Fluorophores Stokes shift

In time-resolved fluorescence, rare earths are frequently used as fluorescent labels. The fluorophores have large Stokes shifts, ie, shifts of the emitted light to a higher wavelength relative to the absorption wavelength, and comparatively long decay times, approximately 0.5 ms. This simplifies the optical... 

Fluorescent probes are divided in two categories, i.e., intrinsic and extrinsic probes. Tryptophan is the most widely used intrinsic probe. The absorption spectrum, centered at 280 nm, displays two overlapping absorbance transitions. In contrast, the fluorescence emission spectrum is broad and is characterized by a large Stokes shift, which varies with the polarity of the environment. The fluorescence emission peak is at about 350 nm in water but the peak shifts to about 315 nm in nonpolar media, such as within the hydrophobic core of folded proteins. Vitamin A, located in milk fat globules, may be used as an intrinsic probe to follow, for example, the changes of triglyceride physical state as a function of temperature [20]. Extrinsic probes are used to characterize molecular events when intrinsic fluorophores are absent or are so numerous that the interpretation of the data becomes ambiguous. Extrinsic probes may also be used to obtain additional or complementary information from a specific macromolecular domain or from an oil water interface. 

Overall, the significant Stokes shift of 100 nm and the good quantum yields make the coumarin dye a powerful fluorescent probe for nucleic acids assays or cell biology. The postsynthetic click chemistry makes this fluorophore readily accessible for fluorescent labeling of nucleic acids. 

Another important feature of fluorophores is the amount of vibrational energy lost in the excited state. The difference between emission and excitation maxima gives a readout in this respect and is referred to as the Stokes shift. In many sensors, a small Stokes shift is unfavorable for FRET ratio measurements due to overlap of emission spectra. 

Photoluminescence (PL) in the polysilanes is well documented,34b,34c and for the poly(diarylsilane)s occurs typically with a small Stokes shift and almost mirror image profile of the UV absorption.59 This is due to the similarity of the chromophore and fluorophore structures in the ground and excited states, respectively, which is a result of the fact that little structural change occurs on excitation of the electrons from the a to the a orbitals. As PL is the emissive counterpart to UV, the emissive counterpart to CD is circularly polarized pho-toluminescence (CPPL). Where the fluorophore is chiral, then the photoexcited state can return to the ground state with emission of circularly polarized light, the direction of polarization of which depends on the relative intensities of the right-handed and left-handed emissions (/R and /l, respectively), which in turn depends on the chirality of the material, or more accurately, the chirality... 

Stokes shift The difference between the absorption maximum and emission maximum of fluorescent dyes or fluorophores. The Stokes shift is... 

The broad emission and low-fluorescence quantum yield of PPS suggested a distribution of trapping sites in the Si skeleton, which were also considered responsible for the lower-than-expected conductivity. The far-IR spectrum of PPS suggested the existence of cyclohexasilane rings connected by linear chains.361,362 Subsequent investigations by Irie et al. on the electronic absorption spectra of radical ions of poly(alkylsilyne)s were taken to indicate the presence of various cyclic silicon species, in corroboration of this conclusion.363 The large Stokes shift and broadness of the fluorescence emission indicate a range of fluorophore structures, different from the chromophore structures. This is... 

This fluorophore has an excitation maximum at 502 nm and an emission maximum at 510 nm. Tlie small Stokes shift of only 8 nm creates some difficulty in discrete excitation without contaminating the emission measurement with scattered or overlapping light. The extinction coefficient of the molecule in methanol is about 77,000 M-1cm-1 at 502 nm. 

The excitation maximum for the molecule occurs at 5 35 nm and its emission at 552 nm. Its Stokes shift is slightly greater than some of the other BODIPY fluorophores, but a 17-nm shift still may not be enough to avoid completely problems of excitation-light interference in emission measurements. BODIPY 530/550 C3 has an extinction coefficient in methanol of about 62,000 M cm 1 at 535 nm. 

The spectral characteristics of Lucifer Yellow iodoacetamide produce luminescence at somewhat higher wavelengths than the green luminescence of fluorescein, thus the yellow designation in its name. The excitation maximum for the probe occurs at 426 nm, and its emission at 530 nm. The rather large Stokes shift makes sensitive measurements of emission intensity possible without interference by scattered excitation-light. The 2-mercaptoethanol derivative of the fluorophore has an extinction coefficient at pH 7 of about 13,000 M-1cm-1 at 426 nm. 

Since all novel furans 95 and pyrroles 98 exhibit a strong blue fluorescence with considerable Stokes shifts, where the absorption maxima are found in the range 7.max, abs = 312-327 nm and the emission occurs at 7,max at,s = 401-451 nm, this multi-component approach to fluorophores can be exploited for combinatorial optimization of emission properties. 

FIGURE 23.2 Stokes shift diagram depicting optimal relationships between excitation and fluorescence light of a fluorophore suitable for microarray applications. 

When absorption and/or the emission spectra of a fluorophore possess two or more bands, the Stokes shift is equal to the difference that separates the two most intense bands of the two spectra. 

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