Switchable NLO response induced by redox properties of Keggin-type polyoxometalate hybrids [PX11O39{Sn(p-C6H4I)}]4- (X = Mo, W)

Two host-guest grown ether supramolecules show switchable phase transition, dielectric and second-harmonic generation effect

The excellent properties of host-guest crown ether inclusions in phase transition, dielectric and second-order nonlinear optical properties have attracted much attention. In this paper, we successfully designed and prepared two novel host-guest crown ether supramolecules [(DFBA)(15-crown-5)]X (X = ClO₄^-, 1; ReO₄^-, 2) by reactions of 2,6-difluorobenzylamine (DFBA) with 1,4,10,13-pentaoxacyclopentadecane (15-crown-5) in HClO₄, or HReO₄ aqueous solution. By the introduction of difluoro-substituted benzylamine as a guest cation, the phase transition temperatures are greatly increased to 377 K for 1 and 391 K for 2. More importantly, the space group of 1 has changed from centrosymmetric (CS) P2/c to the non-centrosymmetric (NCS) Pca2₁ in 2 when substituting perchlorate (ClO₄^-) with the larger and heavier perrhenate (ReO₄^-), which leads to 2 showing a switchable and stable second-harmonic generation (SHG) effect. According to the principle of momentum matching between a cation and anion, the perrhenate group increases the energy barrier of the molecular thermal motion, which not only significantly increases the phase transition temperature of 2 but also causes it to be frozen and crystallized in a NCS space group at room temperature. This research demonstrates that a polar molecule can adjust the suitability of anions and cations inside the crystal by practical chemical means.