The E/Z isomerization process of a uracil–azobenzene derivative in which the nucleobase is conjugated to a phenyldiazene tail is studied in view of its ability to form triply H-bonded complexes with a suitably complementary 2,6-diacetylamino-4-pyridine ligand. UV–vis and 1H NMR investigations of the photochemical and thermal isomerization kinetics show that the thermal Z → E interconversion is 4-fold accelerated upon formation of the H-bonded complex. DFT calculations show that the formation of triple H-bonds triggers a significant elongation of the N═N double bond, caused by an increase of its πg* antibonding character. This results in a reduction of the N═N torsional barrier and thus in accelerated thermal Z → E isomerization. Combined with light-controlled E → Z isomerization, this enables controllable fractional tuning of the two configurational isomers.
