We compare two dendrimers, which contain the same luminescent chromophores but differ in dimensionality, for the detection of an explosive analyte via PL quenching. Each dendrimer has first generation biphenyl dendrons with 2-ethylhexyloxy surface groups but differ in the core units. One dendrimer has a bifluorene core and hence has a “planar†structure, whereas the second has four bifluorene units tetrahedrally arranged around an adamantyl center and hence has a “three-dimensional†structure. Solution Stern–Volmer measurements have previously been reported to show that the three-dimensional dendrimer has a higher binding constant than that of the more planar compound. Films of the dendrimers rapidly detect 2,4-dinitrotoluene (DNT) with thinner films (∼25 nm) being more responsive than thicker films (∼85 nm). Neutron reflectometry measurements show that the analyte can diffuse completely through the films with the three-dimensional dendrimer absorbing more of the analyte. The rate of recovery of the PL was faster for the planar dendrimer than the three-dimensional material showing that large binding constants are not necessary for reversible detection of analytes.© 2011, American Chemical Society