Spatially separated exciplex emission with spacer thickness- and content-tunable properties: Experimental and computational investigations

Abstract

The tunability of long-range charge-transfer (CT) states in spatially separated exciplex systems presents a fundamental challenge in excited-state physics. This study systematically examines how controlled donor–acceptor (D–A) separation modulates the electroluminescent properties and excited-state electronic structures of TAPC–PO-T2T exciplex systems, using mCP as a spacer introduced via interfacial layering or bulk doping. Due to weakened Coulombic interaction by extended D–A spacing, increasing mCP spacer thickness (0–6 nm) or doping concentration (0–80 wt. %) induces a systematic blue shift (Δλ = 35 nm for interlayer and 18 nm for doping) in exciplex emission. Through precise spacer engineering, we achieve remarkable EQE enhancements: (i) 170% improvement with an optimal 8 nm mCP interlayer and (ii) 61% increase at 80 wt. % mCP doping content. To uncover the underlying electronic origins, we construct a series of donor–spacer–acceptor (D–S–A) model systems (DnSA/DnSAS, n = 0–5) and perform density functional theory (DFT) calculations. DFT calculation results show excellent agreement with experiments, particularly in predicting the blue-shifted emission through calculated increases in TAPC → PO-T2T CT state energy with spacer incorporation. Furthermore, more near-degenerate singlet–triplet states could promote efficient reverse intersystem crossing, explaining the enhanced electroluminescent performance. These insights collectively demonstrate that molecular spacer engineering is a powerful strategy for (i) spectral tuning via precise D–A distance control and (ii) efficiency optimization through excited-state energy alignment, offering new insights into the design of high-performance exciplex-based optoelectronic materials and devices.