Fabrication And Third-order Nonlinear Optical (NLO) Performance Of Coordination Polymer Films Based On Azobenzene

With the research deepening of nonlinear optics(NLO),frequency conversion,optical communication,and optical storage have been rapidly developed.Under this background,third-order NLO materials with nonlinear linear absorption and refraction properties play important roles.Although researchers have made great efforts in the development of coordination polymers(CPs)based third-order NLO materials,the third-order NLO properties of most CPs are studied in the liquid dispersion.It is difficult to avoid the decomposition of the frameworks into metal-organic oligomers,or even a mixture of metal ions and organic ligands.To achieve feasible practical applications,it is very urgent to exploit the third-order NLO performance of solid-state CPs,which can not only effectively avoid the influence of solvents,but also be more easily processed and applied in practical devices.So far,the study of the third-order NLO properties of CPs based on solid samples is still very difficult due to the harsh fabrication conditions of solid samples and susceptibility to damage by high-power light field.Doping CPs into soft transparent matrix to prepare hybrid films can effectively avoid the contact of CPs with the outside environment and improve the stability of CPs.And the excellent photophysical properties of the polymer matrix endow the film with extremely high light transmittance even under low-power light field.Importantly,the self-supporting ability and convenient processability of the highquality soft films are beneficial for practical device applications.Thus,it is very meaningful to design and develop soft CPs films with third-order NLO properties to broaden the application range of solid CPs materials in nonlinear optical.This research mainly includes the following parts:(1)we successfully prepared five coordination polymers using azobenzene derivatives H4L1 and H2L2 as ligands,[Zn(L1)0.5(4,4’-azobpy)0.5DMF]n(1),{[Zn2(L1)0.5(1,3-bpea)0.5]·DMF}n(2),[Mn(L1)0.5(4,4’-azobpy)0.5DMF]n(3),[Cd(L2)(DMAc)(H2O)]n(4)and {[Cu(L2)(4,4’-azobpy)]·3H2O}n(5).SC-XRD results show that 1 and 3 are 3-fold interpenetrating 3D structures,2 is non-interpenetrating 3D structures,and 4 and 5 are 1D and 2D structures,respectively.Considering the dispersing ability of CPs in polymer matrix,we synthesized corresponding CPs microcrystal X-MC(X=1-5)with grain size 3 μm on a large scale through a convenient route.Smaller grain size facilitates the penetration of photons and reduces light scattering.It is worth noting that the solid UV-Vis diffuse reflection spectra and CO2 adsorption isotherms of 4-MC and 5-MC before and after UV irradiation indicate that the azobenzene moiety undergo trans→cis structural transition.This is attribued to the fact that one side of the azobenzene is not restricted by the coordination network.Finally,we doped coordination polymers microcrystal X-MC(X=1-5)into polyvinyl alcohol(PVA)to prepare the CPs films X-MC/PVA(X=1-5)with excellent optical properties.(2)The Z-scan results of X-MC/PVA(X=1-5)show that they both have pulse energy-dependent NLO absorption optical switching behavior.The saturable absorption(SA)at low pulse energy and the reverse saturable absorption(RSA)at high pulse energy are attributed to the inability to induce excited state absorption in coordination polymer films at low pulse energy.The NLO refractive properties of XMC/PVA(X=1-5)exhibit self-defocusing behavior.By comparing the nonlinear performance of X-MC/PVA(X=1-3),it can be known that the smaller central metal ion radius and the non-interpenetrating 3D structure are beneficial to intramolecular electron transfer,which leads to better third-order NLO performance.(3)4-MC/PVA and 5-MC/PVA also possess photo stimulated energy-dependent tunable NLO behavior.After photo stimulation input energy,Z-scan results showed that the third-order NLO performance of 4-MC/PVA was weakened,while that of 5MC/PVA was enhanced.The changes in the nonlinear properties of 4-MC/PVA and 5MC/PVA are mainly attributed to the trans→ cis structural transition of the azobenzene unit after the input energy of photo stimulation.The DFT calculation results show that the third-order NLO response of X-MC/PVA originates from the azobenzene groups and the coordination between the central metal and the organic ligands.And the different electron densities after photo stimulation input energy enable 4-MC/PVA and 5-MC/PVA to have tunable NLO behavior.