Polarizable donor-acceptor complexes

Electron Transfer in Polarizable Donor-Acceptor Complexes... 

In principle, the behaviour of any molecular species in forming donor-acceptor complexes depends on its ionization potential, electron affinity and polarizability. However, the donor (or acceptor) ability of a substance depends strongly on the requirements and properties of its partners. The same compound may act as a donor towards strong acceptor compounds or as an acceptor towards donor compounds. This is the case of the TT-amphoteric p-tricyanovinyl-AA/V-dimcthylaniline (41) which is a donor towards 2,4,7-trinitrofluorenone and an acceptor towards /V,/V-dirnclhy Ian Mine138. 

The ability of molecules to form donor-acceptor complexes depends not only on their ionization potential, electron affinity and polarizability, but also on the requirements and properties of partners. 

C6o fullerene is a new type of n-acceptors with a number of essential dissimilarities from another acceptor molecules large size, spherical form, unique electron structure, high symmetry and polarizability. These peculiarities introduce a certain specificity in donor-acceptor interactions in fullerene compounds. Fullerene is a rather weak acceptor. Adiabatic affinity for an electron in the solution is 2.1-2.2 eV [1]. One molecule of C(,o fullerene can accept to 12 electrons [1-4] and donate one electron [5], i.e. the charge on a C6o molecule can vary from +1 to -12. Polarizability of a C6o molecule is high (a 85 A3) and several times greater than that of other ji-acceptor molecules. Because of this, polarization Van der Waals forces are of essential importance in the formation of donor-acceptor complexes. 

When the electron is partially delocalized, one should switch to the adiabatic representation in which the upper and lower CT surface are split by an energy gap depending on P. If this energy gap is expanded in P with truncation after the second-order term, we come to the model of a donor-acceptor complex whose dipolar polarizabilities are different in the ground and excited states. The solute-solvent interaction energy then attains the energy of solute polarization that is quadratic in P... 

Both electronic delocalization and polarizability of the donor-acceptor complex lead to a significant asymmetry between the absorption and emission optical lines as is often observed in experiment.The importance of this effect can be assessed by comparing the dependence of the observed spectral width on solvent polarity with the prediction of MH theory. Equations [6] and [12] can be combined to give... 

We consider a generic donor-acceptor complex solute at infinite dilution in a polyatomic solvent. Both the solute and solvent molecules are represented by rigid and non-polarizable ISM models. In the ISM models the potential energy of interaction between two molecules is a sum of pairwise-additive site-site terms, including Coulombic interactions between partial charges located at the molecular sites. Throughout the paper the subscript A refers to interaction sites of the solute, while the subscript aj refers to interaction site j of solvent molecule a. 

This event leads to the formation of an electron donor-acceptor (EDA) complex involving the formation of a coordinate link between D and M. The availability of the additional electron pair at M causes an increase in electron density at X due to further polarization of the M-X bond. It is apparent that the amount of polarization will depend on both the polarizability of the covalent bond as well as the extent of interaction between D and M. For a given substrate the latter will depend on the donor properties of the donor8). 

Later on, Pearson [75] introduced the concept of hard and soft acid and bases (HSABs) hard acids (defined as small-sized, highly positively charged, and not easily polarizable electron acceptor) prefer to associate with hard bases (i.e., substances that hold their electrons tightly as a consequence of large electronegativities, low polarizabilities, and difficnlty of oxidation of their donor atoms) and soft acids prefer to associate with soft bases, giving thermodynamically more stable complexes. According to this theory, the proton is a hard acid, whereas metal cations may have different hardnesses. 

If a highly polarizable group is introduced into a receptor molecule, substrate binding should cause substantial perturbations, so that the recognition event would be converted into a non-linear optical signal. Such recognition-dependent nonlinear optical probes may be derived for instance from polyenes such as those shown in Figure 20, from inclusion complexes [8.94a] or from donor-acceptor calixarenes [8.94b]. 

However, it is sometimes profitable to compare the relative stabilities of ions differing by unit charge when surrounded by similar ligands with similar stereochemistry, as in the case of the Fe3+—Fe2+ potentials (Table 17-1), or with different anions. In these cases, as elsewhere, many factors are usually involved some of these have already been discussed, but they include (a) ionization enthalpies of the metal atoms, (b) ionic radii of the metal ions, (c) electronic structure of the metal ions, (d) the nature of the anions or ligands involved with respect to their polarizability, donor pir- or acceptor d77-bonding capacities, (e) the stereochemistry either in a complex ion or a crystalline lattice, and (f) nature of solvents or other media. In spite of the complexities there are a few trends to be found, namely ... 

In the simplest case of a donor-acceptor (D-A) molecule, the nonlinear optical activity arises from the electric-field-induced mixing of electronic states such as D-A and D+-A . This makes the response (polarizability) of the molecule different according to the sense of the electric field, and a second-order hyperpolarizability fi coefficient) is observed. If D and A are connected by some bridge, its role in promoting the electronic interaction will be quite similar to the bridge role in mixed-valence complexes. Metal complexes can play the role of donor or acceptor groups. Recent examples have been described with ferrocene or ruthenium(pentaammine) groups [48], but they are either monometallic or too short to be considered in this review. 

In fact, as coordination and organometallic complexes may possess intense, low-energy MLCT, ligand-to-metal CT (LMCT), or intraligand CT (ILCT) excitations, the metal can effectively act as the donor, the acceptor, or the polarizable bridge of a donor-acceptor network. Finally, metal ions are well suited to build molecular structures based on octupolar coordination of organic ligands with D2 or D3 symmetry. 

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