Institute of Organic Chemistry
and Biochemistry of the CAS

Photochromic systems with ultrahigh rate of thermal relaxation are highly desirable for development of new efficient photochromic oscillators. Based on DFT calculations, we designed a series of 5‐phenylazopyrimidines with strong push‐pull character in silico and observed very low energy barriers of thermal (Z)‐to‐(E) isomerization. The structure of the (Z)‐isomer of the slowest derivative in the series was confirmed by NMR with in situ irradiation at low temperature. The substituents can tune the lifetime of thermal back isomerization from hundreds of ms to several ns (8 orders of magnitude). The photoswitching parameters were extracted from transient absorption techniques and a dominant rotation mechanism of the (Z)‐to‐(E) thermal fading was proposed based on DFT calculations.