π-Functionalized 1,5-diazocines with diverse intramolecular connectivities to modulate photophysical and electroluminescence properties

Abstract

The effect of diverse intramolecular connectivities on the photophysical and hole-transporting attributes was studied for six butterfly-shaped π-functionalized 1,5-diazocines (Dzs). These molecules were developed by slight structural tweaking of the central bridging Dz core and varying the end terminal substitution. Here, the flexibility of Dzs was systematically modulated by tuning the intramolecular methylene connectivity between the Dz nitrogens and by changing the end arms from rigid carbazole to flexible diphenylamine. A gradual change in the shape and rigidity led to a transformation from the amorphous to crystalline nature of these systems. A series of organic light-emitting diodes (OLEDs) with these differently bridged Dzs as HTMs were fabricated to systematically compare the hole-transport and electroluminescence properties of these Dz-based systems. The device with N–CH₂–N–Dz-1 as a HTM showed the best performance among the series with the lowest turn-on voltage of 3.0 V and an external quantum efficiency (η_ext) of 8.18%, attributed to high T_g, amorphous nature, and good film-forming properties. Although slightly less efficient than N–CH₂–N-Dz-1, owing to their crystalline or semicrystalline nature, the devices with N–N-Dz-2 and N-0-N-Dz-3 as HTMs exhibited an η_ext of 4.35% and 7.75%, respectively. The tailoring of electronic distribution and hole transport by varying the Dz core and the terminal substituents was supported by density functional theory (DFT)-based calculations and the space charge limited current (SCLC) method. The study showed the significance of judicious structural engineering by tuning intramolecular bridging to modulate photophysical properties in order to impact the electroluminescence. Another highlight is mCPBA-mediated new process chemistry for converting N–CH₂–N-bridged molecules into N–N-bridged derivatives synthetically.