Electron paramagnetic resonance studies of the lowest triplet states of 1,2-benzanthracene- d12, chrysene- d12, and 1,2;5,6-dibenzanthracene- d14 in single-crystal p-terphenyl in the range from 4 to 240 K, and their implications concerning phase transitions in p-terphenyl below 240 K

Abstract

Low-field electron paramagnetic resonance studies from 65 to 240 K of the zero-field splittings of triplet 1,2-benzanthracene- d 12 dilutely substituted in single-crystal p-terphenyl have yielded 193.0 ± 0.3 K as the best value of the temperature at which a phase transition sets in upon cooling, and at which it is completed upon warming, these crystals. Similar studies of triplet chrysene- d 12 (from 7.7 to 80 K) and of 1,2;5,6-dibenzanthracene- d 14 (from 7.4 to 100.7 K) dilutely substituted in single-crystal p-terphenyl give evidence of a previously unreported second phase transition intrinsic to p-terphenyl, but influenced measurably by the presence of dilute guests. In chrysene substituted p-terphenyl, the system for which our present data most clearly indicate a second phase transition, the transition temperature lies in the range from 7.9 to 8.2 K. Our results for these phase transitions are discussed in the context of recent experimental and theoretical results for crystalline p-terphenyl below room temperature. Multiple orientation of these triplet guests and their interactions with p-terphenyl host are discussed for all three systems, and several notable features of the spectra are described.