Following photochemical reactions by thermogravimetry

A new thermal analysis system has been designed to follow the influence of light on solid reactants. This method is based upon the association of a thermogravimetric system for the kinetic measuring method, and a U. V. irradiation system for the activation method. This instrument allows comparative studies of a thermal reaction and the corresponding photochemical one under exactly the same experimental conditions. The experiments, carried out in silver carbonate, have provided new information on photochemical kinetics, but also on solid-state reactivity and even on thermal processes. More generally, this device, designed for studies on reactivity, can be applied to any other system needing the action of light. With the device, the influence of the light intensity, the wavelength and the lifetime of light effects on many reactions can be studied.

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Zusammenfassung Ein neues thermisches Analysensystem zur Untersuchung der Einwirkung von Licht auf feste Reaktanden wird beschrieben. Die Methode beruht auf einer Kombination eines thermogravimetrischen Systems für kinetische Meßmethoden mit einem UV-Bestrahlungssystem für die Aktivierungsmethode. Das Gerät ermöglicht vergleichende Untersuchungen einer thermischen und der entsprechenden photochemischen Reaktion unter gleichen Versuchsbedingungen. An Silbercarbonat ausgeführte Untersuchungen haben neue Informationen über die photochemische Kinetik, aber auch über die Festkörperreaktivität und sogar über thermische Prozesse ergeben. Allgemeiner gesagt, das für Reaktivitätsstudien konstruierte Gerät kann auch zur Untersuchung anderer die Einwirkung von Licht erfordernden Systeme benutzt werden. Mit dem Gerät können der Einfluß der Lichtintensität, der Wellenlänge und der Wirkungsdauer der Lichteffekte auf viele Reaktionen untersucht werden.

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Chemiluminescence determination of carbofuran and promecarb by flow injection analysis using two photochemical reactions

A flow injection-chemiluminescent method for the determination of carbofuran and promecarb is proposed. The assay was based on the on-line conversion of the pesticides into methylamine by irradiation with UV light. The methylamine generated was subsequently reacted with tris(2,2'-bipyridine)ruthenium(III), which was generated through the on-line photo-oxidation of tris(2,2'-bipyridine)ruthenium(II) with peroxydisulfate. The emitted light showed a linear relationship with the concentration of the pesticide over the ranges 0.22-11.2 microg ml(-1) for carbofuran and 0.41-16.6 microg ml(-1) for promecarb. The repeatability was 1.6% expressed as relative standard deviation (n = 10) and the reproducibility, studied on five different days, was 2%. The sample throughput was 200 injections per h. The reliability of the method for routine analysis of these pesticides in water, soil and grains is demonstrated.     

The photochemical reactions of the halogenated N-benzylanilines. Mechanism of the reactions

Several N-(2-halobenzyl)anilines and N-benzyl-2-haloanilines have been synthesized and their photochemical reactions studied. Upon irradiation, the aqueous acetonitrile solution of N-benzyl-2-chloroaniline was cyclized and reduced to give phenanthridine, 5,5′,6,6′-tetrahydro-6,6′-biphenanthridyl (THBP), N-ben-zylaniline, and bibenzyl. Similar products were produced in the photochemical reactions of other halo-genated N-benzylanilines, except iodo-substituted N-benzylanilines. No dimer (THBP) was produced from the iodo-substituted N-benzylanilines. Both singlet and triplet states are involved in the photochemical reactions of the haloarenes.     

Alignment modulation of azobenzene-containing liquid crystal systems by photochemical reactions

Recent progress in alignment modulation of azobenzene-containing liquid crystal systems by photochemical reactions has been reviewed by dividing the modulation methods into two types: phase transitions (order–disorder change) and change of liquid crystal directors (order–order change). First, photochemical phase transitions and alignment changes of liquid crystals in guest/host mixtures and polymers are summarized. Then, alignment control of liquid crystals by linearly polarized light and photoactive surface layers is discussed. Finally, recent applications of alignment change and photochemical phase transitions of liquid crystals in holographic technology and photomechanical effects are introduced. In addition, future possible applications for a variety of practical devices, such as display devices, optical switching and reversible optical image storage, are mentioned.     

Mechanism of the photochemical reactions of substituted benzoquinones

Photoreactions of 2,6-dimethyl-, 2,6-di-tert-butyl-, and 2,6-diphenyl-1,4-benzoquinones with alcohols, organic acids, thioalcohols, amines (primary, secondary, and tertiary), cyclic ethers, tri- and tetraethoxysilanes are accompanied by the effect of chemical polarization of nuclei (CINDP) and lead, along with the formation of hydroquinones, to bicyclic and tricyclic structures and the products of radical substitution in the aromatic ring. The mechanisms of elementary photoreactions were elucidated. The intermediate excited states and radical structures were considered. For the first time permanent generation of radio waves in the course of the reversible photochemical reaction of substituted 1,4-benzoquinones with alcohols was detected.     

Novel photochemical reactions of enamino ketones

Irradiation of the conjugated N-aryl enamino ketone (V) yielded the indole (VI) resulting from a novel decomposition of the ylide intermediate.     

On selective laser photochemical reactions by means of photopredissociation of molecules

The possibility of selective photochemical reactions in gas mixtures by means of excitation of predissociation states of the specific molecules by laser radiation is considered.     

Radical reactions initiated by the photochemical cleavage of carbon-indium bonds of organoindium compounds

Intra- and intermolecular reactions of carbon-centered radicals generated by photolysis of organoindium compounds were examined. The photolysis of vinylindium compounds and indium acetylides provided vinyl and alkynyl radicals, respectively, which were trapped with ethyl iodoacetate giving the corresponding β,γ-unsaturated esters. Allylic indium compounds, prepared from 8-bromo- or 8-iodooct-1,6-dienes and powdered indium metal, underwent an intramolecular radical cyclization to afford the 5-exo-trig product.     

State correlation diagrams for the photochemical reactions of organometallics

State correlation diagrams are proposed for a number of photochemical reactions of organometallics. Spin-orbit coupling is found important to understand the reactivity of these organometallics.     

CASCSF study on the photochemical transposition reactions of pyrazines

Several reaction pathways for the photochemical transformations of methyl-substituted pyrazine in its first excited state 1(pi --> pi*) have been determined using the CASSCF (six-orbital/six-electron active space) and MP2-CAS methods with the 6-311G(d) basis set. Our model investigations suggest that conical intersections play a crucial role in the photoisomerization of pyrazines. Moreover, the present theoretical findings indicate that all of the photoisomerizations of pyrazines adopt the same reaction path as follows: pyrazine --> Franck-Condon region --> conical intersection --> pyrimidine. That is, although an excited-state pyrazine molecule can initiate a phototransposition process easily, this process can be completed on the ground-state potential energy surface after passage through a conical intersection where a fast, radiationless decay is possible. The existence of these nonadiabatic reaction pathways is consistent with the available experimental observations of the photochemistry and photophysics of pyrazine and its methyl derivatives. In the present work, we propose a simple p-pi orbital topology model, which can be used as a diagnostic tool to predict the location of the conical intersections, as well as the geometries of the phototransposition products of various heterocycles.     

Controlling the photochemical reactions of alkenes by light-path length effects of a microchannel reactor

Photoirradiation of Me₂CO–H₂O solution of pent-4-en-1-ol (1a) with a high-pressure mercury lamp in a test tube gave 8-hydroxyoctan-2-one (3a) in 66 % yield along with oxetane (4a) and the isomer (4a′) in 10 % yield. Irradiation of the running Me₂CO–H₂O solution of 1a in the flow system of a microchannel reactor (MCR) gave mainly 4a. The photoreaction of 1,1-diphenylethene (2a) with triethylamine gave a Markovnikov-type adduct (5a) and an anti-Markovnikov-type adduct (6a). The use of the MCR enhanced the production of 5a. These phenomena were explained by the light-path length effects of the MCR.     

External and internal quenching in photochemical reactions of vinylbenzophenone-vinyl ferrocene copolymers: The problem of triplet energy transfer

The quantum yield of the photochemical reaction of the benzophenone group, either free or bound to a macromolecular chain in polyvinylbenzophenone, with a proton donor (isopropanol) has been measured in the absence and in the presence of an efficient triplet acceptor (Ferrocene), either free (external) or copolymerized (internal quenching). From an analysis of the quantum yields and Stern-Volmer constants using the proper local concentration, it is concluded that energy migration along the polymer chain is negligible in this system at the time scale of the triplet life-time (≈ 10^?7 sec). Re-examination of data of Luká? et al. on polyvinylphenylketone, taking in account the local concentration, shows that in this last system energy migration is however efficient even with a shorter triplet life-time.     

Silyl migration in the photochemical reactions of 2-trimethylsilylmethylphenyl ketones

[reaction: see text]. Irradiation of 2-trimethylsilylmethylphenyl ketones 1 gives benzocyclobutenols 2 and 5 together with siloxybenzocyclobutenes 4. Compound 4 arises from the novel photoinduced 1,5-silyl migration of 1. Irradiation of 1 in the presence of magnesium perchlorate gives the less hindered benzocyclobutenols 5 without the formation of 4. The mechanism of the photochemical reaction of 1 is discussed.     

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.     

Expected and unexpected photochemical reactions of acyl iodides

Photolysis of acyl iodides RCOI (R = Me, Me₂CH, Ph) under UV irradiation in toluene environment for 20–55 h proved to be a simple and efficient method of preparation of symmetrical α-diketones RCOCOR. In contrast, the photolysis under the same conditions of acyl iodides RCOI [R = Me(CH₂)₃, Me₃C] did not lead to the formation of the corresponding diacyls, and the reaction products were unexpected 1,1-bis(4-methylphenyl)pentane and a mixture of isomeric 3- and 4-methyl(tert-butyl)benzenes respectively. The most probable mechanism of their formation is the primary photochemical acylation of toluene in the aromatic ring followed by the photochemical reduction of the arising butyl 4-methylphenyl ketone in the case of the valeroyl iodide or the photochemical Norrish type I cleavage of isomeric 3- and 4-methylphenyl (tert-butyl) ketones in event of the pivaloyl iodide. In the photolysis of acetyl iodide (R = Me) in benzene or toluene alongside the diacetyl formation polyarylation process was observed of acylated and iodinated into the aromatic ring solvents with the formation of polymeric products with semiconductor and paramagnetic properties.     

Enantioselective photochemical reactions of 2-pyridones in solution

[reaction: see text] Two key photochemical reactions of prochiral 2-pyridones were studied in the presence of a chiral host. The [4 + 4]-photocycloaddition with cyclopentadiene (CpH) proceeded smoothly and with high enantioselectivity (84-87% ee). The absolute configuration of the endo-diastereoisomer was established by X-ray crystallography. The electrocyclic [4pi]-ring closure to 3-oxo-2-azabicyclo[2.2.0]-5-hexenes occurred with lower enantioselectivity (20-23% ee at -20 degrees C). The velocity of the latter reaction slowed significantly with decreasing temperature.