Intramolecular and Intermolecular Electron Transfer

Electron transfer between two molecules, e.g. M and N, is one of the most fundamental and widespread of all photoinduced chemical reactions. It is at the basis of photosynthesis (section 5.1) in nature, and of photography (section 6.1) in industrial applications. 

In the first case, M is the electron donor and N is the electron acceptor, these roles being reversed in the second case. These properties of donor and acceptor are relative, the same molecule M being a donor towards some species N and an acceptor towards other partners N. It will be seen presently that the direction of electron transfer depends simply on the energy balance of the reactions. 

In a point charge model the Coulomb term is simply C = qq jrD where q and q are the charges of the ions which are separated by a distance r in a solvent of dielectric constant D. This usual form of the Coulomb term may not be valid when the molecules are very close together, because there is then no solvent between the point charges, and a modified expression is then C = qq far. 

In the presence of oxygen the radicals can react to form oxidation products such as carbonyl derivatives, carboxylic acids, etc. 

The importance of electron transfer as a primary chemical reaction has become increasingly recognized in recent years, and its detailed mechanism is a subject of active research. In most cases the direct contact of the donor and acceptor molecules seems to be necessary for efficient electron transfer, particularly for photoinduced electron transfer which is inevitably limited by the lifetime of the excited state molecule M. In principle, electron transfer could also take place between distant molecules by the mechanisms of electron hopping or of electron tunnelling . 

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Application of pulse-radiolysis techniques revealed that the following intramolecular and intermolecular electron-transfer reactions all exhibit a significant acceleration with increasing pressure. The reported volumes of activation are -17.7 0.9, 18.3 0.7, and... 

Coropceanu, V., Andre, J.M., Malagoh, M., and Bredas, J.L., The role of vibronic interactions on intramolecular and intermolecular electron transfer in p-conjugated oligomers, Theor. Chem. Accounts, 110, 59, 2003. 

Another example has been reported of a direct comparison of rates of intramolecular and intermolecular electron transfer between the same or very similar pairs of oxidizing and reducing centres. Previously, Hofmann and Simic had generated... 

Photoinduced electron transfer occurs through excitation of the 400-nm absorption bands of the donor chromophores based on the aminonapthalene-dicarboximide derivatives. The tails of the dopants absorption bands extend to at least 500 nm, which allows for the use of an Ar+ laser. Figure 8 illustrates the ground-state absorption spectra of the donor and acceptor for both the intramolecular and intermolecular charge transfer dopants in toluene. The spectra are similar for all of the dopants, with the exception of 2, which has a 50-nm red-shifted absorption band. The inset illustrates the broadened spectra in the liquid crystalline environment. The extinction coefficient at 457 nm varies from approximately 1000 M-1 cm-1 for 4, 2000 M-1 cm-1 for 1, 5000 M-1 cm-1 for 3, and 10,000 M-1 cm-1 for 2. 

Switching systems based on photochromic behavior,I29 43,45 77-100 optical control of chirality,175 76 1011 fluorescence,[102-108] intersystem crossing,[109-113] electro-chemically and photochemical induced changes in liquid crystals,l114-119 thin films,170,120-1291 and membranes,[130,131] and photoinduced electron and energy transfer1132-1501 have been synthesized and studied. The fastest of these processes are intramolecular and intermolecular electron and energy transfer. This chapter details research in the development and applications of molecular switches based on these processes. 

The next important phenomena that the result of supramolecular effect are the concentration and proximity effects concerning the components of analytical reaction, even through they are considerably different in hydrophobicity, charge of the species, complexing or collisional type of interaction. The concentration and proximity effects determine the equilibrium of analytical reaction, the efficiencies of intramolecular or intermolecular electronic energy or electron transfer and as a result the sensitivity of analytical reactions. 

We will use here the main results obtained for two complex and distinct situations the structural and spectroscopic information gathered for D. gigas [NiFe] hydrogenase and AOR, in order to discuss relevant aspects related to magnetic interaction between the redox centers, intramolecular electron transfer, and, finally, interaction with other redox partners in direct relation with intermolecular electron transfer and processing of substrates to products. 

Nevertheless, there are two highly efficient CL systems which are believed to involve the CIEEL mechanism in the chemiexcitation step, i.e. the peroxyoxalate reaction and the electron transfer initiated decomposition of properly substituted 1,2-dioxetanes (Table 1)17,26 We have recently confirmed the high quantum yields of the peroxyoxalate system and obtained experimental evidence for the validity of the CIEEL hypothesis as the excitation mechanism in this reaction. The catalyzed decomposition of protected phenoxyl-substituted 1,2-dioxetanes is believed to be initiated by an intramolecular electron transfer, analogously to the intermolecular CIEEL mechanism. Therefore, these two highly efficient systems demonstrate the feasibility of efficient excited-state formation by subsequent electron transfer, chemical transformation (cleavage) and back-electron transfer steps, as proposed in the CIEEL hypothesis. 

The above CT systems represent the case for intermolecular electron transfer. There is some analogy to proton transfer in acid-base reactions [5]. We have also examined intramolecular electron transfer systems and studied the influence of IVR and geometric changes this work is detailed elsewhere [5]. Other reactions involving ultrafast electron transfer are those of harpooning in Xe + I2) Nal, and more recently Xe/Ch (see Ref. 1). 

Spectroscopic methods can be used to specify the position of donors and acceptors before photoexcitation [50]. This spatial arrangement can obviously influence the equilibrium eomplexation in charge transfer complexes, and hence, the optical transitions accessible to such species [51]. This ordered environment also allows for effective separation of a sensitizing dye from the location of subsequent chemical reactions [52], For example, the efficiency of cis-trans isomerization of A -methyl-4-(p-styryl)pyridinium halides via electron transfer sensitization by Ru(bpy) + was markedly enhanced in the presence of anionic surfactants (about 100-fold) [53], The authors postulate the operation of an electron-relay chain on the anionic surface for the sensitization of ions attached electrostatically. High adsorptivity of the salt on the anionic micelle could also be adduced from salt effects [53, 54]. The micellar order also influenced the attainable electron transfer rates for intramolecular and intermolecular reactions of analogous molecules (pyrene-viologen and pyrene-ferrocene) solubilized within a cationic micelle because the difference in location of the solubilized substances affects the effective distance separating the units [55]. 

In a supramolecular approach to fullerene-porphyrin hybrids, the assembly of a rigidly connected dyad, in which a zinc tetraphenylporphyrin, Zn(TPP), is noncovalently linked to a C60 derivative via axial pyridine coordination to the metal, was reported [219-222]. Photo excitation of the dyad Zn-complex led to electron transfer with very long lifetimes of the charge-separated pairs, as revealed by optical spectroscopy and confirmed by time-resolved electron paramagnetic resonance spectroscopy. Accordingly, two different solvent-dependent pathways can be considered for the electron-transfer processes. Either the excitation of the porphyrin chromophore is followed by fast intramolecular electron transfer inside the complex, or alternatively the free porphyrin is excited undergoing intermolecular electron transfer when the acceptor molecules ap-... 

Chromate(VI) has been reported to undergo reduction to Crv as a result of PET between its LMCT excited state and an external electron donor. In the study carried out for several aliphatic alcohols (methanol, ethanol, propan-2-ol, butan-1-ol, butan-2-ol, 2-methyl-propan-2-ol) two pathways of PET were identified one-electron transfer for intermolecular and two-electron transfer for intramolecular systems [96,97]. The intermolecular mechanism of the CrVI excited state quenching was also found for phenol or its derivatives [98], whereas in the case of an anion donor (such as oxalate) an effect of external cations was observed [99],... 

In an intramolecular example of photodechlorination implying a phenolate anion, a triplet sensitizer and triethylamine, a cyclization of perchlorinated o-phenoxyphenol to octachlorodibenzodioxin is observed. An intramolecular electron transfer from the polychlorinated phenolate moiety to the perchloro-phenyl residue is considered together with an intermolecular electron transfer from external triethylamine followed by an intramolecular SrnI path for the ring closure [177]. 

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