Supramolecular steric effects as the means of making reactive carbon radicals persistent. Quantitative characterization of the external surface of MFI zeolites through a persistent radical probe and a langmuir adsorption isotherm

The photochemistry of tetraphenylacetone (1) adsorbed on the external surfa ce of a MFI zeolite (the sodium form of LZ-105) has been investigated in co mbination with computational chemistry, surface area measurements, EPR anal ysis, and classical adsorption isotherms. All of the methods are consistent with a supramolecular structural model in which 1 is first adsorbed strong ly through intercalation of a single benzene ring into a hole on the LZ-105 external surface (site I) followed by a weaker binding to the external fra mework between the holes (site II) until a monolayer of 1 is formed. From b oth computational and surface area measurements, it is estimated that the s ite I holes. on the external surface will be filled at ca. 0.3-0.5 wt %/wt loading of 1/LZ-105, which corresponds to 6.5 x 10(18) (ca. 10(-5) mel) of holes or molecules of 1 adsorbed in holes per gram of zeolite. The supramol ecular composition of ca. 0.3-0.5% of 1 on LZ-105 characterizes a "break po int" for the photochemistry and the EPR measurements, since it represents t he value for saturation of the site I holes with 1. These conclusions are s upported quantitatively by experimental isotherms of the adsorption of 1 on LZ-105. Photolysis of 1 intercalated in the site I holes causes fragmentat ion into two isomeric supramolecular diphenylmethyl (DPM) radicals, one (DM P)(in) which is adsorbed into the internal surface and becomes strongly per sistent (half-life of many weeks) and the other (DMP)(ex) which diffuses on -the external surface and rapidly dimerizes (less than a few minutes) to pr oduce the radical-radical combination product tetraphenylethane (2). Photol ysis of 1 adsorbed on the solid external surface produces two supramolecula rly equivalent DPM radicals (DMP)(ex) that diffuse on the external surface and rapidly dimerize to produce 2, and do not produce persistent DPM radica ls.