Excited state - nonfluorescing

Finally, a nonfluorescent YFP variant has been constructed for use as a FRET acceptor for GFP [89]. This allows the detection of the complete emission spectrum of GFP, while the FRET efficiency is high (R0 = 5.9 nm) due to strong overlap of the GFP fluorescence and YFP absorbance band. The occurrence of FRET was detected by a reduction in the excited state lifetime of the GFP by FLIM. The main disadvantage is that the presence of the acceptor cannot be detected in living cells. 

In 1977, Koo and Schuster studied the CL emission produced when diphe-noyl peroxide was decomposed at 24°C in dichloromethane in the dark producing benzocoumarin and polymeric peroxide [111, 112]. No CL emission was observed directly as benzocoumarin is nonfluorescent however, in the presence of aromatic hydrocarbons light was produced because of the fluorescence of these hydrocarbons. The explanation of this phenomenon was based on the above-mentioned CIEEL the aromatic hydrocarbons, which have a low oxidation potential, transfer one electron to diphenoyl peroxide to form a charge-transfer complex, from which benzocoumarin and the corresponding hydrocarbon in the excited state are produced (Fig. 13). 

Peroxyoxalate-based CL reactions are related to the hydrogen peroxide oxidation of an aryl oxalate ester, producing a high-energy intermediate. This intermediate (l,2-dioxetane-3,4-dione) forms, in the presence of a fluorophore, a charge transfer complex that dissociates to yield an excited-state fluorophore, which then emits. This type of CL reaction can be used to determine hydrogen peroxide or fluorophores including polycyclic aromatic hydrocarbons, dansyl- or fluores-camine-labeled analytes, or, indirectly, nonfluorescers that are easily oxidized (e.g., sulfite, nitrite) and quench the emission. The most widely used oxalate... 

Electron-pair donation to the metal, removing the possibility of the low-lying n — it excited state, which would cause the reagent itself to be nonfluorescent. 

In fac-(bpy)Re(I) (CO)3-A (where bpy is 2,2 -bipyridine and A is an aromatic amine), the d-7t(Re)—>jr (bpy) MLCT fluorescent excited state is strongly quenched via intramolecular aniline-Re charge transfer leading to a nonfluorescent LLCT state. By incorporating the donor amino group belonging to the A moiety into a crown-macrocycle, Schanze and Mac Queen(137) have provided a new luminescent cation sensor whose quantum yield of fluorescence raises from 0.0017 (without cation) to... 

Excitation of TPE in hexane with a 305-nm, 0.5-ps laser pulse led to the appearance of absorption bands at 423 and 650 nm during the time duration of the excitation pulse. The 630-nm band decays and the 423-nm band shifts to 417 nm with a lifetime of 5 1 ps, and the 423-nm band shifts to 417 nm on this time scale. The 417-nm absorption then decays with a lifetime of 3.0 0.5 ns. The 630-and 417-nm bands were assigned to an electronic transition from the vertical singlet excited state (5iy) and to an electronic transition from the nonfluorescent S p, respectively. ... 

Although carbonyl compounds are generally nonfluorescent because of fast intersystem crossing to generate the phosphorescent lowest n,n triplet excited state [187], irradiation of the absorption band due to Mg2+ complex of 1-naphthaldehyde (1-NA) or 2-naphthaldehyde (2-NA) formed in the presence of Mg(C104)2 causes strong fluorescence at 430-440 nm as shown in a general manner (Scheme 22) [188]. 

The detection of nonfluorescent ground states within a large population distribution is a difficult problem, as we have just seen for Ca-HCl. A new and general method has been proposed by John Polanyi s group to address this problem in clusters. Here also the reacting system composed of a metal and a molecule or an aggregate of molecules is prepared in a locally excited state of the metal, yielding short-lived quasi-bound states whose resonances are analyzed to unravel the electron-transfer reaction. 

In a molecular crystal the fluorescence decay reflects the sum of the rate constant for radiative and nonradiative decay of the excited state of the crystal. Information on energy transfer requires a fluorescent or nonfluorescent dopant that depletes the exciton reservoir. In a disordered system, in which the excited state is inhomo-geneously broadened a fluorescence decay study does yield information without requiring an excitation scavenger provided that the emission is spectrally resolved [see Section 3.2.3.1]. When measuring the decay within a spectrally narrow detection window one monitors the relaxation across the spectrally assessed energy slice of the EDOS. Such experiments were first done on films of PPV [70]. 

The fluorescent excited state (F ) can transfer its energy to Q (which is usually nonfluorescent) over very short (i.e., contact) distances and is reflected by a decrease in the fluorescence properties such as the quantum yield (), intensity of fluorescence (/), and lifetime (t). 

The absorption and emission properties of bis[4-(dimethylamino)-phenyljsquaraine (3), bis[4-(dimethylamino)-2-hydrox5q)henyl]squaraine (21), bis(2,4,6-trihydroxyphenyl)squaraine (18), and an a2ulyl derivative of squaraine in poly(methyl methacrylate) and polystyrene have been reported [73]. The fluorescence lifetimes of new nanoseconds were observed for 3 and 21 in the polymer films, whereas for 18, the fluorescence lifetime was much shorter (10 ps). As discussed above, the neutral form of 18 is relatively nonfluorescent. A rapid excited state proton transfer tautomerization or deprotonation was proposed for the rapid deactivation of the excited state of 18. 

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