Quencher carbon tetrachloride

Several examples of heavy atom quenching of aromatic hydrocarbon states are known for example, carbon tetrabromide is an efficient quencher of the fluorescence of anthracene167 and carbon tetrachloride behaves similarly with p-terphenyl.188 Since quenching results in formation of the triplet state, it has been possible to use the heavy atom effect to measure intersystem crossing efficiencies ( ). Because of the elegance of this technique 169 and the importance of the results in photochemistry, we shall cover it in some detail. 

While no spectroscopic evidence of a ground-state complex between anthracene and carbon tetrachloride, naphthalene or 1,2-benzanthracene and carbon tetrabromide has been found, Nemzek and Ware [7] were unable to explain their steady-state fluorescence quenching measurements with the parameters deduced from the determination of the time-dependent rate coefficients unless a ground-state complex was present. This cannot be regarded as a satisfactory and consistent analysis because the time-dependent rate coefficient would be modified by the presence of the initial distribution of quencher and fluorophor in the ground state. 

Since these first time-resolved studies, similar quenching methods (i.e., choosing an exclusive upper state quencher) have been used in conjunction with the two-color approach to demonstrate energy transfer from anthracene T2 to acrylonitrile, benzonitrile, dimethoxybenzene [52] and from the upper triplets of 70, 71 and p-terphenyl to a variety of quenchers including carbon tetrachloride [53]. 

Carbon tetrachloride is an apolar quencher which mainly resides in the micellar phase. The decrease of the quenching efficiency for several probe molecules is compared in SDS and CTAC (Table 2) [99]. For most probe molecules, the decrease is moderate and independent of the nature of the micelle. 

Styrene, a powerful quencher for the trichloromethy1 radical, showed no measurable effect on the selective carboxylation using B-CyD at the initial molar ratio 0.04 to carbon tetrachloride. Consequently, the selective carboxylation using 3-CyD as catalyst probably proceeds with trichloromethyl cations, formed in situ from carbon tetrachloride by the catalysis of copper powder, as the active species. 

---