“Cold” photo-induced crystal-to-liquid transition of acylhydrazones: leveraging slight molecular modification to expand the usable temperature range

Abstract

The relationship between the eutectic behavior and photoinduced crystal-to-liquid transition (PCLT) of acylhydrazone derivatives was explored through the comprehensive analyses of their chemical and physical changes using microscopy, spectroscopy, and quantum chemical calculations. Both E and Z isomers were successfully isolated because of the high E-to-Z conversion efficiency of acylhydrazones, thereby allowing thermal analyses of E/Z binary mixtures for each derivative. This study revealed that acylhydrazones undergo PCLT at temperatures below the eutectic point of the binary mixtures, which we termed “cold PCLT”. A detailed investigation revealed that cold PCLT was facilitated by an appropriate molecular design for diversifying metastable conformers during photoisomerization, liquefying at a significantly lower temperature than the eutectic point of the E/Z binary system. Introducing specific side chains retarded the crystallization of the Z isomer, which made the cold PCLT recognizable over a wider temperature range. These fundamental insights into cold PCLT are expected to aid the design of advanced phase change materials and control the usable temperature range.