Flash Photolysis Investigations

In studies of the properties of semiconductor particles or corresponding colloidal solutions, the formation of intermediates needs to be measured. This is usually done by employing a laser flash apparatus, as has been commonly used in photochemistry for many decades. The principle is as follows. 

It is also possible to determine transients in the pico- and femtosecond range. Fairly sophisticated equipment has been developed, which cannot be described here. 

---

Roberts, C. B. Zhang, J. Brennecke, J. F. Chateauneuf, J. E. Laser Flash Photolysis Investigations of Diffusion-Controlled Reactions in Supercritical Fluids. J. Phys. Chem. 1993a, 97, 5618-5623. 

Recent flash photolysis investigations have provided direct evidence that the primary photochemical process of these... 

The [4 + 2]-cycloaddition of 02( Ag) to rubrene has been shown to be a simple method for the determination of oxygen concentrations in organic solvents.Irradiation of benzene solutions of tropone (86) in the presence of 9,10-dicyanoanthracene leads to the formation of four products, (87), (88), (89) and (90), and in acetonitrile-dichloromethane there is also an [8 + 4]tc adduct (91) produced. It has been suggested that this latter compound arises by coupling of the radical cation of (86) with the radical anion of the dicyanoanthracene (Scheme 2). Solvent-dependent quenching of the lowest excited state of 9,10-dibromoanthracene by 2,5-dimethylhexa-2,4-diene has been studied and appears to proceed by an exciplex. Flash photolysis investigations have shown that a neutral radical species is an intermediate in the formation of the [4 + 2] adduct which is a dibenzobicyclo[2.2.2]octadiene-type compound. Irradiation of 9,10-dicyanophenanthrene (DCA) in the presence of buta-1,3-diene gives a mixture of the product of [3 + 2]-photo-... 

Laser flash-photolysis investigations have been carried out for benzylchloro-carbene, phenylchlorocarbene, methoxyphenylcarbene and fluorenylidene, and some other carbenes, generated from diazirine, diazo and other precursors, in the presence of oxiranes and thiiranes. These carbenes abstract oxygen or sulfur atoms with bimolecular rate constants in the range 10 to 10 s ... 

The initial step in the mechanism outlined in Scheme 2 is electron transfer quenching of the singlet arene by DCNB. Nucleophilic addition of the amine to the arene cation radical followed by proton and electron transfer steps yields the adduct and regenerates the sensitizer. Adduct formation requires the use of polar solvents, and yields are higher in aqueous vs. dry acetonitrile. Adduct formation is observed in moderately polar solvents (ethers) in the presence, but not in the absence, of an added salt, n-Bu4NBF4. The solvent and salt effects were interpreted as evidence for C-N bond formation via the free arene radical cation, rather than via an ion pair (CRIP or SSRIP), However, Nieminen et ah concluded that nucleophilic attack involves a radical ion pair on the basis of their laser flash photolysis investigation. In addition to this unresolved controversy, the timing of the subsequent proton transfer and electron transfer steps remains to be established. 

Workentin et al. have recently reported the results of an extensive laser flash photolysis investigation of the reactions of the cation radicals of 9-phenyl- and 9,10-diphenylanthracene (PA and DPA, respectively) with amines. Primary amines react with both cation radicals via nucleophilic addition with rate constants which reflect both the amine basicity and a steric requirement for bond formation. Steric effects are more pronounced for addition of DPA " vs. PA ", presumably due to the presence of substituents at both the 9- and 10-position. Tertiary amines and anilines react with PA " and DPA " via electron transfer with rate constants which correlate with amine ionization potentials. Rate constants for nucleophilic addition of primary amines are faster in acetonitrile than in acetonitrile/water solution. The rate-retarding effect of water is attributed to an equilibrium between the fiee amine (reactive) and hydrated amine (unreactive). The beneficial effect of water on preparative ET-sen itized photoamination may reflect its role as a catalyst for the proton transfer processes which follow C-N bond formation (Scheme 2). Hydration of the amine also should render it less reactive in primary and secondary electron transfer processes which can compete with the formation of the arene cation radical. 

Venkatachalapathy B, Ramamurthy P (1996) Reactions of nitrate radical with amino acids in acidic aqueous medium a flash photolysis investigation. J Photochem Photobiol A Chem 93 1-5... 

Kuhknann R, Schnabel W. Flash-photolysis investigation on primary processes of sensitized polymerization of vinyl monomers. 2. Experiments with benzoin and benzoin derivatives. Polymer 1977 18 1163-1168. 

CB Roberts, J Zhang, JF Brennecke, JE Chateauneuf. Laser flash photolysis investigations of diffusion-controlled reactions in supercritical fluids. J Phys Chem 97 5618, 1993. 

M Kremer, KA Connery, MM DiPippo, J Feng, JE Chateauneuf, JF Brennecke. Laser flash photolysis investigation of the triplet-triplet annihilation of anthracene in supercritical water. J Phys Chem A 103 6591, 1999. 

The key step in the proposed oxetane repair mechanism is the rapid fragmentation of the oxetane radical anion. This ring-opening reaction was recently studied with model compounds. Falvey and coworkers " prepared stable oxetane adducts from dimethylthymine and various benzaldehyde derivatives. Laser flash photolysis investigations in the presence of external electron donors hke dimethyl aniline showed efficient oxetane sphtting, which allowed investigation of the radical intermediates. The radical cations of the various sensitizers and the radical anions of the carbonyl products formed after... 

---