Exciton Transport in an Organic Semiconductor Exhibiting Thermally Activated Delayed Fluorescence

Organic semiconductors characterized by a small singlet-triplet exciton energy splitting exhibit efficient reverse intersystem crossing and thermally activated delayed fluorescence. Consequently, exciton transport may occur along both the singlet and the triplet excited states, each with unique photophysical behavior and exciton energy transfer mechanisms. Delayed fluorescence systems, therefore, provide a unique test bed for characterizing the role of exciton spin in transport and diffusion. Concentration- and temperature-dependent photophysical characterization combined with measurements of the exciton diffusion length (L_D) for 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) elucidate the relative degree and magnitude of transport along the singlet and triplet molecular excited states as well as the role of the local dielectric environment in determining the intersystem balance.