Quantum yield spectroscopic techniques

Fluorescence quenching studies of these alkylated FBAs in aqueous solution were carried out using spectroscopic techniques. For the higher members of each series, plots of fluorescence quantum yield against agent concentration showed a sharp decrease in fluorescence intensity at a specific concentration, in contrast to the smoothly decreasing curve characteristic of the lower alkyl or unsubstituted FBAs. The kinetics of fluorescence... 

The quantum yield for the primary photochemical process differs from that of the end product when secondary reactions occur. Transient species produced as intermediates can only be studied by special techniques such as flash photolysis, rotating sector devices, use of scavengers, etc. Suitable spectroscopic techniques can be utilized for their observations (UV, IR, NMR, ESR, etc.). A low quantum yield for reaction in solutions may sometimes be caused by recombination of the products due to solvent cage effect. 

To study the excited state one may use transient absorption or time-resolved fluorescence techniques. In both cases, DNA poses many problems. Its steady-state spectra are situated in the near ultraviolet spectral region which is not easily accessible by standard spectroscopic methods. Moreover, DNA and its constituents are characterised by extremely low fluorescence quantum yields (<10 4) which renders fluorescence studies particularly difficult. Based on steady-state measurements, it was estimated that the excited state lifetimes of the monomeric constituents are very short, about a picosecond [1]. Indeed, such an ultrafast deactivation of their excited states may reduce their reactivity something which has been referred to as a "natural protection against photodamage. To what extent the situation is the same for the polymeric DNA molecule is not clear, but longer excited state lifetimes on the nanosecond time scale, possibly of excimer like origin, have been reported [2-4],... 

When deciding to study the dynamics of electronic excitation energy transfer in molecular systems by conventional spectroscopic techniques (in contrast to those based on non-linear properties such as photon echo spectroscopy) one has the choice between time-resolved fluorescence and transient absorption. This choice is not inconsequential because the two techniques do not necessarily monitor the same populations. Fluorescence is a very sensitive technique, in the sense that single photons can be detected. In contrast to transient absorption, it monitors solely excited state populations this is the reason for our choice. But, when dealing with DNA components whose quantum yield is as low as 10-4, [3,30] such experiments are far from trivial. 

Various spectroscopic techniques and probes have been used to investigate solubilization of probe molecules, mostly using UV/visible spectroscopy, fluorescence spectroscopy, ESR spectroscopy [64, 74, 217, 287] and NMR-spectro-scopy [367-369]. Fluorescence spectroscopy is particularly versatile [370], as various static and dynamic aspects can be covered by studying excitation and emission spectra, excimer or exciplex formation, quantum yields, quenching, fluorescence life-times, fluorescence depolarization, energy transfer etc. 

Many tetrazines undergo an irreversible photochemical fragmentation with reasonable quantum yield even at very low temperatures. This fact has been used in several applications of the new spectroscopic technique of photochemical hole burning (HB). In the following sections brief descriptions of the principles of this method with references to applications to tetrazines, especially 3,6-dimethyl-1,2,4,5-tetrazine are given. 

Molecules that are not amenable to LIF detection because of their low absorption cross sections or low fluorescence quantum yields may be analyzed through a variety of Indirect fluorescence techniques. For example, non-fluorescent solutes may be detected by the extent to which they enhance (52) or quench (53) the emission of a fluorescent dye added to the mobile phase. Alternatively, simple displacement of the dye molecule In the column effluent by a non-fluorescent solute will result In a reduction of the fluorescence Intensity, which can provide nearly universal detection capability with adequate sensitivity for many applications (54). Finally, non-fluorescent molecules may be rendered detectable through the incorporation of a fluorescent label that Is optimized for the laser excitation wavelength. Derlvatlzatlon methods are particularly attractive because they introduce an additional dimension of chemical and spectroscopic selectivity that can simplify analyses In a predictable and reproducible manner. 

Electronic (absorption and emission) spectroscopies are among the most widely applied experimental techniques in supramolecular chemistry [1]. This section provides a condensed overview of the principles and uses of UV-Vis absorption and emission (fluorescence and phosphorescence) spectroscopies in the study of cydodextrin (CyD) indusion complexes. The emphasis will be on a presentation of the main effects of complex formation on measured spectra, quantum yields, and kinetics. This latter point will be treated in a separate section as it exemplifies the power of spectroscopic techniques in supramolecular studies. Only nonderiva-tized CyDs will be discussed. This is not a comprehensive review, cited references, taken from the literature of the literature of the past ten years, are mainly intended to provide illustrative examples. 

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