Large-sized benzo[e]indolium salt single crystals with high optical nonlinearity

In this paper, a series of D–π–A type benzo[e]indolium salts with highly efficient nonlinear optical performance has been synthesized. Out of the three new benzo[e]indolium salts, (E)-2-(2-hydroxy-3-methoxystyryl)-1,1,3-trimethyl-1H-benzo[e]indol-3-ium iodide (HMTB-1) and (E)-2-(2-hydroxy-4-methoxystyryl)-1,1,3-trimethyl-1H-benzo[e]indol-3-ium iodide (HMTB-2) possess SHG properties, and their maximum powder second harmonic generation (SHG) efficiency is around 0.6 times that of DAST (4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate) at a non-resonant wavelength of λ = 1570 nm. Single crystals of the two noncentrosymmetric salts were successfully grown by using a slow cooling method, and we were able to obtain very large bulk single crystals, about 17 mm × 9 mm × 3 mm, of HMTB-2. X-ray crystallographic analysis indicated that both of the new salts belong to the monoclinic space group Cc (point group m, Z = 4). HMTB-1 forms a noncentrosymmetric phase with methanol through hydrogen bonding.