Annina Aebischer

Intrinsic quantum yields and radiative lifetimes of lanthanide tris(dipicolinates)

The efficiency with which the surroundings of trivalent lanthanide ions sensitize their luminescence (eta(sens)) is a key parameter in the design of highly emitting molecular edifices and materials. Evaluation of eta(sens) requires the measurement of the overall and intrinsic quantum yields obtained upon ligand and metal excitation, respectively. We describe a modified integration sphere enabling absolute determination of these quantities on small amounts of solid samples or solutions (60 muL). The sphere is tested for linear response of emitted versus absorbed light intensities with increasing concentration of Cs(3)[Ln(dpa)(3)] solutions (Ln = Eu, Tb). The overall (Q = 29 +/- 2%) and intrinsic (Q = 41 +/- 2%) quantum yields obtained for Eu allow the direct calculation of eta(sens) (71 +/- 6%) while the radiative lifetime (tau(rad) = 4.1 +/- 0.3 ms) is calculated from Q and the observed lifetime. The intrinsic quantum yield matches the value extracted from emission parameters using the simplified equation proposed by Werts et al. but, on the other hand, the theoretical estimate using spontaneous transition probabilities calculated from Judd-Ofelt (JO) parameters is off by -25% (3.15 ms). In the case of Cs(3)[Tb(dpa)(3)], the molar absorption coefficient of the (5)D(4)<--(7)F(6) transition is too small to measure Q for the solution but this quantity could be determined for the microcrystalline sample (72 +/- 5%, tau(rad) = 1.9 +/- 0.1 ms). In this case, the JO theoretical estimate leads to a much too short tau(rad) value. The large difference in eta(sens) for microcrystalline samples of Eu (85%) and Tb (42%) tris(dipicolinates) is attributed to back energy transfer in the latter compound consecutive to a sizeable overlap between the (5)D(4)-->(7)F(6) emission and the absorption spectrum of the dipicolinate triplet, this overlap being smaller in the case of the solution. The overall quantum yield of Na(3)[Yb(dpa)(3)] in aqueous solution is very low (0.015 +/- 0.002%) due to both poor sensitization efficiency (8%) and small intrinsic quantum yield (Q = 0.178 +/- 0.003%; tau(rad) = 1.31 +/- 0.02 ms). For evaluating intrinsic quantum yields of Yb in aqueous solutions of coordination compounds from lifetimes, a value of 1.2-1.3 ms is recommended.

Near-Infrared→Visible Light Upconversion in a Molecular Trinuclear d-f-d Complex

Giving the green light: The connection of two CrIII sensitizers around a central ErIII acceptor in a self-assembled cation provides high local metal concentrations that favor efficient nonlinear energy transfer upconversion luminescence (see picture). Upon selective low-energy near-infrared irradiation of CrIII-centered transitions, 1 displays an unprecedented molecular two-photon upconverted green ErIII-centered emission. Copyright

Indo-1 derivatives for local calcium sensing.

The role of calcium in signal transduction relies on the precise spatial and temporal control of its concentration. The existing means to detect fluctuations in Ca2+ concentrations with adequate temporal and spatial resolution are limited. We introduce here a method to measure Ca2+ concentrations in defined locations in living cells that is based on linking the Ca2+-sensitive dye Indo-1 to SNAP-tag fusion proteins. Fluorescence spectroscopy of SNAP-Indo-1 conjugates in vitro showed that the conjugates retained the Ca2+-sensing ability of Indo-1. In a proof-of-principle experiment, local Ca2+ sensing was demonstrated in single cells dissociated from muscle of adult mice expressing a nucleus-localized SNAP-tag fusion. Ca2+ concentrations inside nuclei of resting cells were measured by shifted excitation and emission ratioing of confocal microscopic images of fluorescence. After permeabilizing the plasma membrane, changes in the bathing solution induced corresponding changes in nuclear [Ca2+] that were readily detected and used for a preliminary calibration of the technique. This work thus demonstrates the synthesis and application of SNAP-tag-based Ca2+ indicators that combine the spatial specificity of genetically encoded calcium indicators with the advantageous spectroscopic properties of synthetic indicators.

Optimizing Millisecond Time Scale Near-Infrared Emission in Polynuclear Chrome(III)–Lanthanide(III) Complexes

This work illustrates a simple approach for optimizing long-lived near-infrared lanthanide-centered luminescence using trivalent chromium chromophores as sensitizers. Reactions of the segmental ligand L2 with stoichiometric amounts of M(CF(3)SO(3))(2) (M = Cr, Zn) and Ln(CF(3)SO(3))(3) (Ln = Nd, Er, Yb) under aerobic conditions quantitatively yield the D(3)-symmetrical trinuclear [MLnM(L2)(3)](CF(3)SO(3))(n) complexes (M = Zn, n = 7; M = Cr, n = 9), in which the central lanthanide activator is sandwiched between the two transition metal cations. Visible or NIR irradiation of the peripheral Cr(III) chromophores in [CrLnCr(L2)(3)](9+) induces rate-limiting intramolecular intermetallic Cr→Ln energy transfer processes (Ln = Nd, Er, Yb), which eventually produces lanthanide-centered near-infrared (NIR) or IR emission with apparent lifetimes within the millisecond range. As compared to the parent dinuclear complexes [CrLn(L1)(3)](6+), the connection of a second strong-field [CrN(6)] sensitizer in [CrLnCr(L2)(3)](9+) significantly enhances the emission intensity without perturbing the kinetic regime. This work opens novel exciting photophysical perspectives via the buildup of non-negligible population densities for the long-lived doubly excited state [Cr*LnCr*(L2)(3)](9+) under reasonable pumping powers.

Near infrared to visible photon upconversion in Re4+ doped Cs2ZrBr6

Two visible luminescence bands are observed in Re4+:Cs2ZrBr6 under NIR excitation at temperatures between 10 and 300 K. We perform absorption, luminescence, excitation, and time resolved spectroscopic studies at various temperatures to characterize this upconversion luminescence and its driving mechanism. From the time dependent measurements we assign a ground state absorption/energy transfer upconversion type mechanism to the upconversion process. The visible to near-infrared photon ratio is 0.008 at T=50 K with a laser power density of 415 W/cm2. Most of the visible luminescence is quenched by a strong cross-relaxation process out of the highest emitting level as well as intense radiative interexcited state transitions. The ReBr62− unit is one of very few transition metal complexes with three emitting electronic levels in the near-infrared and visible range of the electromagnetic spectrum.

Near-infrared to visible photon up-conversion in V3+, Re4+ co-doped Cs2NaYCl6

Cs 2 NaYCl 6 co-doped with V 3+ and Re 4+ was studied by optical absorption and luminescence spectroscopy. In the cubic host lattice both dopant ions substitute for the octahedrally coordinated Y 3+ . Visible by eye red up-conversion luminescence centered around 14000cm -1 is observed upon near-infrared excitation within the energy range from 10 000 to 12000cm -1 between 15 and 300 K. For V 3+3 T 2 g excitation at 11 350 cm -1 ,the efficiency of this up-conversion process increases with increasing temperature. An energy transfer step from V 3- to Re 4+ followed by an energy transfer up-conversion process on the Re 4+ system is shown to be the likely mechanism responsible for this phenomenon.

Photon upconversion emission in transparent solutions of nanocrystalline hexagonal NaGdF/sub 4/:Yb/sup 3+/, Er/sup 3+/

Photon upconversion (UC) is a nonlinear optical process in which long-wavelength (NIR) radiation is converted into short-wavelength (VIS) radiation. We have extended our synthetic and spectroscopic work towards Yb/sup 3+//Er/sup 3+/ codoped nanoparticles of NaGdF/sub 4/. The /spl beta/-NaGdF/sub 4/:Yb/sup 3+/, Er/sup 3+/ nanoparticles of roughly spherical shape and a size of about 30 nm can be transparently dissolved in DMSO. The UC emission spectra upon laser excitation of the transparent solution of /spl beta/-NaGdF/sub 4/:Yb/sup 3+/, Er/sup 3+/ nanoparticles in DMSO are observed.