Time-resolved electronic absorption

Time resolved electronic absorption and emission spectroscopy has been extensively used for solution phase kinetic studies of fast chemical processes initiated by ionizing radiation. A wealth of information on rate parameters and reaction mechanisms on a variety of chemical reactions has been obtained by this technique. As valuable as these techniques are, they have limitations. In particular, the electronic spectra in solution are often broad and featureless and offer little structural information. As a consequence, the identification of a reaction intermediate is based on chemical intuition and not on its spectral characteristics. Moreover, when more than one transient is present in the system with overlapping electronic absorption, the kinetic monitoring of the individual concentration becomes difficult. Vibra-... 

Koyama Y and Mukai Y (1993) Excited states of retinoids, carotenoids and chlorophylls as revealed by time-resolved, electronic absorption and resonance Raman spectroscopy. In ... 

Vibrational Relaxation. Stochastic processes, including vibrational relaxation in condensed media, have been considered from a theoretical standpoint in an extensive review,502 and a further review has considered measurement of such processes also.503 Models have been presented for vibrational relaxation in diatomic liquids 504 and in condensed media,505 using a master-equation approach. An extensive development of quantum ergodic theory for relaxation processes has been published,506 and quantum resonance effects in electronic to vibrational energy transfer have been considered.507 A paper has also considered the coupling between vibrational relaxation and molecular electronic transitions.508 A theory has also been outlined for the time-resolved electronic absorption spectrum of a molecule undergoing collisional vibrational relaxation.509... 

Del Greco and Kaufman established the mechanism of the decay reaction of OH, using time-resolved electronic absorption spectrophotometry with an OH lamp. Rapid bimolecular disproportionation of 20H occurs, leading to oxygen atoms. 

Fig. 3 Electronic absorption spectra of (A) the EDA complex of 9-cyanoanthracene (CNA) and tetracyanoethylene (TCNE) in comparison with the uncomplexed CNA and TCNE (B) the donor cation CNA+- and acceptor anion TCNE -generated spectroelectrochemically and (C) time-resolved picosecond absorption spectra taken at 25, 50, 75 and lOOps following the charge-transfer excitation at 532 nm of [CNA, TCNE] with a 25-ps laser pulse.

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]. 

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... 

Research on carbon-centered radical cations in solution accelerated dramatically with the development of time-resolved optical absorption and emission techniques. The research group of Th. Forster in Germany pioneered photochemical methods of production of radical cations and anions, as well as exciplexes." While the Forster group focused on structure and lifetimes, the later work of D. R. Arnold in Canada, and of H. D. Roth in the United States," reported the reactivity of photochemically generated radical cations from a mechanistic perspective. These studies of radical ion chemistry evolved into the field we now know as electron donor-acceptor interactions, arich area of science in which carbon-centered radical cations are stiU actively smdied. 

The dynamics of both static and dynamic quenching of the fluorescent singlet states of diazapyrenium salts by nucleotides has been investigated by Brun and Harriman using sub-nanosecond time-resolved transient absorption spectroscopy [88]. Observation of the reduced acceptor DAP+ (Table 5) supports an electron transfer mechanism for fluorescence quenching. Diffusion-controlled rate constants were observed for quenching of DAP + by all four deoxynucleotides. Excitation of... 

TA-transient absorption TRANIP = time-resolved anisotropy TRPL = time-resolved photoluminescence TRIR = time-resolved IR TR = EPR-time-resolved electron paramagnetic resonance spectroscopy. 

There continues to be an enormous amount of activity in the area of PET, much of it directed towards the development of systems capable of delivering artificial photosynthesis. Many of these systems involve porphyrin units as electron-donors and thus it is appropriate to consider them in this section of the review. A number of new fullerene-porphyrin dyads have been reported. A pyrazolinofullerene (155) has been constructed which facilitates efficient PET when strong donors such as iV,Ar-diethylaniline or ferrocene are linked to the pyrazoline ring. A photosynthetic multi-step ET model (156) based on a triad consisting of a meso,meso- inked porphyrin dimer connected to ferrocene and Ceo as electron-donor and electron-acceptor, respectively, has been synthesized and its ET dynamics (Scheme 38) have been investigated using time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. ... 

Cluster properties, mostly those that control electron transfer processes such as the redox potential in solution, are markedly dependent on their nuclearity. Therefore, clusters of the same metal may behave as electron donor or as electron acceptor, depending on their size. Pulse radiolysis associated with time-resolved optical absorption spectroscopy is used to generate isolated metal atoms and to observe transitorily the subsequent clusters of progressive nuclearity yielded by coalescence. Applied to silver clusters, the kinetic study of the competition of coalescence with reactions in the presence of added reactants of variable redox potential allows us to describe the autocatalytic processes of growth or corrosion of the clusters by electron transfer. The results provide the size dependence of the redox potential of some metal clusters. The influence of the environment (surfactant, ligand, or support) and the role of electron relay of metal clusters in electron transfer catalysis are discussed. 

Nanosecond Flash Photolysis Measurements.—A computer-controlled ns flash photolysis spectrometer has been described. " The system was employed in a study of the photochemistry of xanthene dyes in solution. A nitrogen laser was used to provide 2—3 mJ excitation pulses at 337.1 nm for a ns flash photolysis study of electron-transfer reactions of phenolate ions with aromatic carbonyl triplets. " A PDP II computer was used to control the transient digitizer employed for detection, and to subsequently process the data. A nanosecond transient absorption spectrophotometer has been constructed using a tunable dye laser in a pulse-probe conflguration with up to 100 ns probe delayA method for reconstructing the time-resolved transient absorption was discussed and results presented for anthracene in acetonitrile solution. The time-resolution of ns flash photolysis may be greatly increased by consideration of the integral under the transient absorption spectrum. Decay times comparable to or shorter than the excitation flash may be determined by this method. 

This problem is related to the question of appropriate electronic degeneracy factors in chemical kinetics. Whereas the general belief is that, at very low gas pressures, only the electronic ground state participates in atom recombination and that, in the liquid phase, at least most of the accessible states are coupled somewhere far out on the reaction coordinate, the transition between these two limits as a function of solvent density is by no means understood. Direct evidence for the participation of different electronic states in iodine geminate recombination in the liquid phase comes from picosecond time-resolved transient absorption experiments in solution [40, 44] that demonstrate the participation of the low-lying, weakly bound iodine A and A states, which is also taken into account in recent mixed classical-quantum molecular d5mamics simulations [42. 43]. 

Several completely different experiments support our interpretation of the time-of-flight transport process and the conclusions we have drawn about the distribution of band-tail states. The time-resolved photoinduced absorption experiments of Ray etal. 9% ) support the view that the photogenerated holes are concentrated in the vicinity of an energy E, which moves deeper into the localized state distribution, linearly with temperature and logarithmically with time. Furthermore, the time decay of the photoinduced absorption, which is controlled by the more mobile of the two carriers (electrons), has the t form expected from the multiple trapping model (see, for example, Orenstein eta/., 1982). Thea = r/300°K temperature dependence for a reported by Tauc (1982) is in excellent agreement with the electron time-of-flight results. 

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