Flash photolysis transient detection

FIGURE 6.8 Typical side-on (top) and front-face (bottom) optical arrangements for laser flash photolysis to detect transient changes in the absorption spectrum. Abbreviations S = light source (probe) L = lens C = cell holder + cell (typical path lengths 1-0.5 cm (top) and 0.5-0.2cm (bottom)) M = mirror Mo — monochromator P = photomultiplier. The most commonly used lasers deliver powers equal to or larger than 0.5 MW per pulse. 

Transient species, existing for periods of time of the order of a microsecond (lO s) or a nanosecond (10 s), may be produced by photolysis using far-ultraviolet radiation. Electronic spectroscopy is one of the most sensitive methods for detecting such species, whether they are produced in the solid, liquid or gas phase, but a special technique, that of flash photolysis devised by Norrish and Porter in 1949, is necessary. 

The prototype o-quinone methide (o-QM) and / -quinone methide (p-QM) are reactive intermediates. In fact, they have only been detected spectroscopically at low temperatures (10 K) in an argon matrix,1 or as a transient species by laser flash photolysis.2 Such a reactivity is mainly due to their electrophilic nature, which is remarkable in comparison to that of other neutral electrophiles. In fact, QMs are excellent Michael acceptors, and nucleophiles add very fast under mild conditions at the QM exocyclic methylene group to form benzylic adducts, according to Scheme 2.1.2a 3... 

Laser flash photolysis techniques offer the possibility of examining in detail the transient processes responsible for the photostabilizing effect discussed above. The triplet lifetimes are frequently too short, even for this technique however, they can still be estimated using as a probe the quenching by 1-methyl-naphthalene, which leads to the formation of its easily detectable triplet. The optical absorbance due to the 1-methylnaphthalene triplet (Aft) produced as a result of energy transfer is related to the Stern-Volmer slope by equation 5, where N stands for... 

The laser flash photolysis (Xex=351 nm) of a TBP/BP-MDI solution in benzene (Figure 7) yields a transient spectra with distinct maximum at 370 nm which can most likely be attributed to a substituted diphenylmethyl radical. (Similar results are obtained in other solvents such as DMF). No detectable transient species were generated above 350 nm by the laser flash photolysis (Xex=351 nm) of the 60/40 mixture of TBP and benzene alone. Results for the TBP/MDI-PU (7.0 X 10 2 g/dL) system in Figure 8 show, as in the case of the model BP-MDI (Figure 7), that the transient spectrum of MDI-PU obtained indirectly through tert-butoxy radicals has a maximum at 370 nm. This provides additional support for assignment of the transient species responsible for the 370 nm absorbance to a diphenylmethyl radical. 

Epr is most effective for detecting free radicals that may occur as intermediates in oxidation and reduction reactions involving transition metal ions. Since these transients are invariably quite labile, epr is combined with continuous flow, (more conveniently) stopped-flow, flash photolysis, and pulse radiolysis. 

An important development for detection of transient organometallic species is fast time-resolved infrared. The transient is generated rapidly by uv-visible flash photolysis and monitored by ir with ps resolution. Spectra are obtained from a series of kinetic traces recorded at about 4 cm interval. 

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... 

Sundberg and co-workers studied the photochemistry of phenyl azide by conventional flash photolysis in 1974. They detected the transient UV absorption of ke-tenimine 30 and measured its absolute rate constant of reaction with diethylamine to form the IH azepine, which subsequently rearranges to Doering and Odum s 3H-azepine (27). ... 

Laser flash photolysis methods have also been applied to the study of nitrenium ion trapping rates and hfetimes. This method relies on short laser pulses to create a high transient concentration of the nitrenium ion, and fast detection technology to characterize its spectrum and lifetime The most frequently used detection method is fast UV-vis spectroscopy. This method has the advantage of high sensitivity, but provides very little specific information about the structure of the species being detected. More recently, time-resolved infrared (TRIR) and Raman spectroscopies have been used in conjunction with flash photolysis methods. These provide very detailed structural information, but suffer from lower detection sensitivity. 

Abramovitch and Takeuchi had explored the use of N-aminopyridinium ions as precursors to nitrenium ions in both photolysis and thermolysis reactions, but had not attempted to use these precursors under flash photolysis conditions. Moran and Falvey demonstrated that the LFP of 138 in CFI3CN (Scheme 57) generated a short-lived intermediate (1.5 jus) that had characteristics similar to the ions l31b-f. The transient was identified as the singlet ion 116. " Evidence from product analysis and spectrophotometric detection of the cation radical Ph2NH- as a long-lived intermediate was initially thought to indicate that there were parallel path-... 

The key intermediate of Scheme 1 is singlet phenylnitrene the only intermediate which in 1992 had not been detected directly or chemically intercepted in the parent system. In 1997 our group" " and the Wirz group, simultaneously reported that laser flash photolysis of phenyl azide or phenyl isocyanate 5 produces a previously undetected transient with = 350 nm and a lifetime of ns at ambient temperature. 

The electronic absorption spectrum of PN was first detected only in 1997. Laser flash photolysis (266 nm, 35 ps) of PA in pentane at 233 K produces a transient absorption spectrum with two sharp bands with maxima at 335 and 352 nm (Fig. 2). Spectrum 1 was measured, point by point, 2 ns after the laser pulse. In later work, the spectrum of PN was reinvestigated and an additional very weak, long-wavelength absorption band at 540 nm was observed (Spectrum 2). The transient spectrum of Fig. 2 was assigned to singlet phenylnitrene in its lowest open-shell electronic configuration ( A2). 

The different techniques of flash photolysis are used to detect transient species, that is atoms, molecules and fragments of molecules which have very short lifetimes. These cannot be observed by usual experimental techniques which require rather long observation times. For example, the measurement of an absorption or fluorescence spectrum takes several seconds, and this is of course far too long in the case of transient species which exist only for fractions of a second. In some cases these transient species can be stabilized through inclusion in low-temperature rigid matrices, a process known as matrix isolation . 

There are other spectroscopic techniques that can be used as probes in flash photolysis experiments, in particular electron spin resonance (ESR), which detects free radicals and radical ions, and microwave transmission, which detects dipolar transients. We shall not consider these special techniques in this text, references being made at the end of this chapter. 

Following this reaction sequence established by Kumada and Ishikawa, Leigh and coworkers detected the transient 1-silaallene 597 in a laser flash photolysis of the ethynyl-disilane 619. 597 was identified by its characteristic UV absorptions at 275 nm and 325 nm278 and by the trapping reaction with methanol. It is formed, however, in a mixture with silacyclopropene 620, dimethylsilylene 621 and acetylene 622. Based on the quantitative analysis of the products of methanolysis (623-626) a chemical yield of 597 of 12-15% was deduced278 (equation 205). 

Miranda MA, Perez-Prieto J, Font-Sanchis E, Scaiano JC (2000) Five-membered-ring 9-1-2 radicals direct detection and comparison with other hypervalent iodine radicals. Org Lett 1 1587-1589 Muiioz F, Schuchmann MN.OIbrich G, von Sonntag C (2000) Common intermediates in the OH-radi-cal-induced oxidation of cyanide and formamide. J Chem Soc Perkin Trans 2 655-659 Nagarajan V, Fessenden RW (1985) Flash photolysis of transient radicals. 1. X2" with X = Cl, Br, I, and SCN. J Phys Chem 89 2330-2335... 

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