The dispersion of lipophilic perylene bisimides down to nanoparticle dimensions opens the aqueous phase to these highly fluorescent, water insoluble materials. Perylene dyes 1 1, perylene tetracarboxylic bisimides, characterised by their high stability, are used as high performance pigments. 2 The attachment of solubility increasing groups such as long-chain secondary alkyl groups 3 ("swallow-tail groups") to the nitrogen atoms of 1 results in readily soluble, highly fluorescent dyes for

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... lic solvents with fluorescence quantum yields 4 close to 100%. However, the application of such dyes in aqueous solution turned out to be appreciably more difficult. Strongly hydrophilic groups such as sulfonic acid groups, 5 polyether groups, 6 and pyridyl substituents, 7 basically induce water solubility of 1; however, these materials exhibit a strong tendency to aggregate in the highly hydrophilic medium. Moreover, such dyes exhibit a high aptitude for the collection of heavy metal ions 6 and this interferes with the preparation of the dyes in high purity. A dispersion 8 of a special derivative of 1 with a labile crystal lattice was only successful for low concentrations of particles and a spontaneous dispersion of a crown ether 9 derivative of 1 is limited to this special dye. A direct application of the lipophilic, water-insoluble materials would bring about an appreciable improvement. We have applied a dispersing detergent of maleinated linseed oil 10 as an alternative; commercial name 11 Bomol-4N. The lipophilic dyes 1 were dissolved in the dispersant and spread in more than a hundred times the quantity of water; for the preparation of nanoparticles by the application of detergents see ref. 12. Water-insoluble dyes were therefore required that are both lipophilic and readily soluble in the dispersant. High solubility of 1 was attained by the attachment of longchain secondary alkyl groups to the nitrogen atoms. 3 Such dye preparations 1a form strongly yellow fluorescent liquids, remaining unchanged with storage, where the dispersion of the dye is so fine that it is not retained by a D5 glass filter, not even in traces; see Fig. 1 . The porosity 13 of D5 glass filers is 1.0-1.7 mm and particles can be retained that are smaller by a factor of three 14 or even more. The complete passing of particles leaving the glass filter colourless indicates a size of the order of nanometres. The dispersion of the dye must have progressed far beyond the size of the previously reported 8, 9 particles because the latter could be completely collected by a D4 glass filter with pores of 9-15 mm size. The small size of the particles was verified by dynamic light scattering (DLS) measurements where a maximum of the distribution was found at 60 nm; see Fig. 2 . The comparably narrow distribution is as remarkable as the result that no particles were found larger than 600 nm. Thus, complete dispersion down to nano-dimensions proceeded. Information about the direct environment of the nano-dispersed dye molecules was obtained by UV/Vis spectroscopy; the chromophore of 1 is very weakly, but characteristically, solvatochromic. 15 Surprisingly, the UV/Vis spectra of nano-dispersed 1b in water are identical within 2 nm with the spectra in homogeneous solution in the lipophilic chloroform; see Fig. 1 . The unaltered spectral shape indicates isolated chromophores in the nanoparticles where exciton interaction 16 with neighbouring chromophores can be excluded. Such a type of nano-dispersion is very different from previously described microcrystalline particles with interacting chromophores; see for example ref. 17. Moreover, rather high concentrations of dyes in water can be obtained and are only limited by the solubility of Fig. 1 UV/Vis absorption and fluorescence spectra of 1. Left: absorption spectra from bottom to top: 1b in chloroform (red), 1b nanoparticles in water (black), 1a nanoparticle in water (blue). Right: fluorescence spectrum of 1b nanoparticles in water (black).