Theoretical study on the isomerization mechanisms of phenylazopyridine on S0 and S1 states

In this work, some important structures tied closely to the isomerization of 2‐(phenylazo)pyridine (2‐PAPy) and 4‐(phenylazo)pyridine (4‐PAPy) on the S₀ and S₁ states were characterized in detail by using the complete active space SCF (CASSCF) theory. The isomerization mechanism was discussed on the basis of the mapped potential energy surfaces (PESs) and conical intersections (CIs). A comparison of PAPy with azobenzene was carried out to stress the effect of molecular structure on the photoisomerization details. The results indicate that the thermal isomerization for both 2‐PAPy and 4‐PAPy are mainly attributed to the inversion of CNN angle on the side of the pyridine ring. In view of the energy, an optimized CI_rot with a twisting structure supports the rotation mechanism in the photoisomerization of PAPy on the S₁ state. However, it was found that another conical intersection (CI_inv) with a planar structure is higher in energy than the corresponding trans‐FC structure, via which only the PAPy excited on S₂ state or vibrationally hot S₁ state can relax their excitation energy. Minimum energy paths (MEPs) showed that the relaxation process of cis‐PAPy being excited on the S₁ state is characterized by a smoothly falling curve, which is very similar to that of azobenzene. Furthermore, a S₁ minimum and a transition state (TS₁) were found to exist on the MEP starting from the trans‐FC point to the CI_rot for 4‐PAPy, but these two typical structures were not found on the MEP of 2‐PAPy. Compared with azobenzene, 4‐PAPy exhibits a very similar photoisomerization PES, but a subtly different one can be predicted in the case of 2‐PAPy. The present results are expected to provide useful information for the design of photoresponsive materials based on the PAPy units. Copyright © 2009 John Wiley & Sons, Ltd.