Laser flash photolysis method

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... 

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. 

Using /ra .r-l,l,3,3-tetraphenyl-2,4-dineopentyl-l,3-disilacyclobutane 35a as the precursor, the reactivity of the transient l,l-diphenyl-2-neopentylsilene has been studied in various solvents by laser flash photolysis methods. These experiments verified that the isomeric compounds m-35b and ( )-36 and the expected adduct of MeOH to silene 37 are the only primary products of photolysis of 35a at low (ca. 10%) conversion (Equation 9). Similar experiments with m-35b showed it to be ca. 4 times more photoreactive than the trans-isomer, producing tram-35a, 37, and -36 with initial yields of 69%, 11%, and 19%, respectively <20050M2309>. 

Polyamide (6) was irradiated with a single 20 ns flash (530 nm) in A. iV-dimethyl-ac tamide. The kinetics of the cis to trans isomerization of backbone azobenzene residues was followed by time resolved optical absorption, and the subsequent conformation change of the total polymer chain by time resolved light scattering. Before each laser e eriment, the polymer was brought to a compact conformation by eontiimous ultraviolet irradiation, and then the unfolding proce was traced by the laser flash photolysis method. 

The nanosecond-picosecond two-color two-laser flash photolysis method is also useful to study the excited state of radicals, that is, the D, state. We applied nanosecond-picosecond two-color two-laser flash photolysis to detect the absorption and fluorescence spectra of the ketyl radical of benzophenone and its derivatives (BPH and BPDH ) in the excited state in the UV-visible region [111], since BPH and BPDH are well investigated radicals in various fields. Since BPH and BPDH are generated from irreversible ways, such as photoionization, we employed a streak camera to realize the single-shot detection of these intermediates. 

To date it has not been possible to observe directly radical cations in any of the j8-(acyloxy)alkyl migrations studied by the time-resolved laser flash photolysis method... 

Monitoring a transient absorption spectrum with a laser flash photolysis method especially when the rates of electron transfer and the back reaction (recombination) are rapid. 

The final product ArCH ONO is formed in further oxidation of ArCH/ to ArCH/ by CAN and the subsequent reaction with NOj . For toluene derivatives with electron-donating substituents such as the methoxy group, the electron transfer reaction (Equation 4.73) was confirmed by the laser flash photolysis method [44]. For toluene, there is a probability for direct H-atom abstraction (Equation 4.72) with a highly polar transition state. Furthermore, for toluene derivatives with electron-withdrawing substituents, the addition ability of NO3 to phenyl 7t-bonds can be considered on the basis of data for reactions with phenols [41] and furan [45]. To clarify the interchanges in the reaction paths by the substituent in toluene, reaction rate constants for various toluene derivatives were evaluated by flash photolysis [44]. The substituent effect of the rate constants for toluene derivatives was correlated with ionisation energies (lEs) of these substances. The reaction rate for anisole is too fast to obtain accurate rate constants, and only lower limits of the rate constants are obtained (anisole) >310 M -s h For p-nitrotoluene, the rate constant is 2.3T0 M -s IE = 9.5 eV. A deuterium kinetic isotope effect of 1.6 was observed for the reaction of NO3 with toluene and toluene - dg. This indicates that NO3 predominantly abstracts the H atom from methyl groups. In the case of p-xylene, the deuterium isotope effect was not observed [43]. The rate constant forp-xylene (> 2 x 10 M/s) is close to the diffusion-controlled limit in acetonitrile, and consequently selectivity becomes low. 

Using fast time-resolved pulse radiolysis and laser flash photolysis methods, the authors have been able to directly probe the dynamics of electron and triplet exciton transport along the conjugated segments. The results show that transport of both carriers along these organometallic molecular wires is relatively rapid, occurring on timescales less than 200 ps over a... 

TRUV-VIS studies have also provided evidence to support the role of conformation in the rearrangement reaction. For example, a series of simple diazoketones RCONjH, where R = H, Me, i-Pr, and t-Bu, has been examined by laser flash photolysis methods and also by picosecond transient grating spectroscopy. Within this series, as the steric bulk of R increases, the extent of the anti diazoketone conformer present at equilibrium decreases relative to the syn conformer. Consistent with a direct rearrangement pathway leading to ketene via only the syn conformer, the amount of trappable carbene intermediate decreases as R becomes more bulky. 

Studies by Saveant and co-workers provide a more comprehensive picture of how steric and electronic effects govern the rates of aminium radical a-CH deprotonation. In this account, a combination of double potential step chronoamperometry, cyclic voltammetry, and laser flash photolysis methods were used to measure the rates of 9-CH deprotonation of aminium radicals 9, produced by SET oxidation of 9-substituted-N-methylacridanes (Scheme 8). Amines with differing base strength and steric constraints were employed as bases. The data (Table 101.3) clearly show that the deprotonation rate constants are significantly reduced when sterically encumbering substituents are present at C-9 of the acridanium... 

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