Enhanced quenching of anthracene fluorescence by nitroalkanes in zeolite X and Y

Quenching of the first excited singlet state of anthracene ((1)An*) by nitromethane (NM) and 2-methyl-8-nitropropane (2M2NP) has been evaluated by picosecond, fluorescence-based methodologies in alkalimetal, ion-exchanged zeolite X and Y. Three forms of quenching were observed in NaY including static quenching, dynamic quenching on time scales of <0.3 ns, and dynamic quenching on time scales of >1 ns. Static quenching of (1)An* by NM was extensive. For example, a 50% drop in the initial intensity of (1)An* decay profiles was observed at NM loadings equivalent to 0.10 M in the zeolite (or 0.13 NM/sc, where sc = supercage). This level of quenching was not predicted by a random or Poisson distribution of NM among the An-occupied cages. In this case, only 13% of the An-occupied supercages should be occupied by NM at loadings of 0.13 NM/sc. To explain the enhanced static quenching, the action of conjugate acid-base sites has been invoked whereby An adsorbs to acidic, cationic sites and induces the adsorption of NM to nearby negatively charged oxygen atoms or basic sites. In NaX, ionization of NM to aci-NM was observed. Thus, to test quenching in NaX, the less acidic nitro compound 2M2NP was used. In NaY, static quenching of 1An* by 2M2NP was indistinguishable from quenching by NM and in NaX was much less extensive relative to NaY. The latter result is explained by a higher number of basic adsorption sites (or negative charges) as well as a higher degree of intrinsic basicity of adsorption sites in NaX, relative to NaY. Through competitive effects, these sites lower the extent of adsorption of 2M2NP to the An-induced basic sites. The dynamic quenching of (1)An* on time scales of <0.3 ns is explained by intracage diffusion of quenchers randomly loaded into An-occupied supercages whereas that on time scales of > 1 ns is explained by electron tunneling to quenchers located outside the An-occupied supercages. The effects of temperature and added solvent on all three forms of quenching as well as comparisons of the quenching of (1)An* with that of the first excited singlet state of pyrene are examined and discussed.