Laser-induced Dynamics Simulation Of The Azobenzene Isomerization

Azobenzene is a prototype chromophoric molecule and undergoes trans (?) cis isomerization under ultraviolet or visible radiation. The two isomers display clearly different UV absorption spectra. These make azobenzene and its derivatives become good candidates for many applications, including light-triggered switches, constituents of erasable holographic data, image storage devices and in biology. For this reason, the photochemical and photophysical features of azobenzene has attracted extensive research interests for many years.The proposal by Rau that the azobenzene molecule takes inversion path from the reactant to the product for the nÏ€^* excitation and rotation path for theππ^* excitation opens a debate on the photoisomerization mechanism of the azobenzene molecule. Many high level quantum chemical calculations have been applied to study the mechanism and provided valuable information in understanding this important process. In this thesis, we present a realistic dynamics simulation study of the trans-azobenzene photoisomerization under nÏ€^* andππ^* excitations by semiclassical electron radiation ion dynamics (SERID) approach.In the SERID approximation, the state of the valence electrons is calculated by the time-dependent Schrddinger equation, but the radiation field and the motion of the nuclei are treated classically. In other words, the forces for driving nuclear motion are calculated by quantum chemical calculations but nuclear trajectories are updated by solving Newton's motion equation (or Ehrenfest equation). The most important characteristic of SERID is the direct inclusion of a laser pulse that interacts with the molecule. SERID has following features:(1) The laser pulse is characterized by the vector potential A that is coupled to the Hamiltonian through the time-dependent Peierls substitution. This explicitly incorporates the interaction between laser field and electrons and allows us to investigate the effects of laser pulses on the products of a photochemical reaction. This is an important topic in the laser control of chemical reactions.(2) The time-dependent Schrodinger equation is solved using a unitary algorithm obtained from the equation for the time evolution operator. Nuclear positions are updated by numerically integrating Newton's motion equation with the velocity Verlet algorithm which preserves phase space and satisfies the Pauli principle.(3) Electronic structure is calculated by the density functional based tight-binding (DFTB) method. The basis functions are the non-orthogonal basis set and only valence electrons are calculated. Nuclei and core electrons together are treated as an ion. Hamiltonian matrix elements and ion-ion repulsive interaction are calculated by the density functional method and then tabulated as a function of distance of two ions for time-dependent calculations. This allows an effective calculation of forces and energies.(4) The model calculates the forces on-the-fly. This is in contrast to the Molecular Dynamics simulation in which the potential energy surface is constructed before any simulation is conducted.These features make the model as a realistic technique for simulating large systems. Its shortcomes are described below: (A) DFTB is a decent from DFT at LDA level, which may not be suitable for the description of the breaking of chemical bonds. (B) Electronic states and spin multiplicity are not clearly defined and therefore it is not always possible to directly compare our simulation results to high level quantum chemical calculations.We have carried out several hundreds simulation calculations by adjusting frequency, flux, and duration of the laser pulse and obtained the following conclusions:1. For trans-azobenzene, if the laser pulse with frequency of 1.75 eV and duration of 100 fs is used the simulation will lead to nÏ€^* (S₁) excitation and then the S₁ state evolves with time and decays to the ground state S₀ through the drive of the potential energy. The lifetime of S₁ state is about 1450 fs-1650 fs. Laser pulse with frequency of 2.15 eV and 2.2 eV and duration of 100 fs will lead toππ^* excitation. The excited molecule decays from the S (ππ^*) state to the S((Ï€)²(Ï€^*)²) state within 30 fs and immediately relax to a S₁ state. The S₁ state evolves with time and decays to the ground state S₀. The lifetime of S₁ state induced by pulse of 2.15eV and 2.2eV is about 800 fs-1000 fs and 500 fs-600 fs, respectively.2. The N=N bond stretching soon after laser pulse react to azobenzene molecule and rotary motion around central "-N=N-" group company with two CNN angles concerted expanding to 15°-20°are observed either nÏ€^*orππ^*excited. Without finding semi-linear structure indicates the "inversion mechanism" is not supported by simulation results.3. Both nÏ€^*andππ^*excitation in which closely relation between N-N bondlength versus LUMO energy, CNN angles versus HOMO energy and CNNC dihedral versus energy between HOMO and LUMO are observed when azobenzene molecule locates in S₁ excited state.4. The S₁ state can occur either from nÏ€^* orππ^* excitations. When its CNN angles increase the N-N bond shorten sharply, which might be brought about owing to change of vibrational states.5. Decay channels from the S₁ state to the S₀ state at about the perpendicular structure of two phenyl rings in azobenzene. "Perpendicular structure" will lead to formation either trans or cis isomer according to orientation of the force on the two carbon atoms which are adjacent to the nitrogen atom.6. There is an "twisted structure" with 140°dihedral onππ^*excitation besides the above-mentioned "perpendicular structure". Decay channel at "twisted structure" onππ^* excitation will only result in the formation of trans-product. The existence of a decay channel at geometry far from the "perpendicular structure" accounts for the poor E-Z photoisomerization quantum yield forππ^* excitation.7. The bending vibration of the CNN bond are significantly enhanced after the S₂ (ππ^*)/S₃ ((Ï€)²(Ï€^*)² ) decay. The energy for this excitation comes from internal energy conversion at the S(ππ^*)/S((Ï€)²(Ï€^*)²) decay. This excitation promotes the CNN bond angles expansion that makes significant contribution to the decay channel at the geometry of "twisted structure."8. Results of Multiple trajectory simulations show :1) For nÏ€^* excitation at pulse frequency of 1.75eV, the smaller fluence will bring larger possibility of the formation of cis-structure, and vice versa.2) Forππ^* excitation at pulse frequency of 2.15eV, the formation of cis-isomer has larger possibility, while at pulse frequency of 2.2eV, trans-isomer has larger possibility.