Abstract
Three novel small organic heterocyclic compounds: 2-(1,2-diphenyl)-1H-benzimidazole-7-tert-butylpyrene (compound A), 1,3-di(1,2-diphenyl)-1H-benzimidazole-7-tert-butylpyrene (compound B), and 1,3,6,8-tetra(1,2-diphenyl)-1H-benzimidazolepyrene (compound C) were synthesized and characterized for possible applications as blue OLED emitters. The specific molecular design targeted decreasing intermolecular aggregation and disrupting crystallinity in the solid-state, in order to reduce dye aggregation, and thus obtain efficient pure blue photo- and electroluminescence. Accordingly, the new compounds displayed reasonably high spectral purity in both solution- and solid-states with average CIE coordinates of (0.160 ± 0.005, 0.029 ± 0.009) in solution and (0.152 ± 0.007, 0.126 ± 0.005) in solid-state. These compounds showed a systematic decrease in degree of crystallinity and intermolecular aggregation due to increasing steric hindrance, as revealed using powder X-ray diffraction analysis and spectroscopic studies. An organic light-emitting diode (OLED) prototype fabricated using compound B as the non-doped emissive layer displayed an external quantum efficiency (EQE) of 0.35 (±0.04)% and luminance 100 (±6) cd m−2 at 5.5 V with an essentially pure blue electroluminescence corresponding to CIE coordinates of (0.1482, 0.1300). The highest EQE observed from this OLED prototype was 4.3 (±0.3)% at 3.5 V, and the highest luminance of 290 (±10) cd m−2 at 7.5 V. These values were found comparable to characteristics of the best pure blue OLED devices based on simple fluorescent small-molecule organic chromophores.
Highlights
Organic light emitting diodes (OLEDs) have been steadily present over the past30 years, from a laboratory concept stemming from the pioneering work of Ching W
The powder X-ray diffraction (XRD) data indicates that compounds A, B, and C are predominantly amorphous, as their XRD plots display two intense broad scattering bands at 2θ approx. 10◦ and 20◦, while displaying systematic reduction of the intensity of sharp Bragg diffraction peaks stemming from the crystalline phase
Showed the highest photodegradation, with an approximately 30% reduction of the initial fluorescence intensity under the given experimental conditions (Figure 3). This high photobleaching rate observed for BCP can be attributed to the high-energy excited state of BCP, as indicated by its large highest occupied molecular orbitals (HOMO)–lowest unoccupied molecular orbitals (LUMO) energy gap value, which leads to a high susceptibility towards photo-induced degradation reactions [39]
Summary
30 years, from a laboratory concept stemming from the pioneering work of Ching W. The use of multifunctional emissive materials could effectively reduce the number of supporting organic semiconducting layers in OLEDs, e.g., charge injectors and transporters, lowering the complexity and cost of OLED devices [4] In this manuscript, we report the design, synthesis, and characterization of three novel, structurally similar, multifunctional small organic molecules. Our design utilizes multiple phenyl and/or tertiary butyl groups attached to the pyrene-benzimidazole cores of compounds A, B, and C These groups were found to effectively reduce π-π stacking of pyrenyl moieties in the solid-state by causing substantial steric hindrance. Addition to further inducing steric hindrance, electron deficient benzimidazole moieties in the target compounds were added in order to facilitate electron transport since these
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