Photoswitchable Intramolecular Hydrogen Bonds in 5-Phenylazopyrimidines Revealed By In Situ Irradiation NMR Spectroscopy.

NMR spectroscopy with in situ irradiation uncovered unique photoswitchable intramolecular hydrogen bonds (IMHBs) in 5-phenylazopyrimidines with two hydrogen bond donors. These compounds form two stable rotamers, each with one IMHB, and the rotamer ratio changes reversibly upon UV or visible light irradiation. Strong substituent dependence of photoinduced structural changes was observed; using suitable substituents, orthogonal photoswitching can be achieved. For example, whereas UV irradiation caused switching between the two rotamers of the trans isomer of a compound with electron-donating methoxy substituent, visible light enabled to obtain the cis photoisomer. No cis isomer was detected for compounds with electro-neutral or electron-accepting substituents, but photoswitching between the two trans isomers was observed. On the other hand, compounds without hydrogen-bond donors or with one donor only formed stable cis isomers. A mechanism of the photoswitching was proposed by DFT computations.