Charge separated ion pair

Reaction of MgMe2 with iV,./V, iV"-trimethyl-l,4,7-triazaacyclononane (tacn) in C6H6 gave the charge-separated ion pair [(tacn)Mg(/i-Me)3Mg(tacn)]z+[Mg3Me8]2 43 (Figure 19) in 46% yield. The triple-decker cations have Mg-C distances of 2.35 A (av.).168... 

Figure 19 Structure of the charge-separated ion pair [(tacn)Mg(/i-Me)3Mg(tacn)]2+[Mg3 vie8]2- 43...

The spin properties of charge-separated ion pairs can also be exploited for the purposes of all optical switching. Radical pair intersystem crossing (RP-ISC) of the form [D+ -A- ] D+ -A- ] to yield the spin-correlated triplet state is observed in... 

The inhomogeneity of the micellar aggregate also affords assisted spin trapping and the exploitation of magnetic field effects on the charge separated ion pairs [48]. Optical modulation spectroscopy can be used, for example, to follow the decay of radicals formed in homogeneous solution and in SDS micelles. Enhancements of a factor of about 50 in the lifetimes and the steady state concentrations of the radical were observed in the micelle, and a kinetic analysis led to a value of 2 x 103 s 1 for the exit rate constant from the micelle [49]. 

Base-Initiated Polymerization. Although the base-initiated mechanism for reaction 2 was once thought to proceed via free ionic intermediates 31), little evidence supported this mechanism. Conductivities of solutions of alkali metal silanolates in moderately polar solvents are essentially nil (32), and most workers now agree that reactive intermediates consist of ion pairs or charge-separated ion pairs 4-5). The key intermediate is believed to involve coordination of the countercation of the ion pair at the chain end, for example potassium, with the cyclosiloxane in a manner analogous to the crown ethers or cryptates (33), as shown by structure 1. 

The electronic state of the switch is often read by using optical transient absorption and fluorescence emission spectroscopies. Fluorescence is a much more sensitive technique, and can be used even at the single molecule level. It also requires a much smaller density of photons to provide a stable signal however, very few charge-separated ion pair states exhibit radiative charge recombination. In these cases transient absorption is often used because it allows direct nondestructive observation of a molecule s electronic configuration. Problems with this technique... 

It is probably more like a dipolar charged ion, e.g. CaOH" (aq) or a charge-separated ion pair (Ca2+OH-)(aq). 

The lifetime of the final charge-separated ion pair, C -P-Q" , is about 300 ns in a dichloromethane solution (and about 2 jus in a butyro-nitrile solution). This lifetime is substantially longer when compared to porphyrin-quinone ion pairs, P -Q (ca. 100 to 200ps). Other D-S-A assemblies containing porphyrins as chromophores are exemplified in structures 3 and 4. In these triad assemblies an aniline residue provides the electron donor component [35], System 3, for example, exhibits structural rigidity, and thus defined spatial orientation between the donor and acceptor components is established. The sequence of ET reactions occurring upon excitation of the porphyrin is summarized by... 

The primary ion pair decays to the charge-separated state D -P-Q with a quantum efficiency corresponding to 0 = 0.71 and a rate constant of 1.4 X 10 °s The lifetime of the final charge separated ion pair is 2.45 fis, substantially longer than ordinary porphyrin quinone ion pairs, P -Q . 

Charge separation Ion — pair fmmation by ionization of exciton, applied potential or reaction with impurities Electron transfer reaction of the excited state with acceptor or donor... 

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