Dimethylanthracene (DMA), which exhibits almost no self-association in bulk organic solvents, forms a dimer and emits excimer-like fluorescence in a gamma-cyclodextrin nanocavity in a dilute aqueous solution. The B-1(b) and L-1(a) electronic transitions of the DMA dimer split by 2230 and 344 cm(-1), respectively, in a fluorescence excitation spectrum obtained with the excimer-like emission. From these energy splits, the structure of dimer in relation to a dielectric constant inside gamma-CD was discussed on the basis of atom-atom Lennard-Jones potential calculations including Coulombic interactions. Excitation relaxations of DMA in the presence of alpha-, beta-, and gamma-CDs in aqueous solutions were investigated by time-resolved fluorescence. The results suggest that both the hydrated and anhydrated species exist in the alpha- and gamma-CD complexes, while only the anhydrated species exists in the beta-CD complex.