Laser Photolysis Studies Research Articles

Energy transfer between two light harvesting phthalocyanine derivatives as model for artificial photosynthetic antenna: Laser photolysis studies

The singlet-singlet and triplet–triplet energy transfer studies between two light harvesting phthalocyanine derivatives namely, zinc tert-butyl phthalocyanine ZnTBPc, and zinc octabutyloxy phthalocyanine ZnOBuPc, have been investigated to mimic the efficient energy-transfer process among the chlorophyll units in the natural photosynthesis. Optical absorption and emission studies showed the strong absorption and fluorescence bands for ZnOBuPc in the NIR region, which are largely red-shifted compared to that of the ZnTBPc. The strong overlap between the emission spectrum of ZnTBPc and the absorption spectrum of ZnOBuPc in toluene suggests the occurrence of the energy transfer from the singlet ZnTBPc (1.80 eV) to the singlet ZnOBuPc (1.59 eV). From the time-resolved fluorescence technique, the rate of energy transfer process was found to be 6.79 × 107 s−1. From the nanosecond laser photolysis measurements, the energy transfer from the triplet ZnTBPc to the lower lying triplet ZnOBuPc was clearly observed with a rate of 5.77 × 105 s−1. These observations suggest the usefulness of the examined combination (ZnOBuPc/ZnTBPc) as a simple model for the artificial antenna in the photosynthetic systems.

Laser photolysis studies of ω-bond dissociation in aromatic carbonyls of five-membered rings induced by triplet sensitization

Photoinduced ω-bond cleavage was investigated in seven newly synthesized aromatic ketones having five-membered rings (furan and thiophene) and/or phenyl group connected to the phenylthiyl moiety with CS bond. Following direct laser excitation of these ketones in acetonitrile, the transient absorption spectra that appeared were shown to be due to the corresponding triplet states. No chemical reactions occurred upon direct excitation. However, triplet sensitization of these aromatic ketones using xanthone revealed no formation of their triplet states. Instead absorption spectra of the phenylthiyl radical (PTR) appeared as a result of CS bond cleavage. These observations indicate that higher triplet states were reactive leading to CS bond dissociation. The CS bond dissociation mechanism upon triplet sensitization for the studied carbonyl compounds was discussed considering energy diagrams of the higher excited states.

Laser photolysis studies of ω-bond dissociation in aromatic carbonyls with a C-C triple bond stimulated by triplet sensitization.

We have prepared three types of carbonyl compounds, benzoylethynylmethyl phenyl sulfide (2@SPh), (p-benzoyl)phenylethynylmethyl phenyl sulfide (3@SPh) and p-(benzoylethynyl)benzyl phenyl sulfide (4@SPh) with benzoyl and phenylthiylmethyl groups, which are interconnected with a C-C triple bond and a phenyl ring. Laser flash photolysis of 3@SPh and 4@SPh in acetonitrile provided the transient absorption spectra of the corresponding triplet states where no chemical reactions were recognized. Upon laser flash photolysis of 2@SPh, the absorption band due to the phenylthiyl radical (PTR) was obtained, indicating that the C-S bond cleaved in the excited state. Triplet sensitization of these carbonyl compounds using acetone and xanthone was conducted using laser photolysis techniques. The formation of triplet 3@SPh was seen in the transient absorption, whereas the PTR formation was observed for 2@SPh and 4@SPh, indicating that the triplet states were reactive for the C-S bond dissociation. The C-S bond dissociation mechanism for 4@SPh upon triplet sensitization is discussed in comparison with those for 2@SPh and 3@SPh.

Steady state and laser photolysis studies of keto-enol tautomerizations in 2-alkyl-1,3-diketones having five-membered rings in acetonitrile: Temporal UV-A sunscreen

Abstract Keto-enol tautomerization in 2-alkyl-1,3-diketons having five-membered rings was studied using steady state and laser flash photolysis techniques in solution. The alkyl keto-diketones undergo photoinduced tautomerization mainly in the triplet state to the enol in acetonitrile. The alkyl enol-diketones, thus formed, undergo thermal tautomerization to the original keto forms in a few days. The alkyl enol-diketones show fast internal conversion from the excited singlet state to the ground state without yielding the corresponding isomeric forms (rotamer). Based on the computation for state energies of the keto, enol and isomer forms, a schematic energy diagram for the tautomerization was drawn.

Appropriate excitation wavelength removes obfuscations from pyrene excimer kinetics and mechanism studies

The experimental procedures of time-resolved laser photolysis studies of pyrene excimer formation in solution have been scrutinized and the appropriate modifications have been implemented. Contrary to the experimental methods applied in all related previous work, the selection of a suitable excitation wavelength (such that the corresponding pyrene absorbance is less than 0.5 absorbance units) utilized in our study results in simple homogeneous kinetics. Consequently, the rate parameters obtained and the mechanism proposed differ significantly from those published previously. The rate constant values of the unimolecular decay of the pyrene monomer, the unimolecular decay of the pyrene excimer, and the excimer formation in decane solution (η = 0.860 mPa s) at 25 °C are (2.38 ± 0.01) × 10(6) s(-1), (2.78 ± 0.02) × 10(7) s(-1), and (3.11 ± 0.06) × 10(9) M(-1) s(-1), respectively. The dissociation of the excimer to form a singlet excited state pyrene and a ground state pyrene was shown to be negligible. The energies of activation corresponding to the monomer and excimer unimolecular decays were found to be 2.51 ± 0.07 and 25.7 ± 0.7 kJ mol(-1), respectively. Also, our temperature resolved laser photolysis data revealed that the excimer formation has a negative energy of activation equal to -11.2 ± 0.5 kJ mol(-1). This unique phenomenon may be attributed to steric effects in the collision of the reactants. The current findings are important for the correct data analysis and interpretation in many applications of the pyrene excimer.

Laser photolysis study of the triplet states of phthalocyanines on the surface of silica nanoparticles in aqueous solutions

Aluminum and zinc phthalocyanines (AlPc and ZnPc, respectively) adsorbed on the surface of silica nanoparticles (60 nm in diameter) in aqueous solutions have been found to form H-aggregates, which possess characteristic absorption spectra with bands (having a maximum at 640 nm) in a shorter wavelength region with respect to the main Q-band of the monomer (having a maximum at 670 nm). For AlPc on the surface, J-aggregates of two types (long-wavelength bands with maximums at 740 and 770 nm) are also observed. Using nanosecond laser photolysis (with the excitation wavelength of 337 nm) in deoxygenated solutions of AlPc on the surface, the formation of the triplet electronically excited states of J-aggregates has been detected, which are characterized by a broad absorption spectrum in the region of 400–800 nm and a lifetime of 360 μs. No intermediate products have been detected during the photolysis of H-aggregates of ZnPc on the surface.

Laser photolysis study of spectral and kinetic characteristics of the triplet state of all-trans-retinal in aqueous solutions of retinal-binding proteins and liposomes

Spectral and kinetic characteristics of the triplet state of all-trans-retinal (3ATR) in aqueous solutions of bovine serum albumin (BSA), interphotoreceptor retinal-binding protein (IRBP), and phosphatidylcholine (PC) liposomes have been studied by nanosecond flash photolysis. In the protein solutions, the decay kinetics of 3ATP obeys the two-exponential law (with the rate constants of 5 × 105 and 5 × 104 s−1). The introduction of liposomes into the protein solutions increases the contribution of the fast component and slows it down. The corresponding concentration dependence indicates that ATR remains localized in the proteins at the PC/protein molar ratio up to 100. The introduction of oxygen into the protein solutions has no effect on the rate constant of the slow component, but accelerates the fast component and increases its contribution. The efficiency of 3ATP quenching by molecular oxygen in the protein solutions is much lower than in water and liposomes. The photoprotective role of IRBP in the retinoid cycle is discussed.

Femtosecond Laser Photolysis Studies on Temperature Dependence of Cyclization and Cycloreversion Reactions of a Photochromic Diarylethene Derivative

Temperature dependencies of cyclization and cycloreversion processes of a photochromic diarylethene derivative, 1,2-bis(2-methyl-3-benzothienyl)perfluorocyclopentene (BT), were investigated by steady-state spectroscopy and femtosecond laser photolysis methods. Steady-state measurements revealed that the cyclization reaction quantum yield and the fraction of the conformer with C2v symmetry favorable for the cyclization (antiparallel, AP conformer) were independent of temperature in the range of 253–343 K. These results indicated that the cyclization reaction of the AP conformer in the open-ring isomer in the excited state had no apparent temperature dependence and suggested that the fate of the excited AP conformer in the open-ring isomer, such as cyclization or deactivation to the ground state, was determined at the conical intersection. On the other hand, the cycloreversion reaction was dependent on the temperature; the reaction quantum yield increased together with a decrease in the lifetime of the excited state of the closed-ring isomer with increasing temperature. On the basis of the adiabatic energy surface for the reaction profiles, it was deduced that the rapid deactivation into the ground state took place in the S1 state in competition with the activated pathways leading to the conical intersection where the cycloreversion occurred.

Laser photolysis studies of quaternary ammonium ionic liquids aqueous solution

The reactions between quaternary ammonium ionic liquids ((Me3NC2 H4OH) ? (Zn2Cl5) - , abbrev. Ch-Zn2Cl5) and one-electron oxidant (SO4- ), have been studied by nano-second laser photolysis techniques. The mechanism of monopho- tonic ionization by 266 nm laser excitation was suggested and the quantum yield was estimated to be 0.04. The second-order decay rate constant of SO4- oxidation reactions at 460 nm, 1.3 9 10 9 M -1 s -1 , was almost equal to the product rate constant of 1.5 9 10 9 M -1 s -1 at 320 nm in Ch-Zn2Cl5 aqueous solution showing that the decay and the product were synchronic. Comparison of Ch-Zn2Cl5 with chloride choline and ZnCl2 showed that the anion Zn2Cl5 - played an important role in photoionization while choline cation had little function on its photolysis and radiolysis. The present study would be helpful for understanding the application of ionic liquids in the field of electrochemical deposition.

A laser photolysis study of the quenching kinetics of triplet-state all-trans-retinal by oxygen in aqueous albumin and liposome solutions

The kinetics of quenching of the all-trans-retinal triplet state by air oxygen in aqueous solutions of bovine serum albumin and in a cardiolipin liposome suspension was investigated by nanosecond laser photolysis. It was established that the quenching reaction rate constant in the albumin solution (1.8 × 108 l mol−1 s−1) was an order of magnitude less than in liposomes (3.1 × 109 l mol−1 s−1). These constants were 5.0 × 109 and 1.1 × 109 l mol−1 s−1 in methanol and aqueous solutions containing 10 vol % methanol, respectively. The effect of hindered oxygen access to the Lall-trans-retinal attached to albumin is discussed in terms of its influence on the photooxidation processes in the retina.

Laser photolysis study of the transient products of 4-carboxybenzophenone-sensitized photolysis of chlorophenoxyacetic acid-based herbicides in aqueous micellar solutions

The kinetics of the reaction of the triplet states and radicals of 4-carboxybenzophenone (CB) in aqueous sodium dodecyl sulfate micellar solutions in the absence of additives and in the presence of 2,4-dichlorophenoxyacetic acid (DCPA) or 2-methyl-4-chlorophenoxyacetic acid (MCPA) were studied. The rate constants of 3CB quenching by DCPA and MCPA were found to be 8.5 × 105 and 1.4 × 106 s−1, respectively (as calculated for a reactant pair in the micelle). As a result of quenching, radical pairs are produced in the micelle, as well as the ketyl radicals CBH· that lost their partners because of the fast disappearance of some of the DCPA and MCPA radicals. An applied external magnetic field substantially retards the recombination of radical pairs in the micelle.

ChemInform Abstract: A Laser Photolysis Study of the Pressure-Induced Viscosity Dependence of the Primary Process of Benzophenone Photoreduction in Alcohol Solutions

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

A laser photolysis study of the decay kinetics of the triplet states and radicals of flavins in the bovine eye lens

The decay kinetics of the triplet states and radicals of riboflavin and flavin mononucleotide, which were introduced into the bovine eye lens as kinetic photochemical probes, was studied by nanosecond laser photolysis, as a function of dilution of the lens with water. Correlations were found between the kinetic parameters and the degree of ordering of crystallins in the aqueous solution of the lens.

Photochemical formation and decay of tocopheroxyl radical in vitamin E emulsion: A laser-photolysis study

In order to investigate effect of the inhomogeneous dispersion system on photogeneration and decay dynamics of the tocopheroxyl radical (Toc ), transient absorption and steady-state and time-resolved EPR measurements on photolysis of vitamin E (VE) were carried out. Tocopherol (TocH) is photo-decomposed through electron-detachment in the excited state and the subsequent proton releasing, which produce Toc and the solvated electron ( e solv − ) . The decay of Toc is explained by (i) a rapid decay within 2 μs by recombination between Toc and e solv − , (ii) a second-order decay observed in the range of 5–150 μs by reaction between Toc and the superoxide anion radical (O 2 −), and (iii) a slow decay in >10 ms by bimolecular self-quenching of Toc . The Toc decay in the VE emulsion was very different from that in the homogeneous solution. In the emulsion, the Toc yield was small, the contribution of the reaction with O 2 − was rather small, and the decay due to the bimolecular reaction of Toc was much accelerated. In the emulsion, Toc could not be detected by steady-state EPR, although it could in ethanol. These differences should be caused by the oil-droplet formation in the emulsion. From these results, we can expect that the photostability of the VE emulsion is superior to that of the VE homogeneous solutions.

222 nm Photo-induced radical reactions in silazanes. A combined laser photolysis, EPR, GC-MS and QC Study

The initiation mechanism of the VUV-induced conversion of polyorganosilazanes into methyl-Si-O-Si networks was studied by means of model disilazane compounds. A combined experimental approach was chosen to determine the primary radicals and their properties (lifetimes, spectra) as well as the major final products. It was verified that both Si-N and Si-CH(3) cleavage occur in the condensed phase, the former with higher yield. The lifetime of the primary Si- and N-centred radicals in de-oxygenated n-hexane solution is less than </=10 mus. N-centred radicals transform into amines by H abstraction, the availability of weakly bonded H as in the case of tetramethyldisilazane accelerates the reaction considerably. In rigid matrix (frozen solutions) CH(3), silyl radicals and methylene radicals CH(2)R are trapped. In the presence of oxygen, peroxyl radicals are formed and serve as precursors of the subsequent oxidative conversion. Product analysis by GC-MS reveals linear R-(Si-O)(n)- chains rather than branched compounds as the initial products of the oxidative conversion of tetramethyldisilazane. It was shown that reactive silylene intermediates do not play a role in the conversion process. Quantum chemical calculations assist in the interpretation.

Laser photolysis study of the triplet states of fulvic acids in aqueous solutions

The spectral and kinetic characteristics of short-lived intermediates formed during the photolysis of aqueous and alkaline (0.1 mol l−1 NaOH) solutions of fulvic acids (FA) were studied by the nanosecond laser photolysis technique. Laser photolysis of FA at 337 nm leads to the formation of short-lived triplet states of FA (3FA) with a quantum yield of about 1% and different relatively long-lived intermediates (with decay rate constants in deoxygenated solutions of 1.8 × 103–2.1 × 105 and 80–160 s−1, respectively), which are characterized by absorption spectra with maximums at λ ≤ 400 nm. 3FA are quenched by atmospheric oxygen with rate constants of 5.4 × 108–1.1 × 109 l mol−1 s−1. Introduction of phenols into the solutions at concentrations up to 0.1 mol l−1 has no appreciable effect on the decay kinetics of the detected intermediate products of FA photolysis.

Laser photolysis and pulse radiolysis studies on silybin in ethanol solutions

In the present work, laser photolysis techniques have been used to study the transient species of silybin (abbreviated as SLB) in ethanol solutions. It has been demonstrated that the exposure of silybin to 248 nm excimer laser gives rise to monophotonic ionization with a high quantum yield. The transient species have been identified, and a series of related kinetics parameters have also been obtained. For the sake of comparison, we approached the problem using another way to reduce silybin, i.e. attack of solvated electrons, e sol −, produced in pulse radiolysis. The rate constant for reaction of e sol − was determined to be 3.8×10 9 dm 3 mol −1 s −1 at 720 nm, which is consistent with 4.6×10 9 dm 3 mol −1 s −1, obtained by laser photolysis.

Stepwise two-color laser photolysis studies of α-cleavage in highly excited triplet states of α-acyl-4-phenylphenols

The photochemical properties of alpha-cleavage of C-O bond in highly excited triplet states (T(n) with n>2) of p-biphenyl acetate and p-biphenyl benzoate (Me-OBP and Ph-OBP) in solution were investigated in comparison with those in the lowest excited singlet and triplet states by using single laser and sequential two-color two-laser photolysis techniques. Upon 266 nm laser photolysis of Me-OBP and Ph-OBP, occurrence of C-O bond cleavage in the excited singlet state was recognized from the observation of the formation of p-phenylphenoxy radical (PPR) in the transient absorption. The quantum yields (Phi(rad)) of the PPR formation were determined to be 0.29 and 0.24 for Me-OBP and Ph-OBP, respectively. Triplet sensitization using acetone (Ac) provided efficient formation of the lowest triplet states (T(1)) of Me-OBP and Ph-OBP, and the molar absorption coefficients of the triplet-triplet absorption were determined. No photochemical reactions were found in the T(1) state. Upon 355 nm laser flash photolysis of the T(1) states of Me-OBP and Ph-OBP, formation of PPR accompanied with decomposition of the triplet state was confirmed in the transient absorption. This observation indicated that alpha-cleavage proceeds in the highly excited triplet state. The quantum yields (Phi(dec)) of the decomposition in the dissociative highly excited triplet state (T(R)) were determined to be 0.25 and 0.15 for Me-OBP and Ph-OBP, respectively. The reaction mechanism for alpha-bond cleavage in the T(R) state was discussed.

Photochemical reaction of dimethylarsinous iodide in aerated methanol: A contribution to arsenic radical chemistry

Continuous and laser photolysis studies of (CH 3) 2As (III)I were carried out in methanol solutions. The photoproduct obtained with the continuous photolysis of the aerated solutions was identified as {(CH 3) 2As (V)(OH) 2} +{(CH 3) 2As (V)O(OH)} 2][I 3 −] by X-ray crystallography. The laser flash photolysis of (CH 3) 2As (III) I in a degassed methanol gives a transient spectrum of dimethylarsine radical, (CH 3) 2As (III) , with a broad peak centered around 560 nm. The arsinous radical, (CH 3) 2As (III) , decays according to second order kinetics to regenerate (CH 3) 2As (III)I. In the presence of oxygen, (CH 3) 2As (III) reacts with oxygen to yield the peroxy radicals, [(CH 3) 2As (III) (O 2)] with the bimolecular rate constant, 2.0 × 10 9 M −1 s −1. The peroxy radical having weak and broad absorption in the wavelength region λ > 500 nm decays according to first order kinetics with a rate constant 5.0 × 10 5 s −1. It is suggested that the peroxy radical unimolecularly converts to (CH 3) 2As (V) (O)O , which eventually turns to the product {(CH 3) 2As (V)(OH) 2} +{(CH 3) 2As (V)O(OH)} 2][I 3 −].

Ultrafast laser photolysis study on photodissociation dynamics of a hexaarylbiimidazole derivative

Dynamics of the photodissociation process of a hexaarylbiimidazole derivative, 2,2′-di( ortho-chlorophenyl)-4,4′,5,5′-tetraphenylbiimidazole ( o-Cl-HABI), in benzene solution was investigated by means of the femtosecond–nanosecond laser spectroscopy. The time evolution of the transient absorption of the lophyl radical, the product of the photodissociation, was biphasic with the faster and the slower time constants of 80 fs and 1.9 ps, respectively. The faster time constant was ascribable to the formation of the lophyl radical through the bond fission after the excitation, and the slower time constant may be attributed to the conformational change and/or vibrational cooling of nascent radicals.