A water-soluble luminescent tris(2,4,6-trichlorophenyl)methyl radical-carbazole dyad.

Organic luminescent radicals are a new class of materials with potential applications not only in light-emitting devices but also in the biochemistry field. New tris(2,4,6-trichlorophenyl)methyl (TTM) radicals with alkoxy-substituted carbazole donors were synthesized and characterized. PEG-substituted carbazole-TTM was found to be water-soluble. The water-soluble TTM radical aqueous solution showed fluorescence at 777 nm and the ability to shorten the longitudinal relaxation time (T1) of water. The concept of water-soluble luminescent radicals is expected to be used to develop a potential fluorescence and MR dual-use imaging moiety.

Effects of halogen atom substitution on luminescent radicals: a case study on tris(2,4,6-trichlorophenyl)methyl radical-carbazole dyads.

A series of halogen-substitute carbazole TTM radicals was synthesized. The effect of halogen substituents on radical luminescence was systematically evaluated. It was found that the well-known heavy atom effect does not work in the emission of radicals and that halogen substitution of the donor carbazole can change the HOMO and alter the absorption and emission wavelengths. In addition, the photostability was found to be improved with respect to TTM but not significantly different from that of closed-shell fluorescent molecules.

Surface modification of gold by carbazole dendrimers for improved carbon dioxide electroreduction.

Four types of carbazole dendrimers were applied as modification molecules of Au surfaces to improve carbon dioxide electroreduction. The reduction properties depended on the molecular structures: the highest activity and selectivity to CO was achieved by 9-phenylcarbazole, probably caused by the charge transfer from the molecule to Au.

Carbazole-Dendronized Luminescent Radicals.

A series of carbazole-dendronized tris(2,4,6-trichlorophenyl)methyl (TTM) radicals have been synthesized. The photophysical properties of dendronized radicals up to the fourth generation were compared systematically to understand how structure-property relationships evolve with generation. The photoluminescence quantum yield (PLQY) was found to increase with the increasing generation, and the fourth generation (G4TTM) in cyclohexane solution showed a PLQY as high as 63 % at a wavelength of 627 nm (in the deep-red region) from the doublet state. The dendron modification strategy also showed a blue-shift of the emission on increasing the generation number, and the photostability was also increased compared to the bare TTM radical.

Photostability of luminescent tris(2,4,6-trichlorophenyl)methyl radical enhanced by terminal modification of carbazole donor.

Stable organic luminescent radicals have attracted much attention, but their stability under light irradiation is not yet satisfactory. New luminescent radicals (TTMs) based on terminal benzene ring modified carbazole donors were synthesized and evaluated. Their photostability (half-life under continuous laser irradiation) has improved by 1 order of magnitude compared to simple carbazole donors. This is a new molecular design strategy to improve the photostability of luminescent radicals without reducing other photophysical properties.

Novel Series of Mononuclear Aluminum Complexes for High-Performance Solution-Processed Organic Light-Emitting Devices.

Light metal complexes, such as lithium (Li), sodium (Na), magnesium (Mg), and aluminum (Al) complexes, are attractive candidates for the fabrication of thermally activated delayed fluorescent (TADF) materials. Nevertheless, mononuclear Al complexes with delayed fluorescence have not been developed so far. In this study, we successfully developed a novel series of highly luminescent Al complexes with two phenylacridine-modified asymmetric acetylacetonate-type ligands. These complexes exhibit high photoluminescence quantum yields (PLQYs) of up to 79 % in the solid state with a short delayed fluorescence lifetime of approximately 4 µs. Solution-processed organic light-emitting devices (OLEDs) using these Al complexes exhibit excellent performance with an external quantum efficiency of 17.5 % at 100 cd m-2 . This is the best performance in light metal-based TADF OLEDs reported so far. The results are expected to guide the advancement of the next-generation solid-state lighting technology.

π-Extended Carbazole Derivatives as Host Materials for Highly Efficient and Long-Life Green Phosphorescent Organic Light-Emitting Diodes.

High-performance organic light-emitting diodes (OLEDs) that use phosphorescent and/or thermally activated delayed fluorescence emitters are capable of realizing 100 % electron-to-photon conversion. The host materials in these OLEDs play crucial roles in determining OLED performance. Carbazole derivatives are frequently used as host materials, among which 3,3-bis(9H-carbazol-9-yl)biphenyl (mCBP) is often used for lifetime testing in scientific studies. In this study, the π conjugation of the carbazole unit was expanded to enhance OLED lifetime by designing and developing two benzothienocarbazole (BTCz)-based host materials, namely m1BTCBP and m4BTCBP. Among these host materials, m1BTCBP formed a highly efficient [Ir(ppy)3 ]-based OLED with an operational luminescence half-life (LT50 ) of over 300 h at an initial luminance of approximately 12000 cd m-2 (current density: 25 mA cm-2 ). The LT50 value at 1000 cd cm-2 was estimated to be about 23 000 h. This performance is clearly higher than that of mCBP-based OLEDs (LT50 ≈8500 h).

Unique Solid-State Emission Behavior of Aromatic Difluoroboronated ß-Diketones as an Emitter in Organic Light-Emitting Devices.

Aromatic difluoroboronated ß-diketone (BF2 DK) derivatives are a widely known class of luminescent organic materials that exhibit high photoluminescent quantum efficiency and unique aggregation-dependent fluorescence behavior. However, there have been only a few reports on their use in solid-state electronic devices, such as organic light-emitting devices (OLEDs). Herein, we investigated the solid-state properties and OLED performance of a series of π-extended BF2 DK derivatives that have previously been shown to exhibit intense fluorescence in the solution state. The BF2 DK derivatives formed exciplexes with a carbazole derivative and exhibited thermally activated delayed fluorescence (TADF) behavior to give orange electroluminescence with a peak external quantum efficiency of 10 % that apparently exceeds the theoretical efficiency limit of conventional fluorescent OLEDs (7.5 %), assuming a light out-coupling factor of 30 %.

Manipulating the Electronic Excited State Energies of Pyrimidine-Based Thermally Activated Delayed Fluorescence Emitters To Realize Efficient Deep-Blue Emission.

The development of efficient and robust deep-blue emitters is one of the key issues in organic light-emitting devices (OLEDs) for environmentally friendly, large-area displays or general lighting. As a promising technology that realizes 100% conversion from electrons to photons, thermally activated delayed fluorescence (TADF) emitters have attracted considerable attention. However, only a handful of examples of deep-blue TADF emitters have been reported to date, and the emitters generally show large efficiency roll-off at practical luminance over several hundreds to thousands of cd m-2, most likely because of the long delayed fluorescent lifetime (τd). To overcome this problem, we molecularly manipulated the electronic excited state energies of pyrimidine-based TADF emitters to realize deep-blue emission and reduced τd. We then systematically investigated the relationships among the chemical structure, properties, and device performances. The resultant novel pyrimidine emitters, called Ac-XMHPMs (X = 1, 2, and 3), contain different numbers of bulky methyl substituents at acceptor moieties, increasing the excited singlet (ES) and triplet state (ET) energies. Among them, Ac-3MHPM, with a high ET of 2.95 eV, exhibited a high external quantum efficiency (ηext,max) of 18% and an ηext of 10% at 100 cd m-2 with Commission Internationale de l'Eclairage chromaticity coordinates of (0.16, 0.15). These efficiencies are among the highest values to date for deep-blue TADF OLEDs. Our molecular design strategy provides fundamental guidance to design novel deep-blue TADF emitters.

Highly Luminescent π-Conjugated Terpyridine Derivatives Exhibiting Thermally Activated Delayed Fluorescence.

Typically, luminescent π-conjugated 2,2':6',2''-terpyridine (tpy) derivatives are versatile components for tridentate metal ligands, supramolecular materials, two-photon absorption bioimaging probes, fluorescent ion sensors, and organic light-emitting devices. However, a limited number of luminescent tpy materials, other than metal complexes, have been reported. This study introduces a series of π-conjugated tpy derivatives that exhibit strong thermally activated delayed fluorescence (TADF). We have observed that a blue tpy emitter outperforms conventional fluorescent emitters. Additionally, a green tpy emitter has exhibited a performance that is almost comparable to that of its green phosphorescent counterparts, realizing an external quantum efficiency close to 25 % and a power efficiency exceeding 80 lm W-1 with an exceptionally low efficiency roll-off. This study demonstrates the first example of highly luminescent tpy-based TADF emitters.