Electron spin resonance study of phosphorescent centres in biphenyl, biphenyl–tetracyanoethylene, and biphenyl–tetracyanoethylene–naphthalene molecular crystals

Abstract

Phosphorescent centres in single crystals of zone-purified biphenyl have been examined by their triplet e.s.r. spectra. These centres are attributed to impurities, possibly naphthalene, trapped at crystal defect sites, rather than conformationally strained biphenyl molecules. The strong electron acceptor tetracyanoethylene is found to enter the lattice isomorphically, and to have negligible, if any, contribution to the triplet through its own triplet state or that of a donor–acceptor charge-transfer complex. Naphthalene in the presence of an excess of tetracyanoethylene forms a donor–acceptor charge-transfer complex which crystallises out of biphenyl solid solution. Insofar as a residue of this complex is retained in the lattice there is, within the limits of the experimental technique, no detectable charge-transfer contribution to the triplet of the phosphorescent centres of the host biphenyl.