Photoinduced reaction of arene singlets with carbon tetrachloride in zeolite Y

Picosecond fluorescence methodologies combined with nanosecond, laser flash photolysis have been applied to examine the reaction of arene excited sing lets with CCl4 in dehydrated, zeolite NaY. Fluorescence quenching by CCl4 w as shown to originate mainly from a static process, with a much greater dro p in fluorescence intensity than could be accounted for by a random distrib ution of CCl4 among the anthracene-occupied supercages. A similar effect wa s observed in a previous study of arene fluorescence quenching by nitroalka nes in NaY, and was explained by a mechanism involving coadsorption of the arene and nitroalkane to conjugate acid-base sites. This places the reactan ts in close proximity - a requirement for the static quenching. The results of the present study support this quenching mechanism by showing that a di fferent electrophilic species (i.e., CCl4) also behaves like nitroalkanes, while reductive quenching by NN-dimethylaniline and triethylamine does not. Quenching of pyrene fluorescence by CCl4 produced pyrene radical cations t hrough electron transfer to CCl4, with the cation yield proportional to the level of fluorescence quenching. One-third of these radical cations reacte d with the fragmentation products of CCl4 anions to form stable products; t he remaining fraction was stable, but reacted with added ferrocene. These r esults demonstrate that the addition of small quantities of CCl4 to the zeo lite leads to efficient charge separation by a single-photon mechanism. (C) 2001 Elsevier Science B.V. All rights reserved.