Pyrene fluorescence quantum yield

Alternatively one can use steady state fluorescence intensities to obtain information about the cyclization process. For example, the ratio of excimer to pyrene fluorescence quantum yields is... 

Montalti and co-workers studied dansyl [27] and pyrene [28] derivatives and found the fluorescence quantum yields and excited-state lifetime of these two dyes increased in DDSNs. They attributed the enhancements to the shielding effect from the quenchers or polar solvent in the suspension. Their studies also demonstrated that the lifetime of the doped dye molecules was also dependent on the size of the DDSNs. Small DDSNs had a larger population of the short-living moieties that were more sensitive to the environment outside the DDSN. In contrast, the large DDSN had a larger population of the long-living moieties that were not sensitive to the environment. 

PET-17 has been designed for selective recognition of sodium (Figure 10.15). It contains four carbonyl functions, two of them being linked to pyrene and nitrobenzene at opposite sites on the calixarene lower rim. Complexation with Na+ prevents close approach of pyrene and nitrobenzene and thus reduces the probability of PET. The fluorescence quantum yield increases from 0.0025 to 0.016. 

Photolytic. Schwarz and Wasik (1976) reported a fluorescence quantum yield of 0.3 for benzole]pyrene in water. 

In a 5-m deep surface water body, the calculated half-lives for direct photochemical transformation at 40 °N latitude in the midsummer during midday were 5.9 and 4.2 d with and without sediment-water partitioning, respectively (Zepp and Schlotzhauer, 1979). Schwarz and Wasik (1976) reported a fluorescence quantum yield of 0.69 for pyrene in water. 

PAHs also generally have well-structured emission spectra (see Figs. 10.6-10.10) and relatively large fluorescence quantum yields. For example, in degassed n-heptane at room temperature, the fluorescence quantum yields are as follows fluoranthene, 0.35 benz[ ]anthracene, 0.23 chrysene, 0.18 BaP, 0.60 BeP, 0.11 and benzo[g/zi]perylene, 0.29 (Heinrich and Giisten,1980). Cyclopenta[crf]pyrene, however, does not fluoresce. 

Fluorescence techniques have been used with great success in the study of PEO-fe-PSt micelles [64]. In this study, the effect of polymer concentration on the fluorescence of pyrene present in water at saturation was studied. Three features of the absorption and emission spectra change when micellization occurs. First, the low-energy band of the (S2-So) transition is shifted from 332.5 to 338 nm. Second, the lifetime of the pyrene fluorescence decay increases from 200 to ca. 350 ns, accompanied by a corresponding increase in the fluorescence quantum yield. Third, the vibrational fine structure changes, as the transfer of pyrene from a polar environment to a nonpolar one suppresses the permissibility of the symmetry-forbidden (0,0) band. 

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]. 

The fluorescence spectrum of dilute pyrene (<10 4m) in degassed cyclohexane exhibits vibronic structure and has a maximum at 395 nm. As the concentration of pyrene is increased, the fluorescence quantum yield of pyrene decreases and a broad, structureless emission band with a maximum at about 480 nm gains in intensity (Figure 2.22, left). 

The fluorescence quantum yields of pyrene-1-carboxaldehyde in water and methanol are 0.98 and 0.07/ an effect attributed to solvent effects on 7c,n and n,n states. Cycloaddition reactions of 1-naphthonitrile to 1,2-dimethyl-cyclopentene are attributed to both and Lj, states.It is pointed out that although dual fluorescence is known, this is the first example of divergent reaction from two nearly isoenergetic singlet states. An analysis of the u.v. spectra of some acyl pyridines, including a theoretical examination of the molecular geometry, and excited states of bipyrimidine compounds have also been made. Photo tautomerism and the fluorescence of the cation of 4-amino-pyrazole[3,4-iflpyrimidine, an analogue of adenine, has been published by Wierzchowski et Intramolecular heteroexcimer formation in... 

Steady-state fluorescence spectra were recorded ( ex = 347 nm) on a Perkin-Elmer model 650-10S fluorescence spectrometer. The absorbance of pyrene (10" M) in each solution was between 0.01 and 0.05 at the excitation wavelength. Fluorescence quantum yields were obtained by comparison with a quinine bisulfate... 

Without interactions with potential host molecules and in diluted solutions to avoid excimeric formations, pyrene presents in solution an intense and anisotropic fluorescence, as well as a high fluorescence quantum yield [34-37], Direct evidence of ground-state interactions of pyrene with potential host molecules is provided by the emission spectra. The vibrational structure of the emission spectrum of pyrene is constituted by five fine peaks, named I, I2, h, I4, and I5 (Fig. 13.2) [38]. An increase of the intensity of peak Ii is observed in polar solvents, while I, is solvent insensitive. Thus, the evolution of the ratio of intensities /1//3 gives information on the evolution of the polarity of the environment close to molecular pyrene, and consequently on the encapsulation of this guest in a host molecular or supramolecular object [39]. This sensitivity of pyrene, and of peri-fused polycyclic aromatic hydrocarbon molecules in general, to the polarity of the environment is a photophysic property that is extensively used to study host-guest interactions [40]. 

Maeda H, Maeda T, Mizuno K, Fujimoto K, Shimizu H, Inouye M. Alkynylpyrenes as improved pyrene-based biomolecular probes with the advantages of high fluorescence quantum yields and long absorption/emission wavelengths. Chem Eur J 2006 12 824-31. 

The quantum yield is an indicator of how efficient a particular process is. However, some care must be taken in comparing quantum yields for different systems, because the quantum yield is always measured relative to other processes in the molecule. For example, pyrene derivative. The reason this is not reflected in the quantum yields is that we must also consider competing processes. The ISC rate for pyrene-3-carboxaldehyde is also much faster than that for benzene due to the ability to access a ( ,tt ) state that is not available for benzene. This competing ISC process limits the amount of fluorescence, and by coincidence the two compounds end up with the same fluorescence quantum yield. Thus, while the quantum yield tells you about the efficiency of a process for a given molecule, it alone cannot tell you why the process is or is not efficient. 

Figure 20 Fluorescence quantum yields of pyrene in supercritical CO2 (35 C) at... 

The attachment of pyrene or another fluorescent marker to a phospholipid or its addition to an insoluble monolayer facilitates their study via fluorescence spectroscopy [163]. Pyrene is often chosen due to its high quantum yield and spectroscopic sensitivity to the polarity of the local environment. In addition, one of several amphiphilic quenching molecules allows measurement of the pyrene lateral diffusion in the mono-layer via the change in the fluorescence decay due to the bimolecular quenching reaction [164,165]. 

Fluorescence spectra and quantum yields of pyrene in supercritical CO2 have been determined systematically as functions of temperature, CO2 density, and pyrene concentration. Under near-critical conditions, contributions of the pyrene excimer emission in observed fluorescence spectra are abnormally large. The results cannot be explained in the context of the classical photophysical mechanism well established for pyrene in normal liquid solvents. The photophysical behavior of pyrene in a supercritical fluid is indeed unusual. The experimental results can be rationalized with a proposal that the local concentration of pyrene monomer in the vicinity of an excited pyrene molecule is higher than the bulk in a supercritical solvent environment. It is shown that the calculated ratios between the local and bulk concentrations deviate from unity more significantly under near-critical conditions (Sun and Bunker, 1995). 

Table II. Quenching of Pyrene Fluorescence by MV and Quantum Yield of MV Formation...

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. 

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