Viologen/Pyrenetetrasu1Fonic Acid Charge Transfer Crystals: Preparation And Photoconductive Property

Charge transfer interaction, which usually occurs between the electron-rich molecules and electron-withdrawing molecules, is a very important intramolecular interaction for supramolecular chemistry. After the first systematic describing in1950s, tens of thousands studies were focused on this interaction, especially in recent decades for the ultrafast development of supramolecular chemistry. From the self-assembly behavior in solution to the crystal engineering in solid state, charge transfer interaction involves everywhere in supramolecular science. It should be mentioned that a large number of organic fuctional materials were developed and preperated based on the charge transfer interactions, so more and more attention now is paid to the charge transfer interactions for designing and investigating micro/nano photoelectric functional materials.To avoid the complicated molecular strucrture and the sophisticated synthesis, we try to use molecules with simple structures and easy availability to construct the charge transfer complexes. In our study, we choose viologen molecules which can be synthetized concisely as acceptors and commercial1,3,6,8-pyrenetetrasulfonic acid tetrasodium salt (PyTs) as donor.Firstly, the recognition and self-assembly behavior of charge transfer pair containing butyl viologen (BuV) and PyTs was investigated. The obvious fluorescent quenching behavior could be found in the aqueous solution, which showed a typical photoinduced charge transfer interaction between BuV and PyTs, and the quenching behavior was fit for the mechanism of "action of sphere", which means the donors and acceptors were closely to each other by electrostatic interaction, then photoinduced charge transfer happened to result in fluorescent quenching. The tandem mass spectrometry (MS/MS) was used to investigate the detailed structures of the charge transfer aggregates, the MS result showed a kind of face-to-face dimerization stacker with donor and acceptor, this could be also conformed by2D NOESY spectrum. In the second stage of MS/MS, the ionic product peak of dimer stacker was chosen to the later analysis, the dynamic stacking-destacking process was showed via varied bombardment voltages. The use of the tandem mass spectrometry in this energy depending stacking-destacking process showed a noval powerful tool to investigate the charge transfer supramolecular structures in methodology. Compared with the random aggregates constructed by electrostatic interaction, the dimerized aggregates of BuV and PyTs by charge transfer interaction directed the forming of the further highly ordered, which could provide a possibility to study the propertities of organic functional crystallized materials in solid states.Secondly, a single crystal of BuV-PyTs was prepared by a solvent evaporation method. The single crystal X-ray diffraction analysis showed one PyTs molecule and two BuV molecules per unit and only one was stacked with PyTs, importantly, the stacking of BuV and PyTs was a mixed stacking mode. XPS and Raman spectra showed a detailed change of molecular structures in the crystal lattice, this change of molecular structures was a key factor to indicate the existence of change transfer interaction in solid state, the UV diffuse reflectance also supported this inference. Comparing the study of the assembly behavior in solution, the progress of preparing the crystal focuses on the charge transfer interaction in solid state directly, which conduces to comprehend this intramolecular interaction deeply.Thirdly, the crystal device of BuV-PyTs ID charge transfer single crystal fiber was prepared by micro/nano-fabrication technologies successfully. TEM and SAED showed the stacking direction was corresponding to the growth direction of crystal fiber. The photoconductive measurement of the crystal device with the mixed stacking mode showed a photocurrent, which exhibited a strongly sublinear dependence on the voltage and it reached ca.2.25nA at a bias of30V. The photocurrent could be switched promptly several times by irradiated light-on and-off at a bias of30V, without any appreciable attenuation at an on/off ratio about100. So the fast photoresponsibility and the high stability under the light turning on and off implied the charge transfer crystal with the molecular heterojunctions in the lattice and the unique "radical cation-radical cation"interaction in the direction of the mixed stacking profitable for application in organic optoelectronic devices. The mixed stacking mode in the charge transfer crystal with latticelike distribution of molecular heterojunctions is a idealized model for further theoretical studies as well.At last, a series of derivatives of viologen with different R groups or topological molecular structure were designed and synthesized successfully, basing on which the several kinds charge transfer crystals were prepared successively. UV spectra showed the different of the energy gap for viologen molecules which was influenced by the R groups, and the difference of the molecular structure could controlled the charge transfer behavior of viologens. In crystals, intermolecular bond lengths and intramolecular rotating angles conformed the crystal structures influenced by the steric hindrance and electronic effect obviously. ESR spectra showed confined electron behaviors in charge transfer crystals, which was potentially applied on generation and stabilization of radicals within the charge transfer crystals. FC and ZFC M-T curves showed a branch between the two curves of BuV-PyTs, it was also related on the spin behavior of the electron in crystal. A novel topological structure of covalent-linked viologen dimer was synthetized as HexdBuV. The quenching behavior of this acceptor molecule was more obvious, and the structure of the charge transfer crystal did not have the stacking structure but a layer-by-layer structure. That was the result of the covalent-linked structure which favored the progress of ordered constructing more prompt and efficient. The influence of steric, electronic effect and the topological structure on the crystal structure indicate that the charge transfer interaction between viologens and the derivatives of pyrene is a very important phenomenon existing extensively, and it lays a good foundation for the further study for this kind of charge transfer system.