Fluorescent quenching groups

Heat Treatment, Acids and Bases. Many pesticides are non-fluorescent or only barely fluorescent because they contain fluorescence quenching groups such as nitro, carboxyl or phosphate. The removal of these groups liberates the aromatic moiety which may be fluorescent depending on the structure. One obvious approach to remove these groups is to use an acid or a base, but sometimes heat is sufficient to break down the molecule into new fluorescent species. 

The fluorescence quenching depends on the content of the Phen units (the x values) in APh-x. An aqueous solution of APh-9 contained as many charged groups (SOJ) as about 10 times that of APh-50, when compared at the same molar concentration of the Phen residues. When AMPS homopolymer (PAMPS) was added to a solution of APh-50 so that the SOJ residue concentration was equal to that for APh-9, the kq value for the APh-50 quenching by MV2 + decreased from 2.1 x 1012 to 4.2 x 1011 M-1 s 1, which is close to the kq value for APh-9 (Table 2). From these facts the lower kq values for APh-x with lower x (higher... 

More recently, several groups have investigated electrostatic effects on the fluorescence quenching of hydrophobic chromophores covalently attached to various polyanions. The photophysics of the chromophores incorporated in the polyeletrolytes at small mole fractions is relatively simple, because no interaction is expected to occur between the incorporated chromophores. For this reason, most of the studies have focused on amphiphilic polyeletrolytes loaded with a low amount of hydrophobic chromophores. 

It has been shown in Chapter 5, the fluorescence quenching of the DPA moiety by MV2 + is very efficient in an alkaline solution [60]. On the other hand, Delaire et al. [124] showed that the quenching in an acidic solution (pH 1.5-3.0) was less effective (kq = 2.5 x 109 M 1 s 1) i.e., it was slower than the diffusion-controlled limit. They interpreted this finding as due to the reduced accessibility of the quencher to the DPA group located in the hydrophobic domain of protonated PMA at acidic pH. An important observation is that, in a basic medium, laser excitation of the PMAvDPA-MV2 + system yielded no transient absorption. This implies that a rapid back ET occurs after very efficient fluorescence quenching. 

The second group of intermolecular reactions (2) includes [1, 2, 9, 10, 13, 14] electron transfer, exciplex and excimer formations, and proton transfer processes (Table 1). Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. PET is involved in many photochemical reactions and plays... 

The next group of bimolecular interactions (3) shown in Table 1, includes noncontact interactions, in which fluorescence quenching occurs due to radiative and nonradiative excitation energy transfer [1, 2, 13, 25, 26]. Energy transfer from an excited molecule (donor) to another molecule (acceptor), which is chemically different and is not in contact with the donor, may be presented according to the scheme ... 

Most fluorescent PET molecular sensors, including pH indicators of this type, consist of a fluorophore linked to an amine moiety via a methylene spacer. Photo-induced electron transfer (see Chapter 4, Section 4.3), which takes place from amino groups to aromatic hydrocarbons, causes fluorescence quenching of the latter. When the amino group is protonated (or strongly interacts with a cation), electron transfer is hindered and a very large enhancement of fluorescence is observed. 

Calixarene containing a dioxotetraaza unit, PET-18, is responsive to transition metal ions like Zn2+ and Ni2+. Interaction of Zn2+ with the amino groups induces a fluorescence enhancement according to the PET principle. In contrast, some fluorescence quenching is observed in the case of Ni2+. PET from the fluorophore to the metal ion is a reasonable explanation but energy transfer by electron exchange (Dexter mechanism) cannot be excluded. 

Fluorescent cellulose triacetate membranes were prepared by incorporation of pyrene-butyric acid (219), and were applied to in situ detection of ground water contamination by explosives, based on fluorescence quenching by the nitro groups LOD 2 mg/L of DNT (220) and TNT (221) and 10 mg/L for RDX (276) the response follows the Stern-Volmer law for DNT and TNT442. 

This type of probe, often called fluorescent photoinduced electron transfer (PET) sensors, has been extensively studied (for reviews, see Refs. 22 and 23). Figure 2.2 illustrates how a cation can control the photoinduced charge transfer in a fluoroiono-phore in which the cation receptor is an electron donor (e.g., amino group) and the fluorophore (e.g., anthracene) plays the role of an acceptor. On excitation of the fluorophore, an electron of the highest occupied molecular orbital (HOMO) is promoted to the lowest unoccupied molecular orbital (LUMO), which enables photoinduced electron transfer from the HOMO of the donor (belonging to the free cation receptor) to that of the fluorophore, causing fluorescence quenching of the latter. On... 

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