Absorption time-resolved transient

Fig. 16. Time-resolved transient absorption spectra for poly(A/St/Phen) (29) in aqueous solution in the presence of 5 mM MV2 + [Phen] (residue) = 0.66 mM. Delay times after the laser pulse are indicated [102]...

Employing all of the TR results in conjunction with results from femtosecond time-resolved transient absorption (fs-TA) and femtosecond time-resolved Kerr gated fluorescence (fs-KTRF) experiments enables a reaction mechanism to be developed... 

Finally a few sentences are deserved for the vast area of DNA photochemistry. Thymine dimerization is the most common photochemical reaction with the quantum yield of formation in isolated DNA of all-thymine oligodeoxynucleotides 2-3% [3], Furthermore, a recent study based on femtosecond time-resolved transient absorption spectroscopy showed that thymine dimers are formed in less than 1 ps when the strand has an appropriate conformation [258], The low quantum yield of the reaction in regular DNA is suggested to be due to the infrequency of these appropriate reactive conformations. 

Strand cleavage studies have provided relative rate constants for hole transport versus the rate constant for the initial chemical event leading to strand cleavage [18-20]. However, they do not provide absolute rate constants for hole transport processes. Several years ago we introduced a method based on femtosecond time-resolved transient-absorption spectroscopy for investigating the dynamics of charge separation and charge recombination in synthetic DNA hairpins [21, 22]. Recently, we have found that extensions of this method into the nanosecond and microsecond time domains permit investigation of the dynamics of hole transport from a primary hole... 

The dynamics of photoinduced charge separation, kcs, and charge recombination, kcr (Fig. 2a), have been studied in several families of hairpins containing an Sa linker and a single G C base pair by means of femtosecond time-resolved transient absorption spectroscopy [27, 28]. Both the singlet state and anion radical of Sa have strong transient absorption centered at 575 nm. The difference in the independently determined band shapes for Sa ... 

Have we pushed the use of time resolved transient absorption with kinetic modeling to its limit We think not. Studies of the temperature dependence of hole transport in hairpins such as 3GAZ should provide additional details about the mechanism of this processes. The use of donor-acceptor triplexes... 

Figure 2 Time-resolved transient absorption spectra obtained by flash photolysis of phenyl acetate in methylcyclohexane (266 nm excitation). Delay times 50, 100, 200, and 400 msec. (Adapted from Ref. 41.)...

Tomov IV, Rentzepis PM (2004) Ultrafast time-resolved transient stmctures of solids and liquids by means of extended X-ray absorption fine stmcture. ChemPhysChem 5 27-35... 

Figure 20 Time-resolved transient absorption spectra observed at 0 nsec (solid line), 100 nsec (dashed line), and 1 psec (dotted line) after e and transient fluorescence spectrum of TMB observed at 300 nsec after e during PR-LFP of TMB (1.0 x 10 M) in DCE. Energy of AV532, 130 mJ.

A transient absorption spectrum with a peak at 575 nm assigned to DMP was observed immediately after e during PR of DMP (5.0 x 10 M) in DCE. The time-resolved transient absorption spectra showed a slow growth of a new peak at 500 nm assigned to 3,5-dimethoxyphenoxy radical (DMP ) (Fig. 22). The peak at 575 nm due to DMP decreased with time, and most of DMP disappeared at 2 psec after e to yield DMP through deprotonation of DMP It has been shown that bimolecular deproto-... 

The maximum IPCE was 19% at the peak of the visible absorption band of the dye. They also observed a fast-electron injection of below 100 fsec from cyanine dye into the conduction band of Ti02 by time-resolved transient absorption spectroscopy [36]. 

Time-resolved transient absorption spectra of native PYP... 

The lifetimes of the charge separated state in degassed toluene for 1 through 4 are 5, 104, 150, and 300 ns, respectively, as determined by time-resolved transient absorption experiments [60-62], As Table 3 illustrates, this trend is preserved in the liquid crystalline environment, with the caveat that the ion pair lifetimes are increased by at least one order of magnitude. This variation allows for the study of photorefractivity as a function of the lifetime of the intramolecular charge separated state. 

Figure 6.17(a) shows the time-resolved transient absorption spectra obtained for [Ru(dcbpy)2(bpzt)] in aqueous solution at pH 7. The important features are the bleaching of an absorption feature with a maximum at 450 nm and the growing in of a new band at 350 nm. The bleaching process can be attributed to the disappearance of the metal-to-ligand charge-transfer (MLCT) transitions in the excited state, while the feature at 350 nm is indicative of intraligand transitions of the dcbpy radical formed upon excitation of the complex. This type of behavior which is typical... 

Figure 6.17 Time-resolved transient absorption spectra of [Ru(dcbpy)2(bpzt)] (a) in aqueous solution (pH 7), with spectra taken at 0,25,50,100 and 150 ns, respectively (b) immobilized at nanocrystalline TiC>2, with spectra taken at 0, 50, 250,1000 and 2000 ns, respectively. Reproduced by permission of Wiley-VCH from A. C. Lees, B. Evrard, T. E. Keyes, J. G. Vos, C. J. Kleverlaan, M. Alebbi and C. A. Bignozzi, Eur. J. Inorg. Chem., 2309 (1999)...

Figure 6.24 Time-resolved transient absorption spectra in 0.1 M LiClC /acetonitrile, at a leXc of 532 nm (a) Ru(lII)-Ru(ll) in solution, with transients taken at 0,20,40 and 70 ns (b) the TiC>2-Rh(lIl)-Ru(ll) triad, with transients taken at 0,10, 20, 50,100,500 and 2000 ns. Reprinted with permission from C. J. Kleverlaan, M. T. Indelli, C. A. Bignozzi, L. Pavanin, F. Scandola, G. M. Hasselmann and G. J. Meyer,. Am. Chem. Soc., 122,2840 (2000). Copyright (2000) American Chemical Society...

The recombination behavior of this immobilized dyad is illustrated in Figure 6.30, which shows the time-resolved transient absorption spectra both without and in the presence of I—/13. This figure shows clearly that recombination in the absence of the redox relay is slow and takes place on the microsecond time-scale. 

Before 1975 such studies were mainly based on the detection of time-resolved transient absorptions produced by an ultrashort energy pulse (5,6). As in a conventional flash photolysis experiment, the exciting pulse produces transients and a probe pulse detects them as a function of a delay time. For detection to be possible, a large fraction of the parent molecules must be converted to the transient species and an intense absorption band of this species must occur in a suitable region of the spectrum. 

Static electron transfer from photoexcited particles to adsorbed substrates has been observed for a wide range of semiconductor and organic materials respectively. For example, the photoinduced reduction of zwit-terionic viologen (ZV) by CdS conduction band electrons is found by time resolved transient absorption spectra to occur with a risetime for ZV -formation of less than 20 ps [116], consistent with a rate constant for interfacial electron transfer of >5 x 10los-1. Strong adsorption of the... 

Fig. 15, were initially obtained from the DMN fluorescence lifetime data,92,102 but they have recently been measured using subpicosecond time-resolved transient absorption spectroscopy (pump-probe).105... 

Fig. 15 Rate data for photoinduced charge separation and subsequent charge recombination in the dyads 18(h)- Charge separation rates, cs, in THF at 20°C were determined both from fluorescence lifetimes92,102 and by pump-probe (time-resolved transient absorption) spectroscopic measurements.105 The mean lifetimes towards charge recombination, rcn were obtained from time-resolved conductivity measurements in 1,4-dioxane.99 101,103,104...

The transient absorption spectra show that besides the ring-closure product (C), a new transient species was found that could be quenched by oxygen. The authors assigned the new species to an excited triplet state. The time-resolved transient absorption spectra indicated that when the time delay was increased, the excited triplet state was decreased and the ring closure product was increased slightly. The decay of the excited triplet state can be fit to first-order kinetics. The lifetime ( ) of the transient is 0.8 ps in cyclohexane. The authors noted that the photocyclization process proceeds mainly via the excited singlet state, but the excited triplet state is... 

Fig. 8. Time-resolved transient absorption spectra obtained in the reaction of "OH with -hydroxyhenzaldehyde 1 x 10" mol dm" at 1.5 (o) and 10 ( ) ps after the pulse dose/pulse = 1.5 krad. [Reprinted with permission ftom Ref. 28. Copyright 2001, Elsevier. 1...

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