Flash spectroscopy

Sporadic reports of the 2//-thiopyran transformation are still available. The electrocyclic ring-opening of 2-benzyl-2,4,6-triphenyl-2//-thiopyran (131, X = PhCH2) was detected by laser flash spectroscopy (86MI1). 2-Amino-2//-thiopyran was predicted to be more stable than its valence-bond isomer on the basis of semiempirical MINDO/3 and MNDO calculations (84JPR955). 

Nano- and microsecond laser flash spectroscopy was employed to clarify photophysical effects on benzyl group migration in 2//-thiopyran 60 (R = PhCH2, 86MI1). 

Sx, Ti -> Tx). Figures 3.2 and 3.3 illustrate the principle of flash spectroscopy/65 If the second light source is continuous, the change in optical density due to the transient species can be monitored as a function of time at a particular wavelength selected on a monochromator. This type of system is illustrated in Figure 3.4. 

Using picosecond flash spectroscopy Gupta et al. 2k) reported for 2-hydroxyphenylbenzotriazole in ethanol a short-lived transient (6 ps) followed by a transient absorption whose lifetime is estimated to be 600 ps. The authors assigned the short-lived transient to the "vertical singlet" while the long-lived transient is presumably the "proton transferred species". These measurements of transient absorptions with the picosecond flash method confirm our results derived from the fluorescence emission using the phase fluorimetric method. 

The apparatus used for picosecond flash spectroscopy on these systems has been described before(8 10). Figure 3a and b show typical transient absorption data obtained on 2-hydroxybenzophenone and the copolymer. Summary of these spectral data are given in Table 3. The transient observed at the shortest delay time (7ps) is the first excited singlet in all systems. The spectral data (at delay times > 50ps) permit placement of upper limits on triplet yields in CH2CI2 for both 2-hydroxy benzophenone itself and the copolymerized chromophore. 

In flash spectroscopy a second spectroscopic flash is fired a short time after the photolysis flash and the transient absorption spectrum is registered on a photographic plate (Fig. 13). Repeating the experiments with different delay times gives complete information about the wavelength and the time behavior of the intermediate absorptions. 

As in the former cases, k2 was calculated from the integrated extinction coefficients,149 k3 + kt was derived from fluorescence quantum yields,149 while k3 and k4 were separately estimated from the maximum quantum yields of photooxygenations at high oxygen concentrations.150 Flash spectroscopy techniques were used in order to determine k5 and k7, while kB was obtained from the Stern-Volmer quenching constant of oxygen.149 The ratio ke/kg was determined from the variation of AOz with the concentration of the anthracene.71 When photodimerization occurred, k13l(kia + k13) was calculated from the maximum yield of... 

Flash spectroscopy techniques have been applied in order to determine triplet lifetimes (1 /ks) and the quenching constant, kl0, for the triplet-quenching reaction. 

Figure 10.10 Instrumentation for flash photolysis. (A) Flash spectroscopy (B) flash kinetic spectrophotometry.

G. Porter, Flash spectroscopy , Angew Chem. fat. Edit, 80, 1968, 852. 

The biradical intermediates have in some cases been detected by flash spectroscopy, or trapped by added reagents such as a t-alkyl nitroso compound. The enol produced by cleavage of the biradical is relatively inert at low temperatures, and it can be studied spectroscopically after irradiation of the ketone in solutions cooled below —50°C. There have been many mechanistic studies of the Norrish... 

Some other experimental methods also require brief discussion here. The technique of microwave-pulse flash-spectroscopy is similar to that of flash photolysis, except that excitation is achieved by means of a powerful single pulse of microwave radiation from a magnetron19. The gas is contained in a quartz reaction vessel placed along the axis of a cylindrical cavity, tuned to the frequency... 

Solutions of MeTND in cyclohexane show a fluorescence maximum at 355 nm when excited at 313 nm. The fluorescence quantum yield, Of, of MeTND was found to be 0.020. The Of is 0.011 for 2-bromothianaphthene 1,1-dioxide. Flash spectroscopy showed that the lifetime of excited singlet MeTND was <3 ns. 

In a recent paper, Soumillion and co-workers [49] were able to identify CIP and SSIPin the P-naphtholate anion/alkali cation/tetrahydrofuran system. They found out that with lithium, a CIP is formed whereas with sodium/crown ether, a SSIP results. Using uncomplexed sodium or potassium counterion, mixtures of CIPs and SSIP s were detected. All their conclusions are based on spectral shifts in the transient absorption and emission spectra which were gained using laser flash spectroscopy. 

Dorr, Lewis, and co-workers found evidence through quenching experiments and flash spectroscopy for a triplex in the system trans-stilbene — amine — benzene — [105]. They quenched singlet excited trans-stilbene with various mono- and diamines and found a steric effect on the quenching constant The a, co-diamines (dabco, diaminoethane, -propane and -butane) quenched the stilbene fluorescence more efficiently than the monoamines, depending on the chain length between the amino groups. This was ascribed to the formation of cyclic radical cations, with a N-N three electron a-bond. In this case, an exciplex between diamine and stilbene is formed. 

Arbour, C. Sharma, D. K. Langford, C. H. Picosecond flash spectroscopy of Ti02 colloids with adsorbed dyes, J. Phys. Chem. 1990, 94, 331. 

Using nanosecond laser flash spectroscopy, it has been shown that excited ketones can abstract hydrogen of hydroperoxide groups and generate free radicals as follows [18] ... 

Fig. 4 Determination of acetophenone enol acidity constant by flash spectroscopy. Enolate absorption (310 nm) as a function of pH (Haspra et al., 1979). (Reprinted by courtesy of Angewandte Chemie, Verlag Chemie, GmbH, Weinheim, Germany)...

The five spiroimidazodihydroquinoline compounds shown in Figure 1.3 exhibit reverse photochromism and were studied in detail by both stationary illumination and nanosecond time-resolved laser flash spectroscopy. 

In fast-bleaching betaines 9, the UV maxima were recorded using flash spectroscopy. A typical setup is described in the following section.42,43... 

The dependence of kp on the type of a-hydrogen is not known, nor can variations in ke be predicted easily. Mataga has studied amine quenching of triplet benzophenone by flash spectroscopy. N,N-dialkylanilines are the only amines which actually yield radical ions, and then only in polar solvents 157). He suggests two competing decay modes of the exciplex. Monoalkylanilines and tertiary aliphatic amines in any solvent and dialkylanilines in nonpolar solvents yield only radicals, presumably from the exciplex. Even though the oxidation potentials of tertiary aliphatic amines are so low that they quench triplet ketones at rates... 

The seminal work of Marcus and Hush has had a significant impact on the development of PET. Pioneering efforts by Sutin, Hopfield, Jortner, and others established the connection between thermal electron transfer and photoelectron transfer [6]. This work set the stage for a notable series of experiments where laser flash spectroscopy [7], chemically induced nuclear polarization [8], resonance Raman spectroscopy [9], time-resolved microwave conductivity [10], and time-resolved photoacoustic calorimetry [11], to site only a few examples, have been successfully employed to chart the dynamics of PET in homogeneous solution, the solid-state, and organized assemblies. 

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