Orientation of optically nonlinear chromophores at the air-water interface

The isotherms of E-N-aIkyl-4-[2-(4-(N-octadecyl-N-ethylaminophenyl)]-etheny l]-pyridinium halide, represented here by CmH2m+1-D-pi-A(+)-CnH2n+1X-, Sugg est that this two-legged molecule initially adopts a "U-shaped" conformatio n at the air-water interface but becomes "stretched" as the surface pressur e is increased. The area of 0.25 +/- 0.05 nm(2) molecule(-1) prior to colla pse, is independent of the second alkyl group and approximates to the molec ular cross-section. When deposited, the area in contact with the substrate is 0.25 +/- 0.05 nm(2) molecule(-1) and the layer thickness compares favour ably with the molecular length. The second-harmonic intensity is strong whe n the alkyl chain lengths are sufficiently different, i.e. \(m - n)\ greate r than or equal to 4, but is suppressed when m approximate to n. In this li miting case, the second-order properties suggest a centrosymmetric arrangem ent and, therefore, a random orientation of the molecular dipoles, up and d own, within the monolayer film.