Quenching naphthalene

Similarly, 2,3-dimethyl-l,3-butadiene quenches naphthalene fluorescence (1.7 x 107 M-1 sec-1) but does not rearrange ... 

Dienes quench naphthalene fluorescence although they have no excited state of lower energy than the Sx state of naphthalene. Apparently, quenching is due to an exciplex that either undergoes radiationless transition to ground-... 

The first concept to be considered is cation displacement of a complexed quencher of fluorescence. Scheme I illustrates the concq>t. To be operable the scheme requires a quencher that is complexed by a crown ether, a metal ion of interest that is not a quencher but is complexed effectively, and a crown ether ring that orients the complexed quencher so it will effectively quench the chromophore fluorescence. Scheme II shows a relatively simple crown ether system that we hoped would fulfill these requirements(22,2i). The l,5-naphtho-22-crown-6 compound was selected because of the ability of its crown ether band to hold a quencher against the face of the pi system of the naphthalene chromophore. The heavy atom ion Cs+ was selected as a quencher based on its propensity to increase inter-system crossing from the fluorescent Sj state to the nonfluorescent T state(i,2). It was likely, based on previous results(2), that potassium ion would be complexed by the crown, but not quench naphthalene fluorescence appreciably. 

The ease of oxidation of a strained hydrocarbon is a good measure of the relative energies of the HOMOs. Thus the ability of strained hydrocarbons to quench naphthalene fluorescence, which also reflects the energy of the HOMO, correlates reasonably well with their oxidation potentials. 

Which of the following compounds will quench naphthalene fluorescence at a diffusion-controlled rate benzene, benzophenone, biacetyl, fluorene, quinoline, or triphenylene ... 

Davies and Warren" found that when 1,4-dimethylnaphthalene was treated with nitric acid in acetic anhydride, and the mixture was quenched after 34 hr, a pale yellow solid with an ultraviolet spectrum similar to that of a-nitro-naphthalene was produced. However, if the mixture was allowed to stand for 5 days, the product was i-methyl-4 nitromethylnaphthalene, in agreement with earlier findings. Davies and Warren suggested that the intermediate was 1,4-dimethyl-5 nitronaphthalene, which underwent acid catalysed rearrangement to the final product. Robinson pointed out that this is improbable, and suggested an alternative structure (iv) for the intermediate, together with a scheme for its formation from an adduct (ill) (analogous to l above) and its subsequent decomposition to the observed product. 

An electrostatic precipitator is used to remove more tar from coke oven gas. The tar is then sent to storage. Ammonia liquor is also separated from the tar decanter and sent to wastewater treatment after ammonia recovery. Coke oven gas is further cooled in a final cooler. Naphthalene is removed in a separator on the final cooler. Light oil is then removed from the coke oven gas and is fractionated to recover benzene, toluene, and xylene. Some facilities may include an onsite tar distillation unit. The Claus process is normally used to recover sulfur from coke oven gas. During the coke quenching, handling, and screening operation, coke breeze is produced. The breeze is either reused on site (e.g., in the sinter plant) or sold offsite as a by-product. 

Note. Both the rearrangement In t-ButanoI) and the double bond isomerization of (114) (In Benzene) are quenched in a diffusion-controlled process by suitable triplet acceptors e.g., naphthalene or 2,5-dimethylhexa-2,4-diene). The rearrangement (114) (118) -I- (120) is also observed on irradiation in... 

P 24] The nitration of naphthalene was carried out with dissolved or in situ generated N2O5 gas [37]. The temperatare was set to -10 to 50 °C and residence times to 15—45 s. The reaction mixtare processed in the micro reactor was quenched with water, extracted and analyzed by HPLC or GC with mass-selective detection. 

Wan also studied hydroxybenzyl alcohols based on the naphthalene chromo-phore.40 Naphthols 50 and 51 were both examined for their ability to photogenerate naphthoquinone methides 52 and 53, respectively (Eqs. 1.12 and 1.13). While 50 underwent very efficient photosolvolysis, presumably via naphthoquinone methide 52, 51 was essentially unreactive when exposed to light. The inability of 51 to photogenerate 53 is a rare example where the generality of the photodehydration of benzyl alcohols fails. LFP of 50 yielded a very strong visible absorption (A x = 410 and 700 nm) that decayed in aqueous acetonitrile with a lifetime of 34 ps (Fig. 1.2). This transient was assigned to naphthoquinone methide 52 due in part to its efficient quenching when the nucleophilic ethanolamine was added. 

Proteases are enzymes that break peptide bonds in proteins. As such they lend themselves to a variety of homogeneous assay techniques. Most employ labeling both ends of the substrate with a different tag, and looking for the appearance (disappearance) of the signal generated in the intact substrate (product). As an example, for a fluorescence quench assay, the N-terminal of a peptide is labeled with DNP and the C-terminal with MCA. As such, the peptide is fluorescently silent since the fluorescence from DNP is quenched by absorption by the MCA. Another very popular donor/acceptor pair is EDANS 5-[(2-aminoethyl)amino] naphthalene-1-sulfonic acid and DABCYL 4-(4-dimethylaminophenylazo)benzoic acid) (a sulfonyl derivative (DABSYL) [27], Upon peptide cleavage, the two products diffuse, and due to a lack of proximity, the fluorescence increases. 

The high-spin/low-spin interconverison in a Ni11 complex of the cyclam derivative (639) bearing a luminescent naphthalene substituent has been used as a fluorescent molecular thermometer.161 The Ni11 tends to quench fluorescence of the proximate naphthalene subunit, but the two spin states exert a different influence on the emission properties. Emission is temperature dependent, since the high spin —> low spin conversion is endothermic, i.e., a temperature increase favors formation of the low-spin form. 

In Section 3.1 it was shown that the photoreduction of benzophenone can be quenched by addition of small amounts of triplet quenchers such as oxygen or ferric dipivaloylmethide.<60) In fact this was presented as evidence that the benzophenone triplet was involved in the photoreduction. This reaction can also be quenched by naphthalene. In the presence of naphthalene, light is still absorbed by benzophenone and thus benzophenone triplets are produced. However, photoreduction products are decreased. On examining this reaction with flash photolysis, triplet-triplet absorptions were observed but these absorptions corresponded to those of the naphthalene triplet. Thus the triplet excitation energy originally present in the benzophenone triplet must have been transferred to naphthalene and since little of the photoreduction product was observed, this transfer must have been fast in relation... 

The results of this study are presented in Table 4.7. As can be seen from the data in Table 4.7, decarbonylation with hydrogen or deuterium transfer to the resulting radical is a relatively efficient process. The failure to observe this reaction using acetone or acetophenone as photosensitizer would suggest a singlet pathway for the direct photolysis of the aldehyde. In agreement, decarbonylation could not be quenched by naphthalene, piperylene, or 1,3-cyclohexadiene when the aldehyde was excited directly. The reaction could, however, be somewhat quenched by the addition of tri-n-butylstannane. The products in this case were... 

A recent study of Murov and co-workers<106) indicates that the activation energies for isomerization are not the controlling factors. Thus the fluorescence of naphthalene is quenched (5 x 108M-1sec-1) by cis-trans-1,3-cyclooctadiene with isomerization to form cis-cis-1,3-cyclooctadiene. However, the compound bicyclo[4,2.0]oct-7-ene is not formed despite the low activation energy for this process ... 

This reaction is sensitized with acetophenone and quenched by naphthalene, di-f-butylnitroxide, or piperylene. 

As with 2-cyclopentenone, the ratio (70) (71) varies with the molar concentration of the enone, the head-to-head dimer (71) becoming increasingly important at higher concentrations/133 This reaction is efficiently sensitized by acetophenone, benzophenone, thioxanthone, and naphthalene. The same enone concentration effect was observed in the sensitized photo-dimerization as in the direct photolysis. Similarly, quenching of the dimerization by piperylene was not accompanied by a change in dimer ratio. Systematic... 

A singlet naphthalene or a singlet exciplex is thought to be the reactive species in this reaction since the quantum yield of cycloaddition parallels the quenching of naphthalene fluorescence by acrylonitrile. 

Comparison of these experimental results with the calculated charge densities (S0 and Si) at the 2 and 3 positions (Table 11.5) shows that this is the expected result. Except for those compounds discussed below, the failure to observe quenching with triplet quenchers and reaction in the presence of a photosensitizer indicated singlet reactions. Compound (89) was found to also undergo benzophenone-photosensitized substitution, indicating that the triplet state of this compound is also reactive. The reaction, however, was less clean than that observed in the direct photolysis. Similarly, 1,6-dinitro-naphthalene was found to undergo both direct and benzophenone-photosensitized substitution ... 

Laser flash photolysis techniques offer the possibility of examining in detail the transient processes responsible for the photostabilizing effect discussed above. The triplet lifetimes are frequently too short, even for this technique however, they can still be estimated using as a probe the quenching by 1-methyl-naphthalene, which leads to the formation of its easily detectable triplet. The optical absorbance due to the 1-methylnaphthalene triplet (Aft) produced as a result of energy transfer is related to the Stern-Volmer slope by equation 5, where N stands for... 

Figure 3. Stern-Volmer plots for quenching of yellowing following photolysis of PVCa solutions in methylene chloride by (a) piperylene and (b) naphthalene. Yellowing is measured as the increase in absorption at 390 nm.

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