Solvation effects on molecular pure radiative lifetime and absorption oscillator strength in clusters

The solvation effects on the molecular pure radiative lifetime, its absorption line shape, oscillator strength, and spectral red shift are studied for 9, 10-dichloranthracene embedded in clusters of argon, krypton, and xenon. The clusters are synthesized in a supersonic free-jet coexpansion of the organic molecule with the rare gas. Cluster size is controlled by the nozzle backing pressure and its pure radiative lifetime is obtained by measuring both the fluorescence lifetime and the absolute emission quantum yield. For small (jet atom) clusters the pure radiative lifetime is increased by subsequently adding rare gas atoms. For up to two rare gas atoms this increase correlates with the atomic polarizabilities and with the spectral red shift. For Ar and Kr this trend of increasing radiative lifetime continues up to clusters of six rare gas atoms. These results are in good agreement with calculations based on classical electromagnetic theory using point polarizable dipoles for the atom and a simplified charge distribution for the molecule. For large clusters of Ar (up to ∼1000 atoms) both the spectral red shift and the lifetime become cluster size independent. This is also in agreement with classical electromagnetic theory using a model of a point (molecular) dipole embedded in a dielectric (rare gas) sphere. Both experiment and theory indicate that the radiative lifetime in this limit is still larger than that of the free molecule, which in itself is longer than that expected in the bulk solvent. This implies that a further cluster size evolution of this quantity is expected upon increasing the cluster size above the radiation wavelength. We also report on a sudden lineshape broadening and a slight spectral blue shift that accompanies the growth of large molecule-argon clusters and which we interpret as originating from a possible transition towards the surface of the molecule in order to minimize growth strain.