Flash photochemistry

Contradictory evidence regarding the reaction to fonn 8 and 9 from 7 led the researchers to use TREPR to investigate the photochemistry of DMPA. Figure B1.16.15A shows the TREPR spectrum ofthis system at 0.7 ps after the laser flash. Radicals 6, 7 and 8 are all present. At 2.54 ps, only 7 can be seen, as shown in figure B1.16.15B. All radicals in this system exliibit an emissive triplet mechanism. After completing a laser flash intensity sPidy, the researchers concluded that production of 8 from 7 occurs upon absorption of a second photon and not tiiemially as some had previously believed. 

Despite the unpromising UV-visible spectra and flash photolysis studies, the carbene complexes presented in this chapter have a rich photochemistry at wavelengths exceeding 300 nm. A wide range of synthetically useful transformations has been developed, and continued studies are likely to reveal more. 

In studies of this kind, methods developed in radiation chemistry and photochemistry are often applied The methods of pulse radiolysis and flash photolysis allow one to investigate the mechanism of reactions in which free radicals, electrons and positive holes are the intermediates. In order to understand the mechanisms of processes that occur on colloidal particles it is important to know how free radicals... 

As is the case with singlet states, absorption of light by the triplet can occur to produce higher triplet levels. We have previously seen how triplet-triplet absorption by flash photolysis is used as a tool in photochemistry. 

Thus, overall, it is clear that flash photolysis with uv-visible detection is effective in establishing the broad outlines of the photochemistry of a particular metal carbonyl. Intermediates can be identified from their reaction kinetics, and sometimes, with the help of uv-vis data from matrix isolation experiments. Structural information from uv-vis flash photolysis is at best sketchy. Many questions remain unanswered. Time-resolved IR measurements can fill in some of these answers. 

IR spectroscopy is a powerful spectroscopic technique for examining the structure and behavior of intermediates involved in organometallic photochemistry. Examples are given of the combination of IR spectroscopy with matrix isolation, with liquid noble gases as solvents, and with flash generation, for probing novel transients and intermediates. 

The matrix experiments thus reveal some complex photochemistry of relevance to solution chemistry but the experiments do not provide information about kinetics. For this we need a fluid medium e.g. gas or liquid, and we consider such experiments in the next two sections. Flash photolysis suggests itself as the technique for detecting a species as reactive as Cr(C0)5 but before describing these experiments we show what can be achieved from low-temperature solutions. 

The limitation in all of these flash experiments is that only broad featureless UV/vis bands are observed and hence assignment has to rely on comparison with matrix data and/or kinetic consistency. How much more informative vibrational spectroscopy would be There is good reason to be optimistic as in the recent work of Schaffner (8), where, incidentally, it is shown how important a role is played by traces of H2O in the detailed mechanism of the photochemistry of Cr(C0)6 ... 

Another dinuclear carbonyl which presents interesting problems is ](ri C5H5)Fe(CO) ] 2 Does the photochemistry proceed exclusively through homolytic fission to produce two (13 -05 )Fe-(C0)2 radicals or by other possible routes The discussion of this reaction has involved mechanistic and synthetic studies (77), flash photolysis (78) and low-temperature photolysis (29) - the latter work, in THF or ethyl chloride at -78°C, invokes an intermediate in which the Fe-Fe direct bond is broken but the two halves of the molecule are held together by a CO bridge. Clearly such an intriguing problem merits more detailed investigations. 

In the introduction to Volume 1 of this series, the founding editors, J. N. Pitts, G. S. Hammond and W. A. Noyes, Jr. noted developments in a brief span of prior years that were important for progress in photochemistry flash photolysis, nuclear magnetic resonance, and electron spin resonance. A quarter of a century later, in Volume 14 (1988), the editors noted that since then two developments had been of prime significance the emergence of the laser from an esoteric possibility to an important light source, and the evolution of computers to microcomputers in common laboratory use of data acquisition. These developments strongly influenced research on the dynamic behavior of the excited state and other transients. 

The direct irradiation of 1,3,5-cyclooctatriene (184) in ether or hydrocarbon solvents leads to the slow formation of two stable isomers corresponding to disrotatory 47T-electrocyclization (185) and bicyclo[3.1.0]pentene (186) formation along with small amounts of the reduced product 187 (equation 69)279-281. Conventional flash photolysis experiments later showed that, in fact, the main primary photochemical process is the formation of a short-lived stereoisomer (r = 91 ms)282, most likely identifiable as ,Z,Z-184. The transient decays to yield a second transient species (r = 23 s) identified as Z,Z-l,3,5,7-octatetraene (188), which in turn decays by electrocyclic ring closure to regenerate 184282 (equation 70). The photochemistry of 184 has been studied on the picosecond timescale using time-resolved resonance Raman spectroscopy49. 

Previous studies of the photochemistry of alkylchlorodiazirines have shown that the yield of trappable carbene is sensitive to the alkylcarbene structure. A laser flash photolysis study of phenanthridenes (91), precursors of alkylchlorocarbenes, in the presence of pyridine, has ruled out the intermediacy of a carbene-pyridine complex which partitions between pyridine-ylide formation and [1,2]-H shift. ... 

The photochemistry of [Cr(CO)e] has been investigated in several studies. Flash photolysis of cyclohexane solutions of [CrfCO) ] affords two species one has a of 470 nm and a lifetime of 5 ms and the other, = 440 nm, has a lifetime > 1 s. The relationship between photolysed species of [CrfCO) ] and photochemical substitution reactions described in Scheme 4 has been suggested from i.r. and u.v. spectroscopic studies of matrix-isolated species. ... 

Platinum(ni).—Unlike the photochemistry of [PtXg] " (X = Br or I) in which only photoaquation is observed,flash photolysis of [PtClsV" in its Cl Pt charge-transfer band has been shown to result also in photoreduction. The unusual Pt" complex [PtC ]" is produced via the mechanism... 

A study of the photochemistry of frans-[PtBr2(NH3)4] has shown that photoaquation is the only reaction occurring. However, the observation of transient BrJ in the flash photolysis, together with the formation of HCHO when methanol is solvent, suggests that the [PtBr(HjO)(NH3)4] product is formed not only by simple photoaquation but also via the ion-pair mechanism... 

Progress in photochemistry could only be made following progress in spectroscopy and, in particular, the interpretation of spectra in at least semiquantitative terms, but history has shown that this was not enough. The arrival of new methods of analysis which permit determination of small amounts of products, the development of flash photolysis, nuclear magnetic resonance, and electron spin resonances which can yield valuable information about the natures of intermediate excited states, as well as of atoms and radicals, all have permitted the photochemist to approach the truly fundamental problem of photochemistry What is the detailed history of a molecule which absorbs radiation ... 

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). ... 

F. Wilkinson and G. Kelly, Diffuse Reflectance Flash Photolysis, in Handbook of Organic Photochemistry, Vol. I, J. C. Scaiano, Ed., CRC Press, Boca Raton, EL, 1989, pp. 293f. 

So far the methods described for measuring excited state lifetime, and hence reactivity, have been indirect methods that rely on a comparison with some standard le.g. actinometer quantum yield or quenching rate constant) that has already been measured. A direct method for measuring the lifetime of short-lived species produced photochemically is flash photolysis. This is a very important technique in photochemistry, though only the basic ideas as they apply to mechanistic studies are outlined here. In flash photolysis a high concentration of a short-lived species (electronically excited state or... 

Recently Fouassier and Chesneau [219] studied the photochemistry of the system Eosin-PDO-MDEA in aqueous acetonitrile using steady-state irradiation and laser flash photolysis. The photopolymerization of methyl methacrylate (MMA) sensitized by the photoreduction of Eosin is investigated in acetronitrile to understand the mechanism of initiation and the enhancement in the rate of polymerization caused by the presence of PDO, 3. Rates, quantum yields of photopolymerization, and number average molecular weights of the polymer are determined with MMA (7 M), Eosin (3 x 10 5 M), and MDEA (0.1 M) in the presence and in the absence of 2 x 10-3 M PDO. 

---