• photoisomerization
• isomerization mechanism
• fluorescence depolarization
• dynamics simulations
• azobenzene
• anisotropy
• solvent effects
• transition dipole moment

Azobenzene and its derivatives are molecules very often used to construct photomodulable materials and molecular devices. The main characteristic of this kind of molecules is the efficient and reversible trans → cis photoisomerization, that occurs in either sense, without secondary processes. Using the appropriate wavelength, one can convert either isomer into the other one. The photoisomerization mechanism of azobenzene has been debated, during the last decades, because of the peculiar wavelength dependence of the quantum yields and because at least two standard possibilities exist: N=N double bond torsion and N inversion.
Our research group has performed simulations of the photodynamics of azobenzene molecule by mixed quantum-classical methods. Such simulations have been successful in explaining the dependence of the quantum yield on the excitation wavelength. However, these simulations have been conducted on the isolated azobenzene molecule, while almost all the experimental data have been obtained in condensed phase. In particular, Diau's group, from Taiwan, has shown a strong dependence of the excited states dynamics on the solvent viscosity.
The general aim of this work is to study the excited state dynamics of azobenzene in solution, in order to obtain its transient spectra and to produce data directly comparable with the experiments. In particular, we have studied the quantum yields, the isomerization mechanism and the reorientation of the transition dipole moment during the excited state relaxation, in order to understand the time resolved fluorescence anisotropy measurements obtained by Diau and collaborators. This research will also permit to study the reorientation of the whole molecule, which leads to alignment of an azobenzene sample in a polarized laser field.
A basic issue for the interpretation of the fluorescence anisotropy and of the orientation of azobenzene samples in polarized light is related with the direction of the transition dipole vector for the forbidden n-π* transition of trans-azobenzene. Therefore, we have carried out a preliminary ab initio study of the n-π* transition dipole moment, considering the vibrational motions that contribute to the oscillator strength, and focusing on the most effective ones, i.e. those of lowest frequency. The most effective coordinate in promoting this transition is the symmetric torsion of the phenyl groups. Other important coordinates are the antisymmetric phenyl torsion and the torsion of the N=N double bond. The transition dipole vector turns out to lie essentially in the molecular plane, almost parallel to the N-C bonds and to the longest axis of the molecule. Semiempirical calculations are in sufficiently good agreement with those obtained by ab initio methods.
The main part of the thesis work has been devoted to the simulation of the dynamics of the photoisomerization process of azobenzene in solution. We have made use of a mixed quantum-classical method of the surface hopping family. The electronic energies and wavefunctions are computed on the fly, by a semiempirical method modified by our group. A reparameterization of the semiempirical AM1 Hamiltonian has been carried out, considering new ab initio results used as reference values, in order to improve the accuracy of the semiempirical PES. The solvent effects have been introduced in a preliminary way by brownian dynamics, simulating two different solvent viscosities, and then explicitly, with a QM/MM approach. In this approach, the solvent itself is represented by a Molecular Mechanics force-field (OPLS) and the QM/MM interactions are made of electrostatic and Lennard-Jones terms. We have first determined, by ab initio calculations, the solute-solvent interaction potential between azobenzene and two simple molecules, methane and methanol (representatives of non-polar and of protic compounds). In this way we have obtained the necessary QM/MM interaction parameters, and we have run simulations with two solvents used in the experiments, methanol and ethylene glycol (simulations with n-hexane are in progress).
We obtain very good results for the dependence of the quantum yields on the solvent viscosity, and in this way we can confirm that the photoisomerization mechanism is dominated by the torsion of the N=N double bond. The simulations also provide the necessary information to compute the time-resolved fluorescence spectra and anisotropy, i.e. for a complete reproduction of the experimental results. We have obtained a good agreement with the measured time-dependent intensities and anisotropies, but our explanation of the mechanism partly differs from that put forward in the experimental work.