STRUCTURAL EVIDENCE IN 2-AMINO-5-NITROPYRIDINIUM HALIDES (CL-, BR-) OF HERRINGBONE MOTIFS FAVORABLE TO EFFICIENT QUADRATIC NONLINEAR-OPTICAL PROPERTIES

2-Amino-5-nitropyridinium halides have been synthesized and their crys tal structures determined. The structures of acentric phases reveal ne tworks of non-linear chromophores arranged in herringbone motifs. Thei r second-harmonic generation responses vary in magnitude between that of 3-methyl-4-nitropyridine N-oxide and 2-methyl-2,4-dinitrophenyl ami nopropanoate molecular compounds. Chained anions screen favourably the interactions between the non-linear cations in acentric phases, while the reverse situation is observed in the centric phase of 2-amino-5-n itropyiridinium bromide built up with open pseudo-hexagonal anionic la yers.