Stoke’s shift

Figure 9.2 Typical spectral scan of a fluorescent compound showing its absorbance peak or wavelengths of most efficient excitation and its emission peak or wavelengths where light emission occurs. The Stoke s shift is the distance in nanometers between the absorbance peak and the emission peak. The larger the Stoke s shift, the less interference that will occur from excitation light when measuring fluorescence emission.

The emission spectra of BODIPY derivatives normally display narrow bandwidths, providing intensely fluorescent labels for biomolecules. Unfortunately, they also have very small Stoke s shifts, typically on the order of only 10-20 nm. Excitation at the optimal wavelength may cause some interference in measurements at the emission wavelength due to light scattering or cross-over from the wide bandwidth of the excitation source. The dyes usually require excitation at sub-optimal wavelengths to prevent this problem. 

This fluorophore has an excitation maximum at 502 nm and an emission maximum at 510nm. The small Stoke s shift of only 8nm 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 77,000M 1cm 1 at 502nm. 

The excitation maximum for the molecule occurs at 535 nm and its emission at 552 nm. Its Stoke s shift is slightly greater than some of the other BODIPY fluorophores, producing a 17nm separation between excitation and emission peaks. BODIPY 530/550 C3 has an extinction coefficient in methanol of about 62,000 M 1 cm-1 at 535 nm. 

The excitation maximum for BODIPY 493/503 C3 hydrazide occurs at 498 nm and its emission at 506 nm. Since this is an extremely small Stoke s shift, it may be difficult to avoid completely problems of excitation-light scattering interference in critical emission measurements unless sub-optimal excitation wavelengths are used. The molecule has an extinction coefficient in methanol of about 92,000M-1cm 1 at 493 nm. 

The excitation maximum for Br-BODIPY 493/503 is 515nm and its emission occurs at 525 nm when dissolved in methanol. Upon coupling to a sulfhydryl compound, however, the excitation wavelength of the adduct decreases to 493 nm and its emission drops to 503 nm. The very small lOnm Stoke s shift may be a problem, particularly in avoiding interference due to of excitation-light scattering in critical emission measurements. Sub-optimal excitation wavelengths... 

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 Stoke s 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 cm-1 at 426nm. 

STOICHIOMETRIC NUMBER Stoichiometry of elementary reactions, CHEMICAL KINETICS MOLECULARITY UNIMOLECULAR BIMOLECULAR TRANSITION-STATE THEORY ELEMENTARY REACTION STOKE S SHIFT... 

The excited state lifetimes and luminescence properties of metal complexes are related to the relative positions of the potential energy wells shown in Figure 4.77. On the left we have a lowest excited state which resembles geometrically the ground state (the internuclear distances, r, are similar). The crossing between these states requires a high activation barrier E (in a classical picture) and the excited state lifetime is therefore relatively long. The Stoke s shift between the absorption band (a) and the emission band (e)... 

There is another artefact that can arise with concentrated samples, and this is the reabsorption of emission light by the sample itself. Light which is emitted at the centre of the cell must travel through a pathlength of a few mm of the sample, and absorption will take place in the wavelength region of overlap of the emission and absorption spectra. This problem can be serious when the Stoke s shift of these spectra is small. Reabsorption then results in an apparent red shift of the emission maximum with increasing concentration. 

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