Exciplex, charge-transfer

Franck-Condon terms are neglected, the exciplex (charge-transfer) energy relative to the unexcited configuration energy is given approximately by... 

It is evident from the exceptions noted that the mechanism proposed above does not fully capture the pathways open to the Patemo-Biichi reaction. A great deal of effort has been devoted to deconvoluting all of the possible variants of the reaction. Reactions via singlet state carbonyls, charge-transfer paths, pre-singlet exciplexes, and full electron transfer paths have all been proposed. Unfortunately, their influence on product... 

The reaction between the photoexcited carbonyl compound and an amine occurs with substantially greater facility than that with most other hydrogen donors. The rate constants for triplet quenching by amines show little dependence on the amine a-C-H bond strength. However, the ability of the amine to release an electron is important.- - This is in keeping with a mechanism of radical generation which involves initial electron (or charge) transfer from the amine to the photoexcited carbonyl compound. Loss of a proton from the resultant complex (exciplex) results in an a-aminoalkyl radical which initiates polymerization. The... 

The best evidence for a charge-transfer exciplex (hetero excimer) has been provided by Thomaz and Stevens.<148,149) They note a reduction in fluorescence yield of pyrene with increasing heavy-atom concentration and proposed the following set of reactions to explain their results ... 

Not all sensitized photochemical reactions occur by electronic energy transfer. Schenck<77,78) has proposed that many sensitized photoreactions involve a sensitizer-substrate complex. The nature of this interaction could vary from case to case. At one extreme this interaction could involve a-bond formation and at the other extreme involve loose charge transfer or exciton interaction (exciplex formation). The Schenck mechanism for a photosensitized reaction is illustrated by the following hypothetical reaction ... 

A and D are the exciplex or excimer components, denotes the primarily excited species, k is the limiting photoassociation equilibrium constant, AHat ASa, and are the thermodynamic parameters for the exciplex-excimer, and p is the excited state dipole moment of the complex. Note that the large dipole moment for the exciplex indicates almost complete charge transfer in the excited state, (D+, A-). rfc and r, are the fluorescence lifetimes for the complex and the component. 

The acetone-sensitized photodehydrochlorination of 1,4-dichlorobutane is not suppressed by triplet quenchers (20), but the fluorescence of the sensitizer is quenched by the alkyl chloride (13). These observations imply the operation of a mechanism involving collisional deactivation, by the substrate, of the acetone excited singlet state (13,21). This type of mechanism has received strong support from another study in which the fluorescence of acetone and 2-butanone was found to be quenched by several alkyl and benzyl chlorides (24). The detailed mechanism for alkanone sensitization proposed on the basis of the latter work invokes a charge-transfer (singlet ketone)-substrate exciplex (24) and is similar to one of the mechanisms that has been suggested (15) for sensitization by ketone triplets (cf. Equations 4 and 5). 

Studies with model compounds have demonstrated that photodehydrochlorination is sensitized by jj-cresol triplets via a charge-transfer exciplex intermediate in which the alkyl chloride is the electron acceptor (15). The detailed mechanism suggested for this process (15) is outlined in Equations 11 and 12. 

The chemical association of the exciplex results from an attraction between the excited-state molecule and the ground-state molecule, brought about by a transfer of electronic charge between the molecules. Thus exciplexes are polar species, whereas excimers are nonpolar. Evidence for the charge-transfer nature of exciplexes in nonpolar solvents is provided by the strong linear correlation between the energy of the photons involved in exciplex emission and the redox potentials of the components. 

Bhattacharyya K, Chowdhury M (1993) Environmental and magnetic field effects on exciplex and twisted charge transfer emission. Chem Rev 93 507-535... 

When the emissive state is a charge transfer state that is not attainable by direct excitation (e.g. which results from electron transfer in a donor-bridge-acceptor molecule see example at the end of the next section), the theories described above cannot be applied because the absorption spectrum of the charge transfer state is not known. Weller s theory for exciplexes is then more appropriate and only deals with the shift of the fluorescence spectrum, which is given by... 

Class 3 fluorophores linked, via a spacer or not, to a receptor. The design of such sensors, which are based on molecule or ion recognition by a receptor, requires special care in order to fulfil the criteria of affinity and selectivity. These aspects are relevant to the field of supramolecular chemistry. The changes in photophysical properties of the fluorophore upon interaction with the bound analyte are due to the perturbation by the latter of photoinduced processes such as electron transfer, charge transfer, energy transfer, excimer or exciplex formation or disappearance, etc. These aspects are relevant to the field of photophysics. In the case of ion recognition, the receptor is called an ionophore, and the whole molecular sensor is... 

The SET between amine and acceptor may be enhanced by photoexcitation and may lead to the formation of exciplexes2 or molecular complex with charge transfer character3. The photochemistry between aromatic acceptors and amines via the exciplexes has been discussed earlier (Scheme l)4. 

Fig. 7 (a) Molecular orbital (MO) description for the charge-transfer state formation in organic donor/ acceptor systems, (b) Description for CT state emission energy using exciplex MOs in (a)... 

If the exciplex M"Q+ is treated as a pure charge-transfer state with (cf. Eq. 13)... 

The interaction of nondegenerate molecular or charge-transfer states is insufficient to describe the stability of photoassociation products of molecules with different electronic energy levels, ionization potentials, and electron affinities. On the other hand, treatments26-26 of the exciplex as a pure charge-transfer state afford a quantitative description of the shift in fluorescence peak with solvent polarity and with electron affinity of the (fluorescent) donor in the same quencher-solvent system (Eq. 13) moreover, estimated values for the dipole moment of the emitting species (Table VI) confirm its pronounced charge-transfer character. 

An example of exciplex formation in the solid state may be afforded by perylene doped crystals of pyrene which emit a green structureless fluorescence in addition to the blue and orange-red excimer bands of pyrene and perylene, respectively. Hochstrasser112 has shown that the energy of the emitting species is consistent with that of a charge transfer complex of pyrene and perylene molecules in a bimolecular unit of the pyrene lattice. 

It is instructive to consider the stability of other excitations in DNA, ex-cimers and exciplexes. An excimer (exciplex) is formed when two identical (nonidentical) molecules that do not interact in their ground states do so when one of the molecules is in an excited state. As a result of charge-transfer and exchange interactions of the overlapping n electrons of the two molecules on the one hand, and their mutual repulsion on the other, the molecules are drawn together in a potential minimum at a separation smaller... 

Solute-solvent interactions are of two types (1) universal interaction, and (2) specific interaction. Universal interaction is due to the collective influence of the solvent as a dielectric medium. It depends on the dielectric constant D and refractive index n of the solvent and the dipole moment g of the solute molecule. Such interactions are van der Waals type. Specific interactions are short range interactions and involve H-bonding, charge-transfer or exciplex formation. H-bonding ability may change on excitation specially for n-yxt transitions. 

Charge Transfer Mechanism Exciplex Formation and Decay... 

---