Spin-Dependent Exciton Funneling to a Dendritic Fluorophore Mediated by a Thermally Activated Delayed Fluorescence Material as an Exciton-Harvesting Host

Thermally activated delayed fluorescence (TADF) materials generate energetically equivalent spin-singlet and spin-triplet excited states. In the presence of an energy acceptor, each excited state undergoes energy transfer on different length scales. However, the lack of quantitative understanding of the length dependence of the excited energy-transfer processes hampers the rational design of molecular systems that control exciton transport in organic light-emitting diodes (OLEDs) using TADF. We herein utilize a dendritic fluorophore G1, which consists of an anthracene-based fluorescent core encapsulated by four insulating tris(4-tert-butylphenyl)methyl groups as an energy acceptor. By combining transient photoluminescence measurements and kinetic modeling, we demonstrate the spin-dependent energy transfer in a binary host guest system composed of a TADF material as the exciton-harvesting host and G1 as the guest fluorophore. The encapsulated structure with the dendritic shell effectively inhibits triplet excitons on the TADF host from funneling to the fluorescent core, thus allowing efficient reverse intersystem crossing and singlet energy transfer. The utilization of GI in solution-processed OLEDs leads to a maximum external electroluminescence quantum efficiency as high as 5.2%, which is equivalent to an enhancement by a factor of 1.6 over the corresponding nondendritic fluorophore.