Sensing excimer

Various photochemical reaction mechanisms lend themselves for variation of the above parameters, and the more important ones are summarized above. In general, one can expect for the volume necessary for the reaction to occur that it decreases in the sense Excimer > TICT > Butterfly > ESIPT mechanism. Therefore, for a given size distribution of microscopic free volume voids, the fraction of the total free volume usable for the reaction and thus amenable to probing increases in the same sense with the ESIPT mechanism being the outstanding extreme, because this reaction cannot even be stopped in a rigid matrix at very low temperature. 39 ... 

There are many possibilities to use these complex formations in fluorescence sensing. If the excimer is not formed, we observe emission of the monomer only, and upon its formation there appears characteristic emission of the excimer. We just need to make a sensor, in which its free and target-bound forms differ in the ability of reporter dye to form excimers and the fluorescence spectra will report on the sensing event. Since we will observe transition between two spectroscopic forms, the analyte binding will result in increase in intensity of one of the forms and decrease of the other form with the observation of isoemissive point [22]. 

Ueno et al. also prepared the b i s (2 - n ap h t h y I s u I fe n y 1) - y - CD series in which the naphthyl moieties are very limited in their movement because the linker between naphthalene and CD is only sulfur [37], All isomers of 12(AB), 13(AC), 14(AD), and 15(AE) exhibit only monomer fluorescence due to the rigid linker. It means that the two naphthalene moieties cannot take face-to-face orientation because of the limited flexibility. Although the excimer cannot be used for sensing molecules, Ueno et al. found that the monomer fluorescence intensity increases with increasing guest concentration. Thus, this modified CD series can be used as chemosensors of a different type. 

It is believed that two crowns bind a K+ ion, thus leading to loosely preassembled bis-pyrene subunits. When the metal ion recognition event occurs in the presence of y-CD, dramatic enhancements of excimer emission are observed, thus providing a method to selectively detect K+ ion in aqueous solution. This ability to sense an ionic analyte is unusual for CD chemosensors. Modification of 28 with other ion binding sites should provide a general method for cation detection. 

The exciraer decay at 475 nm was not adequately matched by dual exponential fits would be expected according to the conventional monomer/excimer scheme 1. Due to uncertainties with the method in correction for photomultiplier response as a function of wavelength, analysis of excimer kinetics in terms of three exponential terms, although indicated by the data, is not justifiable in any quantitative sense. Decay parameters obtained are given in Table 4. 

Photodimerization often involves an excimer that can be treated as a su-permolecule. (Cf. Section 6.2.3.) Then, the state correlation diagram for the singlet process (Figure 7.27a) ordinarily calls for a two-step return from S, to So along the concerted reaction path. First, an excimer intermediate E is formed. Second, a thermally activated step takes the system to the diagonally distorted pericyclic funnel P" (cf. Section 4.4.1), and the return to So that follows is essentially immediate. The reaction will be stereospecific and concerted in the sense that the new bonds form in concert. However, it will not be concerted in the other sense of the word, in that it involves an intermediate E. ... 

Alcohol sensing. An etched FBG was used [28] to discriminate between methanol (n - 1.326), ethanol (n - 1.359), and isopropyl alcohol (IPA) n =1.378) (Fig. 9). The FBG was written using a KrF excimer laser (248 nm) with a length of 2.5 mm. It was etched down to 15 pm diameter using a 52% solution of hydrofluoric (HF) acid and then to a diameter of 6 pm with a 13% HF solution. The plot of the center wavelengths of the thinned FBG vs ambient refractive index is shown in the next figure. 

In fact, dimers are often interesting and important. For example, alkali dimers are thought to undergo triplet-triplet electronic transitions which are excimer-like, in the sense that the lower state is repulsive (cf section 2.27). They are therefore potentially useful as broadband tunable laser sources [652, 653]. 

Tan and colleagues also used the aptamer beacon strategy to develop realtime sensing of thrombin.In addition to using fluorescence quenching, they also used both FRET and excimer strategies that allowed for significant... 

There are a number of studies on pyrene labelled or doped polymer materials. Dendritic size and electrostatic forces in poly(amido) dendrimers with pyrene residues have been determined through the use of a variety of quenchers. As the dendritic size increases so the Stern-Volmer quenching constant decreases, owing to blocking of the pyrene chromophore by the growing dendritic network. The photophysical properties of poly(acrylic acids) tagged with pyrene has been measured in micellar media at various pHs." The ratio of monomer to excimer gave valuable information on the micropolarity sensed by the pyrene label, as well as the influence of external stimuli. The surfactant... 

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