Takayuki Shoda

Theoretical study on the photoisomerization of azobenzene

Ab initio calculations are performed to elucidate the mechanism of the photoisomerization of azobenzene. We obtain the excitation energies of the S1(n→π*), S2(π→π*), and S3(n2→π*2) states by complete active space self-consistent field (CASSCF) and multireference single double configuration interaction (MRSDCI) calculations. Two-dimensional potential surfaces of the ground- and excited states are obtained at the CASSCF level in order to investigate the isomerization pathways. A conical intersection between the ground state and the S1 state is found near the midpoint of the rotation pathway, and causes a radiationless transition. On the other hand, the S2 state has local minima at the cis and trans structures, so that the isomerization proceeds at the S2 surface following the deexcitation.

Molecular packing analysis of benzene crystals. Part 2. Prediction of experimental crystal structure polymorphs at low and high pressure

Abstract Benzene crystal structures were obtained by pseudoannealing initial structures sampled from a trajectory of constant moles, volume, and temperature molecular dynamics (NVT MD). The crystal structure of the observed low pressure phase I was reproduced as the most stable state and other crystal structures, which included high pressure phase III, were obtained as metastable states at zero pressure. The pressure dependence of enthalpy was analyzed for these crystal structures, and the relative stability among these crystal structures is discussed, at pressures up to 4 GPa. The observed low pressure phase I was the most stable state up to 1.4 GPa and the high pressure phase III was the most stable state above 1.4 GPa. The reported high pressure phase II was not obtained in this work at any pressure up to 4 GPa.

Molecular packing analysis. Part 3. The prediction of m-nitroaniline crystal structure

Abstract Molecular packing analysis of m-nitroaniline was carried out in order to study whether it is possible to predict the crystal structure of systems that include hydrogen bonds. Off-ridge eigenvector minimization with annealing (OREMWA) was carried out with Biosyms potential parameters and potential-derived charges obtained from an ab initio Hartree-Fock calculation using the 6-31++G∗∗ basis set. Two types of initial structure were prepared for the OREMWA calculations (ten structures were created for each type of initial structure). The first-type initial structures were created by deforming the observed crystal structure up to 1 A in the unit cell lengths and molecular translations, and up to 10 ° in the unit cell angles and molecular rotations. The second-type initial structures were created by four molecules satisfying space symmetry relationships of the observed space group Pbc21 in a 12 A × 12 A × 12 A cubic unit cell. In the second-type initial structures, the orientation of a reference molecule was determined randomly. Molecular translation variables of three molecules, molecular rotation valuables of all four molecules, and six unit cell parameters were varied in the OREMWA calculations starting from the first-type initial structures. Molecular translation variables and molecular rotation variables of a reference molecule, and cell lengths only, were varied in the OREMWA calculations starting from the second-type initial structures in order to retain the observed space group symmetry, because the Pbc21 space group is orthorhombic with 90 ° cell angles. This space group is also polar in the c-axis direction, so that one molecular origin in this direction is fixed. The observed crystal structure of m-nitroaniline was reproduced as the most stable crystal structure starting from both types of initial structure.

Hole transport in hydrazone-doped polymer films

Hole drift mobilities were measured in hydrazone-doped polycarbonate films under various concentrations, temperatures and electric fields. Hydrazone molecules used were 9-ethylcarbazole-3-carbaldehyde diphenylhydrazone (CTM1), 9-ethylcarbazole-3-carbaldehyde methylphenylhydrazone (CTM2), and 1-pyrenecarbaldehyde diphenylhydrazone (CTM3). Charge transport in the films obeyed the nonadiabatic small-polaron hopping formalism with almost the same activation energies at concentrations investigated. CTM2 exhibited a different concentration dependency and relatively small mobility especially at high concentrations. Analysis by disorder formalism indicated that the energetic disorder parameter (σ) is almost the same and the non-energetic disorder parameter (Σ) is different. We found some correlations between the mobilities of the CTMs and molecular overlap and orientation in the single crystalline state of CTMs.

Thermally Stimulated Currents by Charge-Generation Layer Excitation in Azo-based Layered Organic Photoconductors

Thermally stimulated currents (TSCs) were measured by exciting the charge-generation layer of layered organic photoconductors. Carrier-generation efficiency was measured in a very wide temperature range of irradiation. By this technique, the formation and successive processes of the initial ion pairs are separated because recombination and separation of the initial ion pairs are suppressed at low temperature. The carrier-generation efficiency of the layered photoconductors using azo pigments shows extremely weak dependences on both irradiation temperature and electric field below 160 K.