Kinetics Of Photochemical Reactions Research Articles

Temporal mapping of photochemical reactions and molecular excited states with carbon specificity.

Photochemical reactions are essential to a large number of important industrial and biological processes. A method for monitoring photochemical reaction kinetics and the dynamics of molecular excitations with spatial resolution within the active molecule would allow a rigorous exploration of the pathway and mechanism of photophysical and photochemical processes. Here we demonstrate that laser-excited muon pump-probe spin spectroscopy (photo-μSR) can temporally and spatially map these processes with a spatial resolution at the single-carbon level in a molecule with a pentacene backbone. The observed time-dependent light-induced changes of an avoided level crossing resonance demonstrate that the photochemical reactivity of a specific carbon atom is modified as a result of the presence of the excited state wavefunction. This demonstrates the sensitivity and potential of this technique in probing molecular excitations and photochemistry.

Kinetics of photochemical reactions of a new biphotochromic compound upon photolysis with light of different wavelengths

Using the method of microsecond flash photolysis with UV and visible light, the spectral and kinetic characteristics of intermediate products of photolysis in toluene and methanol solutions of a new biphotochromic compound have been studied, in which two photochromic moieties, spironaphthoxazine and azomethine, are linked to each other in such a way that there is conjugation between π-electronic systems of the moieties in the ground state of the molecule. Two intermediate products have detected, whose relative efficiency of formation substantially depends on the solvent and the wavelength of excitation light.

Kinetics of photochemical reactions of biphotochromic compounds based on spironaphthopyran and enamine--conjugation effect.

A novel biphotochromic compound (BPC) with two photochromic fragments, namely spironaphthopyran and hydroxyazomethine, was synthesized and studied by nanosecond laser flash photolysis using the excitation wavelengths of 337, 430, 470 and 500 nm in methanol and toluene. The photoexcitation of BPC results in the formation of at least two colored transients. The first one is the merocyanine form B (the maximum in the absorption spectrum is near 600 nm and the lifetime is 0.1 and 0.05 s in methanol and toluene, respectively) due to the spiro-bond break followed by isomerization. The second one is the trans-keto form A(Kt) (the maximum in the absorption spectra is near 480 nm and the lifetime is 0.1 and 5 ms in methanol and toluene, respectively) as a result of cis-enol or cis-keto tautomer transformations. The relative yields of B and A(Kt) depends essentially on the wavelength of excitation. The form A(Kt) is the key transient formed under excitation with the visible light, whereas its yield under excitation with UV light is comparable with that of B. The specific solvation by methanol molecules favors the spirocycle opening even under visible light excitation. The results obtained for novel BPC were compared with those for other BPC where the same fragments are combined in such a way that the form B is the major one under excitation with UV light whereas it virtually is not observed under visible light excitation. The difference in both BPC is discussed in terms of conjugation (π-coupling) between photochromic fragments.

Photostability and Photostabilization of Drugs and Drug Products

Photostability studies of drugs and drug products are an integral part of the product development process in the pharmaceutical industry. These studies are carried out to ensure quality, efficacy, and safety of the formulated products during manufacture, storage, and use. This review deals with the concept of photostability and related aspects and the literature available in the field. It highlights the role of the photochemistry in the photostability studies, describes the functional groups important for the photoreactivity of drugs, explains photophysical processes, and deals with the kinetics of photochemical reactions. The various modes of photodegradation of drugs with examples of selected compounds are presented. The biological consequences of the effect of light on the drug degradation are described. The photostability testing of drugs and drug products and the requirements under ICH guideline are discussed. Some information on the packaging requirements for the formulated products is provided. The various methods used for the photostabilization of solid and liquid dosage forms are also discussed.

Analysis of reaction kinetics in the photomechanical molecular crystal 9-methylanthracene using an extended Finke–Watzky model

In order to develop an improved description of the photomechanical response of 9-methylanthracene (9MA) microcrystals, a detailed study of its solid-state photochemical reaction kinetics is performed. The reaction progress is monitored through the decrease in absorption of an optically microcrystalline thin film. The evolution of the time-dependent photoluminescence during the reaction is also measured. Both the photochemical reaction and nonradiative relaxation rates increase as more photoproduct is formed. In order to analyze the data, an extended version of the Finke-Watzky kinetic model for photochemical reactions is derived, denoted the FW-P model. This extended version enables a systematic analysis of photochemical reaction kinetics in solid-state molecular systems at varying levels of approximation. The FW-P model describes the non-exponential decrease in reactant and also correctly predicts the magnitude of the observed decrease in photoluminescence lifetime over the course of the reaction. The lifetime analysis is complicated by the fact that the microcrystalline 9MA sample contains multiple emitting species, and the extended FW-P model fails to capture the exact dependence of the photoluminescence on product formation. Analysis of the 9MA data indicates that both the photodimerization and the nonradiative relaxation rates can be accelerated by a factor of 10 over the course of the reaction. The results in this paper demonstrate that autocatalytic photodimerization kinetics are present in crystalline 9MA and may influence its photomechanical response.

Thermal and Photochemical Effects on the Fluorescence Properties of Type I Calf Skin Collagen Solutions at Physiological pH

Mammalian collagens exhibit weak intrinsic UV fluorescence that depends on the age and previous history of the sample. Post-translational modifications result in additional fluorescent products (e.g. DOPA, dityrosine, and advanced glycation end products (AGE)). UV radiation can cause longer wavelength fluorescent oxidative bands. These alterations can assess the extent of photolysis. We describe the ground- and excited-state oxidative transformations of newly-purchased type I calf skin collagens (samples #092014 and #072012) and a 7-year-old sample (#072005). We compare the effects of UV radiation (mainly 254 nm) and age on the photochemical reaction kinetics and fluorescence spectral distribution of type I calf skin collagen at pH 7.4. The fluorescence spectra of samples #072012 and #092014 were similar but not identical to pure tyrosine, whereas #072005 indicated significant “dark” oxidation at the expense of tyrosine. Fading of oxidized product(s) at 270/360 nm is second-order. Build-up of 325/400 nm (dityrosine) fluorescence is linear with time. Rate parameters r2 and r1 were respectively proportional to second order disappearance of ground state oxidation products and the quasi–first-order photochemical formation of dityrosine. There is a reciprocal relationship between the rates of decrease in the 270/360 nm fluorescence and concomitant increase in 325/400 nm fluorescence. Their relative rates depend on the age of the collagen sample. There is a reciprocal relationship between r1 and r2. This relationship results because both ground state autoxidation and excited state photo-dimerization proceed via a common tyrosyl radical intermediate. Water of hydration appears to play a role in generating tyrosyl radical.

Photochemical Reaction Kinetics of Duroquinone in a Mixture of Ionic Liquid [BPy][BF<sub>4</sub>] and Acetonitrile

Photochemical reactions of duroquinone (DQ) in solutions consisting of mixtures of the ionic liquid N-butylpyridinium tetrafluoroborate ([BPy][BF4]) and acetonitrile (MeCN) were studied using the 355 nm laser flash photolysis technique. Increasing the ratio of [BPy][BF4] had no obvious impact on the absorption peak of the triplet excited state of DQ ((3)Da), which followed monoexponential kinetics in the N-2-saturated solution. However, increasing the ratio of [BPy] [BF4] did affect the photoinduced electron transfer process from triethylamine to (3)DQ in the mixture by decreasing the reaction rate and the quantum yield of the transient radical. The results highlight the ability to tune the efficiency and the rate of the photoinduced electron transfer by changing the proportion of [BPy][BF4] in the mixture.

Understanding photochirogenesis: solvent effects on circular dichroism and anisotropy spectroscopy.

The basic units that constitute essential biopolymers (proteins and nucleic acids) are enantiomerically biased. Proteins are constructed from L-amino acids and nucleic acids possess a backbone composed exclusively of D-sugars. Photochirogenesis has been postulated to be the source of this homochirality of biomolecules: Asymmetric photochemical reactions were catalyzed by circularly polarized light (cpl) in interstellar environments and generated the first chiral prebiotic precursors. Enantiomers absorb cpl differently and this difference can dictate the kinetics of asymmetric photochemical reactions. These differences in absorption can be studied using circular dichroism (CD) and anisotropy spectroscopy. Rather than measuring the CD spectrum alone, the anisotropy factor g is recorded (CD divided by absorption). This factor g is directly related to the maximum achievable enantiomeric excess. We now report on the substantial influence of solvent and molecular surroundings on CD and anisotropy spectroscopy. This shows for the first time that CD and anisotropy signals depend just as much on the molecular surroundings of a molecule as on the nature of the molecule itself. CD and g spectra of amino acids in different solvents and in the solid state are presented here and the influence of these different surroundings on the spectra is discussed.

Kinetics of photochemical reactions of multifunctional hybrid compounds based on spironaphthoxazines upon photoexcitation with light of different wavelengths

Abstract Four hybrid compounds (HCs), which molecules contain two or three photochromic fragments of different nature, have been examined by the nanosecond laser flash photolysis techniques using excitation wavelengths of 337 and 430 nm in methanol solutions. In HC molecules the photochromic spironaphthoxazine fragment, which is able to generate a colored merocyanine form B upon UV excitation, is linked to the substituted salicylideneaniline (SA) fragment, where reversible photochemical transformations from cis -enol to trans -keto form having distinguishably different absorption spectra can occur. An azobenzene group prones to trans–cis photoisomerization is the substituent in SA fragment in the fourth compound. Four intermediates of different nature were found. A decrease in the relative efficiency of the ESIPT process with respect to that of spiro-bond cleavage was found to be the result of the influence of the electronegative substituents in SA fragments. The excitation light wavelength was shown to be the main factor determining the relative yields of the intermediates of different nature for the hybrid compound with the azobenzene group. The mechanism of different intermediates formation is discussed.

Kinetics of photochemical reactions of spironaphthopyran azomethine upon photoexcitation with light of different wavelengths

Spectral and kinetic characteristics of photolysis products in methanol of the photobifunctional compound, whose molecule contains the photochromic spironaphthopyran fragment as well as the hydroxynaphthylmethylenimine fragment in which intramolecular proton transfer can occur in the ground and excited states of the molecule, were studied by the method of nanosecond laser photolysis with excitation with light of wavelengths 337, 430, and 470 nm. The relative quantum yields of the formation of different photoproducts and their kinetic characteristics were measured. The dependence of the relative yield of the photoproducts of different nature on the wavelength of excitation light was revealed.

Interfacial Chemistry of Poly(methyl methacrylate) Arising from Exposure to Vacuum-Ultraviolet Light and Atomic Oxygen

We herein report on the chemical and physical changes that occur in thin films of poly(methyl methacrylate), PMMA, induced by exposure to high-energy vacuum ultraviolet radiation and a supersonic beam of neutral, ground electronic state O((3)P) atomic oxygen. A combination of in situ quartz crystal microbalance and in situ Fourier-transform infrared reflection-absorption spectroscopy were used to determine the photochemical reaction kinetics and mechanisms during irradiation. The surface morphological changes were measured with atomic force microscopy. The results showed there was no enhancement in the mass loss rate during simultaneous exposure of vacuum ultraviolet (VUV) radiation and atomic oxygen. Rather, the rate of mass loss was impeded when the polymer film was exposed to both reagents. This study elucidates the kinetics of photochemical and oxidative reaction for PMMA, and shows that the synergistic effect involving VUV irradiation and exposure to ground state atomic oxygen depends substantially on the relative fluxes of these reagents.

Photochemical reaction mechanisms and kinetics with molecular nanocrystals: surface quenching of triplet benzophenone nanocrystals

AbstractOrganic molecular nanocrystals suspended in water are useful when studying reactions that occur in the solid state because they retain not only the reactive and supramolecular properties of bulk crystals, but are also amenable to transmission spectroscopy. Having previously studied the triplet state of benzophenone nanocrystals by laser flash photolysis transmission spectroscopy, we now report nanosecond experiments in the presence of several possible quenchers: anionic and cationic surfactants, dissolved oxygen, and as a function of solvent deuteration (H2O and D2O). After finding these to have no effect, several anionic quenchers (I−, Br−, and N) were tested by Stern–Volmer analysis. Significant correlation between the quenching rates in solution and in nanocrystals suggests that the electronic excitation is accessible to quenchers at the surface. Copyright © 2010 John Wiley & Sons, Ltd.

Kinetics of Photochemical Reactions under Restricted Geometry Conditions

Kinetics of ground state reactions under restricted geometry conditions have been rationalized generally taking as a basis the Pseudophase Model, whose basic hypothesis is: The distribution of the reactants between the pseudophases is at equilibrium, that is, the rate of the reaction is slow in relation to the rates of entry and exit of the reactant to and from the receptor. However, photochemical reactions are generally very rapid, so the following question emerges: is the Pseudophase Model still applicable to photochemical reactions? To answer this question has been the primary objective of this work in which photochemical reactions in micelles, polymers and cyclodextrins have been reviewed. The lifetime of the fluorophore and the distributions of the quencher and the probe are crucial properties; they must be taken into account to obtain a proper description of the behaviour of a given system. The models of Infelta, Almgren and Quina visualise different scenarios with varied combinations of these properties. It is shown that, in some cases, the Pseudophase Model holds at least formally.

Thermal-lens study of photochemical reaction kinetics

We consider the time dependence of the absorption coefficient due to the photoinduced chemical reaction (PCR) and species diffusion to calculate the temperature rise in the thermal-lens (TL) effect. The TL signal at the detector plane is also calculated. This theoretical approach removes the restriction that the PCR time constant is much greater than the characteristic TL time constant, which was assumed in a previously published model. Hydrocarbon fuel and aqueous Cr(VI) samples are investigated, and quantitative experimental results for the thermal, optical, and PCR properties are obtained. While similar results were obtained for the Cr(VI) solution using the previous and present models, the relative difference between the PCR time constants extracted from the same experimental data for a hydrocarbon fuel sample is found to be more than 220%. This demonstrates the significant difference of the two models.

Photochemical kinetics for holographic grating formation in phenanthrenequinone doped poly (methyl methacrylate) photopolymer

The photochemical kinetics of phenanthrenequinone (PQ) doped poly (methyl methacrylate) photopolymer in holographic recording was studied theoretically and experimentally. The diffusion of PQ molecules during holographic recording was negligible because of its small diffusion coefficient at room temperature. A photochemical reaction kinetics model of PQ/PMMA was established. The analytical expressions for the temporal variations of transmittance and diffraction efficiency were derived. By fitting the experimental curves, some parameters related with the polymer components were obtained by the proposed model, which can be used to analyze the photochemical process and will be helpful to the optimization of material preparation.

The kinetics of photochemical processes in polymer-salt systems

The kinetics of photochemical reactions in aqueous polymer-salt systems containing ammonium heptamolybdate, dodecatungstate, or metavanadate and polyvinyl alcohol or polyvinylpyrrolidone was studied by measurements of photoinduced electrode potential difference. The rate of primary accumulation of reduced d metal forms was evaluated for different systems. Possible reasons for complex oscillatory processes in the systems were analyzed. Comparative data were obtained for compositions containing polyoxometallate shaped like buckyball:(NH4)42[Mo72VIMo60VO372(HCOO)30(H2O)72] · 30HCOONH4 · 250H2O. UV irradiation of this system caused the oxidation of molybdenum(V).

Synthesis and kinetics of photochemical reactions of novel bifunctional salicylideneiminospironaphthoxazines

A series of novel photochemically bifunctional compounds was synthesized. Their molecules combine the photochromic spironaphthoxazine fragment with a salicylideneimine fragment containing various substituents. The latter can undergo intramolecular proton transfer in the electron-excited state. Photolysis products of three types, namely, open merocyanine forms having an enol or cis-keto form of the salicylideneimine moiety and a trans-keto form with the closed spiro fragment, were detected using pulse photolysis technique in toluene and methanol solutions. The spectral kinetic characteristics of the photoproducts were studied, and their quantum yields were measured. The effects of substituents in the salicylideneimine fragment and the solvent nature were discussed.

Kinetics of photochemical reactions in optically dense media with reagent diffusion

The kinetics of photochemical reactions in optically dense media with reagent diffusion is considered. It is shown, by example of the photochromic isomerization A ↔ B, that if the diffusion coefficient of A differs from the diffusion coefficient of B the disturbance of the initial uniform distribution of the total photochrome concentration ( A + B) along the light irradiation direction is observed. The greater the difference in diffusion coefficients, the greater the observed change in the total concentration distribution. The effect of change in total concentration along the light irradiation direction is found experimentally by the example of azobenzene dissolved in hydrogel and irradiated by UV light.

Radiation field modeling of multi-lamp, homogeneous photoreactors

For modeling and optimization of photoreactors, knowing radiation distribution inside the reactor is essential because the kinetics of photochemical and photocatalytic reactions are strongly dependent on the local incident radiation. In this work, a Monte Carlo multi-lamp radiation model is proposed. The model takes into account the reflection on the surface of the lamps, the refraction in the quartz lamp envelope, as well as the adsorption and the isotropic re-emission of radiation in the mercury vapor of the lamp. To quantify the shadowing effect among the lamps, the effective transmittance through a UV-lamp turned on, a lamp turned off, and a lamp quartz envelope have been measured. For validation purposes, experimental measurements have been performed using one, two, and three lamps. The model predictions show good agreement with the experimental results. Finally, in order to simplify the numerical solution of the model, the validity of assuming the lamps 100% opaque and 100% transparent have been analyzed.

Photochemical reaction kinetics in optically dense media: The influence of thermal reactions

The kinetics of photochemical reactions in optically dense media essentially free from diffusion was considered. The photochromic isomerization A ↔ B was studied as an example. If thermal isomerization is possible, a stationary state is achieved in time determined by rate constants for the thermal reactions. The concentration wave profile is changed during the photochemical reaction propagation. Low values of thermal reaction constants and decrease in sample optical density during photochemical isomerization were found to be essential for maximal wave penetration into the sample. Sharp concentration gradients of A and B can be observed when both the optical density is increased during photochemical isomerization and the quantum yield of the direct photochemical reaction A → B is higher than that of the reverse photochemical reaction B → A.