Thermally activated delayed fluorescence (TADF) emitters are promising dopants for organic light-emitting diodes, including those containing highly twisted donor− acceptor-type structures. However, highly twisted structures limit the variety of chemical structures applicable as TADF emitters. We present a strategy for designing electron donors that can eliminate this requirement and increase the structural diversity of TADF emitters. Using this strategy, we developed an electron donor containing carbazolyl and diphenylamino groups by carefully controlling its electron-donating ability. By combining this donor with a quinoxaline-based acceptor, we obtained the efficient green TADF emitter, N 3 ,N 3 ,N 6 ,N 6 -tetraphenyl-9-(4-(quinoxalin-6-yl)phenyl)-9H-carbazole-3,6-diamine (DACQ), without a highly twisted structure. DACQ exhibits high photoluminescence and electroluminescence efficiencies, comparable to those of a highly twisted TADF emitter containing the same electron-accepting unit. Quantum chemical calculations showed that the diphenylamino groups within the carbazolyl moiety effectively withdraw the HOMO distribution. This reduces the singlet−triplet energy gap, thus inducing TADF. The photophysical properties of TADF compounds depend on the twisting angle between the electron-donating and accepting units. Eliminating the highly twisted structure increases the diversity of potential TADF emitters and allows their photophysical properties to be controlled by changing the twisting angle. ■ INTRODUCTIONThermally activated delayed fluorescence (TADF) emitters have attracted much attention because they can effectively convert triplet excitons into singlet excitons. TADF emitters can improve the electroluminescence (EL) efficiency of organic light-emitting diodes (OLEDs) using conventional carbonbased aromatic compounds. The EL efficiency of TADF-based OLEDs is now close to the theoretical maximum, and a paradigm shift from phosphorescence to TADF has occurred in OLED design. 1 TADF emitters can realize high EL efficiency in the absence of rare metals such as platinum and iridium 2−4 and so are promising alternatives to phosphorescent emitters. The optimum molecular design for TADF emitters is not well understood. Guidelines for rational molecular design are required for the practical application of TADF-based OLEDs, such as in flexible flat-panel displays 5 and solid-state lighting. 6,7 TADF emitters usually consist of electron-donating and -accepting moieties, and their excited states are of a chargetransfer (CT) character. The key process of TADF involves reverse intersystem crossing (RISC) from the lowest triplet state (T 1 ) to the lowest excited singlet state (S 1 ) and radiative decay from S 1 to the ground state (S 0 ). Thus, TADF efficiency depends largely on the efficiency of S 1 ← T 1 RISC and S 1 → S 0 radiative decay. The RISC rate increases with a decreasing energy difference between S 1 and T 1 (ΔE ST ), 8 so minimizing ΔE ST more effectively generates TADF. Separating the spatial distribution of t...

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The use of thermally activated delayed-fluorescence (TADF) allows the realization of highly efficient organic light-emitting diodes (OLEDs) and is a promising alternative to the use of conventional fluorescence and phosphorescence. Recent research interest has focused on blue TADF emitters. In this study, we use quantum mechanics to reveal the relationship between the molecular structures and the photophysical properties of TADF emitters and derive a direction for the molecular design of highly efficient blue TADF emitters. Theoretical analyses show that the luminescence efficiency of TADF emitters largely depends on the overlap density (ρ10) between the electronic wave functions of the ground state and the lowest excited singlet state. By increasing ρ10, we develop an efficient sky-blue TADF emitter material, 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)­phenyl)-9′-phenyl-9H,9′H-3,3′-bicarbazole (BCzT). When doped into a host layer, BCzT produces a high photoluminescence quantum yield of 95.6%. From the transient photoluminescence decays of the doped film, the efficiency of excited triplet state conversion into light is estimated to be 76.2%. An OLED using BCzT as a sky-blue emitter produces a maximum external quantum efficiency (EQE) of 21.7%, which is much higher than the EQE range of conventional fluorescent OLEDs (5–7.5%). The high EQE is a result of the high triplet-to-light conversion efficiency of BCzT. Our material design based on ρ10 distribution provides a rational approach for developing TADF emitters for high-efficiency blue OLEDs.

The use of organic radicals as building blocks is an effective approach to the production of open-shell coordination polymers (CPs). Two-dimensional (2D) CPs with honeycomb spin–lattices have attracted attention because of the unique electronic structures and physical properties afforded by their structural topology. However, radical-based CPs with honeycomb spin–lattices tend to have low chemical stability or poor crystallinity, and thus novel systems with high crystallinity and persistence are in strong demand. In this study, a novel triangular organic radical possessing three pyridyl groups, tris­(3,5-dichloro-4-pyridyl)­methyl radical (trisPyM) was prepared. It exhibits luminescence, high photostability, and a coordination ability, allowing formation of defined and persistent 2D CPs. Optical measurements confirmed the luminescence of trisPyM both in solution and in the solid state, with emission wavelengths, λem, of 665 and 700 nm, respectively. trisPyM exhibits better chemical stability under photoirradiation than other luminescent radicals: the half-life of trisPyM in CH2Cl2 was 10 000 times that of the tris­(2,4,6-trichlorophenyl)­methyl radical (TTM), a conventional luminescent radical. Complexation between trisPyM and ZnII(hfac)2 yielded a single crystal of a 2D CP trisZn, possessing a honeycomb lattice with graphene-like spin topology. The coordination structure of trisZn is stable under evacuation at 60 °C. Moreover, trisZn exhibits luminescence at 79 K, with λem = 695 nm, and is a rare example of a luminescent material among 2D radical-based CPs. Our results indicate that trisPyM may be a promising building block in the construction of a new class of 2D honeycomb CPs with novel properties, including luminescence.

A theoretical design principle for enhancement of the quantum yield of light-emitting molecules is desired. For the establishment of the principle, we focused on the S1 states of blue-emitting anthracene derivatives: 2-methyl-9,10-di(2'-naphthyl)anthracene (MADN), 4,9,10-bis(3',5'-diphenylphenyl)anthracene (MAM), 9-(3',5'-diphenylphenyl)-10-(3'',5''-diphenylbiphenyl-4''-yl) anthracene (MAT), and 9,10-bis(3''',5'''-diphenylbiphenyl-4'-yl) anthracene (TAT) [Kim et al., J. Mater. Chem., 2008, 18, 3376]. The vibronic coupling constants and transition dipole moments were calculated and analyzed by using the concepts of vibronic coupling density (VCD) and transition dipole moment density (TDMD), respectively. It is found that the driving force of the internal conversions and vibrational relaxations originate mainly from the anthracenylene group. On the other hand, fluorescence enhancement results from the large torsional distortion of the side groups in the S1 state. The torsional distortion is caused by the diagonal vibronic coupling for the lowest-frequency mode in the Franck-Condon (FC) S1 state, which originates from a small portion of the electron density difference on the side groups. These findings lead to the following design principles for anthracene derivatives with a high quantum yield: (1) reduction in the electron density difference and overlap density between the S0 and S1 states in the anthracenylene group to suppress vibrational relaxation and radiationless transitions, respectively; (2) increase in the overlap density in the side group to enhance the fluorescence.