Trans-cis isomerization energies of azopyridines: a calorimetric and computational study

Azobenzenes undergo reversible trans-cis photo-isomerization and have been studied extensively as photo-responsive materials. Despite their similar photochemistry, azopyridines have received relatively little attention; for example, their isomerization energies are presently unknown. In comparison with azobenzenes, azopyridines offer additional opportunities for materials design through hydrogen bonding and coordination chemistry. Here we report the isomerization energies for all three symmetrical azopyridines (i.e., the 2,2'-, 3,3'-, and 4,4'-isomers) through a combined experimental and computational study. Heat of isomerization was measured in the liquid state, with o-terphenyl introduced to suppress crystallization. We obtain a dagger E (iso) = 25.2 +/- 0.6, 42.6 +/- 0.6, and 35.0 +/- 1.8 kJ mol(-1) for 2,2', 3,3', and 4,4'-azopyridine, respectively. For azobenzene, we obtain a dagger E (iso) = 47.0 +/- 1.3 kJ mol(-1), in agreement with the literature value and validating our method. Theoretical calculations yielded gas-phase a dagger E (iso) in reasonable agreement with experiment and explain the low isomerization energy of 2,2'-azopyridine on the basis of a low-energy cis conformer. Because of the smaller van der Waals volume of the pyridine N relative to the phenyl CH, the two aromatic rings in the cis isomer can approach closer to coplanarity, leading to greater pi-conjugation and lower conformational energy.