Exciplexes, definition

This instability of the ground-state complex is a somewhat arbitrary feature of the exciplex definition. 

If definite stoichiometry is maintained in the exciplex formation, an isoemissive point similar to isosbestic point in absorption miy be observed. An interesting example of intra-molecular exciplex formation has been reported foi 9-methoxy-10-phenanthrenecarboxanil. The aniline group is not necessarily coplanar with the phenanthrene moiety but is oriented perpendicular to it. The u-elcctron located on its N-atom interacts with the excited -electron system and an intramolecular exciplex with T-bone type structure is formed in rigid glassy medium where rotation is restricted. Temperature dependence of fluorescence of this compound in methylcyclohexane-isopentane (3 1) solvent shows a definite isoemissive point (Figure 6.8). As the solvent melts and movement is restored to the molecule, structured fluorescence reappears. 

From the evidence thus far available, it cannot be concluded that exciplexes are intermediate species in the ortho photocycloaddition of arenes to alkenes. In those cases in which exciplex emission has been observed, there exists no definitive proof that the exciplex is able to undergo bond formation leading to the adduct. For the numerous cases in which no exciplexes have been detected, of course no statements about their intermediacy can be made. 

Changes in the electronic and molecular structures after CT excitation can also result in chemical bond formation between the excited complex [AB[ and another molecule (Q) of the system, yielding an encounter complex AB-Q. The complex formed by interaction of an excited molecular entity with a ground state partner of the same structure is called an excimer, whereas an electronically excited dimer of definite stoichiometry, formed by interaction of an excited molecular entity with a ground state partner of a different structure, is named an exciplex [29], Both excimers and exciplexes have their own properties and can thus be regarded as new chemical species. Their generation is well documented by the concentration effect on the fluorescence of some solutes or by flash photolysis measurements [11,24],... 

Exciplex An electronically excited complex, of definite stoichiometry, nonbonding in the ground state. For example, a complex formed by the interaction of an... 

The term exciplex (excited complex) is used to describe an electronically excited molecular complex of definite stoichiometry. Complexes which fall into this broad classification include ... 

Other reactions of aromatic hydrocarbon anion radicals and amine cation radicals lead to exciplex emission, particularly in nonpolar solvents [15], Luminescence from exciplexes is most definitively observed in systems for which the redox reaction is energetically unable to yield a localized excited state. The free energy of exciplex formation, jE exc, is associated with solvation and geometry optimization in the encounter complex of and A+. 

T vo different molecules M and Q can also form complexes with a definite stoichiometry (usually 1 1). If complexation is present already in the ground state and leads to CT absorption (Section 2.6), which is absent in the individual components, the complex is referred to as a charge-transfer or donor-acceptor complex. If, however, the complex shows appreciable stability only in the excited state, it is called an exciplex (excited complex)-. 

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