Quan-Song Li

Theoretical Studies on Optical and Electronic Properties of Propionic-Acid-Terminated Silicon Quantum Dots.

The origin and stability of photoluminescence (PL) are critical issues for silicon nanoparticles to be used as biological probes. Optical and electronic properties of propionic-acid (PA) -terminated silicon quantum dots (SiQDs) were studied using the density-functional tight-binding method. We find that the adsorbed PA molecules slightly affect the structure of silicon core. The PA adsorption does not change the optical properties of SiQDs, while it substantially decreases the ionization potentials in the excited state and results in some new active orbitals with adjacent energies around the Fermi energy level. Accordingly, the modified surface of SiQDs can serve as a reaction substrate to oxygen and solvent molecules, which is responsible for the increase in both PL stability and water solubility.

Dynamic Characteristics of Aggregation Effects of Organic Dyes in Dye-Sensitized Solar Cells

Two organic dyes (LS-1 and IQ4) containing identical electron donor and acceptor units but distinct π units result in significantly different power conversion efficiency of the corresponding dye-sensitized solar cells (DSSCs): LS-1, 4.4%, and IQ4, 9.2%. Herein, we combine first-principle calculations and molecular dynamics to explore the aggregation effects of LS-1 and IQ4 by comparing their optical properties and intermolecular electronic couplings. The calculated absorption spectra are in good agreement with the experimental observations and reveal them to be evidently affected by the dimerization. Furthermore, molecular dynamics simulations show that steric hindrance induced by the diphenylquinoxaline unit in IQ4 can elongate the distances between intermolecular π units or electron donors, which are responsible for the fact that the intermolecular electronic coupling of LS-1 is about 10 times larger than that of IQ4. More importantly, the aggregated IQ4 remains almost perpendicular to the TiO2 surface, whereas LS-1 gradually tilts during the dynamic simulation, impacting electron injection and recombination in several ways, which clarifies why IQ4 leads to larger photocurrent and higher conversion efficiency. The deep understanding of the dye aggregation effects sheds new light on the complex factors determining DSSC function and paves the way for rational design of high-efficiency self-anti-aggregation sensitizers.

Molecular design of N–NO2 substituted cycloalkanes derivatives Cm(N–NO2)m for energetic materials with high detonation performance and low impact sensitivity

For novel high-energy low-sensitivity energetic materials, a series of novel cycloalkanes derivatives Cm(N–NO2)m (m = 3–8) were theoretically designed by substitution of the hydrogen atoms with N–NO2 group. Density functional theory (DFT) calculations in combination with the isodesmic reaction and the Kamlet–Jacobs equations were employed to predict the heats of formation (HOFs) and the detonation properties. We found that the designed compounds have large positive HOFs, which are proportional to the amount of N–NO2 groups. Importantly, these compounds possess high crystal densities (1.85–1.95 g cm−3) and heats of detonation (1811–2054 kJ g−1), which lead to remarkable detonation properties (detonation velocities = 9.37–9.61 km s−1 and detonation pressures = 38.03–42.48 GPa) that are greater than those of the well-known energetic compounds 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), 1,3,5-trinitro-1,3,5-triazinane (RDX), and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX). Moreover, the bond dissociation energy and the impact sensitivity index h50 values suggest that the title molecules are less sensitive than CL-20, and comparable to HMX and RDX. Therefore, our results show that the designed compounds may be promising candidates for energetic materials with notable detonation performance and low impact sensitivity.

Molecular engineering of quinoxaline dyes toward more efficient sensitizers for dye-sensitized solar cells

D–A–π–A-featured organic dyes incorporating diphenylquinoxaline unit (such as IQ4) have shown great potential in anti-aggregation and broadening spectral response in the field of dye-sensitized solar cells (DSSCs). The crucial restriction for quinoxaline-based cell to attain higher efficiency is the relatively low photocurrent density (JSC). In the present work, three novel push–pull dyes only differing in electron donors, have been designed based on the IQ4 backbone, in order to further improve the light-harvesting capability of quinoxaline dyes and to examine the donor influence on dye performance. Theoretical analysis of the factors correlated with the JSC and open-circuit photovoltage (VOC) demonstrate that, relative to the parent IQ4 dye, the NIQ4 dye bearing the elegant N-annulated perylene donor shows a good performance in light harvesting, electron injection, and dye regeneration, indicating an increased JSC potential for the related cell. Furthermore, despite possessing a smaller vertical dipole moment, the improved blocking effect of NIQ4 not only prevents unfavorable self-aggregation, but also effectively inhibits the parasitic back-recombination. Therefore, the NIQ4 is proposed to be a potential dye in DSSC applications.

Rational design of Co-based redox mediators for dye-sensitized solar cells by density functional theory

Density functional theory (DFT) calculations were carried out to explore the effects of chemically modifying the polypyridine ligands and design efficient Co-based redox mediators for dye-sensitized solar cells (DSSCs). Our results showed that the redox properties of cobalt complexes can be well tuned by altering the number and position of nitrogen atoms on the ligand ring. Adding oxygen atoms on the ligand ring will evidently increase the redox potential, which might be unfavorable for the dye regeneration. The designed good redox mediators possess similar redox potential and reorganization energy to the current high-efficiency redox couples, thus are promising to be used in prospective DSSCs.

Theoretical design of porphyrazine derivatives as promising sensitizers for dye-sensitized solar cells

Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations have been carried out on the electronic structure and optical properties of a set of heterocycle-fused zinc porphyrazine (ZnPz) derivatives, aiming at screening efficient sensitizers for dye-sensitized solar cells (DSSCs). Our results show that the absorption spectra of the designed dyes shift to longer wavelengths and the light harvesting efficiencies are much higher than isolated ZnPz. Moreover, the designed dyes have larger contributions of the anchoring group to the lowest unoccupied molecular orbitals (LUMOs) compared with the currently best sensitizer YD2-o-C8, indicating enhanced electron injection ability from the sensitizer to the semi-conductor. Furthermore, the designed dyes exhibit good performance in terms of the charge transfer characteristics, the driving force of electron injection and dye regeneration, and the excited-state lifetime. Overall, the designed dyes, especially indigo blue fused ZnPz and acridine fused ZnPz, are revealed to be promising sensitizers for high-efficiency DSSCs.

Theoretical study on the adsorption mechanism of iodine molecule on platinum surface in dye-sensitized solar cells

Abstract By means of density functional theory calculations, the adsorption process of I2 at Pt (111) surface in dye-sensitized solar cells (DSSCs) has been investigated. The obtained adsorption energies and stable structures depending on the adsorption sites of the Pt surface are in good agreement with experimental values. Our results show that the dissociative chemisorption and the non-dissociative chemisorption are competitive for the adsorption of I2 on the Pt surface, and the dissociative pathway is more favored in energy. This study is expected to enrich the understanding on the origin of the excellent heterogeneous catalytic performance of Pt for triiodide reduction and the complex iodine chemistry in DSSCs. Understanding of this adsorption mechanism is helpful for rational screening for redox couple and the Pt-free alternative counter electrode materials.

Theoretical design of highly energetic poly-nitro cage compounds

In this work, we report on the design and full prediction of four poly-nitro cage compounds, octanitrooctaprismane (ONOP), octanitrooctaazaprismane (ONOAP), tetranitrooctaprismane (TNOP), and tetranitrooctaazaprismane (TNOAP) at the B3LYP/6-31G (d,p) level using density functional theory (DFT). The results show that all compounds possess large positive heats of formation (HOF) and specific enthalpies of combustion (ΔHC). The detonation velocity (D) and pressure (P) are calculated using Kamlet–Jacobs equations, and ONOP, ONOAP, and TNOAP showed a superior performance in comparison to commonly used energetic materials, 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) and 1,3,5-trinitro-1,3,5-triazinane (RDX). Calculation of the bond dissociation energy (BDE) is carried out and reveals good thermal stabilities for all compounds. In terms of sensitivity, molecules with four nitro groups (TNOP and TNOAP) display lower sensitivity than those with eight nitro groups (ONOP and ONOAP). Importantly, TNOAP outshines other molecules due to its superior energetic properties, compared to those of HMX, and good sensitivity, less than that of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and comparable to that of RDX, so we recommend TNOAP as a promising HEDM candidate.

Insights into the Thermal Eliminations and Photoeliminations of B,N-Heterocycles: A Theoretical Study

Understanding the photochemistry of organoboron compounds is essential to expand optoelectronic applications. In this work, the complete active space self-consistent field (CASSCF) and its second-order perturbation (CASPT2) methods combining with density functional theory (DFT) have been employed to investigate the elimination mechanisms of compound 6,7-dihydro-54-benzo[d]pyrido[2,1-f][1,2]azaborininr (B4) on the ground state (S0) and the first excited state (S1). B4 is one of the 1,2-B,N-heterocycles that undergo competitive thermal elimination and photoelimination depending on the substitution groups on the B atom and the chelate backbone, thus providing a high-selectivity synthesis strategy for luminescent compounds. Since the energy barrier from B4 to BH3-pyrido[1,2-a]isoindole (D1) and pyrido[1,2-a]isoindole (A1) on the ground state is lower than that from B4 to 54-benzo[d]pyrido[2,1-f][1,2]azaborininr (C4), the retraction ring reaction is expected to proceed with larger probability than the H2 elimination upon heating. On the contrary, photoelimination of H2 may take place easily due to the almost barrierless pathway on the S1 state. Remarkably, we have located an energetically available conical intersection (S1/S0)X-1, which allows for ultrafast S1 → S0 decay and subsequently generation of C4. Our results not only throw light on the experimental observations of the selectivity of thermal elimination and photoelimination but also provide detailed information on the excited state as instructional implications for further synthesis and application of B,N-embedded aromatics.