Photostabilization of pesticides by clays and chromophores

Mechanisms for the photostabilization of pesticides by the use of clays and chromophores are described. The main mechanisms include energy transfer and steric hindrance, whereas the importance of mechanisms such as light scattering and hypsochromic shift is marginal. As an example for energy transfer we describe a procedure where the potent and safe but photolabile insecticide bioresmethrin (BR) is coadsorbed on montmorillonite together with the organic cation methyl green (MG). It is shown that the effective photostabilization of BR occurs by the process of energy transfer, which depends on the matching of energy levels of donor (BR) and acceptor (MG) chromophores, on the distance between them and on their relative orientations. The methods for studying the intermolecular interaction between two coadsorbed organic molecules include Fourier‐transform infrared (FTIR) and ¹³C solid state magic angle spinning nuclear magnetic resonance (MAS‐NMR) spectroscopies. Energy transfer processes can also occur from the pesticide to the clay. The presence of transition metal ions in the clay can make it an efficient energy or charge acceptor. Such a mechanism was utilized in the photostabilization of the insecticide tetrahydro‐2‐(nitromethylene)‐2H‐1, 3‐thiazine (NMH). In the latter case, improved photostabilization was achieved by an addition of the cationic dye acriflavine (AF). Steric hindrance: here the clay introduces a stereochemical factor in preventing or slowing down certain photochemical reactions. An example is the photostabilization of the dinitroaniline herbicide trifluralin (TF). The energy transfer approach was also shown to be effective in the photostabilization of microbial insecticides, such as the Bacillus thuringiensis (B.t.) toxin. Photoprotection was achieved by adsorption of cationic chromophores such as AF, MG and rhodamin B to B.t. © 1993 Wiley‐Liss, Inc.