Tag: M.W=300-400

Authors Lyu, H; Diercks, CS; Zhu, CH; Yaghi, OM in AMER CHEMICAL SOC published article about in [Lyu, Hao; Diercks, Christian S.; Yaghi, Omar M.] Univ Calif Berkeley, Dept Chem, Berkeley, CA 94720 USA; [Lyu, Hao; Diercks, Christian S.; Yaghi, Omar M.] Lawrence Berkeley Natl Lab, Div Mat Sci, Berkeley, CA 94720 USA; [Lyu, Hao; Diercks, Christian S.; Yaghi, Omar M.] Kavli Energy NanoSci Inst, Div Mat Sci, Berkeley, CA 94720 USA; [Yaghi, Omar M.] King Abdulaziz City Sci & Technol, UC Berkeley KACST Joint Ctr Excellence Nanomat Cl, Riyadh 11442, Saudi Arabia; [Zhu, Chenhui] Lawrence Berkeley Natl Lab, Adv Light Source, Berkeley, CA 94720 USA in 2019, Cited 24. Recommanded Product: 92-86-4. The Name is 4,4′-Dibromobiphenyl. Through research, I have a further understanding and discovery of 92-86-4

The first unsubstituted olefin-linked covalent organic framework, termed COF-701, was made by linking 2,4,6-trimethyl-1,3,5-triazine (TMT) and 4,4′-biphenyldicarbaldehyde (BPDA) through Aldol condensation. Formation of the unsubstituted olefin (-CH=CH) linkage upon reticulation is confirmed by Fourier transform infrared (FT-IR) spectroscopy and solid-state C-13 cross-polarization magic angle spinning (CP-MAS) NMR spectroscopy of the framework and of its C-13-isotope-labeled analogue. COF-701 is found to be porous (1715 m(2) g(-1)) and to retain its composition and crystallinity under both strongly acidic and basic conditions. The high chemical robustness is attributed to the unsubstituted olefin linkages. Immobilization of the strong Lewis acid BF3 center dot OEt2 in the pores of the structure yields BF3 subset of COF-701. In the material, the catalytic activity of the guest is retained, as evidenced in a benchmark Diels-Alder reaction.

Welcome to talk about 92-86-4, If you have any questions, you can contact Lyu, H; Diercks, CS; Zhu, CH; Yaghi, OM or send Email.. Recommanded Product: 92-86-4

Reference:
Benzoxazole – Wikipedia,
,Benzoxazole | C7H5NO – PubChem

An article Scalable synthesis of multi-substituted aryl-phosphonates: Exploring the limits of isoretical expansion and the synthesis of new triazene-based phosphonates WOS:000489474300001 published article about BOND FORMATION; MAGNETIC-RESONANCE; PHOSPHITES; ACID; ARYLPHOSPHONATES; CHEMISTRY; NETWORKS; GALLIUM; SPECTRA; LIGAND in [Barbee, Derek; Barron, Andrew R.] Rice Univ, Dept Mat Sci & Nanoengn, Houston, TX 77005 USA; [Barron, Andrew R.] Rice Univ, Dept Chem, Houston, TX 77005 USA; [Barron, Andrew R.] Swansea Univ, Energy Safety Res Inst, Swansea, W Glam, Wales in 2020, Cited 59. Name: 4,4′-Dibromobiphenyl. The Name is 4,4′-Dibromobiphenyl. Through research, I have a further understanding and discovery of 92-86-4

The development of novel multi-substituted aryl-phosphonate compounds offers promise as new building blocks for metal-organic frameworks (MOFs) materials with excellent properties in regards to porosity and gas sorption. We demonstrate the efficiency of the palladium-catalyzed Hirao cross-coupling reaction in the synthesis of substituted phosphonates; however, attempts to prepare derivatives with isoretical expansion through the cyclization of 4-(4?-bromophenyl)acetophenone resulted in an extremely low yield, with the isolation of the dimer intermediate. Ab initio calculations showed that while the trimerization of acetophenone is exothermic, that of 4-phenyl acetophenone is endothermic. By contrast, the cyclization of 4-(4?-bromophenyl)benzonitrile is exothermic and allows for the formation of the appropriate phosphonic acid. The benzonitrile methodology also allows for the formation of ortho methyl derivatives with high steric hindrance. All the multi-substituted aryl-phosphonate compounds reported herein can be prepared on a multi gram scale enabling researchers a wider range of building blocks for phosphonate MOFs.

Name: 4,4′-Dibromobiphenyl. Bye, fridends, I hope you can learn more about C12H8Br2, If you have any questions, you can browse other blog as well. See you lster.

Reference:
Benzoxazole – Wikipedia,
,Benzoxazole | C7H5NO – PubChem

Zhou, CW; Zhu, CR; Huang, ZJ; Zhang, WJ; Tang, Q; Gong, CB in [Zhou, Chuan-wen; Zhu, Chun-rong; Huang, Zhen-jie; Zhang, Wei-jing; Tang, Qian; Gong, Cheng-bin] Southwest Univ, Coll Chem & Chem Engn, Key Lab Appl Chem Chongqing Municipal, Tiansheng St, Chongqing 400715, Peoples R China published Di(pyridin-4-yl)aniline Derivatives with a Push-Pull Electronic Structure: Synthesis and Electrochromic Properties in 2019, Cited 46. Computed Properties of C12H8Br2. The Name is 4,4′-Dibromobiphenyl. Through research, I have a further understanding and discovery of 92-86-4.

Traditionally, electrochromic materials rely on counter redox materials like ferrocene to realize redox processes. In this work, two novel, closely related series of electrochromic materials bearing push-pull electronic structure were designed and synthesised (N,N,N’,N’-tetra(pyridin-4-yl)-1,4-phenylenediamine derivatives (TPPDs) and N,N,N ‘,N ‘-tetra(pyridin-4-yl)benzidine derivatives (TPBDs)). When stimulated by an external electric field, both series of compounds exhibited intramolecular charge transfer because of their push-pull electronic structures. Therefore, the TPPDs and TPBDs could undergo redox processes without the assistance of counter electrode chemicals. Furthermore, the TPPDs and TPBDs could replace the electrolyte that is required in conventional electrochromic devices (ECDs) because of their conductivity. This allowed the fabrication of a simple, single-component ECD.

Welcome to talk about 92-86-4, If you have any questions, you can contact Zhou, CW; Zhu, CR; Huang, ZJ; Zhang, WJ; Tang, Q; Gong, CB or send Email.. Computed Properties of C12H8Br2

Reference:
Benzoxazole – Wikipedia,
,Benzoxazole | C7H5NO – PubChem

Formula: C12H8Br2. Recently I am researching about POLYBROMINATED DIPHENYL ETHERS; DIBENZO-PARA-DIOXINS; DECABROMODIPHENYL ETHER; THERMAL-DECOMPOSITION; PHOTODEGRADATION MECHANISM; PHOTOCHEMICAL DEGRADATION; QUANTUM YIELDS; PBDES; TETRABROMOBISPHENOL; PRODUCTS, Saw an article supported by the Australian Research Council (ARC)Australian Research Council; Murdoch University, Australia. Published in ACADEMIC PRESS INC ELSEVIER SCIENCE in SAN DIEGO ,Authors: Saeed, A; Altarawneh, M; Siddique, K; Conesa, JA; Ortuno, N; Dlugogorski, BZ. The CAS is 92-86-4. Through research, I have a further understanding and discovery of 4,4′-Dibromobiphenyl

This study investigates the geometric and electronic properties of selected BFRs in their ground (S-0) and first singlet excited (S-1) states deploying methods of the density functional theory (DFT) and the time-dependent density functional theory (TDDFT). We estimate the effect of the S-0 -> S-1 transition on the elongations of the C-Br bond, identify the frontier molecular orbitals involved in the excitation process and compute partial atomic charges for the most photoreactive bromine atoms. The bromine atom attached to an who position in HBB (with regard to C-C bond; 2,2′,4,4′,6,6′-hexabromobiphenyl), TBBA (with respect to the hydroxyl group; 2,2′,6,6′-tetrabromobisphenol A), HBDE and BTBPE (in reference to C-O linkage; 2,2′,4,4′,6,6′-hexabromodiphenylether and 1,2-bis(2,4,6-tribromophenoxy)ethane, respectively) bears the highest positive atomic charge. This suggests that, these positions undergo reductive debromination reactions to produce lower brominated molecules. Debromination reactions ensue primarily in the aromatic compounds substituted with the highest number of bromine atoms owing to the largest stretching of the C-Br bond in the first excited state. The analysis of the frontier molecular orbitals indicates that, excitations of BFRs proceed via pi ->pi*, or pi ->sigma* or n ->sigma* electronic transitions. The orbital analysis reveals that, the HOMO-LUMO energy gap (EH-L) for all investigated brominesubstituted aromatic molecules falls lower (1.85-4.91 eV) than for their non-brominated analogues (3.39-8.07 eV), in both aqueous and gaseous media. The excitation energies correlate with the EH-L values. The excitation energies and EH-L values display a linear negative correlation with the number of bromine atoms attached to the molecule. Spectral analysis of the gaseous-phase systems reveals that, the highly brominated aromatics endure lower excitation energies and exhibit red shifts of their absorption bands in comparison to their lower brominated congeners. We attained a satisfactory agreement between the experimentally measured absorption peak (lambda(max)) and the theoretically predicted oscillator strength (lambda(max)) for the UV-Vis spectra. This study further confirms that, halogenated aromatics only absorb light in the UV spectral region and that effective photodegradation of these pollutants requires the presence of photocatalysts.

Formula: C12H8Br2. Welcome to talk about 92-86-4, If you have any questions, you can contact Saeed, A; Altarawneh, M; Siddique, K; Conesa, JA; Ortuno, N; Dlugogorski, BZ or send Email.

Reference:
Benzoxazole – Wikipedia,
,Benzoxazole | C7H5NO – PubChem

HPLC of Formula: C12H8Br2. I found the field of Chemistry very interesting. Saw the article Reducing the Exciton Binding Energy of Donor-Acceptor-Based Conjugated Polymers to Promote Charge-Induced Reactions published in 2019, Reprint Addresses Chen, X; Wang, XC (corresponding author), Fuzhou Univ, Coll Chem, State Key Lab Photocatalysis Energy & Environm, Fuzhou 350116, Fujian, Peoples R China.. The CAS is 92-86-4. Through research, I have a further understanding and discovery of 4,4′-Dibromobiphenyl.

Exciton binding energy has been regarded as a crucial parameter for mediating charge separation in polymeric photocatalysts. Minimizing the exciton binding energy of the polymers can increase the yield of charge-carrier generation and thus improve the photocatalytic activities, but the realization of this approach remains a great challenge. Herein, a series of linear donor-acceptor conjugated polymers has been developed to minimize the exciton binding energy by modulating the charge-transfer pathway. The results reveal that the reduced energy loss of the charge-transfer state can facilitate the electron transfer from donor to acceptor, and thus, more electrons are ready for subsequent reduction reactions. The optimized polymer, FSO-FS, exhibits a remarkable photochemical performance under visible light irradiation.

Bye, fridends, I hope you can learn more about C12H8Br2, If you have any questions, you can browse other blog as well. See you lster.. HPLC of Formula: C12H8Br2

Reference:
Benzoxazole – Wikipedia,
,Benzoxazole | C7H5NO – PubChem

An article A comparative study on NHC-functionalized ternary Se/Te-Fe-Cu compounds: synthesis, catalysis, and the effect of chalcogens WOS:000477966800040 published article about TRINUCLEAR COPPER(I) ACETYLIDES; N-HETEROCYCLIC CARBENES; CARBONYL-COMPLEXES; CLUSTERS; ELECTROCHEMISTRY; NANOPARTICLES; CHEMISTRY; TE; CONSTRUCTION; SPECTROSCOPY in [Shieh, Minghuey; Liu, Yu-Hsin; Wang, Chih-Chin; Jian, Huan; Lin, Chien-Nan; Chen, Yen-Ming; Huang, Chung-Yi] Natl Taiwan Normal Univ, Dept Chem, Taipei 11677, Taiwan in 2019, Cited 80. The Name is 4,4′-Dibromobiphenyl. Through research, I have a further understanding and discovery of 92-86-4. SDS of cas: 92-86-4

A novel family of N-heterocyclic carbene (NHC)-incorporated Se-Fe-Cu compounds, bis-1,3-dimethylimidazol-2-ylidene (bis-Me-2-imy)-containing compound [(mu(4)-Se)Fe-3(CO)(9){Cu(Me-2-imy)}(2)] (2), bis-N-methyl- or bis-N-isopropyl-substituted benzimidazol-2-ylidene (bis-Me-2-bimy or bis-Pr-i(2)-bimy)-incorporated compounds [(mu(4)-Se)Fe-3(CO)(9){Cu(Me-2-bimy)}(2)] (3) or [(mu(4)-Se)Fe-3(CO)(9){Cu(Pr-i(2)-bimy)}(2)] (4), and a bis-1,3-dimethyl-4,5-dichloroimidazol-2-ylidene (bis-Me-2-Cl-2-imy)-containing compound [(mu(3)-Se)Fe-3(CO)(9){Cu(Me-2-Cl-2-imy)}(2)] (5), were synthesized in moderate yields in facile one-pot reactions of the ternary pre-designed compound [(mu(3)-Se)Fe-3(CO)(9){Cu(MeCN)}(2)] (1) with the corresponding imidazolium salts and (KOBu)-Bu-t in THF in an ice-water bath. Single-crystal X-ray analyses revealed that the Me-2-imy compound 2 or the Me-2-bimy compound 3 each exhibited a trigonal bipyramidal SeFe3(CO)(9)Cu geometry with an Fe2Cu plane further capped by a Cu(Me-2-imy) or Cu(Me-2-bimy) fragment, respectively, with one long Cu-Cu covalent bond. In addition, compound 4 also comprised a trigonal bipyramidal SeFe3(CO)(9)Cu core structure, but the second Cu(Pr-i(2)-bimy) group bridged the equatorial Fe-Fe edge with two unbonded Cu atoms, due to the presence of a sterically bulky Pr-i(2)-bimy fragment. On the other hand, the strong electron-withdrawing chloro-containing NHC compound 5 showed a comparatively open tetrahedral SeFe3(CO)(9) metal core, where two Fe-Fe edges each were further bridged by a Cu(Me-2-Cl-2-imy) fragment. Due to the nonclassical C-H center dot center dot center dot O(carbonyl) hydrogen bonds between the CO groups of the SeFe3(CO)(9)Cu-2 core and CH moieties of the neighboring NHC ligands, both compounds 2 and 3 comprised a one-dimensional network, while compounds 4 and 5 each were made up of a two-dimensional framework in the solid state, which efficiently enhanced the stability of these Se-Fe-Cu NHC compounds. Importantly, all of these synthesized Se-Fe-Cu NHC compounds 2-5 had pronounced catalytic activities for the homocoupling of arylboronic acids with high catalytic yields. Finally, these Se-containing Fe-Cu NHC compounds further represented excellent models for studying chalcogen effects in comparison to their Te analogs, as demonstrated by their catalytic performances and electrochemical behaviors, and by DFT calculations.

SDS of cas: 92-86-4. Bye, fridends, I hope you can learn more about C12H8Br2, If you have any questions, you can browse other blog as well. See you lster.

Reference:
Benzoxazole – Wikipedia,
,Benzoxazole | C7H5NO – PubChem

Kolivoska, V; Sebera, J; Sebechlebska, T; Lindner, M; Gasior, J; Meszaros, G; Mayor, M; Valasek, M; Hromadova, M in [Kolivoska, Viliam; Sebera, Jakub; Sebechlebska, Tana; Gasior, Jindrich; Hromadova, Magdalena] Czech Acad Sci, J Heyrovsky Inst Phys Chem, Dolejskova 3, Prague 18223, Czech Republic; [Sebechlebska, Tana] Comenius Univ, Fac Nat Sci, Dept Phys & Theoret Chem, Ilkovicova 6, Bratislava 84215 4, Slovakia; [Lindner, Marcin; Mayor, Marcel; Valasek, Michal] KIT, Inst Nanotechnol, POB 3640, D-76021 Karlsruhe, Germany; [Meszaros, Gabor] HAS, Res Ctr Nat Sci, Magyar Tudosok Krt 2, H-1117 Budapest, Hungary; [Mayor, Marcel] Univ Basel, Dept Chem, St Johanns Ring 19, CH-4056 Basel, Switzerland; [Mayor, Marcel] Sun Yat Sen Univ, Lehn Inst Funct Mat, Sch Chem, Guangzhou 510275, Guangdong, Peoples R China; [Lindner, Marcin] Polish Acad Sci, Inst Organ Chem, Kasprzaka 44-52, PL-01224 Warsaw, Poland published Probabilistic mapping of single molecule junction configurations as a tool to achieve the desired geometry of asymmetric tripodal molecules in 2019, Cited 29. Recommanded Product: 4,4′-Dibromobiphenyl. The Name is 4,4′-Dibromobiphenyl. Through research, I have a further understanding and discovery of 92-86-4.

Four molecules containing identical tripodal anchors and p-oligophenylene molecular wires of increasing length were used to demonstrate tuning of the asymmetric molecular junction to the desired geometry by probabilistic mapping of single molecule junction configurations in a scanning tunnelling microscopy break junction experiment.

Recommanded Product: 4,4′-Dibromobiphenyl. Welcome to talk about 92-86-4, If you have any questions, you can contact Kolivoska, V; Sebera, J; Sebechlebska, T; Lindner, M; Gasior, J; Meszaros, G; Mayor, M; Valasek, M; Hromadova, M or send Email.

Reference:
Benzoxazole – Wikipedia,
,Benzoxazole | C7H5NO – PubChem

Computed Properties of C12H8Br2. Authors Lyu, H; Diercks, CS; Zhu, CH; Yaghi, OM in AMER CHEMICAL SOC published article about in [Lyu, Hao; Diercks, Christian S.; Yaghi, Omar M.] Univ Calif Berkeley, Dept Chem, Berkeley, CA 94720 USA; [Lyu, Hao; Diercks, Christian S.; Yaghi, Omar M.] Lawrence Berkeley Natl Lab, Div Mat Sci, Berkeley, CA 94720 USA; [Lyu, Hao; Diercks, Christian S.; Yaghi, Omar M.] Kavli Energy NanoSci Inst, Div Mat Sci, Berkeley, CA 94720 USA; [Yaghi, Omar M.] King Abdulaziz City Sci & Technol, UC Berkeley KACST Joint Ctr Excellence Nanomat Cl, Riyadh 11442, Saudi Arabia; [Zhu, Chenhui] Lawrence Berkeley Natl Lab, Adv Light Source, Berkeley, CA 94720 USA in 2019, Cited 24. The Name is 4,4′-Dibromobiphenyl. Through research, I have a further understanding and discovery of 92-86-4

The first unsubstituted olefin-linked covalent organic framework, termed COF-701, was made by linking 2,4,6-trimethyl-1,3,5-triazine (TMT) and 4,4′-biphenyldicarbaldehyde (BPDA) through Aldol condensation. Formation of the unsubstituted olefin (-CH=CH) linkage upon reticulation is confirmed by Fourier transform infrared (FT-IR) spectroscopy and solid-state C-13 cross-polarization magic angle spinning (CP-MAS) NMR spectroscopy of the framework and of its C-13-isotope-labeled analogue. COF-701 is found to be porous (1715 m(2) g(-1)) and to retain its composition and crystallinity under both strongly acidic and basic conditions. The high chemical robustness is attributed to the unsubstituted olefin linkages. Immobilization of the strong Lewis acid BF3 center dot OEt2 in the pores of the structure yields BF3 subset of COF-701. In the material, the catalytic activity of the guest is retained, as evidenced in a benchmark Diels-Alder reaction.

Welcome to talk about 92-86-4, If you have any questions, you can contact Lyu, H; Diercks, CS; Zhu, CH; Yaghi, OM or send Email.. Computed Properties of C12H8Br2

Reference:
Benzoxazole – Wikipedia,
,Benzoxazole | C7H5NO – PubChem

An article A linear D-pi-A based hole transport material for high performance rigid and flexible planar organic-inorganic hybrid perovskite solar cells WOS:000498852400008 published article about LIGHT-EMITTING-DIODES; EFFICIENT; DEVICE; ADDUCT; LAYERS in [Kwon, Haeun; Reddy, Saripally Sudhaker; Arivunithi, Veera Murugan; Jin, Hyunjung; Park, Ho-Yeol; Cho, Woosum; Jin, Sung-Ho] Pusan Natl Univ, Inst Plast Informat & Energy Mat, Dept Chem Educ, Grad Dept Chem Mat, Busandaehakro 63-2, Busan 46241, South Korea; [Song, Myungkwan] Korea Inst Mat Sci, Mat Ctr Energy Convergence, Surface Technol Div, 97 Changwondaero, Chang Won 642831, Gyeongnam, South Korea in 2019, Cited 51. Quality Control of 4,4′-Dibromobiphenyl. The Name is 4,4′-Dibromobiphenyl. Through research, I have a further understanding and discovery of 92-86-4

A facile and less expensive hole transport material is essential to enhance the power conversion efficiency (PCE) of perovskite solar cells (PSC) without compromising the ambient stability. Here, we designed and synthesized a new class of HTM by introducing donor-pi-acceptor (D-pi-A). The HTM was synthesized by combining the moieties of triphenylamine, biphenyl and oxadiazole derivatives as electron donating, pi-spacer and electron withdrawing moieties, respectively, named 4 ”’-(5-(4-(hexyloxy)phenyl)-1,3,4-oxadiazol-2-yl)-N,N-bis(4-methoxyphenyl)-[1,1′:4′,1 ”:4 ”,1 ”’-quaterphenyl]-4-amine (TPA-BP-OXD). The pi-pi conjugation is increased by introducing the biphenyl pi-spacer. The HTM was terminated with an OXD-based moiety and framed as a D-pi-A-based HTM that trigged improvement in the charge transportation properties due to its pi-pi interactions. We rationally investigated the HTM by characterizing its photophysical, thermal, electrochemical, and charge transport properties. The great features of the HTM stimulated us to explore it on rigid and flexible substrates as a dopant-free HTM in planar inverted-perovskite solar cells (i-PSCs). The device performance in solution processed dopant-free HTM based i-PSC devices on both rigid and flexible substrates showed PCEs of 15.46% and 12.90%, respectively. The hysteresis is negligible, which is one of the most effective results based on a TPA-BP-OXD HTM in planar i-PSCs. The device performance and stability based on the TPA-BP-OXD HTM are better due to higher extraction and transportation of holes from the perovskite material, reduced charge recombination at the interface, and enhanced hydrophobicity of the HTM to compete for a role in enhancing the stability. Overall, our findings demonstrate the potentiality of the TPA-BP-OXD based HTM in planar i-PSCs.

Welcome to talk about 92-86-4, If you have any questions, you can contact Kwon, H; Reddy, SS; Arivunithi, VM; Jin, H; Park, HY; Cho, W; Song, M; Jin, SH or send Email.. Quality Control of 4,4′-Dibromobiphenyl

Reference:
Benzoxazole – Wikipedia,
,Benzoxazole | C7H5NO – PubChem

Zhao, YH; Feng, XJ; Zhang, S; Yamamoto, Y; Bao, M in [Zhao, Yuhui; Feng, Xiujuan; Zhang, Sheng; Yamamoto, Yoshinori; Bao, Ming] Dalian Univ Technol, State Key Lab Fine Chem, Dalian 116023, Peoples R China; [Yamamoto, Yoshinori] Ritsumeikan Univ, Res Org Sci & Technol, Kusatsu 5258577, Japan; [Bao, Ming] Dalian Univ Technol, Sch Chem Engn, Panjin 124221, Peoples R China published Hydrodebromination of Aromatic Bromides Catalyzed by Unsupported Nanoporous Gold: Heterolytic Cleavage of Hydrogen Molecule in 2020, Cited 46. Quality Control of 4,4′-Dibromobiphenyl. The Name is 4,4′-Dibromobiphenyl. Through research, I have a further understanding and discovery of 92-86-4.

Unsupported nanoporous gold (AuNPore) is a highly efficient, practically applicable, and recyclable catalyst for hydrodebromination of aromatic bromides. The AuNPore-catalyzed hydrodebromination of aromatic bromides proceeded smoothly at relatively low hydrogen pressure and temperature to achieve good to excellent yields of the corresponding non-bromine variants. The selective hydrodebromination reaction occurred exclusively in the coexistence of chlorine atom. For the first time, a mechanistic study revealed that the H-H bond splits in a heterolysis manner on the surface of AuNPore to generate Au-H hydride species.

Welcome to talk about 92-86-4, If you have any questions, you can contact Zhao, YH; Feng, XJ; Zhang, S; Yamamoto, Y; Bao, M or send Email.. Quality Control of 4,4′-Dibromobiphenyl

Reference:
Benzoxazole – Wikipedia,
,Benzoxazole | C7H5NO – PubChem