The spin polarization and emissive character of the magnetic resonance spectrum of mobile triplet excitons in tetracene siqlc crystals observed by Yannus, Roscnthal, and Chopp are interpreted in terms of selective population of the Zeeman subtevels by singlet exciton fission. in addition. the selective popuhtion of the triplet sublevel by direct singlet-triplet nbwrption is calculated. The differences to be expected in the EPR spectrum for these two metbods of esciton generation are discussed.
Yarmus et al.
[ 1] have detected electron paramagnetic resonance of tripiet excitons in soli tetracene which were generated from the singlet manifold by exciton fission. Spin polarization was observed for all magnetic field orientation: in the a'b plane, and for regions outside the pair state !evel crossings emissive character was observed. Previously Hadrer and Wolf [2] had reported spin pokarization in naphthalene single crystal and emissive behavior for naphthalene crystals doped with quinoxaline. The emksive behavior presumably originated from selectively populating the M, = Cl (It@h field spin representation) level by intersystem crossing in conjunction with a proper inter-pIay of the depopulating and relaxation rates of the excitons. We show here that singlet exciton fission resuits in selective population of the Zeeman levels and we discuss several interes&!g features associated with this process.
SingIet exciton fission ti a correlative exciton pro-* The au?hor wishes to acknowledge support from the National Science Foundation lnstitutianal Grant to New York University. l * National Science Four&tin Predoctoral Folibw 1971-72 *** Supported by the Atomic Energy Commission. cess known to occur in solid tetracene [3-S] _ It can be described by the two state equation [4] K-1 K2 I-+1'+-(-r-r) =+ s, K-2