Photo-and thermoresponsive N-salicylideneaniline derivatives: solid-state studies and structural aspects

N-Salicylideneaniline (SA) and its derivatives are known to possess chromism upon exposure to external stimuli. Herein, we present mechanochemical synthesis of a series of photo-and thermoresponsive SA-derivatives and report on solid-state stabilisation of their tautomeric forms either by change in temperature or by photoirradiation. The influence of UV light on proton transfer between the enol-imine (EI) and keto-amine (KA) forms was investigated at lambda(1) = 254 and lambda(2) = 365 nm. Differential scanning calorimetry (DSC) measurements provided extra information on the thermodynamic relationship between the prototropic tautomers, and their exposition to liquid nitrogen, combined with variable temperature single-crystal X-ray diffraction (VT-SCXRD) and spectroscopic data, ascertained structural reasons for the intrinsic thermo-optical properties of the compounds. A series of structural determinations between 150 and 300 K further shed light on the thermomechanical behaviour exhibited by the thermoresponsive compounds. By virtue of calorimetry we were able to demonstrate proton transfer via the intramolecular OMIDLINE HORIZONTAL ELLIPSISN hydrogen bond over the temperature range 193-453 K. This present work demonstrates the importance of applying complementary analytical techniques and appropriate approaches for understanding the switching behaviour between the EI and KA forms. Furthermore, the assertion that it is predominantly the planarity (phi < 25 degrees) that determines thermochromaticity is questioned.

Automated summary

This study presents the mechanochemical synthesis of photo- and thermoresponsive N-Salicylideneaniline derivatives and investigates their solid-state stabilization through temperature variation and photoirradiation. It demonstrates the influence of UV light on proton transfer between different tautomeric forms and provides insights into the thermomechanical behavior and thermochromaticity. The importance of applying complementary analytical techniques for understanding the switching behavior between different forms is emphasized.