EPR of triplet excitions in tetracene crystals: spin polarization and the role of singlet exciton fission

The room temperature electron paramagnetic resonance spectrum of triplet excitons in single-crystal tetracene is reported. From experiment performed at 24 GHz the exciton spin hamiltonian ≻/sc;* = gβH·S + D* [S2z*→13S(S + 1)] + E* [S*→S2v*], is fit to the measured angular dependence in the a'b plane. The spin hamiltonian parameters are g = 2.002,D* = −0.0062 cm−1, and E* = +0.0248 ± 0.0005 cm−1. Extensive spin polarization and emission have been observed and are attributed to the formation of the triplet population from singlet exciton fission.