Fluorescence quenching benzene

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.

Figure 5 Plots of log versus AG°et for fluorescence quenching of RAcrH" (2.0 x 10 " M) by various electron donors (a) in MeCN and (b) in benzene at 298 K. Numbers refer to electron donors benzene (1), toluene (2), ethylbenzene (3), cumene (4), ra-xylene (5), o-xylene (6), 1, 3, S-ttimethylbenzene (7),/ -xylene (8), 1,3-trimethylbenzene (9), 1, 2,3,4-tetramethylbenzene (11), 1,2,3,5-tetramethylbenzene (12), 1,2,4,5-tetramethylben-zene (13), pentamethylbenzene (14), hexamethylbenzene (15), triphenylamine (16), N,N-dimethylaniline (17), ferrocene (18), and decamethylferrocene (19). (From Ref. 79.)...

Table 1 Electron-Transfer Rate Constants (ked Determined from the Fluorescence Quenching of RAcrH" by BuTlNAH and R Sn, Determined from the Dependence of the Quantum Yields on [BuTlNAH] and [R Sn] in the PhotoaUtylation of RAcrH with Bu BNAH and R Sn in MeCN, CHCI3 and Benzene at 298 K, and the Driving Force of Back-Electron Transfer (—AGbet) from RAcrH" to Bu BNAH and R Sn in MeCN...

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],...

Since the efficiency of fluorescence quenching of the sensitizer paralleled the oxidizability of the arene in a series of substituted alkyl benzenes, the reaction was thought to proceed through electron transfer followed by protonation and trapping of the radical by oxygen. 

The addition of benzene to 2,3-dihydropyran was examined in the presence of cyclopropyl bromide [177] in an attempt to determine the nature of the excited state of benzene responsible for the formation of the ortho adduct [11,12], The absorption of the bromide at 254 nm was taken into account (90% of the radiation was absorbed by benzene) and a decrease of the rate of formation of 50% in the heavy-atom solvent was observed. Cyclopropyl bromide also quenched the fluorescence of benzene (kq = 3.82 X 10s L/mol 1 /s ). These data are interpreted as enhanced intersystem crossing of Si benzene and the necessary involvement of the Si state in formation of the ortho cycloadduct. 

Triethylamine not only quenches the fluorescence of the exciplex and the formation of product but it also quenches the fluorescence of benzene. In dioxane,... 

Benzene is photoreduced by primary, secondary and tertiary amines (Bellas, et al., 1977) and the various products have been identified. The reaction with tertiary amines is accelerated by the addition of small amounts of protic solvents and use of CH3OD leads to deuterium incorporation. These findings are strongly indicative of the participation of radical ions. Primary and secondary amines quench the fluorescence of benzene but whether or not this leads to radical ions is not known. Product studies have been made of the... 

Many surfactants, e.g. benzenesulphonates, contain aryl groups and it is found that they will form charge-transfer complexes with 1,2,4,5-tetracyano-benzene which can be detected by ultraviolet absorption and fluorescence spectroscopy (Masuhara et al., 1979) a similar result was obtained with an amphiphatic system. Fluorescence quenching in such micelles has been studied, an example being the quenching of the fluorescence of benzyl anthroate by triethylamine in Triton X (Costa and Macanita, 1978). 

Butene-2 even at high pressures does not quench the fluorescence of benzene vapor. 

The main conclusion to be drawn from the application of the benzene photosensitization method to the decomposition of cyclobutanone is that the Cj-hydrocarbons originate from the low-lying triplet state of the ketone. However, use of this method in the investigation of cyclopentanone decomposition indicated that reactions I, II and III (if it is a separate primary process) occur from the first excited state of the ketone. This conclusion was based on the quantitative agreement found between the pressure dependence of the decarbonylation-product formation and the fluorescence quenching by cyclopentanone. 

Figure 5.12. Qualitative state diagram for the fluorescence quenching of benzene by radiationless transition into one of the higher vibrational levels of the isomeric benzvalene. The back reaction is a hot ground-state reaction.

Kasha s Rule (44) requires that emission should occur from the lowest B2 and Bj y states in benzene. This is essentially the case though a predictable very weak emission has been reported recently from higher states of oxygen-quenched benzene in gaseous (45) and solution (46) phases. The major fluorescence... 

In Scheme 16, D and A are proton donor molecules and proton acceptor molecules, respectively. The quantity K represents the stability of the complex DH A, formed by hydrogen bonding interactions in the ground state. For the 2-naphthylamine/pyridine system, the K value in hexane (obtained by fluorescence quenching measures) is 0.6 dm3 mol-1. This value is reduced by using benzene as a solvent (K = 0.2 dm3 mol-1) while, in cyclohexane, K = 12. The observed variations confirm the importance of solvents in influencing solute/solute interactions149. 

The relative rate constants for the fluorescence quenching of benzene and a series of its simple derivatives by some chloro- and fluoro-methanes and CDC13 are found to follow qualitatively but not quantitatively the hypothesis that the rate-determining step in the quenching process is formation of a donor-acceptor exciplex. The study examined the kinetics following excitation to S2 and S3 as well as Si. 115 The lack of quantitative correlation with the theoretical model used may be due to a deficiency in the model for excited-state quenching or due to the... 

Heavy-atom Quenching. From a study of the quenching effect on the fluorescence of 20 polynuclear hydrocarbons and heterocycles by 1-iodopropane in benzene solutions, it has been shown that the rate constants for fluorescence quenching decrease exponentially with E, the energy difference between the fluorescing 128 A. V. Pocuis, /. Chem. Phys., 1974, 61, 2779. 

Table 6 Wavelengthjnm of exciplex emission in benzene solution or rate constant/1 mol-1 s-1 0.5 for fluorescence quenching ...

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