Low-Temperature Spectroscopy of Chromophores in Geometrically Restricted Amorphous Phases

Recent low-temperature spectroscopic studies of chromophores encapsulated in the Å-scale porous cage and channel structures of inorganic molecular sieves have revealed a number of properties absent for chromophors in solid bulk solutions. For example, thionine in a dehydrated zeolite-Y exhibits two distinguishable ground state forms which are associated with a reorientation of the chromophor with respect to the surrounding zeolite cage [1]. This additional degree of freedom which depends on specific relationship between the size of the molecular guest and void of the host gives rise to a new kind of low-temperature photochemistry. In general, however, spectral hole-burning is only feasible if the pores of the molecular sieve are filled with solvent as additional coadsorbate. In samples of this kind the coadsorbate acts as an amorphous solvent shell containing two-level systems (TLSs) which gives rise to non-photochemical hole-burning similar to bulk amorphous systems. In fact, a phthalocyanine-zinc derivative embedded in a hydrated AIPO4-5 molecular sieve allow efficient hole formation up to 80 K, the highest temperature for which stable non-photochemical holes have been reported so far [2].