Potential Energy Surface Character Of Photoisomerization Of Azo Compounds

Azophanes, azocrowns and azocyclodextrins have been used as switch molecules in information storage, ion transfer and other fields. There has been a disputation on the trans-cis photoisomerization mechanism of this kind of molecules for a long time: one is dihedral mechanism(rotation mechanism) with rotation about N=N axes; the other is angular mechanism (inversion mechanism)with rotation about one N atom in the molecular plane.In this paper, a series of azo compounds are employed to investigate the photoisomerization mechanism by means of 3-dimension potential energy surface (3-D PES) method . Two different transition types, n->?* and ?->?*, are determined by analyzing the coefficients of molecular orbitals.The 3-D PES is visualized by programming with MATLAB 6. The location of the important points on PES, minimum , maximum, saddle points, is automated by our program, thus make it easier than traditional observation. For PES with more than 3 dimensions, this location method becomes more complicated. As the calculation consumption time increases sharply with the increase of PES dimensions, the choice of proper scanning step-size is necessary. Calculation availability is evaluated by comparing the errors with different scanning step-sizes and interpolation methods. Three two-dimension interpolation methods, nearest,bi-cubic and bi-linear, are used to treat with the original 3-D PES, and a comparison of time consumption between different step-sizes is made. The result indicates that bi-cubic interpolation method brings the smallest error as compared with the other two and a sharp decrease of calculation time. Visualization, calculation availability and easy location of important points on PES make it possible to study the microcosmic mechanism of our systems.A detailed discussion is made on continuum model of solvent effects. Electrostatic model and self-consistent reaction field model are thoroughly reviewed. For the non-continuum model, we propose our primary solution to treat with temperature effect by introducing a relationship between temperature and dipole polarizability. In this paper's calculation, we take polarizable continuum model to treat with solvent effects on our system. The results indicate that solvent effects are different for two geometry types-trans and cis, but the effects are not huge enough to change the reaction path.The substitution effect on the transition wavelength is obtained. For the n?7t* transition related with lone-pair electron on N atom, the electron-pulling effect of substitutions is dominant. The stronger the electron-pulling effect, the shorter the transition wavelength is. For the potential barrier energy, it is the conjugate effects which play a dominant role rather than the electron-pulling effects.Our results offer a direction for the choice of transition wavelength and the development of new switch molecules. The 3-D PES method proposed in this paper can also be used in other fields.