Abstract
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 (nazoπ*2 and nazo2π*2 states), from a π delocalized molecular orbital (ππ* state), or from the lone pair of the N atom of the pyridine moiety (npyπ* state). Our results show that the mechanism proceeds mainly along the rotation coordinate in both the nazoπ* and ππ* excited states, although the nazo2π*2 state can also be populated temporarily, while the npyπ* 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 nazoπ* and ππ* potential energy surfaces, while the nazo2π*2 and npyπ* 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.
Highlights
Azo compounds constitute a well-known organic family with photochromic properties due to the easy interconversion between their trans and cis isomers, which have different absorption spectra.The parent system—azobenzene— and its derivatives have a wide range of applications due to the possibility to tune their properties through substitutions to control the rates of the forward and back reactions, their fatigue resistance, and their readiness to be incorporated in different kinds of material [1,2].Given its role as a molecular switch, azo compounds have been used in coordination complexes as a trigger of photoresponsive properties
Using a code designed in our group [19], the vertical energies and transition dipole strengths of the nine lowest excited states were computed at 400 snapshots along the molecular dynamics (MD) trajectory using the SA-CASSCF/CASPT2 protocol, introducing the effect of the solvent with the polarizable continuum model (PCM)
In this work we have studied the photoisomerization of 3-phenylazopiridine by ab initio calculations to elucidate the details of the reaction mechanisms that explain the experimental evidence compiled in the literature
Summary
Azo compounds constitute a well-known organic family with photochromic properties due to the easy interconversion between their trans and cis isomers, which have different absorption spectra. Given its role as a molecular switch, azo compounds have been used in coordination complexes as a trigger of photoresponsive properties. Isomerization of phenylazopyridine (PAPy) (Scheme 1) plays a fundamental role in the isomerization ofswitchable phenylazopyridine (PAPy)of One of these complexes, where where the photoreactive ligand is the 3-PAPy, has already been investigated by our group [3].
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