Azobenzene-based molecular photoswitches have led to the design of novel photoresponsive, functional soft-matter systems involving microgels, polymer brushes or DNA compacting agents. When microgel particles are immersed into a solution of azo-containing surfactants, cis-trans isomerization within the latter allows to change size and shape of the soft colloid just by applying an optical stimulus. Whereas the optically induced switching process is fast and occurs via excited electronic states, thermal relaxation of the azo-moiety is determined by rare barrier crossing events on the ground state Born-Oppenheimer surface. A rational design of functional azo derivatives also requires a comprehensive theoretical understanding of their relaxation mechanism as it crucially affects the stability of the corresponding material phase. Although standard computational approaches employing Eyring’s transition state theory (TST) are straightforward to apply, they have limitations when a complex environment of the azo moiety is to be considered, for instance a polymer network or extensive substituents. Here we connect to a recent TST study on a series of azo derivatives, and consider two distinct examples: pure azobenzene (AB) and a push-pull (ppAB) compound in a QM/MM setting in which the solvent is treated at the MM level, in combination with a transition path sampling approach. The path ensembles for ppAB reveal that cis-trans isomerization assumes a more pronounced rotation-like character when the solvent is changed from toluene to DMSO. On the other hand, even for AB where TST predicts a pure inversion mechanism, a small admixture of rotation is found for all solvent types. The two examples also show that in general the (iso-committor) reaction coordinate cannot be expressed as a combination of variables pertaining to the azo molecule alone, but must involve solvent degrees of freedom.

Thermal isomerization relaxation of azobenzene derivatives studied with QM/MM and transition path sampling

253rd National Meeting (2017)

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