Laser flash photolysis excited states

In an entirely different experimental approach the unsymmetrical mixed-valence ion shown in equation (76) was subjected to laser flash photolysis. " Excitation was carried out into the MLCT absorption band of the Ru -+ n (pz) chromophore. Following excitation, one of the deactivation channels leads to the unstable mixed-valence isomer and its subsequent relaxation to the final, stable oxidation state distribution was observed directly using picosecond laser techniques. 

At its best, the study of solvent kies by the formalism given can be used to learn about proton content and activation in the transition state. For this reason it is known as the proton inventory technique. The kinetics of decay of the lowest-energy electronic excited state of 7-azaindole illustrates the technique.25 Laser flash photolysis techniques (Section 11.6) were used to evaluate the rate constant for this very fast reaction. From the results it was suggested that, in alcohol, a double-proton tautomerism was mediated by a single molecule of solvent such that only two protons are involved in the transition state. In water, on the other hand, the excited state tautomerism is frustrated such that two water molecules may play separate roles. Diagrams for possible transition states that can be suggested from the data are shown, where of course any of the H s might be D s. 

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... 

For the investigation of triplet state properties a laser flash photolysis apparatus was used. The excitation source was a Lambda Physik 1 M 50A nitrogen laser which furnished pulses of 3.5 ns half-width and 2 mJ energy. The fluorescence decay times were measured with the phase fluorimeter developed by Hauser et al. (11). 

Ikegami M, Arai T (2000) Laser flash photolysis study on hydrogen atom transfer of 2-(2-hydn>xyphenyl)benzoxazole and 2-(2-hydroxyphenyl)benzothiazole in the triplet excited state. Chem Lett 9 996-997... 

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. 

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. 

The photodecarboxylation of p-(nitrophenyl) glyoxylic acid 156, which was studied by time-resolved and steady-state methods at room temperature93, leads to p-nitrosobenzoic acid and carbon dioxide in good yields with = 0.28 in aqueous solution at pH 2-12 and excitation at 313, 280 or 254 nm (equation 76). An intermediate (Xmax = 350, r 2 xs) observed by nanosecond laser flash photolysis is assigned to the aci-form of the nitroketene... 

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 experiments showed that the Ceo triplet state ( C q), obtained by 532 nm excitation, reacts with poly(methylphenylsilane) by an... 

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. 

Porter et al. studied the photorelease of various alcohols from ester 65, which release their alcohol moiety and form lactones 67 and 68 in solvents with abstractable H-atoms (Scheme 36). They also reported that the photorelease from 65 could be initiated by electron transfer from amines. We used transient spectroscopy to elucidate the photorelease mechanisms of 65 in 2-propanol. Laser flash photolysis of 65 resulted in the formation of the triplet excited state of 65, which has a A ax at 330 and 530 nm. The triplet excited state of 65 decayed with a rate constant of 3 x 10 s in 2-propanol to form radical 66. The quantum yield for photorelease from 65 was determined to be 0.62 in 2-propanol. 

Figure 7.31 Diagram of a ns, kinetic, laser flash photolysis apparatus. F, photolytic laser beam B, continuous analytical beam S, sample cell d, light detector M, monochromator D, photomultiplier 0, oscilloscope with t (time-base trigger) andy (vertical signal) inputs, (b) Point-by-point absorption spectra of chloranil in acetonitrile at 20 ns, 1 [xj after excitation. T corresponds to the absorption by the triplet state, C by the radical anion...

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