Acceptor quenched

Pt2(P205H2) - (d8-d8), and Mo6Clft ( )6. Two- electron oxidations of Re2Cl and Pt2(P205H2)it have been achieved by one-electron acceptor quenching of the excited complexes in the presence of Cl, followed by one-electron oxidation of the Cl -trapped mixed-valence species. Two-electron photochemical oxidation-reduction reactions also could occur by excited-state atom transfer pathways, and some encouraging preliminary observations along those lines are reported. 

In recent experiments D. G. Nocera has extended this work to include two-electron photochemical oxidation of Re2ClQ. The strategy involved here is to generate Re2Cl by acceptor quenching... 

The use of ketones with (ir, n) triplet states to sensitize photoreductions will be attended by chemical sensitization for example, with benzophenone in isopropyl alcohol and 0.5M acceptor, 1% of the sensitizer triplet will still abstract from the solvent even if the acceptor quenches at the diffusion controlled rate. Failure to determine that the quantum yield of reduction was greater than 0.01 might lead to the conclusion that triplet sensitization was occurring. 

The electron- and hole-trapping dynamics in the case of WS2 are elucidated by electron-quenching studies, specifically by the comparison of polarized emission kinetics in the presence and absence of an adsorbed electron acceptor, 2,2 -bipyridine [68]. In the absence of an electron acceptor, WS exhibits emission decay kinetics similar to those observed in the M0S2 case. The polarized emission decays with 28-ps, 330-ps, and about 3-ns components. For carrier-quenching studies to resolve the dynamics of electron trapping, it is necessary that the electron acceptor quenches only conduction-band (not trapped) electrons. It is therefore first necessary to determine that electron transfer occurs only from the conduction band. The decay of the unpolarized emission (when both the electron and the hole are trapped) is unaffected by the presence of the 2,2 -bipyridine, indicating that electron transfer docs not take place from trap states in the WS2 case. Comparison of the polarized emission kinetics in the presence and absence of the electron acceptor indicates that electron transfer does occur from the conduction band. Specifically, this comparison reveals that the presence of 2,2 -bipyridine significantly shortens the slower decay component of the polarized... 

Quenching the luminescence of TMPD by phthalic anhydride (PA) and of pyromellitic dianhydride (PMA) by hexamethyl triindan (HMTI) in vitreous MTHF at 77 K has been discussed by Miller et al. [34]. The donors were the excited molecules of TMPD or molecules of HMTI in the ground state while PA molecules in the ground state or the excited molecules of PMA served as the electron acceptor. Quenching of both the fluorescence and the phosphorescence of excited molecules was observed. The authors deny the possibility of quenching the luminescence of TMPD and PMA via the mechanism of energy transfer since the luminescence has been quenched by mol-... 

Consistent with the principles outlined in Chapter 1, strong hydrogen-bonded association may be achieved by multiple complementary interactions, as in the barbiturate-appended porphyrin system shown in 11.21 (cf. molecular rosettes, Section 10.6.4). This receptor binds a complementary dansyl (dimethylaminonapthalene-sulfonyl) group with an association constant in excess of 106 M-1. In this case, electron transfer is from the photoabsorbing dansyl group to the porphyrin acceptor quenching is almost total, suggesting very fast electron transfer relative to the rate of fluorescence.12... 

The Ru(II)bipyridylcalix[4]diquinone receptor 43 selectively binds and senses acetate anions (from H NMR titrations in DMSO-d6 solution K= 9,990 M1) [37]. This receptor exhibited only weak luminescence because calix[4]diquinone is an electron acceptor, quenching the [Ru(bpy)3]2+ emission... 

With HTRF, the only possible interferences that are not easily corrected are due to the inner filter effect at the acceptor emission wavelength. However, only very few compounds in the hbraries absorb highly in the near infrared region. Possible cryptate quenching is even corrected to a certain extent by the signal ratio, while only acceptor quenching is not taken into account, similar to the situation with the other technologies. 

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... 

The mechanism of the Patemo-Biichi reaction is not well understood, and while a general pathway has been proposed and widely aceepted, it is apparent that it does not represent the full scope of reactions. Biichi originally proposed that the reaction occurred by light catalyzed stimulation of the carbonyl moiety 1 into an excited singlet state 4. Inter-system crossing then led to a triplet state diradical 5 which could be quenched by olefinic radical acceptors. Intermediate diradical 6 has been quenched or trapped by other radical acceptors and is generally felt to be on the reaction path of the large majority of Patemo-Biichi reactions. Diradical 6 then recombines to form product oxetane 3. 

TT-acceptor function of the C2C3 double bond. Lithiated benzothiophene can be quenched by [Pt(dppe)Cl2] to give 279 [97JCS(D)2955]. 

The absorption and luminescence spectra of imidazo[ 1,2,4]triazines and related compounds were recorded. The phenyl groups on both the 6-and the 7-positions quenched the luminescence. An acceptor substituent such as CHO in position-7 sharply reduced the luminescence quantum yield (82MI4). A detailed study of the infrared spectra of imidazotriazines was carried out (75T433). 

In order to clear up the mechanism of inactivation of excited states, we examined the processes of quenching of fluorescence and phosphorescence in PCSs by the additives of the donor and acceptor type253,2S5,2S6 Within the concentration range of 1 x 1CT4 — 1 x 10"3 mol/1, a linear relationship between the efficiency of fluorescence quenching [(/0//) — 1] and the quencher concentration was found. For the determination of quenching constants, the Stem-Volmer equation was used, viz. 

The data in Figure 14 imply that the efficiency of fluorescence quenching by acceptors grows simultaneously with an increase in electron affinity of the quenchers. 

This makes us conclude that the process of quenching is associated with an electron transfer. The efficiency of phosphorescence quenching by acceptors follows, as well, the growth of electron affinity of the latter. Phosphorescence quenching constants are two orders of magnitude lower than fluorescence quenching constants. This indi-... 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

A good example is the excited state of the tris(bipyridine)ruthenium(2+) ion, Ru(bpy)5+. This species results from the transfer of an electron from the metal to a ligand. In the language of localized valences, it is a ruthenium(3+) ion, coordinated to two bipyridines and to one bipyridyl radical anion in other words, [Ru3+(bpy)2(bpy )]2+. This excited state is a powerful electron donor and acceptor.17 The following equations show an example of each quenching mode ... 

The single-electron transfer from one excited component to the other component acceptor, as the critical step prior to cycloaddition of photo-induced Diels Alder reactions, has been demonstrated [43] for the reaction of anthracene with maleic anhydride and various maleimides carried out in chloroform under irradiation by a medium-pressure mercury lamp (500 W). The (singlet) excited anthracene ( AN ), generated by the actinic light, is quenched by dienophile... 

We have also investigated other oxalate esters as a potential means to improve the efficiency. The most commonly used oxalates are the 2,4,6-trichlorophenyl (TCPO) and 2,4-dinitrophenyl (DNPO) oxalates. Both have severe drawbacks namely, their low solubility in aqueous and mixed aqueous solvents and quenching of the acceptor fluorescence. To achieve better solubility and avoid the quenching features of the esters and their phenolic products, we turned to difluorophenyl oxalate (DFPO) derivatives 5 and 6 (Figure 14). Both the 2,4- and the 2,6-difluoro esters were readily synthesized and were shown to be active precursors to DPA chemiluminescence. In fact, the overall efficiency of the 2,6-difluorophenyl oxalate 5 is higher than for TCPO in the chemical excitation of DPA under the conditions outlined earlier. Several other symmetrical and unsymmet-rical esters were also synthesized, but all were less efficient than either TCPO or 2,6-DFPO (Figure 14). 

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