Intramolecular Through-Space Charge Transfer in Compact Cofacial Thermally Activated Delayed Fluorescence Emitters

In terms of the molecular design for thermally activated delayed fluorescence (TADF), the high degree of spatial separation of frontier molecular orbitals can be achieved through constructing cofacial donor-acceptor alignment in a through-space charge transfer (TSCT) framework. When extremely low Delta E-ST is realized, TSCT-TADF molecules still suffer a low radiative transition rate (k(r)) of the emissive TSCT state. Compact TSCT-TADF emitters, mCz-Xo-TRZ and dCz-Xo-TRZ, have been reported, showing a high photoluminescence quantum yield (PLQY) with large k(r). To explore the physical mechanism of high PLQY and large k(r) in the TSCT conformation, comprehensive theoretical calculations on geometries, excitation energies, transition character, strong and weak electronic coupling, and vibration analysis on structural reorganization accompanied by time-resolved spectroscopy were applied to illustrate the electronic coupling mechanism in the compact and rigid TSCT framework and reveal the limited structural relaxation contributing to the nonradiative dissipation in excited states. This work contributes a deep understanding of the electronic interaction mechanism in TSCT molecular frameworks and provides theoretical guidance for the rational design of TSCT luminescent materials.