Anomalous current-dependent ISC modulated by cascade energy transfer among multiple excited states in exciplex-based...

Exciplex has become a core system for constructing high-efficiency organic light-emitting diodes (OLEDs). However, the microscopic spin dynamic mechanism of systems with coexisting multiple excited states remains unclear. Herein, magneto-conductance (MC) measurements were utilised to explore the intrinsic microscopic mechanisms of two devices with TAPC as the donor, in which exciplex and electromer coexist. At 300 K, Device 1 employing TmPyPB as the acceptor exhibits unsatisfactory optoelectronic performance, with its MC response showing an intersystem crossing (ISC) transition from normal to abnormal current (I)-dependence. In contrast, Device 2 with a PO-T2T acceptor delivers excellent optoelectronic properties and regular I-dependent reverse intersystem crossing (RISC). By tuning temperature and doping ratio, Device 1 only presents regular I-dependent ISC at 95 K or with a 1 : 5 TAPC : TmPyPB blending ratio. These behaviours arise from severely imbalanced carrier distribution in Device 1, which modulates the strength of Dexter energy transfer (DET) among three excited states (^exCT₃ → ^el1CT₃ → ^el2CT₃). At low I and ultra-low temperature, the DET channels are too weak to exert an obvious effect on the I-dependence. At high I and temperatures above 245 K, the intensified DET channels and electric-field-induced dissociation indirectly affect the intensity of CT-RISC, resulting in non-monotonic I-dependence for the PP-ISC-dominated MC response of Device 1. This work provides in-depth insights into the spin evolution of multiple excited states in exciplex-based OLEDs, and offers new ideas for the rational design and optimisation of OLEDs by regulating energy transfer channels.