Base quenching

However, there is an important subtlety in 4. While amine receptor-anthracene lumophore pairs produce excellent PET based quenching, a benzo-15-crown-5 ether receptor is demonstrably incapable of transferring an electron to an excited anthracene lumophore unless the latter carries an electron withdrawing substituent. Then, what is the secret of 4 s success First, only one rapid PET process is... 

Entry Base Quenching Reagent Product Config. op (%) Yield (%) Ref... 

Mixtures of 1,3,5- and 1,3,6-cyclooctatriene were obtained by partial reduction of cyclooctatetraene in ways such as protonation of cyclooctatetraene dianion3d-4 and reduction with zinc-alkali.2 5 1,3,6-Cyclooctatriene is the major product in these reductions. However, since 1,3,6-cyclooctatriene isomerizes to 1,3,5-cyclooctatriene on treatment with base, quenching cyclooctatetraene dianion with methanol and subsequent heating affords 1,3,5-cyclooctatriene in an 80% yield.3d Reduction of cyclooctatetraene with sodium hydrazide and hydrazine also produces 1,3,5-cyclooctatriene.6 Therefore, when cyclooctatetraene is available in quantity, these procedures are the methods of choice. 

C in the presence of 0.1 equiv of catalyst 13c and 0.1 equiv of (1 naphthol as an additive. Too strong acids instead of (1 naphthol as an additive either reduced the yield (PhCO2H) or completely inhibited the reaction (PhSO H) probably due to the acid base quench of active catalyst. 

Fig. 7. A three-input INHIBIT gate exemplified by the tetraanion 54 and /1-cyclodextrin (j8-CD). a With neither protons nor /1-CD present in the solution, phosphorescent output is low, because of both PET from the tertiary amine, and through intermolecular triplet-triplet collisions of the bromonaphthalene phosphor, b Addition of calcium ions leads to a reduction in the PET-based quenching of the phosphorescence - however, intermolecular collisions still lead to a low emission, c Shielding of the phosphor with /l-CD reduces intermolecular triplet annihilations, but quenching still occurs via PET. d Only with both Ca2+ and /1-CD present does the solution phosphoresce, e In any combination of Ca2+ and /1-CD, the solution will yield a low output in the presence of molecular oxygen (the INHIBIT stimulus), as a consequence of triplet-triplet collisions...

Brensted Base Quenching. The trans-Rh(cyclam)(CN)2+ ion (cyclam = 1,4,8,11-tetraazacyclotetradecane) displays luminescence from a ligand field excited state (3LF ) at room temperature, in an aqueous solution with a lifetime (8.1 /is) [53] several orders of magnitude longer than generally observed for rhodium(III) amine complexes [54]. As was observed for some other Rh(III) amines, the 3LF emission from trans-Rh(cyclam)(CN)2 is quenched by OH- in solution (Eq. (13)), a process attributed to amine deprotonation [55],... 

Alexandrescu AT (2001) An NMR-based quenched hydrogen exchange investigation of model amyloid fibrils formed by cold shock protein A. Pac Symp Biocomput 67-78... 

Complementary method to organocatalytic enantioselective BH reaction would be the use of chiral Brpnsted acid to activate Michael acceptors or electrophiles. However, their incompatibility between the acid and base catalysts is important issue to be addressed since any possible acid-base quench would lead to inactive catalysts. Recent results have revealed that effective chiral Brpnsted acid catalysts are hydrogen-bond-donating organic molecules. 

Evidence reported in Section 8.2.3 indicates that the base has a catalytic effect on condensations, but excess of base quenches condensation and favors conjugate addition to dipolarophiles that are Michael acceptors. 

In addition to the reasonable sensitivity toward SO2, which may, in fact, be extended to lower limits of detection with a modification of the experimental setup, p-PS is also relatively robust. PS survives many cycles of exposure to SO2 and Ar without any obvious degradation of PL intensity. Experiments of modulated exposure of PS to SO2 and Ar for over 6 hours at -lOmin. cycles did not result in a significant decrease in the PL intensity. The longer-term stability has not been ascertained, but the PS luminescence is stable over more than a year and exhibits a base quenching phenomena over at least many months. 

The experiments presented here suggest that sensitive surface interactions strongly affect the optical properties of porous silicon. Base quenching titrations clearly identify a surface confined acid which accounts for a majority of the PL emission. This pH-dependent luminescence exhibits a pKa of 3-4. An isolated group of surface confined molecules such as surface hydrides or silanols, cannot be responsible for the PL observed in porous silicon based on the observed pKa s. Therefore, a proton, bound to a silicon-bridging oxygen is implicated in the PL mechanism. However, available data does not necessarily indicate that this emission process is exclusive. 

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