Toward Fractioning of Isomers through Binding-Induced Acceleration of Azobenzene Switching

The <i>E</i>/<i>Z</i> 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 <i>Z</i> → <i>E</i> 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 π_g* antibonding character. This results in a reduction of the NN torsional barrier and thus in accelerated thermal <i>Z</i> → <i>E</i> isomerization. Combined with light-controlled <i>E</i> → <i>Z</i> isomerization, this enables controllable fractional tuning of the two configurational isomers.