Electron-transfer dynamics fluorescence

A series of chlorophyll-like donor (a chlorin) linked having C60 (chlorin-C60) or porphyrin-C60 dyads with the same short spacer have been synthesized as shown in Schemes 13.1 and 13.2 [39, 40]. The photoinduced electron-transfer dynamics have been reported [39, 40]. A deoxygenated PhCN solution containing ZnCh-C60 gives rise upon a 388-nm laser pulse to a transient absorption maximum at 460 nm due to the singlet excited state of ZnCh [39]. The decay rate constant was determined as 1.0 X 10u s-1, which agrees with the value determined from fluorescence lifetime measurements [39]. This indicates that electron transfer from 1ZnCh to C60 occurs rapidly to form the CS state, ZnCh +-C60 . The CS state has absorption maxima at 790 and 1000 nm due ZnCh+ and C60, ... 

A detailed description of experimental setup of the femtosecond broadband pump-probe spectroscopy was given in Ref [11, 16]. Schematic representation of the experimental setup is shown in Figure 11.3. The combination of femtosecond broadband pump-probe spectroscopy, with an expanded spectral range, and picosecond fluorescence decay measurements permitted more detailed analysis of electron transfer dynamics [17-19]. The authors disentangled the various kinetic... 

In order to understand the nature of electron transfer dynamics in polymers, it is necessary to understand the electronic structure of polymer ion radicals and their interaction with the counter ion. These are closely related to configurational and conformational structures of polymers. Similar concerns occur in fluorescence studies of polymers where one finds that the electronic properties of the excited singlet state are sensitive to the structure of the main chain, the molecular weight of the polymer, and the position of the chromophore in the polymer structure. These factors determine the statistical distributions of inter-chromophoric distance and relative orientation resulting in characteristic fluorescence behavior. A similar relationship... 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

Keywords Excited-state intramolecular proton transfer Fluorescence dye Photoinduced electron transfer Proton coupled electron transfer Relaxation dynamics... 

An important question concerning energy trapping is whether its kinetics are limited substantially by (a) exciton diffusion from the antenna to RCs or (b) electron transfer reactions which occur within the RC itself. The former is known as the diffusion limited model while the latter is trap limited. For many years PSII was considered to be diffusion limited, due mainly to the extensive kinetic modelling studies of Butler and coworkers [232,233] in which this hypothesis was assumed. More recently this point of view has been strongly contested by Holzwarth and coworkers [230,234,235] who have convincingly analyzed the main open RC PSII fluorescence decay components (200-300 ps, 500-600 ps for PSII with outer plus inner antenna) in terms of exciton dynamics within a system of first order rate processes. A similar analysis has also been presented to explain the two PSII photovoltage rise components (300 ps, 500 ps)... 

When using DCE as quencher (Fig.3a), the second deactivation channel remains open until Ati=l ns. The small free ion yield for this system indicates that charge recombination is much more efficient and therefore much more rapid than charge separation. Furthermore, we know from time resolved fluorescence decay measurements that electron transfer quenching is still not finished at At =lns. Consequently the absorbance should be dominated by fresh geminate ion pairs, over the whole timescale investigated. Thus, the GSR dynamics of Pe + in presence of DCE should be independent of Ati, which is confirmed by our experimental observations. 

In the case of electron transfer reactions, besides data on the dynamic Stokes shift and ultrafast laser spectroscopy, data on the dielectric dispersion (w) of the solvent can provide invaluable supplementary information. In the case of other reactions, such as isomerizations, it appears that the analogous data, for example, on a solvent viscosity frequency dependence 17 ( ), or on a dynamic Stokes fluorescence shift may presently be absent. Its absence probably provides one main source of the differences in opinion [5, 40-43] on solvent dynamics treatments of isomerization. 

Figure 42 Stern-Volmer plots for fluorescence quenching of PBAC by Co(phen)3+ in the presence of 0.008% BAZrP. Using 200 nsec as the singlet lifetime of PBAC, the rate constant for quenching is calculated to be 3 X 1012 M-1sec. This is much too fast for a dynamic process and may involve long-range electron transfer. (From Ref. 17. Copyright 1995 Overseas Publishers Association.)...

With site-directed mutation and femtosecond-resolved fluorescence methods, we have used tryptophan as an excellent local molecular reporter for studies of a series of ultrafast protein dynamics, which include intraprotein electron transfer [64-68] and energy transfer [61, 69], as well as protein hydration dynamics [70-74]. As an optical probe, all these ultrafast measurements require no potential quenching of excited-state tryptophan by neighboring protein residues or peptide bonds on the picosecond time scale. However, it is known that tryptophan fluorescence is readily quenched by various amino acid residues [75] and peptide bonds [76-78]. Intraprotein electron transfer from excited indole moiety to nearby electrophilic residue(s) was proposed to be the quenching... 

Very fast electron transfers from P+ to bacteriochlorophyl (Bchl) and from (Bchl)- to QA do not depend on media dynamics and occur via conformationally non-equilibrium states (Fig.3.18). The dual fluorophore-nitroxide molecules (D-A) are also convenient objects for analysing the activity-dynamics relationship. The marked irreversible photoreduction of the nitroxide fragment of the dual probe incorporated into the binding site of HSA only took place when the nanosecond dynamical processes around the probe traced by ESR and fluorescence methods were detected (Rubtsova et al., 1993, Fogel et al, 1994 Likhtenshtein, 1986 Lozinsky et al., 2002). Similar results were reported for another model protein system, i.e. a-chymotrypsin with spin labeled methionin-92 groups (Belonogova et al., 1997). In the latter enzyme, the excited tryptophan group serves as an electron donor. 

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