Flash photolysis studies processes

Photoinduced ET at liquid-liquid interfaces has been widely recognized as a model system for natural photosynthesis and heterogeneous photocatalysis [114-119]. One of the key aspects of photochemical reactions in these systems is that the efficiency of product separation can be enhanced by differences in solvation energy, diminishing the probability of a back electron-transfer process (see Fig. 11). For instance, Brugger and Gratzel reported that the efficiency of the photoreduction of the amphiphilic methyl viologen by Ru(bpy)3+ is effectively enhanced in the presence of cationic micelles formed by cetyltrimethylammonium chloride [120]. Flash photolysis studies indicated that while the kinetics of the photoinduced reaction,... 

Flash photolysis studies with absorption or delayed fluorescence detection were performed to compare the binding of ground and excited state guests with DNA.113,136 The triplet lifetimes for 5 and 6 were shown to be lengthened in the presence of DNA.136 The decays were mono-exponential with the exception of the high excitation flux conditions where the triplet-triplet annihilation process, a bimo-lecular reaction, contributed to the decay. The residence time for the excited guest was estimated to be shorter than for the ground state, but no precise values for the rate constants were reported. However, the estimated equilibrium constants for the... 

Flash Photolysis Studies in Bimolecular Electron-transfer Processes... 

In the early 1990s, singlet aryl nitrenes had never been detected and the influence of structure on reactivity was only dimly appreciated. Laser flash photolysis studies in combination with theory has provided insight and has led to a comprehensive theory of substituent effects. We have been proud to be part of this process. 

BM) forms with a quantum yield of 0.017 0.005 when Re2Cl " is irradiated at 313 nm. Cleavage of the quadruple bond to form ReCl4(CH3CN)2 is followed by rapid conversion to the neutral monomer by means of acetonitrile replacement of chloride. No reaction occurs in the absence of irradiation at reflux, and the 88 excited state is not responsible for the observed photochemistry, since 632.8 nm radiation does not lead to cleavage of the rhenium dimer (135). Flash-photolysis studies suggest that the dominant photochemical process involves internal conversion to a transient 65 excited... 

Emission quenching, steady state quantum yield measurements, and flash photolysis studies established that the initiation process could be represented by eqs. 13 or 14, and 15 ... 

Such reactions are, of course, unimolecular decompositions, not free radical chain processes. This fact made perfluorodiacyl peroxides ideal precursors for the laser flash photolysis studies which will be described in Sect. 4. Kinetics for the thermal decomposition of a number of perfluorodiacyl peroxides have been measured and their AH+ values were approximately 24 kcal/mol, about 5 kcal/mol... 

Gopidas, K. R. Kamat, P. V. George, M. V. Photochemical processes on oxide surfaces. A diffuse reflectance laser flash photolysis study, Mol. Cryst. Liq. Cryst. 1990, 183, 403. 

The recent flash photolysis study has revealed, that the [Femedta]- excited state is quenched by electron transfer processes by both electron donors and electron acceptors (see Figure 9.12). The latter cause oxidation of the EDTA ligand and the acceptor reduction, whereas the Fem centre does not change its oxidation state. One of the acceptors is molecular oxygen, which results in direct EDTA oxidation at the expense of 02 reduction [21, 22] ... 

Femtosecond flash photolysis studies on Q-state CdS [107] indicate that reaction (4a) proceeds via two recombination processes a 50 ps decay at low excitation intensities, postulated to correspond to geminate e h+ recombination, and a faster 2 ps decay at higher flash fluences, corresponding to non-geminate or possibly three body Auger charge carrier recombination. Other studies by Nosaka and Fox [118] indicate that the second order rate coefficient for electron-hole recombination within CdS particles is of the order 9 x 10 t7 m3 s l. 

Recent laser flash photolysis studies of the kinetics of the process 7 —> 9 suggest that following photoexcitation with time-resolved spectroscopy, an isomer 8 (E form) is formed in the nanosecond or microsecond time domain which undergoes a first-order conversion to the Z form 9.42 43 Details will be published elsewhere. 

N. P. Gritsan, A. Kellmann, F. Tfibel, and L. S. Klimenko, Laser flash photolysis study of the primary processes in the photochromic reaction of l-acyloxy-2-methoxy-anthraquinones, Mol. Cyst. Liq. Cryst. 246, 259-262 (1994). 

The details of the above process have been refined somewhat by subsequent flash photolysis studies [34-38]. However, in all of the mechanisms that have been discussed, a Fe(III)-oxalate complex is reduced by an intramolecular photochemical process, generating an unbound radical (either C204 or C02 ). If a C2(),r radical is produced, it rapidly decomposes to form C02- [39] ... 

In fact, the addition of 1,4-dimethoxybenzene (DMB) and/or several similar compounds, at concentrations as low as 10 4 M, to a mixture of aryl-olefins and DCA almost completely inhibits the reactions. Concentration dependence and flash photolysis studies confirm that the primary electron-transfer process occurs between the singlet excited sensitizer and DMB (E01 = 1.34 V vs SCE) with the generation of the corresponding radical ion pair. As a consequence, quantum yields lower than 1, even at infinite substrate concentration, are measured [95]. In this regard, valuable confirmations came from the cyanoaromatic photoinduced electron-transfer oxygenation of alkynes [99], Farid and Mattes reported that the photooxygenation of diphenylacetylene DPA (E° = 1.85 V vs SCE) 25, leading to a mixture of benzil 26 and benzoic acid 27, was efficiently sensitized by DCA ( = 0.15), but poorly by TCA ( > < 0.001) [Eq. (12)] [99]. 

Free radical promoted, cationic polymerization also occurs upon irradiation of pyridinium salts in the presence of acylphosphine oxides. But phosphonyl radicals formed are not oxidized even by much stronger oxidants such as iodonium ions as was demonstrated by laser flash photolysis studies [51, 52]. The electron donor radical generating process involves either hydrogen abstraction or the addition of phosphorus centered or benzoyl radicals to vinyl ether monomers [53]. Typical reactions for the photoinitiated cationic polymerization of butyl vinyl ether by using acylphosphine oxide-pyridinium salt combination are shown in Scheme 10. 

The generally observed identity of the r value for solvolysis reactivity and gas-phase stability AAG(c+> of the corresponding carbocation leads to an important prediction concerning the solvolysis transition state. In a typical (limiting) two-step SnI mechanism with a single dominant transition state, the r values of transition states for the various nucleophile-cation reactions should be essentially controlled by the intrinsic resonance demand of the intermediate cation the substituent effect should be described by a single scale of substituent constants (a) with an r value characteristic of this cation. In a recent laser flash-photolysis study (Das, 1993) on the recombination of stable trityl and benzhydryl cations with nucleophiles and solvents, McClelland et al. (1986, 1989) have treated the substituent effects on solvent-recombination processes by (2). 

Evidence for more complex ET processes came from studies in which photo chemically generated reductants injected electrons into preformed Fe-cytochrome b lYt-cytochrome c complexes. In one study, the rate of c ET (1.7 X 10 s ) was reported to depend on viscosity and surface mutations. A later laser-flash photolysis study found a rate-limiting second-order reduction of Fe-cytochrome i s/Fe-cytochrome c complexes and no sign of satmation, suggesting that the intracomplex ET rate was greater than lO s-. ... 

Photoreduction of benzophenone by primary and secondary amines leads to the formation of benzpinacol and imines [145]. Quantum yields greater than unity for reduction of benzophenone indicated that the a-aminoalkyl radical could further reduce the ground state of benzophenone. Bhattacharyya and Das confirmed this in a laser-flash photolysis study of the benzophenone-triethylamine system, which showed that ketyl radical anion formation occurs by a fast and a slow process wherein the slow process corresponds to the reaction of a-aminoalkyl radical in the ground state of benzophenone [148]. Direct evidence for similar secondary reduction of benzil [149] and naphthalimides [150] by the a-aminoalkyl radical have also been reported. The secondary dark reaction of a-aminoalkyl radicals in photo-induced electron-transfer reactions with a variety of quinones, dyes, and metal complexes has been studied by Whitten and coworkers [151]. 

Polymerization of butane-1,4-diol dimethacrylate, sensitized by benzophenone in the presence of three different sulfides, has been described by Andrzejewska et al. [190]. The measurements show that in the absence and in the presence of propyl sulfide and 2,2 -thiobisethanol no polymer was formed. This can be explained by the effective back electron transfer process that occurs in the radical-ion pair in organic solvents. Effective polymerization was observed only in the presence of TMT. Laser flash photolysis studies performed for the benzophenone-TMT pair allow one to construct a scheme (Scheme 23) explaining characteristic features of the mechanism of polymerization initiated by the system. The results prompted the authors to study other symmetrically substituted 1,3,5-trithianes as electron donors for benzophenone-sensitized free-radical polymerization (Figure 38 Table 12) [191]. 

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