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...

A meso,meso-linked porphyrin trimer, (ZnP)3, as a light-harvesting chromophore, has been incorporated for the first time into a photosynthetic multistep electron-transfer model including ferrocene (Fc) as an electron donor and fullerene (C60) as an electron acceptor, to construct the ferrocene-meso,meso-linked porphyrin trimer-fullerene system Fc-(ZnP)3-C60. Photoirradiation of Fc-(ZnP)3-C60 results in photoinduced electron transfer from both the singlet and triplet excited states of the porphyrin trimer, 1(ZnP)3* and 3(ZnP)3*, to the C60 moiety to produce the porphyrin trimer radical cation-C60 radical anion pair, Fc-(ZnP)3*+-C60*-. Subsequent formation of the final charge-separated state Fc+-(ZnP)3-C60*- was confirmed by the transient absorption spectra observed by pico- and nanosecond time-resolved laser flash photolysis. The final charge-separated state decays, obeying first-order kinetics, with a long lifetime (0.53 s in DMF at 163 K) that is comparable with that of the natural bacterial photosynthetic reaction center. More importantly, the quantum yield of formation of the final charge-separated state (0.83 in benzonitrile) remains high, despite the large separation distance between the Fc+ and C60*- moieties. Such a high quantum yield results from efficient charge separation through the porphyrin trimer, whereas a slow charge recombination is associated with the localized porphyrin radical cation in the porphyrin trimer. The light-harvesting efficiency in the visible region has also been much improved in Fc-(ZnP)3-C60 because of exciton coupling in the porphyrin trimer as well as an increase in the number of porphyrins.

Localized surface plasmon resonance (LSPR)-induced hot-carrier transfer is a key mechanism for achieving artificial photosynthesis using the whole solar spectrum, even including the infrared (IR) region. In contrast to the explosive development of photocatalysts based on the plasmon-induced hot electron transfer, the hole transfer system is still quite immature regardless of its importance, because the mechanism of plasmon-induced hole transfer has remained unclear. Herein, we elucidate LSPR-induced hot hole transfer in CdS/CuS heterostructured nanocrystals (HNCs) using time-resolved IR (TR-IR) spectroscopy. TR-IR spectroscopy enables the direct observation of carrier in a LSPR-excited CdS/CuS HNC. The spectroscopic results provide insight into the novel hole transfer mechanism, named plasmon-induced transit carrier transfer (PITCT), with high quantum yields (19%) and long-lived charge separations (9.2 μs). As an ultrafast charge recombination is a major drawback of all plasmonic energy conversion systems, we anticipate that PITCT will break the limit of conventional plasmon-induced energy conversion.

Increased photostability and a theoretical estimation of the radiative and non-radiative rates of luminescent organic radicals were achieved.

The vibronic coupling constants of C − 60 are derived from the photoelectron spectrum measured by Wang et al. [X. B. Wang, H. K. Woo, and L. S. Wang, J. Chem. Phys., 123, 051106 (2005).] at low temperature with high-resolutions. We find that the couplings of the Jahn-Teller modes of C − 60 are weaker than the couplings reported by Gunnarsson et al. [O.