Hydrocarbons fluorescence quantum yields

Most studies of the physical binding of hydrocarbon metabolites and metabolite model compounds have measured the effect of DNA binding on hydrocarbon fluorescence intensities, fluorescence lifetimes and UV absorption spectra Radioactive labelling has also been used, but less frequently. Spectroscopic methods are particularly convenient. These methods, especially fluorescence methods, are also very sensitive. All of the hydrocarbons in Figure 1 except the epoxides have high fluorescence quantum yields, which permit routine detection in the 10 -10 7 M concentration range. 

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.)... 

In some cases it is possible to obtain a measure of the association constant for intercalation directly from fluorescence quenching data. This method is applicable when the dynamic quenching of the hydrocarbon fluorescence by DNA is small and when the intercalated hydrocarbon has a negligible fluorescence quantum yield compared to that of the free hydrocarbon. If these conditions are met, the association constant for intercalation, Kq, is equal to the Stern-Volmer quenching constant Kgy (76) and is given by Equation 1. 

The difficulties in the use of fluorescence for quantitative measurement of hydrocarbons are much like those for the ultraviolet absorption methods. Each compound has its own excitation and emission maxima, with the fluorescence quantum yields varying sometimes by an order of magnitude. Thus the amount of hydrocarbon reported by an analysis will depend upon the emission and excitation wavelengths chosen, and upon the compound selected as the standard. 

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 . 

Compounds called azarenes containing one or more heterocyclic nitrogen atoms (e.g. pyridine, quinoline, acridine) have low-lying n —> n transitions, which explains their relatively low fluorescence quantum yields in hydrocarbons. 

However, the fluorescence characteristics of these compounds are strongly solvent-dependent. In protic solvents such as alcohols, hydrogen bonds can be formed between the nitrogen atoms and the solvent molecules. This results in an inversion of the lowest-lying rt-n and n-n states. As the lowest-lying transition becomes of n —> n character in these solvents, the fluorescence quantum yield is much higher than in hydrocarbon solvents. 

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. 

The theory of rotation effects on prolate luminescent molecules in solution and its experimental verification have been developed and compared. Generalized diffusion equations for the rotational motion of an asymmetric rigid motor have been used to given an expression for steady-state fluorescence depolarization. " The radiationless transition from the first excited singlet state of Eosin has been measured by optoacoustic relaxation, and the absolute fluorescence quantum yields of organic dyes in poly(vinyl alcohol) have also been measured by the photoacoustic method. The accuracy of the method has been discussed in the latter paper. Actinometry in flash photolysis experiments has been assisted by new measurements on the extinction coefficient of triplet benzophenone. Matrix-isolation fluorescence spectrometry has been used to detect polycyclic aromatic hydrocarbons from gas chromatography. ... 

The solvent sensitivity of the emission spectrum and fluorescence quantum yield of pyrene and its derivatives have been used to sense the polarity of microphase interiors. By these methods, pyrene in SDS micelles is located within a microenvironment less polar than water, but more polar than typical hydrocarbons [53]. 

Fluorescence of tertiary amines has been observed by Phillips and co-workers (220), Tsumbomura and co-workers (112), and Halpern and co-workers (221-223,294). Fluorescence is reported from tertiary, but not secondary or primary, amines (220), presumably because of the ease with which the amino group can lose an H atom in these compounds (220). Fluorescence has been observed in hydrocarbon solution (112) as well as the vapor phase, with comparable quantum yields (222). For 1,4-diaza-bicyclo-[2,2,2]octane Che fluorescence quantum yields are 0.9 (vapor) and... 

The fluorescence quantum yield of anilines might depend on the excitation wavelengths, particularly those belonging to different absorption bands. In addition, this effect is generally more pronounced in nonpolar hydrocarbons than in polar hydrogen-bonding solvents,... 

Fc is the fluorescence of the dye in a lipid vesicle suspension with a lipid concentration of C F0 and Fx are the limiting dye fluorescence readings in the absence of lipid and at very high lipid concentrations, respectively. The fluorescence as a function of lipid concentration is fitted to this equation via nonlinear regression. The results of such measurements for a series of styryl dyes (10), as well as the spectral characteristics of the dyes in several solvents, are given in Table I. As expected, the membrane affinity of the dyes increases with the length of the chromophore or the number of carbons in the appended hydrocarbon chains. Also noteworthy is the striking increase in fluorescence quantum yield for the membrane-bound dyes this is probably due to the well-ordered immobile environment of the membrane, which inhibits nonradiative decay processes that require molecular motion in the excited state. 

Fluorescence spectra of Cgo and C70 in solvent glass at low temperature are much better resolved than the spectra in room-temperamre solution (Figs. 5 and 9) [8,10-13]. The spectrum of C70 is particularly well-resolved, making it possible to assign vibronic structures [11]. For Cjo, the broadness in the observed emission spectrum is partially due to a severe overlap between fluorescence and phosphorescence (Fig. 9) [10]. According to an early estimate, [8] the fluorescence quantum yields of C q and C70 in 77K hydrocarbon sol-... 

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