Singlet state laser flash photolysis

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... 

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. 

Nitrenium ions (or imidonium ions in the contemporaneous nomenclature) were described in a 1964 review of nitrene chemistry by Abramovitch and Davis. A later review by Lansbury in 1970 focused primarily on vinylidine nitrenium ions. Gassmann s ° 1970 review was particularly influential in that it described the application of detailed mechanistic methods to the question of the formation of nitrenium ions as discrete intermediates. McClelland" reviewed kinetic and lifetime properties of nitrenium ions, with a particular emphasis on those studied by laser flash photolysis (LFP). The role of singlet and triplet states in the reactions of nitrenium ions was reviewed in 1999. Photochemical routes to nitrenium ions were discussed in a 2000 review. Finally, a noteworthy review of arylnitrenium ion chemistry by Novak and Rajagopal " has recently appeared. 

Photolysis of oe-chloro-o-methylacetophenones yields 1-indanones. The mechanism has been studied by laser flash photolysis (Netto-Ferreira, J. C. Scaiano, J. C., J. Am. Chem. Soc., 1991, 113, 5800). Develop a Dauben-Salem-Turro state correlation diagram for the photochemical step and, on the basis of your diagram, discuss the efficiency of the reaction on the singlet and triplet manifold. Do the experimental results agree with your analysis ... 

A number of minima corresponding to oxonium ylides and H-bonded structures were found on the potential-energy surface for reaction of singlet carbenes with water and alcohols." Laser flash photolysis revealed that the rates of reaction between cyclopentadienylidene or fluorenylidene and alcohols increased with alcohol acidity and had linear Bronsted plots with slopes of 0.061 and 0.082, respectively.100 These results point to protonation with a very early transition state or to concerted OH insertion. For tetrachlorocyclopentadienylidene, the results showed that ylide formation (100) is predominant. 

Photoisomerization of 4-methoxy-4/-nitrostilbene 91 suffers from a strong solvent effect (Sch. 35). The (Z/E)pss ratio upon excitation at 366 nm is 91 9 in nonpolar petroleum ether, but the Z/E preference is dramatically switched to give the Z/E ratio of 17 83 in polar dimethylformamide [218]. Laser flash photolysis studies revealed the multiplicity of the excited state involved. The singlet mechanism is operative in the photoisomerization of ( )-4-cyano-4/-methoxystilbene and ( )-4-methoxy-4/-nitrostilbene [219,220], while the photoisomerization of nitrostilbenes involves the triplet state [221]. 

In compiling the information in this chapter, I have relied heavily on several very comprehensive reviews that have appeared over the past few years [1-7]. In particular, the 1978 review by T irro et al. [1] is extremely thorough in describing the intra- and intermolecular photophysics and chemistry of upper singlet and triplet states. In fact, rather than reproduce the same details here, I direct the reader to this review for a summary of upper state behavior reported prior to 1978. (A description of azulene and thione anomalous fluorescence is included since these systems are the best-known systems that display upper state behavior.) I also direct readers to the reviews by Johnston and Scaiano [2] and Wilson and Schnapp [3] which focus on the chemistry of both upper triplet states and excited reaction intermediates as studied by laser flash photolysis (one- and two-color methods) and laser jet techniques. Also, Johnston s thorough treatment of excited radicals and biradicals [4] and the review of thioketone photophysics and chemistry by Maciejewski and Steer [5] are excellent sources of detailed information. 

A recent study by laser flash photolysis showed that a triplet state is not involved in the coloration mechanism of spirooxazines when in solution. A similar conclusion was reported, noting that the photocoloration occurs only in the excited singlet state because of the independence of the reaction to the presence of oxygen.29... 

Fig. 3 shows the spectra of short lived species produced in the laser flash photolysis of PN+ in water, NaLS, and in silica particles. Previous work enable us to identify the excited triplet and singlet states of the pyrene moiety, together with the pyrene cation and hydrated electron, e g q... 

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