Luminescence frozen solutions

James Dewar observed in 1894 phosphorescence from frozen solutions utilizing liquid air [5], Jean Becquerel discovers in 1907 that samples frozen at liquid air temperatures considerably narrow the spectral shape and increased information is obtained from the luminescence spectra [26],... 

Biological, chemical. X-ray diffraction, infrared absorption, e.s.r., n.m.r., luminescence, and quantum studies show that dimer formation is universally observed in irradiated frozen solutions of thymine, thymidine, uridine, thymidylic acid and related compounds, and in DNA [560—576]. The purines of DNA, on the other hand, are little affected [577, 578]. Thymine dimers obtained in frozen solution can be converted to the original monomers by ultraviolet... 

The three complexes are luminescent in the solid state at room temperature and at 77 K or in frozen solutions of dichloromethane or acetone. The optical behavior in frozen solution is similar for the three complexes and also similar to that of the precursor gold derivatives [AuR(tht)], but not in the solid state. The differences in this state are attributed to the different number and types of metal-metal interactions present in each complex. In agreement with this assumption,... 

It has long been recognized that both the diffuse spectra and quenching problems can be alleviated by performing the fluorescence measurement in a low-temperature solid matrix, rather than in a fluid solution. The most common low-temperature matrices used in molecular fluorometric analysis are frozen liquid solutions the analytical characteristics of frozen-solution luminescence spectrometry have been discussed extensively in the literature (2-10). Obviously, MI represents an alternative technique to use of frozen liquid solutions for low-temperature fluorometric analysis. There are two principal advantages of MI over frozen-solution fluorometry. First, in MI, any material which has an appreciable vapor pressure at room temperature can be used as a matrix one is not limited by the... 

Fig. 5. Sample cell compartment unit for cryogenic luminescence measurements of frozen solutions. (1) metallic sample cell (2) analyte solution (3) liquid nitrogen layer (4) sample cell compartment (5) mirror (6) lenses (7) excitation source (8) filter (9) monochromator...

Cr(III) exhibits luminescence in crystallophosphors, in frozen solutions and in extracts of Cr(III) complexes with inorganic and organic ligands309,337 544 5fi3). 

Where kp and km are the rate constant of fluorescence and non-radiative processes, respectively. The fluorescence quantum yield (Of) value in the range of 0.0 to 1.0. If the non-radiative relaxation is fast compared to fluorescence (km > k,), O will be small, that is the compound will fluoresce very little or not at all. Often different non-radiative events are limited in the solid phase, and long-lived luminescence (e.g. phosphorescence) is often studied in frozen solution or other solid phases. Quenchers make non-radiative relaxation routes more favorable and often there is a simple relation between 0 and the quencher concentration. The hest-known quencher is probably O2, which quench almost all fluorophores other quenchers only quench a limited range of fluorophores. If a molecule is subject to intramolecular quenching, O may yield information about the structure. 

This Cl model has been widely employed for explaining the low quantum yield of ESIPT chromophores. However, two ciystal polymorphs and the amorphous solid of 11 exhibit bright ESIPT luminescence with relatively high quantum yields (Table 2), clearly demonstrating emission enhancement in the solid state. Since the amorphous powder and the frozen solution also show bright yellow luminescence, no specific mode of molecular packing is required for the emission enhancement. Therefore, the observed emission enhancement is ascribed to the result of RIR by inhibition of the radiationless decay pathway through Cl. 

The emission spectra obtained from these frozen solutions of [(C6HnNC)2 Au ](PF6) also vary as the solvent is changed [38]. Visually the effect is not as striking as it is in the case of frozen solutions of [Au C(NHMe)2 2](PF6)-0.5(acetone). Relevant spectra are shown in Fig. 31 for 6.0 mM solutions of [(C6HiiNC)2Au ](PF6). Dilution of the solutions of [(C6HnNC)2Au ](PF6) can also produce significant changes in the luminescence from some solutions as was the case with [Au C(NHMe)2 2](PF6) 0.5(acetone) as well. 

Freeze-dried aequorin is also quite stable, but the process of drying always causes a loss of luminescence activity (see the last part of Section 4.1.2). All forms of aequorin are satisfactorily stable for many years at — 50°C or below, but all rapidly deteriorate at temperatures above 30-35° C. A solution of aequorin should be stored frozen whenever possible because repeated freeze-thaw cycles cause little harm to aequorin activity. 

An ingenious variation on the standard fluorescence methods was proposed by Red kin et al. [50]. Water samples were extracted with non-polar solvents, transferred into hexane and the hexane solution frozen at 77 K. At that temperature the normally diffuse luminescence emission bands are present as sharp emission lines, making identification of fluorescing compounds considerably simpler. In the case of a complex mixture, some separation by column or thin layer chromatography might be necessary. 

Most of the described liquid CL systems are employed at ambient temperatures although CL emission may also be generated at very low temperatures. For instance, a low-temperature interaction of U(IV) and Xe03 in frozen aqueous H2S04 solutions accompanied by CL emission was studied by Lotnik et al. [17]. It was shown that the peak of luminescence at 195-200 K is related to CL of... 

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