Fluorescence in nonpolar solvents

Anilinonaphthalene-l-sulfonic acid ammonium salt, which scarcely fluoresces in aqueous solution, is stimulated to intense fluorescence by long-wavelength UV light (A = 365 run) if it is dissolved in nonpolar solvents or adsorptively bound to nonpolar molecular regions [3]. 

In 1982 Wei et al. [78,79] studied the quenching of N,N-dimethyltoiuidine (DMT) fluorescence by adding the electron-accepting monomer MA or MMA and successfully observed broad and structureless exciplex fluorescences at longer wavelengths in nonpolar solvents for the first time. 

DNA and RNA quantification, SNP typing, hybridization, and structural alteration have been widely carried out by modified oligonucleotides possessing pyrene derivatives [104-113]. As is known, pyrene-1-carboxaldehyde fluorescence is considerably dependent on solvent polarity [114], being strong in methanol but insignificant in nonpolar solvents [115]. Owing to this property, Tanaka and collaborators developed a pyrenecarboxamide-tethered modified DNA base, PyU 46, and applied it to SNP discrimination in DNA [116-120],... 

Anthracene has also been used as an acceptor (Fig. 10). In solution, 26 emits a single fluorescence band that is somewhat structured in nonpolar solvents and becomes broad and structureless with increasing polarity [58]. The strongly hindered molecule 27 also exhibits a similar behavior, but its absorption spectrum is better structured [59]. The rate of formation of a charge transfer state is higher for 27 than for 26. Based on this observation, it appears that the twist around the anthryl-phenyl C-C bond plays a significant role in the fluorescence profile of the probes [60]. Acridines, such as 28, behave similarly to anthracene except that acridine is a better electron acceptor [61]. 

Another example of fluorescence intensity modulation in cou-marins is the 3-azido substitution that quenches the fluorescence completely. These compounds are used as starting material for the synthesis of fluorescent triazolocoumarins by click chemistry [31], Interestingly, the fluorescence of some coumarins depends strongly on the solvent. This is the case for 7-alkoxycoumarins that have been used as probes for microenvironments [32], 7-hydroxycoumarin that is pH sensitive, and 7-NR2 substituted coumarins such as coumarin 120 whose quantum yield is reduced in nonpolar solvents due to a change in the 3D structure [33],... 

A. Weller and K. Zachariasse 157-160) thoroughly investigated this radical-ion reaction, starting from the observation that the fluorescence of aromatic hydrocarbons is quenched very efficiently by electron donors such as N,N diethylaniline which results in a new, red-shifted emission in nonpolar solvents This emission was ascribed to an excited charge-transfer complex 1(ArDD(H )), designated heteroexcimer, with a dipole moment of 10D. In polar solvents, however, quenching of aromatic hydrocarbon fluorescence by diethylaniline is not accompanied by hetero-excimer emission in this case the free radical anions Ar<7> and cations D were formed. 

The fluorescence quantum yield of pyrenecarboxaldehyde is very low in nonpolar solvents such as u-hexane (< 0.001) but it drastically increases in polar solvents... 

N. Mataga and Y. Kaifu, Intermolecular proton transfer in the excited hydrogen-bonded complex in nonpolar solvent and fluorescence quenching due to hydrogen bonding, J. Phys. Chem. 36, 2804-2805 (1962). 

Quinolone carboxylic acid antibacterials are synthetic compounds whose basic nuclear structure includes a quinolone ring and a carboxylic acid group. Fluoroquinolones are second-generation quinolones that contain in their molecule a fluorine and a piperazine ring. Quinolones are amphoteric compounds slightly soluble in polar solvents such as water, and insoluble in nonpolar solvents such as benzene and hexane. Most of these drugs are fluorescent and are quite stable in aqueous solution toward light, except miloxacin, which is reported to be unstable. 

Indole in cyclohexane and ethanol is excited at 270 nm populating a mixture of the La and Lb states. The La and Lb states differ in their spectral structure (Fig. la)) and Stokes shifts [3]. The unstructured spectrum of the La state shows a large Stokes shift due to the large change of the dipole moment upon electronic excitation. The dipole moment of the Lb state is similar to the ground state value. In nonpolar solvents like cyclohexane, the Lb state is energetically below the La state and its emission spectrum exhibits vibronic structure. In the polar solvent ethanol state reversal occurs after the electronic excitation and the La state becomes responsible for the more red shifted fluorescence [4],... 

The effects of substituents and solvent polarity on the luminescence properties also have been evaluated for of a series of bichromophoric anthronyl-substituted anthracenes 98 and 99. It can be concluded from the quantum yield data summarized in Table 20 for spiro-substituted compounds 98a e that, dependent on solvent polarity, two different modes of intramolecular interactions between the electronically excited anthracene chromophore and the ground state ketone typically are operative, and both types of interaction result in fluorescence quenching. In nonpolar solvents, fluorescence quenching apparently involves endothermic intramolecular... 

Frequency of the exciplex fluorescence maximum in nonpolar solvent. ... 

Stern-Volraer fluorescence quenching constant in nonpolar solvent. 

The fluorescence of 3-t (113-117) and 3-7 (118) is quenched by secondary and tertiary amines. Rate constants for quenching of It by tertiary amines increase with decreasing amine ionization or oxidation potential (Table 11), as expected for the formation of a charge-transfer stabilized exciplex in which the amine serves as the electron donor. Electron transfer quenching in nonpolar solvent is calculated to be exothermic for amines with E 2 < 1 34 V. Thus, it is not surprising that secondary and tertiary amines quench 3-t with rate constants which approach or even exceed the rate of diffusion. The inefficient quenching of It and 3-7 by primary amines is consistent with their higher oxidation potentials. 

Exciplex fluorescence is observed upon quenching of 3-t (but not 3-7 ) by N-methylpyrrole (117) and tertiary aliphatic amines (113-116) with < 0.90 in nonpolar solvents, but not for... 

The last three entries in Table 13 reflect a marked preference for a-methylene versus methyl proton transfer for electron-withdrawing a substituents. These amines are also unusual in that they react with t in nonpolar solvents and do not display exciplex fluorescence. While this unusual behavior was initially attributed to a free radical hydrogen atom transfer mechanism leading to the formation of exceptionally stable "merostabilized" a-aminoallyl radicals (115), our current view is that the high kinetic acidity of the a-C-H bond of these amines when complexed with t is responsible for their behavior. 

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