Spectroscopy flash absorption

Electronically Excited Halogen Atoms (2/ y2) Observed by Kinetic Spectroscopy in Absorption Following the Flash Photolysis of Polyatomic Halides... 

Until recently the lifetimes of the triplet states of aromatic hydrocarbons in fluid solution at room temperature had been investigated exclusively by the technique of flash absorption spectroscopy. The lifetimes reported for many hydrocarbons, e.g., anthracene or phenan-threne, had been below 1 msec, and it had been assumed that radiationless conversion processes were so rapid under these conditions that the competing radiative triplet-singlet transition would be too slow to per-... 

More recently, powerful time-resolving techniques began to evolve. Nanosecond [13] and picosecond [14] flash absorption and emission spectroscopy made it possible to obtain UV spectra of transient species with very short lifetimes. 

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)...

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. 

Exciplex and Excimer Formation. The formation of excited complexes (excimers and exciplexes) in quenching reactions of excited organic compounds is a well-known phenomenon (cf. 70,230, 231,249). Evidence for the intermediacy of exciplexes is most readily obtained when exciplex emission is observed. However, other techniques. Including flash absorption spectroscopy (250-252) and kinetic analysis of both steady-state and dynamic quenching rate data (253) have also been used to obtain evidence for the existence of these transients. 

Besides the atomic reactions of the group 16 elements such as oxygen, sulfur, and the previously mentioned selenium, the reactions of tellurium atoms with alkenes have also been reported. In the case of tellurium, the ultraviolet photolysis of dimethyl telluride (DMT) is a usable source of 3P2j0 tellurium atoms (69PCS304). In flashed mixtures of DMT vapor (10—3 10 1 torr with COz diluent) using kinetic absorption spectroscopy, intense absorptions at 2143 2259 A, which correspond to known transitions of Te(3i>2), and at 2386 and 2383 A corresponding to Te(3/)1) and (VJ0), respectively, were observed (69JA5695). 

E Schlodder and K Brettel (1988) Primary charge separation in closed photosystem II with a lifetime of 11 ns. Flash-absorption spectroscopy with 02-evolving photosystem II complexes from Synechococcus. Biochim Biophys Acta 933 22-34... 

Fig. 10. (A) Flash-induced absorbance changes at 387,430 and 455 nm in a sample of PS-I particles from Synechococcus sp. under a mild reducing condition (ascorbate+ DCIP) or containing ferricyanide (FeCy) (B) difference spectra constructed from the aA amplitudes at the beginning and end ofthe 200-ns phase, i.e., at 5 ns and 1.6 /iS, respectively. The spectrum in the inset of (B) represents AA [P700 A,-] - [P700 A,] measured at 10 K (taken from Fig. 4 above). The dotted-line difference spectrum in (B) is that for [P700 -P700]. Figure source Brettel (1988) Electron transfer fifom Ay to an iron-sulfur center with tw=2O0 ns at room temperature in photosystem I. Characterization by flash absorption spectroscopy. FEBS Lett239 95,96.

K Brettel (1988) Electron transfer from Af to an iron-sulfur center with ty=200 ns at room temperature in photosystem I. Characterization by flash absorption spectroscopy. FEES Lett 239 93-98... 

Fig. 17. Flash-induced absorbance changes in a PS-I monomer complex of Synechocystis 6803 at 820 nm (A) and at 480 and 580 nm (B), without and with Fd (isolated from Synechocystis). Net Fd signals at 480 and 580 nm compensated for the antenna-pigment triplet contributions are aiso shown. Figure source S6tif and Bottin (1994) Laser flash absorption spectroscopy study of ferredoxin reduction by photosystem i in Synechocystis sp. PCC 6803 Evidence for submicrosecond and microsecond kinetics. Biochemistry 33 8497.

H Bottin and P Mathis (1985) Interaction ofplastocyanin with photosystem I reaction center. A kinetic study by flash absorption spectroscopy. Biochemistry 24 6453-6460... 

PQY S0tif and H Bottin t 995) Laser flash absorption spectroscopy study of ferredoxin reduction by photosystem I Spectral and kinetic evidence for the existence of several photosystem l-ferredoxin complexes. Bb-chemistry 34 9059-9070... 

Redox titrations using EPR and laser flash absorption spectroscopy of the electron acceptor complex in Phormidium laminosum photosystem 2 particles which apparently lack (4) are reported as these preparations offer a model system to determine the identity of the components of the electron acceptor complex. The g=1.6 EPR signal previously described in Synechocystis has also been observed in P. laminosum (5). The redox properties and effect of inhibitors and pH on the g=1.6 signal suggest its appearance reflects the redox behaviour of Qj. ... 

Bottin H, and Mathis P. Interaction of Plastocyanin with the Photosystem-I Reaction Center - a Kinetic-Study by Flash Absorption-Spectroscopy. 

In polar solvents, such as acetonitrile, organic donor-acceptor systems such as those listed in Table 6.2 show only the fluorescence due to A no new fluorescence appears as in exciplex formation. Flash spectroscopy shows absorption spectra characteristic of the hydrocarbon radical anion and the amine radical cation. The product in these solvents is either an ion-pair or two free ions, stabilised no doubt by solvation, and the reaction is a complete transfer of an electron from one molecule to another, rather than exciplex formation. The reaction goes effectively to completion, and so (with only one fluorescence lifetime to be considered) the kinetic equations for the intensity and lifetime reduce to the simple Stem-Volmer forms (Equations (6.16) and (6.19)). The rate constants for the reactions of aromatic hydrocarbons with various amines in acetonitrile are found to be correlated with the free-... 

After the laser flash, one then monitors the progress of events by some rapidly responding method. Conductivity, absorption spectroscopy, and fluorescence spectroscopy are the methods most commonly used. If a reaction product has a characteristic absorption band of sufficient intensity, one can monitor its buildup with time. This might be a UV, visible, or IR band. The need for a band with a high molar absorptivity arises because the reactive transient is usually present at a relatively low concentration, KT6-lCr5 M being typical. If the species of interest is phosphorescent, then the timed decay of its phosphorescence intensity can be recorded. 

Melo, T. B. lonescu, M. A. Haggquist, G. W. Naqvi, K. R. (1999). Hydrogen abstraction by triplet flavins. I time-resolved multi-chaimel absorption spectra of flash-irradiated riboflavin solutions in water Spectrochimica Acta, Part A Molecular and Biomolecular Spectroscopy, Vol.55, No.ll, (September 1999), pp. 2299-2307, ISSN 1386-1425. 

The optical absorption spectra of sulfonyl radicals have been measured by using modulation spectroscopy s, flash photolysis and pulse radiolysis s techniques. These spectra show broad absorption bands in the 280-600 nm region, with well-defined maxima at ca. 340 nm. All the available data are summarized in Table 3. Multiple Scattering X, calculations s successfully reproduce the experimental UV-visible spectra of MeSO 2 and PhSO 2 radicals, indicating that the most important transition observed in this region is due to transfer of electrons from the lone pair orbitals of the oxygen atoms to... 

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. 

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