Excimer nonpolar

A. Weller and K. Zachariasse 157-160) thoroughly investigated this radical-ion reaction, starting from the observation that the fluorescence of aromatic hydrocarbons is quenched very efficiently by electron donors such as N,N diethylaniline which results in a new, red-shifted emission in nonpolar solvents This emission was ascribed to an excited charge-transfer complex 1(ArDD(H )), designated heteroexcimer, with a dipole moment of 10D. In polar solvents, however, quenching of aromatic hydrocarbon fluorescence by diethylaniline is not accompanied by hetero-excimer emission in this case the free radical anions Ar<7> and cations D were formed. 

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. 

A well-known example of an exciplex is the excited-state complex of anthracene and N,N-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene molecule. In nonpolar solvents such as hexane, the quenching is accompanied by the appearance of a broad structureless emission band of the exciplex at higher wavelengths than anthracene (Figure 4.9). The kinetic scheme is somewhat similar to that of excimer formation. 

DMABN suffers from the fact that dual fluorescence is only observable for polar media. Therefore, the pretwisted ester DMPYRBEE has been developed which shows dual fluorescence also in alkane solvents. 9 This probe allowed measurement of nonpolar polymeric siloxane oils and a comparison with the corresponding measurements using an EXCIMER probe. As expected from the decreased reaction volume necessary for the TICT photoreaction, the latter is usable down to much lower temperatures (higher viscosities) and probes a larger fraction of free volume. 26 ... 

In summary, the intrinsic quantum yield and lifetime of the 2-naphthyl excimer is not significantly affected by the mode (intra vs. inter) of excimer formation, at least in a nonpolar solvent. The next question to be pursued is whether the similarity between intra- and intermolecular excimers is maintained in a rigid matrix. 

Some properties of the t -dimethylhexadiene exciplex are summarized in Table 7. Its fluorescence maximum is at slightly shorter wavelength than that of the anthracene-dimethylhexadiene exciplex (435 nm) (51). While data on other unsubstituted arene-diene exciplexes are not available, t appears to be more reactive and to form more stable exciplexes with dienes than arenes of comparable electron affinity (101). The dipole moment of the - -t -dimethylhexadiene exciplex is estimated to be 7 D from the solvent dependence of its fluorescence maxima (36). This value is substantially lower than those for pure charge-transfer exciplexes (p > 15 D) and indicates that this exciplex is relatively nonpolar and might be better categorized as a hetero-excimer, than as an exciplex (83). That is, using the normal resonance description of an exciplex... 

We will discuss briefly the reactive species such as an exciplex and radical ion species generated by the excitation of organic molecules in the electron-donor (D)-acceptor (A) system. An exciplex is produced usually in nonpolar solvents by an interaction of an electronically excited molecule D (or A ) with a ground-state molecule A (or D). It is often postulated as an important intermediate in the photocycloaddition between D and A. In the case of D = A, an excimer is formed as an excited reactive species to cause photodimerization. In some cases, a ter-molecular interaction of an exciplex with another D or A generates a triplex, which is also a reactive intermediate for photocycloaddition. The evidence for the formation of excimers, exciplexes, and triplexes are shown in the fluorescence quenching. Excimer and exciplex emission is, in some cases, observed and an emission of triplex rarely appears. 

As has been mentioned earlier, it is often very difficult to distinguish between and identify the roles of exciplexes (and excimers) and biradicals in cycloaddition reactions. Caldwell and Creed (1978b) have studied the cycloaddition of dimethyl fumarate to phenanthrene and found that the quantum yield of the cyclobutane photoaddition product is increased in the presence of oxygen. It was suggested that oxygen enhances intersystem crossing in the triplet biradical formed between the two reactants. Nitroxide radicals have also been found to increase intersystem crossing (Sj -> Tj) in carbocyanines when nonpolar solvents are used (Kuzmin et al., 1978). When polar solvents are employed full electron transfer takes place. 

From the vibrational structure in the monomer fluorescence spectrum of Py, the Ham-effect (54-58), information was obtained on the polarity of the surface where the excimer and dimer formation takes place. It appears that Py on silica/decanol resides in an environment similar to a homogeneous decanol solution (36), whereas for Py on dry silica a considerably higher polarity was found between that of methanol and water, see Fig. 8. For rever-sed-phase Si-C.jg in contact with methanol/water 3/1 (Section 4.2.3), it was concluded that Py finds itself in a nonpolar medium (21). 

The study of radical ion recombination chemiluminescence of this sort emerged from observations of, so to speak, the reverse reaction. The fluorescence of aromatic hydrocarbons was found to be effectively quenched by certain electron-donor molecules, e.g. N,N-diethylaniline. A new emission appeared in such cases which exhibited a strong red shift, and was ascribed to an excited charge transfer complex ( exciplex , hetero-excimer) (see e.g. [31]), when a nonpolar solvent was used. 

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