Phenylazopyridine as Switch in Photochemical Reactions. A Detailed Computational Description of the Mechanism of Its Photoisomerization

Azo compounds are organic photochromic systems that have the possibility of switching between cis and trans isomers under irradiation. The different photochemical properties of these isomers make azo compounds into good light-triggered switches, and their significantly different geometries make them very interesting as components in molecular engines or mechanical switches. For instance, azo ligands are used in coordination complexes to trigger photoresponsive properties. The light-induced trans-to-cis isomerization of phenylazopyridine (PAPy) plays a fundamental role in the room-temperature switchable spin crossover of Ni-porphyrin derivatives. In this work, we present a computational study developed at the SA-CASSCF/CASPT2 level (State Averaged Complete Active Space Self Consistent Field/CAS second order Perturbation Theory) to elucidate the mechanism, up to now unknown, of the cis–trans photoisomerization of 3-PAPy. We have analyzed the possible reaction pathways along its lowest excited states, generated by excitation of one or two electrons from the lone pairs of the N atoms of the azo group (n(azo)π*(2) and n(azo)(2)π*(2) states), from a π delocalized molecular orbital (ππ* state), or from the lone pair of the N atom of the pyridine moiety (n(py)π* state). Our results show that the mechanism proceeds mainly along the rotation coordinate in both the n(azo)π* and ππ* excited states, although the n(azo)(2)π*(2) state can also be populated temporarily, while the n(py)π* does not intervene in the reaction. For rotationally constrained systems, accessible paths to reach the cis minimum along planar geometries have also been located, again on the n(azo)π* and ππ* potential energy surfaces, while the n(azo)(2)π*(2) and n(py)π* states are not involved in the reaction. The relative energies of the different paths differ from those found for azobenzene in a previous work, so our results predict some differences between the reactivities of both compounds.