An ambipolar BODIPY derivative for a white exciplex OLED and cholesteric liquid crystal laser toward multifunctional devices

A new interface engineering method is demonstrated for the preparation of an efficient white organic light-emitting diode (WOLED) by embedding an ultrathin layer of the novel ambipolar red emissive compound 4,4-difluoro-2,6-di(4-hexylthiopen-2-yl)-1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-s-indacene (bThBODIPY) in the exciplex formation region. The compound shows a hole and electron mobility of 3.3 × 10–4 and 2 × 10–4 cm2 V–1 s–1, respectively, at electric fields higher than 5.3 × 105 V cm–1. The resulting WOLED exhibited a maximum luminance of 6579 cd m–2 with CIE 1931 color coordinates (0.39; 0.35). The bThBODIPY dye is also demonstrated to be an effective laser dye for a cholesteric liquid crystal (ChLC) laser. New construction of the ChLC laser, by which a flat capillary with an optically isotropic dye solution is sandwiched between two dye-free ChLC cells, provides photonic lasing at a wavelength well matched with that of a dye-doped planar ChLC cell

Spin- and Voltage-dependent emission from Intra- and Intermolecular TADF OLEDs

Organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) utilize molecular systems with a small energy splitting between singlet and triplet states. This can either be realized in intramolecular charge transfer states of molecules with near-orthogonal donor and acceptor moieties or in intermolecular exciplex states formed between a suitable combination of individual donor and acceptor materials. Here, we investigate 4,4'-(9H,9'H-[3,3'-bicarbazole]-9,9'-diyl)bis(3-(trifluoromethyl) benzonitrile) (pCNBCzoCF3), which shows intramolecular TADF but can also form exciplex states in combination with 4,4',4''-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA). Orange emitting exciplex-based OLEDs additionally generate a sky-blue emission from the intramolecular emitter with an intensity that can be voltage-controlled. We apply electroluminescence detected magnetic resonance (ELDMR) to study the thermally activated spin-dependent triplet to singlet up-conversion in operating devices. Thereby, we can investigate intermediate excited states involved in OLED operation and derive the corresponding activation energy for both, intra- and intermolecular based TADF. Furthermore, we give a lower estimate for the extent of the triplet wavefunction to be >1.2 nm. Photoluminescence detected magnetic resonance (PLDMR) reveals the population of molecular triplets in optically excited thin films. Overall, our findings allow us to draw a comprehensive picture of the spin-dependent emission from intra- and intermolecular TADF OLEDs.Comment: 9 pages, 5 figure

A novel donor-acceptor carbazole and benzothiadiazole material for deep red and infrared emitting applications

A novel organic material (C1) with the structure D-π-A-π-D was synthesised and characterised. Carbazole was utilised as the electron donor and benzothiadiazole as the electron acceptor unit. The electrochemical, optical and electronic properties of the synthesised compound were studied. Compound C1 exhibits absorption in the visible and ultraviolet range with a high molar absorption coefficient. A strong solvatochromic effect was observed in its emission spectra. Electrochemical and spectroelectrochemical measurements were performed in order to estimate the properties of the molecule in different redox states. Electron paramagnetic resonance (EPR) measurements indicate the delocalisation of radical cations and radical anions over different moieties. Interpretations of the electrochemical and optical results are supported by DFT calculations. OLEDs based on C1 present efficient emission in red and infrared spectral ranges, with a quantum efficiency of 3.13% and a current efficiency of 6.8 cd A-1. The performance is considerably better than what has been reported for analogous devices, based on other carbazole and benzothiadiazole units

Three-terminal light-emitting device with adjustable emission color

A three-terminal organic light-emitting device with a periodic interrupted middle electrode is developed to allow for an adjustable emission color. The emission results from three independent light-emitting diodes with one diode utilizing exciplex emission. An equivalent electrical circuit is suggested taking the current–voltage characteristics and the direction of current flow through the organic structure into account. Two diodes are formed between the embedded middle electrode and the LiF/Al top and ITO bottom electrode, respectively, and the third diode utilizes that part of the device without the middle-electrode exhibiting exciplex emission. It will be shown that the spectrum of the emitted light can be tuned from blue to orange by controlling the applied potentials to the device terminals

Star-Shaped Carbazole Derivatives for Bilayer White Organic Light-Emitting Diodes Combining Emission from Both Excitons and Exciplexes

Star-shaped carbazole-based compounds were synthesized by the Buchwald–Hartvig method. The materials were examined by various experimental and theoretical methods, including differential scanning calorimetry, UV spectrometry, electron photoemission, time-of-flight techniques, and DFT (B3LYP) calculations. The synthesized compounds showed high thermal stability with the initial weight loss temperature higher than 400 °C. The electron photoemission spectra of the layers of the amorphous materials showed ionization potentials of 4.9 eV. Tri­(9-hexylcarbazol-3-yl)­amine showed high hole mobility (μ = 10^–3 cm² V^–1 s^–1 at an electric field of 3.6 × 10⁵ V/cm). The star-shaped compounds were used for the preparation of bilayer white organic light-emitting diodes which combine emission from both excitons and exciplexes. The brightness of the white organic light emitting diode at 7 V is 300 cd/m² with current efficiency 2.3 cd/A and CIE coordinates (0.37, 0.35) which are very close to the equienergy white point (0.33, 0.33)

Highly Efficient Blue Organic Light-Emitting Diodes Based on Intermolecular Triplet–Singlet Energy Transfer

Diphenilamino-substituted carbazoles were used as guest compounds for the preparation of highly efficient blue organic light-emitting diodes based on the phenomenon of delayed fluorescence. It was shown that the spectra of the delayed fluorescence of host–guest systems are identical to those of the prompt fluorescence and in general coincide with the photoluminescence spectra of the guest films. The congruence of the prompt and delayed fluorescence spectra is explained by the effective intermolecular triplet–singlet (T → S) energy transfer from the excited T states of the host to the S states of the guest molecules. High external electroluminescence efficiency of the fabricated electroluminescent devices, reaching 17%, is comparable to that achieved in phosphorescence-based organic light-emitting diodes

Nine-ring angular fused biscarbazoloanthracene displaying a solid state based excimer emission suitable for OLED application

A new biscarbazoloanthracene consisting of nine fused aromatic rings, including two pyrrole units, has been obtained in a straightforward and convergent synthesis. Computational chemistry and conformational analysis revealed that the semiconductor’s molecule is not planar, the two carbazole moieties being helical twisted from the plane of the anthracene unit. Photophysical and electrochemical measurements showed that this angular fused heteroacene has a low lying HOMO energy level with a wide band gap despite its extended p-conjugated molecular framework. Based on its relatively low-lying HOMO level, the semiconductor promises a high environmental stability in comparison to other related linear fused acenes and heteroacenes. The biscarbazoloanthracene has been applied as the light emitting layer in a white light emitting diode (WOLED). It is proposed that the white OLED feature is due to dual light emission properties from the active semiconductor layer being based on both the molecular luminescence of the small molecule and a discrete excimer emission made possible by suitable aggregates in the solid state. Noteworthy, this is the first reported example of such a behavior observed in a small molecule heteroacene rather than an oligomer or a polymer