Fluorescent dimer

This chapter shows that zeolite L is a very suitable host for the arrangement of a wide variety of chromophores. The structure of zeolite L is such that the formation of non-fluorescent dimers inside the channels can be prohibited and chromophores can be aligned in a certain direction. We have shown that this host-guest system can be used to make very efficient nanoscale two-directional photonic antenna systems. A broad spectral absorption range can be achieved by using several different cationic and neutral dyes. 

The authors report detection limits of 8x10 ° mol/dm for TATP and 8 X lO mol/dm for HMTD. When p-hydroxyphenylacetic acid (p-HPAA) (6) was used as the oxidation substrate instead of ABTS, a highly fluorescent dimer (7) was formed. This dimer could be detected spectrophotometricaUy, although the sensitivity dropped (Eq. (12)). Both methods also enabled a semi-quantitative estimation of TATP and HMTD concentrations [86]. Dimerization of p-hydroxyphenylacetic acid (p-HPAA) by hydrogen peroxide in presence of peroxidase [86] is as foUows ... 

The homolog of 106a, 3-(4-hydroxyphenyl)propionic acid (106b), undergoes a similar oxidation by H2O2 to a fluorescent dimer (107b, = 305 nm, 7.3 = 405 nm), catalyzed... 

The type II direct photooxygenation of anthracene and its derivatives has been studied quantitatively by Bowen,2 Livingston,3 and their co-workers. Quantum yields of fluorescence, dimerizations, and... 

Fluorescence measurements were made on a dimer of pentazocine produced by on-column oxidation with O.oh M K3Fe(CN)6. This product was separated from a dihydromorphine fluorescent dimer and a pentazocine-dihydromorphine product (93). 

Analysis of the fluorescence decay curves in water indicated that the intracoil process is static and that anthryl aggregation induces non-exponentiality in the fluorescence decay associated with a dynamic equilibrium between the singlet diphenyl-anthracene and a non-fluorescent dimer state. Further evidence was also presented to show that these polymers self-organise into hydrophobic and hydrophilic regions Energy migration in alternative and random copolymers of 2-vinylnaphthalene and methyl methacrylat methacrylic acid... 

Chemical ionization mass spectrometric detection has been explored for the detection of methyl hydroperoxides However, fluorometry has dominated the current detection schemes for the organic peroxides. Typically, a nonfluorescent substrate is oxidized by the peroxide to generate a fluorescent product. These methods are sufficiently sensitive for accurate measurement of the peroxides in the low ppt by volume. For example, the peroxidase-catalyzed dimerization of p-hydroxyphenylacetic acid (POPHA) occurs in the presence of a peroxy group at elevated pH. The formation of the fluorescent dimer, detected by excitation at 310 nm and emission at 405 nm, is proportional to the concentration of the peroxide. The most common peroxidase catalyst used for this reaction is horseradish peroxidase (HRP). Cost and stability issues with the use of HRP led to the use of other catalysts, such as metalloporphyrins or phthalocyanine complexes. Another fluorescent reaction scheme involves the oxidation of the nonfluorescent thiamine (vitamin Bi) to the fluorescent thiochrome by the peroxide group. This reaction is catalyzed by bovine hematin. This reaction is 25-fold more sensitive for H2O2 than for the organic peroxides. 

Energy transfer from the fluorescent dimer to a nonfluorescent aggregate would also shorten the singlet lifetime. The rate of energy transfer is very sensitive to concentration however, the lifetime at any given temperature appeared to be free of any concentration effects. Moreover, an estimate of the Forster transfer rate at 10 M for even the most favorable conditions indicated that transfer would occur at a much slower rate than the observed quenching. 

The term exdmer is used when the excited dye forms a transient fluorescent dimeric complex with another fluorophore of the same kind. The exdmer fluorescence is usually red shifted with respect to that of the monomer (see Fig. 6.28) The most widely used types of eocdmer-forming probes are pyrene (see Fig. 6.28) and perylene and their derivatives. The ratio of the maxima of the excimer to the monomer spectra can be used to judge the efficiency of exdmer formation. This (Ex/Mo)-ratio depends on the concentration of the dye and is controlled by the diffusion properties. It allows, when using pyrene or perylene labeled fatty acids or phospholipids (see Fig. 6.28), the estimation of the probe s lateral diffusion coefficients in lipid bilayer membranes. Thus, membrane fluidity can be measured by monitoring the fluorescence spectra of such an exdmer probe. 

Eor complexation studies with a guest that exhibits intermolecular self-association, the association should also be considered in the analysis. An example for this is the complexation of phenothiazine dyes with CDs. The dyes form non-fluorescent dimers. The monomers form 1 1 complexes with /i-CD, while the dimers form 1 1 complexes with y-CD. Evaluation of equilibrium constants... 

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