The Photoisomerization Mechanism of Azobenzene: A Semiclassical Simulation of Nonadiabatic Dynamics

• Published in: Chemistry : a European journal 2004-05, Vol.10 (9), p.2327-2341
• Main Authors: Ciminelli, Cosimo, Granucci, Giovanni, Persico, Maurizio
• Format: Article
• Language: English
• Subjects: azobenzene; nonadiabatic dynamics; photoisomerization; semiclassical wavepacket dynamics; semiempirical calculations
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200305415

We have simulated the photoisomerization dynamics of azobenzene, taking into account internal conversion and geometrical relaxation processes, by means of a semiclassical surface hopping approach. Both n→π* and π→π* excitations and both cis→trans and trans→cis conversions have been considered. We show that in all cases the torsion around the NN double bond is the preferred mechanism. The quantum yields measured are correctly reproduced and the observed differences are explained as a result of the competition between the inertia of the torsional motion and the premature deactivation of the excited state. Recent time‐resolved spectroscopic experiments are interpreted in the light of the simulated dynamics. The semiclassical surface hopping approach was used to simulate the nonadiabatic photoisomerization dynamics of azobenzene. The preferred mechanism is the torsion around the NN bond in all four “simulated experiments”: n→π* or π→π* excitation, cis→trans or trans→cis conversion. We offer new interpretations of the observed trends in the photoisomerization quantum yields and of recent time‐resolved spectroscopic experiments.