Study On The Photochromism Of Calixarene-Schiff Bases

Information technology has revolutionized daily life in the last decades and the continuously increasing amount of data to be stored and manipulated strongly stimulated the search for switching and memory elements as tiny as a single molecule. Molecular switches can be converted from one state to another by an external stimulus such as light, electricity and a chemical reaction. Like with their macroscopic counterparts, one is able to control numerous functions and properties of materials and devices. Molecular switches will be potentially applied in the fields of computer chips, biomedicine and so on. In this paper, we synthesized three host compounds by linking two photochromic groups-imine with good photoswitchable properties and calixarene cavities which are able to recognize molecules and ions. Our mainly work includes synthesis of target molecules and some research on molecular recognition and molecular switches. The detailed results are listed as follows:1. Three double arms-host compounds were synthesized by incorporating two photochromic imine groups into the upper rims of calix[4]arene framework. These compounds were identified by IR, 1H NMR, 13C NMR, MALDI-TOF MS and Element analysis. The 1H NMR data reveal that these host compounds are in the cone conformation.2. Photochromic properties were systemically studied in dichloromethane by UV/Vis spectra. (i) The series of Schiff base-calix[4]arenes showed favorable photochromic property in dichloromethane. (ii) The calixarene which was incorporated into the Schiff bases can weaken the effects ofπ-πstacking and effectively improve the photochromic properties of Schiff bases. (iii) The strong withdrawing electron function of nitro group could change the characteristic of intramolecular hydrogen bonding from the neutral hydrogen bonding ([O-H…N]) to the ionic hydrogen bonding ([-δO…H-N+δ]), thus decrease the photochromic ability of Schiff base. (iv)The intermolecular hydrogen bonding of the tautomer-O…H-solvent helps the equilibrium shift towards the quinone form in dichloromethane; however, the enol-zwitterion mixed dimmers prevented the equilibrium from shifting to the quinone form in CH3CN. (v) These compounds show favorable photochromic properties with only one internal H-bond transfer mechanism.3. The host compounds HN and HH can selectively recognize lanthanide ions- Dy3+ over alkali metal ions, alkali earth metal ions, transition metal ions or other lanthanide ions (such as La3+, Pr3+, Eu3+, Gd3+, Er3+, Yb3+). Upon adding the lanthanide ion-Dy3+ to the host solutions, the both solutions changed the color from colorless to yellow. It is interesting that the double methoxyl arms-host (HM) compound (HM) can not only selectively recognize lanthanide ions-Dy3+ with the color of the solution changed form colorless to pink, but also recognize Er3+ with the color changed to pale yellow after stood up for 24 hours in the dark condition. Therefore, the double arms-host (H) can selectively recognize Dy3+ by naked eyes. And the reason of the Er3+ mixed solution changed color after 24 hours could be that the binding for Er3+ is a slow process.4. It suggests that the model 7 showed poor selectivity for the lanthanide ions. And the host compounds 4 can selectively recognize Dy3+ with naked eyes implies that the recognition process is relative to the size-fit effect. The introduction of calix[4]arene blocks increase the selectivity of compounds 4, and it enhances the coordination ability simultaneously for the calix[4]arene cavity playing an important role for the stability of the complexes.5. A nonlinear least-squares analysis provided the stoichiometry of the complex formed by double nitro arms-host compound (HN) and Dy(NO3)3 was 1:1 (HN: Dy3+) and the stability constant log K=5.41±0.2 with correlation coefficient 0.9857; the stability constant log K=5.61±0.4 with correlation coefficient 0.9884 whose stoichiometry was 1:1 (HH: Dy3+) for double H arms-host compound (HN). The analysis provided the stoichiometry of the complex formed from the double methoxyl arms-host compound (HM) and Dy(NO3)3 was 1:1 (HM: Dy3+) and the stability constant log K=5.65±0.14 with correlation coefficient 0.9899; the stability constant log K=5.77±0.06 with correlation coefficient 0.9804 whose stoichiometry was 1:1 (HM: Er3+) for double methoxyl arms-host compound (HM) and Er(NO3)3. 6. The host compound (HN) can selectively recognize the ion-Copper (II) over other transition metal ions (Pb2+, Zn2+, Co2+, Ni2+, Fe3+) and alkali earth metal ions (Ca2+, Mg2+). The stoichiometry of the complex formed from the double nitro arms-host compound and Cu(NO3)2 was 1:1 (HN: Cu2+) and the stability constant log K = 5.2±0.2 with correlation coefficient 0.9957.7. After irradiation of ultraviolet light, fluorescence intensity of the superamolecular system HN-Cu2+ was increased, and it was decreased by irradiation of visible light. The mechanism of photoswitching fluorescent intensity may involve internal charge transfer (ICT), tautomerization of the host HN (between enol-form and keto-form) and the coordinate-release of metal ions. However, after irradiation of ultraviolet light, fluorescence intensity of the superamolecular system HH-Dy3+ was decreased; it was increased by irradiation of visible light. The mechanism of photoswitching fluorescent intensity may be relative to fluorescence resonance energy transfer (FRET) induced by Co-absorption and Re-absorption between the free ligands and HH-Dy3+ complexes. These processes are reversible. It suggests that the fluorescence intensity of two complexes may be switched by light.