Surface Complexes for NIR-to-NIR Single Nanoparticle Imaging

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Article

One–photon NIR sensitization of well-defined Yb(III) surface complexes for NIR–to–NIR single nanoparticle imaging. Giuseppe Lapadula, David Trummer, Matthew P. Conley, Martin Steinmann, Ying-Fen Ran, Sophie Brasselet, Yannick Guyot, Olivier Maury, Silvio Decurtins, Shi-Xia Liu, and Christophe Coperet Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00306 • Publication Date (Web): 23 Feb 2015 Downloaded from http://pubs.acs.org on February 27, 2015

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Chemistry of Materials

One–photon NIR sensitization of well-defined Yb(III) surface complexes for NIR–to–NIR single nanoparticle imaging. Giuseppe Lapadula,1 David Trummer,1 Matthew P. Conley,1 Martin Steinmann,2 Ying-Fen Ran,2 Sophie Brasselet,4 Yannick Guyot,5 Olivier Maury,3 Silvio Decurtins,2 Shi-Xia Liu,2,* Christophe Copéret1,* 1

ETH Zürich, Department of Chemistry and Applied Biosciences, Vladimir Prelog

Weg 2, CH−8093 Zürich, Switzerland 2

Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-

3012 Bern, Switzerland 3

Université de Lyon, CNRS, UMR5182 – CNRS – Ecole Normale Supérieure de

Lyon, Université de Lyon 1, 46 alleé d’Italie, 69007 Lyon, France. 4

Institut Fresnel, UMR CNRS 7249, Université Aix-Marseille, Ecole Centrale

Marseille, 13397 Marseille Cedex 20, France 5

Université de Lyon, Institut Lumière Matière, UMR 5306 CNRS–Université Lyon1,

10 rue Ada Byron, 69622 Villeurbanne Cedex, France

ACS Paragon Plus Environment

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Abstract. Silica nanoparticles of 12 nm were surface–doped with ca. 350 (TTFdppz)Yb(III) surface species, containing bis(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2’,3-c]phenazine (TTF-dppz) as an antenna ligand through a Surface Organometallic Chemistry approach. These nanoparticles absorb and emit in the NIR (λabs = 750 nm, λem = 983 and 1050 nm) with a lifetime of 2.8 µs, similarly to the corresponding Yb(III) molecular complex (λabs = 750 nm, λem = 975, 986, 1009 and 1020 nm with τ1 = 6.93 µs). The silica materials were fully characterized using combined spectroscopic techniques (IR, NMR, UV-vis, luminescence and lifetime), molecular models and isostructural diamagnetic yttrium containing materials for easier characterization by NMR spectroscopy. Having established the surface structures and photophysical properties of these nanoparticles we transposed this methodology to larger silica particles with a diameter of ca. 100 nm. These larger nanoparticles have similar photophysical properties and contain ca. 30’000 chromophores, making possible one-photon NIR-to-NIR emission optical microscopy imaging of single nanoparticles.


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1 Introduction Near infrared light (NIR, 650-1800 nm) is particularly attractive for imaging applications compared to UV-visible wavelengths due to the increased transparency of biological tissues in this spectral range. In the context of bioimaging there are several spectral windows where absorption, scattering and auto fluorescence are reduced in biological tissues (λ = 650 – 900 nm, 1000 – 1350 nm and 1500 – 1800 nm). 1,2 This triggered the design of luminescent probes featuring combined excitation and emission in the NIR with an optimized brightness (defined as the product of the extinction coefficient by quantum yield). During the last decades numerous NIR to NIR biomarkers have been reported including organic chromophores, inorganic nanoparticles or quantum dots. 1-5 In this context, f-elements are attractive candidates due to their unique photophysical properties in the NIR. For example, Yb(III) derivatives exhibit a sharp emission at 980 nm assigned to the 2F5/2 → 2F7/2 transition with µs lifetimes.6,7 The linear excitation of Yb(III) is generally not efficient because of the forbidden f-f transition, which results in very low absorption coefficients. This process is used in the case of up-converting Y2O3:Er3+/Yb3+ nanoparticles, where high local concentrations in Yb(III) are present. However, the low absorption coefficients of the f-f transitions are prohibitive for diluted solutions containing molecular derivatives unless high power excitation sources are used.8,9 An organic antenna ligand that strongly absorbs and transfers energy to the metal excited state by various photophysical processes leads to enhanced f-f emission in molecular species in solution.10 The ideal organic ligand would combine both excitation and emission in the NIR spectral range. However, the sensitizing ligand usually absorbs in the UV-visible region. The longest wavelengths reported for Yb(III) sensitization are around 550 nm


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using antenna ligands containing xanthene (fluorescein, eosin, erythrosine, or rhodamine 6G), Bodipy, murex or tetrathiafulvalene based dyes.11-16 Nonlinear twophoton excitation can displace the ligand excitation to the NIR spectral range.17-21 However, this process requires an intense laser source and is less efficient than the more straightforward linear sensitization of Yb(III) luminescence using an antenna ligand absorbing in the NIR. Here we report that a highly π−conjugated, fused donor – acceptor system, 4’,5’bis(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2’,3’-c]phenazine

(TTF-dppz),22-25

sensitizes Yb(III) molecular and surface complexes, up to 800 nm and can be used to prepare bright 100-nm particles, which can be imaged at the single-particle level. Functionalized TTF derivatives are frequently used in the field of molecular26 and optoelectronics.27-29 TTF-dppz does complex lanthanides,30 but f-block luminescence sensitization was not reported to the best of our knowledge. In contrast to many other TTF containing complexes,29,31,32 the fused TTF-dppz ligand strongly absorbs in the green to red (500 – 700 nm) of the visible spectrum, and this absorption is shifted deeper into the NIR upon coordinated to metal ions.22 In general, Yb(III) complexes have low emission quantum yields, rarely exceeding 3-4% for molecular complexes.33 One strategy to enhance the overall brightness of the TTF-Yb(III) complexes is to incorporate them into nano-objects containing a large number of emitting species. We recently reported the preparation of ultra-bright silica nanoparticles that can be detected by optical two-photon NIR-to-NIR microscopy.34 These 12-nm diameter silica particles were functionalized through grafting M(N(SiMe3)2)3 complexes (M = Y and Yb) followed by a thermolysis step to form isolated M@SiO2 surface species using surface organometallic chemistry.35-39 M@SiO2 contains open coordination sites that can bind bipyridyl antenna ligands.


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While the silica particles pre-treated at 700 °C contain roughly 350 metal sites, the luminescent material contains only ca. 180 well-defined chromophores on the surface, because of the large bipyridyl ligand. We reasoned that a planar TTF-dppz ligand would coordinate to more surface Yb-sites due to the lower projected surface area of this ligand than DEAS bipyridyl antenna ligands. We synthesized densely functionalized silica-nanoparticles with roughly 350 Yb-(TTF-dppz) centers per 12nm nanoparticle, along with its diamagnetic Y-analogue and isostructural Yb/Ymolecular complexes to ease the characterization of the surface complex at the molecular level. The Yb-surface species absorb and emit in the NIR, allowing the preparation and the imaging of 100-nm particles in the NIR at the single particle level due to their exceptional brightness as a result of the presence of ca. 30’000 Yb-(TTFdppz) centers per particle. 2 Results 2.1 Molecular complexes The reaction of M(OSi(OtBu)3)3(κ2–HOSi(OtBu)3)34,40 (M = Y, Yb) with deeppurple TTF-dppz in CH2Cl2 solution (Scheme 1) results in the formation of deeply colored M(OSi(OtBu)3)3(TTF-dppz) complexes in good yields (81% and 78% for M = Yb and Y, respectively).

Scheme 1: Synthesis of M(OSi(OtBu)3)3(TTF-dppz) (M = Y and Yb)


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The infrared (IR) spectrum of M(OSi(OtBu)3)3(TTF-dppz) contains blue-shifted vibrations for C=C relative to free TTF-dppz (Figure S1). The 1H NMR of Y(OSi(OtBu)3)3(TTF-dppz) (Figure S2) shows resonance signals for the protons attached to the dppz core that are shifted to higher frequencies with respect to those of the free ligand. The resonance intensity between the aromatic and the –OtBu are in a 2:81 ratio, in agreement with a 1:1 TTF-dppz:Y stoichiometry. In contrast, the

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C

NMR of the compound Y(OSi(OtBu)3)3(TTF-dppz) is less informative and contains signals that are only slightly shifted compared to the free ligand (Figure S3). The carbon atoms at the 2,9-position of the coordinated TTF-dppz ligand falls at 155 ppm, a downfield shift by only 3 ppm with respect to the free ligand. As expected, the 1H NMR spectrum of paramagnetic Yb(OSi(OtBu)3)3(TTF-dppz) contains broad and unresolved peaks, however the broad peak centered at –2.2 ppm (Δ1⁄2 = 102 Hz) can be assigned to the –OtBu from the siloxides (Figure S4). Due to the reluctance of M(OSi(OtBu)3)3(TTF-dppz) to form X-ray quality crystals, we synthesized the parent 1,10-phenanthroline (Phen) analogues in order to gain insights about the coordination sphere of the molecular species. The reaction of M(OSi(OtBu)3)3(κ2–HOSi(OtBu)3) with one equivalent of Phen in dichloromethane forms M(OSi(OtBu)3)3(Phen) in good yields (77% and 78% for M = Y and Yb, respectively; see NMR of the Y complex in Figures S5 and S6). Single crystals of M(OSi(OtBu)3)3(Phen) (M = Y and Yb) were grown from concentrated CH2Cl2 solution. The crystals of both M(OSi(OtBu)3)3(Phen) shattered at ca. 190 K, which required us to record their structures at 200 K. The single crystal X-ray diffraction measurements at the higher temperature result in increasing dynamic disorder that enlarges the thermal ellipsoids. As depicted in Figure 1, Y(OSi(OtBu)3)3(Phen) and Yb(OSi(OtBu)3)3(Phen)


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are nearly isostructural. The coordination geometry around metal ions is a slightly distorted square based pyramid where the two nitrogen atoms from the ligand and two anionic oxygens form the basal plane while the remaining siloxide is in the apical position. The τ value, a dimensionless parameter used to estimate the distortion of a 5 coordinated species where for τ = 0 the structure is an ideal square based pyramid and for τ = 1 the species would be an ideal trigonal bipyramid41 is close to 0 (τ = 0.07 and 0.1 for Y and Yb, respectively) indicating that both complexes have geometries closer to square based pyramids. The M-ligand distances in the first coordination sphere are similar, but slightly longer in the case of Y because of its slightly larger ionic radius. Additional NMR, crystallographic details, and the full list of distances and angles can be found in the supporting information.

Figure

1:

Single

crystal

structures

of

a)

Yb(OSi(OtBu)3)3(Phen),

b)

Y(OSi(OtBu)3)3(Phen). Ellipsoids are shown at 50% probability, tBu groups and hydrogen atoms are omitted for clarity.

The binding of the antenna ligand to the metal center can be measured by titration using UV-Vis spectroscopy. The absorption spectrum of Y(OSi(OtBu)3)3(TTF-dppz) displays a maximum at 541 nm with an extinction coefficient (ε) of 23000 M-1cm-1, while Yb(OSi(OtBu)3)3(TTF-dppz) exhibits a maximum at 610 nm and ε = 28000 M1

cm-1 (Figure S7). Titrating a solution of M(OSi(OtBu)3)3(κ2–HOSi(OtBu)3) with


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aliquots of TTF-dppz results in two isosbestic points at 383 and 594 nm for M = Yb and at 383 and 558 nm for M = Y, indicating that the metal complex and the ligand are related by linear stoichiometry (Figure S8). The resulting titration curves were fitted to a one-site binding curve to determine the equilibrium constant Keq. The binding constant of TTF-dppz to Yb is Keq = 7.7(3)×103, while the binding constant for the Y derivative is slightly lower (2.7(4)×103). 2.2 Yb(III) Surface Complexes [M@SiO2] (M = Y, Yb) was prepared by the reaction of SiO2-700 (0.26 mmol SiOH g-1) with M(N(TMS)2)3, followed by a thermal treatment under vacuum. Previous studies established that [M@SiO2] contains isolated lanthanide centers integrated onto the silica surface.34 Contacting a deep purple CH2Cl2 solution of TTF-dppz with solid [M@SiO2] yields [TTF-dppz•M@SiO2] as shown in Scheme 2.

Scheme 2: Grafting the TTF-dppz antenna ligand on the M-doped silica surface to give [TTF-dppz•M@SiO2].

The IR spectra of [TTF-dppz•M@SiO2] (M = Y, Yb) are identical and contain aromatic C=C vibration bands at 1361, 1442 and 1448 cm-1 that are blue shifted by ∆ν = 6, 9 and 18 cm-1 compared to the free ligand, consistent with TTF-dppz coordination to the metal sites (Figure S1). Elemental analysis of [TTF-


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dppz•Yb@SiO2] gives 3.94%wt Yb and 1.59%wt, 1.09%wt and 4.94%wt of S, N and C, respectively. These values correspond to 42.4 ±1 C/Yb, 3.9 ±1 N/Yb, and 5.9 ±1 S/Yb. The S/Yb ratio in [TTF-dppz•Yb@SiO2] indicates a near quantitative coordination of the ligand to the ytterbium centers. The higher than expected carbon and nitrogen ratios result from the formation and the subsequent competitive reaction of (Me3Si)2NH with surface silanols, leading to the partial passivation of the surface (formation of OSiMe3 groups in [Yb@SiO2]). For [TTF-dppz•Y@SiO2], 2.11%wt of Y, 1.15%wt, 1.01%wt and 3.81%wt of S, N and C, respectively are found. The 4 ± 1 S/Y ratio indicates that roughly 30% less chromophores are coordinating Y3+ ions compared to the Yb3+ analogue. Furthermore, the energy dispersed X-ray (EDX) analysis

before

and after coordination

of

TTF,

[Yb@SiO2]

and

[TTF-

dppz•Yb@SiO2], shows the presence of ytterbium at the surface of the particles on both samples, consistent with surface doping (Figure S9). The 1H Magic Angle Spinning (MAS) NMR of [TTF-dppz•Y@SiO2] contains broad peaks in the aromatic region from 10.5 to 6 ppm corresponding to the coordinated TTF-dppz ligand (Figure S10). In the aliphatic region, in addition to the peaks assigned to the CH3 and CH2 moieties at 1.3 and 0.9 ppm the peak of the SiMe3 groups is present at 0 ppm. The

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C Cross Polarization Magic Angle Spinning

(CPMAS) of [TTF-dppz•Y@SiO2] is not very informative since the peaks in the aromatic region have similar chemical shifts for the free and coordinated ligand as already found for the corresponding molecular complex (Figure S3). 2.3 Photophysical properties The UV-vis solid-state spectra of the molecular Yb(OSi(OtBu)3)3(TTF-dppz) compounds contain strong displacement of the absorption band into the NIR spectral range (Figure S7), exhibiting an absorption maximum at 600 nm, which continues up


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to the cut-off at 850 nm. The luminescence spectrum of the complex shows four main transitions at 975, 986, 1009 and 1020 nm, as shown in Figure 2. Decreasing the temperature to 77 K results in a significant decrease of the vibronic broadening leading to an enhanced resolution, resolving the band at 1020 nm in two bands at 1010 and 1027 nm respectively. a)

b)

Figure 2: a) Solid state emission spectrum of Y(OSi(OtBu)3)3(TTF-dppz) at room temperature and b) 77 K, normalized NIR emission measured with λex = 600 nm. The molecular Yb(OSi(OtBu)3)3(TTF-dppz) in the solid state showed a monoexponential decay of the luminescence lifetime (eq 1), leading to τ1 = 6.9 µs (Figure S11). The same mono-exponential decay was observed in a 10-4 mM toluene solution, however the lifetime is slightly shorter (τ1 = 5.5 µs), possibly due to competitive dissociation of the ligand under these conditions.

= / +

(1)

The grafted complex [TTF-dppz•Yb@SiO2] contains similar features in the UV-vis


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spectrum, with a strong displacement of the absorption band further into the NIR spectral range. In [TTF-dppz•Yb@SiO2] the absorption is further shifted to the NIR and the maximum absorption wavelength is now at 650 nm while the absorption continues above the cut-off at 850 nm. This result indicates that [TTF-

1

1.5

2

2.5

dppz•Yb@SiO2] absorbs in the NIR in the solid state (Figure 3).

Abs (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


350 450 400 400 500

500 550 600 600 650 700 700 Wavelength (nm)

750 800 800

Figure 3: Solid state UV-vis absorption spectrum of [TTF–dppz•Yb@SiO2]. Upon excitation of [TTF-dppz•Yb@SiO2] at 600 nm, the typical 2F5/2 → 2F7/2 transition of Yb(III) is present with a sharp band at 983 nm and a broad band around 1040 nm (Figure 4) indicating that the TTF-dppz ligand is able to act as an efficient antenna for the sensitization of Yb(III) luminescence. It should be noted that the TTFdppz is excited at 600 nm to optimize the intensity of the excitation of our experimental set-up, but identical spectra have been obtained upon excitation at lower energy (Figure S12). Decreasing the temperature for the measurement does not resolve the broad band at 1040 nm for [TTF-dppz•Yb@SiO2]. The higher energy transition becomes sharper but the envelope is still present indicating an actual distribution of geometries around the Yb(III) metal centers at the surface. This result is not too surprising because the Yb centers are supported on an amorphous surface that will provide different chemical environments.42


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Figure 4: a) Solid state emission spectrum of [TTF-dppz•Yb@SiO2] at room temperature and b) 77 K, normalized NIR emission measured with λex = 600 nm. For [TTF-dppz•Yb@SiO2] the lifetime decay is not well described by a monoexponential decay. The fit improves when a stretched exponential is used (eq. 2).43 This model provides a distribution of lifetimes centered at τ1 where the width is proportional to 1/β. β is a dimensionless parameter and varies between 0 and 1. β is 1 for uniform chemical environments, as it is the case for most molecular species. This model has been applied to amorphous solids to describe the distribution of lifetimes for ytterbium doped silica surface,34 dyes of pyrene on alumina surface,44 and distribution of radicals dispersed in mesoporous silica.45

= / +

(2)

Since the lifetime is fitted by a stretched exponential there is a distribution of lifetimes that are centered at τ1 with a width proportional to the dimensionless


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parameter β. In this case the lifetime of the surface species is τ1 = 2.8 µs (Figure S13), longer than what was observed in the case of DEAS species, possibly because of the rigidity of TTF-dppz compared to DEAS. However β = 0.59 is very close to what was found for DEAS material (β = 0.55), showing that the distribution of Yb sites are similar in both systems. 2.4 Microscopy Once that the surface properties were established on 12-nm silica nanoparticles we applied the same strategy to dope larger 100-nm silica particles (Figure 5). As for the 12-nm particles, the nanospheres were partially dehydroxylated at 700 °C. Titration of the OH of the resulting material with MeMgBr shows the presence of 0.088 mmol/g, which corresponds roughly to 1.5 OH/nm2 for a nanosphere of 100-nm diameter, i.e. 47000 OH per nanoparticles. In contrast to Aerosil silica, some silanols are not accessible according to IR spectroscopy and are likely incorporated in the silica bulk. Contacting the nanospheres partially dehydroxylated at 700 °C with a solution of Yb(N(TMS)2)3 yielded a pale yellow solid [Yb(N(TMS)2)3@SiO2-SPHERES], which was thermally treated at 500 °C at 10-5 mbar yielding [Yb@SiO2-SPHERES] (white solid) and then subsequently contacted with TTF-dppz to yield [TTF-dppz•Yb@SiO2-SPHERES]. The compound shows similar properties to the corresponding 12-nm particles, the only difference is the presence of residual silanols, which are not accessible as already evidenced during the Grignard titration (vide supra). Elemental analysis shows 0.08%wt of Yb, which corresponds to ca. 30’000 Yb centers per particle. The nitrogen and carbon content cannot be used to evaluate the number of chromophores bound to the metal center because the values are within the error of the technique, however assuming similar reactivity than on 12-nm nanoparticles, the 100-nm particles contain ca. 30’000 chromophores.


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Figure 5: TEM pictures of [TTF-dppz•Yb@SiO2-SPHERES] obtained on a Phillips CM12 transmission electron microscope. The filament used to generate the electron beam is a tungsten filament and the accelerating voltage is 120 kV. Images were required at 100 000 time magnification. The photophysical properties of the silica nanospheres were also investigated, showing the same luminescence features at 983 and 1040 nm (Figure S14 and S15) as in the case of [TTF-dppz•Yb@SiO2]. The lifetime distribution is also similar, although with a slightly lower β value of 0.53, indicating a slightly broader distribution of the sites available for coordination. The lifetime decay is also slightly reduced on this support (τ1 = 1.5 µs) compared to the 12-nm particles, but remains at the same order of magnitude (Figure S16). Particles of [TTF-dppz•Yb@SiO2-SPHERES] were imaged using a NIR-to-NIR microscope. The images are obtained from a small amount of powder directly deposited on a coverslip. Upon laser excitation at 800 nm, large size objects appear as


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bright spots that are small aggregates of silica nanoparticles (Figure 6). The majority of the observed objects are on averaged 500 nm in diameter, which is above the diffraction limit of our microscopy set-up (~300 nm). This size is expected from the size of the nanoparticles, which is convoluted by the optical resolution in the final image. The signal is measured in a configuration of low input power to avoid photobleaching. The relatively low detection efficiency is due to the detection of the NIR emission, which explains the fact that the observed signal to noise, although sufficient for imaging, is somewhat below what is generally measured for visible-range nanoparticles. This result establishes that single nanoparticle luminescence is achievable with relatively small silica nanoparticles with single photon NIR excitation and detection. 1600

2 µm

1400 1200 1000 800 600 400 200

-20

-10

0

10

20

1 µm

Figure 6. Scanning microscopy image of [TTF-dppz•Yb@SiO2-SPHERES] nanoparticles, using a NIR-NIR microscope configuration (λex = 800 nm, detection at 1000 nm (integration time per pixel: 60 µs, the intensity scale is in voltage of analog detectors). The inset shows zoomed image evidencing single isolated nano-particles. 3 Conclusions


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Luminescent well-defined Yb(III) surface complexes were prepared via the controlled functionalization of 12- and 100-nm silica nanoparticles through the grafting of a molecular Yb(III) precursor, Yb(N(SiMe3)2)3, followed by a postthermolysis and coordination of TTF-dppz. These surface species were sensitized in the NIR up to 850 nm by the highly efficient chromophore TTF-dppz, and the nanoparticles show similar photophysical properties (λAbs = 600 - 850 nm, τ1 = 2.8 µs and 1.5 µs for 12-nm nanoparticles and 100-nm nanospheres respectively, β = 0.59 and 0.53 for 12 nm and 100 nm nanoparticles). The corresponding Yb(III) molecular complex shows similar properties albeit with slightly blue shifted absorbance and longer τ1 (λAbs = 550 - 750 nm, τ1 = 6.9 µs). High density of sensitized centers was achieved since TTF-dppz coordinated to all Yb-centers, the 12-nm and 100-nm silica nanoparticles having ca. 350 and 30’000 chromophores per particle, respectively. The lifetime decay of these luminescent nanoparticles is best described by a stretched exponential, with a similar distribution of sites in both cases (β = 0.59 and 0.53), likely resulting from the amorphous character of the silica surface in both systems. The large amounts of chromophores (ca. 30’000) in 100-nm nanoparticles make possible imaging of a single particle in the NIR, a critical step for in vivo imaging. Further work is currently under way towards this goal.


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4 Experimental Section

General considerations. All the experiments were carried under dry, oxygen free argon using Schlenk and glove box techniques for the organometallic synthesis. For the syntheses and the treatment of the surface species, reactions were carried out using high vacuum lines (10-5 mbar) and glove box techniques. Dichloromethane, pentane and toluene were purified using a double MBraun SPS alumina column, degassed before use, and stored on molecular sieves. Deuterated solvents were directly distilled by vacuum transfer into a J-Young NMR tube, and degassed before use. C6D6 was stored on sodium and benzophenone, CDCl3 was used out of the bottle. The yttrium and ytterbium amides were synthesized modifying a procedure reported in literature and the M(OSi(OtBu)3)3(κ2–HOSi(OtBu)3) precursors were synthesized as reported in literature. Silica (AEROSIL 200 m2/g) was dehydroxylated according to a published procedure, the surface species [Y@SiO2] and [Yb@SiO2] were prepared according to a previous procedure. All infrared (IR) spectra were recorded using a Bruker  spectrometer placed in the glovebox, equipped with OPUS software. Typically 32 scans were accumulated for each spectrum. All the UV-visible spectra were recorded in dichloromethane solution using quartz Schlenk cuvettes with a Cary 5000 UV-vis-NIR spectrometer at 600 nm/min with a resolution of 1 nm. The 1

H, 13C, and 29Si-NMR spectra were obtained on Bruker DRX 300 spectrometer. The

solution spectra were recorded in the given solvent at room temperature. The 1H and 13

C chemical shifts were referenced relative to the residual solvent peak. For the

solid-state spectra, a Bruker DRX 400 was used. The MAS frequency was set at 10 kHz for all 1H and 13C spectra. The samples were introduced in a 4 mm zirconia rotor in the glove box. 1H, and 13C chemical shifts were referenced to external TMS.


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X-ray crystallography. The crystals were dispersed in paratone oil and mounted in the beam on a flow of nitrogen at 200 K on a Bruker SMART diffractometer equipped with an APEX II CCD area detector. Kα radiation of molybdenum was used. Empirical absorption correction was performed using SADABS-2008/1 (Bruker). The structures were solved with direct methods (SHELXS-97) followed by refinement with least squares methods (SHELXL-97) using OLEX2-1.2 suite of programs.46 The non-hydrogen atoms were refined anisotropically while the hydrogen atoms were placed at calculated positions. Additional crystallographic details are given in the Supporting Information (Figures S17-S18 and Table S1).

Luminescence spectroscopy. The luminescence spectra were measured using a Horiba–JobinYvon Fluorolog–3® spectro-fluorimeter, equipped with a three slit double grating excitation and emission monochromator with dispersions of 2.1 nm/mm (1200 grooves/mm). The steady–state luminescence was excited by unpolarized light from a 450 W xenon CW lamp and detected at an angle of 90° for diluted solution measurements or at 22.5° for solid state measurement (front face detection) by a Peltier–cooled red–sensitive Hamamatsu R2658P photomultiplier tube (300–1010 nm). Spectra were reference corrected for both the excitation source light intensity variation (lamp and grating) and the emission spectral response (detector and grating). Uncorrected near infrared spectra were recorded at an angle of 45° using a liquid nitrogen cooled, solid indium/gallium/arsenic detector (850–1600 nm). The luminescence decay of ytterbium complexes was determined using a home–made set– up. The excitation of the Yb(III) luminescence decays was performed with an optical parametric oscillator from EKSPLA NT342, pumped with a pulsed frequency tripled YAG:Nd laser. The pulse duration was 6 ns at 10 Hz repetition rate. The detection was performed by a R1767 Hamamatsu photomultiplier through a Jobin–Yvon


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monochromator equipped with a 1 μm blazed grating. The signal was visualized and averaged with a Lecroy digital oscilloscope LT342.

Microscopy. The microscope imaging set-up is similar to the one previously developed for similar detection schemes.21 A Ti:sapphire laser beam at 800 nm (150 fs, 80 MHz) is reflected on a dichroic mirror (FF720-SDi01, Semrock) which passes the visible (< 720 nm) and NIR (> 900 nm) regions. The beam is focused by a high numerical aperture microscope objective (x40, NA 1.15 water immersion) with an average power of about 12mW, and scanned by galvanometric mirrors. The fluorescence in both the visible (600/60 nm band pass filter) and NIR (850 nm high pass) spectral regions are collected in the epi direction (non descanned mode) and focused on two large surface photomultipliers (R9110, Hamamatsu). The lateral resolution of the microscope is about 300 nm.

Sample preparation: Preparation of [TTF-dppz•Y@SiO2]. A mixture of 0.50 g of [Y@SiO2] was reacted with a dichloromethane solution of 70 mg (0.110 mmol, 1.1 equiv) of TTFdppz. Contacting the solution to the white yttrium doped silica the solid became immediately deep purple and the solution lost its color. The mixture was reacted for 3 h and the purple solid was washed 5 times with dichloromethane. The residue was dried at 10-5 mbar for 2 h. 1H MAS NMR (400MHz) δ/ppm 10.3, 9.5, 8, 6 (Har), 1.4, 0.8 (HCH2,CH3), 0 (SiMe3).

13

C CPMAS NMR δ/ppm 155, 148, 144, 140, 137, 120,

112, 74, 30, 22, 12, 0. IR ν(C=C) = 1361, 1442, 1488 cm-1. Elemental analyses: Y = 2.11%wt, S = 1.15%wt, N = 1.01%wt and C = 3.81%wt. Preparation of [TTF-dppz•Yb@SiO2]. The material was prepared as described above. IR ν(C=C) = 1361, 1442, 1488 cm-1. Elemental analyses: Yb = 3.94 %wt, S =


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1.59%wt, N = 1.09%wt and C = 4.68%wt. Preparation of [TTF-dppz•Yb@SiO2-SPHERES]. The material was prepared as described above. IR ν(C=C) = 1361, 1442, 1488 cm-1. Elemental analyses Yb = 0.08 %wt, H = 0.13%wt, N = 0.13%wt and C = 0.46%wt. Preparation of Yb(OSi(OtBu)3)3(TTF-dppz). To a solution of 0.25 mg of Yb(OSi(OtBu)3)3(κ2–HOSi(OtBu)3) (0.22mmol, 1 equiv) in 20 mL of CH2Cl2 130 mg of TTF-dppz (0.22 mmol, 1 equiv) in CH2Cl2 were added. After 8 h the solvent was evaporated on reduced pressure. The HOSi(OtBu)3 was sublimed off at 10-5mbar at 40 °C. Then resulting material was recrystallized in CH2Cl2 at -40 °C. The precipitate was filtrated and dried on reduced pressure and stored in the glovebox. The yield was 81% (262 mg). 1H NMR (300MHz, C6D6) δ/ppm 24.2 (Δ1⁄2 = 80 Hz, 2 H), 15.2 (Δ1⁄2 = 25 Hz, 2 H), 3.75 (Δ1⁄2 = 26 Hz, 2 H), 2.6 (Δ1⁄2 = 36 Hz, 4 H), 1.74 (Δ1⁄2 = 22 Hz, 4 H), 1.37 (Δ1⁄2 = 28 Hz, 4 H), -2.2 (Δ1⁄2 = 162 Hz, 81 H). IR ν(C=C) = 1363, 1442, 1488 cm-1. One molecule of CH2Cl2 crystallizes per molecule of compound. Anal. Calcd: C65H105Cl2N4O12S6Si3Yb C, 47.17%wt, H, 6.39%wt, N, 3.38%wt, S, 11.62%wt. Found C, 47.22%wt, H, 6.29%wt, N, 3.37%wt, S, 11.60%wt Preparation of Y(OSi(OtBu)3)3(TTF-dppz). The material was prepared as described above in 78% yield. IR ν (C=C) = 1363, 1442, 1488 cm-1. 1H NMR (300 MHz) d/ppm 10.30 (d, J = 3Hz, 2H), 9.35 (d, J = 5Hz, 2H), 7.68 (s, 2H), 7.59 (t, J = 8Hz, 2H), 2.53 (t, J = 7.5Hz, 4H), 1.54 (s, 81H), 1.47 (m, J = 8Hz, 4H), 0.77 (t, J = 7Hz, 6H). 13C NMR δ/ppm 155, 146, 144, 140, 137, 131, 120, 73, 38, 31, 21, 12. IR ν(C=C) = 1363, 1442, 1488 cm-1. One molecule of CH2Cl2 crystallizes per molecule of compound. Anal. Calcd: Anal. Calcd: C67H109Cl4N4O12S6Si3Y C, 49.69%wt, H,


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6.74%wt, N, 3.57%wt, S, 12.25%wt. Found C, 49.22%wt, H, 6.79%wt, N, 3.57%wt, S, 12.60%wt. Preparation of Y(OSi(OtBu)3)3(Phen). The material was prepared as described above in 78% yield. 1H NMR (300 MHz) δ/ppm 10.28 (dd, J = 4.5Hz, J2 = 1.7 Hz, 2H), 7.49 (dd, J = 8.2 Hz, J2 = 1.3 Hz, 2H), 7.40 (dd, J = 8.5 Hz, J2 = 4.7 Hz, 2H), 7.06 (s, 2H), 1.49 (s, 81 H). 13C NMR δ/ppm 154.0, 137.8, 128.0, 127.4, 125.7, 123.7. Anal. Calcd. C49H99N2O12Si3Y, C, 54.42%wt, H, 8.47%wt, N, 2.64%wt. Found C, 54.66%wt, H, 8.63%wt, N, 2.55%wt. Preparation of Yb(OSi(OtBu)3)3(Phen). The material was prepared as described above in 78% yield. 1H NMR (300 MHz) δ/ppm 28.5 (Δ1⁄2 = 35 Hz, 2 H), 20.9 (Δ1⁄2 = 35 Hz, 2 H), -1.1 (Δ1⁄2 = 76 Hz, 81 H), -108.1 (Δ1⁄2 = 296 Hz, 2 H). Anal. Calcd. C49H99N2O12Si3Yb, C, 50.41%wt, H, 7.84%wt, N, 2.45%wt. Found C, 49.89%wt, H, 7.94%wt, N, 2.30%wt. References (1) Hemmer, E.; Venkatachalam, N.; Hyodo, H.; Hattori, A.; Ebina, Y.; Kishimoto, H.; Soga, K. Nanoscale 2013, 5, 11339. (2) Frangioni, J. V. Curr. Opin. Chem. Biol. 2003, 7, 626. (3) Kiyose , K.; Kojima , H.; Nagano , T. Chem. –As. J. 2008, 3, 506. (4) Achilefu, S. Angew. Chem., Int. Ed. Engl. 2010, 49, 9816. (5) Choi, H. S.; Nasr, K.; Alyabyev, S.; Feith, D.; Lee, J. H.; Kim, S. H.; Ashitate, Y.; Hyun, H.; Patonay, G.; Strekowski, L.; Henary, M.; Frangioni, J. V. Angew. Chem., Int. Ed. Engl. 2011, 50, 6258. (6) Faulkner, S.; Pope, S. J. A.; Burton‐Pye, B. P. Appl. Spectr. Rev. 2005, 40, 1. (7) Eliseeva, S. V.; Bunzli, J.-C. G. Chem. Soc. Rev. 2010, 39, 189. (8) Soga, K.; Tokuzen, K.; Tsuji, K.; Yamano, T.; Hyodo, H.; Kishimoto, H. Eur. J. Inorg. Chem. 2010, 2010, 2673. (9) Kamimura, M.; Kanayama, N.; Tokuzen, K.; Soga, K.; Nagasaki, Y. Nanoscale 2011, 3, 3705. (10) Bünzli, J.-C.; Eliseeva, S. V. In Lanthanide Luminescence; Hänninen, P., Härmä, H., Eds.; Springer Berlin Heidelberg: 2011; Vol. 7, p 1. (11) Huang, W.; Wu, D.; Guo, D.; Zhu, X.; He, C.; Meng, Q.; Duan, C. Dalton Trans. 2009, 2081.


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TOC


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Surface Complexes for NIR-to-NIR Single Nanoparticle Imaging

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