The novel fused Zn(II)porphyrin arrays (Tn, porphyrin tapes) in which the porphyrin macrocycles are triply linked at meso-meso, beta-beta, beta-beta positions have been investigated by steady-state and time-resolved spectroscopic measurements along with theoretical MO calculations. The absorption spectra of the porphyrin tapes show a systematic downshift to the IR region as the number of porphyrin pigments increases in the arrays. The fused porphyrin arrays exhibit a rapid formation of the lowest excited states (for T2, approximately 500 fs) via fast internal conversion processes upon photoexcitation at 400 nm (Soret bands), which is much faster than the internal conversion process of approximately 1.2 ps observed for a monomeric Zn(II)porphyrin. The relaxation dynamics of the lowest excited states of the porphyrin tapes were accelerated from approximately 4.5 ps for the T2 dimer to approximately 0.3 ps for the T6 hexamer as the number of porphyrin units increases, being explained well by the energy gap law. The overall photophysical properties of the porphyrin tapes were observed to be in a sharp contrast to those of the orthogonal porphyrin arrays. The PPP-SCI calculated charge-transfer probability indicates that the lowest excited state of the porphyrin tapes (Tn) resembles a Wannier-type exciton closely, whereas the lowest excited state of the directly linked porphyrin arrays can be considered as a Frenkel-type exciton. Conclusively, these unique photophysical properties of the porphyrin tapes have aroused much interest in the fundamental photophysics of large flat organic molecules as well as in the possible applications as electric wires, IR sensors, and nonlinear optical materials.

The carbon-oxygen double bond of ketones (R(2)C=O) makes them among the most important organic compounds, but their homologues, heavy ketones with an E=O double bond (E = Si, Ge, Sn or Pb), had not been isolated as stable compounds. Their unavailability as monomeric molecules is ascribed to their high tendency for intermolecular oligomerization or polymerization via opening of the E=O double bond. Can such an intermolecular process be inhibited by bulky protecting groups? We now report that it can, with the first isolation of a monomeric germanium ketone analogue (Eind)(2)Ge=O (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl), stabilized by appropriately designed bulky Eind groups, with a planar tricoordinate germanium atom. Computational studies and chemical reactions suggest the Ge=O double bond is highly polarized with a contribution of a charge-separated form (Eind)(2)Ge(+)-O(-). The germanone thus exhibits unique reactivities that are not observed with ordinary ketones, including the spontaneous trapping of CO(2) gas to provide a cyclic addition product.

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...