Optical properties of transparent polymer films of polymethylmethacrylate doped (up to 25 wt.%) by dipolar N,N-dimethylaminobenzylidene 1,3-indandione (DMABI) molecules were studied. Formation of DMABI nanocrystallites, increasing in their density and size with dopant concentration, was revealed by optical microscopy. Transformation of the fluorescence spectrum from the molecular-like emission (for the low dopant concentration below I wt.%) to highly red-shifted fluorescence corresponding to self-trapped excitons in the crystallites was observed. It was shown, that due to the resonant energy transfer in the blend, the DMABI nanocrystals can serve as efficient fluorescence markers. A reverse dependence on the dopant concentration was observed for high intensity excitation, when the DMABI nanocrystals act as nonradiative traps due to efficient exciton-exciton annihilation. (c) 2007 Elsevier B.V. All rights reserved.