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

Monolayers of surface-active hemicyanine dyes have been extensively st udied owing in part to their potential importance for fabrication it t o non-linear optical m rials. In an effort to increase the dipole mome nt to a value larger than that present in the typical hemicyanine, a n ew compound was characterized, having the structure: (CH3)2N-C6H4-CH=C H-CH=CH-C6H4N+C22H45Br- (1) which contains a more extended pi-bonded s ystem compared with some more commonly studied hemicyanines. It is kno wn that hemicyanines may be present in various aggregated states as we ll 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 hemicyan ine in a highly compressed monolayer had a narrow, blue-shifted absorp tion 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 c hloroform solution for disaggregated species, with a broad absorption band at 485 nm, shifting only slightly as the monolayer was progressiv ely compressed. Atomic force microscopy visualizing the two different films indicated that monolayers transferred from pure water subphase a ppeared inhomogeneous, discontinous and indicative of island structure s possibly present in the original spread monolayer, or formed during transfer. Monolayers formed on Cd2+-containing subphase were uniform e xcept for some small surface defects.