IMAGING OF STRUCTURED AND DISORDERED HEMICYANINE MONOLAYERS BY ATOMIC-FORCE MICROSCOPY

Monolayers of surface-active hemicyanine dyes have been extensively studied owing in part to their potential importance for fabrication it to non-linear optical m rials. In an effort to increase the dipole moment to a value larger than that present in the typical hemicyanine, a new compound was characterized, having the structure: (CH3)2N-C6H4-CH=CH-CH=CH-C6H4N+C22H45Br- (1) which contains a more extended pi-bonded system compared with some more commonly studied hemicyanines. It is known that hemicyanines may be present in various aggregated states as well as species having varying degrees of order when in monolayer form. For the new compound, monolayers could be produced having both ordered and disordered structures. On a pure water subphase, the new hemicyanine in a highly compressed monolayer had a narrow, blue-shifted absorption band with a maximum at 435 nm. Such a spectrum is characteristic of H-aggregate species. When the subphase contained cadmium chloride (1 x 10(3) M) the optical absorption band was similar to that seen in chloroform solution for disaggregated species, with a broad absorption band at 485 nm, shifting only slightly as the monolayer was progressively compressed. Atomic force microscopy visualizing the two different films indicated that monolayers transferred from pure water subphase appeared inhomogeneous, discontinous and indicative of island structures possibly present in the original spread monolayer, or formed during transfer. Monolayers formed on Cd2+-containing subphase were uniform except for some small surface defects.