Xuehai Ju

Synthesis and characterization of 1,1′-azobis(5-methyltetrazole)

A high-nitrogen compound (N10 structure), 1,1′-azobis(5-methyltetrazole) which is relatively stable, was obtained by azo coupling reactions with three different oxidants such as trichloroisocyanuric acid (TCICA), sodium dichloroisocyanurate (SDIC) and tert-butyl hypochlorite (t-BuOCl). In particular, TCICA has been used for the first time to oxidize N–NH2 to the N–NN–N linkage. The structural elucidation of the title compound was made by spectral and X-ray crystallographic analyses. The new N10 linkage containing compound exhibits both relative thermal stability and physical stability.

Effects of structural optimization on the performance of dye-sensitized solar cells: spirobifluorene as a promising building block to enhance Voc

We prepared three new push–pull dyes JA3, JA4 and JA0 through structure optimization, and applied them in DSSCs. Judicious molecular structure optimization of dyes can significantly improve the performance of DSSCs. Spirobifluorene as a building block was introduced onto the phenothiazine donor groups. Compared to JA1, the steric effect of the spirobifluorene building block can effectively reduce charge recombination by preventing the I3− of the electrolyte penetrating into the TiO2 surface, thus the Voc of JA3 increased from 701 mV to 800 mV. In order to further improve the light-harvesting ability of the DSSC, we introduced a benzothiadiazole unit (BTD) and prepared D–A–π–A dye JA4. As expected, the Jsc of JA4 increased from 12.32 to 14.43 mA cm−2 compared to that of JA3, and the device reached the highest PCE of 7.0%. Moreover, since extending the π-conjugated system is known to decrease the Voc of DSSC, the porphyrin unit was removed and dye JA0 was synthesized. Notably, the DSSC based on JA0 reached a very high Voc of 840 mV and a high PCE of 6.69%: to our knowledge, this is the highest Voc for DSSCs based on phenothiazine dyes with I−/I3− electrolyte.

Enhanced performance of dye-sensitized solar cells with Y-shaped organic dyes containing di-anchoring groups

Two new Y-shaped D–π–(A)2 type phenothiazine-based dyes (ZJA2 and ZJA3) were designed and synthesized for dye-sensitized solar cells (DSSCs). Compared to the single D–π–A analogue dye ZJA1, the Y-shaped dye with two carboxylic acid anchors was in favour of enhancing the performance of DSSCs. The FTIR spectra revealed that the dye ZJA2 molecules were anchored onto the TiO2 surface by two carboxylic acid anchors, which effectively increased the electron extraction channels. The steady-state emission spectra and time-resolved fluorescence experiments all indicated that the electron injection efficiency of dye ZJA2 was improved by the increased electron extraction channels, thus the Jsc of ZJA2 was greatly improved compared to dye ZJA1. In addition, benzene was used as the π-bridge for linking di-anchoring groups, combined with CDCA, thereby reducing the charge recombination. As a result, the PCE of the DSSC based on ZJA2 (4.55%) was 67% higher than the DSSC based on ZJA1 (2.72%). The Y-shaped D–π–(A)2 type phenothiazine-based dye ZJA3 containing two pyridine anchors was also investigated, and the DSSC based on ZJA3 showed a poor PCE (0.45%) due to the low dye loading.

A novel N–N bond cleavage in 1,5-diaminotetrazole: synthesis and characterization of 5-picrylamino-1,2,3,4-tetrazole (PAT)

The reaction of 1,5-diaminotetrazole with picryl chloride (PiCl) forms 5-picrylamino-1,2,3,4-tetrazole (PAT) rather than the expected 1-picrylamino-5-amino-1,2,3,4-tetrazole or 5-picrylamino-1-amino-1,2,3,4-tetrazole. The structure of PAT was confirmed by single-crystal X-ray diffraction. Some of the energetic properties of the synthesized compound were also studied.

Initial decomposition reaction of di-tetrazine-tetroxide (DTTO) from quantum molecular dynamics: implications for a promising energetic material

Di-tetrazine-tetroxide (DTTO) was predicted in 2001 to have a density (up to 2.3 g cm^(−3)) and heat of detonation (up to 421.0 kcal mol^(−1)) better than other explosives, making it the “holy grail” of energetic materials (EMs), but all attempts at synthesis have failed. We report Density Functional Theory (DFT) molecular dynamics simulations (DFT-MD) on DTTO crystal for the two most stable monomers. We predict that the most stable isomer (c1) has a density of ρ = 1.96 g cm^(−3) with an estimated detonation velocity (D_v) of 9.70 km s^(−1) and a detonation pressure (D_p) of 43.0 GPa, making it comparable to RDX (ρ = 1.82 g cm^(−3), D_v = 8.75 km s^(−1), D_p = 35.0 GPa), HMX (ρ = 1.91 g cm^(−3), D_v = 9.10 km s^(−1), D_p = 39.3 GPa) and CL-20 (ρ = 2.04 g cm^(−3), D_v = 9.38 km s^(−1), D_p = 44.1 GPa). The DFT-MD studies find that the initial reaction at lower pressure is unimolecular decomposition to form two N_2O molecules (barrier 45.9 kcal mol^(−1)), while at higher pressure it is intermolecular oxygen-transfer with a barrier of 40.1 kcal mol^(−1). For the c2 isomer (less stable by 1.2 kcal mol^(−1)) the initial reaction involves two DTTO molecules reacting to form a dimer which then releases N_2 as a direct product (barrier 48.1 kcal mol^(−1)), a unique initial reaction among EMs. These results suggest that DTTO may have a higher thermal stability (barrier >7.0 kcal mol^(−1) higher) than RDX, HMX, and CL-20.

Improvement of photovoltaic performance of DSSCs by modifying panchromatic zinc porphyrin dyes with heterocyclic units

A series of novel panchromatic D–D–π–A porphyrin dyes have been synthesized and applied to dye-sensitized solar cells. Three porphyrin dyes named JP1, JP2 and JP3, and their photophysical and electrochemical properties and photovoltaic performance were investigated and compared with reference dye YD2-O-C8. 2-Hexylthiophene chromophores were introduced to the donor groups, which extended the π-conjugation system effectively, then broadened the range of spectral response and improved the charge separation between the donor and acceptor moieties in the excited state. Moreover, this paper used thiophene-2-carboxylic acid instead of the traditional benzoic acid as an anchor group, which can make the molecules arrange to tilted orientation when adsorbed on the TiO2 surface, and this may effectively suppress the dye aggregation and prevent charge recombination. These dyes were clearly red-shifted when compared with dye YD2-O-C8. Especially for dye JP3, its maximum absorption peak was red shifted 20 nm with respect to dye YD2-O-C8 from 645 to 665 nm, and the molar extinction coefficient (6.2 × 104 M−1 cm−1) of JP3 is double that of YD2-O-C8 (3.1 × 104 M−1 cm−1) at the Q band. Dye JP3 extended the spectral response to 750 nm. The density functional theory (DFT) calculations indicated that the electronic density of the HOMO was increased by the additional thiophene units in these dyes when compared with YD2-O-C8, and this will improve the conjugation and electron donating ability. The power conversion efficiencies of JP1, JP2 and JP3 are 5.09%, 5.62% and 6.40% respectively under AM 1.5G irradiation, which are 74.5%, 82.3% and 93.7% of the YD2-O-C8 based-device (6.83%) under the same conditions.

Improvement of dye-sensitized solar cells performance through introducing different heterocyclic groups to triarylamine dyes

This paper focuses on the structure modification of triphenylamine dyes for efficient dye-sensitized solar cells (DSSCs). Three D–D–π–A dyes (TTR1–3), with triphenylamine moiety and its derivatives as the electron donor, thiophene ring as the π-bridge, and 2-(1,1-dicyanomethylene)rhodanine (DCRD) as the electron acceptor, were synthesized and fully characterized. Nanocrystalline TiO2-based DSSCs were fabricated using these dyes to investigate the effect of different donor groups introduced into triphenylamine on their photovoltaic performances. The overall power conversion efficiency (PCE) of DSSCs based on TTR1–3 with chenodeoxycholic acid (CDCA) coadsorbant are 5.20%, 5.71% and 6.30%, respectively, compared to 6.62% achieved with N719. Introduced heterocyclic group with alkyl lain into triphenylamine decreased dye absorbed amount but significantly improved the value of the open circuit voltage (Voc) and the short-circuit photocurrent (Jsc), which result from the fact that they can effectively suppress the charge recombination and prevent aggregation between adjacent molecules on TiO2. We also researched the effect of sensitization for single dyes on their photovoltaic performances. The PCEs of DSSCs soaked for 32 h increase slightly compared to those of DSSCs soaked for 16 h, which result from the adsorption quantity on the TiO2 surface. We found that, with soaking twice in 32 h, the Jsc and Voc were both obviously improved compared with soaking once in 32 h. These results provide a new approach for enhancing the photovoltaic performances of DSSCs based on single dye.

Can elongation of the π-system in triarylamine derived sensitizers with either benzothiadiazole and/or ortho-fluorophenyl moieties enrich their light harvesting efficiency? – a theoretical study

The structural and electronic properties of five known triarylamine derived sensitizers (A1, A1-F, C218, D2 and Y123) and their associated hypothetical dyes (C218-F, D2-F, Y123-F, Y1234 and Y1234-F) have been studied using density functional theory and time-dependent density functional theory. The sensitizers primarily comprise of a triphenylamine, a 4,4′-dihexylcyclopenta[2,1-b:3,4-b]dithiophene and a cyanoacrylic acid as the electron donating, π-spacer and accepting units, respectively. The π-system is extended by incorporation of either a benzo[c][1,2,5]thiadiazol-4,7-diyl unit or an ortho-fluorophenyl unit or both. To gain insight into the effect of elongation of the π-system on the electronic properties of dye sensitized TiO2 interfaces, first-principles calculations have been carried out on sensitizer molecules co-adsorbed on the (101) surface of the anatase TiO2. The theoretical results revealed that elongating the π-system of the sensitizers with both the benzothiadiazole and ortho-fluorophenyl units increases the molecular extinction coefficient, the excited state lifetime and the light harvesting efficiency but decreases the band gap and the reorganization energy relative to the structurally comparable reference dye Y123. The calculated short circuit current density and level alignment quality showed that the π-system in the triarylamine sensitizers elongated with both benzothiadiazole and ortho-fluorophenyl units broadens their potential use in DSSCs due to the enhanced values as compared to the reference dye. The results obtained in this study will provide a valuable reference for the strategy of inserting various π-spacers in triarylamine sensitizers for dye sensitized solar cell applications.

Reaction mechanism from quantum molecular dynamics for the initial thermal decomposition of 2,4,6-triamino-1,3,5-triazine-1,3,5-trioxide (MTO) and 2,4,6-trinitro-1,3,5-triazine-1,3,5-trioxide (MTO3N), promising green energetic materials

Klapotke and co-workers recently designed two new materials, 2,4,6-triamino-1,3,5-triazine-1,3,5-trioxide (MTO) and 2,4,6-trinitro-1,3,5-triazine-1,3,5-trioxide (MTO3N), envisioned as candidates for green high-energy materials. However, all attempts at synthesis have failed. In order to validate the expected properties for these systems and to determine why these materials are too unstable to synthesize, we used the PBE flavor of Density Functional Theory (DFT) to predict the crystal structures for MTO and MTO3N and then we carried out DFT molecular dynamics simulations (DFT-MD) to determine the initial reaction mechanisms for decomposition. Klapotke estimated that MTO would have a density of ρ = 1.859 g cm−3 with an estimated detonation velocity (Dv) of 8.979 km s−1, making it comparable to RDX (ρ = 1.82 g cm−3, Dv = 8.75 km s−1) and β-HMX (ρ = 1.91 g cm−3, Dv = 9.10 km s−1). His estimated impact sensitivity >30 J, make it much better than HMX (7 J) and RDX (7.5 J). Our predicted crystal structure for MTO (P2(1) space group) leads to ρ = 1.859 g cm−3, in good agreement with expectations. Our DFT-MD studies find that the first step in the decomposition of MTO is intermolecular hydrogen-transfer reaction (barrier 3.0 kcal mol−1) which is followed quickly by H2O and NO release with reaction barriers of 46.5 and 35.5 kcal mol−1. In contrast for MTO3N (P2(1)/c predicted space group), we find that the first steps are a bimolecular decomposition to release NO2 (ΔH = 44.1 kcal mol−1, ΔG = 54.7 kcal mol−1) simultaneous with unimolecular NO2 cleavage (ΔH = 59.9 and ΔG = 58.2 kcal mol−1) a unique initial reaction among EMs. These results suggest that MTO3N would be significantly more thermally stabile (barrier > 6.0 kcal mol−1 higher) than RDX and HMX, making it an excellent candidate to be insensitive new green energetic materials. However we find that MTO leads to very favorable hydrogen transfer reactions that may complicate synthesis and crystallization, making MTO3N the more promising system.

Molecular dynamic simulations on the interaction between an HTPE polymer and energetic plasticizers in a solid propellant

In order to study the interaction between a polymer and plasticizers in a solid propellant and their underlying mechanisms, molecular dynamics (MD) simulations with compass force fields were performed to investigate the Hydroxy Terminated PolyEther (HTPE) polymer and some energetic plasticizers including nitroglycerin (NG)/butanetriol trinitrate (BTTN) mixture, bis(2,2-dinitropropyl)acetal (BDNPA)/bis(2,2-dinitropropyl)formal (BDNPF) mixture and N-butyl-N-(2-nitroxy-ethyl)nitramine (Bu-NENA). Also, the mechanical properties for the HTPE polymer containing energetic plasticizers were theoretically and experimentally studied. It was shown that the HTPE polymer is miscible with all involved energetic plasticizers which can improve the mechanical property of the HTPE polymer. The order of binding energies between HTPE and the energetic plasticizers are found to be HTPE/Bu-NENA > HTPE/BDNPA/BDNPF > HTPE/NG/BTTN.