Water-soluble and fluorescent dendritic perylene bisimides for live-cell imagingw

Molecular imaging, especially fluorescence imaging, has been proven to be useful in clinical diagnosis and drug discovery because it has fine temporal and spatial resolution and can be safely performed with simple instruments and facilities. There are several characteristics of a successful optical molecular probe for medical imaging including water solubility, brightness, photostability, and low cytotoxicity. Although there are a variety of water soluble chromophores commercially available today, each optical molecular probe has its own disadvantage, such as low fluorescence quantum yields, photobleaching, or high cytotoxicity. Recently, Li and coworkers reported heavy-metal complexes as phosphorescent dyes for living cell imaging. Perylene bisimides (PBIs), the well-known fluorescent dyes, display exceptional chemical, thermal, and photochemical stability with high fluorescence quantum yields close to unity. However, PBIs exhibit poor water solubility and very weak fluorescence in water because of aggregation of perylene chromophores. Although water-solubility has been achieved by introducing ionic groups on the imide position, the fluorescence quantum yields of these perylene chromophores remain low. Müllen and coworkers reported a new strategy for water-soluble and fluorescent perylene chromophores. The introduction of hydrophilic substituents onto the bay region of the perylene chromophore resulted in a loss in planarity of the aromatic scaffold, obtaining water-soluble chromophores with moderate fluorescence quantum yields. Recently Würthner and co-workers reported a set of highly water-soluble polyglycerol-dendronized PBIs with outstanding fluorescence quantum yields in water. Furthermore, low cytotoxicity PBIs with high fluorescence in aqueous medium are highly desired. Poly(ethylene glycol) (PEG) is currently the most used polymer in the biomedical field of drug delivery and the only polymeric therapeutic that has a market approval for different drugs. The success of PEG is based on its hydrophilicity, decreased interaction with blood components, and high biocompatibility. In this study, we present the synthesis and property of PEG encapsulation of perylene bisimides (PEPBIs). The chemical structures of PEPBIs are shown in Scheme 1. We prepared these PEPBIs with triblock structures: perylene bisimide moieties as fluorescence cores, branched oligo (glutamic acid)s as scaffolds for suppressing aggregation of the central perylene bisimide chromophores, and polyethylene glycol chains as hydrophilic shells for inducing water solubility and reducing cytotoxicity. These PEPBIs are expected to have low cytotoxicity, good solubility, and high fluorescence quantum yields in water. These PEG encapsulations of perylene bisimides were synthesized by multi-stepwise coupling reactions with perylene tetracarboxylic acid bisanhydride 1, L-aspartic acid 2 and 2,5,8,12,15,18-hexaoxa-10-nonadecanamine 4, as shown in Scheme S1 (ESIw). The attachment of dendritic wedges to 3,4,9,10-perylenetetracarboxyldiimide required initial modification of 3,4,9,10-perylenetetracarboxyldiimide with suitable functional groups. N,N0-Di((S)-1,4-dicarboxy)-3,4,9, 10-perylenetetracarboxyldiimide 3 was easily prepared by twofold condensation between 3,4,9,10-perylenetetracarboxyldianhydride and L-aspartic acid in imidazole at 120 1C for 1 h. The 2,5,8,12,15,18-hexaoxa-10-nonadecanamine 4, a key intermediate for the synthesis of PEPBIs, was obtained according to literature procedures. G0 was obtained via a coupling reaction between 3,4,9,10-perylenetetracarboxyldianhydride and 4. The reaction of 4 with Cbz-protected L-aspartic acid yielded compound 5, and then the Cbz group was removed by Scheme 1 Chemical structures of PEPBIs.