Laser flash photolysis derivatives

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... 

Laser flash photolysis (LFP) of quinone diazide 2d in Freon-113 at room temperature produces carbene Id, which could be monitored indirectly by addition of trapping reagents.25 At 2.0 xs the lifetime of Id is slightly longer than that of la (1.65 xs), otherwise the reactivities of these carbenes are very similar. The Id —> 11 rearrangement is not observed in the LFP experiments. All trapping products with a variety of reagents (O2, acetonitrile, pyridine etc.) are derived from carbene Id. 

The iron(II) complexes of the hexadentate 2-pyridylmethyl derivatives of bipy (94) and (95) are spin cross-over compounds, whose light-induced high-spin to low-spin conversion has been monitored in solution by laser flash photolysis. Single exponential kinetics (A h l = 6.7 x 10 s , at 273 K) were observed for [Fe(94)] ", but for [Fe(95)] " kinetics were biphasic, with the spin-conversion step (ku i = 2.5 x 10 s ) followed by a slower step (k = 3.7 x 10 s ) involving rearrangement of the pyridylmethyl pendant arms. ... 

Several researchers have investigated the photoenolization of various o-methyl acetophenone and o-methyl benzophenone derivatives. The mechanism of photoenolization of o-methyl benzophenone, 1, has been studied with laser flash photolysis and can be described as follows (Scheme 2) Irradiating 1 forms its first... 

McClelland and co-workers identified the initial adduct detected in laser flash photolysis experiments involving the reaction of 75g with d-G as 111 (Ar = 2-fluorenyl, Y = H, R = 2 -deoxyribose)." This identification was based on the absorption spectrum of the intermediate, which extends out to 400 nm suggesting a highly conjugated species, by the observed pXa of 3.9 of the intermediate, which is consistent with deprotonation of 111 to form 112, by the lack of dependence of the rate constant for decomposition of the intermediate on the nature of Ar for the intermediates derived from 75g, 75n, 75p, and 75q, and by the kinetics of the decomposition of the intermediate into the stable C-8 adduct 102, which includes a pH-rate profile that showed both ionization states were reactive, buffer catalysis of decomposition of the... 

We have investigated the reactions of the COs " radicals with double-stranded DNA by laser flash photolysis techniques [15]. In these time-re-solved experiments, the COs radicals were generated by one-electron oxidation of HCOs by sulfate radical anions, SO4 the latter were derived from the photodissociation of persulfate anions, S20s initiated by 308-nm XeCl excimer laser pulse excitation. In air-equilibrated buffer solution containing the self-complementary oligonucleotide duplex d(AACGCGAATTCGCGTT), 208 , and an excess of HCO3., the decay of the CO3 radical anion absorption band at 600 nm is associated with the concomitant formation of the characteristic narrow absorption band of the G(-H) radicals near 310 nm. 

Laser flash photolysis has been performed with the halogenated derivatives in ethanol [374], Measurements of the triplet lifetimes as a function of dye concentration, laser power, and in the presence of electron donors allowed us to determine the rate constants collected in Table 17. 

TABLE 17 Laser Flash Photolysis Results with Novel Xanthene Dyes Derivatives 1... 

By using nanosecond laser flash photolysis of 51, Leigh and coworkers have studied alcohol addition reactions of the l,3,5-(l-sila)hexatriene derivative 52 (Scheme 16)80,81 and of 1,1-diphenylsilene82-84. 

Bradaric and Leigh have also measured absolute rate constants for the reaction of a series of ring-substituted 1,1-diphenylsilene derivatives with methanol, f-butyl alcohol and acetic acid in acetonitrile by similar nanosecond laser flash photolysis techniques87 (Table 7). 

Ito T, Shinohara H, Hatta H, Nishimoto S-l (1999) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers product and laser flash photolysis studies on the oxidative splitting mechanism. J Phys Chem A 103 8413-8420 ItoT, Shinohara H, Hatta H, Fujita S-l, Nishimoto S-l (2000) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers. 2. Conformational effects on the reductive splitting mechanism. J Phys Chem A 104 2886-2893 ItoT, Shinohara H, Hatta H, Nishimoto S-l (2002) Stereoisomeric C5-C5 -linked dehydrothymine dimers produced by radiolytic one-electron reduction of thymine derivatives in anoxic solution structural characteristics in reference to cyclobutane photodimers. J Org Chem 64 5100-5108 Jagannadham V, Steenken S (1984) One-electron reduction of nitrobenzenes by a-hydroxyalkyl radicals via addition/elimination. An example of an organic inner-sphere electron-transfer reaction. J Am Chem Soc 106 6542-6551... 

Flash photolysis and laser flash photolysis are probably the most versatile of the methods in the above list they have been particularly useful in identifying very short-lived intermediates such as radicals, radical cations and anions, triplet states, carbenium ions and carbanions. They provide a wealth of structural, kinetic and thermodynamic information, and a simplified generic experimental arrangement of a system suitable for studying very fast and ultrafast processes is shown in Fig. 3.8. Examples of applications include the keton-isation of acetophenone enol in aqueous buffer solutions [35], kinetic and thermodynamic characterisation of the aminium radical cation and aminyl radical derived from N-phenyl-glycine [36] and phenylureas [37], and the first direct observation of a radical cation derived from an enol ether [38],... 

Conlin and coworkers reported a rate constant for the thermal electrocyclic ring closure of the transient l,3-(l-sila)butadiene derivative 8a in cyclohexane at 25 C, k = (1.19 0.06) x 105 s l31. The rate constant corresponds to the inverse of the lifetime of the silene in the dry solvent, as measured by laser flash photolysis of the isomeric silacyclobutene derivative 7a, the stable product of the thermal ring closure reaction... 

Only two studies have been reported on the quantitative aspects of 1-silaallene reactivity54,55. The two compounds (26a and 26b) were generated and detected by laser flash photolysis of the corresponding alkynyldisilane derivatives 25a and 25b, respectively... 

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