Tuning strength of Donor-Acceptor Unit: Enabling transition between Through-Bond and Through-Space charge transfer in HLCT emissions.

Through-space charge transfer (TSCT) mechanism has emerged as key strategies for the creating of hybrid localized and charge transfer (HLCT) emitters. We present a systematic study of U-shaped molecules incorporating benzofuran (BDF) as electron acceptors and carbazole as the bridge-connected donor (carbazole, phenoxazine, and dihydrophenazine, Cz/PXZ/DHPZ), named BDF-Cz-Cz, BDF-Cz-PXZ, BDF-Cz-DHPZ, BDFCN-Cz-Cz, BDFCN-Cz-PXZ, and BDFCN-Cz-DHPZ. The photophysical properties of these molecules were systematically analyzed based on density functional theory (DFT) and time-dependent DFT (TD-DFT). By adjusting the donor-acceptor strength, theproportionof charge transfer (CT) between the donor and acceptor through space versus through bonds can be altered, enabling the switching between through-bond charge transfer (TBCT) and TSCT. Strong donors and acceptors possess a significant driving force, and analysis of the weak interactions within the molecule revealed that BDFCN-Cz-DHPZ possesses TSCT character. Significantly, PXZ-based and DHPZ-based molecules exhibit the high-lying reverse intersystem crossing (hRISC) process from the triplet excited state (T, n ≥ 2) to the singlet excited state (S), with fast reverse intersystem crossing rate (k) values (in the order of 10 to10 s) and high fluorescence radiative rates (k) values (in the order of 10 s). Therefore, U-shaped molecules are promising candidates for efficient HLCT molecules. This innovative design strategy offers new ideas and avenues for regulating and optimizing the performance of TBCT-HLCT and TSCT-HLCT emitters.