Illhun Cho

Highly efficient and stable deep-blue emitting anthracene-derived molecular glass for versatile types of non-doped OLED applications

New anthracene-based deep-blue emitting molecular glass, 9-(9-phenylcarbazole-3-yl)-10-(naphthalene-1-yl)anthracene (PCAN), which is asymmetrically functionalized with N-phenylcarbazole and naphthalene, has been designed, synthesized, and characterized. The deep-blue emitting PCAN is efficiently secured for color purity, has high glass transition temperature (Tg = 151 °C), and has excellent solubility (>100 mg mL−1 in toluene), due to its highly tilted asymmetric molecular conformation. Using processing versatility of PCAN, vacuum-deposited and solution-processed non-doped deep-blue fluorescent organic light-emitting diodes (OLEDs) were prepared, which employ PCAN as an emitter. The vacuum deposited, non-doped EL device exhibited not only the excellent luminance efficiency and external quantum efficiency (as high as 3.64 cd A−1 and 4.61%, respectively) for the saturated deep-blue CIE chromaticity coordinates of (0.151, 0.086), but also stable performance and a good device lifetime. Furthermore, the solution processed EL device also exhibited an encouraging level of performance (1.24%, 1.15 cd A−1) and deep-blue emission (0.159, 0.105).

Stimuli-Responsive Reversible Fluorescence Switching in a Crystalline Donor-Acceptor Mixture Film: Mixed Stack Charge-Transfer Emission versus Segregated Stack Monomer Emission.

We report on a molecularly tailored 1:1 donor-acceptor (D-A) charge-transfer (CT) cocrystal that manifests strongly red-shifted CT luminescence characteristics, as well as noteworthy reconfigurable self-assembling behaviors. A loosely packed molecular organization is obtained as a consequence of the noncentrosymmetric chemical structure of molecule A1, which gives rise to considerable free volume and weak intermolecular interactions. The stacking features of the CT complex result in an external stimuli-responsive molecular stacking reorganization between the mixed and demixed phases of the D-A pair. Accordingly, high-contrast fluorescence switching (red↔blue) is realized on the basis of the strong alternation of the electronic properties between the mixed and demixed phases. A combination of structural, spectroscopic, and computational studies reveal the underlying mechanism of this stimuli-responsive behavior.

Concurrent supramolecular gelation and fluorescence turn-on triggered by coordination of silver ion

By adding silver ion to the non-fluorescent solution of a pyridine functionalized α-cyanostilbene derivative, an immediate gelation accompanying a high-contrast fluorescence turn-on was effectively generated. The silver ion-coordinated highly fluorescent gel underwent reverse gel-to-sol transition restoring the non-fluorescent sol state upon reaction with tetrabutylammonium fluoride. Silver nanoparticles embedded in the fluorescent gel film were fabricated by in situ UV-light irradiation, which induced the surface plasmon effect on the fluorescence emission.

Highly Luminescent 2D-Type Slab Crystals Based on a Molecular Charge-Transfer Complex as Promising Organic Light-Emitting Transistor Materials

A new 2:1 donor (D):acceptor (A) mixed-stacked charge-transfer (CT) cocrystal comprising isometrically structured dicyanodistyrylbenzene-based D and A molecules is designed and synthesized. Uniform 2D-type morphology is manifested by the exquisite interplay of intermolecular interactions. In addition to its appealing structural features, unique optoelectronic properties are unveiled. Exceptionally high photoluminescence quantum yield (ΦF ≈ 60%) is realized by non-negligible oscillator strength of the S1 transition, and rigidified 2D-type structure. Moreover, this luminescent 2D-type CT crystal exhibits balanced ambipolar transport (µh and µe of ≈10-4 cm2 V-1 s-1 ). As a consequence of such unique optoelectronic characteristics, the first CT electroluminescence is demonstrated in a single active-layered organic light-emitting transistor (OLET) device. The external quantum efficiency of this OLET is as high as 1.5% to suggest a promising potential of luminescent mixed-stacked CT cocrystals in OLET applications.

Patterned Taping: A High-Efficiency Soft Lithographic Method for Universal Thin Film Patterning

As a universal lithographic technique for microscale/nanoscale film patterns, we develop a strategy for the use of soft lithographically patterned pressure-sensitive tape (patterned tape) as a pattern-transporting stamp material. Patterning was successfully implemented through the selective detachment and/or attachment of various thin films, including organic and metallic layers demanding no subsequent physical, thermal, or chemical treatment, as this incurs the risk of the deformation of the thin film and the deterioration of its functionalities. Its features of universal adhesion and flexibility enable pressure-sensitive tapes to form patterns on a variety of surfaces: organic, polymeric, and inorganic surfaces as well as flat, curved, uneven, and flexible substrates. Moreover, the proposed technique boasts the unique and distinct advantages of short operation time, supreme patterning yield, and multilayer stacking capability, which suggest considerable potential for their application to advanced optoelectronic device fabrication.

Dicyanovinyl-substituted indolo[3,2-b]indole derivatives: low-band-gap π-conjugated molecules for a single-component ambipolar organic field-effect transistor

A series of low-band-gap π-conjugated molecules comprising N,N′-dihexylindolo[3,2-b]indole as an electron donor (D) and dicyanovinyl as an electron acceptor (A) with A–π–D–π–A architecture have been designed and synthesized to fabricate a single-component ambipolar organic field-effect transistor (OFET). Molecules with different π-bridging units (none, thiophene, and bithiophene) were synthesized and characterized to investigate their structure–property correlation. Via the cooperative effects of intramolecular charge transfer (ICT) interactions and extension of conjugation, the band gap of the newly synthesized molecules was reduced to 1.41 eV in the solution state. Among other compounds, 2H2TIDID-DCV (with a thiophene π-spacer) exhibited highly balanced ambipolar charge transport with hole and electron mobilities of 0.08 cm2 V−1 s−1 and 0.09 cm2 V−1 s−1, respectively, from a vacuum-deposited OFET device. Spin-coated OFET devices using OD2TIDID-DCV, in which the hexyl side chains of 2H2TIDID-DCV are replaced by 2-octyldodecyl groups, also exhibited an ambipolar charge-transporting nature (mobilities of 9.67 × 10−2 cm2 V−1 s−1 for holes and 3.43 × 10−3 cm2 V−1 s−1 for electrons). Both 2H2TIDID-DCV and OD2TIDID-DCV exhibited favorable thin-film morphology for the formation of charge-transporting channels, and structural analyses of these films revealed the same molecular packing characteristics of a three-dimensional lamellar π-stacking structure.

Structure–Property Correlation in Luminescent Indolo[3,2-b]indole (IDID) Derivatives: Unraveling the Mechanism of High Efficiency Thermally Activated Delayed Fluorescence (TADF)

A series of indolo[3,2-b]indole (IDID) derivatives are designed as a novel structural platform for thermally activated delayed fluorescence (TADF) emitters. Intramolecular charge transfer (ICT)-type molecules consisting of IDID donor (D) and various acceptor (A) moieties are synthesized and characterized in the protocol of the systematical structure-property correlation. IDID derivatives exhibit high efficiency, prompt fluorescence as well as TADF with emission ranges tuned by the chemical structure of the acceptor units. Interestingly, almost all of the IDID derivatives show an identical energy level of the lowest triplet excited state (T1) attributed to the locally excited triplet state of the IDID backbone (3LEID), while that of their lowest singlet excited state (S1) is largely tuned by varying the acceptor units. Thus, we demonstrate the underlying mechanism in terms of the molecular engineering. Among the compounds, Tria-phIDID and BP-phIDID generate efficient delayed fluorescence based on the small energy gap between the lowest singlet and triplet excited states (ΔEST) and mediation of the 3LEID state. Organic light-emitting diodes with these Tria-phIDID and BP-phIDID as a dopant in the emitting layer show highly efficient electroluminescence with maximum external quantum efficiencies of 20.8% and 13.9%, respectively.