Quencher benzene

Fig. 14. Quenching of fluorescence of PP solution in benzene. Quenchers (Q) (1) tetracyanoethylene, (2) chioranil, (3) trinitrobenzene / fluorescence in relative units in the absence of a quencher, / in the presence of a quencher...

Figure 5.16. Plot of data for the external heavy-atom quenching of pyrene fluorescence in benzene at 20°C. Polaro-graphic half-wave reduction potentials Ein are used as a measure of the electron affinity of the quencher containing chlorine (O), bromine ( ), or iodine (3). From Thomaz and Stevens<148) with permission of W. A. Benjamin, New York.

The three unsymmetrical carotenoids such as asteroidenone, adonixanthin, and adonirubin are not well known and their structures are shown in Figure 14.1. However, they have been studied in detail as 02 quenchers both in benzene and methanol as shown in Table 14.2. 

As will be discussed later, it is possible a4> that the thermolysis involves a metal-nitrene complex whereas the photolysis involves the free nitrene. The product distribution is not affected by the presence of a photosensitizer, but since ferrocene itself is both an efficient triplet quencher as well as a sensitizer 26,27) jt is very difficult to probe the spin state of ferrocenyl nitrene at the moment of reaction. The cycli-zation appears to be a singlet reaction since the yield of 27 in benzene solution is essentially unaffected by oxygen or the presence of hydro-quinone a5>. 

In comparison, irradiation of 33 in benzene resulted in a transient spectra with Amax = 390 nm. The decay of transient absorption was resolved into a slow and a fast component (t= 10 ms and 225 ns), which were assigned to E-34 and Z-34, respectively. Enol Z-34 is short-lived in benzene because it reketonizes efficiently via sigmatropic 1,5-H atom shift, which is not retarded with H-atom bonding, as occurs in methanol. Addition of triplet quenchers in benzene solutions did not completely quench the formation of Z-34 thus, the authors concluded that both triplet and singlet pathways were prevalent in benzene. However, the low quantum yield, 0 11,150,152 j-Qj. formation from 33 in benzene suggests that only E-34 elim-... 

When a cr ( = 0.8 0.2 nm) the reaction probability is 1 at the contact distance. This condition is fulfilled for good quenchers such as CCI4, SFg, fluorinated hydrocarbons, and in some experiments alkenes or benzene [70,128,130,132]. When (/< a, the term NVi = 0. 

The application34 of time-dependent diffusion theory to the quenching constant kCMTf for the perylene-dimethylaniline system in acetonitrile, which exhibits a pronounced dependence on quencher concentration, provides a value of 6 A for the quenching encounter diameter as the adjustable parameter which considerably exceeds that of 3 A computed for the same system in benzene. This accords with the conclusions of Leonhardt and Weller35 that electron transfer may occur with direct contact of donor and acceptor in polar media (Section V.E). 

More recently, photochemical reactions of 138 with cyclic and acyclic olefins have been described. When 138 is irradiated (Pyrex) with cyclo-heptatriene, [4 + 4]- and [4 + 6]-adducts (247-250, Scheme 16) are obtained in addition, photodimer 242 and o-dibenzoylbenzene (140) were isolated. The ratio of the [4 + 6]-adducts to the [4 + 4]-adducts [(249 + 250)/(247 + 248)] is increased in air-saturated benzene solutions compared with oxygen-free benzene solutions, and enhanced in heavy atom solvents (e.g., chloroform compared with cyclohexane) furthermore, this ratio is decreased in the presence of the triplet quencher azulene. These observations suggest that [4 + 6]- and [4 + 4]-adducts are formed by different mechanisms. 

The earliest estimates of G (triplet) from y irradiation of benzene were approximately 1.0133,134 but it is now thought that the olefin concentrations used were too low to quench all triplets. Several different workers have now measured G values around 5 for production of olefin triplets.237-238 A G value as high as 10 has been estimated from the benzene radiosensitized isomerization of the stilbenes.240 It is possible that the stilbenes are better quenchers than are simple olefins, so that they quench benzene triplets formed in a spur. A large amount of T-T annihilation within the spur may normally lower the yield of scavenge-able triplets.240 Alternately, direct radioionization at high stilbene concentrations could increase G (isomerization).239... 

Several flavonoids have given mixtures of products on irradiation for example, the 3-phenylflavone (380) in methanol is converted into the isocoumarins (381) and (382) the latter is the major product and is derived from the former (75CC241). Quercitin pentamethyl ether (383) in methanol and in the absence of oxygen yields four tetracyclic products. In deoxygenated benzene, the reaction rate is doubled but only (384) and (385) are formed. This photocyclization is not inhibited by triplet quenchers and the primary photointermediate is in the triplet state (67JA6213). 

Figure 10. Plots of log feel vs the oxidation peak potentials of methyl- and methoxy-substituted benzenes ( x) for the fluorescence quenching of flavin analogs (la,2a-c) by the quenchers in the absence (O) and presence of O.lOmol dm-3 Mg(C10J2 ( ) or Zn(C10J2 (3) in MeCN. Numbers refer to the quenchers (e.g., 1 = MeC6H5, 14 = p-(MeO)2C6H4) [154],...

It is not always easy to ascertain if the addition reaction proceeds via the triplet excited state of the arene. Benzene and its simple derivatives such as anisole and benzonitrile have high triplet energies (benzene, 84 kcal/mol 1 anisole, 81 kcal/mol" benzonitrile, 77 kcal/mol-1) which makes sensitization impracticable. Results of quenching experiments are sometimes difficult to interpret, as has become evident from the work of Cantrell. He found [109] that the formation of adducts from benzonitrile and 2,3-dimethylbut-2-ene, vinyl acetate, and 2-methoxypropene in solutions 0.5 M in cri-l,3-pentadiene occurs at a rate only approximately one-fourth that in the absence of added quencher. Five years later, the author reported [110] that m-l,3-pentadicnc itself adds slowly to benzonitrile to give ortho adducts. When a correction was made for the reaction of benzonitrile with the quencher, it became apparent that little, if any, triplet quenching had occurred. 

A kinetic scheme was proposed [122] with the fluorescent exciplex as precursor of the photoproducts (ortho as well as meta adducts). Quantum yields of adduct formation, exciplex emission, and benzene fluorescence were measured as a function of alkene concentration. The kinetic data fit the proposed reaction scheme. The authors have also attempted to prove the intermediacy of the exciplex in the photoaddition by adding a quencher to the system benzene + 2,2-dimethyl-... 

The rate constants for quenching of the triplet state of 3,4-dimethoxyacetophenone by methoxybenzene or phenol in benzene solution were determined using samples of 3,4-dimethoxyacetophenone with an absorbance of 0.1 (ca. 2 x 10 3 mol.1. 1) at the laser excitation wavelength (354.7 nm) in the presence and absence of the quenchers. Neither phenol nor methoxybenzene absorb at the laser excitation wavelength. 

Figure 5a. A Plot of the Psuedo First Order Rate Constant k as a Function of Quencher Concentration [Q] for 3,4-dimethoxyacetophenone in Benzene Quenched by Phenol.

The rates at which several substituted benzenes quench triplet benzophenone have been measured 17 8). No single linear free energy relationship can be derived. For alkoxybenzenes, alkylbenzenes, benzene itself, and benzotrifluoride as quenchers, one finds a linear plot of log vs. IP with a slope similar to that found for the plot of all substituted benzenes and triplet a-trifluoroacetophenone. A given aromatic such as benzene quenches the fluorinated ketone triplet, which has an E - -E(A /A) value of only 16 kcal, some 50 times faster than it quenches triplet acetophenone or benzophenone 132>. This rate difference reflects only 20% of the full 12 kcal difference in thermodynamic redox potentials. However, the halobenzenes and benzonitrile quench triplet benzophenone faster than does benzene 178>. It seems likely that with these electron poor benzene derivatives, some alternate chemical reaction becomes dominant. Although a reverse CT process has been suggested, with the triplet ketone as donor, it is perhaps more likely that some sort of radical addition occurs with conjugating substituents on... 

In the ensuing sections many more examples will be cited, such as the cycloaddition reactions of alkenes to benzene and related compounds (Cantrell, 1977 Gilbert and Heath, 1979 Bryce-Smith et al., 1980b) where the quenching of fluorescence can be related to the redox properties of the quencher and yet it has proved impossible to detect the formation of fluorescent complexes or radical ions. On the basis of this negative evidence it has to be assumed that reaction proceeds via a non-relaxed exciplex. 

Primary process (11) is also believed to proceed through a vibrationally excited ground-state molecule since it is also subject to pressure quenching, even at low pressures. This would fall in line with studies on the pyrolysis of l,S cyclohexadiene, which show that hydrogen, acetylene, ethylene, and benzene are products of the thermal reaction. Besides self-quenching, the use of inert gases such as xenon, carbon dioxide, propylene, and diethyl ether as quenchers has been investigated. ... 

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