Photo-crosslinkable second order nonlinear AB2-type monomers: convenient synthesis and the enhanced NLO thermostability

In this article, two AB2-type second-order nonlinear optical monomers (DN-SH, DS-SH) containing two double bonds and one thiol group respectively, were synthesized successfully. These two monomers could be in-situ photo-crosslinking via thiol-ene click reaction under 365 nm UV light without photoinitiator after poling. Thanks to the photo-crosslinking process of thin films, the thermostability of the second-order nonlinear optical property was much enhanced. DS-SH exhibited larger NLO effect and better NLO thermostability in comparison with that of DN-SH, because of the introduction of sulfonyl-based chromophores which can decrease the electrostatic interaction between nitro-based chromophore moieties. T80% of the photo-crosslinked DS-SH films increased to 112.5 °C, much higher than that of uncrosslinked ones (75 °C), while T80% of DN-SH films increased from 58 to 95 °C. More excitedly, the doped thin films of DN-SH and DS-SH at different molar ratios (1:1, 2:1, 1:2) showed better NLO properties (123, 122, 114 pm/V, respectively) after photo-crosslinking than that of single DN-SH (89 pm/V) and DS-SH (108 pm/V) respectively.