Photochromic, Thermosalient, Photophysical, and Electrochemical Properties of Pyridyl–Quinolinyl–Ethylene-Derived Multifunctional Organic Materials

This study offers a comprehensive structure-property correlation of a novel olefin <i>E</i>-1-(4'-pyridyl)-2-(4″-quinolinyl) ethylene (<b>PQE</b>) and its <i>N</i>-methylated derivative (<b>MPQE</b>) and demonstrates how crystal packing, hydration, and conjugation collectively dictate their solid-state photoreactivity and photochromic, thermosalient, photophysical, and electrochemical properties. <b>PQE</b> exhibited a quantitative [2 + 2] photocycloaddition reaction forming its <i>head-to-tail</i> dimer, which further exhibited a reversible cleavage of the cyclobutane ring. Crystals of <b>PQE</b> exhibited a color change under UV light, indicating photochromic behavior. Two different crystal forms, namely, <b>MPQE</b> and hydrated <b>MPQE·2.25H</b>_<b>2</b><b>O</b>, were obtained by tuning the crystallization medium. Despite having suitable stacking of <b>MPQE</b> cations, <b>MPQE·2.25H</b>_<b>2</b><b>O</b> remained photoinert; however, it exhibited a thermosalient behavior due to dehydration-induced lattice strain. <b>MPQE</b> displayed bathochromic spectral shifts in comparison to neutral <b>PQE</b>, indicating the effect of <i>N</i>-quaternization. The dimer, <b>BPBQCB</b>, displayed hypsochromic and hypochromic spectral shifts compared to <b>PQE</b>, for loss of conjugation. Their redox characteristics have been explored. Such observation of multifunctional behavior is rare and offers potentials for various applications.