Low-temperature spectroscopy and persistent spectral hole-burning have been used to characterize several aspects of the dynamics of physiadsorbed chromophores and amorphous phases confined to the Angstrom-lengthscale voids of molecular sieves. It is shown that subtle differences of geometric factors, i.e. the relationship between molecular size and form and the confining void play a decisive role in the mobility and dynamics of thiazine dyes encapsulated in solvent-free faujasite structures. Additional solvent in the pores forms an amorphous phase which allows the formation of persistent spectral holes. Spectral diffusion in some of the solvent/molecular sieve systems is largely reduced due to the effect of the restricted geometry on the structure of the amorphous phase and/or the spatial distribution of two-level systems. For a hydrated A1PO(4)-5 host with phthalocyanine-zinc derivates efficient hole formation has been demonstrated up to 80 K, the highest temperature for which stable non-photochemical holes have been reported so far.