Nonadiabatic ab initio molecular dynamics of photoisomerization in bridged azobenzene

Abstract

The photoisomerization mechanisms of bridged azobenzene are investigated by means of surface hopping dynamics simulations based on the Zhu-Nakamura theory. In the geometry optimizations and potential energy surface calculations, four minimum-energy conical intersections between the ground state and the lowest excited state are found to play important roles in the trans-cis and cis-trans isomerization processes. The trans-cis photoisomerization proceeds through two minimum-energy conical intersections. Ultrafast pedal motion of the N atoms and twisting of phenyl rings around their N-C bonds allows the molecule to move to a minimum-energy conical intersection, after which surface hopping from S 1 to S 0 occurs. In the S 0 state, further rotation occurs around the N=N bond and two N-C bonds until the azo moiety and phenyl rings complete their isomerization. Finally, the cis form is achieved by subsequent adjustment of the ethylene bridge. In the cis-trans photodynamics, there is one rotational pathway, in the middle of which two CIs are responsible for the surface hopping to the S 0 state. After the nonadiabatic transition, the molecule reaches the trans form through a barrierless pathway and the two phenyl rings and the additional bridge complete their reorientation almost at the same time.

Nonadiabatic dynamics study of bridged-azobenzene by the time-dependent density functional tight-binding method

“…This mechanism revealed is in accordance with the DFT based molecular dynamics simulations performed by Allen and co-workers [30]. It also agrees well with the CASSCF simulation results performed by Gao et al [33]. The hopping geometries where the transitions to the ground state take place are investigated in Fig.…”

“…The forces and nonadiabatic couplings needed to propagate the nuclear trajectories are obtained with the TD-DFTB method. The nonadiabatic dynamics approach is similar to the study of Br-AB in the previous paper by one of our co-workers and the whole procedure can be summarized as follows [33]. The system is propagated on the excited state potential energy surface and the energy difference between the two adjacent adiabatic potential energy surfaces is monitored.…”

“…In the work by Gao et al, CASSCF(14,12)/6-31G calculations showed there were three equilibrium geometries in the S 0 state and four minimum energy CIs between the S 0 and S 1 states. CASSCF(6, 5)/6-31G nonadiabatic dynamics simulation revealed that two CIs were found to be responsible for the nonadiabatic transitions during the photoisomerization [33]. Now we insert the CIs found by the CASSCF method into the figure to make a comparison.…”

“…The absorption spectra of Br-AB (E structure) obtained with the (a) the TDDFT method using hybrid B3LYP functional and the (b) TD-DFTB method. Table 1 The first excite energies (nm) of Br-AB obtained with the TD-DFTB method, the TDDFT method using hybrid B3LYP functional and the CASSCF(6, 5)/6-31G method [33]. The according experiment results are also shown [25].…”

“…Density-functional-based dynamical simulations conducted by Jiang et al later confirmed that the bridging feature did not hinder the rotation and rotational pathway was not restricted [32]. Nonadiabatic ab initio molecular dynamics simulations were carried out with the CASSCF method and the PESs of the S 0 and S 1 states were constructed at the two-state-averaged CASSCF(10,8)/ 6-31G * level to reveal the mechanisms of the trans-cis and cistrans photoisomerizations process [33], four minimum-energy conical intersections between the ground state and the lowest excited state were found to play important roles in the trans-cis and cis-trans isomerization processes. In the above studies of the Br-AB, density functional theory is frequently used as the electronic structure methods for the dynamics simulation because density functional theory can reach a good balance between the accuracy and time requirement.…”