Critical Role of Intermolecular-Interaction-Mediated Conformational Dynamics in Sensitized OLEDs

Sensitized organic light-emitting diodes (OLEDs) based on a thermally activated delayed fluorescence sensitizer require high Förster resonance energy transfer efficiency (Φ_FRET) to achieve high efficiency and color purity. Since the energy transfer process involves complex interactions between two distinct molecules in amorphous solid film, it remains far from being understood and needs deep insights. Herein, taking two representative sensitizers, nonrigid donor-π-acceptor type DMAC-TRZ and rigid donor-spiro-acceptor type ACRSA as examples, we first established a reliable multiscale model based on the picture of the energy transfer process and obtained experimentally verified energy transfer rates (k_FRET∼10⁷ s^-1) and then performed molecular-level calculations to explore the interplay of intermolecular interactions and molecular rigidity in governing Φ_FRET. For nonrigid DMAC-TRZ, it is not the commonly believed intrinsic conformer variation via the dihedral angle between the donor and π unit but strong intermolecular interactions with a terminal emitter at short distance that induce undesirable low-energy conformers with large variation in the dihedral angle between the acceptor and π unit, which further result in an anomalous reduction of Franck-Condon factor weighted density of states (FCWD) and thus lower-than-expected k_FRET and Φ_FRET. For ACRSA, it is a rigid spirocyclic structure that greatly weakens intermolecular interactions and preserves an almost distance-independent molecular conformation, thus facilitating nearly uniform FCWD parameters and superior k_FRET and Φ_FRET. Our study provides fresh insights into the critical role of intermolecular-interaction-mediated conformational dynamics in energy transfer, revealing a blind and vital point behind molecular rigidity in developing high-performance sensitized OLEDs.