Novel tetra-cationic xanthene derivative (Flu) was synthesized. Its adsorption and photochemical behaviors on the clay surface were investigated. Fluorescence quantum yield (ϕf) and fluorescence lifetime were 0.50 and 2.9ns for Flu/clay complex. ϕf of Flu was enough high (>0.1) even at high density conditions (0.080moleculesnm−2). It is supposed that the strong interaction between clay and Flu by the ‘Size-Matching Effect’ realizes the highly emissive clay complexes at high density adsorption condition by a suppression of a molecular aggregation, which tends to decrease the photochemical activity.