A Second-Order Nonlinear Optical Material Prepared through In Situ Hydrothermal Ligand Synthesis

The in situ hydrothermal reactions of ZnCl<sub>2</sub> with benzonitrile, 2-amino-5-cyanopyridine, and <i>trans</i>-2,3-dihydro-2-(4‘ ‘-cyanophenyl)-benzo[<i>e</i>]indole in the presence of NaN<sub>3</sub> and water afford two 3D-diamond-like networks, (CN<sub>4</sub>−C<sub>6</sub>H<sub>5</sub>)<sub>2</sub>Zn (<b>1</b>) and (NH<sub>2</sub>−C<sub>5</sub>H<sub>3</sub>N−CN<sub>4</sub>)<sub>2</sub>Zn (<b>2</b>), and one 2D square grid network, [(CN<sub>4</sub>−C<sub>6</sub>H<sub>4</sub>−C<sub>12</sub>H<sub>7</sub>N−C<sub>5</sub>H<sub>4</sub>N)<sub>2</sub>Zn]·1.5H<sub>2</sub>O (<b>3</b>), in which these ligands gradually involve a noncenter-A−D (acceptor−donor) system, a one-center-A−D system, and a two-center-A−D system, respectively. All three compounds crystallize in noncentrosymmetric space groups (<i>I</i>4̄2<i>d </i>for <b>1</b> and <b>2</b> and <i>Fdd</i>2 for <b>3</b>) and display strong second harmonic generation (SHG) responses. Among the three new complexes, <b>3</b> shows the largest SHG effect, which is about 50 and 500 times that of urea and KDP (KH<sub>2</sub>PO<sub>4</sub>), respectively. The two-center-A−D system (multicenter push−pull electronic effect) in <b>3</b> may be responsible for it having the largest SHG effect. Interestingly, the three compounds exhibit strong fluorescent emissions at different wavelengths, <b>1</b> and <b>2</b> with blue fluorescent emissions at 390 and 415 nm and <b>3</b> with yellow-green fluorescent emissions at 495 and 532 nm.