Isomerization of N₂O₄ in Solid N₂H₄ and Its Implication for the Explosion of N₂O₄–N₂H₄ Solid Mixtures

The mechanism responsible for the explosion of solid mixtures of nitrogen tetroxide (N2O4) and hydrazine (HZ) or methyl-substituted HZs, detected experimentally by slow warming from 77 to 203–223 K, has been elucidated by quantum chemical calculations using the Vienna ab initio simulation package code. The result of the calculation for the reaction of a N2O4 molecule embedded in the middle of the N2H4 molecular crystal, N2O4@HZ23, indicates that a loose nonconventional transition state (TS) occurring by stretching the O2N–NO2 bond up to 2.18 Å with the concerted rotation of one of the NO2 groups producing the reactive ONONO2 isomer (ONONO2@HZ23) has a low 13.1 kcal/mol barrier at TS1; the process is exothermic by 45 kcal/mol, reflecting the much stronger ONONO2 binding with N2H4. A further simultaneous reaction of ONONO2 with 2N2H4 in the same unit cell occurs with a small 1.4 kcal/mol barrier producing NO3– + NH2N­(H)­NO + N2H5+ with an overall exothermicity of 70.2 kcal/mol. The mechanism for this last-step reaction is distinctly different from that in the gas phase taking place via a five-centered concert mechanism giving N2H3NO and HNO3, which can further produce N2H5+NO3– by the rapid acid–base neutralization process. On the basis of the predicted structure, energy, and vibrational frequencies at TS1, we estimated the rate constant at 218 K for the N2O4–ONONO2 isomerization reaction in solid N2H4 to be 1.35 s–1, giving the half-life of N2O4@HZ23 to be as short as 0.5 s. This result can explain why the slow warming of the solid mixtures of N2O4 and N2H4 from 77 K exploded reproducibly at 218 K

Development of Novel Mixed Halide/Superhalide Tin-Based Perovskites for Mesoscopic Carbon-Based Solar Cells

Tin perovskites suffer from poor stability and a self-doping effect. To solve this problem, we synthesized novel tin perovskites based on superhalide with varied ratios of tetrafluoroborate to iodide and implemented them into solar cells based on a mesoscopic carbon-electrode architecture because film formation was an issue in applying this material for a planar heterojunction device structure. We undertook quantum-chemical calculations based on plane-wave density functional theory (DFT) methods and explored the structural and electronic properties of tin perovskites FASnI3–x(BF4)x in the series x = 0, 1, 2, and 3. We found that only the x = 2 case, FASnI­(BF4)2, was successfully produced, beyond the standard FASnI3. The electrochemical impedance and X-ray photoelectron spectra indicate that the addition of tin tetrafluoroborate instead of SnI2 suppressed trap-assisted recombination by decreasing the Sn4+ content. The power conversion efficiency of the FASnI­(BF4)2 device with FAI and Sn­(BF4)2 in an equimolar ratio improved 72% relative to that of a standard FASnI3 solar cell, with satisfactory photostability under ambient air conditions

Location and Number of Selenium Atoms in Two-Dimensional Conjugated Polymers Affect Their Band-Gap Energies and Photovoltaic Performance

We synthesized and characterized a series of novel two-dimensional Se-atom-substituted donor (D)−π-acceptor (A) conjugated polymersPBDTTTBO, PBDTTTBS, PBDTTSBO, PBDTSTBO, PBDTTSBS, PBDTSTBS, PBDTSSBO, and PBDTSSBSfeaturing benzodithiophene (BDT) as the donor, thiophene (T) as the π-bridge, and 2,1,3-benzooxadiazole (BO) as the acceptor with different number of Se atoms at different π-conjugated locations, including the π-bridge, side chain, and electron-withdrawing units. We then systematically investigated the effect of different locations and the number of Se atoms in these two-dimensional conjugated polymers on the structural, optical, and electronics such as band-gap energies of the resulting polymers, as determined through quantum-chemical calculations, UV–vis absorption spectra, and grazing-incidence X-ray diffraction. We found that through the rational structural modification of the 2-D conjugated Se-substituted polymers the resulting PCEs could vary over 3-fold (from 2.4 to 7.6%), highlighting the importance of careful selection of appropriate chemical structures such as the location of Se atoms when designing efficient D−π-A polymers for use in solar cells. Among these tested BO-containing polymers, PBDTSTBO that has moderate band gaps and good open-circuit voltages (up to 0.86 V) when mixed with PC₇₁BM (1:2, w/w) provided the highest power conversion efficiency (7.6%) in a single-junction polymer solar cell, suggesting that these polymers have potential applicability as donor materials in the bulk heterojunction polymer solar cells

Synthesis of Upper-Rim Sulfanylpropyl- and <i>p-</i>Methoxyphenylazo-Substituted Calix[4]arenes as Chromogenic Sensors for Hg²⁺ and Ag⁺ Ions

A series of calix[4]­arenes with upper-rim sulfanylpropyl and p-methoxyphenylazo groups (compounds 8–10) were synthesized and found to be effective chromogenic sensors for selectively detecting Hg2+, Hg+, and Ag+ ions among 18 screened metal perchlorates. In comparison to previously reported diallyl- and dithioacetoxypropyl-substituted calix[4]­arenes (5, 6, 14, 15, and 16) and the newly synthesized compound 7, the distal (5,17)-disulfanylpropyl-substituted di-p-methoxyphenylazocalix­[4]­arene 9 demonstrated superior performance with a limit of detection of 0.028 μM for Hg2+ ions in a chloroform/methanol (v/v = 399/1) cosolvent. Job’s plot revealed 1:1 binding stoichiometry for all these upper-rim sulfanylpropyl- and p-methoxyphenylazo-substituted calix­[4]­arenes 8–10 with Hg2+ ions, and Benesi–Hildebrand plots from ultraviolet/visible (UV–vis) titration spectra were used for the determination of their association constants. Our findings indicated that the distal orientation of two p-methoxyphenylazo and two sulfanylpropyl groups in calix[4]­arenes 8–10 is more favorable for binding Hg2+ ions than the proximal (5,11-) orientation; moreover, the adjacent sulfanylpropyl groups exhibited superior coordination as ligands compared to the allyl and thioacetoxypropyl groups. Notably, compounds 8–10 displayed a comparable trend in their association with Ag+ ions, albeit with 1 order of magnitude lower binding constants and a distinct binding mode compared to Hg2+ ions. UV–vis spectroscopy, Job's plots, high-resolution mass spectrometry, and 1H nuclear magnetic resonance titration studies are presented and discussed

Symmetry and Coplanarity of Organic Molecules Affect their Packing and Photovoltaic Properties in Solution-Processed Solar Cells

In this study we synthesized three acceptor–donor–acceptor (A–D–A) organic molecules, <b>TB3t-BT</b>, <b>TB3t-BTT</b>, and <b>TB3t-BDT</b>, comprising 2,2′-bithiophene (BT), benzo­[1,2-b:3,4-b′:5,6-d″]­trithiophene (BTT), and benzo­[1,2-b;4,5-b′]­dithiophene (BDT) units, respectively, as central cores (donors), terthiophene (3t) as π-conjugated spacers, and thiobarbituric acid (TB) units as acceptors. These molecules display different degrees of coplanarity as evidenced by the differences in dihedral angles calculated from density functional theory. By using differential scanning calorimetry and X-ray diffractions for probing their crystallization characteristics and molecular packing in active layers, we found that the symmetry and coplanarity of molecules would significantly affect the melting/crystallization behavior and the formation of crystalline domains in the blend film with fullerene, PC₆₁BM. <b>TB3t-BT</b> and <b>TB3t-BDT</b>, which each possess an inversion center and display high crystallinity in their pristine state, but they have different driving forces in crystallization, presumably because of different degrees of coplanarity. On the other hand, the asymmetrical <b>TB3t-BTT</b> behaved as an amorphous material even though it possesses a coplanar structure. Among our tested systems, the device comprising as-spun <b>TB3t-BDT</b>/PC₆₁BM (6:4, w/w) active layer featured crystalline domains and displayed the highest power conversion efficiency (PCE) of 4.1%. In contrast, the as-spun <b>TB3t-BT</b>/PC₆₁BM (6:4, w/w) active layer showed well-mixed morphology and with a device PCE of 0.2%; it increased to 3.9% after annealing the active layer at 150 °C for 15 min. As for <b>TB3t-BTT</b>, it required a higher content of fullerene in the <b>TB3t-BTT</b>/PC₆₁BM (4:6, w/w) active layer to optimize its device PCE to 1.6%

KSCN-induced Interfacial Dipole in Black TiO₂ for Enhanced Photocatalytic CO₂ Reduction

Tuning the electronic band structure of black titania to improve photocatalytic performance through conventional band engineering methods has been challenging because of the defect-induced charge carrier and trapping sites. In this study, KSCN-modified hydrogenated nickel nanocluster-modified black TiO2 (SCN–H–Ni–TiO2) exhibits enhanced photocatalytic CO2 reduction due to the interfacial dipole effect. Upon combining the experimental and theoretical simulation approach, the presence of an electrostatic interfacial dipole associated with chemisorption of SCN has dramatic effects on the photocatalyst band structure in SCN–H–Ni–TiO2. An interfacial dipole possesses a more negative zeta potential shift of the isoelectric point from 5.20 to 3.20, which will accelerate the charge carrier separation and electron transfer process. Thiocyanate ion passivation on black TiO2 demonstrated an increased work function around 0.60 eV, which was induced by the interracial dipole effect. Overall, the SCN–H–Ni–TiO2 photocatalyst showed an enhanced CO2 reduction to solar fuel yield by 2.80 times higher than H–Ni–TiO2 and retained around 88% product formation yield after 40 h