Spectroscopy transient absorption

The difference in absorption AA(X,t) compared with the ground state absorption is measured for various delay times at different K. AA(X,t) contains information on how the absorption spectrum is changed due to the initial excitation. The time of absorption corresponds to t = 0. These changes have different origins. The excited molecules may undergo internal conversion, ISC, or chemical reactions. 

The experiment is repeated for different delay times. As the time proceeds, old absorptions at a certain wavelength may die out, while another will start at a different wavelength. From the absorption pattern, it is possible to deduce what dynamics takes place in the system. 

Abstract The concepts at the basis of transient absorption measurements were illustrated with particular reference to nanosecond kinetic spectrophotometry and femtosecond pump and probe methods. The main features of the typical experimental setups for both techniques were illustrated as regards optical parts, geometrical layout of components and light detection systems. Examples of application of transient absorption spectroscopy were illustrated for the elucidation of photoinduced processes in supramolecular systems like a fullerenepyrrolidine-oligophenyleneethynylene hybrid derivative, a drug-protein complex and a tri-chromophoric system consisting of two porphyrins and one perylene bisimide. 

One of the most powerful tools in photochemistry and photophysics is represented by the flash photolysis method. The fundamental idea is to use an intense light pulse with a suitably short time duration to perturb the equilibrium of a system and follow the appearance and the evolution of photochemically formed transients by detecting, in particular, their electronic absorption. This technique was developed 

Istituto per la Sintesi Organica e la Fotoreattivitk (ISOF), Consiglio Nazionale delle Ricerche (CNR), Area della Ricerca, Via Kero Gobetd 101, 

Ceroni (ed.). The Exploration of Supramolecular Systems and Nanostructures by Photochemical Techniques, Lecture Notes in Chemistry 78, 

Analogous results were obtained for the uncomplexed CD-St heptachromophoric cyclodextrin discussed in Section 7.2.4.2. In this case however, and owing to the larger Forster radius for homotransfer (22 A), the leveling of the anisotropy occurs in ca. 100 ps, and the average reciprocal rate constant for transfer is only 2.4 ps [14]. Nevertheless, a similar nearest neighbor distance (7 A) is recovered. 

In the previous sections, it has been shown how powerful the time-resolved fluorescence techniques are in real time probing of photoinduced processes and in allowing the determination of reaction rates from fluorescence lifetimes. The present section is devoted to the method of UV/vis transient absorption spectroscopy, which is a key method in probing non emissive species and is thus crucial to detect photoreaction products or intermediates following optical excitation of molecules in their electronic excited states. When carried out on short time scales, i.e. with femtosecond to subnanosecond excitation sources, fluorescent species can also be detected by their stimulated emission. Combining time-resolved fluorometry and transient absorption spectroscopy is ideal for the study of photochemical and photophysical molecular processes. 

Few instrumental developments have appeared in the field of conventional (ps) flash photolysis, so the description of this technique will be limited to a selection of applications illustrating the diverse use and results obtained from the method. 

A long-lived transient was observed following the excitation of C2H2 using a 3000 J 5 ps flash in Nj. A 2 ps spectroscopic flash, triggered by a set delay after excitation, monitored the transient absorption in the wavelength region 140— 157 nm. The reactions of OH and OD with butane, [ H io]-butane, and pentane,  

Kawasaki, K. Kasatani, and H. Sato, Chem. Phys. Lett., 1980, 75, 128. 

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A suitable method for a detailed investigation of stimulated emission and competing excited state absorption processes is the technique of transient absorption spectroscopy. Figure 10-2 shows a scheme of this technique. A strong femtosecond laser pulse (pump) is focused onto the sample. A second ultrashort laser pulse (probe) then interrogates the transmission changes due to the photoexcita-lions created by the pump pulse. The signal is recorded as a function of time delay between the two pulses. Therefore the dynamics of excited state absorption as... 

Structure, then the time-resolved photoelectron spectra [20, 21] could reveal signatures of two different intermediate structures, representing two different pathways on the PES. Transient absorption spectroscopy and other femtosecond time-resolved techniques may also be applicable to this problem. 

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 ... 

In good electron acceptor solvents, such as carbon tetrachloride and chloroform, the photodegradation of carotenoids is significantly increased as compared to other solvents (Christophersen et al. 1991, Mortensen and Skibsted 1999), because of a direct photoinduced electron-transfer reaction from the excited singlet state of the carotenoids to the solvent, as determined by transient absorption spectroscopy (Jeevarajan et al. 1996, Mortensen and Skibsted 1996,1997a,b, El-Agamey et al. 2005), Equation 12.2 ... 

Figure 3. Transient absorption spectroscopy on 2-hydroxybenzophenone and its derivative copolymer (A) 2-hydroxybenzophenone in EtOH solution and 2-hy-droxybenzophenone in CHtClt solution (B) copolymer in CHuCU. solution...

Lessing HE, Von Jena A (1976) Separation of rotational diffusion and level kinetics in transient absorption spectroscopy. Chem Phys Lett 42 213-217... 

Alfano, J. C., Walhout, P. K., Kimura, Y. and Barbara, P. F. Ultrafast transient-absorption spectroscopy of the aqueous solvated electron, J.Chem.Phys., 98 (1993), 5996-5998... 

Kasatani K, Kawasaki M, Sato H (1984) Lifetime shortening of the photoisomer of a cyanine dye by inclusion in a cyclodextrin cavity as revealed by transient absorption spectroscopy. J Phys Chem 88 5451-5453... 

The photophysics of fluorophores undergoing photoinduced charge transfer and/or internal rotation(s) is often complex. Time-resolved fluorescence experiments, transient absorption spectroscopy measurements, quantum chemical calculations, and comparison with model molecules are helpful in understanding their complex photophysical behavior. 

Figure 8.7. Delayed fluorescence and diffuse reflectance transient absorption spectroscopy on scattering substrates. Example terthicnyl on silica gel excited with = 354 nm (neodymium/yttrium-aluminum-garnet) (Nd/YAG) laser pulse of 10 nsec, 20 mj), recorded with a gated diode array spectrometer.

In the experiment, ZnCcP is produced by laser photolysis, then transient absorption spectroscopy follows the formation (k ) and decay (/Ceb) of ZnCcP + in wild-type and mutant crystals. Kang and Crane have studied the effects of interface mutations on electron transfer rates in single crystals using complexes between a zinc-substituted cytochrome c peroxidase (ZnCcP) and site-directed mutants of yeast cytochrome c (yCc).The mutants replaced the... 

We have observed the unligated B. excited state in noncoordinating solvents (11,45-47), buf not in ones that coordinate. Using transient absorption spectroscopy, Holten and coworkers found that the lifetime of the state for Ni... 

The nickel proteins and the native iron globins show the same behavior in regard to fifth-ligand photodissociation but for totally different reasons. Heme proteins and other metal-reconstituted heme proteins have been investigated by transient absorption spectroscopy (49-53) and transient Raman spectroscopy (16,54-62). In none of the Fe proteins is loss of the histidine ligand observed even on a picosecond timescale. Soret excitation of carbonmonoxy and oxy Hb and Mb photolyzes CO or O2, but not the histidine fifth ligand. 

Optical Spectroscopy General principles and overview, 246, 13 absorption and circular dichroism spectroscopy of nucleic acid duplexes and triplexes, 246, 19 circular dichroism, 246, 34 bioinorganic spectroscopy, 246, 71 magnetic circular dichroism, 246, 110 low-temperature spectroscopy, 246, 131 rapid-scanning ultraviolet/visible spectroscopy applied in stopped-flow studies, 246, 168 transient absorption spectroscopy in the study of processes and dynamics in biology, 246, 201 hole burning spectroscopy and physics of proteins, 246, 226 ultraviolet/visible spectroelectrochemistry of redox proteins, 246, 701 diode array detection in liquid chromatography, 246, 749. 

Probing Metalloproteins Electronic absorption spectroscopy of copper proteins, 226, 1 electronic absorption spectroscopy of nonheme iron proteins, 226, 33 cobalt as probe and label of proteins, 226, 52 biochemical and spectroscopic probes of mercury(ii) coordination environments in proteins, 226, 71 low-temperature optical spectroscopy metalloprotein structure and dynamics, 226, 97 nanosecond transient absorption spectroscopy, 226, 119 nanosecond time-resolved absorption and polarization dichroism spectroscopies, 226, 147 real-time spectroscopic techniques for probing conformational dynamics of heme proteins, 226, 177 variable-temperature magnetic circular dichroism, 226, 199 linear dichroism, 226, 232 infrared spectroscopy, 226, 259 Fourier transform infrared spectroscopy, 226, 289 infrared circular dichroism, 226, 306 Raman and resonance Raman spectroscopy, 226, 319 protein structure from ultraviolet resonance Raman spectroscopy, 226, 374 single-crystal micro-Raman spectroscopy, 226, 397 nanosecond time-resolved resonance Raman spectroscopy, 226, 409 techniques for obtaining resonance Raman spectra of metalloproteins, 226, 431 Raman optical activity, 226, 470 surface-enhanced resonance Raman scattering, 226, 482 luminescence... 

Ohkita H, Cook S, Astuti Y, Duffy W, Tierney S, Zhang W, Heeney M, McCulloch I, Nelson J, Bradley DDC, Durrant JR (2008) Charge carrier formation in polythiophene/fullerene blend films studied by transient absorption spectroscopy. J Am Chem Soc 130 3030... 

Pagsberg, P., O. J. Nielsen, and C. Anastasi, Gas Phase Studies in Atmospheric Chemistry Using Pulse Radiolysis and Transient Absorption Spectroscopy, in Spectroscopy in Environmental Science (R. J. H. Clark and R. E. Hester, Eds.), Chap. 6, Wiley, 1995. 

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