Intramolecular through-space heavy-atom effect in multiresonant thermally activated delayed fluorescence emitters with [2.2]paracyclophane

The heavy-atom effect (HAE) is a powerful strategy to enhance spin–orbit coupling (SOC) for accelerating reverse intersystem crossing (RISC) in multiresonant thermally activated delayed fluorescence (MR-TADF) emitters, which is relevant to the performance of organic light-emitting diodes (OLEDs). Here, we demonstrate the effectiveness of a through-space HAE using a [2.2]paracyclophane (PCP) scaffold with a co-facially aligned MR-TADF emitter and bromine atom. By placing a bromine atom at pseudo-ortho (po) or pseudo-meta (pm) positions relative to the tCzBN MR-TADF core, po-PCP-tCzBN-Br and pm-PCP-tCzBN-Br were obtained. The two emitters have nearly identical emission spectra to the reference PCP-tCzBN, yet exhibit faster k_RISC: 5.3 × 10⁴ s⁻¹ (po) and 4.1 × 10⁴ s⁻¹ (pm) compared to 2.8 × 10⁴ s⁻¹ (PCP-tCzBN). Despite this improvement in k_RISC, the corresponding OLEDs showed no alleviation in efficiency roll-off and lower maximum external quantum efficiency (EQE_max). These findings suggest that a through-space HAE using a PCP is an effective strategy to accelerate k_RISC while largely preserving spectral purity; however, this improvement in k_RISC alone is insufficient to produce a superior device performance.