Fluorescence bands

PMDs demonstrate pronounced absorption and contain fluorescence bands that are relatively narrow and highly intense, which arise from electron transitions occurring within the polymethine chromophore... 

Quinacrine concentrates in the scolex of the parasite and causes the muscles needed for holding onto the intestinal wall to relax. The worms are stained yellow and pass from the body, still aUve. Quinacrine can intercalate with DNA and inhibit nucleic acid synthesis. It creates fluorescent bands in deoxyadenylate—deoxythmidylate-rich regions of DNA and has been used as a stain in the study of human genetics. 

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

In 159 and 163-166 the tertiary amine function is coordinated to the boron atom and transmits the electronic change due to the ester formation to the chromophore. In 160-162 the boron atom is directly connected to the chromophore. After the complexation of the saccharide, the change of the charge transfer, e.g., for 159 [249-251], or the fluorescence bands, e.g., for 160-166 [252-255], can be measured and interpreted. The most selective binding of n-glucose has been achieved with host 164 that forms a 1 1 complex with a macrocyclic structure (Scheme 1). 

Luminescence studies revealed that compounds (119)-(121) displayed broad emission bands at 298 K, with ranging from 387 nm to 371 nm for the P to Bi derivatives, respectively [92]. On cooling to 77 K, compound (119) displays both a fluorescent band nm) and a phosphorescent... 

In a recent study, poly(aryl ether) dendritic branches terminated with triethyleneglycol chains were attached to Cgg [66] dendrimer 32 represents the fourth generation. The photophysical properties of these fullerodendrimers have been systematically investigated in three solvents, namely toluene, dichloromethane, and acetonitrile. On increasing dendrimer generation, it has been found that in each solvent (i) the maximum of the fullerene fluorescence band is red-shifted... 

The first observations on the fluorescence of colloidal CdS were made using a colloid stabilized by colloidal silicon dioxide . The fluorescence spectrum consisted of a broad band with the maximum between 580 nm and 650 nm. The reproducibility of this red fluorescence was very poor. In the presence of excess Cd ions the intensity of the fluorescence was increased, which indicates that anion vacancies were centers of luminescence. Aging of the sol for a few weeks in the dark and in the absence of air was accompanied by an increase in fluorescence intensity by a factor of ten and a gradual red shift of the fluorescence band. However, even after this increase, the fluorescence quantum yield was still below 10 . ... 

Figure 28 (unbroken line) shows the absorption and fluorescence spectra of colloidal ZnO in aqueous solution at pH = 11.7 The weak fluorescence band at wavelengths close to the onset of absorption was attributed to the fluorescence of the... 

The Q-materials of CdjPj and CdjAsj mentioned in the preceding section fluoresce strongly both in solution and in the solid state. The fluorescence band, which often shows a structure, has its maximum at a wavelength substantially longer than that of the onset of absorption. 10 nm particles fluoresce mainly in the infrared, 2nm particles fluoresce green. All kinds of fluorescence color were observed for particles within this size range 62,63.240)... 

We see then that the relative fluorescence quantum yield can be determined by measuring the areas under the fluorescence bands of the sample and the fluorescent standard. However, these spectra must be corrected before their true areas can be determined. Several factors are responsible for this. The most important of these are the phototube and monochromator responses. For most phototubes the maximum response occurs within a limited wavelength range, falling off rather sharply in some cases at the short-and long-wavelength ends. This is illustrated in Figure 2.14. Similarly,... 

If the refractive indices of the solvents used for the sample and the fluorescence standard are not the same, a further correction must be made. For example, quinine sulfate in 0.1 N H2S04 (Or = 0.5) is commonly used as a fluorescence standard. If the fluorescence of the sample whose relative quantum yield is desired is determined in benzene, a correction factor of 27% must be applied in determining the relative areas under the fluorescence bands. If ethanol is used, this correction is only 5.5%. 

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. 

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... 

Dependencies of luminescence bands (both fluorescence and phosphorescence), anisotropy of emission, and its lifetime on a frequency of excitation, when fluorescence is excited at the red edge of absorption spectrum. Panel a of Fig. 5 shows the fluorescence spectra at different excitations for the solutes with the 0-0 transitions close to vI vn, and vra frequencies. Spectral location of all shown fluorescence bands is different and stable in time of experiment and during lifetime of fluorescence (panel b)... 

These experiments have firmly established that the red shift of the fluorescence band in polar solutions with the change of excitation quanta energy is caused by inhomogeneous configurational broadening of electronic energy levels. Later, it was found that not only singlet but also triplet states of dyes are broadened... 

Red shift of fluorescence band with the increase of dye concentration due to directed nonradiative energy homotransfer (DHT) (owing to FRET mechanism) from the blue to the red centers of sample (see panel b in Fig. 5). 

There are single- and multiparameter approaches for determining the polarity and separation of contribution of different interactions to the total effect of polarity on spectroscopic characteristics. They are based on different theories of solvatochromic shifts of absorption and fluorescence bands. 

Anthracene has also been used as an acceptor (Fig. 10). In solution, 26 emits a single fluorescence band that is somewhat structured in nonpolar solvents and becomes broad and structureless with increasing polarity [58]. The strongly hindered molecule 27 also exhibits a similar behavior, but its absorption spectrum is better structured [59]. The rate of formation of a charge transfer state is higher for 27 than for 26. Based on this observation, it appears that the twist around the anthryl-phenyl C-C bond plays a significant role in the fluorescence profile of the probes [60]. Acridines, such as 28, behave similarly to anthracene except that acridine is a better electron acceptor [61]. 

Among other factors, the quantum yield of fluorescence determines the intensity of light emission in a CL. This, as well as the position in the spectrum occupied by the fluorescence band, is largely a function of the molecular structure. 

Shim and coworkers [129] synthesized poly(2-fluoro-l,4-phenylene vinylene) 75 by the thermal conversion method. This polymer exhibits almost the same absorbance spectra as PPV 1 (Amax 410 nm), but the fluorescence band (Amax = 560 nm) is red-shifted by ca. 20 nm. The LUMO level was shifted down by ca. 0.15eV, facilitating electron injection but, in contrast to the above polymer 74, no fluorescence quenching was observed. Consequently, the PLED devices fabricated as ITO/75/A1 have about ten times higher EL efficiency than those fabricated with PPV 1 under identical conditions. 

Fluorescence-activated cell sorter, 26 971 Fluorescence band maxima, 20 512 Fluorescence detection, 17 635 Fluorescence detectors, liquid... 

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