Second-order optical nonlinearities and photostabilities of 2-N-methylstilbazolium salts

We have synthesized a number of 2-N-methylstilbazolium salts and measured their powder second harmonic generation (SHG) efficiencies. The pale yellow salt 3'-methoxy-2-N-methylstilbazolium triflate was found to exhibit an SHG efficiency 24 times that of a urea reference standard at 532 nm, and an X-ray diffraction study showed the crystals to be noncentrosymmetric (space group Cc; a = 18.126 (4) Å, b = 7.040 (1) Å, c = 13.315 (2) Å, β = 93.46 (1)°, Z = 4). These crystals are photochemically stable under visible irradiation. On the other hand, 2'-methoxy-2-N-methylstilbazolium triiflate shows no SHG signal; it forms centrosymmetric crystals (space group P2_1/n; a = 11.999 (2) Å, b = 13.458 (2) Å, c = 11.043 (2) A, β = 108.47 (1)°, Z = 4) that undergo photochemical 2 + 2 cycloaddition.