Fluorescence, aromatic hydrocarbons

Since its discovery by Chandross and to this day, peroxy-oxalate chemiluminescence has been controversial because of its enormous complexity in view of the many alternative steps involved in this process. The principal mechanistic feature of the peroxy-oxalate chemiluminescence pertains to the base-catalyzed (commonly imidazole) reaction of an activated aryl oxalate with hydrogen peroxide in the presence of a chemiluminescent activator, usually a highly fluorescent aromatic hydrocarbon with a low oxidation potential . A variety of putative high-energy peroxide intermediates have been proposed for the generation of the excited states . In the context of the present chapter, it is of import to mention that recent work provides experimental evidence for the intervention of the 1,2-dioxetanedione 18 (Scheme 11) as the high-energy species responsible for the chemiexcitation. Furthermore, clear-cut experimental data favor the CIEEL mechanism as a rationalization of the peroxy-oxalate chemiluminescence . 

Recent work by Schmidt and Schuster (1978a, 1980a) has shown that the addition of any of several easily oxidized, fluorescent aromatic hydrocarbons or amines to solutions of [21] results in greatly enhanced chemiluminescence. Moreover, addition of these molecules accelerates the rate of reaction of [21]. The catalyzed reaction is first order in both [21] and aromatic hydrocarbon or amine (which is termed the activator, act). Acetone is still produced quantitatively, and the activator is not consumed in the reaction, but rather serves as a catalyst for the decomposition of the dioxetanone. The kinetic behavior is thus described by the simple rate law (27), where kt is the rate... 

Originally invoked for electroluminescent reactions, this idea has now been developed for the reaction of peroxides with fluorescent compounds of low ionisation potential [51]. Many of these reactions are discussed in Chap. XI, but Fig. 2 can be most succintly exemplified by the radical ion annihilation shown, where Ar is a fluorescent aromatic hydrocarbon such as diphenylanthracene, LUMO is the lowest, normally unoccupied molecular orbital and HOMO is the highest occupied molecular orbital. 

An apparently bimolecular decomposition mechanism is observed in the presence of fluorescent aromatic hydrocarbons. Rubrene was the first catalyst used [25], considerably enhancing the chemiluminescence of the a-peroxy lactones. Triplet-triplet annihilation of rubrene triplets was suggested, these being produced by energy transfer from the dioxetanone decomposition products. However, this suggestion was not corroborated by further experimental evidence. 

It is very difficult to follow these alternatives to the very well defined dioxetan route right through to the light emitting step. A variety of fates for the intermediate peroxide, and a variety of electron transfer mechanisms are possible. Very many heterocycles and fluorescent aromatic hydrocarbons are chemiluminescent in dipolar aprotic solvents with base in the presence of oxygen. The quantum yields are rarely high, and the identification of a single well defined pathway is extremely difficult. 

In its simplest and classical form a fluorescent aromatic hydrocarbon, such as 9,10-diphenyl anthracene (DPA) undergoes alternating oxidation and reduction at an electrode with an A. C. supply. 

DPA, rubrene, fluorene and other fluorescent aromatic hydrocarbons are reported to yield chemiluminescence when electrolyzed together with alkyl and aryl halides such as (6) or (7)... 

The radical anions can be prepared by the reaction of a fluorescent aromatic hydrocarbon such as 9,10-dimethylanthracene, with an alkah metal (lithium, sodium or potassium) in an ether solvent (1,2-dimethoxyethane, tetrahydrofuran or 2-methyltetrahydrofuran. 

Nevertheless, this type of analysis, usually done by chromatography, is not always justified when taking into account the operator s time. Other quicker analyses are used such as FIA (Fluorescent Indicator Analysis) (see paragraph 3.3.5), which give approximate but usually acceptable proportions of saturated, olefinic, and aromatic hydrocarbons. Another way to characterize the aromatic content is to use the solvent s aniline point the lowest temperature at which equal volumes of the solvent and pure aniline are miscible. 

There are two approaches to estimation of AG fThe first is an empirical approach (36) based on dynamics of fluorescence quenching of aromatic hydrocarbons ia acetonitrile solution. Accordingly,... 

Principal component analysis has been used in combination with spectroscopy in other types of multicomponent analyses. For example, compatible and incompatible blends of polyphenzlene oxides and polystyrene were distinguished using Fourier-transform-infrared spectra (59). Raman spectra of sulfuric acid/water mixtures were used in conjunction with principal component analysis to identify different ions, compositions, and hydrates (60). The identity and number of species present in binary and tertiary mixtures of polycycHc aromatic hydrocarbons were deterrnined using fluorescence spectra (61). 

The performance of microwave-assisted decomposition of most difficult samples of organic and inorganic natures in combination with the microwave-assisted solution preconcentration is illustrated by sample preparation of carbon-containing matrices followed by atomic spectroscopy determination of noble metals. Microwave-assisted extraction of most dangerous contaminants, in particular, pesticides and polycyclic aromatic hydrocarbons, from soils have been developed and successfully used in combination with polarization fluoroimmunoassay (FPIA) and fluorescence detection. 

ACID-BASED SURFACTANT CLOUD POINT EXTRACTION AND PRECONCENTRATION OF POLYCYCLIC AROMATIC HYDROCARBONS PRIOR TO FLUORESCENCE DETERMINATION... 

Fig. 29 Fluorescence scans of polycyclic aromatic hydrocarbons at various excitation wavelengths in combination with various secondary filters.

Sodiiun dodecylsulfate, cetyltrimethylam-monium chloride, sodium cholate, -cyclodextrin dansylated amino acids and polycyclic aromatic hydrocarbons > 45-fold 1% in water the greatest enhancement of fluorescence is that of sodium cholate on pyrene [263]... 

E. R. Brouwer, A. N. J. Elermans, El. Lingeman and U. A. Th Briknman, Determination of polycyclic aromatic hydrocarbons in surface water by column liquid cliromatogr a-phy with fluorescence detection, using on-line micelle-mediated sample preparation , J. Chromatogr. 669 45-57 (1994). 

Another useful standard Is SRM 1647, priority pollutant polynuclear aromatic hydrocarbons (in acetonitrile). It can be used to calibrate liquid chromatographic Instruments (retention times. Instrument response), to determine percent recoveries, and to fortify aqueous samples with known PAH concentrations. Figure 8 Illustrates the HPLC separation and UV detection (fluorescence is also used extensively) for the 16 priority pollutants. 

C.N. Ho, G.D. Christian and E.R. Davidson, Application of the method of rank annihilation to fluorescent multicomponent mixtures of polynuclear aromatic hydrocarbons. Anal. Chem., 52 (1980) 1071-1079. 

In LIF detection systems, excitation power may be increased up to six orders of magnitude compared to CF detection. Most LC-LIF detection concerns under-ivatised polynuclear aromatic hydrocarbons (PAHs) and fluorescing dyes (e.g. polymethines). Because only a limited number of analytes possess native fluorescence, derivatisation of the analyte before detection is normally required in trace analysis of organic solutes by means of LIF detection. LIF detection in HPLC was reviewed... 

The fluorescence of a range of polycyclic aromatic hydrocarbons is found to be quenched in the presence of alkyltriphenyl-phosphonium salts via electron-transfer from the hydrocarbon to... 

Thus we see that in molecules possessing ->- 77 excited states inter-combinational transitions (intersystem crossing, phosphorescence, and non-radiative triplet decay) should be efficient compared to the same processes in aromatic hydrocarbons. This conclusion is consistent with the high phosphorescence efficiencies and low fluorescence efficiencies exhibited by most carbonyl and heterocyclic compounds. 

The fact that quadricyclene and dienes quench the fluorescence of aromatic hydrocarbons despite the fact that the energetics for classical energy transfer are very unfavorable has been rationalized by the formation of an exciplex. A general mechanism is as follows ... 

Polycyclic aromatic hydrocarbons (PAH) are important air pollutants that have to be detected at very low concentrations. Fig. 2.4h shows the separation of a synthetic mixture of very low levels of PAH. They are barely detectable using uv absorption, but are easily monitored by fluorescence. 

Different aromatic hydrocarbons (naphthalene, pyrene and some others) can form excimers, and these reactions are accompanying by an appearance of the second emission band shifted to the red-edge of the spectrum. Pyrene in cyclohexane (CH) at small concentrations 10-5-10-4 M has structured vibronic emission band near 430 nm. With the growth of concentration, the second smooth fluorescence band appears near 480 nm, and the intensity of this band increases with the pyrene concentration. At high pyrene concentration of 10 2 M, this band belonging to excimers dominates in the spectrum. After the act of emission, excimers disintegrate into two molecules as the ground state of such complex is unstable. 

With spectroscopic methods it is possible to obtain information about the conformation of hydrocarbon-DNA complexes. The fluorescence quantum yields of aromatic hydrocarbons are greatly reduced when they bind to DNA in intercalated conformations. Figure 3 shows how the intensity of the emission spectrum of DMA decreases with increasing concentrations of DNA in 15% methanol. (In Figure 3 and throughout this discussion DNA concentrations and association constants have been reported in terms of PO molarity unless otherwise indicated. The solution content of organic solvents is given in percent volume.)... 

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