Deuterium isotope effects in zero-field splittings of oriented triplet states of 1,2;5,6-dibenzanthracene, phenanthrene, and naphthalene

Changes in the zero-field splitting parameters of phosphorescent 1,2;5,6-dibenzanthracene, phenanthrene, and naphthalene oriented in single-crystal hosts have been determined at temperatures near 77°K upon perdeuterating the phosphorescent molecules, the host molecules, or both. Through control and determination of appropriate experimental variables these changes have been shown to arise strictly from the isotopic substitution. For the naphthalene-biphenyl and phenanthrene-biphenyl systems readily measurable changes were observed upon isotopic substitution of either guest or host molecules. The data support the distinction of ``guest'' and ``host'' isotopic effects for zero-field splitting parameters, but definitive analysis of the origin of these effects awaits further work.