PHOTOCHEMISTRY OF ORGANIC-MOLECULES ENTWINED IN SPIDERWEBS - THE USE OF POLY(METHYL METHACRYLATE) GLASS FOR RESTRICTING EXCITED-STATE MOTION

A study of several unimolecular photochemical rearrangements in poly(methyl methacrylate) glass was carried out with the intent of comparing the behavior of excited-state species trapped in a polymer matrix with both the corresponding solution photochemistry and that in crystal lattices. Three reactants were studied: (a) 4,5,5-triphenylcyclohex-2-en-1-one, (b) 1,1-dicyano-3,3,5,5-tetraphenyl-1,4-pentadiene, and (c) 2,2-dimethyl-1,1-diphenyl-3-(2,2-diphenyl)cyclopropane. The solution photochemistry of the three compounds had previously been studied in our laboratory, and the photochemistry in the crystal lattice had been similarly investigated for the first two. The crystal lattice photochemistry of the last of the three was investigated in the present study. For each of the three reactants; different photochemical behavior was observed depending on the environment-polymer glass, solution, or crystal lattice. The experimental behavior correlated nicely with theoretical assessment of molecular motion of the component atoms in proceeding onward from that point along the reaction coordinate where multiple reaction pathways are available. The preferred reaction pathway proved consistently to require the least motion and the minimum molecular volume displacement. A concept of least motion dependent on its application to the ''branch point'' rather than the entire conversion of reactant to product was established.