Squaraine chemistry. Effect of N-pyrrolidino substitution on the synthesis and solid state properties of squaraines

Abstract

Several symmetrical and unsymmetrical squaraines bearing N-pyrrolidino groups have been synthesized by condensation of N-pyrrolidinoaniline derivatives with squaric acid or a 1-aryl-2-hydroxycyclobutene-3,4-dione derivative. As compared to squaraines synthesized from N,N-dimethylanilines, a consistently higher synthetic yield was obtained for the N-pyrrolidino-squaraines synthesized in the work. The yield improvement is attributable to the high nucteophilicity of N-pyrrolidinoanilines, owing to the rigidized N-pyrrolidino ring structure. N-Pyrrolidino substitution is shown to have very little effect on the physical and spectroscopic properties of squaraine. Examinations of the solid state properties of N-pyrrolidino substituted squaraines, by solid state absorption spectroscopy and by X-ray powder diffraction reveal that the N-pyrrolidino group is the only N-alkyl substituent identified thus far that exerts no effect on the aggregation of squaraine molecules in the microcrystalline state. This implies that N-pyrrolidino squaraines should be photoconductive and indeed photoconductivities have been observed in xerographic devices incorporating them.