Nitrates have long been ignored for practical uses as nonlinear optical (NLO) materials because they are usually very easy to dissolve in water; despite this, the π-conjugated [NO₃]^- is among the most desirable NLO-active structural units. 

Recent studies led by Prof. LUO Junhua and ZHAO Sangen from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, resulted in a new water-resistant NLO material, Bi₃TeO₆OH(NO₃)₂ (BTOHNO), which consists of three structural chromophores including Bi₃O₆OH short chains with 6s² lone pair electrons, distorted TeO₆ octahedra with d¹⁰ electrons, and π-conjugated  [NO₃]^- triangles.

Owing to the cooperative effect of three chromophores, BTOHNO show an enhanced phase-matchable NLO response of 3 × KH₂PO₄, which exceeds those of most nitrate NLO materials.  

Remarkably, the authors found BTOHNO was highly resistant to water as it did not show obvious weight loss and decay of NLO response after being dipped into de-ionized water for 24 h.  

Co-authors Prof. LIN Zheshuai et al from Technical Institute of Physics and Chemistry, Chinese Academy of Sciences performed theoretical calculations and found that the adsorption of a foreign water molecule in the BTOHNO crystal lattice is an endothermic process with an adsorption energy of +790 meV, theoretically conformed the water-resistance of BTOHNO.  

The authors deem that it is an effective approach to developing practically desired nitrate NLO materials of high water-resistance and good optical performance by introducing diverse chromophores into the nitrate systems. 

The study entitled “Cooperation of Three Chromophores Generates the Water-Resistant Nitrate Nonlinear Optical Material Bi₃TeO₆OH(NO₃)₂” has been published in Angew. Chem. 

The cooperation of three chromophores in Bi₃TeO₆OH(NO₃)₂ leads to a strong second-harmoinc generation that does not degenerate in water (Image by Prof. LUO's group)