Recent breakthroughs in through-space charge transfer in π-stacked molecules as thermally activated delayed fluorescent emitters for OLED applications

Abstract

Intramolecular charge transfer through covalent bonds has been a conventional strategy for designing thermally activated delayed fluorescence (TADF) emitters by integrating donor and acceptor moieties along a conjugated backbone for organic light-emitting diode (OLED) applications. However, this approach often faces challenges, including red-shifted emissions, large singlet–triplet energy gaps (ΔE_ST), and significant non-radiative decay losses. To mitigate these constraints, through-space charge transfer (TSCT) has emerged as a potential alternative in TADF emitter design. TSCT emitters exhibit small ΔE_ST, reduced non-radiative decay, high photoluminescence quantum yields (PLQYs) and accelerated reverse intersystem crossing rates (k_RISC). By minimizing direct conjugation between donor and acceptor units, TSCT enables precise tuning of photophysical properties, making it particularly advantageous for achieving deep-blue and blue emission. TSCT in TADF emitters can be realized either through non-conjugated (σ-bonded) or conjugated backbones. However, TSCT TADF emitters with non-conjugated linkers often suffer from thermal stability issues. As a result, TSCT TADF emitters with conjugated backbones have gained considerable attention for overcoming these obstacles. Designing such molecules requires precise structural alignment, particularly with optimal face-to-face stacking and inter-ring distances ideally under 3.5 Å to facilitate TSCT, thereby enhancing TADF properties and device performance. To date, various π-stacked TSCT TADF emitters have been developed, with ongoing research aimed at refining their properties. This review provides a comprehensive overview of the design strategies, synthetic routes, and structure–property relationships of π-stacked TSCT TADF emitters and highlights their performance in OLEDs. Schemes illustrating chemical structures and a table summarizing key photophysical and electroluminescence performance data of reported materials are provided. Finally, we discuss the existing challenges and future directions in this field to guide further advancements.