Piezomodulation study of Langmuir-Blodgett films of copper phthalocyanine derivatives

A method for quantitative analysis of piezomodulation spectra of a thin film on a substrate, including the internal reflection in the system, is presented. This method is applied to Langmuir–Blodgett (LB) films of copper phthalocyanine (CuPc) derivatives on glass substrates. A uniaxial stress was applied along the symmetry axis of the film. The spectra were measured in the region from 1.5 to 2.4 eV in three configurations; transmission, reflection from the film side, and reflection from the substrate side. The optimum Lorentzian parameters and their changes due to the stress are determined by the least-mean-square fit. The microstructures and electronic states of the LB film are discussed from the stress effects. The very small feature, which is hardly identified in the absorption spectra because of the large Frenkel exciton band, is attributed to a charge transfer (CT) excitation.