First principles studies of cis-trans photoisomerization dynamics and excited states in ethylene, stilbene, azobenzene and TATB.

摘要

Chemical reactions that involve multiple electronic states can be stimulated by the absorption of light by a molecule. Photochemical reactions are plentiful in nature and technologically useful but, as a group, are not well understood. We have studied the photochemistry of three paradigmatic compounds, ethylene, stilbene and azobenzene, using first principles molecular dynamics calculations. After absorption of a photon, the ethylene molecule twists about the central C–C bond. Pyramidalization of one CH₂ group allows access to a conical intersection through which population quenches nonradiatively to the ground electronic state. The vibrationally hot ground state molecule dissociates to form acetylene and either molecular or atomic hydrogen. Stilbene and azobenzene can undergo photoisomerization between cis and trans isomers, converting light into mechanical energy. In stilbene, non-radiative quenching to the ground state can occur at a twisted and pyramidalized conical intersection that is remarkably similar to that of ethylene. Photocyclization of cis-stilbene, giving 4a,4b-dihydrophenanthrene, competes with the photoisomerization pathway and likely occurs through a conical intersection of S₀ and S₁ in the cis-stilbene configuration. The dynamics of trans-azobenzene induced by photo-excitation to each of the four lowest singlet excited states (S ₁–S₄) help explain the results of pump-probe photoionization spectroscopy and elucidate the photoisomerization mechanism. Population can transfer from S₂, S₃ or S₄ to S₁ while the molecule remains in its planar trans configuration. Internal conversion to the ground state can occur via an inversion or torsional pathway, but the former route is only accessible following photo-excitation to S₂ and higher lying states.; In principle, the energy required for electronic excitation may also arise from collisions with one or more nearby molecules. One might suggest an importance of electronic excitation in chemical processes at high temperatures and pressures, such as detonation. This possibility was tested in solid 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) by comparing the pressures required for metalization and detonation. Our results indicate that solid TATB detonates at lower pressures than those required for metalization.