ABSTRACT For terahertz (THz) applications, we designed new organic salt crystals by introducing a substituent with high conformational freedom. In these crystals, 4‐ethylbenzenesulfonate (EBS) anions with an ethyl substituent exhibit a low rotational energy barrier along the C sp 3 ─C sp 2 (Ar) bond, which can create multiple conformers having substantially different molecular shapes. EBS anions were paired with a highly nonlinear optical cationic chromophore, 7‐chloro‐2‐(4‐(4‐(hydroxymethyl)piperidin‐1‐yl)styryl)‐1‐methyl‐quinolinium (PM7ClQ), to form salt crystals. Two polymorphs were obtained: PM7ClQ‐EBS(I), crystallizing in the Pca 2 1 space group with an out‐of‐plane conformation of the EBS anions, and PM7ClQ‐EBS(II), crystallizing in the P 1 space group with an in‐plane conformation of the EBS anions. Compared to previously reported PM7ClQ‐based crystals with low conformational flexibility substituents, PM7ClQ‐EBS(II) exhibits an isomorphic crystal structure, whereas PM7ClQ‐EBS(I) exhibits a remarkably different crystal structure. While PM7ClQ‐EBS(II) crystals are difficult to grow in bulk, PM7ClQ‐EBS(I) crystals show good bulk crystal growth properties. We demonstrate efficient THz wave generation: under excitation with 130 fs laser pulses at 1500 nm, a 0.24‐mm‐thick quinolinium‐based PM7ClQ‐EBS(I) crystal generates THz waves at 1.3 THz with 2 times higher spectral amplitude compared to a benchmark off‐diagonal benzothiazolium‐based crystal with a thickness of 0.18 mm.
OH et al. (Tue,) studied this question.