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

Abstract

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 kRISC: 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 kRISC, OLEDs fabricated with these emitters show no alleviation in efficiency roll-off and display lower maximum external quantum efficiency (EQEmax). The results highlight the promise of through-space HAE as a strategy to accelerate RISC while evidencing that C-Br bond stability adversely affects OLED performance.