Theoretical Studies On Response Properties Of Azobenzene Derivatives:the Combination Of Quantum Chemical Calculations And Molecular Dynamics Simulations

The organic photoelectric functional materials with particular electronic structures and optical properties played an increasingly important role in material science. The rational designs of novel electro-optical materials have stimulated extensive experimental and theoretical interests. With the development of quantum chemical methods and molecular simulation techniques, various theoretical models have been employed to study the structure-property relationship and the effects of solvent, external electronic field and self-aggregation on optical properties. In order to theoretically design novel photoelectric molecules, we carried out theoretical calculations on the optical properties and photo-induced isomerizations. The influence of media environments (solvents and electric field) and molecular self-aggregations on the structure and optical properties of azobenzene derivatives was systematically explored. The results and conclusions of this thesis are summarized as follows:(I) Modulation of the electronic structures and optical propertiesOrganic nonlinear optical (NLO) materials with both high nonlinearity and good transparency were designed by chemical modifications, such as the changes in molecular topology, substitute group, and the metal encapsulation.DFT calculations show that the H-shaped D-π-A dimers increase the second-order polarizabilities by virtue of the enhanced dipole moment, without a significant shift in the maximum absorption bands. The addition of another azobenzol group between electron-donating and-accepting groups, or replacement with the strong electron-donating and/or-accepting substituent increases the second-order polarizabilities but leads to an evident red-shift in spectra.The4n7r-electron systems with crown-shaped topology have large hyperpolarizabilities because the almost parallel alignment of dipoles of the unit systems induces large dipole moments of the molecules. Especially, the up-crown shaped molecules exhibit higher second-order polarizabilities by about4times of the down-crown counterparts The introduction of two Pd atoms in the extended porphyrin does not increase the hyperpolarizability due to the centrosymmetric structure. However, metal encapsulation in metallocenes enhances the hyperpolarizability significantly.(II) Effects of external electric field, solvents and self-aggregations on optical propertiesThe influence of electric field, solvents and molecular self-aggregations on the structure and optical properties of D-π-A azobenzene derivative (OH-azobenzene-NO2) has been investigated by molecular dynamics (MD) simulations and quantum chemical calculations. Solvents and electric field lead to insignificant effects on the rigid π-conjugated backbone, but the optical properties of both the ground and the low-lying excited states are very sensitive to the intermolecular solute-solvent interactions. The dipole-dipole interactions and intermolecular hydrogen bonds facilitate the self-aggregations of D-π-A monomers in solution. The V-shaped aggregation dimer exhibits a20%enhancement of second-order polarizabilities relative to that of the monomer, whereas the antiparallel alignment leads to a cancellation of dipole moment and hence dramatic decrease in hyperpolarizability (one third of the monomer). The Boltzmann-weighted hyperpolarizability (in consideration of the relative stability of these two aggregations) is close to that of the monomer.(Ⅲ) Molecular dynamics simulations of photo-induced isomerization of azobenzene derivativesThe isomerizations and solute-solvent interactions of azobenzene derivatives in external electric field and solution have been studied by MD simulations. Nonequilibrium MD simulations demonstrate that the light-induced cis-trans isomerization completed during500fs after the system was excited to the low-lying excited states. The steric hindrance of the solvent molecules and intermolecular interactions result in longer relaxation time in THF solution than in gas phase. Due to the complicated reorientations of dipole moments, the isomerization with the applied external electric field needs longer relaxation time than that without electric field.Azo-Gln-Phe-Ala molecule has rich configurations and easily undergoes the folding/unfolding of the end peptide chain in aqueous solution. The unfolding Azo-Gln-Phe-Ala chain structure favors the intermolecular hydrogen bonding and π-stacking in molecular aggregates.The results obtained in this thesis may shed insight on the designs and preparations of novel materials in experiments.