Optical second harmonic generation in achiral bis(n-alkylamino)dicyanoquinodimethanes: alkyl chain length as the design element

Chain length of substituted n-alkyl groups is proposed to be a convenient design element for the generation of noncentrosymmetric crystal lattices of interest in optical second harmonic generation (SHG). Powder studies on 7,7-Bis(n-alkylamino)-8,8-dicyanoquinodimethanes indicate that moderate solid-state optical SHG is obtained when the alkyl chains are of length 4, 5, and 6, and no detectable SHG occurs when the chains are of length 0, 3, 7, 8, and 12. The significant role of the intermediate alkyl chain length in generating noncentric crystal lattices is examined using the crystal structures of the propyl, butyl, and octyl derivatives presented in this paper.