| As a kind of novel supramolecular self-assembly materials,functional small molecule gel can form ordered microstructure by non-covalent bonding to bind organic solvents.Azobenzene compounds can be applied to biomedical,drug release,environmental management and other fields because of their unique photoisomerization.They are widely concerned by researchers.Therefore,using azobenzene as functional group to synthesis highly efficient and stable photoresponsive gel has important scientific significance and application value.In this paper,six novel azobenzene gelators with melamine groups were designed and synthesized on the basis of literature review.The gelation properties of azobenzene gelators were analyzed.The microstructure and photoreaction behavior were investigated.The details are as follows:1.Three kinds of azobenzene compounds containing carboxyl groups were synthesized by using 4,4'-dihydroxyazobenzene,4,4'-diaminodiphenyl ether and 4,4'-diaminodiphenylmethane as raw materials.The azobenzene compounds were condensed with melamine derivatives to form the monoazobenzene gelators AZO-1 and AZO-2 and bisazobenzene gelators D-AZO-1,D-AZO-2 and D-AZO-3,D-AZO-4.Their molecular structure were confirmed.2.The gelation behavior of six gelators in organic solvents was investigated.It was found that they could form gel in a variety of organic solvents,and their gelation properties were good.The UV spectra of AZO-1 and AZO-2 in gel state showed that the blue shift wavelength of AZO-2 was significantly larger than that of AZO-1.It indicated that the non-covalent bonding of AZO-2 in solvents were stronger than AZO-1,which reflected better gelation properties than AZO-1.The concentration of bisazobenzene gel formed in most solvents were less than 5 mmol/L.The average gel-sol transition temperature is higher than 60 'C.The gelation properties of bisazobenzene gel was stabler than that of monoazobenzene gel.3.Compared with the solution state,the UV spectra of azobenzene gel showed a certain degree of blue shift,which indicated that gelators formed H-aggregates in organic solvents.The absorption peak positions of xerogels formed in different solvents were obviously different,which indicated that solvent has a certain influence on the formation of gel.The fibers in the transparent gels,which formed in cyclohexane,methylcyclohexane and xylene,packed tightly.And the fibers in the the opaque gels,which formed in ethyl acetate,acetone and ethanol,intertwined to form looser three-dimensional network structure.4.The photoreaction behavior of azobenzene gelators was characterized by UV absorption spectroscopy and scanning electron microscopy.The trans-structure of azobenzene gelators in the chloroform solution transition to the cis-structure under UV light irradiation.The UV spectrum showed that the absorption peak of trans ?-?*gradually decreased and the absorption peak of n-?*enhanced.The phase of azobenzene gel changed a little after UV irradiation.Gelation behavior of the azobenzene gelators formed after heating and dissolving in the organic solvent and irradiating by UV light for a certain time has been investigated.It could form gel but the color became significantly darker.The characteristic absorption peak position of the gel changed a little after the UV irradiation.It can be seen that the close stacking of trans structures limited the transition of photo-induced cis-trans isomerization.The fibers of gels formed in the polar solvents have thinner orientation after the UV irradiation.And the stacking of fiber is more closely.The results above showed that the azobenzene gelators with melamine groups have excellent gelation properties.Especially the bisazo gelators showed higher efficiency and heat resistant characteristic in some solvents.Although six azobenzene gels did not show the expected photo-phase transition.It also provided scientific basis for further exploration of light responsive azobenzene gel,which is helpful for the development and application of small molecule functional gel. |
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