Primary photoproduct

Fig. 5. Chemistry of cyclized mbbei—bis-a2ide negative acting resist, (a) Preparation of cyclized mbber resin from polyisoprene (b) photochemistry of aromatic bis-a2ide sensiti2ers. The primary photoproduct is a highly reactive nitrene which may combine with molecular oxygen to form oxygenated products, or may react with the resin matrix by addition or insertion to form polymer—polymer linkages.

Catalysis (qv) refers to a process by which a substance (the catalyst) accelerates an otherwise thermodynamically favored but kiaeticahy slow reaction and the catalyst is fully regenerated at the end of each catalytic cycle (1). When photons are also impHcated in the process, photocatalysis is defined without the implication of some special or specific mechanism as the acceleration of the prate of a photoreaction by the presence of a catalyst. The catalyst may accelerate the photoreaction by interaction with a substrate either in its ground state or in its excited state and/or with the primary photoproduct, depending on the mechanism of the photoreaction (2). Therefore, the nondescriptive term photocatalysis is a general label to indicate that light and some substance, the catalyst or the initiator, are necessary entities to influence a reaction (3,4). The process must be shown to be truly catalytic by some acceptable and attainable parameter. Reaction 1, in which the titanium dioxide serves as a catalyst, may be taken as both a photocatalytic oxidation and a photocatalytic dehydrogenation (5). 

Nitrile ylides derived from the photolysis of 1-azirines have also been found to undergo a novel intramolecular 1,1-cycloaddition reaction (75JA3862). Irradiation of (65) gave a 1 1 mixture of azabicyclohexenes (67) and (68). On further irradiation (67) was quantitatively isomerized to (68). Photolysis of (65) in the presence of excess dimethyl acetylenedicar-boxylate resulted in the 1,3-dipolar trapping of the normal nitrile ylide. Under these conditions, the formation of azabicyclohexenes (67) and (68) was entirely suppressed. The photoreaction of the closely related methyl-substituted azirine (65b) gave azabicyclohexene (68b) as the primary photoproduct. The formation of the thermodynamically less favored endo isomer, i.e. (68b), corresponds to a complete inversion of stereochemistry about the TT-system in the cycloaddition process. 

Burhenne J, M Ludwig, M Spiteller (1997a) Photolytic degradation of fluoroquinolone carboxylic acids in aqueous solution. Primary photoproducts and half-lives. Environ Sci Pollut Res 4 10-15. 

There is still controversy over the primary steps in the photolysis of dinuclear metal carbonyls, such as Mn2(CO)10 or [CpFe(CO)2]2, and the field has been recently reviewed (102,103). The controversy has centered on the number of primary photoproducts, their identity, the effects of photolysis wavelength, and the possibility of heterophotolysis (21,103). 

Similar experiments on a large number of transition metal carbonyls have shown that this process favors dissociation to and detection of metal clusters or atoms. Since most metal-(CO)n photofragments are themselves subject to efficient dissociation, MPI experiments do not identify the primary photoproducts. This situation contrasts sharply with electron impact ionization where the parent ion is usually formed and daughter ions are seen as a result of parent ion fragmentation. Figure 4 shows the electron impact mass spectrum of Mn2(C0) Q (33). for comparison with the MPI mass spectrum of Figure 3. 

The development of comprehensive models for transition metal carbonyl photochemistry requires that three types of data be obtained. First, information on the dynamics of the photochemical event is needed. Which reactant electronic states are involved What is the role of radiationless transitions Second, what are the primary photoproducts Are they stable with respect to unimolecular decay Can the unsaturated species produced by photolysis be spectroscopically characterized in the absence of solvent Finally, we require thermochemical and kinetic data i.e. metal-ligand bond dissociation energies and association rate constants. We describe below how such data is being obtained in our laboratory. 

These results were explained by the discovery that the chloro-ruthenium complexes are not the primary photoproducts under CO in 1.0 M CCl /octane solution. Instead Ru(CO>5 proved to be the initial product even after nearly complete photofragmentation of the starting material, and the chlorocarbonyl ruthenium products to be the result of a secondary, dark reaction between the Ru(C0)5 and CCI4 (3) ... 

An alternative explanation, based on a spectroscopic study involving an argon matrix, has been advanced to account for the singlet photochemistry of phenyl azide (457).381 The primary photoproduct is believed to be 1-azacyclohepta-l,2,4,6-tetraene (458). A separate but later study suggests that... 

Note added in typing In a very recent paper (81) Vaida and co-workers have used picosecond laser photolysis to show that, in cyclohexane solution, Cr(CO)5...cyclohexane (Amax 497 nm) is formed within 25 ps of the photolysis of Cr(C0)5 This suggests that, in solution, the primary photoproduct is Cr(C0)5 and that there is essentially no activation energy for the reaction of Cr(C0)5 with the solvent. Clearly, experiments with pulsed KrF lasers on carbonyls in solution and matrix may be very revealing. 

Organometallic Intermediates by CO Loss. There are many studies where the primary photoproduct generated by CO loss has been characterized in a matrix. One example will illustrate. 

In order to rationalize the complex reaction mixtures in these slurry reactions the authors suggested that irradiations of the oxygen CT complexes resulted in simultaneous formation of an epoxide and dioxetane36 (Fig. 34). The epoxide products were isolated only when pyridine was co-included in the zeolite during the reaction. Collapse of the 1,1-diarylethylene radical cation superoxide ion pair provides a reasonable explanation for the formation of the dioxetane, however, epoxide formation is more difficult to rationalize. However, we do point out that photochemical formation of oxygen atoms has previously been observed in other systems.141 All the other products were formed either thermally or photochemically from these two primary photoproducts (Fig. 34). The thermal (acid catalyzed) formation of 1,1-diphenylacetaldehyde from the epoxide during photooxygenation of 30 (Fig. 34) was independently verified by addition of an authentic sample of the epoxide to NaY. The formation of diphenylmethane in the reaction of 30 but not 31 is also consistent with the well-established facile (at 254 nm but not 366 or 420 nm) Norrish Type I... 

Preparative photolysis of AETSAPPE (0.25 M aqueous solution) at 254 nm (Rayonet reactor) resulted in the formation of the disulfide product 2-amino(2-hydroxy-3-(phenyl ether) propyl) ether disulfide (AHPEPED) as the primary photoproduct Photolysis of AETSAPPE at 254 nm (isolated line of medium pressure mercury lamp) resulted in rapid initial loss of starting material accompanied by formation (analyzed by HPLC) of AHPEPED (Figure 12a and 12b) (Scheme IV). Similar results were obtained for photolysis- at 280 nm. Quantum yields for disappearance of AETSAPPE and formation of AHPEPED at 254 nm and 280 nm are given in Table I. The photolytic decomposition of AETSAPPE in water was also accomplished by sensitization ( x =366 nm) with (4-benzoylbenzyl) trimethylammonium chloride (BTC), a water soluble benzophenone type triplet sensitizer. The quantum yield for the sensitized disappearance (Table I) is comparable to the results for direct photolysis (unfortunately, due to experimental complications we did not measure the quantum yield for AHPEPED formation). These results indicate that direct photolysis of AETSAPPE probably proceeds from a triplet state. 

Irradiation rraws-2-[3-(7V-methylamino)propyl] stilbene 89 results in the formation of 7V-methyl-l-benzyltetrahydro-2-benzazepine 90 as the only significant primary photoproduct (equation 26), which in turn undergoes secondary photochemical N-demethylation. The final mixture contains 90 (38%) and 91 (25%) at high (>95%) conversion. Intramolecular photoadditions of these (equations 24-26) secondary (aminoalkyl)-stilbenes are highly regioselective processes24. 

Time-resolved resonance Raman spectroscopy has been used to study the photorearrangement of o-nitrobenzyl esters in polar and protic solvents53 in acetonitrile, the only primary photoproduct is nitronic acid 68 with a lifetime of 80 microsecond, while in methanol the nitronic acid exists in equilibrium with the nitronate anion 69, giving a lifetime of 100 microseconds (equation 41). 

Although the stoichiometry of the reaction (eq 6) and the reactivity experiments detailed below indicate that the primary photoproduct from [ReH (dppe)2] is almost certainly... 

CASRN 51707-55-2 molecular formula C9H8N4OS FW 220.20 Soil. The reported half-life in soil is approximately 26-144 d (Hartley and Kidd, 1987). Photolytic. Klehr et al. (1983) studied the photolysis of thiadiazuron on adsorbed soil surfaces. Irradiation was conducted using artificial sunlight having a wavelength <290 nm. The amount of thiadiazuron remaining after irradiation times of 0.25, 0.5, 1, 2, 3.75, and 18.0 h were 56.4, 42.8, 35.7, 23.8, 25.0, and 67.2%, respectively. The primary photoproduct identified was l-phenyl-3-(l,2,5-thiadiazol-3-yl)urea and five unknown polar compounds. The unknown com pounds could not be identified because the quantities were too small to be detected. 

Irradiation of the precursor I3-N2 in an Ar matrix quickly gives rise to a compound with strong IR bands at 1758 and 1772 cm [75, 84]. However, careful monitoring of the irradiation by IR spectroscopy revealed that this was not the primary photoproduct. As shown in Fig. 4, during irradiation, the concentration of the starting material (monitored by its absorption at 2084 cm ) decreases and gives rise to a new component A. However, before consumption of I3-N2 is completed, the concentration of A (monitored by its absorption at 1063 cm ) decreases and signals due to components B and C (monitored by their absorptions at 1758 and 3280 cm , respectively) appear. 

Early picosecond studies were carried out by Schneider et al, [63] on the parent spiro-oxazine (NOSH in Scheme 8) and similar derivatives. In a back-to-back work, they also described a complimentary CARS (coherent anti-Stokes Raman spectroscopy) investigation [69], Simply put, these authors found that the closed spiro-oxazine ring opened in 2-12 psec after laser excitation. The reaction was slower in more viscous solvents. An intermediate state formed within the excitation pulse and preceded the formation of merocyanine forms. This transient was named X in deference to the X transient named by Heiligman-Rim et al. for the spiropyran primary photoproduct [8], (See also the previous section.) The name X has since been adopted by other workers for the spiro-oxazines [26,65],... 

We examine here possible structural effects that may result from or accompany the generation of the primary photoproducts, and speculate about the consequences of concomitant changes in distances,conformations, relative orientations and charges on the electronic profiles of and interactions between the BChls, BPheos and their radicals. Because the primary events in green plant photosynthesis also involve a series of chlorophyll donors and acceptors ( ), similar trends should therefore prevail for chlorophyll radicals as well. Furthermore, radicals of porphyrins and hydroporphyrins (saturated porphyrins such as chlorins and isobacteriochlorins) have been... 

Any elementary chemical process (also referred to as a primary photoreaction) in which an electronically excited molecular entity yields a primary photoproduct. See Primary Photoproduct... 

PRIMARY PHOTOCHEMICAL PROCESS PRIMARY PHOTOPRODUCT PRIMARY PHOTOPRODUCT PRIMITIVE CHANGE... 

Cyano- and 10-chloro-acridine A-oxides yield 1,2-oxazepines on photolysis in benzene (Scheme 237). When the 10-substituent is methyl the unstable 1,2-oxazepine undergoes electrocyclization to a benzisoxazoline (320). Secondary photoproducts may be formed by direct irradiation of N- oxides or by rearrangement of the primary photoproduct. 

Schmidt, S., R. N. Schindler, and T. Benter, Photodissociation Dynamics of CIO and CIOOCI Branching Ratios, Kinetic Energy and Quantum Yield of Primary Photoproducts, Presented at the XXIII Informal Conference on Photochemistry, May 10-15,... 

The principal disadvantage to the aminopyridinium ion route is the accessibihty of the precursors. None are available commercially, and most require multistep syntheses giving relatively low yields. Another potential pitfall is the formation of the pyridine byproduct (54). Pyridines can function as nucleophiles, attacking the nitrenium ion and creating complex mixmres. Finally, pyiidinium ions are electron deficient and can serve as good ground-state electron acceptors. Many of the stable products generated from nitrenium ion reactions are amines and are relatively easy to oxidize. Thus, a potential problem is secondary reaction, whereby primary photoproducts are oxidized by the precursor. 

A detailed product study of the irradiation of benzil in cumene and in isopropyl alcohol has shown that a complex product mixture, similar to that observed from irradiation of benzil in cyclohexane, is obtained.83 However, from the data in these solvents, as in the case of cyclohexane, it is difficult to draw any firm mechanistic conclusions since it is not known if all the observed products are primary photoproducts or if many of them are formed via secondary reactions of the primary photoproduct or products. The complexity of the product mixture may be due to photodecomposition of 10. 

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