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

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, n ≥ 2) to the singlet excited state (S₁), with fast reverse intersystem crossing rate (k_RISC) values (in the order of 10⁶ to10⁸ s^-1) and high fluorescence radiative rates (k_r) values (in the order of 10⁶ s^-1). 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.