UV-Vis flash photolysis

Thus, overall, it is clear that flash photolysis with uv-visible detection is effective in establishing the broad outlines of the photochemistry of a particular metal carbonyl. Intermediates can be identified from their reaction kinetics, and sometimes, with the help of uv-vis data from matrix isolation experiments. Structural information from uv-vis flash photolysis is at best sketchy. Many questions remain unanswered. Time-resolved IR measurements can fill in some of these answers. 

It had already been established by uv-vis flash photolysis (35) that Cr(CO)5 (solvent) was the first observable intermediate in the photolysis of Cr(CO)6. Figure 9 shows the IR spectrum (96) of the photoproduct Cr(CO)5(C6Hi2) in cyclohexane solution. The spectra were obtained using Cr(CO)5(13CO) (96). The extra spectroscopic information provided by the 13CO group was sufficient to show that the spectrum was consistent... 

The IR kinetic measurements (96) of the rate constants for reaction of Cr(CO)5(C6H12) with CO were very similar to those measured using uv-vis flash photolysis (30,33). In the presence of added ligands, Cr(CO)5(C6H12) decayed to give Cr(CO)5L products. For both L = CO and L = H20 the activation energy was 22 5 kJ mol-1 (96), but surprisingly the rate of addition of H20 was much faster than that of CO. Similar... 

Fast time resolved infrared attached to flow and to uv-vis flash photolysis has been an important development for the study of rapid substitution, e.g. in Co2(CO)r in hexane " and... 

Uv-vis flash photolysis of [CpFe(CO)2l2 (14) and MeCN in cyclohexane at 25 °C gives the ir/time display shown in Figure 3.16. This shows that 14 is destroyed in the flash and that there are two intermediates CpFe(pCO)jFeCp (16) and in much smaller amounts, CpFe(CO)2, (15)-The principal intermediate reacts relatively slowly with MeCN to give Cp2Fe2(CO)j(MeCN) (17) and the decay of 16 mirrors exactly the increase in 17 (A = 7.6 x 10 M s at 24°C... 

UV-Vis flash photolysis is the most widely used technique with which to follow fast photochemical reactions. However, although flash photolysis provides excellent kinetic information about excited states and reaction intermediates, UV-Vis spectra in solution are often broad and featureless and provide little structural information. Moreover, in experiments where several photoproducts are produced, overlapping of the broad absorption bands can make it very difficult to elucidate the mechanism. Transient vibrational spectroscopy provides more detailed structural information to probe photochemical reactions especially since Raman and IR bands of large molecules in solution are generally much narrower than UV-Vis bands. 

Further studies were carried out with halocarbene amides 34 and 357 Although again no direct spectroscopic signatures for specifically solvated carbenes were found, compelling evidence for such solvation was obtained with a combination of laser flash photolysis (LFP) with UV-VIS detection via pyridine ylides, TRIR spectroscopy, density functional theory (DFT) calculations, and kinetic simulations. Carbenes 34 and 35 were generated by photolysis of indan-based precursors (Scheme 4.7) and were directly observed by TRIR spectroscopy in Freon-113 at 1635 and 1650 cm , respectively. The addition of small amounts of dioxane or THF significantly retarded the rate of biomolecular reaction with both pyridine and TME in Freon-113. Also, the addition of dioxane increased the observed lifetime of carbene 34 in Freon-113. These are both unprecedented observations. 

Final resolution of these problems, particularly the complications from multiple matrix sites, came from investigations using spectroscopic methods with higher time resolution, viz. laser flash photolysis. Short laser pulse irradiation of diazofluorene (36) in cold organic glasses produced the corresponding fluorenylidene (37), which could be detected by UV/VIS spectroscopy. Now, in contrast to the results from EPR spectroscopy, single exponential decays of the carbene could be observed in matrices... 

Any new technique relies heavily on what has gone before. In the remainder of this introduction, first we outline briefly the role of matrix isolation in characterizing transition metal fragments and then consider what conventional flash photolysis with uv-vis detection has revealed about the reactivity of these fragments. It is the timescale of these reactions which dictates the speed of the IR spectroscopy required to detect the intermediates. 

Church and co-workers (77) have obtained time-resolved IR spectra of both Mn(CO)5 and Mn2(CO)9 by flash photolysis of Mn2(CO)I0 in solution. The spectra (Fig. 11) were in close agreement with the spectra of matrix isolated Mn(CO)5 (22) and Mn2(CO)9 (5,106). There was a bridging vc 0 band for Mn2(CO)9 showing that it has a CO-bridged structure in solution as well as in the matrix. Structural information of this type could not have been obtained from uv-vis spectroscopy. Similarly, the IR spectra indicated that Mn(CO)5 had the same C4v structure in solution (77) as in the matrix (22). In solution (77), the yield of Mn2(CO)9 was approximately equal to that of Mn(CO)5. Bearing in mind that one molecule of Mn2(CO),0 produces two molecules of Mn(CO)5 [Eq. (14)], CO loss from Mn2(CO)10 [Eq. (15)], must be the major process at these photolysis wavelengths (37,77). 

Another transient aminoxyl radical has been generated , and employed in H-abstraction reactivity determinations" . Precursor 1-hydroxybenzotriazole (HBT, Table 2) has been oxidized by cyclic voltammetry (CV) to the corresponding >N—O species, dubbed BTNO (Scheme 9). A redox potential comparable to that of the HPI —PINO oxidation, i.e. E° 1.08 V/NHE, has been obtained in 0.01 M sodium acetate buffered solution at pH 4.7, containing 4% MeCN". Oxidation of HBT by either Pb(OAc)4 in AcOH, or cerium(IV) ammonium nitrate (CAN E° 1.35 V/NHE) in MeCN, has been monitored by spectrophotometry , providing a broad UV-Vis absorption band with A-max at 474 nm and e = 1840 M cm. As in the case of PINO from HPI, the absorption spectrum of aminoxyl radical BTNO is not stable, but decays faster (half-life of 110 s at [HBT] = 0.5 mM) than that of PINO . An EPR spectrum consistent with the structure of BTNO was obtained from equimolar amounts of CAN and HBT in MeCN solution . Finally, laser flash photolysis (LFP) of an Ar-saturated MeCN solution of dicumyl peroxide and HBT at 355 nm gave rise to a species whose absorption spectrum, recorded 1.4 ms after the laser pulse, had the same absorption maximum (ca 474 nm) of the spectrum recorded by conventional spectrophotometry (Scheme 9)59- 54... 

Laser flash photolysis methods have also been applied to the study of nitrenium ion trapping rates and hfetimes. This method relies on short laser pulses to create a high transient concentration of the nitrenium ion, and fast detection technology to characterize its spectrum and lifetime The most frequently used detection method is fast UV-vis spectroscopy. This method has the advantage of high sensitivity, but provides very little specific information about the structure of the species being detected. More recently, time-resolved infrared (TRIR) and Raman spectroscopies have been used in conjunction with flash photolysis methods. These provide very detailed structural information, but suffer from lower detection sensitivity. 

Whether laser flash photolysis (LFP) is used to detect RIs before they react, or matrix isolation at very low temperatures is employed to slow down or quench these reactions, spectroscopic characterization of RIs is frequently limited to infrared (IR) and/or ultraviolet-visible (UV-vis) spectroscopy. Nuclear magnetic resonance (NMR) spectroscopy, which is generally the most useful spectroscopic technique for unequivocally assigning structures to stable organic molecules, is inapplicable to many types of RI. 

The currently accepted spectroscopic assignments were obtained by a combination of multiple techniques. Leyva et applied matrix absorption and emission spectroscopy along with flash photolysis techniques. Chapman and LeRoux obtained the matrix IR spectrum of cyclic ketenimine K and Hayes and Sheridan obtained the matrix IR and UV-Vis spectrum of triplet phenylnitrene and cyclic ketenimine K. Schuster and co-workers applied time resolved IR and UV-Vis spectroscopy and demonstrated the formation of cyclic ketenimine K in solution, the species that absorbs strongly at 340 nm. 

Several investigations concerning the thermodynamic and kinetic aspects of the thermal reactions of flavylium-type compounds have long been in the literature,133-371 while photochemical and photophysical aspects have been systematically examined more recently.[17-19,38 31 As we shall see below, pH jump, temperature jump, and flash photolysis experiments permit measurement of the rate constants of some of the reactions involved, and steady state titration experiments (using UV/Vis and NMR techniques) allow the measurement of equilibrium constants. In order to illustrate the complex reaction network in which these systems operate, we will now focus on the behavior of the 4 -methoxyflavylium ion (Figure 2 R4 = R7=H, R4- = OCH3).[391... 

Fig. 7.3 Experimental setup for the nanosecond laser Flash Photolysis with a white light continuum. A Brilland-Quantel Nd YAG laser delivers the fundamental pulses (355 and 532 nm). A pulsed XBO lamp is used as white light source. The laser signal is split in order to trigger the digital storage oscilloscope (DSO) utilizing a second photodiode (PD). Two separate detection units in different geometries—photomultiplier (PMT) in front face and a PD in side face—detect the signal in the UV/vis and NIR region, respectively. The monochromator is operated by a standard PC...

The laser flash photolysis technique relies on the use of a pulsed UV laser for the rapid synthesis of the reactive intermediate of interest by photochemical decomposition of a suitable stable precursor, and (most commonly) fast time-resolved UV/VIS spectrophotometry to detect the species and monitor its decay19. The absorbance-time profile so... 

The photochemical properties of [Ru(CO)3(dmpe)j (dmpe = l,2-bis(dimethylphosphino)ethane) have been studied by matrix isolation at 12 K and laser flash photolysis with UV-vis and IR detection at ambient temperatures. UV photolysis (A,ex = 234-376mn) in a matrix resulted in the formation of [Ru(CO)2(dmpe) S] (S = matrix host). Laser flash photolysis in heptane solution (A,ex = 266 or 308 nm) revealed that [Ru(CO)2(dmpe) (heptane)] was a short-lived intermediate fragment that reacted rapidly with [Ru(CO)3(dmpe)]. 

Martin CB, Shi X, Tsao M-L, Karweik D, Brooke J, Hadad CM, Platz MS. (2002) The photochemistry of riboflavin tetraacetate and nucleosides. A study using density functional theory, laser flash photolysis, fluorescence, UV-Vis and time resolved infrared spectroscopy. J Phys Chem B 106 10263-10271. 

Our research focuses on mechanistic and kinetic studies of photochemical and electrochemical CO2 reduction that involves metal complexes as catalysts. This work makes use of UV-vis, NMR, and FTIR spectroscopy, flash photolysis, pulse radiolysis. X-ray diffraction, XANES (X-ray absorption near-edge spectroscopy) and EXAFS (extended X-ray absorption fine structure). Here we summarize our research on photochemical carbon dioxide reduction with metal macrocycles. 

The absolute rate constants have been refined by time-resolved laser flash photolysis in which the change in concentration of the stannyl radical, or a reaction product, or an added probe is monitored by UV/vis spectroscopy.60 Rate constants for the reactions covering the literature up to 1981 are listed in Landoldt-Bomstein (1983, II13c, pp 323-336). A selection of current values is given in Table 20-2. 

Time-resolved UV/vis absorption spectroscopy has been initiated by Norrish and Porter who developed flash photolysis in the late 1940s, opening the way to the detection of transient chemical species with time resolution of a few microseconds [30, 31]. The present state of art transient absorption techniques allow detection of chemical intermediates with less than 10 fs resolution. The techniques used depend on the explored time scale but the principle, which is illustrated in Fig. 7.14, is the same. 

Direct measurement of the reaction of interest is sometimes possible using rapid reaction techniques. In laser flash photolysis, an intense, short-lived pulse of light irradiates the sample and the products are monitored by a variety of techniques, from basic UV/Vis spectroscopy to techniques - such as laser-excited fluorescence -which require a second, analytical pulse of radiation. In pulse radiolysis, a short (1-10 ns) pulse of high-energy (1-10 MeV) electrons irradiates the sample and the decay of the fragments can be analysed in the same way as the fragments from flash photolysis. The equipment for pulse radiolysis is even more complex and costly than that for flash photolysis, and tends to be concentrated in national facilities. 

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