Heavy Atom as a Molecular Sensor of Electronic Density: The Advanced Dimer-Type Light-Emitting System for NIR Emission

The approaches to design and control intermolecular interactions for a selective enhancement of specific process(es) are of high interest in technologies using molecular materials. Here, we describe how π–π stacking enables control over the heavy-atom effect and spin–orbit coupling (SOC) through dimerization of an organic emitter in solid media. π–π interactions in a red thermally activated delayed fluorescence (TADF) emitter Ac-CNBPz afford specific types of dimers. In its brominated derivative Ac-CNBPzBr, the vicinity of the Br atom and the electronic density of the dimer involved in a spin-flip transition afford up to 200-fold increase of the SOC, in the most favorable case, attributed to the external heavy-atom effect (EHAE) of the halogen atom. The presence of such dimers in the films of Ac-CNBPzBr provides enhancement of reverse intersystem crossing, and thus, TADF occurs mostly within a few microseconds, up to 20 times faster than in Ac-CNBPz. For this reason, organic light-emitting diodes using Ac-CNBPzBr as an emitter and an assistant dopant show a decreased efficiency roll-off by a factor of 4 and 1.5, respectively. The crucial aspects of the intermolecular electronic interactions between a chromophore system and an HA together with the particularly favorable dimer geometry not only help to understand the nature of the EHAE but also provide guidelines for the molecular design of emitters for all-organic light-emitting devices with enhanced stability.