High-Temperature Switchable Nonlinear Optical and Dielectric Material Revealed by Molecular Modification

Switchable materials with a high phase transition (PT) temperature have drawn much attention in the past few decades due to the wide application in electronic devices. Herein, 2-methyl-2-azabicyclo[2.2.2]­octan-2-ium perrhenate ([N-Me-isoQH]­ReO₄) and 2-azabicyclo[2.2.2]­octan-2-ium perrhenate ([isoQH]­ReO₄) were designed and synthesized by molecular modification based on the reported ferroelectric compound quinuclidinium perrhenate (HQReO₄). Their crystal symmetries at room temperature decrease mainly owing to the cation-ion interaction and hydrogen-bond interaction between the cations and anions from [N-Me-isoQH]­ReO₄ to the reported HQReO₄ and to [isoQH]­ReO₄. However, their PT transition temperatures increase in the order of [N-Me-isoQH]­ReO₄, HQReO₄, and [isoQH]­ReO₄. [isoQH]­ReO₄ showed a high PT temperature of 393 K, which is 48 K higher than that of reported HQReO₄, and experienced a symmetry change from noncentrosymmetric point group C₂ to centrosymmetric point group C_4h. Its reversible dielectric and SHG behaviors make it a promising high-temperature dielectric and NLO molecular switch.