Optical Properties of Heavily Fluorinated Lanthanide Tris β-Diketonate Phosphine Oxide Adducts

The construction of lanthanide(III) chelates that exhibit superior photophysical properties holds great importance in biological and materials science. One strategy to increase the luminescence properties of lanthanide(III) chelates is to hinder competitive non-radiative decay processes through perfluorination of the chelating ligands. Here, the synthesis of two families of heavily fluorinated lanthanide(III) β-diketonate complexes bearing monodentate perfluorinated tris phenyl phosphine oxide ligands have been prepared through a facile one pot reaction [Ln(hfac)3{(Ar)3PO}(H2O)] and [Ln(F7-acac)3{(Ar)3PO}2] (where Ln = Sm3+, Eu3+, Tb3+, Er3+ and Yb3+). Single crystal X-ray diffraction analysis in combination with photophysical studies have been performed to investigate the factors responsible for the differences in the luminescence lifetimes and intrinsic quantum yields of the complexes. Replacement of both bound H2O and C–H oscillators in the ligand backbone has a dramatic effect on the photophysical properties of the complexes, particularly for the near infra-red emitting ion Yb3+, where a five fold increase in luminescence lifetime and quantum yield is observed. The complexes [Sm(hfac)3{(Ar)3PO}(H2O)] (1), [Yb(hfac)3{(Ar)3PO}(H2O)] (5), [Sm(F7-acac)3{(Ar)3PO}2] (6) and [Yb(F7-acac)3{(Ar)3PO}2] (10) exhibit unusually long luminescence lifetimes and attractive intrinsic quantum yields of emission in fluid solution (ΦLn = 3.4% (1); 1.4% (10)) and in the solid state (ΦLn = 8.5% (1); 2.0% (5); 26% (6); 11% (10)), which are amongst the largest values for this class of compounds to date.


Introduction
The unique optical properties of the trivalent lanthanides continue to garner appeal due to their numerous commercially exploitable applications in the biomedical and materials science fields [1,2].Their inherent luminescent properties, which arise from parity forbidden intra f-f transitions, including long-lived emission (microsecond to millisecond range), high colour purity, insensitivity to dissolved molecular oxygen and resistance to photobleaching and blinking render these ions highly suitable as the emissive components in a number of growing technologies.These include, organic light emitting diodes [3], up and down-converting phosphors [4][5][6], luminescent sensors in biomedical diagnostics [7] and luminescent markers in cell microscopy [8][9][10].More recently, the potential for near infra-red (nIR) emitting lanthanide chelates (Yb 3+ , Nd 3+ , Er 3+ , Pr 3+ ) [11][12][13][14][15][16] to be exploited in mammalian cell imaging and in the telecommunications industry has been realized.Lanthanide ions that are emissive in the near infra-red region of the electromagnetic spectrum are particularly suited to biological imaging using confocal microscopy [17] and in optical amplifiers and waveguides [18][19][20] since their emission coincides with the more transparent wavelengths of biological tissue and silica.Above all however, the long-lived emission of the trivalent lanthanides is ideal for preventing spectral interference from scattered light and autofluorescence via time-gated luminescence detection [21] (pp.[6][7][8], which commonly occurs upon UV excitation in molecular complexes, particularly in biological media. Lanthanide chelates that incorporate organic chromophores offer additional attractive properties for their use as optical materials, namely the use of low power excitation sources, a large Stokes' shift between excitation and emission and theoretically very high quantum yields of emission.This is principally because the parity forbidden f-f transitions can be overcome by efficient energy transfer from the triplet state of the chromophore of the chelated organic ligand (antenna) to the lanthanide excited emissive state upon UV-visible excitation; this is termed the "antenna effect" or "sensitised emission" [22].In this regard, β-diketonates and their substituted derivatives [23] have become synonymous with the development of highly luminescent lanthanide complexes used in optoelectronic devices and in fluoroimmunoassays such as DELFIA (dissociation-enhanced lanthanide fluoroimmunoassay) [21,[24][25][26] (p.5085).These monoanionic ligands form very kinetically and thermodynamically stable 1:3 charge neutral metal:ligand chelates and the appended chromophores can very efficiently sensitise both the visible and nIR f-f based emission from lanthanide ions [27].
However, the majority of these complexes only take advantage of the green and red emission of Tb 3+ and Eu 3+ respectively due to the fact that the excited states of these ions are only marginally quenched by frequency matched vibrational harmonics of proximate O-H and N-H oscillators and the luminescence lifetimes are of millisecond order.By contrast, the relative lower energy of the excited states of the nIR emitting lanthanides means that the emission from these ions is considerably quenched by lower energy vibrational overtones of X-H bond vibrations (particularly C-H oscillators) present within the ligand architecture and/or by closely diffusing solvent molecules [12].Since the magnitude of vibrational quenching of the emissive excited state of a lanthanide ion is dictated by the energy gap law [28,29], X-H quenching is much more pronounced in nIR emitting lanthanides compared to those that emit in the visible region.In general, vibrational quenching is considered negligible if the energy gap between the emissive state and the next lowest lying energy state is greater than or equal to the sixth harmonic of the fundamental vibrational mode.However, in the case of the nIR emissive lanthanides, the emissive states often lie within the first and third vibrational overtones of X-H oscillators [30].In this regard, the fact that O-D oscillators whose fundamental modes vibrate at lower frequency (e.g., 3450 cm −1 in an OH bond compared to 2500 cm −1 in an OD bond), can be exploited to increase the luminescence lifetime and quantum yield of Ln 3+ based emission.Horrocks [31,32] and others [11,33] have exploited this effect to develop an empirical equation to determine the number of O-H oscillators and therefore water or methanol molecules bound to a Ln 3+ ion in aqueous or methanolic solution to great effect (Ln 3+ = Eu 3+ , Tb 3+ , Yb 3+ and in a series of isostructural lanthanide(III) complexes, Nd 3+ ).By extension, the luminescence lifetime of a given lanthanide complex can be increased by simply replacing the spectroscopic solvent to its deuterated counterpart.However, X-D vibrational quenching is often still operative in nIR emissive complexes; as an example, the third C-D overtone overlaps with the 4 I 13/2 excited state of Er 3+ (in comparison to the first vibrational harmonic of C-H bonds).
To further overcome the limitations of vibrational quenching and thereby significantly increase the luminescence quantum yield of nIR (and visible) emitting lanthanide complexes, synthetic approaches based on partial or full deuteration [27,[34][35][36][37] of the C-H bonds in organic ligands can be accomplished.For example, Seitz and Platas-Iglesias have synthesized several series of selectively deuterated Lehn cryptand complexes of Pr 3+ , Nd 3+ , Er 3+ and Yb 3+ and evaluated the spectral overlap integrals of the excited states with aromatic C-H and C-D overtones to develop a comprehensive picture of lanthanide C-H quenching combinations for luminescence enhancement in nIR emitting lanthanide complexes [30].An alternative strategy is partial or perfluorination of C-H bonds of the supporting ligand [38][39][40][41].This has also been utilized very effectively to prepare partially fluorinated complexes mainly based on diamine adducts of tris β-diketonate hexafluoroacetylacetonate (hfac) and heptafluoroacetylacetanoate (F 7 -acac) chelates that exhibit improved photophysical properties [23,42].For example, the radiative lifetimes of the 4 I 13/2 → 4 I 13/2 transition at 1530 nm in crystalline samples of Er(hfac) 3 (H 2 O) 2 and Cs[Er(hfac) 4 ] have been determined to be ~9.5 and 23.4 ms respectively, reflecting the greater degree of fluorination in the latter [38].

Synthesis of the Complexes
A series of lanthanide tris hexafluoroacetylacetonate (hfac) and heptafluoroacetylacetonate (F 7 -acac) bis perfluorinated tris aryl phosphine oxide complexes were originally targeted by two synthetic routes as outlined in Scheme 1.Either treatment of prepared Ln(hfac) 3 •2H 2 O [28] with two equivalents of (Ar F ) 3 PO (tris(pentafluorophenyl)phosphine oxide) in CH 2 Cl 2 at room temperature sonicated for one hour or a one pot reaction of the corresponding lanthanide acetate, protonated β-diketonate and two equivalents of (Ar F ) 3 PO [41] at 2 × 10 −3 M concentrations heated to reflux temperature for one hour yielded the complexes [Ln(hfac) 3 {(Ar F ) 3 PO}(H 2 O)] and (Ar F ) 3 PO (Ln = Sm 3+ , Eu 3+ , Tb 3+ , Er 3+ and Yb 3+ , 1-5) and [Ln(F 7 -acac) 3 {(Ar F ) 3 PO} 2 ] (Ln = Sm 3+ , Eu 3+ , Tb 3+ , Er 3+ and Yb 3+ , 6-10) in moderate yields (26%-59%) after recrystallization from CH 2 Cl 2 solutions (as verified by single crystal X-ray diffraction analysis).Interestingly, conducting the reactions in dry CH 2 Cl 2 under air sensitive conditions at higher concentrations (1 × 10 −2 M) in order to promote bis phosphine oxide substitution with the labile coordinated water molecules in the parent lanthanide acetate or β-diketonate complexes, resulted solely in the isolation of the mono phosphine oxide substituted complexes for the hexafluorinated β-diketonate hfac [47], whereas only the bis phosphine oxide substituted derivatives were isolated with the heptafluoro β-diketonate F 7 -acac using both standard and dry solvents in similar conditions (Scheme 1).This interesting divergence in reactivity can be attributed to the greater electron withdrawing effects of the F 7 -acac ligand relative to the hfac β-diketonate which promotes a higher affinity for the second phosphine oxide to bind to the electropositive lanthanide(III) metal centres.In the case of the hfac reactions, adjusting the stoichiometry of the phosphine oxide accordingly, resulted in slightly improved product yields in most cases apart from complex 4 (44%-53%).
The 31 P and 19 F NMR (nuclear magnetic resonance) spectra of all the complexes exhibited paramagnetically shifted phosphorous and fluorine resonances; the 19 F resonances of the β-diketonate being significantly more shifted and broadened than the corresponding resonances in the coordinated (Ar F ) 3 PO ligands (see supplementary material).Notably, multinuclear NMR analysis of the crude powders from the hfac reactions taken after removal of all volatiles before recrystallization suggested no presence of a minor lanthanide species containing perfluorinated phosphine oxide ligands that could be formulated as [Ln(hfac) 3 {(Ar F ) 3 PO} 2 ] as previously reported for Er 3+ (vide infra) [47].

Photophysical Properties of the Complexes
With two families of complexes incorporating different degrees of fluorination in hand, we next resolved to investigate the effects of perfluorination on the optical properties of the complexes.The UV-visible absorption spectra of all the complexes 1-10 exhibit intense absorptions in the UV region of the electromagnetic spectrum with absorption maxima at ca. 230 and 300 nm (complexes 1-5) attributable to the π-π* transitions of the perfluorinated-phenyl and β-diketonate chromophores respectively.In the absorption spectra of complexes 6-10, an additional maximum centered at ca. 325 nm is also observed.Following UV excitation into all ligand absorption bands at 230, 280 or 355

Photophysical Properties of the Complexes
With two families of complexes incorporating different degrees of fluorination in hand, we next resolved to investigate the effects of perfluorination on the optical properties of the complexes.The UV-visible absorption spectra of all the complexes 1-10 exhibit intense absorptions in the UV region of the electromagnetic spectrum with absorption maxima at ca. 230 and 300 nm (complexes 1-5) attributable to the π-π* transitions of the perfluorinated-phenyl and β-diketonate chromophores respectively.In the absorption spectra of complexes 6-10, an additional maximum centered at ca. 325 nm is also observed.Following UV excitation into all ligand absorption bands at 230, 280 or 355 nm, all the complexes exhibited lanthanide f-centered emission in the visible and near infra-red region at typical wavelengths for a given lanthanide(III) ion (Figure 3).In all cases, ligand sensitised emission was confirmed by recording the excitation spectrum at the emission maximum, which matched well to the absorption spectra indicating that both chromophores are involved in the sensitization process.Representative emission spectra for the [Ln(F 7 -acac) 3 {(Ar F ) 3 PO} 2 ] family of complexes are illustrated in Figure 3. Interestingly, the relative intensities of the visible and near infra-red emission, in particular of the Er 3+ and Yb 3+ complexes (9 and 10), are much greater than those recorded for the Er 3+ and Yb 3+ nIR analogues of [Ln(hfac) 3 {(Ar F ) 3 PO}(H 2 O)] (4 and 5) reflecting the greater degree of vibrational quenching from the coordinated water molecule in the latter.nm, all the complexes exhibited lanthanide f-centered emission in the visible and near infra-red region at typical wavelengths for a given lanthanide(III) ion (Figure 3).In all cases, ligand sensitised emission was confirmed by recording the excitation spectrum at the emission maximum, which matched well to the absorption spectra indicating that both chromophores are involved in the sensitization process.Representative emission spectra for the [Ln(F7-acac)3{(Ar F )3PO}2] family of complexes are illustrated in Figure 3. Interestingly, the relative intensities of the visible and near infra-red emission, in particular of the Er 3+ and Yb 3+ complexes (9 and 10), are much greater than those recorded for the Er 3+ and Yb 3+ nIR analogues of [Ln(hfac)3{(Ar F )3PO}(H2O)] (4 and 5) reflecting the greater degree of vibrational quenching from the coordinated water molecule in the latter.The luminescence lifetimes of the [Ln(hfac)3{(Ar F )3PO}(H2O)] complexes recorded at the emission maxima are typical for both the visible and nIR emitting Ln 3+ complexes, being in the microsecond range (Table 1).Interestingly, for the shorter lived Er 3+ and Yb 3+ complexes 4 and 5, the kinetic traces following 337 nm excitation were best fitted to a biexponential decay, suggestive of two non-interconverting emissive species in solution on the timescale of the experiment.Given that the longer lived component of the kinetic trace of 5 (τ = 1.94 μs) can be compared to the related complex Er(TPIP)3 (TPIP = tetraphenylimidodiphosphinate) [43], where τCDCl3 = 5.0 μs, it seems reasonable to assume that the shorter lived component is due to the aqua species [Ln(hfac)3{(Ar F )3PO}(H2O)], whereas the longer lived species is devoid of a coordinated solvent molecule.In the case of the longer lived visible emitting lanthanide ions in this series of hfac complexes, dynamic exchange of the labile water molecule is faster than the timescale of the experiment leading to the observation of an averaged solution lifetime [48].It is worth noting here, that the luminescence lifetimes of all the complexes measured in solution are exceptionally sensitive to small amounts of water present in the solvent, therefore the solvent was thoroughly dried prior to use and reported values are reproducible in at least three independent measurements.Additionally, initial discrepancies in kinetic measurements led us to additionally record the photophysical properties of the complexes in the solid state (Table 1).

ΦLn = τobs/τrad
(1) The luminescence lifetimes of the [Ln(hfac) 3 {(Ar F ) 3 PO}(H 2 O)] complexes recorded at the emission maxima are typical for both the visible and nIR emitting Ln 3+ complexes, being in the microsecond range (Table 1).Interestingly, for the shorter lived Er 3+ and Yb 3+ complexes 4 and 5, the kinetic traces following 337 nm excitation were best fitted to a biexponential decay, suggestive of two non-interconverting emissive species in solution on the timescale of the experiment.Given that the longer lived component of the kinetic trace of 5 (τ = 1.94 µs) can be compared to the related complex Er(TPIP) 3 (TPIP = tetraphenylimidodiphosphinate) [43], where τ CDCl3 = 5.0 µs, it seems reasonable to assume that the shorter lived component is due to the aqua species [Ln(hfac) 3 {(Ar F ) 3 PO}(H 2 O)], whereas the longer lived species is devoid of a coordinated solvent molecule.In the case of the longer lived visible emitting lanthanide ions in this series of hfac complexes, dynamic exchange of the labile water molecule is faster than the timescale of the experiment leading to the observation of an averaged solution lifetime [48].It is worth noting here, that the luminescence lifetimes of all the complexes measured in solution are exceptionally sensitive to small amounts of water present in the solvent, therefore the solvent was thoroughly dried prior to use and reported values are reproducible in at least three independent measurements.Additionally, initial discrepancies in kinetic measurements led us to additionally record the photophysical properties of the complexes in the solid state (Table 1). 1 Reported lifetimes are subject to an error of ±15%, indistinguishable data were obtained at 230 and 355 nm excitation; 2 The intrinsic quantum yield of Ln 3+ centered emission was calculated using equation 1 using the following τ rad (natural radiative lifetime) values: Sm 3+ 1.98 ms ( 4 G 5/2 ) from reference [50]; Er 3+ 4 ms ( 4 I 13/2 ) from reference [47]; Eu 3+ , 1.11 ms ( 5 D 0 ) from reference [14]; Tb 3+ , 5.1 ms ( 5 D 4 ) from reference [44]; and Yb 3+ 1.3 ms ( 2 F 5/2 ) from reference [51]; 3 Value determined using the longest lifetime component of the emission; 4 Lifetime determined following 337 nm excitation using a ns pulsed N 2 laser; 5 Not determined due to very weak or lack of f-centered emission; 6 Complex unstable when excited with using a ns pulsed N 2 laser at 337 nm and 10 Hz.
Since we were unable to reliably determine the total quantum yield of emission for all the complexes, (due in part to the small f-f molar absorption extinction coefficients) we have instead estimated the intrinsic quantum yield of emission (Φ Ln ) based on calculated and published values of the Ln 3+ radiative lifetime of a given emissive excited state in the absence of any non-radiative deactivation processes, τ rad (Equation ( 1)) [2,37,40,47].The quantum efficiency of the Ln 3+ based emission can be determined by measuring the quantum yield based on emission following excitation into the f-f absorption bands.Alternatively, and in the absence of sufficiently intense emission upon direct excitation, the intrinsic quantum yield can be estimated from the ratio of the observed luminescence lifetime (τ obs ) with the radiative lifetime (τ rad ).However, since τ rad values depend heavily on the coordination environment and refractive index of the medium of a given lanthanide ion, we have here chosen larger τ rad values reported for structurally similar complexes as far as possible to give more reasonable estimations of Φ Ln (Table 1) [14,15,39,[50][51][52][53][54].Nevertheless, caution must be exercised when interpreting these values since very small changes in the ligand structure, solvent and coordination geometry of the complex can lead to large discrepancies in τ rad values.As a result, the values reported here therefore only serve as a guide.For the series of the [Ln(hfac) 3 {(Ar F ) 3 PO}(H 2 O)] complexes (1-5), generally the intrinsic quantum yields in fluid solution are typical for partially fluorinated β-diketonate complexes, but notably, the quantum yield values for the Sm 3+ and Yb 3+ derivatives 1 and 5 both in solution and in the solid state are relatively large (3.4%, 8.5%, 0.3% and 2% respectively).However, concentration quenching effects or local heating in the solid state samples cannot be ruled out and these values may in fact be considerably larger.
The effect of complete fluorination on the photophysical properties of the complexes is further evidenced by the significant increase in the luminescence lifetimes of the [Ln(F 7 -acac) 3 {(ArF) 3 PO} 2 ] series of complexes 6-10 (Table 1) of approximately one order of magnitude.This effect is particularly pronounced for the nIR emitting lanthanide ions Er 3+ and Yb 3+ , where in the case of the Er 3+ derivative, 9, the lifetimes measured in solution and the solid state are very similar (15.3 and 16.8 µs respectively).This observation is in agreement with those of Monguzzi and co-workers who recorded identical lifetimes for this complex in CDCl 3 solution and in the solid state.For complex 9, the similar solution state and solid state lifetimes suggest that the Er 3+ ions may be shielded to the same degree in their immediate coordination environment.Again, the detrimental effect of intermolecular quenching between neighbouring Er 3+ ions and/or heating of the samples may result in an apparent lowering of the actual lifetime and quantum yield values.Here, the intrinsic quantum yield of 0.4% is considerably lower than the theoretical value where the radiative lifetime is equal to 4 ms [47], which suggests that residual CH 2 Cl 2 solvent (as observed by X-ray crystallography) may play a role in lowering the overall quantum yield of emission.This observation is borne out to a certain degree by examination of the kinetic data for the Yb 3+ complex 10, where the Yb 3+ excited state is less susceptible to vibrational quenching by C-H oscillators [42].This complex exhibits a marked enhancement (by a factor of >5) in both solution and solid state luminescence lifetimes and quantum yields when compared to the hexafluorinated aqua complex [Yb(hfac) 3 {(Ar F ) 3 PO}(H 2 O)], 5. Indeed, the fully perfluorinated derivative 10 exhibits a remarkably long luminescence lifetime of 139 µs in the solid phase (18 µs in CH 2 Cl 2 solution) and an intrinsic quantum yield of Yb 3+ based emission of 11%.This compares well to the Yb 3+ bis-bipyridine N-oxide derivative of the Lehn cryptand described by Seitz that exhibits a room temperature solution luminescence lifetime of 26.1 µs in deuterated methanol [31].In this system, perdeuteration of the ligand results in an extraordinarily long solution lifetime of 172 µs and the highest reported intrinsic quantum yield of Yb 3+ emission in solution to date (26%).Together, these observations further highlight the fact that purposeful reduction in competitive vibrational quenching thereby substantially increasing the lifetime of the lanthanide based emission is key to achieving large nIR quantum yields of emission.
Interestingly, in the case of the Sm 3+ complex 6, no f-centered emission was observed in solution at room temperature; the emission spectrum being dominated by residual ligand centered transitions.This clearly indicates that the F 7 -acac ligand is a poor sensitizer for Sm 3+ and that the C-F bond in the β-diketonate unit quenches the Sm 3+ based emission considerably [44].In the solid state however, complex 6 exhibits a typical Sm 3+ based emission profile with a long luminescence lifetime of 506 µs and a large calculated intrinsic quantum yield of 26% [16,55].
Mass spectra were obtained using either MALDI from CH 2 Cl 2 solutions with a dithrinol matrix on a Shimadzu Axima confidence spectrometer (Shimazdu, Kratos site, Manchester, UK, for all complexes) or electrospray mass spectrometry, performed on a Micromass Platform II system (ligands and precursors).For all of the complexes 1-10, no identifiable molecular ion peaks or fragmentation products were observable.
Elemental analyses on the compounds were performed by M. Jennings and colleagues in the microanalytical laboratory in the School of Chemistry at the University of Manchester; a Carlo ERBA Instruments CHNS-O EA1108 elemental analyzer (Carlo ERBA Instruments, Milan, Italy) was used for C, H and N analysis and a Fisons Horizon elemental analysis ICP-OED spectrometer (VG Elemental, Winsford, UK) for metals.
Absorption spectra were recorded in dry CH 2 Cl 2 on a T60U spectrometer (PG Instruments Ltd., Lutterworth, UK) using fused quartz cells with a path length of 1 cm or on a double-beam Cary Varian 500 scan UV-vis-nIR spectrophotometer over the range 300-1300 nm.
All solution luminescence measurements were recorded on compounds dissolved in dry CH 2 Cl 2 solutions using screw or Teflon™ (Hellma UK Ltd., Southend on Sea, UK) capped fused quartz cuvettes with a 1 cm or 0.1 cm path length.All measurements were recorded within 30 min of sample preparation to avoid ingress of oxygen and moisture.Luminescence measurements of solid samples were recorded using finely divided powdered samples held in between two 10 cm 2 quartz plates.All steady state emission and excitation spectra were recorded on an Edinburgh Instrument FP920 Phosphorescence Lifetime Spectrometer (Edinburgh Instruments, Livingston, Scotland) equipped with a 450 watt steady state xenon lamp, a 5 watt microsecond pulsed xenon flashlamp, (with single 300 mm focal length excitation and emission monochromators in Czerny Turner configuration), a red sensitive photomultiplier in peltier (air cooled) housing (Hamamatsu R928P), and a liquid nitrogen cooled nIR photomultiplier (Hamamatsu, Hamamatsu City, Shizuoka Prefecture, Japan).Lifetime data were recorded following excitation with the microsecond flashlamp using time correlated single photon counting (PCS900 plug-in PC card for fast photon counting).Lifetimes were obtained by tail fit on the data obtained and quality of fit judged by minimization of reduced chi-squared and residuals squared.For the Er 3+ and Yb 3+ complexes 4 and 5, the sample was excited using a pulsed nitrogen laser (337 nm) operating at 10 Hz.Light emitted at right angles to the excitation beam was focused onto the slits of a monochromator, which was used to select the appropriate wavelength.The growth and decay of the luminescence at selected wavelengths was detected using a germanium photodiode (Edinburgh Instruments, EI-P, Edinburgh Instruments, Livingstone, Scotland) and recorded using a digital oscilloscope (Tektronix TDS220, Tektronix Inc., Beaverton, OR, USA) before being transferred to a PC for analysis.Luminescence lifetimes were obtained by iterative reconvolution of the detector response (obtained by using a scatterer) with exponential components for growth and decay of the metal centred luminescence, using a spreadsheet running in Microsoft Excel.The details of this approach have been discussed elsewhere [60,61].Unless otherwise stated, fitting to a double exponential decay yielded no improvement in fit as judged by minimisation of residual squared and reduced chi squared.Note that the Tm 3+ complexes [Tm(hfac) 3 {(Ar F ) 3 PO}(H 2 O)] and [Tm(F 7 -acac) 3 {(Ar F ) 3 PO} 2 ] were found to be non-emissive in CH 2 Cl 2 solutions at room temperature upon UV excitation (230-360 nm).According to a modification of a literature procedure [41], tris(pentafluorophenyl)phosphine (1.031 g, 1.9 mmol) was dissolved in chloroform (40 mL), cooled to 0 • C in an ice bath and stirred for 30 min.3-Chloroperoxybenzoic acid (0.3013 g, 2.2 mmol) was dissolved in chloroform (10 mL) and then added dropwise to the reaction mixture over 10 min.The reaction mixture was again cooled to 0 • C in an ice bath and left to warm to room temperature and stirred for 48 h.After this time, saturated sodium hydrogen carbonate solution (50 mL) was added and the reaction mixture stirred for 30 min.The organic soluble products were extracted with chloroform (3 × 50 mL) and dried over MgSO 4 .The organic layer was removed by rotary evaporation and the white powder dried in vacuo to give 0.89 g of tris(pentafluorophenyl)phosphine oxide (81% yield).All data were consistent with those documented in the literature.ES + MS (MeCN) m/z 549 [M + H] + (44%), 571 [M + Na] + (100%), 1119 [2M + Na] + (62%), 1667 (3M + Na] + (67%).

Preparation of Ln(hfac
According to a literature procedure reported for Nd 3+ [34], The corresponding lanthanide acetate hydrate (Ln(OAc) 3 •xH 2 O) (15 mmol) was dissolved in deionised water (20 mL) with stirring in an ice bath.1,1,1,5,5,5-hexafluoro-2,4-pentanedione (hfac) (5.0 g, 24 mmol) was dissolved in methanol and was added dropwise to the lanthanide acetate solution.The reaction mixture was stirred for 3 h in an ice bath and a further 65 h at room temperature.The solvent was then removed using a rotary evaporator and the resulting product was recrystallised from MeOH.

Preparation of [Ln
Under N 2 , an oven dried Schlenk was charged with 0.33 mmol of the appropriate lanthanide acetate hydrate (Ln(OAc) 3 •xH 2 O) and 0.66 mmol (142 mg) (Ar F ) 3 PO.25 mL of dry CH 2 Cl 2 was then added by cannula or syringe.1,1,1,3,5,5,5-heptfluoro-2,4-pentanedione (0.94 mmol) was subsequently rapidly added and the reaction mixture heated at reflux temperature for 1 hour under a flow of N 2 , then cooled to room temperature.After this time, dry pentane (40 mL) was added and the Schlenk flask placed in the freezer at −18 • C.After one week, the crystalline products were isolated by filtration, washed with cold CH 2 Cl 2 and dried under vacuum suction to yield the title compounds.The products are slightly hygroscopic when stored under ambient conditions for extended periods of time (>1 year).All complexes are soluble in acetone and sparingly soluble in CH 2 Cl 2 and CHCl 3 .

Conclusions
In summary, two families of emissive heavily fluorinated lanthanide(III) β-diketonate complexes bearing monodentate perfluorinated tris phenyl phosphine oxide ligands, [Ln(hfac) 3 {(Ar F ) 3 PO}(H 2 O)] and [Ln(F 7 -acac) 3 {(Ar F ) 3 PO} 2 ] (where Ln = Sm 3+ , Eu 3+ , Tb 3+ , Er 3+ and Yb 3+ ) have been prepared and characterized by NMR spectroscopy, single crystal X-ray diffraction and luminescence spectroscopy.In depth photophysical studies on the complexes in CH 2 Cl 2 solution and in the solid state have shown that replacing both the bound inner sphere H 2 O molecule and C-H oscillator in the β-diketonate ligand backbone in [Ln(hfac) 3 {(Ar F ) 3 PO}(H 2 O)] by a second (Ar F ) 3 PO ligand and C-F bond respectively in [Ln(F 7 -acac) 3 {(Ar F ) 3 PO} 2 ] has a dramatic effect on the photophysical properties of the complexes.This effect is particularly notable for the near infra-red emitting ion Yb 3+ , where a five fold increase in luminescence lifetime and quantum yield is observed in [Yb(F 7 -acac) 3 {(Ar F ) 3 PO} 2 ] (10) compared to [Ln(hfac) 3 {(Ar F ) 3 PO}(H 2 O)] (5).The presented data herein conclude that replacing all C-H oscillators in the immediate coordination environment of the Ln 3+ ions with C-F bonds substantially reduces the degree of competitive vibrational quenching, particularly for the nIR emitting trivalent lanthanides in solution which in turn leads to more intense and longer lived f-centered emission.Here, this is particularly evident for the Sm 3+ and Yb 3+ complexes 1, 5 and 10, which exhibit both long luminescence lifetimes and relatively large intrinsic quantum yields of emission.Such an approach combined with intentional reduction of the radiative lifetime of the lanthanide based emission as demonstrated by Seitz [37] may find increasing use in the development of optical imaging probes and highly emissive materials for optoelectronics amongst other applications in the near future.

Table 1 .
Photophysical properties of the complexes recorded in dry CH 2 Cl 2 and in the solid state at 298 K upon excitation at 280 nm.