Hierarchical Nanowire Architectures Self‐Assembled from Ultra‐Deep‐Blue Fluorene‐Based Conjugated Molecules toward Organic Light‐Emitting Diodes with CIE_y = 0.06

Abstract

Organic conjugated molecules with a rigid rod‐like π‐backbone structure automatically and easily self‐assemble into an anisotropic nanostructure. However, supersecondary structures obtained from the hierarchical secondary self‐assembly of nanostructures have rarely been reported for non‐amphiphilic conjugated molecules. Here, a nanowire architecture as a supersecondary structure from an ultra‐deep‐blue fluorene‐based conjugated molecule (FCz‐C8‐Am) to improve the emission efficiency and stability is reported. In significant contrast to the four reference molecules, the FCz‐C8‐Am molecules grow into soft nanowires and further self‐assemble into a series of nanowire architectures in the gelation process. This is associated with the synergistic effect of the hydrogen bonds among the amide units, pendant π–π stacking interactions between pendant Cz units, and appropriate soft steric interaction among side‐chains, which are the three design requirements for preparing these nanowire architectures. Interestingly, this supersecondary architecture of FCz‐C8‐Am has a stable ultra‐deep‐blue emission, with an efficiency of ≈77% and a Commission Internationale de L'Eclairage (CIE) value of (0.16, 0.06) in the solid state. These findings provide a profound understanding of the relationship between the inherent molecular structure, supramolecular interaction, and supersecondary nano‐architecture, offering useful information for the development of new functional optoelectronic materials.

Ultraviolet Organic Laser from Rhombus Microcrystal: Benefits of Single-Molecule Emission from Twisted Structure

“…Therefore, as an alternative to LD-1, CL-1 molecules are rationally designed and easily prepared via a one-step Suzuki reaction, as displayed in the Supporting Information (Scheme S1). 40 The chemical structures are confirmed by NMR analysis and single-crystal measurements (Figures S1 and S2). More interestingly, according to the theoretical simulation in Figure 2a, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of CL-1 molecules are calculated to be about −6.22 and −2.13 eV, respectively.…”

“…Therefore, as showed in Figure , it is clear that the LD-1 molecules had emission peaks at 390 and 410 nm with a lasing peak at 410 nm, which are attributed to the intramolecular planar conformation and weak intermolecular π interaction . In this regard, similar to the discussion above, the twisted molecular conformation with an extremely weak intermolecular interaction may be useful to obtain excellent ultraviolet and deep-blue emission. , As displayed in Figure a, as expected, the diluted CL-1 solution had a maximum absorption peak at 326 nm, attributed to the intramolecular vibration band, which is consistent with previous works. ,, Due to the strong self-absorption induced by the millimeter-scale thickness, the optical spectral analysis of CL-1 single crystal is not provided here. It is easily found that the CL-1 single crystal also had robust ultraviolet and deep-blue emission under UV lamp 365 nm excitation (inset in Figure a).…”

“…29,44 As displayed in Figure 3a, as expected, the diluted CL-1 solution had a maximum absorption peak at 326 nm, attributed to the intramolecular vibration band, which is consistent with previous works. 29,39,40 Due to the strong self-absorption induced by the millimeter-scale thickness, the optical spectral analysis of CL-1 single crystal is not provided here. It is easily found that the CL-1 single crystal also had robust ultraviolet and deep-blue emission under UV lamp 365 nm excitation (inset in Figure 3a).…”

“…In the last decades, fluorene-based LOCMs have become the most promising candidates for fabricating ultraviolet and deep-blue OLEDs and organic lasers, associated with their wide bangap (>3.0 eV) and high fluorescence efficiency. ,− Similar to other types of dyes and emitters, with the aim of efficient and stable emission in solid states, two serious defect structures need to be avoided: physical and chemical defects. ,, The former is defined as the intermolecular aggregation and excited states, which may act as the “guest” center with a relatively narrow band to trap exciton from the matrix molecules. ,, The latter involves chemical oxidation and molecular degradation in the material preparation and long-time device operation . These factors lead to low efficiency and alter color purity in the ultraviolet and deep-blue fluorene-based LOCMs. ,,, Then, introducing the bulky units at the 9-position of fluorene can effectively not only enhance the chemical stability and antioxidation to suppress the undesirable green-band emission but also avoid the serious intermolecular aggregation to reduce the formation of multiexcited states, which is a universal strategy to design and prepare efficient and stable ultraviolet and deep-blue LOCMs. ,− Fortunately, a robust deep-blue fluorene molecule (LD-1, as shown Figure d) is obtained with a high emission efficiency (60%), which also showed a lasing behavior at 390 and 410 nm in the single-crystal states, associated with their typical herringbone molecular packing mode. , In fact, the LD-1 single-crystal presents a red-shifted emission of about 20 nm, compared to their diluted solution, due to their relatively planar conformation in the crystal state. More interestingly, herein, an ultraviolet lasing behavior (379 nm) is obtained from the CL-1 single crystal.…”

“…As discussed above, the electronic structure of OCMs is the fundamental parameter determining their photophysical and electrical properties. , The fluorene-segment is a universal platform to design and prepare ultraviolet and deep-blue light-emitting semiconductors with an ultrawide bandgap. − It is also commonly understood that the incorporation of electron-rich benzene group also slightly influences their electronic structure and bandgap. , Meanwhile, the fluorine substitution of aromatic units may not only enhance the chemical stability and precisely control intermolecular interaction and arrangement but also slightly affect the fundamental optoelectronic property. Therefore, as an alternative to LD-1, CL-1 molecules are rationally designed and easily prepared via a one-step Suzuki reaction, as displayed in the Supporting Information (Scheme S1) . The chemical structures are confirmed by NMR analysis and single-crystal measurements (Figures S1 and S2).…”