Solid-state optical properties of the methyl-exopyridine-anthracene rotaxane

Photophysical properties in the solid state of both, an anthracene grafted rotaxane and the corresponding thread are studied. The thread in liquid and solid states as well as the rotaxane in the liquid phase exhibit only the usual anthracene-like photoluminescence (PL) behaviour, while the rotaxane polycrystalline powder emits a broad structureless red-shifted band. The excitation spectrum of the rotaxane reveals the presence of low-lying states below the absorption feature of the anthracene, the one of the thread reproducing quite accurately the absorption spectrum. Site-selective PL measurements show that the broad emission of the rotaxane is a superposition of two contributions corresponding to energy levels which can be photoexcited directly within the low energy absorption tail. Time resolved PL measurements reveals immediately after excitation the characteristic features of the anthracene-like emission, indicating an energy transfer from the excited anthracene moiety to the structural defect states giving rise to the unusual radiative emission.