Manifestation of π–π Stacking Interactions in Luminescence

Mar 23, 2015 - Synopsis. In the crystal lattices of the trispyrazolylborate-lanthanide complexes described, the molecules are π-stacked into infinite...
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Manifestation of π−π Stacking Interactions in Luminescence Properties and Energy Transfer in Aromatically-Derived Tb, Eu and Gd Tris(pyrazolyl)borate Complexes Elena A. Mikhalyova,† Anastasiya V. Yakovenko,† Matthias Zeller,‡ Mikhail A. Kiskin,§ Yuriy V. Kolomzarov,∥ Igor L. Eremenko,§ Anthony W. Addison,*,⊥ and Vitaly V. Pavlishchuk*,† †

L. V. Pisarzhevskii Institute of Physical Chemistry of the National Academy of Sciences of Ukraine, Prospekt Nauki 31, Kiev, 03028, Ukraine ‡ STARBURSTT CyberInstrumentation Consortium and Department of Chemistry, Youngstown State University, One University Plaza, Youngstown, Ohio 44555-3663, United States § N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninsky Prosp. 31, 119991 Moscow, GSP-1 Russian Federation ∥ Institute of Semiconductor Physics of the National Academy of Sciences of Ukraine, Prospekt Nauki 41, Kiev, 252028, Ukraine ⊥ Department of Chemistry, Drexel University, Philadelphia, Pennsylvania 19104-2816, United States S Supporting Information *

ABSTRACT: The three new complexes Tp P y Ln(CH3CO2)2(H2O) (Ln = Eu (1), Gd(2), or Tb (3)) were prepared and characterized crystallographically. In the crystal lattices of these complexes, separate molecules are connected in infinite chains by π-stacking interactions. Complexes 1 and 3 display intense photoluminescence and triboluminescence (red and green respectively), while compound 3 exhibits electroluminescence commencing at 9 V in an ITO/PVK/3/Al device (ITO = indium-tin oxide, PVK = poly(N-vinylcarbazole)). A series of Eu/Tb-doped Gd compounds was prepared by cocrystallization from mixtures of 1 and 2 or 2 and 3, respectively. It was shown that π-stacking interactions are involved in increasing the efficiency of energy transfer from the gadolinium complex to emitting [TpPyEu]2+ or [TpPyTb]2+ centers, and this energy transfer occurs through hundreds of molecules, resembling the process of energy harvesting in chloroplast stacks.

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INTRODUCTION Interest in complexes of 4f ions has been stimulated by their potential use as materials for biological fluoro-immunoassays, lasers, lighting systems, electroluminescent devices and diodes, cathode ray tubes, sensors, dosimeters, imaging agents, display applications, decoration purposes, etc.1−3 The uniquely diminished radial extension of the 4f orbitals and their consequently weak perturbation by ligand environments leads to line-like emission by lanthanide ions, resulting in high color purity for the emitted light. Tb3+ and Eu3+ compounds are characterized by long-lived and strongly luminescent excited states, so complexes of these ions are considered prospective materials for light-emitting diodes (LEDs).4−6 The emission color is defined by the electronic structure of the lanthanide ion, the band energy being only weakly dependent on the metal ion’s coordination environment. However, the structures of the ligands significantly influence the intensity and quantum yield of the Ln3+ complexes’ luminescence, and the practice of employing UV-absorbing ligands as “antennæ” for excitation © 2015 American Chemical Society

photons has been known and widely used for several decades.7−12 Enhanced luminescence efficiency is associated with two principal ligand properties: (i) the energy difference between the triplet level of the ligand and the lanthanide excited state should be in an appropriate range6,13,14 to provide effective ligand-to-metal energy transfer and to decrease the probability of energy back-transfer to the ligand’s triplet level, which would lead to luminescence quenching; (ii) as the lanthanide ions’ absorptions are not intense, the ligand should have a high extinction coefficient, in order to lead eventually to substantial population of the lanthanide excited level.6,13,14 Despite recognition of the fact that in the solid state the fluorescence efficiency is greatly affected by crystal lattice and molecular organization, the details are elusive. In particular, little is known about the effect on photophysical properties of supramolecular organization via noncovalent interactions such as π-stacking. Received: August 31, 2014 Published: March 23, 2015 3125

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Inorganic Chemistry

or by slow diffusion of hexane into the solution (generally when using CH2Cl2). Yield, 70−80%. Tp P y Eu(CH 3 CO 2 ) 2 (H 2 O) (1). Anal. for 1 ·0.35 CHCl 3 (C28.35H27.35N9O5BEuCl1.05) calcd/found: C, 44.0/44.0; H, 3.53/3.40; N, 16.3/16.1. Mass spectrum (m/z): 656.3 (70%, [Eu(TpPy)(CH3CO2)]+, calcd 656.1); 569.0 (100%, [Eu(TpPy-pzPy)(CH3CO2)2H−]+, calcd 569.1), 299.1 (55%, [Eu(TpPy-pzPy)(CH3CO2)2-H−]+, calcd 299.0). X-ray quality crystals of the composition 1·CHCl3 were obtained by slow diffusion of hexane into a CHCl3 solution of the complex. A crystal for X-ray analysis was taken from the mother liquor immediately before mounting for diffraction, while a sample for C,H,N-microanalyses was air-dried. Tp P y Gd(CH 3 CO 2 ) 2 (H 2 O) (2). Anal. for 2·0.21 CHCl 3 (C28.21H27.21N9O5BGdCl0.63) calcd/found: C, 44.4/44.7; H, 3.57/3.43; N, 16.5/16.2. Mass spectrum (m/z): 661.1 (100%, [Gd(TpPy)(CH3CO2)]+, calcd 661.1); 446.0 (15%, [Gd(pzPy)2-2H ]+, calcd 446.0). X-ray quality crystals of the composition 2·CH2Cl2 or 2·CH3CN were obtained by slow diffusion of hexane into a CH2Cl2 solution of the complex or by recrystallization from CH3CN, respectively. A crystal for X-ray analysis was taken from the mother liquor immediately before mounting for diffraction, while a sample for C,H,N-microanalyses was air-dried. TpPyTb(CH3CO2)2(H2O) (3). Anal. for 3·0.15C6H14 (C28.9H29.1N9O5BTb) calcd/found: C, 46.1/46.0; H, 3.87/4.07; N, 16.8/16.5. Mass spectrum (m/z): 662.2 (100%, [Tb(Tp Py )(CH3CO2)]+, calcd 662.1); 447.0 (18%, [Tb(pzPy)2-2H ]+, calcd 447.0). X-ray quality crystals of the composition 3·CH3CN were obtained by recrystallization from CH3CN. Again, a crystal for X-ray analysis was taken from the mother liquor immediately before mounting for diffraction, while a sample for C,H,N-microanalyses was air-dried. Syntheses of the compounds for doping experiments were performed in the same manner as for pure compounds 1−3, but mixtures with appropriate contents of respective salts were used instead of pure LnCl3· 6H2O. To avoid the influence of different crystallization solvents on luminescence properties of the compounds, all solids were recrystallized from CH2Cl2-hexane (1:2 by volume). Procedure for LED Construction. The substrate, an ITO-coated glass (ITO = indium-tin oxide, Aldrich 636916) with a sheet resistance of 16 Ω/cm2, was washed in distilled water and isopropanol in an ultrasonic bath. Poly(N-vinylcarbazole) (PVK) and 3 were dissolved in chloroform to make solutions of 15 and 4 mg/mL, respectively. The chloroform solutions of PVK and 3 were spin-coated sequentially, twice (1500 rpm, 30 s) onto the ITO substrate to prepare a light emitting layer. The total thickness of the PVK and emitting layers was 100−120 nm. Aluminum as a cathode layer was subsequently deposited onto the emission layer using a conventional vacuum deposition procedure.

This point is of great importance not only for chemistry but also for biochemistry (organization of light-harvesting system in chloroplasts) and for material science  as a structural tool for tuning luminescent characteristics of materials. Indeed, π−π interactions have drawn much attention in the development of our understanding of both inter- and intramolecular phenomena in chemistry and biology.15 Stacking of the bases in nucleic acids has emerged16,17 as a significant feature of their structural integrity, and considerable effort has been expended over the past few decades on detailing the possibilities for π−π-mediated energy- and electron-transfer in semisynthetic versions of those types of molecules.18−21 A natural system of unequivocal importance is represented by Photosystem-I (PS-I), in which “antenna” molecules of chlorophyll gather and transfer solar photon energy to a special pair of reaction center chlorophylls.22,23 A singular feature of PS-I is the efficient transfer of this energy through aromatic systems over long distances. We report here some properties of a “crystal engineered” system, in which π−π interactions are responsible for similar energy transfer over distances of some hundreds of nanometers, not previously observed in synthetical coordination chemistry.

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EXPERIMENTAL SECTION

Commercially available reagents and solvents (Aldrich and UkrReaChim) were used without further purification; 99.99% gadolinium(III) chloride hydrate was from Alfa Aesar. The hydrotris[3-(2′-pyridyl)pyrazol-l-yl]borate ligand was prepared as described previously.24 C,H,N-microanalyses were performed with a Carlo Erba 1106 analyzer. FAB/LSIMS mass spectra were run on a Waters Micromass AutoSpec Ultima instrument, using 3-nitrobenzyl alcohol as a matrix; peak assignments are based on the masses of the most abundant isotopes. Xray diffraction data were collected on a Bruker AXS SMART APEX CCD diffractometer. Structures were solved using SHELXS-9725 and refined with the SHELXL-97 program package26 by full-matrix leastsquares on F2; H atoms were treated by a riding model. Crystallographic data are summarized in Table S1. The photo- and electroluminescence spectra were measured with a PerkinElmer LS55 spectrophotofluorimeter. The luminescence quantum yield (η) measurements were performed with the commercial phosphor Y2O3:Eu3+ as a standard, with excitation at λex = 254 nm (rather than 315 nm) because the absorbance of this luminescent material at 315 nm is low (i.e., its reflectivity (R) is too high), so that the value of 1 − R cannot be measured with good accuracy. The value of η = 0.85 reported by Jüstel et al.27 for Y2O3:Eu3+ was used in the calculations; MgO (R = 0.91) was used as a reflectance standard.28,29 The emission spectra (of the compounds and standard) were recorded on a PerkinElmer LS 55 spectrofluorimeter (operated in the phosphorescence mode) with a gate time of 15 ms (for time window 0− 15 ms); i.e., the gate time substantially exceeded the emission decay time τ. For Eu3+ and Tb3+ complexes, similar to 1 and 3 the values of τ are less than 2.7 ms,30,31 and for Y2O3:Eu3+ τ is 1.76 ms.32 In a separate experiment (Horiba Jobin-Yvon Inc. Fluorolog FL 3−22 spectrometer), the decay times were checked and found to be in the range of 0.7−1.2 ms for 1 and 1.3−1.6 ms for 3. DC potential for electroluminescence measurements was applied using a Keithley 2400 Digital Source/Meter. Layer thicknesses were measured with a Dimension NanoScope IIIa SPM, in the tapping mode. Ratios of Eu or Tb and Gd were determined using the ICP-AES method on an Agilent 7500ce ICP-mass spectrometer. TpPyLn(CH3CO2)2(H2O) (Ln = Eu (1), Gd(2), or Tb (3)). TpPyTl (0.162 g, 0.25 mmol) and CH3COONa·3H2O (0.068 g, 0.5 mmol) were suspended in MeOH (15 mL). Solid LnCl3·6H2O (for 1, Ln = Eu, 0.091 g, 0.25 mmol; for 2, Ln = Gd, 0.093 g, 0.25 mmol; for 3, Ln = Tb, 0.093 g, 0.25 mmol) was added to a stirred mixture. The mixture was further stirred for 2 h, followed by MeOH evaporation under reduced pressure and product extraction using CHCl3 or CH2Cl2. Solid samples were obtained by slow evaporation of solvent (generally when using CHCl3)

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RESULTS AND DISCUSSION In this paper, we report structural organization and luminescence properties focusing on the energy transfer process of three new complexes TpPyLn(CH3CO2)2(H2O) (where Ln = Eu (1), Gd (2), Tb (3); TpPy− = tris(3-[2′-pyridyl]pyrazolyl)borate (Figure 1a)). The TpPy− was chosen as a ligand to match the conditions delineated above. Because of its extensive conjugation, it has a high extinction coefficient, and this is, appropriately, in the UV region. The acetate anion was chosen as a suitable counterion which forms relatively stable complexes with lanthanides.33 The complexes 1−3 were obtained by the reaction of LnCl3, TpPyTl and CH3CO2Na in CH3OH solution followed by extraction of the products with CHCl3. Single crystals were obtained by slow hexane diffusion into a CHCl3 solution for 1, by recrystallization from CH2Cl2-hexane for 2 and by recrystallization from CH3CN for 2 and 3. The X-ray structures of these complexes differ only by solvent molecule content, being CHCl3, CH2Cl2, or CH3CN solvates, respectively, and by details of the packing arrangement. The Ln3+-containing units are virtually identical. For uniformity in doping experiments that were 3126

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Figure 1. (a) Molecular structure of 2 (drawn with thermal ellipsoids at the 50% probability level). Hydrogen atoms and a CH2Cl2 molecule are omitted for clarity of presentation. The insert displays the tris(3-[2-pyridyl]pyrazolyl)borate anion. (b) Coordination polyhedron of Gd3+ ion in 2. (c) Infinite chain motif formed by π-stacked aromatic fragments of TpPy−.

Table 1. Structural Parameters for π Stacks in 1−3

subsequently performed, the compounds were recrystallized from CH2Cl2-hexane, with the crystal structure being determined by the dominant ion. Only the molecular structure of compound 2 from CH2Cl2-hexane will be described in detail, though some structure parameters will be compared among the compounds. Complex 2 consists of individual molecules (Figure 1a) in which the nonacoordinate Gd3+ ions are surrounded by six N atoms from the tris(pyrazolyl)borate, two O atoms from acetate anions, and one O atom from a coordinated water molecule, in a slightly distorted capped square-antiprismatic arrangement (Figure 1b). The “cap” position is occupied by atom N3, and the bases of the prism are formed by the atoms O1, O2, N2, and O4 (maximal deviation from basal plane is 0.042 Å for O1) and N5, N6, N9, and N8 (maximal deviations from their basal plane are 0.031 Å for N5 and N8); the prism skew angle is 44°. The Ln−O and Ln−N distances are typical of such bonds.30,31,34 Individual molecules of 1−3 are packed in infinite 1D chains in the crystal lattice by π-stacking interactions between aromatic fragments of TpPy− (Figure 1c). The distances between adjacent, essentially parallel pyridylpyrazole moieties from the complex molecules organized in these stacks vary only from 3.4 to 3.6 Å (Table 1)  the usual range for such stacking.35,36 Two of the three pyridylpyrazole fragments from each TpPy− ligand are involved in head-to-tail π-stacking; i.e. a pyrazole interacts with a pyridine of another ligand molecule. Two slightly different types of stacks can be distinguished (parameters in Table 1); these stacks differ only slightly in terms of features such as the distance between planes of adjacent pyridylpyrazole fragments, the displacements of their centroids, the angle between the C1− C2 vector and the normal to the plane of the pyridylpyrazole moiety, and the net area of π−π overlap. Meanwhile, in both these types of stacks the pyridylpyrazole fragments are slipped with respect to each other, a consequence being that the π−π overlap between aromatic fragments varies from 19% to 38% of their areas. The consequences of the packing mode are phenomenologically similar to those arising from the organization of chlorophyll molecules in PS-I, where the antenna Chl-a

compound

la, Å

d(C1−C2), Å

θ, deg

δ, %

d(Ln1−Ln2), Å

1, CH3CN-solvate

3.284 3.424 3.265 3.393 3.340 3.503 3.341 3.503

3.466 3.659 3.485 3.628 3.622 3.702 3.627 3.699

19 21 20 21 23 19 23 19

38 21 26 19 28 26 27 27

8.671 8.873 8.665 8.779 8.742 8.910 8.739 8.885

2, CH2Cl2-solvate 2, CH3CN-solvate 3, CHCl3-solvate a

l is the distance between planes of adjacent pyridylpyrazole fragments; d(C1−C2) is the distance between centroids of pyridylpyrazole fragments. θ is the angle between the C1−C2 vector and the normal to the plane of the pyridylpyrazole fragment. δ is the area of π−π overlapping, as % of pyridylpyrazole area.

are far away from the special pair of reaction center Chl-a (P680), for which the Eu3+/Tb3+ molecules are analogues. The metal-tometal ion separations in these 1D chains are 8.7−8.9 Å. Though these separations are within the range for which direct energy transfer between lanthanide ions has been observed,40−43 the distances between covalently bonded fragments and π-stacked pyridylpyrazoles are significantly shorter. Thus, energy transfer through these stacks is more probable than direct energy transfer between lanthanide ions. Luminescence excitation spectra (Figure 2) of solid samples of compounds 1 and 3 contain relatively weak but sharp bands assigned to Eu3+- or Tb3+-centered transitions, respectively. In the spectrum of compound 1 these bands are located at 396, 463, and 531 nm and correspond to the 7F0 → 5L6, 7F0 → 5D2, and 7F0 → 5D1 transitions,44,45 respectively, while in the spectrum of 3 two bands near 374 nm and at 488 nm correspond to the 7F6 → 5 D3 and 7F6 → 5D4 transitions, respectively.44 In addition, for 1 and 3 there are very intense broad bands at shorter wavelengths associated with the organic ligand. The similarities in form and position of these bands in the excitation spectra with the 3127

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Figure 2. (a) Excitation (monitored at λem.= 617 nm; black) and emission (λex. = 275 nm; red) spectra of 1. Diffuse reflectance spectrum of TpPyK (blue). (b) Excitation (monitored at λem. = 545 nm; black) and emission (λex. = 315 nm; red) spectra of 3. Diffuse reflectance spectrum of TpPyK (blue).

Figure 3. Energy levels of ligands and lanthanide ions for complexes 1 (a) and 3 (b).

ment52,53 shown by the crystallography. In addition, in the emission spectrum of 1 several weak bands are present corresponding to transitions from higher excited states (5D1 and 5D2).54 The low intensity of these bands can be ascribed to fast energy transfer from these terms to 5D055 or to quenching of upper levels (5D1 and 5D2) after ion excitation.56 For complex 3, four intense Tb3+ emission bands were observed in the luminescence spectrum (Figure 2b), at 489, 545, 586, and 621 nm, assigned to the 5D4 → 7F6, 5D4 → 7F5, 5D4 → 7 F4, and 5D4 → 7F3 transitions, respectively.50,51,57 The most intense of these corresponds to the 5D4 → 7F5 transition. Some very weak bands at longer wavelengths than 650 nm were also present and could arise from 5D4 → 7F2,1,0 transitions. Luminescence quantum yields (Φ’s) determined as previously indicated27,33 were found to be 24(2)% (λex. = 254 nm) and 43(4)% (λex. = 254 nm) for solid samples of the complexes 1 and 3, respectively. These quantum yield values are relatively high, given that there is a water molecule in the first coordination sphere, which usually significantly quenches the Ln3+-centered emission,58,59 but not unprecedentedly so.48 The emission Φ’s for CH 2 Cl 2 solutions of similar complexes Tp Mepy Ln(NO3)2(H2O) (TpMepy− = tris[3-(6′-methylpyridin-2′-yl)pyrazol-1-yl]-borate) and TpPyLn(NO3)2 are 22% and 41% for the Tb 3+ compounds and 0.01% and 5% for the Eu 3+ compounds.30,31,34 One should note that quantum yields

absorption band in the spectrum of TpPyK (Figure 2) further evidence that the tris(pyrazolyl)borate ligand plays the role of an antenna.46,47 The long wavelength edge of this band of 1 is rather drawn out compared with that of 3, likely due to the presence of an LMCT transition of 1, near 340 nm.48 Since Eu(III) has the highest optical electronegativity, 1.99, among Eu(III), Gd(III), and Tb(III),49 the LMCT in compound 1 would be at the longest wavelength of the three. Excitation spectra show that the maximum luminescence intensity for solid samples of 1 and 3 arises from absorption within the 225−335 nm interval for the Eu complex and 225− 320 nm for the Tb complex. For both compounds, ligandcentered luminescence appeared to be completely quenched, vide inf ra, and only sharp bands assigned to lanthanide f−f transitions were observed in the emission spectra (Figure 2). In the emission spectrum of 1 (Figure 2a), five intense bands were observed, characteristic of Eu3+ luminescence, centered at 580, 593, 616, 652, and 695 nm, assigned to the 5D0 → 7F0, 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3, and 5D0 → 7F4 transitions, respectively.28,50,51 The magnetic dipole-allowed transition 5D0 → 7F1 is less intense than the electric dipole-allowed transition 5 D0 → 7F2, consistent with the absence of an inversion center in the local geometry of Eu3+; each of these two bands has at least two poorly resolved components which might relate to the distorted C4v symmetry of the Eu3+ coordination environ3128

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transmission of energy from an excited ligand to a metal ion via a ligand-to-metal charge-transfer48 or through an intraligand charge transfer state;48,68 (ii) though involving mainly ligand antenna triplet state to metal energy transfer,1,2,5,8 ligand singlet states have also been invoked;61−63 (iii) the donor and acceptor centers are arranged in a highly ordered fashion.66 Meanwhile, for those instances in which energy transfer appears to occur between molecular fragments connected by sp3 centers, it is actually transmitted by through-space transition dipole coupling between the fragments.70,71 To examine the roles of molecular stacking and any potential spin-induced relaxation, we performed experiments in which compounds 1 and 3 were doped into TpPyGd(CH3CO2)2(H2O) (2) by cocrystallization. The mixtures obtained show Eu3+ and Tb3+ luminescence, respectively. The usual observation for such doped/diluted systems, of the relative quantum yield (q; q is the ratio of the Φ of doped compound to that of pure dopant, with qpure’s of pure compounds taken as 1) passing through a maximum with increasing dopant concentration φ (φ is the mole fraction of emitting ion, i.e. Eu3+ or Tb3+) does not hold for the present compounds.72,73 In an ideal case with no quenching processes, the q and Φ would both be unity. The Φ is sustained down to quite low (φ = 0.1) doping levels (Figure 4). Beyond this, both it and q fall off, as a result of transmission losses due to cross-relaxation between Gd3+ units,72 thermal or optical dissipation of energy, and also presumably absorbing trace organic impurities and relaxation quenching associated with solvent (solvating) molecules. We also adopt an enhancement factor Y (relative luminescence intensity per Eu3+/Tb3+ ion, defined as Y = qφ/φ), to evaluate the effect of emitter ion dilution by Gd3+ on the emission intensity. Remarkably, as the φ of the Eu3+/Tb3+ dopant decreases, the Y values increase markedly (Figure 4), indicating the operation of a mechanism for enhancement of Eu3+ and Tb3+ luminescence by the Gd3+ complex. This is taken as evidence for intermolecular energy transfer in the crystal lattices, as the only way for energy to be emitted from the Eu3+/Tb3+ molecules is for them to acquire it from excited Gd3+-containing molecules as donors.66,74,75 For control experiments involving 50:50 mol % mechanically ground mixtures of 1 or 3 with 2, the q’s were 0.69(3) and 0.76(3), respectively, the significantly lower values than for the respective cocrystallization-doped compounds 2, confirming that there are energy transfer interactions at the molecular level in the doped compounds. If intracomplex energy migration occurs mainly through the S1 levels of the ligands, then one mechanism for energy transfer from [TpPyGd]2+ units to Eu3+ or Tb3+ would consist of several steps: (i) excitation of a pyridylpyrazole moiety to its S1 level (λex = 315 nm, 31 750 cm−1), (ii) energy transfer from this pyridylpyrazole to the Gd3+ 6P7/2 level (at 32 100 cm−1), (iii) intracomplex energy transfer from Gd3+ to S1 of another pyridylpyrazole moiety, (iv) energy transfer between two πstacked pyridylpyrazole moieties and repetition of steps ii−iv until the excitation energy reaches the pyridylpyrazole of a TpPy− connected to Eu3+/Tb3+. Such an exciton migration of energy via singlet excited states may account for CH3CO2− not acting as an efficient trap for excitation energy. In the event that intracomplex energy migration occurs mainly through TpPy− T1 levels, then a second mechanistic possibility involves (i) again, excitation of a [TpPyGd]2+ pyridylpyrazole moiety to its S1 level, (ii) intersystem crossing to the TpPy− T1 level, (iii) intracomplex molecular energy transfer to another

determined for solid samples vs solutions are strictly not comparable because of the different natures of the quenching processes in the different phases, so one cannot necessarily compare such data between different phases in a precise, quantitative fashion. The presence and intensity of lanthanide-localized luminescence bands in the emission spectra depend on the relationship between the energies of the ligand singlet and triplet levels and the emissive levels of the lanthanide ions.13,14 The energies of the TpPy− singlet and triplet levels were determined earlier for TpPyGd(NO3)2(H2O) in MeOH solution as 30 000 and 23 800 cm−1, respectively.31 From the long-wavelength edge of the band in the absorption spectrum of CH3CO2Na,60 the acetate anion singlet level is at 40 800 cm−1; two phosphorescence bands with maxima at 465 and 530 nm corresponding to decay from triplet states near 21 500 and 18 900 cm−1 were found for the acetate anion.60 Though the main accepted pathway for energy transfer to the Ln3+ levels is via triplet levels of the ligands,1,2,5,8 it has been shown that direct transfer from a ligand’s singlet level can also occur.61−63 For energy transfer through triplet levels there are thus acetate anion “traps” which should significantly quench Tb3+ emission,55,64,65 but this is not the case here. While the singlet levels of both TpPy− and acetate anion lie significantly higher than the principal luminescence levels of Eu3+ and Tb3+, the energy can, however, be transferred to higher excited levels. The evidence for this energy transfer pathway is the presence of weak bands assignable to transitions from the 5D2 and 5D1 levels in the luminescence spectrum of 1. The attribution of singlet and triplet energy transfer pathways for 1 and 3 is thus reasonable. Figure 3 shows the energy levels along with possible energy transfer events. In a study of the fluorescence enhancement by Gd3+ in Eu3+− thenoyltrifluoroacetone−trioctylphosphine oxide microcrystalline suspensions, it was proposed66 that the energy was transferred by an exciton-migration mechanism; i.e., the energy absorbed by the donor molecule is rapidly delocalized throughout the gadolinium complex matrix until it reaches the recipient species, which then emits the observed intrinsic fluorescence of the Eu3+ ion. Ci and Lan66 also suggested that for efficient luminescence enhancement, the donor and acceptor centers should be arranged in a highly ordered way  exactly the situation we have in the present study where donor (Gd3+ complex) and acceptor (Eu3+ or Tb3+ species) species are highly organized by the stacking interaction. Piguet et al.50 pointed out that intramolecular energy transfer cannot significantly enhance the luminescence and that migration of the excited states among the π-stacked organic ligands is responsible for observation of luminescence of Tb3+ and Eu3+ impurities at levels less than 0.01%. The facilitation of energy transfer between mixed lanthanide ion systems via ligand carriers has also been proposed elsewhere.1,48,67 It has been proposed that the aggregation of emitting molecules into stacks can result in generation of a new charge-transfer state induced by such π-stacking interactions,48,68 but we have no evidence here for the formation of such states in our systems. From the above discussion about intracomplex energy transfer in 1 and 3, we can address two mechanisms for energy transfer in the doped chains. Zurawski et al.69 recently noted that the influence of π-stacking interactions on the luminescence properties is not fully described by the literature. It is clear that the π-stacking of the molecules is responsible for migration of the excitation energy through the doped Gd(III) lattice. Some features of the necessarily multistep mechanisms that have been proposed for this phenomenon include (i) 3129

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Figure 4. (a) Dependence of qφ (unfilled markers) and Y (solid markers) on φ, the mole fraction of Eu3+ (red triangles), and Tb3+ (black squares) in TpPyGd(CH3CO2)2(H2O) (2). Solid lines represent the best fits to the “hopping” model of eq 4. (b) The reciprocal plot of 1/Y vs φ augments visualization of the range and extent of the doped lattice emission enhancement.

pyridylpyrazole moiety by dipole−dipole interactions or by thermal equilibrium between two moieties,55 (iv) again, energy transfer between two π-stacked pyridylpyrazole moieties and repetition of steps iii−iv until the excitation energy reaches the pyridylpyrazole of a TpPy− connected to Eu3+/Tb3+. It is impossible to distinguish which one of the two proposed mechanisms occurs. Moreover, probably both mechanisms occur in the doped chains. Though these mechanisms are different, the next discussion is appropriate for both of them. First of all, it should be noted that at low φ values the energy must transfer through several Gd3+ ions from the first excited Gd(III) molecule in order to meet the first Eu3+/Tb3+ ion. In effect, then, the stack of Gd(III) molecules acts as a “superantenna” for the Eu3+/Tb3+ acceptors. Similar excitation energy migrations through πstacked fragments have been proposed for some solely organic aromatic systems, such as naphthalene−anthracene, anthracene−naphthacene, etc.76−79 Several consequences can be deduced from the proposed model: (i) the energy transfer between the initial antenna molecule and distant recipient occurs via several intermediate transducer stations that only slightly diminish the signal at each stage; (ii) raising the Gd3+ concentration increases the distance between the initially excited Gd3+-containing molecule and the emitting Ln3+ ion, and this increases the probability of nonradiative relaxation. To estimate the number of energy “jumps” i.e. energy transfer between respective fragments of neighboring molecules, the hopping model of exciton diffusion was applied.80−82 It should be noted that this model does not take into account the mechanism of energy transfer in the additional “jump”. According to this model, the luminescence quantum yield can be expressed as Φ=

n=

nr + nl + nj

(2)

Taking eq 2 into account, expression 1 may be rewritten as Φ=

η(1 − F)nφ (1 + (1 − F)nφ)

(3)

For all the compounds in a doping series, η is constant, so the qφ is (1 − F)nφ (1 + (1 − F)n) × (1 + (1 − F)nφ) (1 − F)n (1 + (1 − F)n)φ = (1 + (1 − F)nφ)

qφ =

(4)

The q’s concentration dependences were fitted with eq 4. The best fits are shown in Figure 4 and correspond to parameters (1 − F)n = 347(25) and n = 526(38) with χ2 = 1.9 × 10−3 (χ2 = ∑(qφ,calc − qφ,exp)2/∑(qφ,exp)2) for the Eu3+-doping series and (1 − F)n = 268(26) and n = 406(39) with χ2 = 3.8 × 10−3 for the Tb3+-doping series. Taking into account that the distance between nearest Ln3+ ions in the crystal lattice is 8.53 Å, we estimate distances (between absorbing and emitting fragments) of exciton migration of 300(30) nm and 230(30) nm for Eu3+ and Tb3+ doped compounds, respectively. Energy migration through hundreds of structural fragments is known for some synthetic polymers (poly(N-vinyl)carbazole),82,83 solid solutions (perylene in N-isopropyl carbazole),80 and porphyrin-based compounds (Zn2+ complexes).84 For other Zn2+−porphyrin complexes, the exciton diffusion length was estimated as 15 nm, and energy transfer was proposed to occur between stacks.85 In the case of compound 2 doped by Tb3+ and Eu3+ ions, the number of energy hops is comparable to those for the organic systems above, but the distance of exciton migration is significantly higher due to the larger size of the molecules here. To the best of our knowledge, complex 2 doped by Tb3+ and Eu3+ is the first example of lanthanide complexes in which the possibility of energy transfer over hundreds of nanometers has been studied. These complexes can be considered phenomenological models of natural light-harvesting systems. The luminescence of Eu3+/Tb3+-doped 2 was also studied at 77 K. The concentration dependences of q at 77 K and the best

η(1 − F)njφ (nr + nl + (1 − F)njφ)

nj

(1)

where η is an intrinsic quantum yield; nj, nr, and nl are the relative probabilities (per unit time) of jumping, radiative, and nonradiative deactivation; 1 − F ≈ 0.66 is a factor which takes into account the possibility of back-transfer of the exciton during its 1D walk.81 The number of energy hops during the lifetime may be calculated as 3130

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Inorganic Chemistry fits are presented in Figure SI 2 (Supporting Information) and correspond to parameters (1 − F)n = 29(5) and n = 44(8) with χ2 = 1.3 × 10−2 for the Eu3+-doping series and (1 − F)n = 179(16) and n = 271(24) with χ2 = 5.9 × 10−3 for the Tb3+-doping series. These values lead to exciton diffusion lengths of 25(5) and 150(15) nm, respectively, which are less than these values at room temperature. The reduction of the diffusion distances may point to vibrational relaxation participation in the energy transfer. A similar effect was found for the solid solution of tetracene in anthracene.78 To determine the possibility of using complexes 1 and 3 as emitting layers in LEDs, an ITO/PVK/complex/Al device (with a total PVK/complex layer thickness of 100−120 nm) was fabricated, with PVK as the hole transport material. Visible light emission was observed, turning on at 9 V and reaching a maximum at 17.5 V. The electroluminescence spectrum was measured for the device with the Tb3+ complex 3 and was quite similar to the photoemission spectrum of this compound (Figure 5).

to Eu3+- or Tb3+-containing units in the same manner as photon energy.

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CONCLUSIONS Two new highly luminescent complexes Tp P y Ln(O2CCH3)2(H2O) (Ln = Eu (1) or Tb (3)) were obtained, and their structures and luminescence properties were examined. Intense photoluminescence with quantum yields of 24(2)% and 43(4)% for 1 and 3 were observed. Complex 3 exhibits Tb3+centered electroluminescence with a turn-on potential of 9 V. Both complexes are also characterized by triboluminescence, the color of which coincides with the photoluminescence color. Complexes 1 and 3 may have potential for construction of luminescent devices. It has also been shown that π-stacking interactions are involved in increasing the efficiency of energy flow from the gadolinium complex to emitting TpPy-Eu3+ or -Tb3+ centers, to a remarkable degree. Energy transfer in these systems occurs over hundreds of nanometers, which to the best of our knowledge is an unprecedented distance for coordination compounds. Such supramolecular organization of energy transfer from the center which absorbs the photon toward the emitting center serves as a synthetic model for the organization of the light-harvesting system in chloroplasts. These observations also open prospects for design of highly efficient luminescent materials with very low content of intrinsically luminescent ions.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data for 1, 2, and 3 (Table SI 1, CCDC: 1015071 (1 × CHCl3), 1015072 (3 × CH3CN), 1015073 (2 × CH3CN), 1015074 (2 × CH2Cl2)), additional X-ray figures (Figure SI 1), Enhancement factor equations, dependence of relative quantum yields (qφ) on φ for Eu3+ and Tb3+ in TpPyGd(CH3CO2)2(H2O) (2) at 77 K (Figure SI 2), and video files of triboluminescence of 1 and 3. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 5. Intensity-normalized electro- (black) and photoluminescence (red) spectra of 3.

AUTHOR INFORMATION

Corresponding Authors

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

Both complexes 1 and 3 also generated intense visible light when ground between glass plates (Figure 6). The tribolumi-

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a joint grant of the National Academy of Sciences of Ukraine and the Russian Foundation for Basic Research (No. 03-03-14 and 14-03-90423). The X-ray diffractometer was funded by NSF Grant 0087210, Ohio Board of Regents Grant CAP-491, and by Youngstown State University. A.W.A. thanks the College of Arts & Sciences of Drexel University for support. The authors thank Dr. V.P. Dotsenko of the A.V. Bogatsky Physico-Chemical Institute of the NAS of Ukraine for additional assistance with luminescence measurements.

Figure 6. Triboluminescence images of samples of (a) 1 and (b) 3.

nescence color coincided with that of the photoluminescence (red for complex 1 and green for complex 3). That these crystals are of a low-symmetry space group is consistent with the idea that mechanical fracturing leads to an asymmetric charge distribution between the two sides of a fracture, and this in turn can generate excited states that lead to emission.12,86−88 We take compound 3’s exhibition of electroluminescence as additional evidence for this interpretation. It should be noted that compounds 2 doped by Eu3+/Tb3+ up to a 1 mol % doping level also show visible triboluminescence, indicating that mechanical energy is transformed and transferred

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