For various application of nonlinear optics, π-conjugated organic materials have to exhibit large macroscopic diagonal and off-diagonal nonlinear optical response in crystalline state. We designed new organic ionic crystals consisting of fluorinated benzothiazolium based PM5FB (5-fluoro-2-(4-(4-(hydroxymethyl)piperidin-1-yl)styryl-3-methylbenzo[d]thiazol-3-ium) cation and various counter anions. We observed that PM5FB based chromophores have high thermal stability and microscopic optical nonlinearity. All of PM5FB based organic crystals have isomorphic series-type crystal structure formed by hydrogen bonding between cation and anion which is similar to previously reported PMB-T (2-(4-(4-(hydroxymethyl)piperidin-1-yl)styryl-3-methylbenzo[d]thiazol-3-ium 4-methylsulfonate) crystal. Surprisingly, it was investigated that PM5FB-CBS (5-fluoro-2-(4-(4-(hydroxymethyl)piperidin-1-yl)styryl-3-methylbenzo[d]thiazol-3-ium 4-chlorobenzenesulfonate) has molecular ordering of 56.7° which is very close to optimal angle for maximizing off-diagonal component. In THz generation experiment, off-diagonal term of PM5FB-CBS crystal shows a 3 times more efficient peak-to-peak THz electric field than diagonal term of standard inorganic ZnTe crystal and reduced dimples in the THz wave spectra which indicate that phonon mode was suppressed.
