Laser flash absorption photolysis

The conmron flash-lamp photolysis and often also laser-flash photolysis are based on photochemical processes that are initiated by the absorption of a photon, hv. The intensity of laser pulses can reach GW cm or even TW cm, where multiphoton processes become important. Figure B2.5.13 simnnarizes the different mechanisms of multiphoton excitation [75, 76, 112], The direct multiphoton absorption of mechanism (i) requires an odd number of photons to reach an excited atomic or molecular level in the case of strict electric dipole and parity selection rules [117],... 

Figure lb shows the transient absorption spectra of RF (i.e. the difference between the ground singlet and excited triplet states) obtained by laser-flash photolysis using a Nd Yag pulsed laser operating at 355 nm (10 ns pulse width) as excitation source. At short times after the laser pulse, the transient spectrum shows the characteristic absorption of the lowest vibrational triplet state transitions (0 <— 0) and (1 <— 0) at approximately 715 and 660 nm, respectively. In the absence of GA, the initial triplet state decays with a lifetime around 27 ps in deoxygenated solutions by dismutation reaction to form semi oxidized and semi reduced forms with characteristic absorption bands at 360 nm and 500-600 nm and (Melo et al., 1999). However, in the presence of GA, the SRF is efficiently quenched by the gum with a bimolecular rate constant = 1.6x10 M-is-i calculated... 

Another intermediate of the photolysis of TiO was observed in experiments with platinized particles (in the absence of polyvinyl alcohol). The spectrum shown in Fig. 22 is prraent immediately after the laser flash. The signal decays as shown by the inset in the figure. The rate of decay is not influenced by oxygen but is increased by oxidizable compounds such as Br ions in the solution. The broad absorption band in Fig. 22 with a maximum at 430 nm was attributed to trapped positive holes. Chemically, a trapped hole is an 0 radical anion. In homogeneous aqueous solution, 0 ... 

Gratzel and Serpone and co-workers recently reported on a picosecond laser flash photolysis study of TiO. They observed the absorption spectrum immediately after the 30 ps flash and attributed it to electrons trapped on Ti" " ions at the surface of the colloidal particles. The absorption decayed within nanoseconds, the rate being faster as the number of photons absorbed per colloidal particle increased. This decay was attributed to the recombination of the trapped electrons with holes. 

Much attention has been devoted to the development of methods to generate quinone methides photochemically,1,19-20 since this provides temporal and spatial control over their formation (and subsequent reaction). In addition, the ability to photogenerate quinone methides enables their study using time-resolved absorption techniques (such as nanosecond laser flash photolysis (LFP)).21 This chapter covers the most important methods for the photogeneration of ortho-, meta-, and para-quinone methides. In addition, spectral and reactivity data are discussed for quinone methides that are characterized by LFP. 

Using nanosecond laser flash photolysis techniques, Leigh80 observed transient absorption spectra which he attributed to the silenes derived from photolysis of various methylphenyldisilylbenzenes. Thus the silenes 52,53, and 54 were found to absorb at 425,460, and 490 nm, respectively, in isooctane, and 55 was also found to absorb at 490 nm.75 In other studies, the silene Ph2Si=CH2 derived by laser flash photolysis was found to absorb at 323 nm.111... 

Skibsted and coworkers (Mortensen and Skibsted 1996) have shown that upon the laser flash photolysis of carotenoids in chloroform bleaching of the ground state absorption is observed and there is formation of two near infrared-absorbing species ()tmax 920 and lOOOnm for 0-CAR). The species absorbing at about lOOOnm is 0-CAR + and, as with the carotenoid/CCl302 system noted earlier, the 0-CAR,+ is formed from the other species. The nature of the other species is not defined although an adduct or a neutral carotenoid radical is proposed. 

The formation of 7a was also observed in solution using laser flash photolysis (LFP) with nanosecond time resolution.25,26 In Freon-113 7a shows an absorption maximum at 470 nm, and a life-time of longer than 20 xs.25 The rate of 2.9 x 109 M 1 s-1 for this reaction is almost the diffusion limit and implies a very small or absent barrier. In aqueous solution the rate constant for the reaction of la with 3Oj is 3.5 x 109 M-1 s-1, and the absorption maximum of 7a was determined as 460 nm.26 This clearly demonstrates that the oxidation of carbene la in solid argon and in solution follows the same reaction pathway. 

The chemical properties of BA have been studied in detail (Lapin et al., 1984). Low temperature epr spectroscopy shows clearly that the ground state of BA is the triplet (3BA). The zero field parameters (Table 3) reveal some details of this structure. When the irradiation is performed at 4.6 K in a 2-methyltetrahydrofuran glass no epr signals from radical species are apparent. The optical spectrum under these conditions shows absorptions (Table 4) which disappear when the glass is warmed. From these findings the absorption bands are assigned tentatively to 3BA. This conclusion is strongly supported by results from laser flash photolysis experiments. 

Laser flash photolysis experiments48,51 are based on the formation of an excited state by a laser pulse. Time resolutions as short as picoseconds have been achieved, but with respect to studies on the dynamics of supramolecular systems most studies used systems with nanosecond resolution. Laser irradiation is orthogonal to the monitoring beam used to measure the absorption of the sample before and after the laser pulse, leading to measurements of absorbance differences (AA) vs. time. Most laser flash photolysis systems are suitable to measure lifetimes up to hundreds of microseconds. Longer lifetimes are in general not accessible because of instabilities in the lamp of the monitoring beam and the fact that the detection system has been optimized for nanosecond experiments. 

Toth et al. then used laser flash photolysis as a means to determine the value of k x independently of the above study (8). They used 355 nm laser light to photolyze mixtures of C102 and Br2/Br s. Absorption of this light by Br3 led to the prompt formation of Br2, and the subsequent loss of Br2 was monitored by its absorbance at 360 nm. The loss of Br2 occurred with mixed 2nd- and lst-order kinetics due to the parallel 2nd-order self reaction of Br2 and its pseudo-first-order reaction with C102. These experiments led to a value of 3.6 x 109 M 1 s 1 for kh which is in good agreement with the approximate value (1.1 x 109 M 1 s ) originally obtained. 

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. 

Nanosecond laser flash photolysis was applied to study excited-state 2-nitrothiophene in polar and non-polar solvents76 the transient absorption at 545 5 nm was assigned to its lowest triplet state. The rate constants of the interaction of this triplet excited state, with a number of substrates such as cyanide and hydroxide ions, have been determined77. Similarly, the transient absorption at 490 5 nm was assigned to the lowest triplet excited state of 5-nitro-2-furoic acid78, and that at 500 5 nm to that of 7V-(n-butyl)-5-nitro-2-furamide79. 

Laser flash photolysis at wavelengths within the charge-transfer absorption bands of 2,2,6,6-tetramethylpiperidine-./V-oxyl (TEMPO) and carbon tetrachloride yields theoxoam-monium chloride of TEMPO 291 (Xmax = 460 nm) and the trichloromethyl radical in an essentially instantaneous 18 ps) process152. The primary photochemical reaction is an electron transfer from TEMPO to carbon tetrachloride followed by immediate decomposition of the carbon tetrachloride anion radical to chloride and trichloromethyl radical (equation 140). The laser flash photolysis of TEMPO and of other nitroxides in a variety of halogenated solvents have confirmed the generality of these photoreactions152. 

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. 

Using laser flash photolysis with a frequency-quadrupled neodynium laser, Stevens and al 161b) measured the lifetime of the triplet state of fluoro- and pentafluoro-benzene in the gas phase along with the energy transfer efficiencies to cis-2-butane and oxygen. The triplet transient absorption decay was found to be predominantly first order with a... 

One principally different method of photoexcitation consists in the so-called two-color, two-laser flash photolysis. This method allows exciting ion-radicals already generated during the first laser (first color) flash photolysis. With a short-time delay after the first laser flash, the ion-radicals are selectively excited by the second laser (second color) flash. This flash is of the wavelength tuned to the absorption of the ion-radical initially formed. As a result, the ion-radical transforms into... 

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... 

Another transient aminoxyl radical has been generated , and employed in H-abstraction reactivity determinations" . Precursor 1-hydroxybenzotriazole (HBT, Table 2) has been oxidized by cyclic voltammetry (CV) to the corresponding >N—O species, dubbed BTNO (Scheme 9). A redox potential comparable to that of the HPI —PINO oxidation, i.e. E° 1.08 V/NHE, has been obtained in 0.01 M sodium acetate buffered solution at pH 4.7, containing 4% MeCN". Oxidation of HBT by either Pb(OAc)4 in AcOH, or cerium(IV) ammonium nitrate (CAN E° 1.35 V/NHE) in MeCN, has been monitored by spectrophotometry , providing a broad UV-Vis absorption band with A-max at 474 nm and e = 1840 M cm. As in the case of PINO from HPI, the absorption spectrum of aminoxyl radical BTNO is not stable, but decays faster (half-life of 110 s at [HBT] = 0.5 mM) than that of PINO . An EPR spectrum consistent with the structure of BTNO was obtained from equimolar amounts of CAN and HBT in MeCN solution . Finally, laser flash photolysis (LFP) of an Ar-saturated MeCN solution of dicumyl peroxide and HBT at 355 nm gave rise to a species whose absorption spectrum, recorded 1.4 ms after the laser pulse, had the same absorption maximum (ca 474 nm) of the spectrum recorded by conventional spectrophotometry (Scheme 9)59- 54... 

The transient absorption formed from laser flash photolysis of 15 was not quenched in oxygen-saturated solutions, presumably because the photoenolization to form 16 takes place from a singlet excited state of 15. Furthermore, Schworer and Wirz did not observe any transient absorption that can be attributed to the formation of the triplet excited state of 15 from direct irradiation. In contrast. Hurley and Testa used energy transfer to estimate that the triplet excited state of 15 is formed in 67% yield,whereas Takezaki et al. have measured the yields for forming the triplet excited state of 15 to be slightly higher or above 80%. They estimated the lifetime of the lowest triplet excited state of 15 to be 350 ps in ethanol. 

Scaiano et al. used laser flash photolysis to show that irradiation of 25 forms anion 26A, which has a maximum absorption at 600 Anion 26A is considered... 

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