Second Order Nonlinear Optical Switches Based On The Phase-changable Coordination Polymers And Their Solid Solutions

Nonlinear optical（NLO）materials are of great importance due to their pivotal role in the fields of photoelectronics and photonics.In recent years,the research on NLO switches,of which quadratic NLO effect such as second harmonic generation（SHG）can be on-off switched under external physical stimuli,have aroused intense interest.So far,one most feasible method is to tweak the conformation or orientation of NLO-chromophore molecules in the crystal structures by thermal stimulus,with a mechanism related to structural phase transition,which can realign the NLO-active moieties to alter the SHG signal,especially those with the transition from a noncentrosymmetric space group to a centrosymmetric one.Following this strategy,the thermoresponsive NLO switches within the molecule-based systems,typically represented by the coordination polymers（CPs）,can be achieved with many advantages such as the designable and flexible structures,the relatively mild preparation condition as well as appropriate on/off contrast of SHG signal.Nevertheless,most of the reported thermoresponsive NLO switches do not have the tunability on the regulation of their on-off temperatures（T_S）,especially for the temperature range above and below the room temperature,which is a key issue for practical application.In this thesis,to resolve the above issue,the formation of CP solid solutions or alloys that retain a similar structure to the NLO-switchable parent phase may be an efficient and convenient method,of which the T_Scould be finely tuned by the varied ratio of such binary system.According to the similarity-intermiscibility theory,one key requirement for the syntheses of CP solid solution is that the two mixed CPs should be structurally similar.On the other hand,as is widely used,one most effective procedure for the preparation of solid solution is via the mixed melting-cooling treatment.However,by comparison with metal alloys and more conventional materials,there have been few reports on the preparations of CP-based solid solutions via mixed melting,mainly owing to the near-ubiquitous irreversible thermal decomposition of CPs upon heating to moderate temperatures.In the field of coordination chemistry,thiocyanate,as a heteratomic end-to-end bridging ligands between two metal ions,,can often be used to construct one-dimensional chain-like CPs with various ammonium as template agents.Not only are Such compounds are not only apt to undergo structural phase transitions,but also easy to melt before decomposition.Therefore,considering the above considerations,we deem that chain-like metal thiocyanate CPs with organic ammonium as the template are not only very suitable for exploring NLO switches,but also used to form CP solid solutions to tune the on-off temperature of the NLO effect.Bearing the aforementioned considerations in mind,we have synthesized a series of c admium thiocyanate chain CPs,（i-Pr NHMe₂）[Cd（SCN）₃]（1）,（Me NEt₂）[Cd（SCN）₃]（2）and（Me₂Im）[Cd（SCN）₃]（3）.i-Pr NHMe₂,Me NEt₂,and Me₂Im represent N,N-dimethylisopropy lamine,N,N-diethylmethylamine and 1,3-dimethylimidazole,respectively.Based on structur al analyses,thermal analyses,NLO effect tests,Molecular dynamics simulations,X-ray abs orption near-edge structure analyses,and solid solutionizing,we have studied their structur al phase transition and the“on-off”mechanism of NLO signals.In summary,by combinin g the thermoresponsive NLO-switching function of phase-change material and the dual soli d solution approach within the CP system,via mixed melting treatment,we provide a met hod for producing thermoresponsive NLO switch with highly tunable on-off temperature.