Primary Photochemical Step. Quantum Yield

The photochemical Stark-Einstein law implies that the number of primary steps of a photochemical reaction must be equal to that of light quanta absorbed. Denoting the number of primary steps by ANq, the total amount of absorbed radiant energy by AI, and the light quantum by hv, this law can be written in the form 

In most cases, because of the occurence of secondary processes, the number of reacted molecules AN does not coincide with that of primary photochemical steps. For a general case, the photochemical effectivity of light is specified by the value 

This value known as the quantum yield represents the number of molecules reacting per one quantum absorbed. 

The measured quantum yields for various reactions cover a very wide range. Suffice it to compare the d values for related reactions such as H2 + CI2 - 2HC1 and H2 + Br2 2HBr. V hilst for the first reaction, O can reach 100000 [47], it assumes a value of the order 0.001 (at 160—218 °C) in the second reaction [48]. 

One of the reasons for the often observed low quantum yields lies first of aU in the low rates of secondary processes, which make possible a competing deactivation of primary active centers either by their consumption outside the reaction (e.g. by recombination of atoms and radicals) or by loss of excitation energy (by emission or in collisions with surrounding molecules). 

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Aliphatic sulfides can be efficient co-initiators for the photoinduced polymerization induced by benzophenone [185, 186]. An exceptionally strong effect was observed for 2,4,6-trimethyl-1,3,5-trithiane (TMT). A model reaction for free-radical formation during photoreduction of an initiator triplet state by a sulfide is the photoreduction of benzophenone by dimethyl sulfide [171, 187-189]. In this process it was established that electron transfer from the sulfur atom to the triplet state of the benzophenone is a primary photochemical step. In this step, radical ions are formed. The overall quantum yields of photoproducts (ketyl radicals and radical anions) are low (Ed) 0.26) in aqueous solution, in the range 0.16-0.20 in mixed water-acetonitrile solution and less then 0.01 in pure acetonitrile. These results suggest that, in organic solvents, back electron transfer within the radical-ion pair to regenerate the reactants is the dominant process. 

The fluorescence properties of several europium and samarium ) -diketonates have been measured and assignments of the transitions made. Rare-earth element hexafluoroacetylacetonates with amino-acids have also been reported to fluoresce. The luminescence of the heptafluoroheptane-2,4-dione complexes of Sm, Eu, and Tb has been measured in dilute ethanol at pH8 and 610nm mixed-ligand complexes with 1,10-phenanthroline exhibited an enhanced luminescence. Photolysis of the Tb chelate of 2,2,6,6-tetramethylheptane-3,5-dione has been examined at 311 nm in various alcohols, and loss of one -diketone ligand found to be the primary photochemical step. A linear correlation was demonstrated between the quantum yield of dissociation of the complex and the formation constant of the complex-alcohol adduct. 

High chemical but low quantum yields of the two products (233) and (234) are obtained on irradiation (X > 500 nm) of the tetraketones (235). The primary photochemical step is the conversion of the tetraketones into carbon monoxide and the ketenes (236). The reaction process occurs from the singlet state and is thought to be concerted. The final products are formed by the addition of these ketenes (236) to ground state tetraketone. The ketene (236b) was studied in a little more detail and it was shown that irradiation in benzene or toluene at room temperature results in the formation of the dimer (237). Addition of the same ketene to diketones such as biacetyl was also reported. 

After the primary step in a photochemical reaction, the secondary processes may be quite complicated, e.g. when atoms and free radicals are fcrnied. Consequently the quantum yield, i.e. the number of molecules which are caused to react for a single quantum of light absorbed, is only exceptionally equal to exactly unity. E.g. the quantum yield of the decomposition of methyl iodide by u.v. light is only about 10" because some of the free radicals formed re-combine. The quantum yield of the reaction of H2 -f- CI2 is 10 to 10 (and the mixture may explode) because this is a chain reaction. 

Because of the paucity of rate data for the reactions of F and FO one can only speculate on the course of the reaction subsequent to the initial dissociation. The results of photochemical studies398,399 give some guidance. The quantum yield F2o of photodecomposition is 1.0 at 3650 A, independent of temperature in the range 15-45 °C, pressure of F20 and pressure of oxygen398 the primary step is almost certainly as in (2)398,3". Thus, at room temperature at least, any contribution from... 

The net result of a photochemical redox reaction often gives very little information on the quantum yield of the primary electron transfer reaction since this is in many cases compensated by reverse electron transfer between the primary reaction products. This is equally so in homogeneous as well as in heterogeneous reactions. While the reverse process in homogeneous reactions can only by suppressed by consecutive irreversible chemical steps, one has a chance of preventing the reverse reaction in heterogeneous electron transfer processes by applying suitable electric fields. We shall see that this can best be done with semiconductor or insulator electrodes and that there it is possible to study photochemical primary processes with the help of such electrochemical techniques 5-G>7>. 

Quantum yields of product formation, Ov, can range from <10 6 to >106, depending on the reaction system a quantum yield >1 suggests the secondary reaction courses. A secondary step allows another reactant molecule to be consumed by the primary photoproduct and then Op could reach 2. The quantum yield >2 suggests a chain reaction mechanism. A good example of this is the photochemical synthesis of HC1, for which Op = 106, which means that absorption of one photon by a Cl2 molecule results in the production of a million molecules of HC1. 

The photochemical oxidation of SO2 was studied by several workers in the 1950 s. As the work has been reviewed by Leighton, we will only summarize the more important results. The product of the photo-oxidation is SO3, or, if water is present, H2SO4. Various workers have obtained quantum yields of SO3 ranging from 0.3 to 0.003. As the photo-oxidation is initiated by light of insuflfi-cient energy to break the OS-0 bond, it seems certain that the primary step is the formation of excited SO2 molecules. Presumably these excited species then react with O2 to form a peroxide which subsequently decomposes by various steps, e.g. 

The photochemical decompositions of perfluoroazomethane - and per-fluoroazoethane closely resemble those of the parent azohydrocarbons. Thus perfluoroazomethane when irradiated in the near ultraviolet, decomposes to nitrogen and perfluoroethane via a primary dissociation step in which trifluoro-methyl radicals are formed. Quantum yields of about 0.25 in the gas phase at room temperature - may be the result of collisional deactivation of excited perfluoroazomethane molecules. 

A new and efficient route to endo-hirsutene has been described which uses, as the key step, the oxetane (11) from the intramolecular photocycloaddition of (12) (Rawal et ai), and 3-deoxy-D-arabino-2-heptulopyranosonic acids can be synthesised photochemically from Barton esters such as (13) (Barton and Liu). Interest in the development of photochemically removable protecting groups continues, and the irradiation of the 4-hydroxyphenacyl-protected system (14) is reported to release ATP with a quantum yield of 0.37 (Givens and Park). Benzoylbenzoate esters of primary and secondary alcohols undergo cleavage in the presence of electron donor molecules and it is proposed that such esters could be effective photolabile protecting groups for alcohols and that thiols can be similarly protected (Jones et al.). 

Fig. 5.49. Determination of the partial photochemical quantum yield of the primary photodegradation step of umbeliiferone using the fluorescent intensity corrected by absorbance at...

The influence of temperature on photochemical reaction rates can be very different depending on the type of reaction, almost always following the same order of magnitude as the quantum yield. The primary act is never affected by temperature and thus the change of reaction rate with temperature is always due to other, thermal, steps of the mechanism. 

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