Secondary charge separation

Chemical reaction of the excited state (secondary charge separation, isomerisation, disassociation etc). 

Figure 19. Two types of triads for photoinduced charge separation. Molecular components are designated as P (chromophore) D (donor) A (acceptor) A (secondary acceptor). Electron transfer processes are designated as cs (primary PET) cr (primary charge recombination) cs (secondary charge separation) cr (final charge recombination),...

The photoinduced and thermal behavior is described for the second-generation dendrimer where (i) excitation of ZnP units results in photoinduced energy transfer (PEnT) to the free-base porphyrin layer (ii) the excited P layer transfer electrons to the AuP core and (iii) a secondary charge-separated state is generated by hole transfer from the oxidized P layer to ZnP units, as shown below ... 

There is evidence, both experimental and theoretical, that there are intermediates in at least some Sn2 reactions in the gas phase, in charge type I reactions, where a negative ion nucleophile attacks a neutral substrate. Two energy minima, one before and one after the transition state, appear in the reaction coordinate (Fig. 10.1). The energy surface for the Sn2 Menshutkin reaction (p. 499) has been examined and it was shown that charge separation was promoted by the solvent.An ab initio study of the Sn2 reaction at primary and secondary carbon centers has looked at the energy barrier (at the transition state) to the reaction. These minima correspond to unsymmetrical ion-dipole complexes. Theoretical calculations also show such minima in certain solvents, (e.g., DMF), but not in water. "... 

JCS(P1)1113]. The formation of the trans adduct involves a boatlike endo transition state (110 versus 111), which is enhanced in aqueous solution by some extra charge separation resulting from both secondary orbital interaction and by a hydrophobic packing effect of the substrate (94JOC1358, 94TL595). 

Although pyrrole is a very weak acid (pA 17.7), it can be deprotonat-ed by a strong base, such as butyllithium. Its acidity is much greater than a typical aliphatic secondary amine, say pyrrolidine [tetrahydropyrrole, (pAT ca. 27)]. Unlike pyrrole itself, resonance within the pyrrole anion does not involve charge separation (Scheme 6.8). 

Finally, solute radical ions can be generated by light-induced, one-photon or multiphoton ionization of their parent compounds (Chaps. 5 and 16). This approach is particularly useful in the ultrafast studies of short-lived, unstable radical ions that aim to unravel their solvation, recombination, reaction, and vibrational relaxation dynamics of the primary charges (see, e.g., Chap. 10). Whereas the time scale of radiolytic production of secondary ions is always limited by the rate with which the primary species reacts with the dispersed parent molecules, light-induced charge separation can occur in <100 fsec. There are many studies on photoionization of solute molecules in liquid solutions we do not intend to review these works. 

A related series of complexes is formed with ammonia some of these complexes have considerable stability such as LiI 4NH3, in which the cation is believed to be tetrahedrally coordinated and charge-separated [Li(NH3)4]+. Results of studies on the dissociation properties show that the stability of the ammines falls off in the sequence Li+ > Na+ > K+, and with halides of the same metal in the order I- > Br- > Cl-.38 The ammonia may be replaced by primary, secondary and tertiary amines but the products are less stable. 

Since the initial reports of the C-P-Q triads, a number of other molecules of the D-D -A or D -D-A types have been described. Triad 12, prepared by Wasielewski and coworkers, is a relative of the C-P-Q series in which the secondary donor is an aniline derivative (D), rather than a carotenoid [63]. The bicyclic bridges were introduced in order to add rigidity to the system. The fluorescence lifetime of the porphyrin moiety of 12 was found to be <30ps. This result is consistent with rapid electron transfer to the quinone to yield D-P+-QT. This result was confirmed by transient absorption measurements. The absorption results also revealed that this intermediate charge separated state decays with a rate constant of 1.4 x 1010 s-1 to a final charge separated state D+-P-Qr. Thus, the decay pathways are similar to those shown in Fig. 3 for the C-P-Q triads. This final state has a lifetime of 2.45 ps in butyronitrile (which is similar to that found for 4 in acetonitrile) [44], and is formed with a quantum yield of about 0.71. Thus, the efficiency of the transfer analogous to step 4 in Fig. 3 for this molecule is also about 0.71. 

This exciplex has a large amount of charge transfer character, as shown by the solvent dependence of its fluorescence emission spectrum. The exciplex can then receive an electron from the secondary donor to form the final charge separated state ... 

PET across membranes is extensively discussed in the first contribution with emphasis on primary photochemical charge separation processes and secondary recombination reactions. Mainly vesicles and planar bilayer membranes serve as models which allow the spatial separation of photochemically generated oxidants and reductants. 

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