Transient flash photolysis

The absorption spectroscopy of transients (flash photolysis) is the most valuable tool for studying the decay of electronically excited species and reactive intermediates 360,379) Xhe limiting factor in the observability of a transient is the duration of the flash 10-8 sec. for conventional discharge lamps and 10-9 sec. for a Laser set-up 536)t... 

Transient flash photolysis and time-resolved luminescence experiments were performed using a system similar to one described previously (92), which is diagramed in Figure 1. Aqueous suspensions of ca. 100 mg of solid sample... 

Transient species, existing for periods of time of the order of a microsecond (lO s) or a nanosecond (10 s), may be produced by photolysis using far-ultraviolet radiation. Electronic spectroscopy is one of the most sensitive methods for detecting such species, whether they are produced in the solid, liquid or gas phase, but a special technique, that of flash photolysis devised by Norrish and Porter in 1949, is necessary. 

The flash photolysis of s-trinitrobenzene (TNB) aerated solns in alcohols generated a transient species with absorption maxima at 430 and 51 Onm (Ref 27). The yield of the transient was a function of oxygen concn, and its rate of formation was viscosity dependent. In deaerated solns, instead of the transient, a brown permanent product, identified as a charge transfer... 

Capellos and Suryanarayanan (Ref 28) described a ruby laser nanosecond flash photolysis system to study the chemical reactivity of electrically excited state of aromatic nitrocompds. The system was capable of recording absorption spectra of transient species with half-lives in the range of 20 nanoseconds (20 x lO sec) to 1 millisecond (1 O 3sec). Kinetic data pertaining to the lifetime of electronically excited states could be recorded by following the transient absorption as a function of time. Preliminary data on the spectroscopic and kinetic behavior of 1,4-dinitronaphthalene triplet excited state were obtained with this equipment... 

Nanosecond flash photolysis of 1,4-dinitro-naphthalene in aerated and deaerated solvents showed a transient species with absorption maximum at 545nm. The maximum of the transient absorption was independent of solvent polarity and its lifetime seemed to be a function of the hydrogen donor efficiency of the solvent. The transient absorption was attributed to the lowest excited triplet state of 1,4-dinitronaphthalene. Based on spectroscopic and kinetic evidence, the triplet state of 1,4-dinitronaphthalene behaved as an n - Tt state in nonpolar solvents,... 

More recently, Scaiano et al. (1991) observed (Zs)->(Z)-isomerization of 1,3-di-phenyltriazene also in methanol by using flash photolysis, transient spectroscopy, and laser-induced optoacoustic calorimetry (LIOAC). The interpretation of the data is consistent with the mechanism shown in Scheme 13-4, involving two solvent molecules. 

The absorption of radiation produces unstable species. Flash photolysis does so by interaction of light with a solute. The transient may be a photoexcited state or a molecular fragment. Pulse radiolysis starts with highly reactive entities formed by dissociation of the solvent (e.g., H, eaq, and HO from H20) and consists of a study of their reactions or of reactive transients derived from them. In either case one monitors the ensuing reactions by luminescence (for excited states), light absorption, or conductivity changes. 

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 electron transfer reaction from fluorenyl carbanions adjacent to sulfoxide or sulfone (9) to fluorenone (FIO) has been studied by means of flash photolysis. For n = 1 as well as 71 = 2 the transient FIO, M (contact ion pair) appeared in THF and FIO (free ion)... 

The prototype o-quinone methide (o-QM) and / -quinone methide (p-QM) are reactive intermediates. In fact, they have only been detected spectroscopically at low temperatures (10 K) in an argon matrix,1 or as a transient species by laser flash photolysis.2 Such a reactivity is mainly due to their electrophilic nature, which is remarkable in comparison to that of other neutral electrophiles. In fact, QMs are excellent Michael acceptors, and nucleophiles add very fast under mild conditions at the QM exocyclic methylene group to form benzylic adducts, according to Scheme 2.1.2a 3... 

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... 

Flash photolysis, ESR, and absorption studies of this photolysis have identified a transient species (leading to product) as a complex (Amax 410 nm, r 5 x 10-7 sec) between the triplet excited aromatic compound and its nucleophilic reaction partner. 

On flash photolysis of benzophenone in benzene two transients are observed/9-11 One of these, the shorter lived, is assumed to be the excited triplet of benzophenone, and the longer lived transient would appear to be the ketyl radical by comparison of its spectrum with that of authentic ketyl radical formed in solvents that are good hydrogen donors. This, however, may be an entirely new species which fortuitously absorbs in the same region as the ketyl radical. 

Flash photolysis has been used to study the triplet-triplet absorption spectra (T1 -> Tq) of a number of aromatic molecules both in solution and in the gas phase.<34) A disadvantage associated with the flash technique for obtaining triplet-triplet absorption spectra is that the transient absorption can occur for only a short time after the initial flash (determined by the lifetime of triplet... 

Laser flash photolysis techniques offer the possibility of examining in detail the transient processes responsible for the photostabilizing effect discussed above. The triplet lifetimes are frequently too short, even for this technique however, they can still be estimated using as a probe the quenching by 1-methyl-naphthalene, which leads to the formation of its easily detectable triplet. The optical absorbance due to the 1-methylnaphthalene triplet (Aft) produced as a result of energy transfer is related to the Stern-Volmer slope by equation 5, where N stands for... 

In order to understand these results it is necessary to consider the nature of the intermediates formed upon photolysis of arylamines. The absorption spectra of transients produced upon photolysis of aniline and various alkyl ring-substituted arylamines was obtained by Land and Porter (18) in different solvents using a flash photolysis apparatus. On this basis they identified both an anilinyl radical (PhNH-) and an anilinyl radical cation (PhNHj). The radical cation is present in polar media (H2O) but absent in cyclohexane. From these results, a homolytic cleavage... 

In general, intramolecular isomerization in coordinatively unsaturated species would be expected to occur much faster than bimolecular processes. Some isomerizations, like those occurring with W(CO)4CS (47) are anticipated to be very fast, because they are associated with electronic relaxation. Assuming reasonable values for activation energies and A-factors, one predicts that, in solution, many isomerizations will have half-lives at room temperature in the range 10 7 to 10 6 seconds. The principal means of identifying transients in uv-visible flash photolysis is decay kinetics and their variation with reaction conditions. Such identification will be difficult if not impossible with unimolecular isomerization, particularly since uv-visible absorptions are not very sensitive to structural changes (see Section I,B). These restrictions do not apply to time-resolved IR measurements, which should have wide applications in this area. 

Both Porter s original flash photolysis apparatus and Pimentel s rapid scan spectrometer recorded the whole spectral region in a time which was short compared to the decay of the transient species. Kinetic information was obtained by repeatedly firing the photolytic flash lamp and making each spectroscopic measurement at a different time delay after each flash. The decay rate could then be extracted from this series of delayed spectra. Such a process clearly has limitations, particularly for IR measurements, where the decay must be slow compared to the scan rate of the spectrum. 

Fig. 12. Transient IR difference spectra showing changes in absorbance (a) 5 / seconds, (b) 25 seconds, and (c) 1.25 mseconds after the UV flash photolysis of [CpFe(CO)2]2 in cyclohexane solution under 1 atm pressure of CO. Bands pointing upward represent an increase in absorbance (i.e., formation of a compound) and those pointing downward a decrease [i.e., depletion of starting material, (A)]. The bands are assigned as follows A, [CpFe(CO)2]2 B, CpFe(CO)2 and C, CpFe(p.-CO)3Fe(Cp). Points marked were recorded with a 12CO laser and those marked + with a 13CO laser. [Reproduced with permission from Moore et al. (61).]...

Already a considerable number of transient organometallic species have been characterized by IR kinetic spectroscopy (see Table I). Like most other sporting techniques for structure determination, IR kinetic spectroscopy will not always provide a complete solution to every problem. What it can do is to provide more structural information, about metal carbonyl species at least, than conventional uv-visible flash photolysis. This structural information is obtained without loss of kinetic data, which can even be more precise than data from the corresponding uv-visible... 

First Order Rate Constants for Decay of Transients Seen by Longer Wavelength (A rr > 390 nm) Flash Photolysis of Ru3(C0)]o in Cyclohexane Solutions with Various Added Ligands a... 

The transient seen by flash photolysis in THF is proposed to be II (S - THF), since no transient precursor to substitution with a lifetime > 30 ps was seen in cyclohexane despite the comparable s in both solvents. According to Scheme 2, when L - CO or [L] [CO] the following relationship would hold true ... 

Figure 3. Double reciprocal plots of the kinetics data obtained for the decay of the transients seen for the short wavelength flash photolysis (A rr > 315 nm) of THF solutions of Ru3(CO) 2 in the presence of various ligands L (from reference 5).

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