The influence of pyrrole linked to the π-conjugated polyene on crystal characteristics and polymorphism

Novel pyrrole-based polyene derivatives bearing various different substituents were synthesized to investigate the influence of molecular conformational change (rotational isomerization) of a π-conjugated bridge on crystal characteristics and polymorphism. The pyrrole-based polyene crystals exhibited strong second harmonic generation efficiency that was ∼two orders of magnitude larger than that of urea. Chromophores bearing asymmetric pyrrole readily formed polymorphs whereas crystals having a symmetric dimethylaminophenyl group displayed only one crystal structure. The observed polymorphism in pyrrole-based chromophores was attributed to the existence of rotational isomerization in which the minor rotamer exhibits a “self-additive effect” during the formation of the crystals.