Molecular monolayers of charge-transfer complexes: Protonation and aggregation studied by second harmonic generation

Abstract

Second-order optical nonlinearities of molecules can be strongly affected by the environment. Protonation and aggregation of the dyes 4-amino-4′-nitrostilbene (NS) and hemicyanine (HC), are investigated by second harmonic generation from molecular monolayers floating on water. The observed second-order nonlinearity of such molecules in the form of either a pure monolayer or a monolayer diluted with stearic acid directly reflects the degree of protonation of the molecules. For NS and HC, the variation of the second-order molecular polarizability with protonation is opposite. It is demonstrated that the measured nonlinearity can be used to deduce the effective proton concentration in the surface region. The proton concentration close to a stearic acid monolayer floating on water, for example, is found to be ∼7 orders of magnitude larger than the bulk proton concentration when the latter is low. The effects of stearic acid in diluting a dye molecular monolayer on changing the environment and breaking dye aggregates are discussed.