ESR investigations of triplet naphthalene in plastic hosts

The thermal dependence of the zero field splittings (zfs) %of the photoexcited triplet state of naphthalene-d8 in a variety of plastic hosts of varying macroscopic rigidity has been measured between 5 and 333 K using electron spin resonance spectroscopy. The hosts employed were polycyclohexyl methacrylate (CHMA), polymethylmethacrylate (PMMA), polyvinylacetate (PVA), and polystyrene (PS). The relative macroscopic rigidity of each host matrix as determined by the degree of crosslinking and the glass transition temperature was CHMA≳PMMA≳PS≳PVA. Over the temperature range investigated, the zfs in the CHMA, PMMA, and PVA matrices were the same within experimental error but approximately 1% to 2% greater than those observed in PS. A linear variation in the root mean square zero field splitting with temperature % (∂D*/∂T) was observed in each matrix. Despite significant differences in host macroscopic rigidity, the magnitude of (∂D*/∂T) was the same in the methacrylates and PVA, and slightly less than that observed in PS. Attempts to fit the data to a linear combination of (1/T) or of exponential functions were unsuccessful. The results suggest that the guest triplet thermal dependence is influenced by classical motional processes in the host matrix cage, and that host matrix rigidity does not reflect the local environment of the guest triplet.