Positive and negative magnetic field effects in organic semiconducting materials

It has been found that non-magnetic organic semiconducting materials can exhibit magnetic responses in electrical current, electroluminescence, photoluminescence, and photocurrent when an external magnetic field is applied. These intrinsic magnetic responses can be naturally attributed to (i) magnetic field-dependent singlet/triplet ratio and (ii) singlet/triplet ratio-dependent excited processes. We observe that inter-molecular and intra-molecular excited states are sensitive and insensitive, respectively, to external magnetic field in magnetic field dependences of electroluminescence, photocurrent, and photoluminescence. This experimental phenomenon suggests that the electron-hole separation distance essentially determines whether magnetic field effects can be activated through the competition between spin-exchange interaction-induced singlet-triplet energy difference and magnetic Zeeman splitting when applied magnetic field is stronger than spin-orbital coupling. Furthermore, the dissociation in inter-molecular excited states and exciton-charge reaction in intra-molecular excited states have positive and negative responses to external magnetic field. As a result, controlling the dissociation in inter-molecular excited states and the exciton-charge reaction in intra-molecular excited states provide an effective methodology to tune the magnetic field effects between positive and negative values in organic semiconducting materials. (C) 2009 Elsevier B.V. All rights reserved.