Rigidifying Donor–Acceptor Frameworks via C–C Interlocking to Modulate Excited-State Dynamics in HLCT Emitters

Abstract

Thermally activated delayed fluorescence (TADF) exploits intramolecular charge-transfer states to harvest triplet excitons in organic light-emitting diodes (OLEDs) through the twisted configuration of the donor and acceptor units. However, the highly twisted conformation of TADF molecules results in limited fluorescence yield and leads to energy loss. In this study, we designed rigid and planar fused HLCT emitters by interlocking the C-C bond between the donor and acceptor. Carbazole and π-extended carbazole derivatives were used as donors, and cyano benzene, benzophenone, sulfonyldibenzene, and N-methyl phthalimide, were employed as acceptors. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were used to evaluate the electronic, excited-state, and photophysical properties of the newly designed molecules. The radiative and non-radiative decay rates were estimated using thermal vibrational correlated formalism (TVCF). Furthermore, light emitting properties of organic emitters with and without interlock structures were systematically evaluated. These results reveal the crucial role of molecular rigidity in enhancing radiative and non-radiative properties of organic emitters. Overall, our findings establish intricate relationships between the molecular structure and key photochemical properties, including intersystem crossing rates, reverse intersystem crossing rates, radiative decay rates, and charge transfer characteristics.