A modified method for calculating quantum yields of photochemical reactions

The previously proposed approach to calculation of quantum yields of photochemical reactions has been improved. The new version uses a different procedure of accounting for the asymmetry (anharmonicity) of potential wells of the molecules involved in the photochemical conversion. The form of a doubleminimum potential energy surface has been considered in a more correct manner. It has been shown that the number of model parameters is reduced by two thirds compared with the model used previously, adequately reflecting the characteristics of structural transformations of polyatomic molecules in which a relatively small part of the total number of coordinates are transformed. The quantum yields of photochemical transformations of six dienes into their cyclic isomers have been calculated. Quantitative agreement of the calculated values with the experimental data has been achieved for all the reactions. It has been shown that the model parameters have good transferability in a series of related molecules, thereby rendering the simulation of photochemical processes by the method in question predictive.     

Simulation of the cyclopropanecarbaldehyde and cyclopropylethanone photoisomerization processes and calculation of the quantum yields of the reactions

The cyclopropanecarbaldehyde → 2-butenal and cyclopropylethanone → 3-pentenone-2 photoisomerization processes were simulated. The calculated quantum yields of the products are in quantitative agreement with experimental data. The validity of the method proposed for the calculation of quantum yields of photochemical reactions was confirmed. It was found that if the condition of relative smallness of the optical transition probabilities as compared to the quantum beat frequency is met, the quantum yields can be quantitatively estimated with satisfactory accuracy directly from the transition probabilities without running the full calculation of the phototransformation kinetics. It was shown that the integral characteristics of photochemical reactions (quantum yields) are highly responsive to the conformational state of the molecules involved in the process.     

Simulation of photochemical processes and calculation of the quantum yields of isomerization reactions of substituted dienes

The possibility of construction of a semiempirical method for simulation of photochemical processes and calculation of the quantum yields of reactions has been studied. The practicability of the approach was exemplified with the photochemical transformations of some diene compounds into their cyclic isomers, namely, 2,4-dimethylpentadiene-1,3 → trimethylcyclobutene, 2,3-dimethylbutadiene-1,3 → dimethylcyclobutene, pentadiene-1,3 → 3-methylcyclobutene, cis-butadiene-1,3 → cyclobutene, and 2-methylbutadiene-1,3 → 1-methylcyclobutene. The calculated quantum yields of the reactions are in qualitative and quantitative agreement with experimental data.     

A method for simulation of photochemical processes and calculation of quantum yields of reactions

The possibility of construction of a semiempirical method for simulation of photochemical processes and calculation of quantum yields of reactions has been studied. The practicability of the approach was demonstrated for the o-xylene → m-xylene, m-xylene → p-xylene, m-xylene → o-xylene, and o-diethylbenzene → m-diethylbenzene photoisomerization reactions as an example. The calculated quantum yields of the reactions are in qualitative agreement with experimental data.     

A simple model for predicting the course of photochemical reactions

The possibility of building and using a simplified model with constant (time-invariant) reaction probabilities for rapid simulation of photochemical processes with the required reliability of the prediction has been shown. A relation between the probabilities of transformations and fundamental molecular characteristics (optical transition probabilities and quantum beat frequencies referring to a nonradiative reaction transition) that can be quantitatively determined with a good degree of certainty has been revealed. For monomolecular isomerization reactions as an example, the attainable level of the accuracy of prediction by the simple model has been estimated, which has been shown to be ～10% in most cases. It is important that the prediction of the order of magnitude of the key characteristics of photochemical processes, such as quantum yield and the reaction rate, is definitely correct as is required in most applications of photochemistry.     

A Master Equation for Photochemical Rates

Traditionally, the general photochemical rate equation could be integrated only at two limits (high concentration and low concentration). In this paper, the general photochemical equation has been integrated to yield a master equation that is valid for any chromophore concentration. Hence, in future, data for all concentrations can be utilized in deriving the incident photon flux and/or the quantum yield for a photochemical reaction. Unfortunately, the master equation can only be used for monochromatic light sources.     

P‐Type Photochromism of New Helical Naphthopyrans: Synthesis and Photochemical,Photophysical and Theoretical Study

Two novel helical naphthopyrans have been synthesised. The helical scaffold has the interesting effect of increasing the thermal stability of the transoid‐trans (TT) open isomer formed upon UV irradiation of the closed form (CF), which transforms these naphthopyrans from thermal to photochemical photochromes. The photochromic performance is excellent in both polar and apolar solvents and the conversion percentage from the CF to the TT form can be as high as 92.8 %. We propose a new method to determine the quantum yields of the photochemical processes that lead to transoid‐cis (TC) and TT isomers, and their molar absorption coefficients. The thermal stability of the TT and TC isomers has been studied in different solvents. The quantum yields of fluorescence before and after irradiation, along with the decay lifetimes, have also been measured. TD‐DFT calculations have been performed to determine the relative thermodynamic stability of the species involved in the photochromic mechanism and to rationalise their spectral properties.     

2-Nitrobenzaldehyde as a chemical actinometer for solution and ice photochemistry

2-Nitrobenzaldehyde (2NB) is a convenient, photochemically sensitive, and thermally robust actinometer. Although 2NB has been used in a number of solution and ice experiments in the laboratory, the quantum efficiencies and molar absorptivities of 2NB have not been critically evaluated, especially on ice. Using a series of laboratory and field measurements we have measured the light absorbance and photochemical properties of 2-nitrobenzaldehyde in solution (water and/or acetonitrile) and in/on water ice. Our results show that the molar absorptivities of 2NB are only weakly dependent upon temperature and that the quantum yield is independent of temperature in water; the quantum yield is also independent of wavelength, as shown by past reports. Furthermore, we find that the photochemistry of 2NB in/on water ice is the same as in liquid water. While most studies employing 2NB cite and use a quantum yield of 0.50, based on a review of the literature, and on our new experimental data, we recommend a quantum yield of 0.41 for 2NB photolysis for both solution and water ice.     

ARYL AZIDES AS PROTEIN PHOTOLABELS: ABSORPTION SPECTRAL PROPERTIES AND QUANTUM YIELDS OF PHOTODISSOCIATION

Abstract A series of ring-substituted phenyl and naphthyl azides have been prepared. Electronic absorption spectral properties and quantum yields of disappearance have been determined in aqueous solution. For phenyl azide derivatives, the wavelengths of maximum absorption vary systematically, correlating with electronic effects of the ring substituents. Quantum yield values of 0.1–0.7 are obtained and are independent of a number of experimental conditions, including concentration. There is no simple correlation of the quantum yields of azide disappearance with electronic effects, except that the naphthyl azides have higher photochemical efficiencies than the phenyl azide derivatives. A series of potential protein photolabels has been identified based on electronic absorption characteristics and photochemical efficiencies.     

Spectroscopic properties of colloidal indium phosphide quantum wires

Colloidal InP quantum wires are grown by the solution-liquid-solid (SLS) method, and passivated with the traditional quantum dots surfactants 1-hexadecylamine and tri-n-octylphosphine oxide. The size dependence of the band gaps in the wires are determined from the absorption spectra, and compared to other experimental results for InP quantum dots and wires, and to the predictions of theory. The photoluminescence behavior of the wires is also investigated. Efforts to enhance photoluminescence efficiencies through photochemical etching in the presence of HF result only in photochemical thinning or photooxidation, without a significant influence on quantum-wire photoluminescence. However, photooxidation produces residual dot and rod domains within the wires, which are luminescent. The results establish that the quantum-wire band gaps are weakly influenced by the nature of the surface passivation and that colloidal quantum wires have intrinsically low photoluminescence efficiencies.     

Mechanistic Aspects of the Effects of Stress on the Rates of Photochemical Degradation Reactions in Polymers

Abstract

This review examines the mechanistic origins of the effects of stress on the photochemical degradation rates of polymers. Recent studies have shown that tensile and shear stresses accelerate the rate of the photochemical degradation of polymers. Conversely, compressive stress generally retards the rate of photochemical degradation. After an initial discussion of the photochemical auto‐oxidation mechanism, the three primary hypotheses that purport to explain how stress affects photochemical degradation are examined. The first hypothesis is attributed to Plotnikov, who proposed that stress changes the quantum yields of the reactions that lead to bond photolysis. The second hypothesis, attributed to a number of researchers, says that stress affects the ability of the geminate radical pairs, formed in the photochemical bond cleavage reactions, to recombine. The third hypothesis proposes that stress changes the rates of radical reactions subsequent to radical formation. A further attempt to account for the effects of stress on degradation rates is a modification of the so‐called Zhurkov equation that has been used rather successfully to predict the effects of stress on degradation rates in thermal reactions. This empirical equation relates the quantum yield of degradation to a composite activation barrier for the overall photochemical reaction. Following the discussion of these hypotheses, experimental mechanistic studies of stress effects are summarized, and what little data there is is shown to be consistent with the hypothesis that proposes that stress primarily affects the ability of photochemically generated radical pairs to recombine. By decreasing the efficiency of radical–radical recombination, the effect is to increase the relative efficiencies of the radicals' other reactions and hence the rate of degradation. In addition to stress, other factors can affect the rates of polymer photodegradation. These factors include the absorbed light intensity, the polymer morphology, the rate of oxygen diffusion in the polymer, and the chromophore concentration. Each of these parameters must be carefully controlled in mechanistic studies that probe the effects of stress on degradation rates.     

Nitrate radical quantum yield from peroxyacetyl nitrate photolysis

Peroxyacetyl nitrate (PAN, CH3C(O)OONO2) is a ubiquitous pollutant that is primarily destroyed by either thermal or photochemical mechanisms. We have investigated the photochemical destruction of PAN using a combination of laser pulsed photolysis and cavity ring-down spectroscopic detection of the NO3 photoproduct. We find that the nitrate radical quantum yield from the 289 nm photolysis of PAN is Phi(NO3)PAN = 0.31 +/- 0.08 (+/-2 sigma). The quantum yield is determined relative to that of dinitrogen pentoxide, which is assumed to be unity, under identical experimental conditions. The instrument design and experimental procedure are discussed as well as auxiliary experiments performed to further characterize the performance of the optical cavity and photolysis system.     

Electrochemical observation of the photoinduced formation of alloyed ZnSe(S) nanocrystals

Electrochemical studies of thiol-capped ZnSe nanocrystals in aqueous solution have demonstrated several distinct oxidation and reduction peaks in the voltammograms, with the peak positions being dependent on the size of the nanocrystals and their photoluminescence quantum efficiency. The evolution of the specific features in the cyclic voltammetric curves of ZnSe NCs as a function of their photochemical treatment is studied. The interpretation of the results based on the approaches previously developed for CdTe NCs is found to be in good correlation with the proposed mechanism of the ZnSe NCs phototreatment, i.e., the formation of a sulfur-enriched surface shell. By this, cyclic voltammetry has been demonstrated to be a powerful method for probing surface states of semiconductor NCs as well as for monitoring the evolution of these states during photochemical processing.     

Zur strahlungslosen Desaktivierung des tiefsten angeregten Singulett-Zustandes von 2-Alkyl-indazolen in neutraler und protonierter Form

On the Mechanism of Radiationless Deactivation of the First Excited Singlet State of 2-Alkyl-indazoles The temperature dependence of the quantum yields of fluorescence (Φ_F), intersystem crossing (Φ_T) and photoreactions (Φ_R) have been measured for several neutral and protonated 2-alkyl-indazoles. The experimental data (Fig. 1 and 3) can be interpreted in terms of the reaction schemes 1 and 2 with temperature independent constants for fluorescence emission and intersystem crossing and temperature dependent photochemical primary processes. Whereas at low temperatures the sum of the quantum yields of fluorescence and intersystem crossing equals 1, at higher temperatures a very efficient radiationless deactivation process takes place (Table). Based on kinetic and photochemical data it is concluded that this deactivation proceeds via a hypersurface crossing (Fig. 2) or hypersurface touching (Fig. 4) in the course of the photochemical rearrangements 1 → 2 and 4 → 5 respectively. Similar mechanisms are expected to be responsible for the unusual internal conversion in other heterocyclic compounds.     

A METHOD OF TREATING PHOTOCHEMICAL TRIPLET QUENCHING DATA

Abstract— A modified Stern-Volmer plot method is proposed for treatment of quantum yield data where both reactive photoexcited singlet and triplet states are present in solution. The method may be applied when the reactions occurring are zeroth order in ground state photochemical substrate and allows the separation of singlet and triplet state contributions to the total quantum yield and the calculation of kinetic lifetimes without going to very high quencher concentrations. The method is illustrated through its application to literature data on the Type II photoelelimination reaction of 2-hexanone.     

偶氮苯的光化学顺反异构化反应及其在光强测定中的应用

本文叙述了用偶氮苯露光计测定光强的原理。本方法也同样可用于测定A<﹦>B型光化学异构化反应的量子产率,设计了专用的计算机程序来处理在光强和量子产率的测定中的各种有关数据。     

Study of a reaction between 2,3-dichloro-1,4-naphthoquinone and N,N'-diphenyl thiourea involving an EDA adduct as intermediate

The reaction between 2,3-dichloro-1,4-naphthoquinone and N,N'-diphenyl thiourea in acetonitrile medium, which yields the product, 2,3-(N,N'-diphenylthioureylene)-naphtho-1,4-quinone has been found to take place in two ways--thermal and photochemical. The thermal (dark) reaction occurs through an electron donor-acceptor (EDA) adduct as intermediate with evolution of HCl and kinetic data fit into the scheme A + B<==>AB(fast)-->product(slow) Formation constant of the EDA adduct and the rate constant of the slow process have been determined at four different temperatures from which the enthalpy of formation of AB has been determined. The photochemical reaction has been studied with 360 nm ordinary light and also with 365 and 370 nm laser beams. Use of laser causes about 10(3)-fold increase in the rate of the reaction but does not affect the quantum yield. The final product has been isolated and characterised by elemental analysis, 1H and 13C NMR, IR spectroscopy and mass spectrometry.     

Self-modelling analysis applied to nanosecond transient absorption spectroscopy of flavone: an aid to elucidate and characterise reaction intermediates

A nanosecond transient absorption spectroscopy study of flavone performed with a 248 nm pump radiation has been investigated with the support of a chemometric treatment: SIMPLISMA. The experimental spectra obtained in various solvent with a pump-probe delay lower than about 2 micros are in quite good concordance with those already presented in the literature. Nevertheless after about 10 micros, the spectrum pattern significantly evolves as a function of time particularly for the methanolic solution. A qualitative analysis together with a SIMPLISMA chemometric treatment of the experimental data allowed to elucidate and characterise two interdependent transient species in the alcoholic medium: the lowest T1 triplet state of flavone and the ketyl radical forming by H-abstraction reaction from the solvent. In cyclohexane and acetonitrile, the same species seem to be produced in the studied time-scale but the radical form is generated with variable quantum yield depending on the solvent polarity. The pure spectrum and the photochemical kinetics of each reaction intermediate could have been determined with the help of the second derivative SIMPLISMA calculation procedure.     

Application of a "black body" like reactor for measurements of quantum yields of photochemical reactions in heterogeneous systems

We report for the first time an experimental application of the concept of a "black body" like reactor to measure quantum yields (Phi) of photochemical reactions in liquid-solid heterogeneous systems. A major advantage of this new method is its simplicity since the fractions of reflected and transmitted light are negligible due to reactor geometry and high optical density of the heterogeneous systems. The average quantum yield of a test reaction (phenol photodegradation) over TiO(2) (Degussa P25) as determined by this method was 0.14, identical to the quantum yield measured earlier for this same reaction under similar conditions by Salinaro and Serpone. We also report the quantum yield of phenol photodegradation over N-doped TiO(2) during photoexcitation at the fundamental absorption band (lambda = 365 nm; Phi = 0.12) and at the N-doping induced extrinsic absorption band (lambda = 436 nm; Phi = 0.08) of the photocatalyst.