Laser flash photolysis time-resolved spectroscopy

The format of this year s report is unchanged. The past twelve years growth in the field of polymer photochemistry has continued, not least because of the industrial applications in such fields as photopolymerization. In fact, a new journal specifically devoted to polymer photochemistry is now available (see later references). Modern techniques such as laser flash photolysis, time-resolved emission, and derivative spectroscopy are being used to unravel the complex photophysical and photochemical processes involved in polymers. 

Miller R J and Closs G L 1981 Application of Fourier transform-NMR spectroscopy to submicrosecond time-resolved detection in laser flash photolysis experiments Rev. Sc/. Instrum. 52 1876-85... 

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 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. 

An alternative to laser flash photolysis which is useful for studying opaque (but reflecting) samples, is diffuse reflectance spectroscopy [27]. This spectroscopic technique measures the ratio of the intensity of light reflected from the sample, I, to that reflected from a background or reference reflective surface, Iq. In time-resolved... 

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. 

The experiments described above do not allow us to perform time-resolved measurements and to track the kinetics initiated by irradiation. While the time-resolved infrared spectroscopy was intensively and successfully developed with flash laser photolysis, time-resolved infrared spectroscopy was surprisingly not implemented in radiation chemistry until recently. 

Martin CB, Shi X, Tsao M-L, Karweik D, Brooke J, Hadad CM, Platz MS. (2002) The photochemistry of riboflavin tetraacetate and nucleosides. A study using density functional theory, laser flash photolysis, fluorescence, UV-Vis and time resolved infrared spectroscopy. J Phys Chem B 106 10263-10271. 

Parent radical cations derived from alkanes and alkyl chlorides can be directly observed in the nanosecond time domain by time-resolved spectroscopy such as laser flash photolysis and electron pulse radiolysis. Especially the latter one enables the direct ionization of the solvents independently on the optical properties of the sample and a well-defined electron transfer regime according to Eq. (2) or (3). Representative examples of the radiolyfic generation of solvent radical cations are given in Eqs. (4) and (5a) for the cases of 1-chlorobutane and -decane. ... 

Electron transfer from the conduction band of semiconducting catalysts across the interface into the accepting species in solution has also been investigated extensively with nanosecond laser-flash photolysis and time-resolved fluorescence spectroscopy (109). 

In many synthetically useful radical chain reactions, hydrogen donors are used to trap adduct radicals. Absolute rate constants for the reaction of the resulting hydrogen donor radicals with alkenes have been measured by laser flash photolysis techniques and time-resolved optical absorption spectroscopy for detection of reactant and adduct radicals Addition rates to acrylonitrile and 1,3-pentadienes differ by no more than one order of magnitude, the difference being most sizable for the most nucleophilic radical (Table 8). The reaction is much slower, however, if substituents are present at the terminal diene carbon atoms. This is a general phenomenon known from addition reactions to alkenes, with rate reductions of ca lOO observed at ambient temperature for the introduction of methyl groups at the attacked alkene carbon atom . This steric retardation of the addition process either completely inhibits the chain reaction or leads to the formation of rmwanted products. 

The photochemical and photophysical properties of BPHTs and other phenothiazine derivatives have been investigated by means of a combination of time-resolved laser techniques, including laser flash photolysis and steady-state fluorescence spectroscopy [30,96]. The main goal of these studies was to develop novel photosensitizers for photoresist technology in the photoin-duced crosslinking of polymers. 

The absolute rate constants have been refined by time-resolved laser flash photolysis in which the change in concentration of the stannyl radical, or a reaction product, or an added probe is monitored by UV/vis spectroscopy.60 Rate constants for the reactions covering the literature up to 1981 are listed in Landoldt-Bomstein (1983, II13c, pp 323-336). A selection of current values is given in Table 20-2. 

Laser-flash photolysis of (203) in solution affords the ketene (204) and this is the first time that this ketene has been observed using time-resolved IR spectroscopy, Although other workers have previously reported the formation of this species. The present work reports the kinetics of the reaction of the ketene with water, methanol and diethylamine. The products obtained from the irradiation of a series of AT-acetoacetyl-a-amino acids have been identified by gas chromatography and chemical ionization mass spectrometry. ... 

Amao, Y., Asai, K., and Okura, I. (2000). A novel optical oxygen sensing system based on triplet-triplet reflectance of fullerene Cgg-polystyrene film by time-resolved spectroscopy using diffuse reflectance laser flash photolysis. Analyst, 125, 523-fi. 

On the basis of product studies, it is clear that irradiation of the naphthyl azides leads to loss of nitrogen with the likely consequent formation of nitrenes. Just as for phenyl azide, the initially formed singlet nitrenes may intersystem cross to the triplet and then dimerize to azo compounds. Clearly in the case of 2-naphthyl azide, but not 1-naphthyl azide, a closed-shell ground-state intermediate that can be trapped with diethylamine can be generated. The intermediate was formulated as the azirine on the basis of product studies [57]. Low temperature absorption spectroscopy and time-resolved laser flash photolysis experiments to be described later support the formation of azirines and provide an explanation for the different reactivity observed between the 1- and 2-substituted azides. 

These interpretations were supported by time-resolved spectroscopy. Laser flash photolysis of 4 in CH2C12 produces a transient spectrum (Figure 11) very similar to that obtained by flash photolysis of phenyl azide (Figure 8) and is therefore attributed to pentafluorodehydroazepine. Similar results were obtained in acetonitrile and tetrahydrofuran. However LFP of 4 in methanol gives an entirely different transient spectrum. (Figure 12). The... 

A recent laser flash photolysis study of 13 using time-resolved IR and UV-Vis spectroscopy has confirmed basic conclusions derived from the previous... 

---