Ionic Organic Small Molecules as Hosts for Light-Emitting Electrochemical Cells

Light-emitting electrochemical cells (LEECs) from ionic transition-metal complexes (iTMCs) offer the potential for high-efficiency electroluminescence in a simple, single-layer device. However, LEECs typically rely on the use of rare metal complexes. This has limited their cost effectiveness and put constraints on their applicability. With a view to leveraging the efficient emission of these complexes while mitigating costs, we describe here a host/guest LEEC strategy that relies on the use of carbazole (Cz)-based organic small-molecule hosts and iTMC guests. Three cationic host molecules were prepared via the coupling of 1-(4-bromophenyl)-2-phenylbenzimidazole (PBI-Br) with Cz. This has allowed a comparison between the hosts bearing methoxy (PBI-CzOMe) and tert-butyl (PBI-Cz tBu) substituents, as well as an unsubstituted analogue (PBI-CzH). Cyclic voltammetry and UV-visible absorption revealed that all three host materials have wide band gaps characterized by reversible oxidation and irreversible reduction events. On the basis of electronic structure calculations, the host highest occupied molecular orbital (HOMO) resides primarily on the Cz moiety, whereas the lowest unoccupied molecular orbital (LUMO) is located primarily on the phenyl-benzimidazolium unit. Photoluminescence analysis of thin-film blends of PBI-CzH with iTMC guests confirmed that the emission was blue-shifted relative to pristine iTMC films, which is consistent with what was seen in dilute dichloromethane solution. LEEC devices were prepared based on thin films of the pristine hosts, pristine guests, and 90%/10% (w/w) host/guest blends. Among these host/guest blends, LEECs based on PBI-CzH displayed the best performance, particularly when an iridium complex was used as the guest. The system in question yielded a luminance maximum of 624 cd/m² at an external quantum efficiency of 3.80%. This result stands in contrast to what is seen with typical organic light-emitting diode host studies, where tert-butyl substitution of the host generally leads to a better performance. To rationalize the present observations, the host materials were subject to single-crystal X-ray diffraction analysis. The resulting structures revealed clear head-to-tail interactions in the case of both PBI-CzH and PBI-CzOMe. No such interactions were evident in the case of PBI-Cz tBu. Furthermore, PBI-CzH showed a relatively smaller spacing between the successive HOMO and successive LUMO levels relative to PBI-CzOMe and PBI-Cz tBu, a finding consistent with more favorable charge transport and energy transfer. The results presented here can help inform the design and preparation of host materials suitable for use in single-layer iTMC LEECs.