Hydrocarbons, aromatic fluorescence yields

Iangelaar, J., Retschnik, R.P.H., and Hoijtink, G. (1971) Studies on triplet radiative lifetimes, phosphorescence, and delayed fluorescence yields of aromatic hydrocarbons in liquid solutions. 

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.)... 

Hydrazide chemiluminescence has been investigated very intensively during recent years (for reviews, see 1>, p. 63, 2>, 90>). Main topics in this field are synthesis of highly chemiluminescent cyclic diacyl hydrazides derived from aromatic hydrocarbons, relations between chemiluminescence quantum yield and fluorescence efficiency of the dicarboxylates produced in the reaction, studies concerning the mechanism of luminol type chemiluminescence, and energy-transfer problems. 

The most commonplace substrates in energy-transfer analytical CL methods are aryl oxalates such as to(2,4,6-trichlorophenyl) oxalate (TCPO) and z s(2,4-dinitrophenyl) oxalate (DNPO), which are oxidized with hydrogen peroxide [7, 8], In this process, which is known as the peroxyoxalate-CL (PO-CL) reaction, the fluorophore analyte is a native or derivatized fluorescent organic substance such as a polynuclear aromatic hydrocarbon, dansylamino acid, carboxylic acid, phenothiazine, or catecholamines, for example. The mechanism of the reaction between aryl oxalates and hydrogen peroxide is believed to generate dioxetane-l,2-dione, which may itself decompose to yield an excited-state species. Its interaction with a suitable fluorophore results in energy transfer to the fluorophore, and the subsequent emission can be exploited to develop analytical CL-based determinations. 

Examples of fluorescence quantum yields and lifetimes of aromatic hydrocarbons are reported in Table 3.171. 

However, the heavy atom effect can be small for some aromatic hydrocarbons if (i) the fluorescence quantum yield is large so that de-excitation by fluorescence emission dominates all other de-excitation processes (ii) the fluorescence quantum yield is very low so that the increase in efficiency of intersystem crossing is relatively small (iii) there is no triplet state energetically close to the fluorescing state (e.g. perylene)10 . 

Dimerization. Many polycyclic aromatic hydrocarbons form photodimers via 4 + 4 cycloadditions.58-59 Anthracene (11), for example, dimerizes with a limiting quantum yield of 0.3 when irradiated in benzene.59-60 The reaction takes place from the singlet state and competes with fluorescence, which drops to zero at high anthracene concentrations. More detailed coverage of these reactions is found in the reviews by Bowen59 and by Trecker.58... 

At low enough temperatures vibrational fine structure of aromatic chromophores may be well resolved, especially if they are embedded in a suitable matrix such as argon or N2, which is deposited on a transparent surface at 15 K. This matrix isolation spectroscopy77166 may reveal differences in spectra of conformers or, as in Fig. 23-16, of tautomers. In the latter example the IR spectra of the well-known amino-oxo and amino-hydroxy tautomers of cytosine can both be seen in the matrix isolation IR spectrum. Figure 23-16 is an IR spectrum, but at low temperatures electronic absorption spectra may display sharp vibrational structure. For example, aromatic hydrocarbons dissolved in n-heptane or n-octane and frozen often have absorption spectra, and therefore fluorescence excitation spectra, which often consist of very narrow lines. A laser can be tuned to excite only one line in the absorption spectrum. For example, in the spectrum of the carcinogen ll-methylbenz(a)anthrene in frozen octane three major transitions arise because there are three different environments for the molecule. Excitation of these lines separately yields distinctly different emission spectra.77 Likewise, in complex mixtures of different hydrocarbons emission can be excited from each one at will and can be used for estimation of amounts. Other related methods of energy-... 

In presence of dissolved oxygen, photooxidation may also occur. Bowen and Williams (19) carried out some early experiments with benzene, some of its methyl derivatives, and certain other aromatic hydrocarbons in hexane solutions illuminated with light at 2537 A. They found low values for the quantum yields of fluorescence and of oxygen take-up the sum of these quantities being less than unity. At higher temperatures (>50°C.) chain propagation by a radical mechanism... 

For years it has been known that the quantum yield of fluorescence for a number of aromatic hydrocarbons decreases with increasing concentration, but the cause of this concentration quenching was not well understood. In 1955 it was first noted by Forster that increasing concentration not only quenches the normal fluorescence of pyrene (5), but also introduces a new fluorescent component. 

Perhaps the simplest optically controlled switches are single molecules embedded in a solid host matrix. These systems consist of an amorphous, polycrystalline, or crystalline film doped with dilute concentrations of impurity molecules. The most commonly used dopant molecules are fused polycyclic aromatic hydrocarbons and porphyrins. In addition to facile sample preparation, these planar molecules absorb in the visible to near IR regions of the spectrum, possess large extinction coefficients in both the ground and excited states, and have high fluorescence quantum yields. 

Some years ago, Schaap and co-workers developed a method by which compounds that do not quench the fluorescence of the singlet excited DCA sensitizer may nevertheless be rapidly oxidized [102, 108, 109, 160-162]. For example, epoxides 69a-d, unreactive under standard DCA-sensitized conditions [107,163], can be readily converted into the corresponding ozonides 70a-c, in high yields, by use of a non-light-absorbing aromatic hydrocarbon, i.e., biphenyl (BP) as a cosensitizer in conjunction with DCA. Variable amounts of carbonyl compounds, such as benzophenone from 69 a, b, benzaldehyde from 69 b, c, d are... 

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