The fluorescence efficiency of tetracene single crystals may be enhanced by as much as 38% in a magnetic field $H>\mathrm{}2000\mathrm{}$ G. The enhancement is anisotropic with respect to the orientation of $H$ in the $\mathrm{ab}$ plane. It is shown that a magnetically sensitive coupling of singlet states to a double-triplet-exciton state ($T_1{T}_1$) at ∼2.40 eV is an important channel for radiationless decay in crystalline tetracene above 160°K.

• Received 27 December 1968

DOI:https://doi.org/10.1103/PhysRevLett.22.593