Charge separation affected

We have considered the weak van der Waals forces that cause the condensation of covalent molecules. The formation of an ionic lattice results from the stronger interactions among molecules with highly ionic bonds. But most molecules fall between these two extremes. Most molecules are held together by bonds that are largely covalent, but with enough charge separation to affect the properties of the molecules. These are the molecules we have, called polar molecules. 

Meisel etal. [18-20] were the first to investigate how the addition of a polyelectrolyte affects photoinduced ET reactions. They found that charge separation was enhanced as a result of the retardation of the back ET when poly(vinyl sulfate) was added to an aqueous reaction system consisting of tris(2,2 -bipyridine)ruthenium(II) chloride (cationic photoactive chromophore) and neutral electron acceptors [21]. More recently, Sassoon and Rabani [22] observed that the addition of polybrene (a polycation) had a significant effect on separating the photoinduced ET products in an aqueous solution containing cir-dicyano-bis(2,2 -bipyridine)ruthenium(II) (photoactive donor) and potassium hexacyano-ferrate(III) (acceptor). These findings are ascribable to the electrostatic potential of the added polyelectrolytes. 

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],... 

A para-substituent may stabilize mesomerically either the conjugate acid of an acid-base pair rather more than it stabilizes benzoic acid, or it may stabilize the conjugate base rather more than it stabilizes the benzoate anion. The first situation is found in car-boxonium ions [13], where the delocalization of the positive charge on to a mesomerically electron-donating substituent stabilizes the cation. A similar resonance in the benzoic acid molecule [14] involves a separation of charge and affects the binding of the proton... 

The photoinduced charge separation between N-alkyl-N -methyl-4,4 -dipyridinium dichloride (CnMV2+ Cn = dodecyl, tetradecyl, hexadecyl, and octadecyl) and Ru(bpy)j + was also strongly affected by the presence of CTAC micells 21 . Upon reduction by Ru(bpy)j +, the viologen aquires hydrophobic properties leading to solubilization into micells (Eq. (13)). The subsequent recombination reaction is retarded by the positive surface of the micell. This decreases the rate constant of the back electron transfer at least by 500. 

The ionic domain of polyelectrolytes also affects the photoinduced charge separation between the coexisting low molecular charged compounds38). The quenching of Ru(bpy)2 + by a zwitterion compound, dibenzylsulphonates viologen (18, BSV), was enhanced about 4 times by poly(vinylsulphate) (PVS)39), and the back reaction... 

There are several factors caused by the change of the particle size that affect the activity of particulate photocatalysis (1) surface area, (2) band energy shift, (3) accessibility to the surface, and (4) space for charge separation. 

Assuming that we have such a situation favorable for charge separation, we have to consider what factors influencing the efficiency of charge separation in an illuminated semiconductor electrode are affected by crystal orientation or crystal imperfections. Five such factors are listed in the following table ... 

Azo compound decomposition is much less susceptible to polar substituent effects, and so probably has less charge separation in the transition state,75 but is more sensitive to geometrical restrictions. Bridgehead azo compounds decompose at rates lower than expected on the basis of their tertiary nature, whereas peresters are much less strongly affected.70 The difference can be rationalized by the proposal that the transition state comes farther along the reaction coordinate in azo decomposition, so that the nonplanarity forced on the incipient radical by the ring system is felt more strongly there. 

Under some conditions, effects arising from interconversion of singlet and triplet forms of the charge separated state could also affect the lifetime. Such effects have been ignored in the above discussion. 

Transient absorption studies demonstrated that the yield of the final C+-P-Q T state for 10 is 0.30 in dichloromethane. This increase by a factor of 7 over that found for 4 must be due to a higher efficiency for step 4, as was hoped for. Indeed, the efficiency of this step has been increased from 0.04 in 4 to 0.73 in 10. Thus, fine-tuning of the energetics of the various electron transfer steps in the C-P-Q triads is indeed a powerful tool for controlling quantum yield without necessarily affecting the amount of energy stored in the final charge separated state (this is essentially the same for both 4 and 10). 

A relative of 12 has been prepared by Sanders, van der Plas, and coworkers [64], Triad 13 features an N,A-dimethylaniline-type donor and an anthraquinone acceptor. These moieties are linked to the ortho positions of the porphyrin aryl groups, and this leads to a folded conformation for the molecule, as determined from NMR studies. Both the free base and zinc derivatives of 13 were prepared. The folded conformation might be expected to facilitate electron transfer among the components of the triad, and while this could enhance the quantum yield of the initial charge separated state, it might unfavorably affect the yield of the final D+-P-QT state and its lifetime. Unfortunately, photochemical or spectroscopic studies were not reported. 

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