Thermochromic behaviour of merocyanine Langmuir-Blodgett films

Abstract A kinetic study of the thermal decay of the red-shifted bands of dye-containing Langmuir-Blodgett films was carried out for three different merocyanines. The decay process was traced by repeated application of a cycle consisting of spectral measurements, heating at a constant temperature for a given time and then cooling to room temperature. For one of the three dyes the red-shifted band was found to be restored on cooling, while for the other two the spectral changes were quenched. This irreversible decay behaviour is characterized as a first-order rate process with values of the frequency factor τ −1 0 and the activation energy W of 10 11 –10 15 s −1 and 1.1−1.3 eV respectively. The values of W are much smaller than the photon energy hv ∼ 2 eV for the absorption maxima of the bands or the dissociation energy of the chemical bonds, indicating that the present decay process is a steric rearrangement of chromophores.