Study On Self-assembly Between Viologen Derivates And Cucurbit [8] Uril And Their Applications

The new host family cucurbit[n]uril (CB[n], n=5-10) have a hydrophobic cavity and two identical carbonyl-laced portals. For its properties, cucurbiturils have attracted many scientists’ interests. In contrast to CB[5]-CB[7], the voluminous cavity of CB[8] exhibits remarkable host-guest properties including the encapsulation of two identical guests or a hetero-guest-pair inside the cavity. This discovery led to build several novel supramolecular assemblies such as supramolecular amphiphiles that led to vesicles, molecular loops, supramolecular polymers on surfaces, and molecular necklaces.In this paper, the synthetic process of cucurbituril was simplified and a new intramolecular CT compound PHEV was studied, where electron donor 4-hydroxylphenol was linked to electron acceptor viologen. The results showed that PHEV was assembled with CB[8] by the ratio 1:1, where PHEV molecule was folded, phenol and viologen were forming charge-transfer (CT) complex in the cavity of CB[8]. If PHEV2+ was reduced to PHEV+·cation radical, two radical-assembly forms would exsist in CB[8]:viologen radical dimer and phenol viologen radical partner, and there exsisted a dynamic balance between two forms in solution. This conclusion was confirmed by NMR, electrochemical and uv-vis spectra, where the dynamic balance between viologen cation radical and CB[8] is first reported.To further investigate the influence on the self-assembly between viologen and CB[8] by electron donor of aromatic hydrocarbon, a series of D-A compounds (PEV, AEV, NEV and BEV) was synthesized, where aromatic hydrocarbons (such as pyrene, naphthalene, benzene, anthraquinone) were used as electron donor and viologen was used as electron acceptor. These componds could bind CB[8] with 1:1 confirmed by NMR, electrochemical and UV method. With the addition of CB[8], the fluorescence of aromatic-viologen compounds were increased, which was due to the electropositivity of viologen cation partly neutralized by the electronegativity of CB[8] portals, resulting in the decreasing the of fluorescence quenching by viologen. The results indicated that the benzene ring of BEV and part of viologen moiety were in CB[8] cavity and other viologen moiety was kept outside the cavity of CB[8]. The strctures of PEV-CB[8], AEV-CB[8] and NEV-CB[8] solutions were showed that viologen moiety was included in CB[8] cavity and-CH3 was kept outside the cavity of CB[8], where anthyl or naphthyl also enter into CB[8] cavity by charge transfer interaction between electron-donor and electron-acceptor. Considering theπ-conjugate aromatic moiety and the redox property of viologen moiety, intramolecular electron-transfer (ET) process would exsist after light irradiation of aromatic-viologen compounds. The investigation of the interaction between the compounds and CT DNA showed that PEV or AEV can bind CT DNA with strong intercalation, while NEV and BEV also binded DNA with weaker interaction. With the presence of CB[8], the binding interaction between viologen derivatives and CT DNA was stronged, especially in NEV-CB[8] or BEV-CB[8] complexes and CT DNA systems.Because of the fluorescence increasing of the including complexes and the appeatance of viologen cation radical in absorption spectra under light irradiation, the complexes were used to compared with violgen compounds without CB[8] for DNA photocleavage. The result showed that CB[8] can increase DNA photocleavage efficiency by violgen compounds. Electron transfer would exist from excited aromatic ring to viologen acceptor of the compounds under irradiation, which gives a charge-seperated (CS) excited states Aryl+·～EV+·where EV+·would reduce dissolved oxygen in an aqueous solution to generate a reactive oxygen species to oxidize DNA together with the Aryl+·radical. The lifetime of EV+·is very short, owing to the existing backwards intramolecular ET. However, the inclusion of CB[8] can inhibit the backwards intramolecular ET to some extent, prolonging the lifetime of EV+ and eventually increasing the photocleavage efficiency. The mechanism study show that Aryl+·,·OH and 1O2 were the reactive components to cleave plasmid DNA. The results demonstrate the applicability of this type of host-guest chemistry, providing a general methodology to construct a light sensitized DNA photocleavage system by host-guest chemistry.