Promoting through-space charge transfer-based TADF via rational alignment of quasi-planar donor and acceptor in solid state

Through-space charge transfer (TSCT)-based thermally activated delayed fluorescence (TADF) materials have attracted widespread attention due to their great potential in simultaneously possessing small singlet-triplet splitting energy (ΔEST) and rapid radiative rate (kf), but it remains a great challenge for the rational design of such materials. In this work, we perform a theoretical investigation on the photophysical properties of carbazole-based TSCT TADF materials both in solvent and solid phase using the polarizable continuum model and the combined quantum mechanics and molecular mechanics method to rationalize their experimental performance difference and reveal the molecular design principles. The results indicate that the molecules with gradually enhanced planarity of donor (D) or/and acceptor (A) units exhibit decreased distance and enhanced intramolecular interactions between D and A units originated from the intermolecular interactions of compact molecular arrangement, leading to favorable radiative rate process in solid phase compared with the situation in toluene. And the enhancement of planarity of D and A units significantly restricts the molecular stretching and rotation vibrations and decreases the non-radiative processes, which mainly contributes to the improved performance of the molecules. Moreover, such TSCT-based molecules show an obvious separation of the highest occupied molecular orbital and lowest unoccupied molecular orbital distributions, which is beneficial to achieving small ΔEST values. These theoretical understandings could give an in-depth physical insight into the structure-property relationship of such TSCT-based materials, providing quasi-planar controlled strategies for the exploration of high-performance TADF materials.