Brightly shining nanoparticles: lipophilic perylene bisimides in aqueous phase

However, the application of such dyes in aqueous solution turned out to be appreciably more difficult. Strongly hydrophilic groups such as sulfonic acid groups, polyether groups, and pyridyl substituents, 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 and this interferes with the preparation of the dyes in high purity. A dispersion 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 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 as an alternative; commercial name 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. 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 of D5 glass filers is 1.0–1.7 mm and particles can be retained that are smaller by a factor of three 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 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. 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 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