Expanded donor—acceptor systems

Nine years later, Leonhardt and Weller detected an excimer type emission in solutions containing perylene and dimethylaniline [80]. This first heteroexcimer has become the prototype of an ever expanding area of research. Perhaps the impact of these observations are best illustrated by the monograph dealing with the new phenomenom published only 12 years after the first report [81]. The significance of this research for the proper understanding of photo-induced electron transfer is born out by the first positive identification of a radical anion resulting from the irradiation of a donor-acceptor system in polar solvents (vide infra) [82]. 

Donor-acceptor jr-jr stacking has been recognised as a valuable interaction for many years, with several detailed mmiographs produced on these systems over 50 years ago [39, 40]. Over the intervening years, the number of structural motifs that can form charge-transfer (CT) complexes has expanded to such an extent that a comprehensive list is not practical to compile for illustrative purposes, four of the most commonly used donor and acceptor motifs are shown in Fig. 2. 

On irradiation with light, CT complexes convert to the excited state and charges are transferred. The excited CT complex obtained as a single molecule with an extensive 7r-orbital system represents a single monomer molecule. The expanded 7r-electron system is readily polarized and, so, requires little activation energy. The CT complex is then the most reactive species in a mixture of excited CT complexes with their homopolymerizable electron donors and acceptors. 

If the number of electron pairs for two chemical species (e.g.. reactant and product) is equal, it will matter whether the space (MO) occupied by each electron pair is different in the two systems, e.g., expanded in one case, contracted in the other, delocalized or localized, more or less spherical, more or less crowded due to the vicinity of other electron pairs, etc. Significant differences in the distribution of electron pairs occur if one compares a TS of a pericyclic reaction with reactants or products (reaction barrier), a molecule with classical bonding with an isomer possessing nonclassical (delocalized) bonding (molecular stability), a donor-acceptor complex with its separated parts (complex binding energy), isoelectronic species such as carbocations and boranes, etc. In all these cases, dynamic electron correlation should play an important role. 

The solubility parameter approach was subsequently expanded from three to four terms with the division of the hydrogen-bonding parameter into acidic and basic solubility parameters to quantify electron-donor and electron-acceptor properties [48,49], However, the expansion of these solubility parameter terms did not make the equation any easier to use foptfiBi prediction of solubility in cosolvent systems. 

Lewis defined a base as an electron-pair donor and an acid as an electron-pair acceptor. This definition further expands the list to include metal ions and other electron pair acceptors as acids and provides a handy framework for nonaqueous reactions. Most of the acid-base descriptions in this book will use the Lewis definition, which encompasses the Brpnsted-Lowry and solvent system definitions. In addition to all the reactions discussed previously, the Lewis definition includes reactions such as... 

Let us see whether this mechanism is even more general and consider the electrophilic substitution in the model reaction H+ - - H-H -> H-H - - H+. This time, the role of the donor is played by the hydrogen molecule, while that of the acceptor is taken by the proton. The total number of electrons is only two. The DA structure corresponds to (x) ( ) (x ) - Other structures are defined by a full analogy with the previous case of the system structure D+A means structure D+ A obviously corresponds to (x) ( )Hx ) structure D Ato (x) ( )°(X ) D Ato (x)°( )°(x ) andD+ A to (x)°( ) (x )°-As before, the ground-state Slater determinant may be expanded into the contributions of these structures. The results (the overlap neglected) are collected in Table 14.5. 

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