Michael R. J. Dack Australian Notionol University Canberra, A.C.T. 2600. Australia
Charge-Transfer Complexes and Photochemistry
The formation and properties of ionic and covalent bonds have received a great deal of attention over the years. Much less is known about non-bonded interactions in solution. Molecules attract or repel each other in a variety of ways, the least efficient of these being through van der Waals and dispersion forces. Dipole-dipole and dipoleinduced dipole interactions are very weak compared to bonded interactions, but their importance in a given situation is often out of all proportion to their strength. They are responsible for molecules coming together prior to reaction, for example, and play a crucial part in solvation processes. These interactions are essentially electrostatic in character. The strength of the non-bonded forces between molecules depends upon the relative electronegativities of the molecules, or of certain parts of the molecules. Whether hydrogen bonding could be included in this category of interaction is open to debate. It certainly appears that a t least part of the bond is due to non-bonded electrostatic forces. If the intermolecular forces are strong enough, association between two (or more) molecules may occur to produce a permanent or transient complex. Such molecular complexes have been proposed as intermediates in many reactions. and while some have actuallv been isolated. the existenceof otber complexes has been claimed only b; inference. Various workers have attached labels to their molecular complexes which attempt to describe the nature of the weak interactions. Thus, the terms donor-acceptor, charge-transfer and a-complex have been used+o restrict the attractive forces to a particular mechanism, but all too often this restriction has been arbitrary and has led to some confusion. Banthorpe (I) recently drew attention to the prohlem by separating a-complexes from the other types of molecular complex: the linkage in a-complexes is believed to be covalent and therefore much stronaer than purely electn,rtntic inferarrions. The present article deals with complexes of the chargetransfer variety. It is only during the past twenty years that their widespread involvement in chemistry has been recognized and analyzed. Because of the unique nature of charge-transfer complexes, an examination.of their properties now seems appropriate, especially with reference to photochemical applications.
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