| Nonlinear optical materials with large third-order optical nonlinearities and sensitive response have been applied in optical signal processing,optical limiting,photonic switching and optical devices.The optical nonlinearity of azobenzene derivative is important for its application because of the large nonlinear optical coefficient and fast optical response.The photoisomerization of azo-containing molecules provides an easier way to modify their linear and nonlinear polarizability as well as optical nonlinear refraction.In the past decades,the investigation and improvement on the nonlinear response and property of optically controllable azobenzene molecules in solution and assembly structure have been paid considerable attention in fundamental science and application fields.Whereas,a large number of reported works about azobenzene derivatives were investigated in solution phase,and the macroscopic properties in film are severely dependent on the molecular assembly behaviors and structures.In this context,considerable efforts have been devoted to construct molecular materials with well-defined structures,which can be carried out via different techniques such as layer by layer and self-assembly.The Langmuir-Blodgett(LB)technique provides a good platform to construct molecular assemblies due to the molecular orientation and packing mode can be arranged in a controlled manner,which has been proved working in the fabrication of monolayer and multilayer structured films of azobenzene derivatives.In this work,we systemically studied the photoinduced isomerization behavior and the nonlinear optical properties of the BNB-t4 and AOB-t4 LB films,the involved physical and chemical mechanisms have been discussed as well in this work.The results can be listed as follows:1.The interfacial behaviors and film-forming performances of azobenzene derivatives BNB-t4 with amide groups and AOB-t4 with 1,2,4-oxadiazoles films were studied by LB technology.and its LB films' properties were studied by the methods of isothermal curve of the surface pressure-area(?-A),UV-visible spectroscopy and FT-IR spectra.The results showed that the mono-molecular film formed on the nickel nitrate solution is the most stable.According to the linear diagram of the maximum absorption band's intensity with the numbers of layers,the monolayers formed on the sub-phase were well-distributed,and their stability were good when transferred to the substrate.2.Compare the UV-visible absorption spectra of the LB film with that in solution,obvious redshift in LB film of BNB-t4 and AOB-t4 relative to the dilute solution suggests the formation of J-aggregation mainly through ?–? interactions.With the UV light irradiation,It can be observed that the photoinduced trans-to-cis isomerization takes place in monolayer and multilayer LB films,the isomerization rate of AOB-t4 LB film is faster than BNB-t4,which can be attributed to the weaker intermolecular interactions in AOB LB film than that in BNB-t4 LB film.3.The nonlinear optical properties of the two azobenzene derivative films were investigated with the Z-scan measurements.The picosecond Z-scan results of less layer(7-layer and 9-layer)AOB-t4 LB films exhibit the positive nonlinear refraction and two-photon absorption.And the nonlinear optical refraction and two-photon absorption for thinner film are correspondingly enhanced by several times because of the more delocalized conjugated electron in AOB-t4.Specifically,the third-order optical nonlinearities of AOB-t4 and BNB-t4 thicker LB films(11-layer to 15-layer)demonstrate the negative nonlinear refraction and saturated absorption.Combined with UV-visible absorption spectra,the sign of nonlinear optical absorption is determined by the competition between the two-photon absorption and saturated absorption.For the thinner LB films,the two-photon absorption dominates,and the saturated absorption plays the major role for the thicker LB films.In other words,the nonlinear optical response of AOB-t4 LB film can be tuned by simply changing the number of layers. |
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