Nanoparticles of a 1,2-bis-(3-thienyl)perfluorocyclopentene derivative were first prepared by a reprecipitation method. The photochromism and kinetic behavior of the aggregates were investigated. As nanoparticles formed, the absorption maximum peaks of the closed-ring form were blue-shifted. The blue shift was explained by H-aggregates formation, which was consistent with the decreased fluorescence efficiency of the open-ring form. Spectrokinetic study showed that photochromic transformations were strongly affected by molecular aggregation. The decrease of photocyclization activity was mainly attributed to the intra- and intermolecular effects exerted by aggregation. (C) 2003 Elsevier B.V. All rights reserved.