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Novel multi-carbazole TADF molecules based on a benzocyanide backbone with biphenyl and indolocarbazole, 4CzBN-BP and 4CzBN-ICz, as additional triplet scaffolds.

Metal halide perovskites are attracting a lot of attention as next-generation light-emitting materials owing to their excellent emission properties, with narrow band emission1-4. However, perovskite light-emitting diodes (PeLEDs), irrespective of their material type (polycrystals or nanocrystals), have not realized high luminance, high efficiency and long lifetime simultaneously, as they are influenced by intrinsic limitations related to the trade-off of properties between charge transport and confinement in each type of perovskite material5-8. Here, we report an ultra-bright, efficient and stable PeLED made of core/shell perovskite nanocrystals with a size of approximately 10 nm, obtained using a simple in situ reaction of benzylphosphonic acid (BPA) additive with three-dimensional (3D) polycrystalline perovskite films, without separate synthesis processes. During the reaction, large 3D crystals are split into nanocrystals and the BPA surrounds the nanocrystals, achieving strong carrier confinement. The BPA shell passivates the undercoordinated lead atoms by forming covalent bonds, and thereby greatly reduces the trap density while maintaining good charge-transport properties for the 3D perovskites. We demonstrate simultaneously efficient, bright and stable PeLEDs that have a maximum brightness of approximately 470,000 cd m-2, maximum external quantum efficiency of 28.9% (average = 25.2 ± 1.6% over 40 devices), maximum current efficiency of 151 cd A-1 and half-lifetime of 520 h at 1,000 cd m-2 (estimated half-lifetime >30,000 h at 100 cd m-2). Our work sheds light on the possibility that PeLEDs can be commercialized in the future display industry.

Low stability of perovskite light-emitting diodes (PeLEDs) is the biggest obstacle to the commercialization of PeLED displays. Here, we cover the current status and challenges in analysing and improving the stability of PeLEDs and suggest some advice that will benefit the community to boost the operational lifetime of PeLEDs.

Multi-resonance thermally activated delayed fluorescence (MR-TADF) molecules based on boron and nitrogen atoms are emerging as next-generation blue emitters for organic light-emitting diodes (OLEDs) due to their narrow emission spectra and triplet harvesting properties. However, intermolecular aggregation stemming from the planar structure of typical MR-TADF molecules that leads to concentration quenching and broadened spectra limits the utilization of the full potential of MR-TADF emitters. Herein, a deep-blue MR-TADF emitter, pBP-DABNA-Me, is developed to suppress intermolecular interactions effectively. Furthermore, photophysical investigation and theoretical calculations reveal that adding biphenyl moieties to the core body creates dense local triplet states in the vicinity of S

To survive in dynamic environments, animals make behavioral decisions based on innate and learnt information about the valences of sensory cues. In the brains of multiple species, innate and learnt sensory valence signals are initially encoded by distinct neural populations1–7 but then reconverge in downstream brain structures that guide behavioral choices6–9. This reconvergence hinges on the prior acquisition of learnt valence information, which in turn might depend upon innate valence signals. However, it remains unknown whether and how innate sensory valence cues shape the acquisition of learnt valence information. Here we show that in the fruit fly brain, interactions between innate and learnt sensory valence signals within interconnected short- and long-term memory units of the mushroom body jointly regulate memory formation and expression via modulation of dopamine teaching signals. By using time-lapse, in vivo optical voltage imaging to record neural spiking with millisecond-resolution in flies undergoing olfactory associative conditioning, we found that PPL1 dopamine neurons (PPL1-DANs) heterogeneously and bi-directionally encode punishment, reward, and innate and learnt odor-valence cues. Learning modulates these representations in a way that combines innate and learnt valence information and allows the PPL1-DANs to regulate memory storage in their downstream targets, mushroom body output neurons (MBONs). PPL1-γ1pedc and PPL1-γ2α’1 neurons control short-term memory formation. After repeated conditioning, feedback signals carrying short-term memory data from MBON-γ1pedc>α/β to PPL1-α’2α2 and PPL1-α3 allow these dopamine cells to encode previously learnt valences and promote long-lasting memory formation. A computational model constrained by the mushroom body connectome and our spiking data explains how dopamine signals integrate innate and learnt valence data to regulate memory storage, extinction, and the interactions between short- and long-term memory traces. The model yields non-intuitive predictions about the effects of different training protocols, which our experiments confirm. Overall, the mushroom body achieves flexible learning through dopamine-mediated integration of innate and learnt valences in parallel sets of DAN/MBON learning units with feedback interconnections. This hybrid physiologic-anatomic mechanism may be a general means by which ecological information regulates learning and memory in other species and brain structures relying on dopaminergic signaling, including the vertebrate basal ganglia10.

Avian models are valuable for studies of development and reproduction and have important implications for food production. Rapid advances in genome-editing technologies have enabled the establishment of avian species as unique agricultural, industrial, disease-resistant, and pharmaceutical models. The direct introduction of genome-editing tools, such as the clustered regularly interspaced short palindromic repeats (CRISPR) system, into early embryos has been achieved in various animal taxa. However, in birds, the introduction of the CRISPR system into primordial germ cells (PGCs), a germline-competent stem cell, is considered a much more reliable approach for the development of genome-edited models. After genome editing, PGCs are transplanted into the embryo to establish germline chimera, which are crossed to produce genome-edited birds. In addition, various methods, including delivery by liposomal and viral vectors, have been employed for gene editing in vivo. Genome-edited birds have wide applications in bio-pharmaceutical production and as models for disease resistance and biological research. In conclusion, the application of the CRISPR system to avian PGCs is an efficient approach for the production of genome-edited birds and transgenic avian models.

We report a new thermally activated delayed fluorescence (TADF) molecule developed by the ortho-biphenyl substitution of a multi-carbazole TADF molecule. The new TADF molecule, 4mCzBN-BP, is composed of four dimethylcarbazole donors, a benzonitrile acceptor core and an ortho-biphenyl triplet scaffold. 4mCzBN-BP exhibits a bluish-green emission with a peak wavelength at 491 nm and a very high photoluminescence quantum yield (PLQY) of 95% in an mCBP host. To investigate the effect of the ortho-biphenyl substitution on the excited-state dynamics and photophysical properties of the TADF molecule, we performed time dependent density functional theory (TD-DFT) calculations of 4mCzBN-BP and two multi-carbazole TADF molecules with similar molecular structures. A local excited triplet state (3LE) with an energy (2.81 eV) close to the lowest singlet charge-transfer state (1CT) can be formed at the biphenyl of 4mCzBN-BP. However, due to the planarization of the biphenyl at the triplet state and a large steric hindrance caused by adjacent carbazole donors, 3LE at the biphenyl can only be formed by a subset of possible conformations of 4mCzBN-BP molecules in the solid mCBP host, leading to a multiexponential decay of the delayed fluorescence. Color coordinates of (0.196, 0.452), a high external quantum efficiency of 23.7% and a long operational lifetime (LT50) of 750 hours were achieved with the organic light emitting diodes based on 4mCzBN-BP.

Triplet harvesting is important for the realization of high-efficiency fluorescent organic light-emitting diodes (OLEDs). Triplet-triplet annihilation (TTA) is one triplet-harvesting strategy. However, for blue-emitting anthracene derivatives, the theoretical maximum radiative singlet-exciton ratio generated from the TTA process is known to be 15% in addition to the initially generated singlets of 25%, which is insufficient for high-efficiency fluorescent devices. In this study, nearly 25% of the radiative singlet-exciton ratio is realized by TTA using an anthracene derivative, breaking the theoretical limit. As a result, efficient deep-blue TTA fluorescent devices are developed, exhibiting external quantum efficiencies of 10.2% and 8.6% with Commission Internationale de l'Eclairage color coordinates of (0.134, 0.131) and (0.137, 0.076), respectively. The theoretical model provided herein explains the experimental results considering both the TTA and reverse intersystem crossing to a singlet state from higher triplet states formed by the TTA, clearly demonstrating that the radiative singlet ratio generated from TTA can reach 37.5% (total radiative singlet-exciton ratio: 62.5%), well above 15% (total 40%), despite the molecule having S1 , T2

Innate immune system is triggered by pattern recognition receptors (PRRs) recognition. Retinoic acid-inducible gene 1 (RIG-I) is a major sensor that recognizes RNA ligands. However, chickens have no homologue of RIG-I; instead, they rely on melanoma differentiation-associated protein 5 (MDA5) to recognize RNA ligands, which renders chickens susceptible to infection by influenza A viruses (IAVs). Here, we engineered the cMDA5 viral RNA sensing domain (C-terminal domain, CTD) such that it functions similarly to human RIG-I (hRIG-I) by mutating histidine 925 into phenylalanine, a key residue for hRIG-I RNA binding loop function, or by swapping the CTD of cMDA5 with that of hRIG-I or duck RIG-I (dRIG-I). The engineered cMDA5 gene was expressed in cMDA5 knockout DF-1 cells, and interferon-beta (IFN-β) activity and expression of interferon-related genes were measured after transfection of cells with RNA ligands of hRIG-I or human MDA5 (hMDA5). We found that both mutant cMDA5 and engineered cMDA5 triggered significantly stronger interferon-mediated immune responses than wild-type cMDA5. Moreover, engineered cMDA5 reduced the IAV titer by 100-fold compared with that in control cells. Collectively, engineered cMDA5/RIG-I CTD significantly enhanced interferon-mediated immune responses, making them invaluable strategies for production of IAV-resistant chickens. KEY POINTS: • Mutant chicken MDA5 with critical residue of RIG-I (phenylalanine) enhanced immunity. • Engineered chicken MDA5 with CTD of RIG-I increased IFN-mediated immune responses. • Engineered chicken MDA5 reduced influenza A virus titers by up to 100-fold.

Phenazasiline, a sp3 hybridized silicon-bridged diphenylamine, is a promising donor moiety for deep-blue TADF emitters because of its deep highest occupied molecular orbital, high triplet level of 3.1 eV, and orthogonal connection with acceptor moieties. Herein, we report highly efficient deep-blue TADF emitters composed of phenazasiline donor and triazine acceptor combined with methyl-substituted phenylene linkers. As a series of methyl-substituted linker-based TADF emitters, we also synthesized spiroacridine donor-based sky-blue TADF emitters to study the effect of methyl-substituted linkers on the photophysical properties of TADF emitters. Deeper blue emissions of TADF emitters were obtained by substituting either one or two methyl groups at the phenylene linker of TADF molecules, and a small singlet–triplet gap below 0.2 eV was maintained for efficient reverse intersystem crossing. Organic light-emitting diodes (OLEDs) with spiroacridine-based TADF emitters, TTSA and TXSA, showed external quantum effi...

We report a blue thermally activated delayed fluorescence compound based on a spiro-silicon-connected silafluorene–phenazasiline donor. By incorporating triphenyltriazine as an acceptor, the compound exhibited well-separated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels and charge transfer-type emission, confirmed by density functional theory (DFT) calculations and its solvatochromic characteristics. The compound exhibited blue emission with a photoluminescence (PL) peak at 469 nm, a high photoluminescence quantum yield (PLQY) of 68%, and a single-triplet energy gap of 0.20 eV in a DPEPO host matrix. Due to its co-planar silafluorene and triphenyltriazine moieties, the compound exhibited a high horizontal emission dipole ratio of 79%. Various electroluminescent devices of single host, mixed host, and interface exciplex structures were fabricated using the compound as the emitter. The external quantum efficiency (EQE), maximum brightness, and Commission Internationale de L’Eclairage (CIE) y-values ranged from 0.63 to 20.6%, 210 to 6930 nit, and 0.120 to 0.206, respectively, depending on the host materials and device structures. Among the fabricated EL devices, the DPEPO host device exhibited the highest EQE of 20.6% with CIE color coordinates of (0.150, 0.184).

The acidic nuclear phosphoprotein 32 family member A (ANP32A) is a cellular host factor that determines the host tropism of the viral polymerase (vPol) of avian influenza viruses (AIVs). Compared with human ANP32A (hANP32A), chicken ANP32A contains an additional 33 amino acid residues (176-208) duplicated from amino acid residues 149-175 (27 residues), suggesting that these residues could be involved in increasing vPol activity by strengthening interactions between ANP32A and vPol. However, the molecular interactions and functional roles of the 27 residues within hANP32A during AIV vPol activity remain unclear. Here, we examined the functional role of 27 residues of hANP32A based on comparisons with other human (h) ANP32 family members. It was notable that unlike hANP32A and hANP32B, hANP32C could not support vPol activity or replication of AIVs, despite the fact that hANP32C shares a higher sequence identity with hANP32A than hANP32B. Pairwise comparison between hANP32A and hANP32C revealed that Asp149 (D149) and Asp152 (D152) are involved in hydrogen bonding and electrostatic interactions, respectively, which support vPol activity. Mutation of these residues reduced the interaction between hANP32A and vPol. Finally, we demonstrated that precise substitution of the identified residues within chicken ANP32A via homology-directed repair using the CRISPR/Cas9 system resulted in a marked reduction of viral replication in chicken cells. These results increase our understanding of ANP32A function and may facilitate the development of AIV-resistant chickens via precise modification of residues within ANP32A.

Reverse intersystem crossing (RISC) rate of a thermally activated delayed fluorescence (TADF) molecule is sensitive to the energy alignment of singlet charge-transfer state (1CT), triplet charge-transfer state (3CT), and locally excited triplet state (3LE). However, the energy distribution of the charge-transfer states originating from the conformational distribution of TADF molecules in a solid matrix inevitably generated during the preparation of a solid sample due to the rotatable donor-acceptor linkage is rarely considered. Moreover, the investigation of the energy distribution of the 3CT state is both theoretically and experimentally difficult due to the triplet instabilities of time-dependent density functional (TD-DFT) calculations and difficulties in phosphorescence measurements, respectively. As a result, the relation between conformational distribution, configurations of excited state transition orbitals, and excited state energies/dynamics have not been clearly explained. In this work, we determined the energy distribution of CT states of the TADF emitter TPSA in frozen toluene at 77 K by the measurement of time-resolved spectra in the full time range (1 ns ~ 30 s) of emission including prompt fluorescence, TADF, 3CT phosphorescence, and 3LE phosphorescence. We obtained the energy band of CT states where 1CT and 3CT states are distributed in the range of 2.85-3.00 eV and 2.64-2.96 eV, respectively. We tested various global hybrid and long-range corrected functionals for the TD-DFT calculation of 3CT energy of TPSA and found that only the M11 functional shows consistent results without triplet instability. We performed TD-DFT with the M11* functional optimized for robust dihedral angle scan of 3CT states without triplet instability and reproduced the energy band structure obtained from the experiment. Through TD-DFT and experimental investigations, it is estimated that the dihedral angle of donor-acceptor (θD-A) and acceptor-linker (θA) of TPSA in frozen toluene lie within the range of 70°≤θD-A≤90° and 0°≤θA≤30° respectively. Our results show that the dihedral angle distribution must be considered for further investigation of the photophysics of TADF molecules and the development of stable and efficient TADF emitters.

The innate immune system, which senses invading pathogens, plays a critical role as the first line of host defense. After recognition of foreign RNA ligands (e.g., RNA viruses), host cells generate an innate immune or antiviral response via the interferon-mediated signaling pathway. Retinoic acid-inducible gene I (RIG-1) acts as a major sensor that recognizes a broad range of RNA ligands in mammals; however, chickens lack a RIG-1 homolog, meaning that RNA ligands should be recognized by other cellular sensors such as melanoma differentiation-associated protein 5 (MDA5) and toll-like receptors (TLRs). However, it is unclear which of these cellular sensors compensates for the loss of RIG-1 to act as the major sensor for RNA ligands. Here, we show that chicken MDA5 (cMDA5), rather than chicken TLRs (cTLRs), plays a pivotal role in the recognition of RNA ligands, including poly I:C and influenza virus. First, we used a knockdown approach to show that both cMDA5 and cTLR3 play roles in inducing interferon-mediated innate immune responses against RNA ligands in chicken DF-1 cells. Furthermore, targeted knockout of cMDA5 or cTLR3 in chicken DF-1 cells revealed that loss of cMDA5 impaired the innate immune responses against RNA ligands; however, the responses against RNA ligands were retained after loss of cTLR3. In addition, double knockout of cMDA5 and cTLR3 in chicken DF-1 cells abolished the innate immune responses against RNA ligands, suggesting that cMDA5 is the major sensor whereas cTLR3 is a secondary sensor. Taken together, these findings provide an understanding of the functional role of cMDA5 in the recognition of RNA ligands in chicken DF-1 cells and may facilitate the development of an innate immune-deficient cell line or chicken model.

Background The initial step of influenza infection is binding of the virus to specific sialic acid receptors expressed by host cells. This is followed by cell entry via endocytosis. Cleavage of the influenza virus hemagglutinin (HA) protein is critical for infection; this is performed by host cell proteases during viral replication. In cell culture systems, HA is cleaved by trypsin added to the culture medium. The vast majority of established cell lines are mammalian. Results In the present study, we generated genetically engineered chicken DF-1 cell lines overexpressing transmembrane protease, serine 2 (TMPRSS2, which cleaves HA), ST3 beta-galactoside alpha-2,3-sialyltransferase 1 (ST3GAL1, which plays a role in synthesis of α-2,3 linked sialic acids to which avian-adapted viruses bind preferentially), or both. We found that overexpression of TMPRSS2 supports the virus life cycle by cleaving HA. Furthermore, we found that overexpression of ST3GAL1 increased the viral titer. Finally, we showed that overexpression of both TMPRSS2 and ST3GAL1 increased the final viral titer due to enhanced support of viral replication and prolonged viability of the cells. In addition, overexpression of these genes of interest had no effect on cell proliferation and viability. Conclusions Taken together, the results indicate that these engineered cells could be used as a cell-based system to propagate influenza virus efficiently in the absence of trypsin. Further studies on influenza virus interactions with chicken cell host factors could be studied without the effect of trypsin on cells.

We describe several strategies for the molecular design of high-efficiency blue fluorescent emitters. Asymmetric donor–acceptor (D–A) and symmetric A–D–A-type fluorescent emitters were designed with spiroacridine donors and diphenyltriazine acceptors. Substituting a toluene or xylene moiety for the phenyl group connecting the donor and acceptor and replacing the diphenylsilane group with a fluorene moiety resulted in a deeper blue emission without any losses in luminescence efficiency. On the basis of these substitutions, deep blue organic light emitting diodes (OLEDs) with Commission Internationale de L’Eclairage (CIE) coordinates of (0.149, 0.082) and an external quantum efficiency (EQE) of 7.7% were fabricated using a D–A-type emitter. Symmetrizing the D–A structure to an A–D–A structure increased the proportion of horizontally oriented emission dipoles in the organic film from 70 to 90%. OLEDs incorporating symmetric A–D–A-type emitters had EQEs as high as 8.5% due to increased outcoupling efficiencie...

Our aim was to verify the potential ability of succinylacetone (SA) to inhibit mitochondrial function, thereby suppressing cancer cell proliferation. SA treatment caused apoptosis in HCT116 and HT29 cells, but not in SW480 cells, with mitochondria playing a key role. We checked for dysfunctional mitochondria after SA treatment. Mitochondria of HT29 cells were swollen, indicating damage, whereas in HCT116 cells, several mitochondria had a diminished size. Damaged mitochondria decreased ATP production and induced reactive oxygen species (ROS) in the cells. To understand SA-induced reduction in ATP production, we investigated the electron transfer chains (ETC) and pyruvate dehydrogenase kinase (PDK) activity, which prevents the transfer of acetyl-CoA to the TCA (tricarboxylic acid) cycle by inhibiting PDH (pyruvate dehydrogenase) activity. In each cell line, the inhibitory mechanism of ATP by SA was different. The activity of complex III consisting of the mitochondrial ETCs in HT29 cells was decreased. In contrast, PDH activity in HCT116 cells was reduced. Nicotinamide nucleotide transhydrogenase (NNT)-removing reactive oxygen species (ROS) was upregulated in HT29 cells, but not in HCT116 cells, indicating that in HT29 cells, a defense mechanism was activated against ROS. Collectively, our study showed a differential mechanism occurs in response to SA in colon cancer cells.

BackgroundInfluenza viruses must utilize host factors to complete their lifecycle. Species-specific differences in host factors between birds and mammals mean that avian influenza viruses (AIVs) replicate well in avian hosts but not in human hosts. Acidic nuclear phosphoprotein 32 family member A (ANP32A) has been identified as the host restriction factor for the viral polymerase (vPol) activity of AIVs. The ANP32A belongs to the conserved ANP32 family, the functional roles of which during viral replication remain unclear.MethodsIn this study, we targeted chicken ANP32A using clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9-mediated genome editing to examine the functional roles of ANP32A and other members of the ANP32 family.ResultsWe showed that chicken ANP32A only, not ANP32B and ANP32E, plays a pivotal role in supporting vPol activity of AIVs. Furthermore, we found that the human ANP32C, ANP32D, and ANP32E have suppressive effects on vPol activity in contrast to human ANP32A and ANP32B.ConclusionsChicken and human ANP32 family members had different effects on vPol activity, suggesting that species-specific vPol activity of AIVs could be caused by the differential functions and overall competency of ANP32 family members.

New blue (DBA-SAB) and deep-blue (TDBA-SAF) thermally activated delayed fluorescence (TADF) emitters are synthesized for blue-emitting organic-light emitting diodes (OLEDs) by incorporating spiro-biacridine and spiro-acridine fluorene donor units with an oxygen-bridged boron acceptor unit, respectively. The molecules show blue and deep-blue emission because of the deep highest occupied molecular energy levels of the donor units. Besides, both emitters exhibit narrow emission spectra with the full-width at half maximum (FWHM) of less than 65 nm due to the rigid donor and acceptor units. In addition, the long molecular structure along the transition dipole moment direction results in a high horizontal emitting dipole ratio over 80%. By combining the effects, the OLED utilizing DBA-SAB as the emitter exhibits a maximum external quantum efficiency (EQE) of 25.7% and 1931 Commission Internationale de l'éclairage (CIE) coordinates of (0.144, 0.212). Even a higher efficiency deep blue TADF OLED with a maximum EQE of 28.2% and CIE coordinates of (0.142, 0.090) is realized using TDBA-SAF as the emitter.

In this study, we developed a donor-π-acceptor-π-donor-type thermally activated delayed fluorescence (TADF) emitter (NyDPAc) using 1,5-naphthyridine as a core moiety. NyDPAc exhibited a high horizontal dipole ratio and thus a high external quantum efficiency (20.9%) even at a relatively low photoluminescence quantum yield (57%). 1,5-naphthyridine as an electron acceptor represents a promising core moiety for efficient TADF materials.

Three types of thermally activated delayed fluorescence (TADF) emitters, namely, moTrSAc, tmTrSAc, and motmTrSAc, are reported that emit blue-shifted emission with high external quantum efficiencies (EQEs). These emitters have electron donating spiroacridine in common and various triazine-based electron acceptors. The electronic and structural control on the electron acceptor moiety adjusts the reduction potential of the molecule to shift the emission wavelength to the blue region. Moreover, as these emitters become widened, the light out-coupling efficiency increases due to the enhanced horizontal emitting dipole orientation. As a result, OLEDs based on moTrSAc, tmTrSAc, and motmTrSAc emit blue light in a range of 460–480 nm with high EQEs of 21.3, 15.5, and 19.5%, respectively.

Activated Hepatic Stellate Cells (HSCs), major fibrogenic cells in the liver, undergo apoptosis when liver injuries cease, which may contribute to the resolution of fibrosis. Bisdemethoxycurcumin (BDMC) is a natural derivative of curcumin with anti-inflammatory and anti-cancer activities. The therapeutic potential of BDMC in hepatic fibrosis has not been studied thus far in the context of the apoptosis in activated HSCs. In the current study, we compared the activities of BDMC and curcumin in the HSC-T6 cell line and demonstrated that BDMC relatively induced a potent apoptosis. BDMC-induced apoptosis was mediated by a combinatory inhibition of cytoprotective proteins, such as Bcl2 and heme oxygenase-1 and increased generation of reactive oxygen species. Intriguingly, BDMC-induced apoptosis was reversed with co-treatment of sr144528, a cannabinoid receptor (CBR) 2 antagonist, which was confirmed with genetic downregulation of the receptor using siCBR2. Additionally, incubation with BDMC increased the formation of death-induced signaling complex in HSC-T6 cells. Treatment with BDMC significantly diminished total intracellular ATP levels and upregulated ATP inhibitory factor-1. Collectively, the results demonstrate that BDMC induces apoptosis in activated HSCs, but not in hepatocytes, by impairing cellular energetics and causing a downregulation of cytoprotective proteins, likely through a mechanism that involves CBR2.

The development of humanoid robots is becoming more popular these days. While the robotics community is outputting varieties of enabling results for humanoid robots to perform many different types of tasks such as locomotion or entertainment, there has not been much research on securing their safety in motion. A way to make humanoid robots prepare for some dangerous incidents should be given attention in order to prevent the developers from losing expensive and important parts of robots and efforts they put in. The main focus of this research is to make a robot break-fall when external forces are exerted behind the robot. This paper is organized as follows: the research robotic platform that is used in order to test break-fall motions of robots is introduced. Various methods of break-fall motions are designed and evaluated. We conclude that among the types of motion we investigated, motion type C-2, stretching arms forward & bending knees & damping motion, is the best break-fall motion. Application of our algorithm to other practical robots may result in reduction of precarious incidents of robots falling down, which will be beneficial for robot developers.