Electron Paramagnetic Resonance Absorption in Oriented Phosphorescent Chrysene at Magnetic Fields below 125 G. I. Multiplet Structure in Low-Field EPR Spectra

Abstract

Multiple electron paramagnetic resonance absorptions by phosphorescent chrysene-h12 and -d12 in single crystals of p-terphenyl-h14 and -d14 and symmetric octahydroanthracene (OHA) have been investigated, principally in static magnetic fields below 125 G, and in the temperature range from 64 to 250°K. The zero-field splittings of the lowest triplet state of chrysene-d12 have been determined very precisely in experiments at static fields from ·25 to ·3 G. The high-precision splittings determined for the twenty resonance absorptions observed for chrysene-d12 in p-terphenyl (at ·0.116, 0.063, and 0.053 cm−1) could be correlated into seven triples corresponding to seven distinct triplet-state species. Only six absorptions were observed for chrysene-d12 in OHA, and these could be correlated into two triples of splittings. Decisive evidence is presented that all 26 resonance absorptions observed arise solely from chrysene-d12 and not (in part) from impurities. For p-terphenyl host, direct experimental evidence has been obtained for orientational inequivalence among the chrysene-d12 guest triplet states associated with the different zero-field splitting triples. [In a subsequent report (II), the orientations of the inequivalent guest triplet tensor axes with respect to each other and to the p-terphenyl molecular and crystal axes have been quantitatively calculated from data presented here and from supplemental data.] The temperature dependences of the triplet lifetimes of the sets of chrysene-d12 guests associated with different zero-field splittings have been investigated for both p-terphenyl and OHA hosts. In addition, the temperature dependences of the zero-field splittings of chrysene-d12 in OHA have been determined over an extended range of temperature. The effects on the chrysene zero-field splittings of the deuteration of both guest and host (p-terphenyl) have been investigated. A limited investigation of the system chrysene-d12 in biphenyl is also reported.