Circularly Polarized Luminescence from an Eu(III) Complex Based on

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Circularly Polarized Luminescence from an Eu(III) Complex Based on 2‑Thenoyltrifluoroacetyl-acetonate and a Tetradentate Chiral Ligand Marco Leonzio,† Marco Bettinelli,† Lorenzo Arrico,‡ Magda Monari,§ Lorenzo Di Bari,*,‡ and Fabio Piccinelli*,† †

Laboratorio Materiali Luminescenti, DB, Università di Verona and INSTM, UdR Verona, Strada Le Grazie 15, 37134 Verona, Italy Dipartimento di Chimica e Chimica Industriale, via Moruzzi 13, 56124 Pisa, Italy § Dipartimento di Chimica ‘‘G. Ciamician’’, Università di Bologna, Via Selmi 2, 40126 Bologna, Italy Inorg. Chem. Downloaded from pubs.acs.org by DURHAM UNIV on 08/06/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: A new chiral complex {[EuL(tta)2(H2O)]CF3SO3; L = N,N′-bis(2-pyridylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine; tta = 2-thenoyltrifluoroacetyl-acetonate} has been synthesized and characterized from a structural and spectroscopic point of view. The molecular structure in the solid state shows the presence of one chiral L, two tta, and one water molecules bound to the metal center. L and tta molecules can efficiently harvest and transfer to Eu(III) the UV light absorbed in the 250−400 nm range. The forced electric-dipole 5D0 → 7F2 emission band dominates the Eu(III) emission spectra recorded in the solid state and in solution of acetonitrile or methanol and the calculated intrinsic quantum yield of the metal ion is around 40−50%. The light emitted by the enantiopure complex shows a sizable degree of polarization with a maximum value of the emission dissymmetry factor (glum) equal to 0.2 in methanol solution. If compared with the complex in the solid state or in acetonitrile solution, then the first coordination sphere of Eu(III) when the complex is dissolved in methanol is characterized by the presence of one CH3OH molecule instead of water. This fact is related to different Eu(III) CPL signatures in the two solvents.

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INTRODUCTION Luminescent β-diketonate complexes of lanthanide ions possess attractive properties (i.e., high quantum yield, high brightness, narrow band emission) which candidate them as promising materials for a wide range of applications. In particular, [Ln(β-diketonate)3L] containing an additional bidentate or tridentate chromophore are gaining increasing attention for technological application such as organic emitting layers in lightemitting diodes (OLED)1−4 and as solar concentrators.5−7 The main disadvantages concerning practical applicability of the Ln3+ β-diketonate complexes are connected to the rather low thermal and photochemical stability. In order to overcome these drawbacks, they have been incorporated into polymers,8 mesoporous materials9,10 and organic−inorganic hybrid materials.11−13 When the emissive complex is chiral nonracemic, it may emit left and right circularly polarized light with different intensities. The phenomenon is called circularly polarized luminescence (hereafter CPL) and it is usually quantified through the luminescence dissymmetry factor glum = 2(IL − IR)/(IL + IR), with IL and IR being the left- and right-polarized intensity, respectively. While isolated organic chiral molecules or macromolecules typically display glum values of 10−3−10−2, some © XXXX American Chemical Society

lanthanide complexes show higher values (up to 0.1−1), in particular Tb(III)- and Eu(III)-based ones.14 Circularly polarized (CP) organic light-emitting diodes (CP-OLEDs) form an emerging technology and recently a lanthanide-based OLEDs with remarkable CP electroluminescence have been designed.15 These devices are based on the chiral Eu(III) complex CsEu(hfbc)4 (hfbc = 3-heptafluorobutyryl camphorate, a β-diketonate) showing the highest known CP photoluminescence.16 In this context, an Eu(III) complex based on a new chiral diketonate (carvone) appears a promising candidate for this kind of application.17 To the best of our knowledge, the most famous examples of complexes showing CPL activity in which the chirality is introduced by L ligand in Ln3+ β-diketonate systems, are described by Yuasa et al.18−20 and by some of us,21 in the case of tridentate chiral bis(oxazolinyl)-pyridine ligands. To date, no examples are reported regarding a CPL study of Ln3+ β-diketonate systems containing a tetradentate chiral ligand. Only one family of Ln complexes, based on ternary β-diketonate and an achiral tetradentate ligand as additional chromophore able to sensitize the lanthanide luminescence, has been characterized.22 Received: May 30, 2018

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DOI: 10.1021/acs.inorgchem.8b01480 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

anisotropic thermal parameters for all non-hydrogen atoms. The aromatic and methylene hydrogen atoms were placed in calculated positions and refined with isotropic thermal parameters U(H) = 1.2 Ueq(C) and allowed to ride on their carrier carbons, whereas the H atoms in the coordinated water molecule were located in the Fourier map and refined isotropically [U(H) = 1.2 Ueq(O)]. The two terminal thienyl rings showed disorder due to a 180° rotation around the C21−C22 and C29−C30 axes, respectively; therefore, the sulfur and α-carbon atoms of the rings were refined for the two orientations. Some restraints were applied to the geometries obtained from the refinement of the disordered rings. The final refined occupancy factors were of ca. 0.59% (S2) and 0.62% (S1), respectively for the major orientation. The atomic displacement parameters of the two positions for each disordered atom in the thienyl rings were constrained to be equal. Crystal data and details of data collections for [Eu(N,N′-bis(2pyridylmethylidene)-1,2-cyclohexanediamine)(tta)2(H2O)]CF3SO3 are reported in Table S1. CCDC 1824682 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: + 44−1223−336−033; e-mail: deposit@ccdc. cam.ac.uk). Powder X-ray Diffraction (P-XRD). X-ray powder diffraction patterns were measured with a Thermo ARL X’TRA powder diffractometer, operating in the Bragg−Brentano geometry and equipped with a Cu anode X-ray source (Kα, λ = 1.5418 Å), using a Peltier Si(Li) cooled solid-state detector. The patterns were collected with a scan rate of 0.01°/s in the 5−50° 2θ range. Polycrystalline samples were ground in a mortar and then put in a low-background sample holder for the data collection. Luminescence and Decay Kinetics. Room-temperature luminescence was measured with a Fluorolog 3 (Horiba-Jobin Yvon) spectrofluorometer, equipped with a Xe lamp, a double excitation monochromator, a single-emission monochromator (mod. HR320), and a photomultiplier in photon-counting mode for the detection of the emitted signal. All the spectra were corrected for the spectral distortions of the setup. The spectra were recorded on CH3CN (0.1 mM) and methanol (0.05 mM) solutions, as was done for the CPL spectra (see below). In decay kinetics measurements, a xenon microsecond flashlamp was used, and the signal was recorded by means of multichannel scaling method. True decay times were obtained using the convolution of the instrumental response function with an exponential function and the least-squares-sum-based fitting program (SpectraSolve software package). Circularly Polarized Luminescence. CPL spectra were recorded with the homemade spectrofluoropolarimeter described previously.30 The spectra were recorded on CH3CN (0.1 mM) and methanol (0.05 mM) solutions in a 1 cm cell and at solid state, obtained by deposition of 50 μL of a 5 mM CH3CN solution on a quartz plate. The samples were excited at 365 nm, with a 90° geometry between the detector and the light source. Electronic Circular Dichroism. ECD spectra were recorded with a Jasco J710 spectropolarimeter on CH3CN 1 mM and CH3OH 2 mM solutions in a 0.1 mm cell and at the solid state, obtained by deposition of 50 μL of 1 mM CH3CN solution on a quartz plate. NMR. Proton NMR spectra were acquired on an Agilent Inova 600 working at 14.1T on the sample dissolved in acetonitrile-d3 and methanol-d4, using the residual solvent peak at 1.94 ppm as the internal reference. The standard 1D, longitudinal relaxation time T1 through inversion recovery, and gradient-COSY spectra were acquired. ESI-MS. Electrospray ionization mass spectra (ESI-MS) were recorded with a Finnigan LXQ Linear Ion Trap (Thermo Scientific, San Jose, CA, USA) operating in positive ion mode. The data acquisition was under the control of Xcalibur software (Thermo Scientific). A MeOH solution of sample was properly diluted and infused into the ion source at a flow rate of 10 μL/min with the aid of a syringe pump. The typical source conditions were transfer line capillary at 275 °C; ion spray voltage at 4.70 kV; sheath, auxiliary, and

In this contribution, the synthesis of a highly luminescent heteroleptic β-diketonate complex of europium is presented. The cationic complex, characterized by two anionic tta (2-thenoyltrifluoroacetyl-acetonate) ligands and one chiral tetradentate Schiff base NNNN ligand surrounding the metal ion (Figure 1), shows a strong CPL activity both in solution (acetonitrile and methanol) and in the solid state.

Figure 1. Molecular structure of [EuL(tta)2(H2O)]CF3SO3; L = N,N′-bis(2-pyridylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine.

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EXPERIMENTAL SECTION

Eu(CF3SO3)3 (Aldrich, 98%) was stored under vacuum for several days at 80 °C and then transferred in a glovebox. N,N′-Bis(2pyridylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine (L) and EuL(CF3SO3)3 were synthesized by following the procedures reported in literature.23−25 [EuL(tta)2(H2O)] CF3SO3 was synthesized as follows: At room temperature, 78 mg (0.342 mmol) of Htta have been dissolved in a methanol (2 mL) solution containing 19 mg (0.342 mmol) of KOH. The clear solution was slowly added to a methanol solution (2 mL) of ligand L [50 mg (0.171 mmol)] and Eu(CF3SO3)3 [102 mg (0.171 mmol)]. The final mixture was stirred for 1 h a room temperature, and the solvent was then removed under reduced pressure. The desired product has been obtained in a good yield as white powder upon extraction in dichloromethane (6 mL) followed by the removal of the solvent under reduced pressure. Yield 50%. ESIMS(methanol, Scan ES+; m/z): 293.27 ([L + H]+), 315.26 ([L + Na]+), 809 (100%), 807 (92%), 810 (21%), ([Eu(tta)2(CF3SO3)(CH3OH)2 + H]+). Elemental Anal. Calcd for C35H32EuF9N4O8S3 (MW 1055.8): C, 39.82; H, 3.05; N, 5.31; O, 12.12. Found: C, 39.78 ; H, 2.99; N, 5.22; O, 12.07. IR (KBr pellet, cm−1): ν = 1645 (CN imine stretching), 1537 (carbonyl group stretching), 1029 (pyridine bending), 638 (pyridine bending). X-ray Crystallography. Single Crystal X-ray Diffraction (SCXRD). The X-ray intensity data of [Eu(N,N′-bis(2-pyridylmethylidene)-1,2-cyclohexanediamine)(tta)2(H2O)]CF3SO3 were collected on a Bruker SMART ApexII CCD diffractometer using Mo Kα radiation. Cell dimensions and the orientation matrix were initially determined from a least-squares refinement on reflections measured in three sets of 20 exposures, collected in three different ω regions, and eventually refined against all data. A full sphere of reciprocal space was scanned by 0.3° ω steps. The software SMART26 was used for collecting frames of data, indexing reflections and determination of lattice parameters. The collected frames were then processed for integration by the SAINT program,26 and an empirical absorption correction was applied using SADABS.27 The structure was solved by direct methods (SIR 2014)28 and subsequent Fourier syntheses and refined by full-matrix least-squares on F2 (SHELXL2014)29 using B


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Inorganic Chemistry sweep gas (N2) flow rates were 10, 5, and 0 arbitrary units, respectively. Helium was used as the collision damping gas in the ion trap set at a pressure of 1 mTorr. Elemental Analysis. Elemental analyses were carried out by using a EACE 1110 CHNOS analyzer.

2.612(5) and 2.680(5) Å).34 The four Eu−N distances show significant differences in that the two imine nitrogens form bonds at 2.565(10) and 2.568(10) Å, significantly shorter than those involving the pyridine nitrogens [2.66(1)(9) and 2.63(1) Å] (Table 1). This is probably due to steric effects rather than a consequence of the different donor ability of the nitrogen atoms. In fact, the O4 and F1 atoms belonging to the diketonate unit are close to one hydrogen (H1) bound to a pyridine C atom [H(1)···O(4) 2.45 Å, H(1)···F(1) 2.48 Å] establishing weak intramolecular hydrogen bonds. N(py)−Eu distances shorter than the observed ones would generate steric repulsion between tta ligand and pyridine rings. Previously, a similar behavior has been observed for ligand L when coordinated to the Eu(NO3)3 fragment.24 The N atoms of the tetradentate ligand slightly deviate from the ideal square planar geometry [N(imine): +0.224(6) and −0.218(6) Å, N(py): +0.111(3) and −0.117(3) Å], and the distance of the Eu ion from the N4 least-squares plane of the coordinating N atoms is 0.663(7) Å. The β-diketonate ligands are found on the same side of the plane defined by the tetradentate ligand whereas the opposite side is occupied by the water molecule. In the crystal packing of [EuL(tta)2 (H2O)] CF3SO3 (Figure S1), two cationic [EuL(tta)2(H2O)]+ are held together through two bridging triflate anions that form H bonds with the O5 and O6 oxygens and the two H atoms of the coordinated water molecules [O(8)H(81)···O(6′), 1.90(3), 2.75(1) Å, 177(14)°, O(8)H82···O(5′′) 1.91(3), 2.76(1) Å, 172(9)°; symm. op. (I): x, y − 1, z; (II) −x,-y + 1, −z + 1]. Powder X-ray Diffraction. The chemical composition and the structure of the racemic Eu(III) complex in the powder as it was obtained from the synthesis are the same as those of the one observed in the single crystal. In fact, the X-ray diffraction patterns of the powdered [EuL(tta)2(H2O)]CF3SO3 (L is present as racemate) sample and the one simulated from the SC-XRD molecular structure (Figure 2) are superimposable (Figure S2). UV−Visible Absorption. The UV−visible electronic absorption spectra of [EuL(tta)2(H2O)]CF3SO3 was recorded in acetonitrile solution and compared with those of L, EuL(CF3SO3)3, and Eu(tta)3 (Figure 3). [EuL(tta)2(H2O)]-

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RESULTS AND DISCUSSION Single Crystal X-ray Structural Determination. Single crystals suitable for X-ray diffraction were obtained by slow evaporation of a dichloromethane solution of the complex. Selected bond distances and angles are given in Table 1, while the full crystallographic data are presented in Tables S1. Table 1. Selected Bond Lengths (Å) and Angles (deg) for [Eu(N,N′-bis(2-pyridylmethylidene)-1,2cyclohexanediamine)(tta)2(H2O)]CF3SO3 Eu−O(1) Eu−O(2) Eu−O(3) Eu−O(4) Eu−O(8) N(3)−Eu−N(4) N(2)−Eu−N(3) O(4)−Eu−O(3)

2.384(7) 2.359(9) 2.478(8) 2.327(9) 2.392(10) 63.5(3) 62.4(3) 70.2(3)

Eu−N(1) Eu−N(2) Eu−N(3) Eu−N(4) N(2)−Eu−N(1) O(2)−Eu−O(1)

2.66(1) 2.565(10) 2.568(10) 2.628(10) 62.7(4) 72.6(3)

Racemic [EuL(tta)2(H2O)] CF3SO3 complex crystallizes in the centric triclinic space group P1̅. The Eu(III) ion is ninecoordinated surrounded by one tetradentate ligand L, two bidentate tta, and one water molecule (Figure 2). One triflate

Figure 2. Crystal structure of [EuL(tta)2(H2O)] CF3SO3.

counterion is also present in the outer coordination sphere of the metal ion. The Eu−O bond distances of the β-diketonate units (average 2.39 Å) are slightly longer than those found in octa-coordinated tris-β-diketonate complexes containing bipyridine (mean Eu−O = 2.350 Å in [Eu(tta)3(bipy)])31 or phenanthroline (mean Eu−O = 2.36(1) Å in [Eu(tta)3(phen)])32,33 as a secondary ligand and are almost identical to those found in the ternary nine-coordinated terpyridine complex [Eu(DPM) 3 (terpy)] with Eu−O = 2.38(3) Å (DPM = dipivaloylmethanate).34 It is interesting to note that one Eu−O bond distance involving β-diketonate unit is significantly longer than the others (Eu−O(3) = 2.478(8) Å, Table 1). The Eu−N bond distances ranging from 2.565(10) to 2.66(1) Å are slightly shorter than the values found in the [Eu(DPM)3(terpy)] complex (with Eu−N distances between

Figure 3. Absorption spectra of L, EuL(CF3SO3)3, Eu(tta)3, and [EuL(tta)2(H2O)]CF3SO3 in acetonitrile at 298 K.

CF3SO3 shows two broad absorption bands spanning the UV range from 250 to 400 nm, with two absorption maxima (280 nm with a shoulder around 290 and 350 nm). The Eu(tta)3 C


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Inorganic Chemistry

bound to the metal ion. On the contrary, the luminescence emission spectrum of the powder dissolved in methanol is similar but not superimposable to the one collected in acetonitrile (Figure.S3). This could mean that the solvent plays a non-negligible role in the definition of the geometric environment of Eu(III), which is slightly different in the two solvents. The most significant difference is in the region of the 5D0 → 7 F1 transition (Figure S3). The luminescence decay curves of the 5D0 excited state in the solid state and in acetonitrile and methanol solutions were also collected (Figure S4). In all cases, the curves are well-fitted by a single exponential function, and the observed lifetimes are 0.60(1), 0.57(1), and 0.44(1) ms for the sample in the solid state, in methanol, and in acetonitrile, respectively. By means of the equation ϕ = τobs/τR for the calculation of the intrinsic quantum yield, we found a value around 40 and 52% for the complex in acetonitrile and methanol solution, respectively. The Eu(III) radiative lifetime (τR) is 1.1 ms in both solvents and has been determined from the analysis of the emission spectrum by using the equation reported by Werts et al.36 Even though the precise estimation of the intrinsic quantum yield for the solid state sample appears less obvious, we expect a value at least equal to the one found in acetonitrile and methanol solutions, due to the similar nature of the two complexes. In order to clarify exactly the nature of the coordination environment of the complex in the two solvents (acetonitrile and methanol), we also measured the 5D0 excited state lifetimes in deuterated solvents. Interestingly, the same τobs in CH3CN and CD3CN (around 0.44 ms) was measured, and consequently, the same intrinsic quantum yield (40%) is obtained, which allows us to conclude that the C−H vibrations of the acetonitrile molecules are not capable to substantially quench the 5D0 Eu(III) excited state by means of the multiphonon relaxation process.37 On the contrary, a different τobs was measured when the complex is dissolved in in CD3OD and CH3OH [τobs = 0.75(1) and 0.57(1) ms, respectively]. The value of the intrinsic quantum yield (ΦLn) in CD3OD (68%) is therefore higher than that in CH3OH (52%). From the equation reported in the literature,38 the number of bound methanol molecules (m) can be obtained by m = 2.1(1/ τobsCH3OH − 1/τobsCD3OD). The calculated value of m = 0.9(5) is compatible with the presence of one methanol molecule in the inner coordination sphere of Eu(III) when the sample is dissolved in CH3OH. The decrease of the quantum yield (from 68 to 52%) is connected with the presence of one O−H vibration (coming from CH3OH) close to the metal ion, which is able to quench in a nonradiative way the 5D0 excited state of Eu(III) by multiphonon relaxation. However, the values of τobs and ΦL in CH3CN and CD3CN being lower than that in CH3OH is likely due to the presence in the first coordination sphere of a molecule capable to quench the Eu(III) excited state more efficiently than CH3OH, when the complex is dissolved in acetonitrile. This molecule cannot be CH3CN since (i) C−H and CN vibrations possess lower energy than O−H vibration and (ii) the identical τobs measured in CH3CN and CD3CN proves that CH vibrations are not capable to significantly quench the 5D0 excited state of Eu(III). Consequently, it is highly probable that in place of CH3CN a water molecule is bound to the metal ion. In fact, H2O possesses two high-energy O−H phonons instead of only one in the case of CH3OH, which makes it more efficient as multiphonon quencher. This conclusion is also supported by the evidence of a perfect overlap between the emission spectra

complex presents one main absorption band (λmax = around 350 nm) which is attributed to the ligand centered singlet− singlet π−π* enolic transition.22 The composite absorption band peaking around 260 nm observed for ligand L, assigned to electronic transitions involving both the pyridine ring and the conjugated CN group (i.e., π−π*, n−-π* transitions), is red-shifted to 280−285 nm upon complexation of the ligand with Eu(III) in EuL(CF3SO3)3. On the basis of this evidence, the strongest peaks for [EuL(tta)2(H2O)]CF3SO3 at 285 and 350 nm are assigned to the overlapping absorption bands of L and tta ligands bound to Eu(III), respectively. The latter absorption band in the N−UV region is particularly attractive for obtaining strong absorption at lower energy, which is required in many applications. If the absorption spectrum of [EuL(tta)2 (H2O)]CF3SO3 is recorded in methanol solution, then the same conclusions can be drawn, as long as the concentration of the complex is not decreased below 10−5 M. In such case, while the band of tta ligand is not significantly shifted (λmax = 348 nm), the one of L ligand is blue-shifted (λmax = 260 nm) and is superimposable with the absorption of the “free” ligand in methanol (data not shown). This could mean that in very diluted methanol solution ligand L is completely dissociated. Luminescence. Excitation spectra of [EuL(tta)2(H2O)]CF3SO3 sample in the solid state and dissolved in acetonitrile upon monitoring the 5D0 → 7F2 transition (λem = 615 and 619 nm, respectively) are shown in Figure 4a.

Figure 4. Luminescence excitation spectra (a) and emission spectra (b) of [EuL(tta)2(H2O)]CF3SO3 in the solid state (black lines) and dissolved in acetonitrile (red line) at 298 K. For reasons of clarity, the emission spectra have been shifted on y axis.

In both cases, the typical red luminescence of Eu(III) is effectively sensitized upon excitation of both L (peak around 280 nm) and the tta ligands (peak around 350 nm). As for the emission spectra in the solid state and in acetonitrile (Figure 4b), the typical Eu(III) luminescence stemming from f−f transitions is clearly detected. Since the hypersensitive 5D0 → 7F2 transition dominates the spectrum, the point symmetry of Eu(III) deviates from the inversion symmetry,35 even in acetonitrile solution. As it is well-known that the Eu(III) emission features are strongly connected to the local symmetry at the metal ion and that the emission spectra recorded in the solid state and in acetonitrile solution are superimposable upon several excitation conditions (i.e., λem = 285 and 350 nm), we expect the same Eu(III) geometric environment in the solid state and in acetonitrile solution, characterized by two tta, one L ligand, and one solvent molecule (water or acetonitrile) D


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Inorganic Chemistry in acetonitrile solution and in the solid state where the coordinated water molecule has been unambiguously observed by X-ray diffraction experiment. In conclusion, the different nature of the solvent molecule (water or methanol) bound to the metal ion is responsible for both the small but significant difference of the observed Eu(III) excited state lifetimes and the slightly different luminescence emission spectra of the complex (Figure S3). It is worth noting that the nature of the bound solvent molecule has a more evident impact on the CPL signature of the complex (see discussion below). The excitation and emission spectrum of EuL(CF3SO3)3 in acetonitrile are presented in Figure S5. Also in this case, the metal-centered luminescence is effectively sensitized by ligand L upon excitation around 280 nm, and the emission features originate from Eu(III) located in a strongly asymmetric environment. Also, for this complex, these spectra are superimposable with the ones obtained in the solid state (data not shown). Chiroptical Characterization. The CPL spectra obtained for the two enantiomers of [EuL(tta)2(H2O)]CF3SO3 both in acetonitrile and methanol solutions (Figures 5 and 6,

Figure 6. Normalized CPL spectra (top) and total emission spectra (bottom) of the two enantiomers of [EuL(tta)2(H2O)]CF3SO3 in MeOH. The spectra are normalized to the maximum of the 5D0 → 7F2 emission band. 5 D0 → 7F2 transitions, respectively centered at 590 and 615 nm. The 5D0 → 7F3 (655 nm) and the 5D0 → 7F4 (700 nm) are less CPL-active, whereas the 5D0 → 7F0 (580 nm) transition does not give any CPL signal. The highest glum is reached for the most intense band of the 5D0 → 7F1 multiplet at 592 nm: We measured a value of |0.13| in acetonitrile and |0.2| in methanol. The different numbers of components for 5D0 → 7F2 transition may be associated with the different types of solvent molecule coordinated to Eu(III) when the complex is dissolved in acetonitrile or methanol (see the discussion above). Electronic circular dichroism (ECD) spectra in acetonitrile, in methanol, and in the solid state are shown in Figures S6−S8. In all cases, they display a complex pattern, associated with both tta and L absorption bands. This reveals that the chiral ligand dictates a defined chiral twist in the diketonates as well. There are several small differences between the spectra in the two solvents, which once more suggests some minor structural solventdependent rearrangements. As for what concerns the total emission spectra, some differences in the relative intensities of the two bands associated with the 5D0 → 7F2 transition can be observed in acetonitrile solution. The most blue-shifted component of the multiplet gains intensity upon time (data not shown), probably indicating that the complex undergoes some modification when dissolved in CH3CN for a few hours. Nevertheless, because the CPL spectra (normalized on the emission intensities of the redshifted component of the transition) of the two enantiomers are mirror images, we did not investigate this aspect any further. The CPL spectra of the main bands were also collected for the

Figure 5. Normalized CPL spectra (top) and total emission spectra (bottom) of the two enantiomers of [EuL(tta)2(H2O)]CF3SO3 in CH3CN. The spectra are normalized to the maximum of the 5D0 → 7 F2 emission band.

respectively) are perfect mirror images. We observe several partially resolved transitions thanks to the apparent resolution enhancement typical of spectra with nearby opposite-signed bands. This is particularly evident for the 5D0 → 7F2 transition, which shows four bands in acetonitrile, two positive and two negative, and three bands in methanol (in total emission spectra, only two bands are visible in both the cases). The main CPL signals are associated with the 5D0 → 7F1 and E


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Inorganic Chemistry

found in the CPL and ECD spectra discussed above. Moreover, because the 1H NMR signals are highly structured, due to J-couplings, and have very small half-height line width (Δν1/2), one can rule out line broadening caused by chemical exchange, for example, due to slowly occurring geometrical rearrangement The quantitative analysis of the paramagnetic shifts and relaxation data would require a set of other Ln analogues, in order to separate pseudocontact and contact contributions to the observed NMR shifts and it was considered not necessary for the present discussion.

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CONCLUSIONS A new chiral complex showing a good luminescence emission efficiency (intrinsic quantum yield around 40−50% and up to 68% in deuterated methanol) and a strong CPL activity both in solution (glum 0.13 and 0.2 in acetonitrile and methanol, respectively) and in the solid state (glum around 0.1) has been synthesized and characterized from a spectroscopic and structural point of view, by means of X-ray diffraction on single crystal and by TL and CPL spectroscopy. The synergic use of these techniques has permitted us to discover that while the structure of the complex in the solid state is not disrupted if it is dissolved in acetonitrile when a polar and protic solvent such as methanol is used a substitution of the coordinated water molecule by a solvent one is observed. This is reflected in a different Eu(III) CPL signature. In conclusion, the investigated chiral complex can be considered a promising candidate as red phosphor in CP-OLED thanks to the sizable values of its intrinsic quantum yield, its emission dissymmetry factor (glum), and the presence of the tta ligand being able to ensure an efficient light harvesting.

Figure 7. Normalized CPL spectra (top) and total emission spectra (bottom) of the two enantiomers of [EuL(tta)2(H2O)]CF3SO3 at the solid state. The spectra are normalized on the maximum of the 5D0 → 7 F2 emission band.

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ASSOCIATED CONTENT

* Supporting Information S

two enantiomers in the solid state (Figure 7), obtained by deposition from acetonitrile solutions on a quartz plate. The spectra display the same trends (sequence, relative intensities, and signs of bands) in comparison to the ones recorded in acetonitrile solutions. This indicates that both in solution and in the solid state Eu(III) ion is surrounded by the same dissymmetric environment. In this case, we measured a glum around |0.05| for the CPL band at 593 nm. Finally, we report in a summary table (Table 2) the selected spectroscopic and chiroptical data for the complex. NMR. We decided also to get at least some insight into the solution structure of the complex by means of paramagnetic NMR. The 1H NMR spectra, both in CD3CN and CD3OD, show narrow resonances, thus indicating that the complex has a static C2 symmetry (Figures S9−S11). Because the X-ray diffraction (XRD) structure lacks any symmetry (all the atoms fall in a general position (C1 symmetry), it does not well represent either the solid-state sample (obtained by deposition) or the complex in solution, in agreement with the differences

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01480. Listings of crystal data, luminescence excitation and emission data, excited states decay kinetics data, electronic circular dichroism (ECD) data, and NMR data (PDF) Accession Codes

CCDC 1824682 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Table 2. Intrinsic Quantum Yield, Branching Ratios and glum Values for the [EuL(tta)2(H2O)]CF3SO3 in Methanol and Acetonitrile Solution and in the Solid Statea branching ratios relative to 5D0 → 7F1 and glum on the top of the band physical state

ΦLn (%)

methanol solution acetonitrile solution solid

52 40 ∼40

D0 → 7F0 (glum)

5

0.16 (n.d.) 0.13 (n.d.) 0.13 (n.d.)

D0 → 7F1 (glum)

5

1 (0.202) 1 (0.148) 1 (0.055)

D0 → 7F2 (glum)

5

13.2 (0.011) 13.9 (0.005) 13.9 (0.005)

D0 → 7F3 (glum)

5

0.32 (0.017) 0.35 (0.04) 0.35 (n.d.)

D0 → 7F4 (glum)

5

3.55 (0.018) 3.51 (0.019) 3.51 (n.d.)

a

For each multiplet, the glum values relative to the point of most intense CPL are reported; n.d. stands for not determined. F


Article

Inorganic Chemistry ORCID

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Marco Bettinelli: 0000-0002-1271-4241 Lorenzo Di Bari: 0000-0003-2347-2150 Fabio Piccinelli: 0000-0003-0349-1960 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully thank Prof. A. Melchior (University of Udine) for the ESI-MS measurements. M.L., M.B., and F.P. thank the Facility “Centro Piattaforme Tecnologiche” of the University of Verona for access to the Fluorolog 3 (Horiba-Jobin Yvon) spectrofluorometer and Thermo ARL X’TRA powder diffractometer.

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Circularly Polarized Luminescence from an Eu(III) Complex Based on

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