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The design of P-type photoswitches with thermal stability of the metastable form of hundreds of years that would efficiently transform using excitation wavelengths above 350 nm remains a challenge in the field of photochromism. In this regard, we designed and synthesized an extended set of 13 pyridine/quinoline hydrazones and systematically investigated the structure-property relationships, defining their kinetic and photoswitching parameters. We show that the operational wavelengths of the pyridine hydrazone structural motif can be effectively shifted toward the visible region without simultaneous loss of their high thermal stability. Furthermore, we characterized the ground-state and excited-state potential energy surfaces with quantum-chemical calculations and ultrafast transient absorption spectroscopy, which allowed us to rationalize both the thermal and photochemical reaction mechanisms of the designed hydrazones. Whereas introducing an electron-withdrawing pyridyl moiety in benzoylpyridine hydrazones leads to thermal stabilities exceeding 200 years, extended π-conjugation in naphthoylquinoline hydrazones pushes the absorption maxima toward the visible spectral region. In either case, the compounds retain highly efficient photoswitching characteristics. Our findings open a route to the rational design of a new family of hydrazone-based P-type photoswitches with high application potential in photonics or photopharmacology.

The photoswitching behavior of 5-phenylazopyrimidines was investigated by optical methods and NMR spectroscopy with in situ irradiation sustained by mathematical modeling and DFT calculations. Irradiation of various compounds with electron-donating groups on the pyrimidine ring and substituents with electron-withdrawing as well as electron-donating substituent in the para-position of the phenyl ring were examined. All compounds could be successfully converted to the cis isomer; this isomerization and the subsequent thermal fading were studied. Switching cycles can be repeated without signs of photodegradation for most of the compounds, which makes them adept to molecular photoswitches. Interestingly, the chloro and cyano derivatives can be switched without UV light, which makes them vis(π → π*)-vis(n → π*) photoswitches. Surprisingly equal trans-to- cis photoisomerization quantum yields for π → π* and n → π* excitation indicate the blocking of the inversion pathway following π → π* excitation. In contrast to that, DFT computations suggest the inversion mechanism for the reverse thermal cis-to- trans isomerization of 5-phenylazopyrimidines.