Thermally induced variations in the conformational composition and spatial orientation of molecular components of a dicarbocyanine dye layer

The effect of heating on the spatial orientation and concentration of the components of dicarbocyanine dye layers on a glass substrate is studied experimentally. Layers of two types differing in the spatial orientation angles of all their components (monomeric all-trans and β-mono-cis isomers, dimers, and J aggregates) are considered. It is shown that storing of samples at temperatures of 150–200°C causes partial thermal destruction but does not change the absorption spectra of the molecular components. Heating of the layer of the first type changes neither the orientation nor the relative concentration of the components. Heating of the layer of the second type leads to irreversible changes in the orientation angles of all the molecular components and causes the β-mono-cis isomer and the dimer to transform into the all-trans isomer and the J aggregate. The dependences of the orientation angle on the heating time have the form of saturating functions. The rate of variation of the orientation angle and its limiting value in the saturation region increase with increasing temperature of heating. At high temperatures, the layer of the second type transforms completely into the layer with the first type of spatial orientation. A mechanism of the changes in the angles of the spatial orientation of the layer components, which includes the stages of thermal isomerization and reorientation of each molecular component, as well as rearrangement of its environment, is proposed. Energetic and kinetic parameters of all the stages of layer reconstruction satisfying the experimental results obtained in the region from 150 to 230°C are determined.