Excimer fluorescence, compounds exhibiting

Compounds Exhibiting Excimer Fluorescence in Solutions at Room Temperature11... 

All the above compounds yield excimer fluorescence when excited in room-temperature solution. However, because the rotational potential of the C—X bond and the nonbonded interactions of the substituents of the X atom differ from those of the C—C bond 126), the amount of excimer fluorescence from R(C—X—C)R differs from that of R(C—C—C)R. The heteroatom X can also influence the rotational state of the side groups R, as illustrated by the formation of the anti-photodimer in bis(l-naphthylmethyl)ether u2), but not in l,3-bis(l-naphthyl)propane 10). Finally, compounds having n 3 may exhibit excimer fluorescence, if the linkage contains one or more heteroatoms. For example, the—C—O—C—C— linkage in a,to-bis(2-naphthyl) compound allows excimer fluorescence to be observed in room-temperature solution39). 

A dipalmitoylphosphatidylcholine labelled with pyrene has been synthesised. In all organic solvents this compound exhibits pyrene excimer emission but in aqueous solution there is total fluorescence quenching. It is suggested that aggregation leads to the fluorescence quenching (Sunamoto et al., 1980). 

Bis(crown ether) compound 4 was synthesized in the hope that both crown ether rings would sandwich one metal ion between them, and that this process would bring the two naphthalene chromophores into close proximity so that an excited dimer (excimer) would form and a characteristic fluorescence band would appear. Early examples of metal ion fostered excimer formation were reported by Bouas-Laurent and co-workers(T) and Tundo and Fendler(S). Compound 4 has not shown excimer fluorescence, probably because the decrease in energy is small when two naphthalene chromophores form an excimer and because the 2,3-disubstitution on the naphthalene rings may present steric barriers to excimer formation. However, 4 does exhibit cation enhanced fluorescence as an extraction agent(2i) in dichloromethane(22). 

Dipyrene derivatized molecules are known to form intramolecular excimers. The excimer exhibits a fluorescence emission at a wavelength longer than that of the monomer, and selective analysis of polypyrene-labeled compounds can be carried out using the excimer fluorescence. 

In contrast, the fluorescence spectra of the parent y-cyclodextrins (compounds y-CD1, y-CD2, y-CD3, y-CD4) exhibit both monomer and excimer bands in the absence of guests because the cavity is large enough to accommodate both fluorophores (Figure 10.38). The ratio of excimer and monomer bands changes upon guest inclusion. The ratio of the intensities of the monomer and excimer bands was used for detecting various cyclic alcohols and steroids (cyclohexanol, cyclo-dodecanol, i-borneol, 1-adamantanecarboxylic acid, cholic acid, deoxycholic acid and parent molecules, etc.). 

Since Forster s original work, a large number of aromatic compounds, including benzene, naphthalene, and anthracene, have been found to have concentration-dependent fluorescence spectra under some conditions. Most of these excimers are not as stable a.s the pyrene prototype, and require lower temperatures or higher concentrations to be observed.29 Some crystals also exhibit excimer emission. Crystalline pyrene, for example, has only a single structureless fluorescence band of the same energy as its excimer emission in solution.30... 

The geometrical requirements for excimer emission are stringent. In crystals the molecules must be stacked in columns or arranged in pairs with an interplanar distance of less than 3.5 A.31 Chandross and Dempster carried out an elegant investigation of the geometrical requirements for excimer formation in solution. They studied the fluorescence of compounds 6-10 in methylcyclohexane solution and found that only compounds 7 and 9 exhibit strong emission from an intra-... 

Ad-C3-Ad and poly-VAd exhibited the monomeric fluorescence at about 320 nm in ethanol. This finding suggests that the excimer was not formed for both dimeric and polymeric compounds in ethanol solution. Furthermore, the excimer could not be obtained either in the case of dimers or polymers of 6-methylaminopurine derivatives, though excimer emissions were observed in water-ethylene glycol. The absence of the excimer may be explained by the fact that the stacked forms of the nucleic acid bases are unstable in ethanol solution where the bases appear to be solvated with ethanol molecules23. ... 

TICT formation can also be combined with excimer or exciplex formation. An example is compound Bi-DMABK, which, in addition to the TICT state, can also form head-to-head intramolecular excimers even at very low concentration and therefore exhibits triple fluorescence [114]. At sufficiently high concentrations, DMABN [115,116] and its derivatives like DMABME [117-121,126] are also expected to form, in addition to TICT, (intermolecular) excimers, ground state dimers or larger aggregates. 

Several chiral substituted a-phenyl-alkyl alcohols, which were partially enantio-merically enriched, were successfully purified by this method after esterification with 9-anthroic acid optical purities higher than 95% were obtained.Fluorescence studies of racemic compounds show a characteristic a-type excimer emission, whereas the chiral enantiomer exhibits a typical y-type monomer emission. Figure 7 shows the spectra of l-(2-chlorophenyl)ethyl 9-anthroate for a crystalline sample of enantiomeric (1) and racemic (2) at 77 K. It has been demonstrated that in the absence of configurational disorder chiral impurities in the range of 1 % can be detected by this method. ... 

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