Linear and circular dichroism of polymeric pseudoisocyanine

Journal article, 1977

The dye l,lt-diethyl-2,2’-cyaniinoed ide (pseudoisocyanine = PIC) was studied by circular dichroism (CD) and linear dichroism (LD). PIC forms optically active aggregates in aqueous (+)tartrate solution and as complexes with DNA. They are characterized by J bands at 572 (tartrate) and 553 nm (DNA), with negative and positive CD, respectively, which may be explained by helical band arrays of dye molecules (left- and right-handed helices, respectively). At higher degrees of aggregation in (+)tartrate a positive J band may be due to helical stacking of PIC polymers such as in a cholesteric liquid crystal. Intense LD indicates that the solution is not random but contains large domains with local order. It is shown that earlier conclusions about helicity reversals and mechanical induction of optical activity in aggregated PIC when formed in optically inactive media may have been due to instrumental sensitivity to linear dichroism from possibly nonhelical PIC fibers. The PIC aggregation was also studied by LD at different linear flow gradients in aqueous NaCl solution. From LD relaxation lengths of 4 ~ 1 0 0 0 0 A0 were estimated for the orientable aggregates. The ratio of LD to random absorption at different degree of aggregation suggests that also unalignable (PION oligomers may exhibit J bands. By exciton theory N was tentatively estimated to be more than 10. An earlier “card-pack” model, with angle cy close to 90°, between the monomer long axis and the polymer axis, is discarded. The suggested structure of PIC, is a coiled band with cy = 30-40’. Influence of flow on aggregation was observed: At low gradients the formation of aggregates with low rotational diffusion is favored (by applying a constant gradient apparently very elongated aggregates may be synthesized in this way). Above a certain value (ca. lo00 s-’) the effect of the gradient was the opposite, leading to the destruction of aggregates. PIC was also studied in oriented polyvinyl alcohol film in which it was monomeric, and in polystyrene film where a J band was observed at 585 nm. In the Appendix factors determining the instrumental LD response are analyzed.