A novel pentadecanuclear lanthanide hydroxy cluster [{Ln15(μ3-OH)20(PepCO2)10(DBM)10Cl}Cl4] (Ln = Eu (1), Tb (2)) featuring the first example with peptoids as supporting ligands was prepared and fully characterized. The solid-state structures of 1 and 2 were established via single-crystal X-ray crystallography. ESI-MS experiments revealed the retention of the cluster core in solution. Although OH groups are present, 1 showed intense red fluorescence with 11(1)% absolute quantum yield, whereas the emission intensity and the quantum yield of 2 were significantly weaker. In vitro investigations on 1 and 2 with HeLa tumor cells revealed an accumulation of the clusters in the endosomal-lyosomal system, as confirmed by confocal microscopy in the TRLLM mode. The cytotoxicity of 1 and 2 toward the HeLa cells is moderate.

The fluorophore rhodamine B is often used in biological assays. It is inexpensive, robust under a variety of reaction conditions, can be covalently linked to bioactive molecules, and has suitable spectral properties in terms of absorption and fluorescence wavelength. Nonetheless, there are some drawbacks: it can readily form a spirolactam compound, which is nonfluorescent, and therefore may not be the dye of choice for all fluorescence microscopy applications. Herein this spirolactam formation was observed by purifying such a labeled peptoid with high performance liquid chromatography (HPLC) and monitored in detail by making a series of analytical HPLC runs over time. Additionally, a small library of eight peptoids with rhodamine B as label was synthesized. Analysis of the absorption properties of these molecules demonstrated that the problem of fluorescence loss can be overcome by coupling secondary amines with rhodamine B.

Two-photon excitation induced photoluminescence (2PE-PL) microscopy of CdSe colloidal quantum dots at the single-entity level is demonstrated. We provide evidence for single nanoparticle microscopy in the two-photon excitation regime by varying the laser excitation average power, as well as by measuring the confocal point spread function in three dimensions with a single quantum dot. Model calculations of the point spread function are in good agreement with our experimental findings in the 2PE-PL nonsaturation regime. Ultimately, we observe photon antibunching and triggered single-photon emission at room temperature from those quantum nanostructures under two-photon excitation in a well-defined three-dimensional photonic volume.While two-photon absorption had been theoretically described by Maria Göppert-Mayer as early as in 1931, 1 the field of two-photon excitation microscopy was only opened by W. Denk et al. in 1990. 2 It is of specific interest for many characterization purposes such as subcellular and medical imaging since it can provide a large spectral distance between the excitation light and the observed photoluminescence of suitable chromophores. It thus enables straightforward wavelength filtering, confines the optical detection capability in three dimensions due to its inherent quadratic intensity dependence, and offers improved imaging contrast due to decreased scattering, NIR absorption, and fluorescence in biological environments. Moreover, it can be advantageous in spectroscopy and microscopy for excited-state symmetry selection rule studies.The two-photon excitation imaging technology triggered the search, creation, and characterization of favorable chromophores 3 with low intensity two-photon laser excitation conditions together with long observation time scales. Such organic fluorophores, however, show low two-photon excitation cross sections together with rapid photobleaching properties 4 compared to the colloidal semiconductor CdSe nanocrystals used here (also referred to as quantum dots), with a two-photon excitation cross section as high as 66 000 GM (Göppert-Mayer units GM = 10 −50 cm 4 photon/s). 5 Also, semiconductor quantum nanostructures show at the singleentity level strong photoluminescence rates under ambient conditions that favored quantum optics experiments such as photon antibunching under one-photon excitation 6-8 and even triggered single-photon emission under pulsed one-photon absorption. 9 Fine tuning and an energetic smoothing of the interface of the core-shell nanostructure have recently lead to nonblinking semiconductor nanocrystals with photoluminescence excited-state lifetimes in the range of 4.0-5.6 ns. 10 Although single-photon sources and antibunching mediated by two-photon absorption of a coherent laser beam have been proposed theoretically as early as in 1975, 11,12 a proofof-concept experiment is still missing. This is commonly attributed to the lack of appropriate chromophore systems with high 2PE cross section combined with high photostability. 13 Such experimen...

Incorporation of fluorous ponytails such as polyfluorinated alkyl residues (CH2)m(CF2)nCF3 leads to a novel class of bright rhodamine-based fluorescence dyes. These dyes combine the excellent photophysical properties of the frequently used rhodamine dyes with the unique features of "light" fluorous molecules. One of those features is the possibility to separate substances utilizing fluorous solid-phase extraction (F-SPE), which is based on the specific intermolecular interaction between fluorous compounds. Thus, molecules, which are labeled with these new dyes, are not only accessible to fluorescence experiments, but can also be easily purified (via so-called FluoroFlash columns) prior to use. The dyes were bound to a cell penetrating peptoid (polycationic oligo(N-substituted) glycine) on solid supports. These conjugates were purified with F-SPE before their photophysical and biological properties were investigated.

Single-molecule microscopy is a powerful tool for investigating various uptake mechanisms of cell-penetrating biomolecules. A particularly interesting class of potential transporter molecules are peptoids. Fluorescence labels for such experiments need to comply with several physical, chemical, and biological requirements. Herein, we report the synthesis and photophysical investigation of new fluorescent pyridinium derived dyes. These fluorescent labels have advantageous structural variations and spacer units in order to avoid undesirable interactions with the labeled molecule and are able to easily functionalize biomolecules. In our case, cell-penetrating peptoids are successfully labeled on solid supports, and in ensemble measurements the photophysical properties of the dyes and the fluorescently labeled peptoids are investigated. Both fluorophores and peptoids are imaged at the single-molecule level in thin polymer gels. With respect to bleaching times and fluorescence lifetimes the dye molecules and the peptoids show only slightly perturbed optical behaviors. These investigations indicate that the new fluorophores fulfill well single-molecule microscopy and solid-phase synthesis requirements.