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 E-1-(4′-pyridyl)-2-(4″-quinolinyl) ethylene (PQE) and its N-methylated derivative (MPQE) and demonstrates how crystal packing, hydration, and conjugation collectively dictate their solid-state photoreactivity and photochromic, thermosalient, photophysical, and electrochemical properties. PQE exhibited a quantitative [2 + 2] photocycloaddition reaction forming its head-to-tail dimer, which further exhibited a reversible cleavage of the cyclobutane ring. Crystals of PQE exhibited a color change under UV light, indicating photochromic behavior. Two different crystal forms, namely, MPQE and hydrated MPQE·2.25H2O, were obtained by tuning the crystallization medium. Despite having suitable stacking of MPQE cations, MPQE·2.25H2O remained photoinert; however, it exhibited a thermosalient behavior due to dehydration-induced lattice strain. MPQE displayed bathochromic spectral shifts in comparison to neutral PQE, indicating the effect of N-quaternization. The dimer, BPBQCB, displayed hypsochromic and hypochromic spectral shifts compared to PQE, for loss of conjugation. Their redox characteristics have been explored. Such observation of multifunctional behavior is rare and offers potentials for various applications.