Promoting a Significant Increase in the Photoluminescence Quantum

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

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Promoting a Significant Increase in the Photoluminescence Quantum Yield of Terbium(III) Complexes by Ligand Modification Thaiane Gregoŕ io,† Joyce de M. Leão,‡ Guilherme A. Barbosa,† Jaqueline de L. Ramos,† Siddhartha Om Kumar Giese,† Matteo Briganti,† Paula C. Rodrigues,§ Eduardo L. de Sa,́ † Emilson R. Viana,‡ David L. Hughes,∥ Luís D. Carlos,⊥ Rute A. S. Ferreira,⊥ Andreia G. Macedo,‡ Giovana G. Nunes,† and Jaísa F. Soares*,† Downloaded via NOTTINGHAM TRENT UNIV on August 28, 2019 at 09:52:36 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Chemistry, Federal University of Paraná, Centro Politécnico, Jardim das Américas, 81530-900 Curitiba, Paraná, Brazil ‡ Department of Physics, Federal University of Technology, Av. Sete de Setembro, 3165, 80230-901 Curitiba, Paraná, Brazil § Department of Chemistry, Federal University of Technology, Rua Deputado Heitor Alencar Furtado, 5000, 81280-340 Curitiba, Paraná, Brazil ∥ School of Chemistry, University of East Anglia, Norwich NR4 7TJ, U.K. ⊥ Department of Physics, Campus Universitário de Santiago, University of Aveiro, 3810-193 Aveiro, Portugal S Supporting Information *

ABSTRACT: Two discrete mononuclear complexes, [Tb(bbpen)(NO3)] (I) and [Tb(bbppn)(NO3)] (II), for which H2bbpen = N,N′-bis(2-hydroxybenzyl)-N,N′-bis(pyridin-2-ylmethyl)ethylenediamine and H2bbppn = N,N′-bis(2-hydroxylbenzyl)-N,N’-bis(pyridin-2-ylmethyl)-1,2-propanediamine, were synthesized and characterized by FTIR, Raman, and photoluminescence (PL, steady-state and time-resolved modes) spectroscopy. The attachment of a methyl group to the ethylenediamine portion of the ligand backbone differentiates II from I and acts as a determining feature to both the structural and optical properties of the former. The single-crystal X-ray structure of H2bbppn is described here for the first time, while that of complex II has been redetermined in the monoclinic C2 space group in light of new diffraction data. In II, selective crystallization leads to spontaneous resolution of enantiomeric molecules in different crystals. Absolute emission quantum yields (ϕ) and luminescence excited-state lifetimes (at room temperature and 11 K) were measured for both complexes. Despite their similar molecular structures, I and II exhibit remarkably different ϕ values of 21 ± 2% and 67 ± 7%, respectively, under UV excitation at room temperature. Results of quantum-mechanical (DFT and TD-DFT) calculations and experimental PL measurements also performed for H2bbpen and H2bbppn confirmed that both ligands are suitable to work as “antennas” for TbIII. Considering the 5D4 lifetime profiles and the significantly higher absolute quantum yield of II, it appears that thermally active nonradiative pathways present in I are minimized in II due to differences in the conformation of the ethylenediamine bridge.

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It is well-known that the photoluminescence of Tb3+ compounds originates in the strongly shielded inner 4f orbitals of the metal. Their green emission is based on spin- and parityforbidden 4f−4f transitions, with emission wavelengths almost independent of the coordination environmentusually exhibit-

INTRODUCTION

The luminescent properties of lanthanide complexes, particularly those containing Tb3+ and Eu3+ ions coordinated to a variety of polydentate organic ligands, have attracted considerable attention because of their potential applications in several bioanalytical studies,1−4 photoluminescent displays,5−8 and light-emitting devices.9−11 © XXXX American Chemical Society

Received: May 13, 2019

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DOI: 10.1021/acs.inorgchem.9b01397 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ing an offset lower than 10 nm.12 Ligand field effects can contribute to partial allowance of these 4f−4f transitions by mixing 4f and 5d states; in these cases, emission quantum yields and luminescence lifetimes are strongly affected by the organic ligand and the specific geometries determined by them around the metal ions.13−17 To circumvent the low extinction coefficients and enhance the luminescent properties of the 4f ions, several complexes of Tb3+, as well as other trivalent lanthanide ions (Ln3+) such as Sm3+, Eu3+, and Dy3+, have been synthesized with chelating, strongly absorbing multidentate organic ligands, especially those containing aromatic or other π-chromophores such as βdiketonates, bipyridines, calixarenes, and dipicolinic acids that display the so-called “antenna effect”.10,18−20 This gives highly luminescent complexes in which, following irradiation by UV light and according to the widely known Jablonski model, the chromophore-containing ligand transfers energy to the metal center via the ligand first excited triplet state (T1), populating the emitting Ln3+ levels.21,22 In addition to this, the importance of the so-called “singlet pathway”, in which the energy transfer to the 4f levels occurs directly from the ligand first excited singlet state (S1), has also been recognized, although the longer-lived T1 state is generally accepted as the most suitable intermediate.2 Strong luminescence has also been reported for complexes in which chelating ligands create an asymmetric coordination environment around the Ln3+ ion. Examples are complexes of Eu3+ with phosphine oxide ligands in eight-coordinate trigonal dodecahedral (8-TDH, D2d) and nine-coordinate monocapped square-antiprismatic (9-SAP, C4v) geometries.23−25 More recently, Tb3+ complexes with bulky 2,2,6,6-tetramethyl-3,5heptanedione (tmh) and triphenylphosphine oxide ligands have also shown high absolute emission quantum yield values (ca. 88%), attributed to the monocapped-octahedral (7-MCO, C3v) structure.26 In this context, considerable effort has been made in the development of heterofunctional ligands containing (soft and hard) N, O, and S binding sites, suitable for the coordination of a range of p-, d-, and f-block metal ions. For example, the wellknown proligand88 H2bbpen (N,N’-bis(2-hydroxybenzyl)N,N’-bis(pyridin-2-ylmethyl)ethylenediamine) and its derivatives have been used to prepare complexes with first-row transition-metal ions as models for metalloenzymes,27−30 and with Ln3+ ions for nuclear medicine31 and magnetic studies.32 This potentially hexadentate proligand presents two relatively soft nitrogen atoms on the pyridine rings and four harder donor atoms, two amine nitrogens and two oxygen atoms on the phenol groups.33,34 A variation of H2bbpen, the molecule N,N′bis(2-hydroxylbenzyl)-N,N′-bis(pyridin-2-ylmethyl)-1,2-propanediamine), H2bbppn,35,36 presents one additional methyl group attached to the ethylenediamine backbone and produces complexes with a similar coordination environment about the metal ion, but with lower symmetry in comparison to those obtained with H2bbpen. The chemical structures of the H2bbpen and H2bbppn molecules are represented in Figure 1. To the best of our knowledge, no detailed luminescence study of lanthanide complexes containing bbpen2− ligands or its derivatives has been described, apart from a recent study on Tb3+ and Yb3+ complexes of a di-tert-butyl-H2bbppn derivative that has shown significant luminescence quenching upon ligand oxidation.37 Complex I, [Tb(bbpen)(NO3)] (Figure S1), was first synthesized and characterized by X-ray diffraction analysis by our research group.38 The single-crystal X-ray structure of

Figure 1. Structural representations of H2bbpen and H2bbppn.

[Tb(bbppn)(NO3)] (complex II), in turn, was reported in the C2221 space group by Yamada and co-workers, together with those of an isostructural series of complexes containing Dy, Ho, Er, Tm, and Lu, all in the +III oxidation state.35 In the present work, H 2bbppn and [Gd(bbpen)(NO3 )] (product III, Supporting Information) were characterized by single-crystal X-ray diffraction analysis for the first time, and a new set of crystal data obtained at 100 K for complex II has provided an opportunity to describe its structural features with greater accuracy. We have also studied the photoluminescence properties of complexes I and II, and DFT/TD-DFT studies were performed for the proligands to help shed light on the significant differences observed in the light-emitting behavior of the two complexes.

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EXPERIMENTAL SECTION

General Considerations. Reagents were purchased from SigmaAldrich and used without additional purification. Solvents were dried by standard procedures39 and distilled under N2 prior to use (when required). H2bbpen and H2bbppn were prepared following literature procedures.34,35 Recrystallization of H2bbppn was carried out from a tetrahydrofuran solution (10 mg mL−1) at room temperature. The identities of H2bbpen and H2bbppn were confirmed by elemental analysis and 1H NMR spectroscopic analysis in dmso-d6 at 298 K (400.13 MHz, Figure S2 and Table S1, Supporting Information). Anal. Found for H2bbpen: C, 73.7%; H, 6.43%; N, 12.2%. Calcd for C28H28N5O5Tb: C, 74.0; H, 6.65; N, 12.3. Found for H2bbppn: C, 74.6; H, 6.75; N, 11.6. Calcd for C29H30N5O5Tb: C, 74.3; H, 6.88; N, 12.0. H2bbpen had its unit cell confirmed by single-crystal X-ray diffraction analysis, while the crystal and molecular structures of H2bbppn are presented here for the first time. The syntheses of complexes I and II were carried out under N2(g) using standard Schlenk techniques as described below. After crystallization, all products were air-stable. Data on complexes [Gd(bbpen)(NO3)] (III) and [Gd(bbppn)(NO3)] (IV), which were prepared for the determination of excited triplet state energies,40 are presented in Table S2 and Figure S3 in the Supporting Information. Analytical Methods and Instruments. Carbon, nitrogen, and hydrogen contents were determined by combustion analysis on a Thermal Scientific Flash ES 1112 series Elemental Analyzer run by MEDAC Laboratories Ltd. (Chobham, Surrey, U.K.). 1H NMR spectra were obtained from dmso-d6 (0.1% tetramethylsilane) solutions (ca. 10 mg mL−1) on a Bruker AVANCE 400 spectrometer. FTIR spectra (4 cm−1 resolution) were recorded from KBr pellets in the range of 400− 4000 cm−1 on a BIORAD FTS 3500GX instrument. Raman spectra were obtained on a Renishaw Raman Image spectrophotometer coupled to a Leica optical microscope, which focuses the incident radiation on a 1 μm2 area. Spectra were recorded with an incident power of 0.2 mW using Ar+ (514 nm) and He-Ne (632.8 nm) laser excitation over the range of 200−4000 cm−1. Diffuse reflectance spectra were measured using a Shimadzu UV 3600 instrument, with a spectral range of 200−700 nm and resolution of 2 nm. Computational Details. Density functional theory (DFT) calculations on the electronic structures of the proligands were performed with the B3LYP hybrid functional41 and the 6-311++(d,p) basis set (triple-ζ with double polarization and diffuse functions)42 as B


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Inorganic Chemistry implemented in the Orca 4.0 quantum chemistry software package.43 The auxiliary basis set SARC/J44 was also necessary in order to apply the RIJCOSX approximation and reduce computational time.45 Coordinates obtained from the X-ray structures of the proligands were employed without any further geometry optimization. The calculated UV electronic spectra (20 states) were obtained from a TDDFT approach (also B3LYP/6-311++(d,p)) and were plotted using Vesta and Molden software.46,47 Photoluminescence (PL) Measurements. Emission and excitation spectra were recorded with the front face acquisition mode at both 11 and 300 K on a Fluorolog-3 Jobin Yvon-Spex spectrometer (Model FL3-2T) with modular double-grating excitation (1200 grooves/mm, blazed at 330 nm), fitted with a TRIAX 320 singleemission monochromator (1200 grooves/mm, blazed at 500 nm, reciprocal linear density of 2.6 nm mm−1) coupled to a R928 Hamamatsu photomultiplier. The excitation source was a 450 W Xe arc lamp. The emission spectra were corrected for detection and optical spectral response of the spectrofluorimeter, while the excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector. Time-resolved measurements were carried out with the setup described for the luminescence spectra using a pulsed Xe-Hg lamp (6 ms pulse at half-width and 20−30 ms tail). The measurements were performed using a He closed-cycle cryostat, and the temperature was increased in steps of 10 K using a Lakeshore 330 autotuning temperature controller with a resistance heater. For photoluminescence measurements, all samples were processed in pellets with 0.5 mm thickness. Absolute Emission Quantum Yields. Emission quantum yields were measured at room temperature using a quantum yield measurement system from Hamamatsu (Model C9920-02) equipped with a 150 W Xe lamp coupled to a monochromator for wavelength discrimination, an integrating sphere as the sample chamber, and a multichannel analyzer for signal detection. Results are average values from three measurements on each sample. The method is accurate within 10%. Synthesis of [Tb(bbpen)(NO3)] (I). Complex I was prepared according to our previous report.38 Anal. Found: C, 49.76; H, 3.99; N, 10.59. Calcd for C28H28N5O5Tb: C, 49.93; H, 4.19; N, 10.40. Synthesis of [Tb(bbppn)(NO3)] (II). To a solution of 1.10 g (2.53 mmol) of Tb(NO3)3·5H2O in 35 mL of acetonitrile was added solid H2bbppn (1.19 g, 2.53 mmol). After it was stirred for 1 h at room temperature, to the resulting yellow solution was added 700 μL (0.508 g, 5.02 mmol) of triethylamine and the mixture was stirred for 30 min. After this period, the reaction mixture was cooled to −20 °C, producing after 3 days a light yellow solid that was filtered off and dried under vacuum. Yield: 1.03 g, 59.2%. Suitable colorless crystals for XRD analysis were obtained from recrystallization of 150 mg of this solid by liquid diffusion of glyme in an acetonitrile solution (2:1). Recrystallization yield: 0.020 g, 13%. II is soluble in dimethyl sulfoxide, slightly soluble in acetonitrile, and practically insoluble in hexane, toluene, tetrahydrofuran, dimethoxyethane, chloroform, and dimethylformamide at room temperature. Heating at 70 °C improved the solubility in dichloromethane, chloroform, dimethylformamide, and acetonitrile. Anal. Found: C, 49.62; H, 4.42; N, 10.26. Calcd for C29H30N5O5Tb: C, 50.66; H, 4.40; N, 10.19. Single-Crystal X-ray Diffraction Analyses of H2bbppn and [Tb(bbppn)(NO3)] (II). General Information. Data were collected on a Bruker D8 Venture diffractometer equipped with a Photon 100 CMOS detector, Mo Kα radiation, and graphite monochromator. Colorless crystals of H2bbppn (0.352 × 0.269 × 0.170 mm) and II (0.252 × 0.041 × 0.044 mm) were mounted on MiTeGen micromeshs, and intensity data were measured at 301(2) and 100(2) K, respectively, by thin-slice ω and φ scans. Cell dimensions were based on 3507 (H2bbppn) and 6017 (II) observed reflections (I > 2σI). Data were processed using the APEX3 program. 48 The structures were determined by the intrinsic phasing routines in the SHELXT program49,50 and refined by full-matrix least-squares methods, on F2 values, in SHELXL.51,52 All non-hydrogen atoms were refined with anisotropic thermal parameters. Scattering factors for neutral atoms were taken from the literature.53 All hydrogen atoms, except H(1) and

H(2) for H2bbppn (Figure 2), were included in idealized positions and their Uiso values were set to ride on the Ueq values of the parent carbon

Figure 2. Representation55,61 of the molecular structure of the proligand H2bbppn, with the atom numbering scheme. Hydrogen atoms, except those involved in hydrogen bonds, are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. atoms. Computer programs used in these analyses were noted above and run through WinGX54,55 at the Universidade Federal do Paraná and the University of East Anglia. Crystal data, data collection, and general structure refinement data are presented in Table 1.

Table 1. Crystallographic and Refinement Data for H2bbppn and [Tb(bbppn)(NO3)] (II) [Tb(bbppn)(NO3)] (II)

H2bbppn elemental formula molar mass/g mol−1 cryst syst, space group a/Å b/Å c/Å α = γ/deg β/deg V/Å3 Z density/g cm−3 temp/K F(000) abs coeff/mm−1 cryst color, shape θ range/deg no. of rflns collected no. of unique data no. of obsd data (I > 2σ(I)) no. of params goodness of fit on F2 R1 (I > 2σ(I)), wR2 (I > 2σ(I))a R1, wR2 (all data) Largest diff peak and hole/e Å−3

C29H32N4O2 468.58 monoclinic, P21/c (No. 14) 11.4248(7) 16.8019(11) 14.4159(9) 90 110.197(2) 2597.1(3) 4 1.198 301(2) 1000 0.077 colorless block 3.706−27.500 139892 5962 (Rint = 0.126) 3507

C29H30N5O5Tb 687.50 monoclinic, C2 (No. 5) 19.0143(7) 8.6424(3) 16.8278(6) 90 90.4790(10) 2765.20(17) 4 1.651 100(2) 1376 2.606 colorless prism 2.854−27.500 108114 6360 (Rint = 0.056) 6017

322 1.045 0.054, 0.129

371 1.083 0.017, 0.035

0.088, 0.141 0.020, 0.036 0.27 (close to H10) and 0.46 (close to Tb) and −0.21 −0.36

H2bbppn, w = [σ2(Fo2) + (0.0664P)2 + 0.7179P]−1 with P = (Fo2 + 2Fc2)/3; complex II, w = [σ2(Fo2) + (0.0132P)2 + 2.8316P]−1 with P = (Fo2 + 2Fc2)/3.

a

C


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Inorganic Chemistry

Table 2. Selected Bond Lengths (Å) and Angles (deg) for H2bbppn, with Estimated Standard Deviations in Parenthesesa N(1)−C(7) N(1)−C(8) N(1)−C(29) N(2)−C(21) N(2)−C(22) N(2)−C(28)

1.480(2) 1.467(3) 1.480(2) 1.476(2) 1.452(3) 1.463(2)

N(3)−C(9) N(3)−C(13) N(4)−C(23) N(4)−C(27) O(1)−C(1) O(2)−C(15)

1.326(3) 1.355(4) 1.323(4) 1.340(4) 1.371(3) 1.363(3)

C(6)−C(7) C(8)−C(9) C(20)−C(21) C(22)−C(23) C(28)−C(29) C(29)−C(30)

1.504(3) 1.501(3) 1.498(3) 1.501(4) 1.530(3) 1.524(3)

N(1)−C(7)−C(6) N(1)−C(8)−C(9) N(1)−C(29)−C(28) N(2)−C(21)−C(20) N(2)−C(22)−C(23) N(2)−C(28)−C(29)

112.38(16) 111.78(16) 115.97(15) 112.01(16) 114.0(2) 115.17(15)

C(8)−N(1)−C(29) C(7)−N(1)−C(8) C(7)−N(1)−C(29) C(22)−N(2)−C(28) C(21)−N(2)−C(28) C(21)−N(2)−C(22)

113.49(14) 111.80(15) 113.65(15) 111.51(16) 110.70(15) 110.03(16)

O(1)−C(1)−C(6) O(1)−C(1)−C(2) O(2)−C(15)−C(20) O(2)−C(15)−C(16) C(8)−C(9)−N(3) C(22)−C(23)−N(4)

121.46(19) 118.1(2) 122.7(2) 117.5(2) 116.50(19) 114.0(2)

a

Dimensions involving the carbon atoms in the aromatic rings have not been included.

Notes on the Structure and Refinement Details for II. The data for the structure determination were measured at 100 K and the cell parameters refined from these measurements showed a β angle of 90.4790(10)°; this is significantly different from 90.0° and indicates a monoclinic cell with space group C2 (No. 5), rather than the published orthorhombic cell, space group C2221 (No. 20), from diffraction data measured at room temperature.35 The structure was refined in both crystal systems and, at the molecular level, they are essentially identical; there are no significant differences in the molecular and intermolecular dimensions. The CheckCIF routine, run on the monoclinic results for II, considers it likely that there is additional symmetry in the crystal and that the space group may be C2221. We prefer to accept that the distorted β angle is real and that the space group is C2. Certainly, there are no signs of the peculiar alternation of opposing bond lengths that one normally finds in a symmetry system lacking some symmetry elements. We suggest that the distortion from the orthorhombic system results from the disorder in the methyl−ethyl bridge where the methyl group of C(30) is bonded to either of the bridging carbon atoms, but with a 60/ 40 occupancy ratio (Figure S4); in the orthorhombic system, the ratio is fixed at 50/50. It appears that either arrangement is possible but one is marginally preferred and that the orientation distorts the β angle away from 90°. The absolute configuration at the chiral carbon atom in both arrangements, N−CH(Me)−CH2−N or N−CH2−CH(Me)−N, is R and the Flack x factor for the monoclinic C2 refinement data is −0.005(4), indicating an enantiomerically pure crystal.

formation of these different complexes was shown to depend on the nature and strength of the base (hydroxide or acetate) and the size of the LnIII ion.58 Single-Crystal X-ray Diffraction Analysis of H2bbppn. H2bbppn crystallizes in the centrosymmetric, monoclinic space group P21/c, giving rise to an achiral crystal structure despite the chiral nature of the individual molecular components of the crystals.59,60 This is similar to what is observed for H2bbpen (orthorhombic Pbca space group),57 with which H2bbppn shares the existence of two prochiral nitrogen centers, N(1) and N(2), as shown in Figure 2. In both unit cells, those of H2bbppn (this work) and H2bbpen,57 the presence of an inversion center eliminates crystal chirality; the crystals are therefore racemates. In H2bbppn, the presence of a single methyl group bound to C(29) creates an additional chiral center in comparison to H2bbpen and lowers the molecular symmetry, leading to differences in the overall molecular and crystal structure symmetries of the corresponding terbium complexes I and II, as discussed below. Selected bond lengths and angles for H2bbppn are presented in Table 2 and compared with the corresponding dimensions for H2bbpen57 in Table S3. The most striking, global difference between the two molecules in the solid state lies in the spatial relations within the set of aromatic rings, depending on the presence or absence of the methyl group bound to C(29). In H2bbppn, the two pyridine and two phenol rings “pack” on one side of the molecule, clearing themselves of contact with the −CH3 group on the other side (Figure S5). In the absence of this methyl substituent, in turn, H2bbpen is a more “spatially relaxed” molecule. This reflects on the fact that the longest nonbonding distances across the H2bbppn molecule (Figure 2), those involving (i) C(12) and C(25) of the two opposite pyridine rings (10.44 Å), and (ii) C(3) and C(18) from the phenol moieties (10.61 Å), are similar to one another indicating a compact structureand are both significantly shorter than the largest nonbonding distance in H2bbpen (11.78 Å), defined by C(5)pyridine and C(26)phenol.57 No significant intermolecular interaction was observed in the crystal structure of H2bbppn. On the other hand, in the presence of intramolecular hydrogen bonds involving the phenolic OH groups and the nitrogen atoms of the pyridine or the diamine fragments (Figure 2), the H2bbppn molecules fold in on themselves, contributing to a relatively dense crystal packing (Figure S6). The O1−H1···N1amine and the O2−H2···N4pyridine hydrogen bonds can be classified as medium-strength (mostly electrostatic) interactions (Table S5A).62 When the two

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RESULTS AND DISCUSSION In the present work, structural, spectroscopic, and electronic features of four molecules are compared: namely the organic compounds H2bbpen and H2bbppn and the Tb3+ complexes (I and II) obtained from them. Two other coordination compounds have been prepared, III and IV, which are gadolinium(III) analogues of I and II respectively, to help with the interpretation of photoluminescence properties (Supporting Information). For clarity, the first two molecules are referred to as proligands,56 in order to differentiate them from the deprotonated bbpen2− and bbppn2− ligands in the lanthanide complexes. The molecular and crystal structures of the proligand H2bbpen and its Tb3+ complex [Tb(bbpen)(NO3)] (I) have been reported earlier in the literature by Neves and co-workers57 and by our research group38 respectively, and will be compared to the structural data reported here for H2bbppn and complex II, as discussed below. The literature also reports the synthesis and characterization of [Yb(bbpen)(NO3)],58 a neutral and mononuclear complex analogous to I, together with related monocationic (binuclear and trinuclear) products. The D


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methyl group of C(2) disordered with site occupancies fixed at 0.5. In the present work, the crystal structure has been revisited and solved in the monoclinic C2 space group; this is a revision and extension of the previous work,35 with more precisely measured cell parameters and intensity data from a lowtemperature study (data collection at 100(2) instead of 296(1) K). The choice of C2 was based on the β angle in the unit cell and supported by the refinement of the site occupancy factors of the disordered methyl carbon atom, C(29). For the β angle, results of the low-temperature data collection point to a value of 90.4790(10)°, which is significantly different from the exact 90° angle required for an orthorhombic cell and therefore favors a monoclinic cell. Additionally, on refinement in the C2 symmetry, the site occupancy factors of C(29a) and C(29b) refine to 0.398(12) and 0.602(12), respectively, indicating a preferential orientation (Figure S4). In contrast, the resolution in C2221 requires a 2-fold symmetry axis passing through O(5), N(5), the Tb3+ ion, and the midpoint of the C(27)−C(28) bond, which implies equal occupancies (0.5) of the two C(29) carbon sites: i.e., an either/or situation on the molecule. We suggest that the crystallographic disorder and the existence of a preferred orientation of the methyl group are the factors responsible for modifying the β angle from the orthorhombic to the monoclinic figure. Accordingly, in the C2 case the molecule has pseudo-2-fold symmetry, with the Tb atom slightly removed from the expected axial site. No intramolecular hydrogen bonds are observed for II, in line with the deprotonation of the H2bbppn phenol groups preceding coordination. In contrast, weak C−H···O interactions involving the noncoordinated O(5) center (from the nitrate ligand) give rise to slightly asymmetric contacts to the C(20)− H(20A) and C(7)−H(7A) bonds of two neighboring complex molecules (Figure 4), determining the shortest intermolecular contacts present in the crystal structure of II and building a twodimensional network roughly perpendicular to the b axis (Figure 4). The asymmetry is probably determined by the preferential orientation of the disordered C(29) methyl in the ethylenediamine bridge, as discussed above. A similar hydrogen bond

proligands H2bbpen and H2bbppn are compared,57 the structural difference reveals itself principally in the torsion angles around the N1−C14 and N3−C15 bonds in the former (−69.3°) versus the N2−C28 and N1−C29 bonds in the latter (57.8°), which are affected by the lack (or presence) of the methyl group (C30) in the ethylenediamine bridge and by differences in the hydrogen bonding patterns. Single-Crystal X-ray Diffraction Analysis of [Tb(bbppn)(NO3)] (II). In the solid-state structure of II, the Tb3+ ion is eight coordinate, with the bbppn2− ligand occupying six coordination positions and a chelating nitrate completing the coordination sphere (Figure 3 and Table S4). Our results show

Figure 3. Representation of the molecular structure of [Tb(bbppn)(NO3)] (II), showing the atom labeling scheme and 40% probability ellipsoids. The site occupancy of the methyl carbon, C(29a), refines to 0.398(12) and for the alternative C(29b) the value is 0.602(12). Only one of these disordered orientations is depicted; both are shown in Figure S4.

that the molecular structure of II is essentially the same as that reported by Yamada et al.,35 who worked in the orthorhombic, C2221 space group (see the Experimental Section) with the

Figure 4. Packing of the [Tb(bbppn)(NO3)] molecules evidencing the formation of weak C−H···O hydrogen bonds. The two disordered orientations of the methyl group are depicted. E


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Inorganic Chemistry

Figure 5. Graphic representation of selected frontier molecular orbitals of H2bbpen and H2bbppn. HOMO denotes the highest energy occupied molecular orbital and LUMO the lowest unoccupied molecular orbital. Colors: red, oxygen; blue, nitrogen; brown, carbon; white, hydrogen.

enough to zero (−0.003(9), −0.015(9), and −0.062(8)) to support the occurrence of the spontaneous resolution of II upon crystallization. In two of these crystals, the absolute configuration of the N(1), C(28), and N(3) stereocenters was R,S,R (respectively), opposite to that in the first sample, while the fourth crystal presented the S,R,S configuration, confirming the selective crystallization of the two mirror images of II in different crystals. According to Flack,59 this sort of sample constitutes a “racemic conglomerate”, defined as a “crop of crystals grown under equilibrium conditions from a racemate undergoing spontaneous resolution”. In this context each crystal is enantiomerically pure, but “the overall molecular composition of the crop is that of a racemate”.59 The same spontaneous resolution apparently happens upon crystallization of complex I, since the crystal structure reported earlier by us is also chiral (absolute structure parameter of −0.0107(19) in C2221) and shows the ligand only in the R,R configuration of the nitrogen centers.38 Considering that all syntheses of these complexes (I and II) have started from the racemic mixture of the respective proligand, spontaneous resolution upon complexation appears to be a general feature of this “polydentate ligand/lanthanide” system. This agrees with the low yield of the recrystallization procedures (10−13%, see the Experimental Section) and may be determined both by the low symmetry of the lanthanide coordination sphere and by the establishment of intermolecular interactions involving the additional nitrate ligand, as mentioned above. This is said because for d-block, octahedral complexes of bbpen2− reported in the literature, namely [M(bbpen)]PF6 (M = MnIII,65 VIII,33 RuIII 57), only achiral crystal structures have been reported, in all cases containing, in the unit cell, racemic mixtures of the enantiomeric complex molecules. Vibrational Spectroscopy. A selected region of the IR and Raman spectra of I and II is presented in Figure S7, together with those of H2bbpen and H2bbppn that are displayed for

network was observed in the crystals of complex I,38 with the differences of shorter and strictly symmetric C···O and H···O distances (Table S5B), in line with the absence of the methyl group in the ligand. Although the methyl group of bbppn2− is not directly involved in these contacts with O(5) in II, its presence possibly relates to the fact that the intermolecular interactions described in this paragraph are ca. 0.3 Å longer in complex II in comparison to I (Table S5B). This leads to a more tightly packed crystal structure for I, as shown by differences in the unit cell volumes (2653.6(4) and 2765.20(17) Å3 at 100 K for I and II, respectively). Also, the molecular arrangement determined by these regular interactions may be important in defining the resolution of the optical isomers of II and (therefore) the enantiomeric purity of the crystals, as mentioned below. Indeed, besides the N- and C-centered (R or S) chirality already present in the H2bbppn molecules, which gives rise to several possible diastereoisomers of this proligand, the formation of the five chelate rings upon complexation to the Tb3+ ion also induces metal-centered chirality (Δ versus Λ) in compound II. Interestingly, despite all possible absolute configurations derived from this complexity, the bbppn2− ligand in the analyzed crystal of complex II presents only the S,R,S configuration as far as the N(1), C(28), and N(3) stereocenters (respectively) are concerned. Moreover, each molecule in the crystal structure of II presents the same relative spatial orientation of the chelate rings assigned by Yamada and coworkers to the ΔΛΔ absolute configuration.35,63 These findings apply to every molecule in the crystal, and therefore both the molecular and the crystal structures of II are chiralin agreement with C2 being a Sohncke group59and with the absolute structure (Flack) parameter64 of −0.005(4) obtained for the analyzed crystal. Additionally, Flack parameters59,64 determined after refinement in C2 of the intensity data collected for three other, randomly picked, crystals of II were also close F


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Inorganic Chemistry comparison. The vibrational spectra of the two Tb3+ compounds are very complex, with phenolate and pyridine vibrational bands over the entire spectral range,66−68 some of them superposed with those of other functional groups. The IR spectra of I and II are better defined than those of the proligands in the region of 3200−2600 cm−1 (not shown), presenting only bands assigned to ν(CH)ar and ν(CH2). The absence of the broad band in the region of 3000−3200 cm−1, attributed to the asymmetric and symmetric stretchings of N−H and O−H bonds, confirms the deprotonation of both ligands in I and II. The ν(C−O) band for the phenol moiety at ca. 1240 cm−1 in the spectra of the proligands (Figure S7a) is strongly affected by deprotonation/ coordination69 and appears at ca. 1295 cm−1 in the complexes (Figure S7b).68 Bands at 1480−1485, 1290−1296, and 1030− 1037 cm−1, expected for ν(NO), νa(NO2), and νs(NO2) of the nitrate ion,70 are superimposed on several ligand bands, making their assignment difficult. The fluorescence of the Tb3+ ion interferes with the observation of most of the expected bands in the Raman spectra of the complexes, and this prevents a detailed spectral analysis. The most intense bands of I and II occur at 1035−1060 cm−1 (Figure S7d) and are assigned to the symmetric stretching vibrations of the coordinated nitrate anion71 and to the breathing vibration of the aromatic rings. Bands of medium intensity between 300 and 400 cm−1 in I and II were absent in the spectra of H2bbpen and H2bbppn (Figure S7c versus Figure S7d) and could be tentatively assigned to Tb−O stretching.71 Electronic Structure and Calculated and Experimental Energy Levels for H2bbpen and H2bbppn. The indirect excitation/sensitization of Ln3+ ions through the “antenna effect” is usually associated with metal chelation by chromophore-containing organic ligands capable of harvesting and transferring energy to the metal ions. Although frequently discussed in terms of a simple S1(ligand) ⇒ T1(ligand) ⇒ Ln3+(5D4 for Tb3+) energy pathway, the effect in fact results from a very complex process that depends on a number of conditions to be satisfied by the metal−ligand interaction.72−75 DFT and TD-DFT theoretical calculations were carried out for both neutral H2bbpen and H2bbppn to describe their electronic structures and to evaluate their potential as sensitizers in luminescent Ln3+ complexes. In this context, the energies and the composition of the frontier molecular orbitals of the proligands were accessed in the gas phase with the B3LYP hybrid functional and the 6-311++G(d,p) basis set. Structural parameters employed in these calculations were those obtained from the experiment (X-ray crystallography analyses), without optimization. Contour plots of selected molecular orbitals are shown in Figure 5 and in Tables S7 and S8, while Table S6 summarizes the expected electronic (UV) transitions. For H2bbpen, the calculated HOMO shows 85% of π character and is mostly concentrated on one of the phenol rings (Figure 5), whereas the calculated LUMO lies predominately on one of the pyridine rings with 90% π* character. For H2bbppn, in turn, the HOMO electron density is mainly located on the central ethylenediamine moiety and connecting carbon atoms next to the pyridine and phenol rings, while the HOMO-1 and the LUMO are mainly phenol and pyridine based, respectively. A general inspection of the contour surfaces shown in Figure S6 for both proligands, together with the main calculated transitions in Table S6, indicates that most of the frontier orbitals are involved in n → π*(pyridine) or π(phenol) → π*(pyridine) transitions.

The calculated UV spectrum of H2bbppn is blue-shifted in relation to that of H2bbpen (Figure S8 and Table S6), and this is a relevant feature as far as the necessary correspondence between the energies of the first ligand excited states and the 5D4 level of the Tb3+ ions is concerned. Considering that donor− acceptor transitions with significant overlap of zero-phonon spectral lines participate in active transfer mechanisms,76,77 we compared the relative positions of the electronic transitions arising from the proligands with those involving the 4f levels of the lanthanide ion. For H2bbpen and H2bbppn the theoretical lowest-energy excited S1 states were calculated respectively at 28074 cm−1 (356.2 nm) and 30102 cm−1 (332.2 nm) (Table S6 and Figure S8). These values are in reasonably good agreement with the experimental values determined by diffuse reflectance spectra (Figure S9), for which the S1 states are found at ca. 28600 cm−1 (approximately 350 nm; H2bbpen) and ca. 32200 cm−1 (ca. 310 nm; H2bbppn). These broad absorptions above 300 cm−1, which are not recorded in dilute solutions of the proligands in organic solvents65,78 but appear in the solid state,35,36 may be related to both the photoacidity79 and the tendency to oxidation37 of the phenolic groups (that are) hydrogen-bonded to the pyridine acceptors in the crystals (Figure 2 and Table S5A).57,80−82 Comprehensive electronic spectroscopy studies of this class of proligands in the solid state have not yet been reported, and the full elucidation of the nature of these electronic transitions will require further investigation. The long-lived emission spectra arising from the highestenergy excited triplet (T1) state of both ligands present a zerophonon line at ca. 24390 cm−1 (410 nm) followed by a vibronic progression with a maximum at 22831 cm−1 (438 nm); these values were determined from their 11 K time-resolved emission spectra emission spectra (Figure S10). An additional broad band at 450−600 nm is also observed at the stationary emission spectra acquired at 11 K, being assigned to a lower-energy triplet state. For both proligands, the energy difference (ΔE) between the experimentally determined S1 and T1 states is close to or smaller than 7000 cm−1 (Table 3), and such a difference is Table 3. Calculated and Experimental Energy Levels for H2bbpen and H2bbppn energy/cm−1

a

compound

S1a

S1b

T1b

ΔE (S1b−T1b)

H2bbpen H2bbppn

28074 30102

28600 32200

24390 24390

4210 7810

Theoretical. bExperimental.

reported to be ideal in favoring an efficient intersystem crossing (ISC) to the triplet state (high ηISC) over competitive relaxation back to the ground (S0) state.2 On the other hand, it is also known that the efficiency of the T1 → 5D4 energy transfer (ηET) in TbIII complexes depends on the energy gap between these two excited states (first ligand triplet and metal 5D4 level). In this context, it is believed that the sensitizer should possess T1 energy at least 1800 cm−1 above the 5 D4 level (20400 cm−1 from the literature; 20518 cm−1 from the experiment; Scheme 1) to minimize back energy transfer from the LnIII ion to the ligand T1 state.2,83 According to data in Table 3, the T1 ↔ 5D4 energy difference (ca. 3900 cm−1) is compatible with this threshold for both ligands, a positive feature in favor of efficient photo-antenna performance. G


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Inorganic Chemistry

Scheme 1. Schematic Energy Level Diagram (Left) and 5D4 → 7F5 Transitions (Right) for Complexes I and II at 300 (Black Line) and 11 K (Red Line)a

a On the left-hand side, the higher-energy T1 value corresponds to the edge of the emission band localized at ca. 21300 cm−1 (ca. 470 nm), while the error bars correspond to the FWHM of bands observed at the absorbance and emission spectra acquired for the GdIII complexes III and IV.

Optical Characterization of Complexes I and II. Diffuse reflectance spectra registered for I and II (Figure 6; dashed

its emission intensity. This result agrees with the lower PL quantum yields registered for I, as described below. Photoluminescence, decay time, and quantum yield measurements were carried out to evaluate the performance of bbpen2− and bbppn2− as sensitizing ligands for TbIII in I and II. Excitation spectra (Figures 7a,c) acquired at 11 and 300 K while the emission at 549 nm (5D4 → 7F6) was monitored are composed of two broad bands at 270 and 330 nm attributed to the ligands, together with a series of sharp lines arising from the 7 F6 → 5G4,5 and 7F6 → 5D2−4 intra-4f8 transitions. This is consistent with the energy splitting of the partially filled 4f orbitals by the low-symmetry ligand fields in both I and II.84,85 Accordingly, the good resolution of the emission spectra registered at low temperature allowed the identification of the electronic transitions involving the 11 (2J + 1) Stark components of the 7F5 level (insets, Figure 7b,d), while the magnification of the spectral region between 655 and 700 nm shows the low-intensity 5D4 → 7F2 lines. A representation of the energy levels for these complexes is presented in Scheme 1. At room temperature the emission spectrum is dominated by the 5D4a → 7F5a and 5D4a → 7F5g transitions at 540.6 and 549.0 nm, respectively (Scheme 1), with contributions arising from upper 5D4 sublevels that are populated by low-energy phonons. With a decrease in temperature, only the 5D4a level remains populated and, consequently, the 5D4a → 7F5a transition becomes dominant at 11 K. The same temperature effects on the relative emission intensity are observed for the 5D4 → 7F6,4−3 transitions (Figure 7b,d). The experimental energies of the singlet states shown in Scheme 1 were determined for the complexes by diffuse reflectance spectroscopy; values thus obtained were 24096 cm−1 (415 nm), 30303 cm−1 (330 nm), and 37735 cm−1 (265 nm), see Figure 6 and Figure S11). Additionally, in order to evaluate the energy matching between the triplet states of the ligands and the emitting 5D4 level, emission spectra were recorded for [Gd(bbpen)(NO3)] (III) and [Gd(bbppn)(NO3)] (IV). For

Figure 6. Room-temperature excitation (solid lines, λem = 549 nm, lefthand axis) and UV−visible absorbance (dashed lines, right-hand axis) spectra recorded for I (in red) and II (black line).

lines) revealed the presence of two intense absorption bands at ca. 270 and 330 nm, in accordance with the corresponding excitation spectra. An additional absorption centered at ca. 400 nm was also recorded for both complexes, together with a lower energy band for I that extends to ca. 650 nm and relates to the light pink color of the crystals.38 The absorptions at 270−400 nm are compatible with transitions to the S1−S3 excited singlet states of the complexes, as discussed below. The lower energy band, in turn, overlaps the 5D4 level of TbIII and therefore probably acts as a quenching pathway for complex I, decreasing H


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Inorganic Chemistry

Figure 7. Excitation (a, c) and emission (b, d) spectra acquired at 11 K (red line) and 300 K (black line) from complexes I (top) and II (bottom) in the solid state (0.5 mm thick pellets).

both GdIII compounds, the time-resolved emission spectra corresponding to the lowest-energy excited triplet state presented a broad band running from 440 to 700 nm (Figure S11b) and, due to the lack of vibrationally structured emission bands, it was not possible to determine the zero−zero energy of the ligand triplet states. To circumvent this limitation, the T1 state is represented as a band in Scheme 1, in which the higher energy value corresponds to the edge of the emission band localized at ca. 21300 cm−1 (ca. 470 nm), while the error bar corresponds to the full width at half-maximum (fwhm) of the emission spectra acquired for the GdIII complexes III and IV. Lifetime values for the 5D4 states of both complexes were also measured at room temperature and 11 K by monitoring the emission within the 5D4 → 7F5 transitions (549 nm) after excitation at 330 nm. This excitation wavelength corresponds to one of the ligand excited states, as shown in Figure 7a,c. Each of the four decay curves is well described by a single-exponential function, in good agreement with the presence of a single local environment of the Tb3+ ions in the complexes. The calculated 5 D4 decay times are given in Table 5; the results are on the same order of magnitude as decay time values reported for other TbIII complexes.77

Results in Table 5 evidence that complex II has a significantly higher quantum yield (67 ± 7%) in comparison to I (21 ± 2%) under UV excitation at 320 nm. This is compatible with a more effective contribution of bbppn2− to the Tb3+ excitation, that is, a more efficient “antenna” effect in comparison to bbpen2−, in agreement with data already shown in Figure 6. Moreover, the temperature dependence of the excited 5D4 state lifetime (τ) revealed for I suggests that, in this complex, the energy barriers for quenching processes involving the LnIII excited state are small and are easily overcome at room temperature,86 a feature that also results in the low room-temperature photoluminescence quantum yield in Table 5. This effect is suppressed by lowering the temperature down to 11 K, which results in a higher decay time (τ11 K = 0.824 ± 0.016 ms). On the other hand, τ values change only slightly for II within a large temperature range, indicating the minimization of thermally active, nonradiative quenching processes that possibly involve specific molecular vibrations in the central ethylenediamine portion of the ligand. In this distinguishing part of the ligand backbone, the methyl group attached to C(27)/C(28) in II apparently acts as a structural (vibrational) stabilizer. Finally, we have also performed measurements with TbIII and EuIII complexes of bbpen2− and bbppn2− in which the nitrate ligand of I and II has been replaced with chloride. The europium(III) complexes do not exhibit room-temperature emission and display low-intensity photoluminescence at 11 K, while the terbium(III) analogues are strong emitters. These results indicate that the two ligands are not proper for use with europium(III), possibly because energy differences involving the ligand excited states and the emitting 5D0 level of the EuIII ion are not compatible with an efficient antenna effect.

Table 5. Room Temperature and 11 K 5D4 Lifetimes (λexc = 330 nm) Monitored at λem = 549 nm and Absolute Quantum Yields (ϕ) Acquired upon Excitation at 320 nm τ (ms) complex

room temp

11 K

ϕ (quantum yield, %)

I II

0.670 ± 0.004 0.819 ± 0.006

0.824 ± 0.016 0.847 ± 0.009

21 ± 2 67 ± 7 I


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Inorganic Chemistry

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.

Ligand design effects related to those described here for I and II on nonradiative PL quenching pathways were reported by Hasegawa and co-workers87 in a [TbL1(NO3)2] complex (L1 = 2,2′-bipyridine derivative ligand) whose quantum yield was only 1% at room temperature and increased significantly with temperature decrease. Results of theoretical calculations on energy profiles and corresponding structural changes during emission and quenching processes allowed the identification of specific (ligand) vibrational modes on an azomethine/ethylenediamine bridging group that induced the quenching process.86 The authors have also elegantly predicted that specific structural modifications on this ligand site could result in efficient energy transfer from T1 to the lanthanide-centered 5DJ excited states with minimization of quenching pathways. Although related calculations are still missing for the class of ligands employed in this work, it appears that a similar vibrational mechanism involving the ethylenediamine bridge may be in place in I and II to determine their strikingly different photoemission behavior.

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*E-mail for J.F.S.: [email protected]. ORCID

Luís D. Carlos: 0000-0003-4747-6535 Rute A. S. Ferreira: 0000-0003-1085-7836 Giovana G. Nunes: 0000-0001-7052-2523 Jaísa F. Soares: 0000-0002-2775-131X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Fundaçaõ Araucária (grants 20171010 and 283/2014, protocol 37509), Conselho Nacional de Desenvolvimento Cientı ́fico e Tecnológico (CNPq, grant 308426/2016-9), Coordenaçaõ de Aperfeiçoamento de Pessoal de Nı ́vel Superior (CAPES, PVE A099/2013 and Finance Code 001), and Universidade Federal do Paraná (UFPR). T.G., J.d.M.L., S.O.K.G., D.L.H., A.G.M., and J.F.S. also thank the CNPq, CAPES, and Fundaçaõ Araucária for fellowships. A.G.M. acknowledges financial support from Serrapilheira Institute (grant number Serra-1709-17054).

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CONCLUSIONS Although it is widely known that emission intensities in lanthanide complexes strongly depend on metal environment, the variety of available molecular symmetries, chromophorecontaining ligands, and crystal field interaction strengths makes it difficult to promptly identify the most likely mechanisms of photosensitization (or deactivation) for each class of compound.86 In line with this consideration, the present work shows that a very simple substitution in a ligand backbone is sufficient to effectively impact structural and spectroscopic properties of the resultant complexes and therefore determine remarkably different PL properties. Ethylenediamine-derived ligands constitute a very large, versatile, and useful class of molecules whose coordination capability satisfies the structural and electronic requirements of most metal ions. In the rapidly developing chemistry of lanthanide and actinide ions, the rationalization of luminescent, magnetic, and medicinal properties of the complexes formed by these ligands can pave the way to very productive applications, but this objective requires powerful theoretical and systematic experimental approaches. Analyzing the effect of small differences in ligand design starting from a basic backbone, as discussed herein, appears to be an interesting strategy to a stepwise construction of a model that helps predict and explain specific properties of the derived coordination compounds, aiming at the possible construction of sensors, displays, and/or devices from the best performing complexes.

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AUTHOR INFORMATION

Corresponding Author

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01397. NMR, FTIR, Raman, diffuse reflectance, excitation, and emission spectra for proligands and complexes, additional ORTEP diagrams and structural data for complexes I−III, synthetic details, and calculated electronic transitions and calculated frontier molecular orbitals for H2bbpen and H2bbppn (PDF) Accession Codes

CCDC 1905765, 1905767, and 1905865 contain the supplementary crystallographic data for this paper. These data can be J


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Inorganic Chemistry ligand but which is actually unbound”. See: Leigh, G.J.; de Souza, J.S. Coord. Chem. Rev. 1996, 154, 71−81.

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