The Design、 Synthese And Characteristics Of Order-disorder Phase Transition Compounds

Phase transition is usually associated with the changes of chemical composition,structure, properties or morphology triggered by the external conditions such as temperature, pressure, magnetic fields, etc. At the same time, there will be significant thermal change, dielectric anomaly and unusual SHG optical properties during the phase transition. In this case, phase transition materials have attracted much attention due to their developing applications in communications, memory devices and data storage et al. Organic phase transition materials are anticipated as inexpensive,environment friendly, easy-modified compared to inorganic phase transition materials.Therefore, exploration and preparation of the functional phase transition materials are one of the most attractive subjects in recent years. However, how to design structural phase transition materials with design and crystal assembly strategy is still a challenge.Order- disorder phase transformation is one of the main types of the structural phase transitions. The using of flexible alkylammonium moietives or anionic moieties is an important strategy for the construction of phase change compounds. In this case,the long-chain alkylammonium cation or the flexible cation, which tend to be disordered due to a relatively large freedom of motion and become ordered at lower temperature, will offer more opportunities and potentials to fabricate the functional phase transition materials. We used the flexible dipropylammonium(DPA) cation to design and construct novel functional phase transition materials by the molecular crystal engineering strategy. Three series of representative functional phase transition compounds were classified into three parts:(1). A new phase transition material dipropylammonium picrate(1) has successfully synthetized by simple reaction. 1 undergoes a reversible first-order phase transition at about 180 K, which has been confirmed by the differential scanning calorimetry(DSC). The calculated molecular degree of freedom is 1.55, which indicates the disorder-order character. The further crystal structure analyses reveal that the order–disorder transitions of the DPA cations and picrate anions and the distinct distortion of the ethyl moieties in the flexible chain-like DPA cations dominate the phase transition of 1. Particularly, the dielectric constants of 1 exhibit a marked step-like anomaly tuned between a high dielectric state in the high temperature phase and a low state in the low temperature phase, which is consistent with the characteristic of switchable molecular dielectrics.(2). By introducing the chiral moiety, we will combine the phase transition properties with the second harmonic generation properties to obtain multi-functional phase transition materials. Here, we use the dipropylamine and D-(+)-10-camphorsulfonic acid to obtain a new multi-functional phase transition compound dipropylammonium D-(+)-10-camphorsulfonate(2). DSC curves of 2present a pair of reversible peaks around 205 K with a heat hysteresis of 3.8 K during the heating/cooling cycle, indicating a reversible first-order phase transition character.Single crystal X-ray diffractions, determined at 130 K and 293 K, illustrate that 2crystallizes in polar space group P21 in both phases. Deeper microscopic structural analyses reveal that the dynamically order-disorder transformations of one propyl group in the dipropylammonium(DPA) cation as well as a distinct distortion of terminal ethyl upon cooling down play major roles of the structural phase transition.Step-like dielectric anomaly further confirms the occurrence of phase transition and reveals a switchable feature of dielectric constant in 2. Moreover, The SHG measurements indicate that 2 is phase-matchable material with NLO activities of about 2 times that of KDP.(3). At last, a new molecular phase transiton crystal material dipropylamine dichloroaceticacate(3) was synthesized. Compound 3 undergoes two sequential reversible phase transitions near 215 K and 148 K, respectively. DSC measurements indicate the second order phase transition feature near 215 K and first order character at 148 K. The structure at room temperature reveals that it belongs to space group of P21/c, while at 180 K, the crystal structure determination of the compound reveals that it still belongs to a monoclinic crystal system but with a noncentrosymmetric space group of Pc, which is one of the 88 kinds of ferroelectric phase transitions. The variable- temperature second harmonic generation(SHG) measurements further confirmed the change from the centrosymmetric phase to noncentrosymmetric phase.The microscopic structural analyses reveal that the origin of the phase transition is attributed to the order-disorder transformation of the dichloroaceticacate anion, which is distinct from the above two compounds.Above all, we used the flexible dipropylammonium to react with different anions by the method of crystal engineering and successfully assembled functional phase transition materials, which will provide useful strageties and theory supports for the designing of composite materials.