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 (ΦFRET) 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
(kFRET∼107 s–1) and then performed molecular-level calculations to explore the
interplay of intermolecular interactions and molecular rigidity in
governing ΦFRET. 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 kFRET and ΦFRET. 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 kFRET and ΦFRET. 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.