)CoIIITPP, reacts with potassium cyanide in dichloromethane or benzene containing 18-crown-6 to give a green solution of [crown-K+][(CN)2CoIIITPP-]. The molecular structure of [crown-K+][(CN)2CoIIITPP-] is identified by X-ray crystallography. In methanol, (Cl)CoIIITPP plus KCN also gives a green solution of [(CN)2CoIIITPP-]. The green methanol solution containing 1.4 x 10(-4) M KCN turns orange by continuous photolysis with a 250-W mercury lamp for 5 min. The orange solution returns to green when it is kept in the dark for 5 min. The kinetic study suggests that [(CN)2CoIIITPP-] dissociates CN- by continuous photolysis, giving rise to the formation of the orange species, (CH3OH)(CN)CoIIITPP. The photoproduct, (CH3OH)(CN)CoIIITPP, regenerates the green species, [(CN)2CoIIITPP-], by reaction with CN-. The laser photolysis study of [(CN)2CoIIITPP-] in methanol demonstrates that photodissociation of CN- takes place within 20 ns after the 355-nm laser pulse, resulting in the formation of two transients, I (short-lived) and II (long-lived). The absorption spectra of both transients are similar to that of (CH3OH)(CN)CoIIITPP. These transients eventually return to [(CN)2CoIIITPP-]. The decay of species I follows first-order kinetics with a rate constant k. = 2 x 10(6) s-1, independent of the concentration of KCN. Species II is identified as (CH3OH)(CN)CoIIITPP, which is observed with the continuous photolysis of the solution. The laser photolysis of [crown-K+][(CN)2COIIITPP-] in dichloromethane gives the transient species, which goes back to the original complex according to first-order kinetics with a rate constant k = 5 x 10(6) s-1. [crown-K+][(CN)2CoIIITPP-] is concluded to photodissociate the axial CN- to form [crown-K+CN-][(CN)CoIIITPP] in which an oxygen atom of the crown moiety in [crown-K+CN-] is coordinated to the cobalt(III) atom of [(CN)CoIIITPP] at the axial position. The intracomplex reverse reaction of [crown-K+CN-][(CN)CoIIITPP] leads to the regeneration of [crown-K+][(CN)2CoIIITPP-]. The structure and the reaction of the transient species I observed for [(CN)2CoIIITPP-] in methanol are discussed on the basis of the laser photolysis studies of [crown-K+][(CN)2CoIIITPP-] in dichloromethane.

Laser Photolysis of Chromium(III) Porphyrins with Axial Pyridines in Dichloromethane and Toluene Solutions. Novel Effects of a Hydrogen Bond in the Ligand Exchange Reaction

Laser photolysis studies were carried out for (chloro)(pyridine)(5,10,15,20-tetraphenylporphyrinato)chromium(III), [Cr(TPP)(Cl)(Py)], in both dichloromethane and toluene containing water. The five-coordinate [Cr(TPP)(Cl)] produced by the photoinduced dissociation of pyridine from [Cr(TPP)(Cl)(Py)] initially reacts with H2O to give [Cr(TPP)(Cl)(H2O)], which eventually exchanges the axial H2O with Py to regenerate [Cr(TPP)(Cl)(Py)]. The rate for the ligand exchange of [Cr(TPP)(Cl)(H2O)] with exogenous Py is found to exhibit a bell-shaped pyridine-concentration dependence. Kinetic studies revealed that at a high Py concentration, the exogenous Py probably makes a hydrogen bond with the axial H2O of [Cr(TPP)(Cl)(H2O)] to yield [Cr(TPP)(Cl)(HO−H···Py)] as a dead-end complex. A similar structure of the Cr−TPP complex having 2-methylpyridine molecules bound to the coordinated H2O ligand by a hydrogen bond was determined by X-ray structure analysis. The exchange reaction of the axial HO−H···Py in [Cr(TPP)(Cl)(HO−...

Nanosecond laser photolysis studies of chlorosomes and artificial aggregates containing bacteriochlorophyll e: evidence for the proximity of carotenoids and bacteriochlorophyll a in chlorosomes from Chlorobium phaeobacteroides strain CL1401.

Time-resolved, laser-induced changes in absorbance, delta A(lambda; t), have been recorded with a view to probing pigment-pigment interactions in chlorosomes (control as well as carotenoid-depleted) and artificial aggregates of bacteriochlorophyll e (BChle). Control chlorosomes were isolated from Chlorobium phaeobacteroides strain CL1401, whose chromophores comprise BChle, bacteriochlorophyll a (BChla) and several carotenoid (Car) pigments; Car-depleted chlorosomes, from cells grown in cultures containing 2-hydroxybiphenyl. Artificial aggregates were prepared by dispersing BChle in aqueous phase in the presence of monogalactosyl diglyceride. In chlorosomes delta A(lambda; t) shows, besides a signal attributable to triplet Car (with a half-life of about 4 microseconds), signals in the Qy regions of both BChl. The BChla signal decays at the same rate as the Car signal, which is explained by postulating that some Car are in intimate contact with some baseplate BChla pigments, and that when a ground-state Car changes into a triplet Car, the absorption spectrum of its BChla neighbors undergoes a concomitant change (termed transient environment-induced perturbation). The signal in the Qy-region of BChle behaves differently: its amplitude falls, under reducing conditions, by more than a factor of two during the first 0.5 microsecond (a period during which the Car signal suffers negligible diminution), and is much smaller under nonreducing conditions. The BChle signal is also attributed to transient environment-induced perturbation, but in this case the perturber is a BChle photoproduct (probably a triplet or a radical ion). The absence of long-lived BChle triplets in all three systems, and of long-lived BChla triplets in chlorosomes, indicates that BChle in densely packed assemblies is less vulnerable to photodamage than monomeric BChle and that, in chlorosome, BChla rather than BChle needs, and receives, photoprotection from an adjacent Car.

Characterization of the transient species generated by the photoexcitation of C-phycocyanin from Spirulina platensis: a laser photolysis and pulse radiolysis study

Nanosecond laser flash photolysis and pulse radiolysis were used to generate and characterize the triplet state and cation radical of C-phycocyanin (C-PC) from Spirulina platensis. The transient absorption spectra of C-PC were measured from direct excitation and acetone sensitization in aqueous solution at room temperature by KrF (248 nm) laser flash photolysis. Laser-induced transient species have been characterized by the method of acetone sensitization and one-electron oxidation. In nitrous oxide-saturated phosphate buffer saline (pH=7.0) of C-PC, the produced intermediates are assigned to the excited triplet state and the radical cation. Using acetone as photosensitizer, the C-PC excited triplet states produced via triplet-triplet energy transfer and the C-PC radical cation from electron transfer reaction were further confirmed. Furthermore, the corresponding kinetic parameters were determined. To our knowledge, the transient absorption spectra of C-PC have been reported for the first time.

Nanocrystalline Mesoporous Strontium Titanate as Photoelectrode Material for Photosensitized Solar Devices: Increasing Photovoltage through Flatband Potential Engineering

Nanocrystalline SrTiO3 is synthesized by hydrothermal treatment of nanocrystalline titanium dioxide in the presence of strontium hydroxide. Working photoelectrochemical solar cells are produced using these nanometer-sized semiconductor particles as photoelectrode materials. At AM 1.5, measured open circuit voltages were roughly 100 mV higher than in solar cells produced using nanocrystalline titanium dioxide (anatase), in agreement with a simple relation between semiconductor conduction band edge and open circuit voltage for these cells. Photocurrents measured in the SrTiO3 cells were roughly 1/3 those measured with TiO2 (anatase) -based cells. On the basis of flash laser photolysis and absorptance studies, we suggest that low dye loading and possibly suboptimal dye−oxide interactions can be the cause for the relatively low photocurrents in the SrTiO3 system.

Picosecond laser photolysis studies of DMA–DMPP in solution

Picosecond transient absorption spectra of: 4-(4′- N,N-dimethylaminophenyl)–3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-[3,4-b;4′,3′-e]-pyridine (DMA–DMPP), 3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-[3,4-b;4′,3′-e]-pyridine (BPP) and 3,5-dimethyl-1,4,7-triphenyl-bis-pyrazolo-[3,4-b;4′,3′-e]-pyridine (H-DMPP) were measured in solvents of different polarity. The results revealed the previously postulated change of the character of the fluorescing state from a primary excited, low polar state in non-polar solvents to a CT state in highly polar ones. Transient absorption spectra in the equilibrium fluorescent state of DMA–DMPP in polar solvents comprised the band ascribable to the cation radical of the DMA moiety. The charge transfer process is fastest in methanol and acetonitrile; in the former it is not controlled by a dynamic Stokes shift.

Nanosecond laser photolysis studies of vitamin K3 in aqueous solution

Abstract Vitamin K 3 in aqueous solution was investigated by 248 nm laser flash photolysis. Laser-induced transient species were characterized according to kinetic analysis and quenching experiments by Mn 2+ and O 2 . In neutral solutions, the intermediates recorded were assigned to excited triplet states and dehydrogenated radicals of vitamin K 3 . In comparison with the results of pulse radiolytical experiment, vitamin K 3 not only has strong electron affinity but could also could be photoionized by UV laser light. All this shows that vitamin K 3 acts as an effective electron carrier and electron transfer agent.

Laser Photolysis Studies on Photodissociation of Axial Ligands from Isocyanide Complexes of Cobalt(III) and Rhodium(III) Porphyrins in Toluene Solutions. A Comparison with the Photochemistry of Carbonylrhodium(III) Porphyrin

Chlorocobalt(III) and iodorhodium(III) tetraphenylporphyrins, (Cl)CoIIITPP and (I)RhIIITPP, react with 2,6-dimethylphenyl isocyanide (R−NC) in toluene solutions to give the isocyanide complexes, (C...

Laser-Photolysis Studies of Isocyanide Complexes of Chlorochromium(III) Tetraphenylporphyrin in Toluene Solutions

Chloroaquachromium(III) tetraphenylporphyrin, Cl(H2O)CrIIITPP, reversibly reacts with isocyanide, CN−R, to form Cl(CN−R)CrIIITPP in toluene solutions containing 4.4 × 10-3 M H2O and CN−R. The equilibrium constant for the formation of Cl(CN−R)CrIIITPP obtained by spectroscopic measurements ranges from 10 to 102 depending on the nature of the isocyanide used. The laser-photolysis studies of the toluene solution demonstrate that CN−R is photodissociated from Cl(CN−R)CrIIITPP to produce ClCrIIITPP with a quantum yield ca. 0.1. The five-coordinate ClCrIIITPP reacts with H2O and CN−R to give Cl(H2O)CrIIITPP and Cl(CN−R)CrIIITPP, respectively. The transient product Cl(H2O)CrIIITPP further reacts with CN−R to regenerate Cl(CN−R)CrIIITPP. The rates for the decay of Cl(H2O)CrIIITPP are measured as a function of the concentration of CN−R in order to determine the rate constants of the forward and backward reactions for the formation of Cl(CN−R)CrIIITPP. The equilibrium constants obtained from the kinetic analysis fo...