Density functional theory study on sensing properties of g-C3N4 sheet to atmospheric gasses: Role of zigzag and armchair edges

The ability of the polymer-based graphitic carbon nitride (g-C₃N₄) as a gas sensor toward NO, NO₂, CO, CO₂, SO₂, SO₃, and O₂ gasses is assessed using density functional theory (DFT) calculations in terms of energetic and electronic transport characteristics. In particular, this study is aimed to explore the role of zigzag and armchair edges of the g-C₃N₄ sheet on sensing performances. The electronic properties of adsorption systems, such as Bader charge analysis, band gaps, work function, and density of states (DOS), are used to understand the interaction between the adsorbed gas molecules and the g-C₃N₄ sheet. Our calculated results indicate that SOx (SO₃ and SO₂) gasses have higher adsorption energies on the g-C₃N₄ sheet than other gasses. Furthermore, the transport properties, such as current–voltage (I-V) and resistance-voltage (R-V) curves along the zigzag and armchair directions are calculated using the non-equilibrium Green's function (NEGF) method to understand the performance of the g-C₃N₄ sheet as a prominent conductive/resistive sensor. The I-V/R-V results indicate that the zigzag g-C₃N₄ sheet has excellent sensing ability toward SOx gasses at low applied voltages. However, the presence of H₂O degrades the sensing performance of the armchair g-C₃N₄ sheet. Theoretical recovery time has also been calculated to evaluate the reusability of g-C₃N₄ sheet-based gas sensors. Our results reveal that the zigzag g-C₃N₄ sheet-based sensing device has a remarkably high sensitivity (>300%) and selectivity toward SOx gasses and has the potential to work in a complex environment.     

Theoretical study on halide and mixed halide Perovskite solar cells: Effects of halide atoms on the stability and electronic properties

Increasing the stability of perovskite solar cells is one of the most important tasks in the photovoltaic industry. Thus, the structural, energetic, and electronic properties of pure CH₃NH₃PbI₃ and fully doped compounds (CH₃NH₃PbBr₃ and CH₃NH₃PbCl₃) in cubic and tetragonal phases were investigated using density functional theory calculations. We also considered the effects of mixed halide perovskites CH₃NH₃PbI₂X (where X = Br and Cl) and compared their properties with CH₃NH₃PbI₃. The DFT results indicate that the phase transformation from tetragonal to cubic phase decreases the band gap. The calculated results show that the X‐site ion plays a vital role in the geometrical stability and electronic levels. An increase in the band gap and a reduction in the lattice constants are more apparent in CH₃NH₃PbI₂X compounds (I > Br > Cl).     

Structural and spectral properties of 4-bromo-1-naphthyl chalcones: a quantum chemical study

The structural and optical properties of 4-bromo-1-naphthyl chalcones (BNC) have been studied by using quantum chemical methods. The density functional theory (DFT) and the singly excited configuration interaction (CIS) methods were employed to optimize the ground and excited state geometries of unsubstituted and substituted BNC with different electron withdrawing and donating groups in both gas and solvent phases. Based on the ground and excited state geometries, the absorption and emission spectra of BNC molecules were calculated using the time-dependent density functional theory (TDDFT) method. The solvent phase calculations were performed using the polarizable continuum model (PCM). The geometrical parameters, vibrational frequencies, and relative stability of cis- and trans-isomers of unsubstituted and substituted BNC molecules have been studied. The results from the TDDFT calculations reveal that the substitution of electron withdrawing and electron donating groups affects the absorption and emission spectra of BNC.     

Surprising selectivity in the transformation of dimethoxy azaindoles

Transformation of 4,7-dimethoxy-6-azaindole into 4-hydroxy-7-methoxy-6-azaindole or 7-hydroxy-4-methoxy-6-azaindole can be readily controlled by careful selection of a reagent. Treatment with concentrated HCl results in hydrolysis at the 4-position exclusively, while TMS-I provides demethylation at the 7-position only. Products were unambiguously identified by single crystal X-ray crystallography.     

Tautomerization and solvent effects on the absorption and emission properties of the Schiff base N,N′-bis(salicylidene)-p-phenylenediamine – A TDDFT study

Time-dependent density functional theory combined with a polarized continuum model has been applied to study solvent effects on the geometrical and energetic properties, as well as the absorption and emission properties, of three tautomeric forms of N,N′-bis(salicylidene)-p-phenylenediamine (BSP). The calculated properties are in agreement with the available experimental data. It was observed that the solvent environment does not affect the vertical excitation energies significantly, whereas tautomerization strongly affects both the absorption and emission spectra of BSP.     

Charge transport and optical properties of discotic liquid crystalline molecules THDDP and substituted THDP

The theoretical investigations have been carried out on the discotic liquid crystalline molecules, 2,3,6,7‐tetrakis‐hexyloxy‐9,16‐diaza‐dibenzo[a,c]phenazine (THDDP) and different substituted 2,3,6,7‐tetrakis‐hexyloxy‐dibenzo[a,c]phenazine (THDP) to study their charge transport and optical properties. The key parameters of charge transport such as charge transfer integrals and site energies have been calculated from the matrix elements of Kohn‐Sham Hamiltonian. The reorganization energy for the presence of excess charge and the rate of charge transfer calculated from Marcus theory have been used to find the mobility of the charge carrier in the studied molecules. The results show that the substitution and stacking angle change strongly affect the charge carrier mobility in π‐stacked THDDP and substituted THDP molecules. Molecular dynamics simulations have been performed to find the most favorable conformation. The time‐dependent density functional theory (TDDFT) calculations reveal that for these molecules the different substitutions does not alter the main features of optical properties and the molecules may be used as blue light emitters. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Absorption and emission spectral studies of Eu3+-doped PbO-PbF2 glass system

Judd-Ofelt parameters obtained from the absorption spectra of Eu³⁺ ions doped in PbO-PbF₂ glasses are intermediate between the values for fluoride and phosphate glass matrices. Eu³⁺ ions are coordinated to both oxide and fluoride ions. The calculated transition probabilities (As^-1) for the laser transition⁵D_o→⁷F₂ are 171 and 170 for 30PbO-70PbF₂ and 70PbO-30PbF₂ glasses respectively and are significantly lower compared to phosphate glasses. The calculated (β_R cal) and experimental (β_Rexpt) branching ratios for this transition show good agreement. The emission spectra display high energy transitions in the 440–570 nm region, a characteristic of parent matrices with low energy phonons such as the tellurite, germanate and fluoride glasses. The electron-phonon coupling strengths deduced from the excitation spectra of Eu³⁺ are 10.2 x 10⁻³ and 9.5 x l0⁻³ for 30PbO-70PbF₂ and 70PbO-30PbF₂ glasses respectively. The relative emission intensities of the low energy transitions to high energy transitions and the ratios of the most intense transitions⁵D₀→⁷F₂/⁵D₀→⁷F₇ significantly vary for the two glasses providing evidence for clustering of Eu³⁺ ions with increase in its concentration and increasing PbO content.     

Highly convergent synthesis of a rebeccamycin analog with benzothioeno(2,3-a)pyrrolo(3,4-c)carbazole as the aglycone

A highly convergent, scalable synthesis of the rebeccamycin analog 2 was demonstrated in seven steps and 31% overall yield based on the N-protected building block dibromomaleimide 7. The practical synthesis of other two building blocks, 5,6-difluoro-3-benzothiopheneboronic acid 6 and 5,6-difluoroindole 8, is described.     

Theoretical investigation on intramolecular electron transfer in polypeptides

Theoretical investigations on the intramolecular electron transfer between the intermediate residues of different secondary structures of an oligopeptide have been carried out. Density functional theory calculations have been performed to calculate the charge transfer integral, spatial overlap integral and site-energies for the optimized secondary structures of the glycine oligopeptide by varying the dihedral angles ( and ψ) along the -carbon atom of amino acid subgroups. The reorganization energy has been calculated in the presence of an excess negative charge. The electron transfer rates for the model peptide have been estimated and the dependence of the rate on secondary structures is discussed.