Protonation and monolayer aggregation studied by second-harmonic generation

Various types of aggregates have been studied in Langmuir–Blodgett-type monolayers of selected pyridine and hemicyanine dyes. Environmental changes such as protonation or admixture of chromophores with opposite donor–acceptor characteristics result in dramatic changes of the measured surface susceptibility χ¯(2). Both enhanced χ¯(2) values in H aggregates following the shape of the linear absorption band and vanishing χ¯(2) values of antiparallel dimers or aggregates have been observed. The experimental results suggest that linear and nonlinear measurements need to be considered for a correct interpretation of the experimental results on monolayer aggregation.