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

The in situ hydrothermal reactions of ZnCl₂ with benzonitrile, 2-amino-5-cyanopyridine, and trans-2,3-dihydro-2-(4‘ ‘-cyanophenyl)-benzo[e]indole in the presence of NaN₃ and water afford two 3D-diamond-like networks, (CN₄−C₆H₅)₂Zn (1) and (NH₂−C₅H₃N−CN₄)₂Zn (2), and one 2D square grid network, [(CN₄−C₆H₄−C₁₂H₇N−C₅H₄N)₂Zn]·1.5H₂O (3), 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 (I4̄2d for 1 and 2 and Fdd2 for 3) and display strong second harmonic generation (SHG) responses. Among the three new complexes, 3 shows the largest SHG effect, which is about 50 and 500 times that of urea and KDP (KH₂PO₄), respectively. The two-center-A−D system (multicenter push−pull electronic effect) in 3 may be responsible for it having the largest SHG effect. Interestingly, the three compounds exhibit strong fluorescent emissions at different wavelengths, 1 and 2 with blue fluorescent emissions at 390 and 415 nm and 3 with yellow-green fluorescent emissions at 495 and 532 nm.