Dynamics of adsorbed molecules in zeolites

Abstract Dye molecules adsorbed or embedded in the cages of a microporous zeolite have unique degrees of freedom which are not available to chromophores in e.g. molecular crystals or glasses. We have used low temperature optical spectroscopy and spectral hole burning to study the dynamics of a selected system, namely thionine molecules in a faujasite structure. The spectral properties depend strongly on the water content of the faujasite. Thionine in a hydrated sample shows considerable electron-phonon coupling comparable to the behaviour in a glassy matrix. In a dehydrated sample there exist two stable thionine conformations which can interconvert at low temperatures through ground state as well as excited state reactions. This reversible photocycle explains the appearance of antiholes at the burning wavelength. The electron-phonon coupling in the dehydrated sample is weak, similar to the characteristics of adsorbate/substrate systems.