Flash photolysis compounds

Flash photolysis of coordination compounds. Z. Stasicka and A. Marchaj, Coord. Chem. Rev., 1977,23,131-181 (271). 

There are countless other reactions, many like these and others rather different, but the idea in every case is the same. A sudden flash of light causes an immediate photo-excitation chemical events ensue thereafter. This technique of flash photolysis was invented and applied to certain gas-phase reactions by G. Porter and R. G. W. Nor-rish, who shared with Eigen the 1967 Nobel Prize in Chemistry. High-intensity flash lamps fired by a capacitor discharge were once the method of choice for fast photochemical excitation. Lasers, which are in general much faster, have nowadays largely supplanted flash lamps. Moreover, the laser light is monochromatic so that only the desired absorption band of the parent compound will be irradiated. 

Despite the above, there is also considerable evidence to suggest that oxazole formation proceeds via an intermediate nitrile ylide, particularly in the catalysed reactions (see below). Nitrile ylides have been detected in laser flash photolysis studies of diazo compounds in the presence of nitriles, and stable nitrile ylides can be isolated in some cases.<94CRV1091>... 

In a flash photolysis study of a series of diazo carbonyl compounds, a correlation was found between the amount of carbene that could be trapped by pyridine and the amount of s-trans ketone.240... 

Flash photolysis, ESR, and absorption studies of this photolysis have identified a transient species (leading to product) as a complex (Amax 410 nm, r 5 x 10-7 sec) between the triplet excited aromatic compound and its nucleophilic reaction partner. 

Relatively little work has been done on the flash photolysis of gas phase metal carbonyls, partly because of the low volatility of many of the compounds. Early work by Callear (41,42) provided some evidence for Ni(CO)3 generated from Ni(CO)4 in the gas phase (41) and Fe atoms produced from Fe(CO)5 (42). This latter process has even been used as the basis of an Fe atom laser (43). More recently Breckenridge and Sinai (44) studied the flash photolysis of Cr(CO)6. Their results, interpreted largely on the basis of data from matrix isolation experiments, were in broad agreement with Kelly and Bonneau s solution work (JJ). In particular, they found no evidence for loss of more than one CO group [Eqs. (4) and (5)]. 

Fig. 12. Transient IR difference spectra showing changes in absorbance (a) 5 / seconds, (b) 25 seconds, and (c) 1.25 mseconds after the UV flash photolysis of [CpFe(CO)2]2 in cyclohexane solution under 1 atm pressure of CO. Bands pointing upward represent an increase in absorbance (i.e., formation of a compound) and those pointing downward a decrease [i.e., depletion of starting material, (A)]. The bands are assigned as follows A, [CpFe(CO)2]2 B, CpFe(CO)2 and C, CpFe(p.-CO)3Fe(Cp). Points marked were recorded with a 12CO laser and those marked + with a 13CO laser. [Reproduced with permission from Moore et al. (61).]...

Bradley and Tuffnell133 studied the decomposition of COCl2 and (COCl)2 in a combined shock tube and flash photolysis apparatus. The shock raised the reactant gas mixture to a steady high temperature and steady high velocity the rapid flow was used continuously to remove photolysed gas from the illuminated region. The experiments yielded no new kinetic data but results indicated that (COCl)2 is an intermediate in the photolyses of both compounds. 

Laser Flash Photolysis at 248 nm of TDI-PU. MDI-PUE. and Model Compounds. Figures 1 and 2 show the transient absorption spectra of MDI-PUE (5.5 X lO-3 g/dL) and TDI-PU (2.3 X 10 3 g/dL) in THF at a 2.0 ns delay after pulsing with a krypton fluoride excimer laser (Xex=248 nm) in air and nitrogen saturated samples. Both spectra have common peaks in nitrogen saturated solutions (shown by arrows) at 310 nm, 330-360 nm (broad), and above 400 nm (broad, diffuse absorbance).. The MDI-PUE sample has an additional and quite distinctive peak at 370 nm. In the presence of air, the peak at 370 nm for MDI-PUE is completely extinguished, while the sharp peaks at 310 nm for TDI-PU and MDI-PUE and the broad band above 400 nm are only marginally quenched by oxygen. 

Laser Flash Photolysis at 351 nm of tert-Butyl Peroxide/Benzene Solutions Containing MDI-PUE and Model Compounds. Photolysis of tert-butyl peroxide (TBP) results in a highly efficient production of tert-butoxy radicals. It has recently been shown (15) that tert-butoxy radicals generated by the laser flash photolysis of TBP can rapidly extract hydrogen atoms from appropriate substrates such as aniline and diphenylamine (Scheme III). 

An excess of the twisted form (B) was prepared from the photostable 2,2 -bis(trifluoromethyl)-129c by laser flash photolysis, and its decay to the folded form (A) was followed photometrically. From the rate constants at four temperatures, AH (B- A) = 16.1 0.7 kcal/mol was obtained. For this compound, AH° (A B) = 4.2 kcal/mol had been found, and the sum of these two values agrees well with AG (E- Z) = 21.5 0.3 kcal/mol obtained by band-shape analysis, which probably corresponds to a A// value of 20 to 21 kcal/mol, assuming AS to be -2 to -5 e.u. 

Laser flash photolysis (30 ns, 50 mj, 347 nm) of 4-nitrostilbene, 4,4 -dinitro-stilbene, 4-nitro-4 -methoxystilbene and 4-dimethyl-amino-4 -nitrostilbene permits the observation of transients 133) por the latter two compounds, the lifetime and the absorption spectra of the transients vary strongly with the polarity of the solvent used. First order decay rate constants are given in Table 4. 

The term methylene is exclusively reserved for CH2. Spectroscopic investigations demonstrated that photolysis of diazo compounds in fact produces free carbenes. A flash photolysis of diazomethane gave methylene whose spectrum could be recorded 3i>. ESR-spectra were taken of a series of triplet carbenes which had been obtained by direct irradiation of diazo compounds in various matrices at low temperatures (see p. 97). 

Cr + reacts with H atoms, generated by pulse radiolysis or uv flash photolysis, to give Cr(H20)5H, A = 1.5 X 10 M s . The product may be regarded as the first member of a series of compounds of general formulae Cr(H20)5R which arise from reaction of Cr " with RCl in anaerobic acid solution. It is the most reactive member of the series towards acidolysis (e.g. A (CrH )/fc(Cr(CH3) ) = 2 X 10 ) and in other electrophilic reactions. ... 

The mechanism has been ascertained by laser flash photolysis. Besides aromatic substitution, it is possible to obtain cyclic products by using meta-substituted compounds 279 due to the para-directing effect of the nitro group [222, 223]. 

Photolysis ofbenzylchlorodiazirine (3) in the presence of tetramethylethylene (TME) is known to produce ( )- and (Z)-/l-chlorostyrene (4) and the cyclopropane (5). Plots of [5]/[4] vs [TME] are curved, consistent with the existence of two pathways for the formation of the alkenes (4). Benzylchlorocarbene (BnClC ) was generated by laser flash photolysis of the phenanthrene (6) in the presence of TME. In this case, plots of [5]/[4] vs [TME] are linear, mling out the possibility that the second pathway to the alkenes (4) involves reaction of a carbene-alkene complex. Time-resolved IR spectroscopy revealed that diazirine (3) rearranges to the corresponding diazo compound, but this process is too inefficient to account for the curvatures. It is proposed that the second pathway to alkene formation involves the excited state of the diazirine. 

Laser flash photolysis of phenylchlorodiazirine was used to measure the absolute rate constants for intermolecular insertion of phenylchlorocarbene into CH bonds of a variety of co-reactants. Selective stabilization of the carbene ground state by r-complexation to benzene was proposed to explain the slower insertions observed in this solvent in comparison with those in pentane. Insertion into the secondary CH bond of cyclohexane showed a primary kinetic isotope effect k ikY) of 3.8. l-Hydroxymethyl-9-fluorenylidene (79), generated by photolysis of the corresponding diazo compound, gave aldehyde (80) in benzene or acetonitrile via intramolecular H-transfer. In methanol, the major product was the ether, formed by insertion of the carbene into the MeO-H bond, and the aldehyde (80) was formed in minor amounts through H-transfer from the triplet carbene to give a triplet diradical which can relax to the enol. 

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

Laser flash photolysis experiments showed that the (Reaction 8.17) reacts with cyclic oligosilanes 42, 43 and 44 in benzonitrile by an electron transfer mechanism [43]. The rate constant ( et) for the three-membered cyclic compound 42 is found to be 7.0 x 10 M s, whereas for the other two compounds it was more than two orders of magnitute lower, i.e., (1-2) X 10 M- s-. ... 

Nakamura and co-workers provided detailed mechanistic information for the photoinduced electron transfer from tri-1 -naphthyl phosphate and related compounds to 9,10-dicyanoanthracene yielding binaphthyls The intramolecular nature of the reaction could be established by using laser flash photolysis experiments as well as fluorescence measurements [17],... 

The direct detection of the S <- Sj absorption in organic compounds has so far been achieved by a nanosecond or picosecond laser flash photolysis method. The general features of transient absorption spectra of metalloporphyrins actually suggest the presence of strong absorption bands in visible or ultraviolet region (38-40). However, as the transient absorption of the state often overlaps with that of ground state depletion, it is usually difficult to evaluate the absolute absorption cross sections for the transition by... 

The iron(II) complexes of the hexadentate 2-pyridylmethyl derivatives of bipy (94) and (95) are spin cross-over compounds, whose light-induced high-spin to low-spin conversion has been monitored in solution by laser flash photolysis. Single exponential kinetics (A h l = 6.7 x 10 s , at 273 K) were observed for [Fe(94)] ", but for [Fe(95)] " kinetics were biphasic, with the spin-conversion step (ku i = 2.5 x 10 s ) followed by a slower step (k = 3.7 x 10 s ) involving rearrangement of the pyridylmethyl pendant arms. ... 

A bond-cleavage reaction that has been light-induced. The term has also been used to describe irradiation of a sample by light however, such usage is discouraged. See Flash Photolysis Photoreactive Caged Compounds... 

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