Circular Dichroism and UV−Visible Absorption Spectra of the Langmuir−Blodgett Films of an Aggregating Helicene

Nonracemic helicene 1 forms high-quality Langmuir−Blodgett (LB) films, in which the molecules are organized in parallel columns with their edges exposed to the surface. Evidence for the edge-on exposure is the molecular area of a monolayer on water and, as determined by X-ray diffraction, the similarity of the interlayer spacings of multilayers to the diameters of the molecular columns that constitute nonracemic 1 in bulk. Evidence of the organization into parallel columns includes the linear dichroism of the films and the 180° periodicity with which rotating the films varies the amounts by which plane polarized light is rotated, the intensities of the second harmonics generated, and the films' degree of anisotropy. The circular dichroisms of a 10-layer LB film could be resolved from the effects of linear dichroism and linear birefringence. The CD and UV−vis absorption spectra of the films are similar to those of solutions in which the molecules are aggregated, not isolated. The measurement of a CD spectrum for a monolayer, carried out easily for nonracemic 1, is unprecedented.