Water-soluble, self-assembled nanocapsules composed of a functional bilayer membrane and enclosed guest molecules can provide smart (that is, condition responsive) sensors for a variety of purposes. Owing to their outstanding optical and redox properties, perylene bisimide chromophores are interesting building blocks for a functional bilayer membrane in a water environment. Here, we report water-soluble perylene bisimide vesicles loaded with bispyrene-based energy donors in their aqueous interior. These loaded vesicles are stabilized by in situ photopolymerization to give nanocapsules that are stable over the entire aqueous pH range. On the basis of pH-tunable spectral overlap of donors and acceptors, the donor-loaded polymerized vesicles display pH-dependent fluorescence resonance energy transfer from the encapsulated donors to the bilayer dye membrane, providing ultrasensitive pH information on their aqueous environment with fluorescence colour changes covering the whole visible light range. At pH 9.0, quite exceptional white fluorescence could be observed for such water-soluble donor-loaded perylene vesicles.

A series of four spermine-functionalized perylene bisimide dyes without linkers (1) and with linkers (2-4) between the chromophore and the polyamine was synthesized. Protonation of the spermine moieties resulted in the formation of highly water-soluble dyes with up to six positively charged ammonium ions. The aggregation behavior of these strongly fluorescent bola-amphiphiles was studied in pure water as solvent by UV/Vis and fluorescence spectroscopy, and an astonishingly high fluorescence quantum yield of up to Phi(fl)=0.90 was observed for PBI 1. Atomic force microscopy and transmission electron microscopy were applied for the visualization of the aggregates on surfaces. Molecular modeling studies were performed by force-field calculations to explore the aggregate morphologies, which also provided valuable information on the influence of the additional alkylcarbonyl linkers. Our detailed spectroscopic and microscopic investigations revealed that the excellent optical properties of perylene bisimide chromophores can be used even in pure deionized water if their aggregation is efficiently suppressed.

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