Octupolar (C₃ and S₄) Symmetric Cyclized Indole Derivatives: Syntheses, Structures, and NLO Properties

Several cyclized indole derivatives have been synthesized, and their structures been determined. The C₃-symmetric single-chiral N-phenyltriindole (Tr-Ph3) crystallized in the P1 space group, and the S₄-symmetric saddle-like tetraindole (TTr) crystallized in the I4̅ space group. The Tr-Ph3 and TTr crystals exhibit remarkable powder SHG intensities 5 and 11 times that of KH₂PO₄ (KDP), respectively. TTr is a useful octupolar core to build S₄-symmetric molecules and crystals for second-NLO materials.