Electron-transfer donor-acceptor salts

Unimolecular electron transfer of the salt bridge associated donor-acceptor pair. 

Dye photooxidation leading to subsequent polymerization requires moleeules that are strong electron acceptors in the ground state. Systems eomposed of onium salts, reducing agents, and sensitizer/electron transfer donors are examples of such systems. They are excellent photoinitiators for the polymerization of acrylates [2-4]. 

Zgf 2. In simple salts, it is the electron transfer from each donor to each acceptor molecule. In complex salts, it can be taken as the mean electron transfer per acceptor molecule. Suppose that is merely the bulk equilibrium value of a quantity z which can vary continuously throughout the allowed range of zq, 0 i. z t 2. However, as z varies, no change is allowed in the molecular structure of the crystal the structure is fixed to be that of the equilibrium salt, zq. z = 0 corresponds, therefore,to a system of neutral molecules in the structure of the charge transfer salt. 

Recent advances in laser spectroscopy (femtosecond time scale) have made it possible to determine the rate constants for decarboxylation (fcjJ and back-electron transfer (fc et) in the photolysis of electron donor/acceptor salts such as methylviologen/benzilates (MV +/Ar2C(0H)C02) and methylviologen/ary-lacetates (MV VArGHjCOj) (Scheme 7). The values are much higher for the benzilates (2-8 x 10" s ) than for the arylacetates (1-2 x 10 s" ). Decarboxylation of these donors is thus almost a barrier-free... 

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. 

Much stronger donor-acceptor interactions stabUze D+A too much to give rise to the pseudoexcitation. The electron transferred configuration is stable and predominant. Electrons transfer to generate ion radical pairs or salts. Covalent bonds do not form and electron transfer results. 

For some pairs of strong donors and acceptors, D A is too stabilized for the delocalization and for the pseudoexcitation. One electron transfers from the donors to the acceptors instead. No bonds but ion radical pairs or salts form between the donors and acceptors. However, the electron transfers can be followed by reactions. This mechanistic band is here termed simply, transfer band . 

The wide diversity of the foregoing reactions with electron-poor acceptors (which include cationic and neutral electrophiles as well as strong and weak one-electron oxidants) points to enol silyl ethers as electron donors in general. Indeed, we will show how the electron-transfer paradigm can be applied to the various reactions of enol silyl ethers listed above in which the donor/acceptor pair leads to a variety of reactive intermediates including cation radicals, anion radicals, radicals, etc. that govern the product distribution. Moreover, the modulation of ion-pair (cation radical and anion radical) dynamics by solvent and added salt allows control of the competing pathways to achieve the desired selectivity (see below). 

Importantly, the purple color is completely restored upon recooling the solution. Thus, the thermal electron-transfer equilibrium depicted in equation (35) is completely reversible over multiple cooling/warming cycles. On the other hand, the isolation of the pure cation-radical salt in quantitative yield is readily achieved by in vacuo removal of the gaseous nitric oxide and precipitation of the MA+ BF4 salt with diethyl ether. This methodology has been employed for the isolation of a variety of organic cation radicals from aromatic, olefinic and heteroatom-centered donors.174 However, competitive donor/acceptor complexation complicates the isolation process in some cases.175... 

Reactions of highly electron-rich organometalate salts (organocuprates, orga-noborates, Grignard reagents, etc.) and metal hydrides (trialkyltin hydride, triethylsilane, borohydrides, etc.) with cyano-substituted olefins, enones, ketones, carbocations, pyridinium cations, etc. are conventionally formulated as nucleophilic addition reactions. We illustrate the utility of donor/acceptor association and electron-transfer below. 

Despite considerable efforts, the formulation in equation (42) remains incomplete owing to the high reactivity of organocuprates as well as their oligomeric nature. Accordingly, we select organoborates as stable electron donors to study alkyl additions to various pyridinium acceptors (by thermal and photoinduced electron transfer) via charge-transfer salts as follows. 

The salt effects just considered are counterion effects. Sometimes, however, an added salt can induce electron transfer from a donor to an acceptor. Here are several examples. 

All these data verify that in real systems, the rate of electron transfer between components of a conductive chain is high. There are states of a mixed valence. Enhanced electric conductivity and other unusual physical properties are widespread among those inorganic or coordination compounds that contain metals in intermediate -valence states. In cases of organic metals, nonstoi-chiometric donor/acceptor ratios provide even better results. For example, the salt of (TTF)i (Br)oj composition displays an electric conductivity of 2 X 10 cm while (TTF)i(Br)i salt does not... 

The electrical conductivity of TTF TCNQ is of the order of 10 S m at room temperature and increases with decreasing temperature until around 80 K when the conductivity drops as the temperature is lowered. TCNQ is a good electron acceptor and, for example, accepts electrons from alkali metal atoms to form ionic salts. In TTF-TCNQ, the columns of each type of molecule interact to form delocalised orbitals. Some electrons from the highest energy filled band of TTF move across to partly fill a band of TCNQ, so that both types of columns have partially occupied bands. The number of electrons transferred corresponds to about 0.69 electrons per molecule. This partial transfer only occurs with molecules such as tetrathiafulvalene whose electron donor ability is neither too small nor too large. With poor electron donors, no charge transfer... 

Two questions are inseparable how to optimize ion radical reactions, and how to facilitate electron transfer. As noted in the preceding chapters, electron transfers between donors and acceptors can proceed as outer-sphere or inner-sphere processes. In this connection, the routes to distinguish and regulate one and another process should be mentioned. The brief statement by Hubig, Rathore, and Kochi (1999) seems to be appropriate Outer-sphere electron transfers are characterized by (a) bimolecular rate constants that are temperature dependent and well correlated by Markus theory (b) no evidence for the formation of (discrete) encounter complexes (c) high dependence on solvent polarity (d) enhanced sensitivity to kinetic salt effects. 

The most prospective donors are those with ionization potentials of ID < 6.6 eV. Acceptors with electron affinities of EA > 2.6 eV are suitable. When / EA < 4 eV, donor-acceptor interaction leads to strong molecular complexes with a charge-transfer degree >0.5. Donor-acceptor charge transfer often results in the formation of ion radical salts having metallic conductivity. In terms of charge-transfer degree, ion radical salts have values >0.7. 

Immediately upon excitation of an IPCT band with a fs laser at 400 nm, transient absorption was observed for both salts in solutions with a peak at about 600 nm, characteristic of 4,4/-bipyridinium radical cations. Figure 20 shows the transient absorption spectra of PV2+(I )2 in methanol solution. A marked increase in the absorbance of the 4,4/-bipyridinium radical cations took place within 1 ps after excitation. 4,4/-Bipyridinium radical cations were thus formed in a fs time scale by the photoinduced electron transfer from a donor I- to an acceptor 4,4/-bipyridinium upon IPCT excitation [48], The time profiles of transient absorption at 600 nm are shown in Fig. 21 for (a) PV2+(I )2 in a film cast from DME and (b) PV2+(TFPB )2 in DME solutions. Both of them showed a very rapid rise in about 0.3 ps, which was almost the same as the time resolution of our fs Ti sapphire laser measurement system with a regenerative amplifier. Similar extremely rapid formation of 4,4/-bipyridinium radical cations was observed for PV2+(I )2 salts in methanol and dimethylsulfoxide solutions upon IPCT excitation, respectively. These results demonstrated that the charge separated 4,4/-bipyridinium radical cations were formed directly upon IPCT excitation because of the nature of IPCT absorption bands (that the electrons correlated with the IPCT band are transferred partially at the ground state and completely at the excited state). Such a situation is very different from usual photochromism which is caused by various changes of chemical bonds mainly via the excited singlet state. No transient absorption was observed for PV2+(I )2... 

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