SPV

Figure 10 illustrates Stern-Volmer plots for the fluorescence quenching of APh-x by MV2+ and SPV in aqueous solution [74]. With MV2+, the quenching is so effective that it occurs at very low quencher concentrations (in the range of 10 6 M), whereas with SPV, it proceeds to about the same extent at two-orders of magnitude higher quencher concentration (in the range of 10 4 M). 

Table 2. Rate constants for the fluorescence quenching of APh-8 (8) by SPV (14) and MV2 + (13) in aqueous solution [74]...

These findings can be attributed to the increase in the local concentration of MV2+ on the APh-x molecular surface caused by eletrostatic interactions. In contrast, the quenching constants for MV2 + and SPV show no such large difference in the SDS micellar and AM systems. The addition of NaCl reduces the value of kq to about one-third that for the quenching of APh-9 (APh-x with 9 mol% Phen units) by MV2 + in a salt-free solution. This effect is mainly accounted for by the screening of electrostatic attraction between APh-9 and MV2+. 

The DPA moiety is less active in forming the CT complex with viologens than the pyrene moiety e.g., for PMAvDPA the KCT values with MV2+ and SPV are 1.3 x 103 M 1 and almost zero, respectively, at pH 8-9 [60, 77], whereas for PMAvPY they are 7.8 xlO4 and 6.3 x 102 M, respectively, at pH 11 [77]. Therefore, the polymer-bound pyrene system undergoes much more static quenching than the polymer-bound DPA system. As will be discussed in Chapter 6, it is very important for charge separation whether the fluorescence quenching is static or dynamic. 

Morishima et al. [75, 76] have shown a remarkable effect of the polyelectrolyte surface potential on photoinduced ET in the laser photolysis of APh-x (8) and QPh-x (12) with viologens as electron acceptors. Decay profiles for the SPV (14) radical anion (SPV- ) generated by the photoinduced ET following a 347.1-nm laser excitation were monitored at 602 nm (Fig. 13) [75], For APh-9, the SPV- transient absorption persisted for several hundred microseconds after the laser pulse. The second-order rate constant (kb) for the back ET from SPV- to the oxidized Phen residue (Phen+) was estimated to be 8.7 x 107 M 1 s-1 for the APh-9-SPV system. For the monomer model system (AM(15)-SPV), on the other hand, kb was 2.8 x 109 M-1 s-1. This marked retardation of the back ET in the APh-9-SPV system is attributed to the electrostatic repulsion of SPV- by the electric field on the molecular surface of APh-9. The addition of NaCl decreases the electrostatic interaction. In fact, it increased the back ET rate. For example, at NaCl concentrations of 0.025 and 0.2 M, the value of kb increased to 2.5 x 108 and... 

Retardation of back ET was also observed with phenanthrene solubilized in the SDS micelle (kb = 6.8 x 107 M-1 s-1) (see Fig. 13) [75]. However, as can be seen from Fig. 13, the transient yield of SPV- for the micellar system is extremely low, presumably because only a small fraction of SPV- can escape from the geminate ion pair. This finding implies that SPV preferably resides inside the micelle and that the electron transfer mainly takes place in the micelle, not across the charged surface. 

For QPh-x, the decay of SPV- was faster than that for the APh-x-SPV system, and obeyed first-order kinetics [76]. The addition of NaCl (0.2 M) caused the decay profile to change i.e. the back ET was considerably slowed and the decay kinetics changed from first-order to second-order, with a reaction rate constant of kb = 1.8 x 109 M-1 s-1. These findings suggest the impossibility of escape of... 

SPV- from the electric field of the polycation, which leads to a first-order back ET kinetics. Since the addition of NaCl interferes with the electrostatic binding of SPV- by QPh-14, SPV- can escape into the bulk phase by diffusion. Therefore, the back ET occurs via a bimolecular process when NaCl is added. 

MV2 + or SPV as an acceptor [125, 126], They have concluded that the steric protection of chromophores from the quencher crucially affects the efficiency of the initial charge separation. This conclusion is essentially the same as that reached by Morishima et al. [119, 120],... 

Two key parameters have been found to determine the ultimate eapabili-ties of the MOEMS-based MOS contrast and spectral photometric variation (SPV). Contrast is dehned as the total amount of non-selected flux of light passing through the multi-slit device a contrast requirement of 2000 has been established in order to avoid pollution of the spectra by strong sources (spoilers) and sky background. The SPV is the unpredictable photometric variation due to the random position of the sources on the PMSM grid SPV requirement has been fixed to <10%. 

Solar energy systems are solar home systems, solar photovoltaic (SPV) systems, solar water heating (SWH) systems, solar dryers, and solar cookers. These systems are installed and managed by a household or a small commirrrity. A solar home system is a PV system with a maximum capacity of 40 W. 

The reciprocal vectors defined in Section VIII may be used to construct a more direct derivation. Let b and m refer to reciprocal basis vectors defined using Spv and in Eqs. (2.207) and (2.208), respectively. By substituting definitions (2.207) and (2.208) into completeness relation (2.190), we find that... 

The final silver cluster diameter increases, at given initial Ag and PVA concentrations, for example from 15 to 50 nm (n 50 times larger), when the part of reduction is increasingly achieved by the donor SPV, rather than by radiolytic radicals [31]. A red shift correlates with the growth in size in the final optical surface plasmon band. Nonirradiated solutions of EDTA silver complex are stable because EDTA does not reduce the ions directly. However, after the appearance of the 400-nm spectrum of silver... 

Surface treatments of CD CdSe films deposited from selenosulphate/NTA solutions have a pronounced effect on various optical, electrical, and optoelectronic properties of the films, due to interaction with or modification of such surface states. Mild etching (dilute HCl) of the films reverses the direction of current flow both in CdSe/polysulphide photoelectrochemical cells [108] and in Kelvin probe surface photovoltage (SPV) measurements in air [109], These studies are discussed in more detail in Chapter 9, in Section 9.2 on photoelectrochemical cells. At this point, it is sufficient to state that the effect is believed to be due to preferential trapping of either electrons or holes at surface states that are modified by the etching process. 

Interestingly, non-metallic silver clusters, depending on their sizes, may act either as electron donors or as electron acceptors. Using sulfonatopropyl-viologen, SPV (E° for SPV/SPV- = — 0.41 V/NHE), pulse radiolysis established that small silver clusters (n < 4) acted as electron donors (Le. E° for (Ag /Agn < E° for SPV/SPV - ) while, conversely, large silver clusters (n 2 4) were electron acceptors (i.e. E° for Ag/ /Agn > E° for SPV/SPV- ) [511]. Size-dependent electrochemical potentials of silver aggregates have been elucidated (Fig. 82) [506]. 

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