Exciplex formation concentration dependence

There are several issues involved in formation of exciplexes and excimers that limit the device performance. In the blended emissive layers, the formation of exciplex strongly depends on the concentration, structure and morphology of the film (Mazzeo et al 2002). In such cases fine-tuning of the emission spectrum can be done by concentration variation. However, there is a limit for concentration variation since phase separation or aggregation occurs above an optimum concentration, which in turn, will not allow spatial overlap of the LUMOs of the donor and the acceptor (Mazzeo et al 2002). Another difficulty in exciplex formation is the temperature dependence since at low temperatures the emission by exciton recombination will be predominant while at high temperatures, the exciplex emission will be predominant (Chao and Chen 1998). 

The new samples of polyesters (]5) and methacrylate (16) were fractionated as shown in Table 2. Examples of fluore - scence spectra are presented in Fig. 2. The plots of Fe/Fm (i.e. the intensity ratio of exciplex to monomer emission) vs. concentration shown in Fig. 3 are prominently DP dependent. An important finding is that interpolymer exciplex formation is most efficient at the intermediate DP region. 

Table 10.9. Dependence of X-H2PC photogeneration efficiency (0eh), fluorescence intensity (F), exciplex formation probability (0ct). and exciplex dissociation efficiency to free carriers (0d), on TNF concentration...

In the presence of enantiomerically enriched sensitizer 36 (77% ee), the observed enantiomeric excesses increased from 1% ee at room temperature to 15% ee at -65°C2 With the enantiomerically enriched anthracene derivative 37 (91% ee), the enantioselectivity was enhanced to 23% ee at -65°C, favoring the opposite enantiomer. Based on fluorescence quenching experiments and the concentration dependence of the observed enantioselectivities, the authors suggest the formation of diastereomeric exciplexes by complexation of the different prochiral faces of the styrene to the chiral surface of the sensitizer. The... 

In principle, an equation relating the emission intensity (of fluorescence, excimer or exciplex) to equilibrium constants and concentrations of G and C (and S for GSC2 complex formation) can be derived from dehnitions of the equilibrium constants and mass balances. The equilibrium constants can be obtained by numerical htting of the experimental data to the equation [12]. However, as the upper limit of fluorophore concentration for fluorescence measurement is low, the evaluation of the equilibrium constants involved in the formation of these complexes by using only fluorescence technique is often difficult. In most cases, the concentration dependences of other spectroscopic properties (e.g. absorption, circular dichroism, NMR) of the guest in the presence and in the absence of CD are used as complementary techniques. Detailed description of the methods of analysis can be found in Refs. [25-29]. 

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... 

Bromobiphenyl undergoes photoreduction from the triplet state375. The dependence of the quantum yield upon the concentration of the substrate does indicate the formation of an excimer. Since cpisc = 0.98, it may be concluded that this excimer is formed via the triplet state. The linear solvation energy parameters indicate a weak polarization of the excimer, suggesting a weak radical anion and cation character in the two moieties. The charge separation is smaller than in the exciplex formed from 4-bromobiphenyl and tri-ethylamine. 

The [4 + 2]-cycloaddition of 02( Ag) to rubrene has been shown to be a simple method for the determination of oxygen concentrations in organic solvents.Irradiation of benzene solutions of tropone (86) in the presence of 9,10-dicyanoanthracene leads to the formation of four products, (87), (88), (89) and (90), and in acetonitrile-dichloromethane there is also an [8 + 4]tc adduct (91) produced. It has been suggested that this latter compound arises by coupling of the radical cation of (86) with the radical anion of the dicyanoanthracene (Scheme 2). Solvent-dependent quenching of the lowest excited state of 9,10-dibromoanthracene by 2,5-dimethylhexa-2,4-diene has been studied and appears to proceed by an exciplex. Flash photolysis investigations have shown that a neutral radical species is an intermediate in the formation of the [4 + 2] adduct which is a dibenzobicyclo[2.2.2]octadiene-type compound. Irradiation of 9,10-dicyanophenanthrene (DCA) in the presence of buta-1,3-diene gives a mixture of the product of [3 + 2]-photo-... 

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