We describe herein an adsorption-induced energy transfer between phenanthrene, a major environmental pollutant, and a fluorescently labeled dendrimer acting as a host molecule. We find experimentally that such energy transfer is the most efficient at a solvent pH of 8 and for a phenanthrenedendrimer molar ratio of 12. Using molecular dynamics simulations we show that the strongest binding interactions occur between phenanthrene and the primary amines of the dendrimer. The simulations provide evidence that at low pH, phenanthrene-phenanthrene interactions are favorable and compete with phenanthrene-dendrimer binding. This study offers a new scheme for detecting dendrimer molecular assembly and a physical basis for exploiting dendrimer nanotechnologies for water purification and environmental remediation.