Article pubs.acs.org/JPCA

Enhanced Electric Dipole Transition in Lanthanide Complex with Organometallic Ruthenocene Units Yasuchika Hasegawa,*,† Nao Sato,† Yuichi Hirai,† Takayuki Nakanishi,† Yuichi Kitagawa,† Atsushi Kobayashi,§ Masako Kato,§ Tomohiro Seki,† Hajime Ito,† and Koji Fushimi† †

Faculty of Engineering and §Faculty of Science, Hokkaido University, N13 W8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan S Supporting Information *

ABSTRACT: Enhanced luminescence of a lanthanide complex with dynamic polarization of the excited state and molecular motion is introduced. The luminescent lanthanide complex is composed of one Eu(hfa)3 (hfa, hexafluoroacetylacetonate) and two phosphine oxide ligands with ruthenocenyl units Rc, [Eu(hfa)3(RcPO)2] (RcPO = diphenylphosphorylruthenocene). The ruthenocenyl units in the phosphine oxide ligands play an important role of switching for dynamic molecular polarization and motion in liquid media. The oxidation states of the ruthenocenyl unit (Rc(1+)/ Rc(1+)) are controlled by potentiostatic polarization. Eu(III) complexes attached with bidentate phosphine oxide ligands containing ruthenocenyl units, [Eu(hfa)3(RcBPO)] (RcBPO = 1,1′-bis(diphenylphosphoryl)ruthenocene), and with bidentate phosphine oxide ligands, [Eu(hfa)3(BIPHEPO)] (BIPHEPO =1,1′-biphenyl-2,2′-diylbis(diphenylphosphine oxide), were also prepared as references. The coordination structures and electrochemical properties were analyzed using single crystal X-ray analysis, cyclic voltammetry, and absorption spectroscopy measurements. The luminescence properties were estimated using an optoelectrochemical cell. Under potentiostatic polarization, a significant enhancement of luminescence was successfully observed for [Eu(hfa)3(RcPO)2], while no spectral change was observed for [Eu(hfa)3(RcBPO)]. In this study, the remarkable enhanced luminescence phenomena of Eu(III) complex based on the dynamic molecular motion under potentiostatic polarization have been performed.

■

INTRODUCTION Inorganic phosphors containing rare earth ions, luminescent lanthanide compounds are promising luminescent materials for applications such as displays, fluorescent lights, high-power lasers, and photocouplers for fiber-optic telecommunication systems.1 Various types of luminescent lanthanide compounds such as inorganic crystals, glass materials, semiconductors, nanoparticles, and molecular systems with Eu(III), Tb(III), Sm(III), Yb(III), and Nd(III) have been synthesized, and their luminescence properties have been explored.2 The luminescence properties of lanthanide compounds are mainly derived from electric transitions in the 4f orbitals, which are much different from the luminescent properties generally derived from π (organo-aromatic molecules) and d (transition metal ions) orbitals. The characteristic 4f orbitals promote notably long emission lifetimes (>1 μs), narrow emission bands (full width at half-maximum (fwhm) < 10 nm), and suitable fourlevel electronic transitions for photoamplification.2 The 4f orbitals of lanthanides are generally shielded from direct perturbation by the outer filled 5s and 5p shells.1 The electric dipole transitions from the 4f inner shell of lanthanide ions are intrinsically forbidden because of their odd parity. However, they can be partially allowed upon mixing of the 4f and 5d states through the ligand field effect linked to the coordination structure.3,4 The coordination numbers of lanthanide ions in solution vary from 8 to 12 depending on the nature of the ligating molecules.5 The coordination geometry of lanthanide compounds influences the electronic © 2015 American Chemical Society

transition intensity in the 4f orbitals. It has been widely accepted that the radiative transition probability in 4f orbitals is enhanced by a reduction of the geometrical symmetry of the coordination structures.6,7 The characteristic coordination structures of luminescent lanthanide compounds have been extensively studied in the field of photophysics and coordination chemistry.8−24 One mononuclear lanthanide complex generally provides one coordination structure. Transformation of the coordination structure, which is related to the radiative rate constant kr, is expected to manifest as a luminescence switching phenomenon. According to the luminescence switching phenomenon of lanthanide compounds, lanthanide complexes with switching units as ligands have been previously studied. Faulkner described the luminescence switching of a Eu(III) complex with color-changeable ferrocenyl units under an electric field.25 Zhang and Gunnlaugsson synthesized luminescent Eu(III) complexes linked with pH-sensitive organic ligands.26,27 Luminescence switching using Eu(III) complexes with colorchangeable photochromic ligands under photoirradiation has also been reported.28 Such luminescence of lanthanide compounds is based on the spectral overlap between the changeable absorption band of the switching ligands and the emission bands of the lanthanide ions. The spectral overlap Received: February 24, 2015 Revised: April 12, 2015 Published: April 15, 2015 4825

DOI: 10.1021/acs.jpca.5b01809 J. Phys. Chem. A 2015, 119, 4825−4833

Article

The Journal of Physical Chemistry A

visible region.39 It is considered that the ruthenocenyl unit is the best material for observation of dynamic molecular motion as a switching unit. The low-vibrational-frequency hfa ligand is effective for the suppression of vibrational relaxation.8,40 Two phosphine oxide ligands with ruthenocenyl units also promote the formation of an asymmetric coordination structure, which enhances kr.41,42 A Eu(III) complex attached with bidentate phosphorylruthenocene units, [Eu(hfa)3(RcBPO)] (RcBPO = 1,1′-bis(diphenylphosphoryl)ruthenocene), and a Eu(III) complex attached with bidentate phosphine oxide, [Eu(hfa)3(BIPHEPO)] (BIPHEPO = 1,1′-biphenyl-2,2′-diylbis(diphenylphosphine oxide), were prepared as references (Figure 1c,d). The coordination structures and electrochemical properties of these complexes are analyzed using single crystal X-ray analysis and cyclic voltammetry. The luminescence properties are estimated using an optoelectrochemical cell (Figure 1e). Under potentiostatic polarization, a significant enhancement of luminescence was successfully observed for [Eu(hfa) 3 (RcPO) 2], while no spectral change of [Eu(hfa)3(RcBPO)] was found. In addition, the luminescenceswitching phenomenon is discussed with respect to density functional theory (DFT) calculation data. In this study, luminescence-switching phenomena of a butterfly-shape Eu(III) complex with dynamic molecular active sites have been performed.

between the absorption and emission bands promotes a decrease of the emission quantum yield with an increase of the nonradiative rate constant, knr.25 Filter effects due to the spectral overlap also decrease the emission quantum yield. Thus, luminescence-switchable lanthanide compounds without this spectral overlap are expected to open up new strategies for molecular materials science and engineering. Here, we focus on luminescence-switchable lanthanide compounds that have dynamic molecular motion without a color-changeable absorption band in the visible region. Recently, various supramolecular systems based on dynamic molecular motion, such as molecular shuttles and machines, have been reported.29−37 Luminescence-switchable lanthanide compounds with dynamic molecular motion and without colorchangeable absorption would provide reversible control of the coordination geometry, and thus kr. In this study, we report the first butterfly-shape lanthanide complex with dynamic molecular active sites. The luminescent lanthanide complexes are composed of one Eu(hfa)3 (hfa, hexafluoroacetylacetonate) and two phosphine oxide ligands with ruthenocenyl units, Rc (Figure 1a, [Eu(hfa)3(RcPO)2]; RcPO = diphenylphosphor-

■

EXPERIMENTAL SECTION Materials. Europium acetate monohydrate (99.9%) was purchased from Wako Pure Chemical Industries Ltd. 1,1,1,5,5,5-Hexafluoro-2,4-pentanedione, BIPHEPO (1,1′-biphenyl-2,2′-diylbis(diphenylphosphine oxide), ruthenocene, and chlorodiphenylphosphine were obtained from Tokyo Kasei Organic Chemicals and Aldrich Chemical Co. Inc. All other chemicals and solvents were of reagent grade and were used without further purification. Apparatus. Infrared spectra were recorded on a JASCO FT/IR-350 spectrometer. 1H NMR (270 MHz) spectra were recorded on a JEOL LA400. Chemical shifts are reported in δ ppm, referenced to an internal tetramethylsilane standard for 1 H NMR. Elemental analyses were performed using a Yanaco CNH corder MT-6. Preparation of Tris(hexafluoroacetylacetonato)europium Dihydrates [Eu(hfa)3(H2O)2]. Europium acetate monohydrate (5.0 g, 13 mmol) was dissolved in distilled water (20 mL) in a 100 mL flask. A solution of 1,1,1,5,5,5hexafluoro-2,4-pentanedione (7.0 g, 34 mmol) was added dropwise to the solution. The reaction mixture produced a precipitation of white yellow powder after stirring for 3 h at room temperature. The reaction mixture was filtered, and the resulting powder was recrystallized from methanol/water to afford colorless needle crystals of the title compound. Yield: 9.6 g (95%). IR (KBr): ν = 1650 (st, CO), 1145−1258 (st, C− F) cm−1. Elemental Anal. Calcd (%) for C15H7EuF18O8 (809.91): C, 22.27; H, 0.87. Found: C, 22.12; H, 1.01. Preparation of Diphenylphosphorylruthenocene (RcPO). A solution of n-BuLi (9.0 mL, 1.6 M hexane, 14 mmol), was added dropwise to a solution of ruthenocene (1.0 g, 14 mmol) in dry tetrahydrofuran (THF) (30 mL) and N,N,N′,N′-tetramethylethylenediamine (1.5 mL, 10.0 mmol) at 0 °C. The mixture was allowed to stir for 3 h at 0 °C, after which chlorodiphenylphosphine (PPh2Cl; 2.8 mL, 15 mmol) was added dropwise at 0 °C. The mixture was gradually brought to room temperature, and stirred for 14 h. The product

Figure 1. Chemical structures of (a) [Eu(hfa)3(RcPO)2], (b) postulated model of [Eu(hfa)3(Rc+PO)2] under potentiostatic polarization, (c) [Eu(hfa)3(RcBPO)], and (d) [Eu(hfa)3(BIPHEPO)]. (e) Schematic diagram of optoelectrochemical cell (left) (WE, working electrode; RE, reference electrode; CE, counter electrode) and photographs of [Eu(hfa)3(RcPO)2] in acetonitrile (center) and of luminescent [Eu(hfa)3(RcPO)2] excited at 350 nm under potentiostatic polarization.

ylruthenocene). The ruthenocenyl units in the phosphine oxide ligands play an important role of switching for dynamic molecular polarization and motion (charge repulsion, Figure 1b) in liquid media. The oxidation state of the ruthenocenyl unit (Rc(0)/Rc(1+), ruthenocenyl cation) can be controlled electrochemically.35,38 In addition, the Rc(0) and Rc(1+) ruthenocenyl units do not exhibit absorption bands in the 4826

DOI: 10.1021/acs.jpca.5b01809 J. Phys. Chem. A 2015, 119, 4825−4833

Article

The Journal of Physical Chemistry A

Crystallography. Colorless single crystals of Eu(III) complexes were mounted on a glass fiber using paraffin oil. All measurements were made on a Rigaku R-AXIS RAPID with graphite monochromated Mo Kα radiation. All structures were solved by direct methods (SIR 2004) and expanded using Fourier techniques. All calculations were performed using the Crystal Structure crystallographic software package except for refinement, which was performed using Rigaku Crystal Structure. We confirmed the CIF data using the checkCIF/ PLATON service. All calculations were performed using the crystal structure crystallographic software package. We confirmed the CIF data using the checkCIF/PLATON service. CCDC-1048081 [Eu(hfa)3(RcPO)2] and CCDC-1044882 [Eu(hfa)3(RcBPO)] contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Electrochemical Measurements. Two types of potential controlling polarization of the lanthanide complexes were conducted in acetonitrile solution with 0.1 mol L −1 tetrabutylammonium hexafluorophosphate as supporting electrolyte using a BioLogic SP-150 potentiostat. A Pt mesh, Pt wire, and Ag/AgCl electrode were used as working, counter, and reference electrodes, respectively, in a quartz glass optoelectrochemical cell with a cell length of 1 mm and a solution volume of 10 mL. Cyclic voltammetry was carried out at a potential sweep rate of 20 mV s−1 to survey electrochemical reactivity of the complexes. Potentiostatic polarization at potentials, at which oxidation of the complex was in a masstransfer controlling process, was also carried out to evaluate optoelectrochemical properties of the complexes. Optical Measurements. Absorption and emission spectra of the lanthanide complexes were measured with a JASCO V670 spectrophotometor and JASCO F-6300-H spectrometer which corrected for the response of the detector system. The emission quantum yields of lanthanide complex solutions degassed with argon (10 mM in acetone-d6) were obtained by comparison with the integrated emission signal (550−750 nm) of Eu(hfa)3(TPPO)2 as a reference (Φ = 0.60:50 mM in acetone-d6) with an excitation wavelength of 465 nm (direct excitation of Eu(III) ions) for Eu(III) complexes.42 Emission lifetimes of lanthanide complexes (10 mM in acetone-d6) were measured using the third harmonics (355 nm) of a Q-switched Nd:YAG laser (Spectra Physics, INDI-50, fwhm = 5 ns, λ = 1064 nm) and a photomultiplier (Hamamatsu photonics, R5108, response time ≤1.1 ns). The Nd:YAG laser response was monitored with a digital oscilloscope (Sony Tektronix, TDS3052, 500 MHz) synchronized to the singlepulse excitation. Emission lifetimes were determined from the slope of logarithmic plots of the decay profiles. Computational Details. Time dependent density functional theory (TD-DFT) geometry optimizations and dipole moment calculations of monodentate phosphine oxide ligands were carried out with Gaussian 09W D.01 employing the threeparameter hybrid functional of Becke based on the correlation functional of Lee, Yang, and Parr (B3LYP).43,44 The LANL2DZ-ECP basis set was used for Ru atoms ,and the 631G(d) basis set was used for all other atoms.

was extracted with ethyl acetate; the extracts were washed with brine three times and dried over anhydrous MgSO4. The solvent was evaporated, and the resulting residue was purified by column chromatography (silica, dichloroethane/hexane, 1:4) to afford ruthenocene (0.70 g, yield: 40%). The obtained solid (0.44 g, 1.1 mmol) and acetone (ca. 60 mL) were placed in a flask. The solution was cooled to 0 °C and then 30% H2O2 aqueous solution (2 g) was added to it. The reaction mixture was stirred for 16 h. The product was filtrated, and washed with acetone and diethyl ether three times. A white powder was produced (0.35 mg, yield: 73%). 1H NMR (400 MHz, acetone-d6, 25 °C): δ 7.48−7.72 (m, 10H, Ar), 4.80−4.81 (dd, 2H, Cp), 4.69 (dd, 2H, Cp), and 4.44 (s, 5H, Cp) ppm. IR (KBr): 1118 (s, PO) cm−1. ESI-MS (m/z) = 433.04 [M]+. Preparation of 1,1′-Bis(diphenylphosphoryl)ruthenocene (RcBPO). A solution of n-BuLi (9.0 mL, 1.6 M hexane, 14 mmol), was added dropwise to a solution of ruthenocene (1.0 g, 14 mmol) in dry THF (30 mL) and N,N,N′,N′-tetramethylethylenediamine (1.5 mL, 10.0 mmol) at 0 °C. The mixture was allowed to stir for 3 h at 0 °C, after which chlorodiphenylphosphine (PPh2Cl; 5.6 mL, 30 mmol) was added dropwise at 0 °C. The mixture was gradually brought to room temperature, and stirred for 14 h. The product was extracted with ethyl acetate; the extracts were washed with brine three times and dried over anhydrous MgSO4. The solvent was evaporated, and the resulting residue was purified by column chromatography (silica, dichloroethane/hexane, 1:4) to afford ruthenocene (0.41 g, yield: 16%). The obtained solid (2.0 g, 3.3 mmol) and acetone (ca. 40 mL) were placed in a flask. The solution was cooled to 0 °C, and then 30% H2O2 aqueous solution (1 g) was added to it. The reaction mixture was stirred for 16 h. The product was filtrated, and washed with acetone and diethyl ether three times. A white powder was produced (1.1 mg, yield: 52%). 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.27−7.62 (m, 20H, Ar), 4.83 (dd, 4H, Cp), and 4.52−4.64 (m, 4H, Cp) ppm. IR (KBr): 1118 (s, PO) cm−1. ESI-MS (m/z) = 633.08 [M]+. General Procedure for Preparation of Eu(III) Complexes with Phosphine Oxide Ligands. Phosphine oxide ligand (1 equiv of RcBPO or BIPHEPO (1,1′-biphenyl-2,2′diylbis(diphenylphosphine oxide), 2 equiv of RcPO) and Eu(hfa)3(H2O)2 (1.1 equiv) were dissolved in ethanol (20 mL). The solution was refluxed while stirring for 8 h, and the reaction mixture was concentrated to dryness. The residue was washed with chloroform several times. The insoluble material was removed by filtration, and the filtrate was concentrated. Recrystallization from ethanol gave colorless block crystals of the lanthanide complexes. [Eu(hfa)3(RcPO)2]. Yield: 0.2 g (18%). IR (KBr): 1655 (st, CO), 1254 (st, C−F), 1124 (st, PO) cm−1. FAB-MS (m/ z) = 1430.93 [M − (hfa)] + . Anal. Calcd (%) for EuC59H41O8F18P2Ru2: C, 43.32; H, 2.53. Found: C, 43.16; H, 2.56. [Eu(hfa)3(RcBPO)]. Yield: 0.4 g (17%). IR (KBr): 1655 (st, CO), 1255 (st, C−F), 1125 (st, PO) cm−1. FAB-MS (m/ z) = 1198.94 [M − (hfa)] + . Anal. Calcd (%) for EuC49H31O8F18P2Ru: C, 41.90; H, 2.22. Found: C, 41.71; H, 2.28. [Eu(hfa)3(BIPHEPO)]. Yield: 35%. MALDI TOF-MS (m/z) = 1367.3 ([M + K]+). Anal. Calcd (%) for C51H31EuF18O8P2: C, 46.14; H, 2.35. Found: C, 45.91; H, 2.42.

■

RESULTS AND DISCUSSION Coordination Structures of Eu(III) Complexes with Monodentate and Bidentate Phosphorylruthenocenes. Single crystals of the Eu(III) complexes with monodentate and 4827

DOI: 10.1021/acs.jpca.5b01809 J. Phys. Chem. A 2015, 119, 4825−4833

Article

The Journal of Physical Chemistry A

Eu−O distance = 2.30(0) Å for Eu(hfa)3(RcPO)2). The average Eu−O distances for the Eu(hfa)3(RcBPO) and Eu(hfa)3(BIPHEPO) reference complexes with phosphine oxide ligands are estimated to be 2.39(2) and 2.32(7) Å, respectively. The average distances between the Eu and O atoms were dependent on the ligand moieties of the phosphine oxide ligands. The selected distances and angles between the Eu and O atoms directly reflect the coordination geometry. The coordination geometry of lanthanide complexes is generally categorized to be an eight-coordinated square antiprism structure (8-SAP) or trigonal dodecahedron structure (8TDH). Based on the crystal data, calculations on the shapemeasure factor S were performed to categorize the geometrical structure and estimate the degree of distortion in the first coordination sphere of the coordination structure.15 The S value is given by

bidentate phosphorylruthenocenes were successfully prepared by recrystallization from ethanol solutions for single crystal Xray analysis. The resulting crystal data are summarized in Table 1. ORTEP views of all the Eu(III) complexes show the eightTable 1. Crystal Data for Eu(III) Complexes chem formula formula wt cryst color, habit cryst syst space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z dcalc/g cm−3 T/°C μ(Mo Kα)/cm−1 max 2θ/deg no. measd reflns no. unique reflns R (I > 2σ(I))a Rw (I > 2σ(I))b a

Eu(hfa)3(RuPO)2

Eu(hfa)3(RuBPO)

C59H41EuF18O8P2Ru2 1635.99 colorless, block monoclinic C2/c (No. 15) 19.8841(4) 17.3586(4) 17.7094(5) − 6098.5(3) − 6098.5(3) 4 1.782 −150.0 16.632 54.9 29 011 13 530 0.0409 0.1093

C49H31EuF18O8P2Ru 1404.73 colorless, block orthorhombic P212121 (No. 19) 13.414(1) 16.375(1) 24.158(2) − − − 5306.5(7) 4 1.758 −150.0 16.278 55.0 58 077 12 142 0.1049 0.2068

m

S = min

⎛1⎞ 2 ⎜ ⎟ ∑ (δ − θ ) i i ⎝m⎠ i=1

(1)

where m, δi, and θi are the number of possible edges (m = 18 in this study), the observed dihedral angle between planes along the ith edge, and the dihedral angle for the ideal structure, respectively. The estimated S values for the Eu(III) complexes are summarized in Table 2 (detail data, see Supporting Information Figure S1, Table S1, and Table S2). For [Eu(hfa)3(RcPO)2] and [Eu(hfa)3(RcBPO)], the S values for 8-SAP (point group D4d, S = 6.2 and 5.9°) are smaller than those for 8-TDH (point group D2d, S = 11.1 and 12.5°), which suggests that the 8-SAP structure is less distorted than the 8TDH structure. It was thus determined that the coordination geometry of [Eu(hfa)3(RcPO)2] and [Eu(hfa)3(RcBPO)] is the 8-SAP structure. Here we discuss the coordination geometry based on the positions of the phosphine oxides. The coordination geometries of [Eu(hfa)3(RcPO)2] and [Eu(hfa)3(RcBPO)] are categorized as structures a and b, respectively, as shown in Figure 2c. Structure a contains a symmetry axis and mirror plane, while structure b has no symmetry axis, mirror plane, or inversion center. To enhance kr related to the emission quantum yield, the lanthanide complex should form a low symmetrical structure to increase the probability of a 4f−4f transition. Therefore, structure b of [Eu(hfa)3(RcBPO)] is considered to have a significant advantage for the enhancement of kr. The S values for [Eu(hfa)3(RcPO)2] and [Eu(hfa)3(RcBPO)] are larger than that for [Eu(hfa)3(BIPHEPO)] (point group D4d, S = 3.7°). Electrochemical Properties of Eu(III) Complexes with Ruthenocenyl Units. The electrochemical reactivities of [Eu(hfa)3(RcPO)2] and [Eu(hfa)3(RcBPO)] in acetonitrile solution were surveyed by comparison of cyclic voltammograms with that for ferrocene (Fc; (C5H5)2Fe). The oxidation peak potentials from the cyclic voltammograms are summarized in Table 2. The half-potentials for the oxidation and reduction peaks (redox potential) of ferrocene were the same, regardless of the presence of [Eu(hfa)3(RcPO)2] or [Eu(hfa)3(RcBPO)] in solution. The voltammogram shapes obtained were similar to those for ruthenocene and typical ruthenocenyl derivatives.35,38 The Eu(hfa)3 lanthanide complex is not oxidized at potentials lower than +1.4 V. The electronic and steric structures of the cyclopentadienyl ligands in the ruthenocenyl units affect the oxidation potential of ruthenium (Ru). The oxidation potential

R = ∑∥Fo| − |Fc∥/∑|Fo|. bRw = [(∑w(|Fo| − |Fc|)2/∑wFo2)]1/2.

Figure 2. ORTEP drawings of (a) [Eu(hfa)3(RcPO)2] and (b) [Eu(hfa)3(RcBPO)]. See Figure S1 (Supporting Information) for the detailed structure of [Eu(hfa)3(RcBPO)]. (c) Schematic diagrams of supposable square antiprism structures with phosphine oxide units.

coordinated structures (Figure 2). The coordination sites of [Eu(hfa)3(RcPO)2] host three hfa ligands and two monodentate RcPO ligands (Figure 2a). In contrast, [Eu(hfa)3(RcBPO)] is composed of three hfa and one bidentate RcBPO ligands (Figure 2b). The distance between Eu and O in the hfa ligand (for example, average Eu−O distance = 2.42(3) Å for Eu(hfa)3(RcPO)2) is larger than the distance between Eu and O in the complex with phosphine oxide ligands (average 4828

DOI: 10.1021/acs.jpca.5b01809 J. Phys. Chem. A 2015, 119, 4825−4833

Article

The Journal of Physical Chemistry A Table 2. Shape-Measure Factors and Electrochemical Data for Eu(III) Complexes compd [Eu(hfa)3(RuPO)2] [Eu(hfa)3(RuBPO)] d

[Eu(hfa)3(BIPHEPO)2] ruthenocenee

S(D2d)a/deg

S(D4d)b/deg

determ coord geom

oxidn peakc/V

6.40 5.87 (Eu1) 7.22 (Eu2) 3.7 −

10.4 12.5 (Eu1) 28.0 (Eu2) 13 −

8-SAP 8-SAP

0.70

8-SAP −

1.47 − 0.61

a

S value for eight-coordinated trigonal dodecahedron (8-TDH). bS value for eight-coordinated square antiprism (8-SAP). cCathodic potentials measured using cyclic voltammetry in acetonitrile (Pt electrode, vs Fc/Fc; (Fc, ferrocene), 0.1 mol dm−3 tetrabutylammonium hexafluorophosphate, 20 mV s−1). In CH2Cl2 solution, ruthenocence and ruthenocence compounds provide reversible cyclic voltammogram. In acetonitrile solution, ruthenocence and ruthenocence compounds provided semireversible cyclic voltammogram because of adsorption and elimination of an acetonitrile molecule (ref 48). See Supporting Information, Figure S3. dReference 40. eStructural formula of ruthenocene: (C5H5)2Ru.

+1.4 V, the absorption band at 310 nm decreased, while a new band appeared around 370 nm. Isosbestic points appeared at 290 and 340 nm, which supports the reversibility of the two quasi-components of the Eu(III) complexes, EuIII(Rc(0)/ Rc(0)) for [Eu(hfa)3(RcPO)2] and EuIII(Rc(1+)/Rc(1+)) for [Eu(hfa)3(Rc+PO)2]. The oxidized EuIII(Rc(+)) for [Eu(hfa)3(Rc+BPO)] also exhibits a new absorption band around 370 nm under polarization. The original and oxidized Eu(III) complexes were cycled without marked degradation for more than 20 cycles under the alternating addition of potentiostatic polarization. We observed only one oxidation peak of [Eu(hfa)3(RcPO)2] in acetonitrile. We could not find second oxidation peaks of [Eu(hfa)3(RcPO)2]. We consider formation of EuIII(Rc(1+)/Rc(1+)) for [Eu(hfa)3(Rc+PO)2] under static potentiostatic polarization at +1.4 V. The new absorption bands for the oxidized [Eu(hfa)3(Rc+PO)2] and [Eu(hfa)3(Rc+BPO)] did not overlap with the emission bands of the Eu(III) ion. Therefore, the 4f−4f emission from the Eu(III) ion in the complex would not be quenched by the absorption bands of the cationic Rc+PO and Rc+BPO ligands. Luminescence Properties under Potentiostatic Polarization. Emission spectra of the complexes in acetonitrile solution were measured under potentiostatic polarization. Parts a and b, respectively, of Figure 4 show the spectra for [Eu(hfa)3(RcPO)2] at +1.0 V and [Eu(hfa)3(RcBPO)] at +1.4 V. The emission bands for the EuIII(Rc(0)/Rc(0)) for [Eu(hfa)3(RcPO)2] and EuIII(Rc(0)) for [Eu(hfa)3(RcBPO)] complexes were observed around 578, 592, 613, 650, and 698 nm, and are attributed to the f−f transitions of 5D0−7FJ with J = 0, 1, 2, 3, and 4, respectively. The spectra were normalized with respect to the magnetic dipole transition intensity at 592 nm (5D0−7F1), which is known to be insensitive to the environment surrounding the lanthanide ions.39 The emission bands at 613 nm (5D0−7F2) and 700 nm (5D0−7F4) are due to electric dipole transitions, which are strongly dependent on the coordination geometry. Enhancement of the electric transition bands of EuIII(Rc(1+)/Rc(1+)) for [Eu(hfa)3(Rc+PO)2] under potentiostatic polarization at +1.0 V was successfully observed (Figure 4a). The significant change in the emission band shape suggests transformation of the coordination geometry of EuIII(Rc(1+)/Rc(1+)) for [Eu(hfa)3(Rc+PO)2] under potentiostatic polarization. In contrast, no change was observed in the emission spectra for the Eu(III) complex with one bidentate phosphorylruthenocene, EuIII(Rc(+)) for [Eu(hfa)3(Rc+BPO)], under potentiostatic polarization at +1.4 V (Figure 4b). To estimate the steady-state emission properties of [Eu(hfa)3(RcPO)2] and [Eu(hfa)3(RcBPO)], the emission quantum yields in acetone-d6 were measured for comparison with

of [Eu(hfa)3(RcPO)2] (0.70 V) is thus similar to that of the reference ruthenocene ((C5H5)2Ru, 0.61 V).35,36 The slight difference between [Eu(hfa)3(RcPO)2] and ruthenocene (0.09 V) is due to the addition of phosphine oxide in the cyclopentadienyl rings. However, the oxidation potential of [Eu(hfa)3(RcBPO)] (1.47 V) is much higher than that of [Eu(hfa)3(RcPO)2]. Free rotation of the two cyclopentadienyl rings of RcBPO is restricted by coordination to the Eu(III) ion in [Eu(hfa)3(RcPO)2]. It is considered that the larger positive shift of the oxidation potential may be due to blocking of the free rotation of the cyclopentadienyl rings on the ruthenocene unit. Formation of the ruthenocenyl cations (Rc+) in the Eu(III) complexes was achieved by potentiostatic polarization of [Eu(hfa)3(RcPO)2] at +1.0 V for 1065 s and at +1.4 V for 1440 s for [Eu(hfa)3(RcBPO)2]. Parts a and b, respectively, of Figure 3 show the changes in the absorption spectra for

Figure 3. Absorption spectra for (a) [Eu(hfa)3(RcPO)2] in acetonitrile under potentiostatic polarization at 1.0 V for 0, 20, 340, 545, 825, and 1065 s, and for (b) Eu(hfa)3(RcBPO)] in acetonitrile under potentiostatic polarization at 1.4 V for 0, 20, 380, 730, 1090, and 1440 s.

[Eu(hfa)3(RcPO)2] and [Eu(hfa)3(RcBPO)] under polarization in acetonitrile. The original absorption bands around 310 nm for [Eu(hfa)3(RcPO)2] and [Eu(hfa)3(RcBPO)] without polarization are assigned to the π−π* transitions of the RcPO and RcBPO ligands. Under potentiostatic polarization of [Eu(hfa)3(RcPO)2] at +1.0 V and of [Eu(hfa)3(RcPO)2] at 4829

DOI: 10.1021/acs.jpca.5b01809 J. Phys. Chem. A 2015, 119, 4825−4833

Article

The Journal of Physical Chemistry A

than those for [Eu(hfa)3(RcBPO)] (knr = 2.0 × 102 s−1) and [Eu(hfa)3(BIPHEPO)] (knr = 3.1 × 102 s−1). The emission decay profiles of [Eu(hfa)3(RcPO)2] and [Eu(hfa)3(RcBPO)] in acetonitrile under potentiostatic polarization at +1.0 and +1.4 V are shown in parts c and d (red lines), respectively, of Figure 4. The time-resolved emission of EuIII(Rc(+)) for [Eu(hfa)3(Rc+BPO)] under potentiostatic polarization at +1.4 V shows a single-exponential decay, which was 0.88 ms (red line in Figure 4d). The emission lifetime of cationic EuIII(Rc(+)) for [Eu(hfa)3(Rc+BPO)] under polarization is smaller than that of the original EuIII(Rc(0)) for [Eu(hfa)3(RcBPO)] in acetonitrile (τ = 1.18 ms, blue line in Figure 4d). The slight decrease in the emission lifetime of EuIII(Rc(+)) for [Eu(hfa)3(Rc+BPO)] may be due to luminescence quenching on the Pt electrode. The two components of the emission decay of [Eu(hfa)3(RcPO)2] were observed under potentiostatic polarization at +1.0 V (Figure 4c): the original EuIII(Rc(0)/Rc(0)) for [Eu(hfa)3(RcPO)2] (faster emission decay) and oxidized EuIII(Rc(1+)/Rc(1+)) for [Eu(hfa)3(Rc+PO)2] (slower emission decay). The faster emission decay component of the original EuIII(Rc(0)/Rc(0)) for [Eu(hfa)3(RcPO)2] (τfast = 0.15 ms) may also be due to luminescence quenching on the Pt electrode. The slower emission decay component (τslow) for oxidized EuIII(Rc(1+)/Rc(1+)) for [Eu(hfa)3(Rc+PO)2] was 0.46 ms, which is longer than that for the original EuIII(Rc(0)/ Rc(0)) for [Eu(hfa)3(RcPO)2] (τ = 0.41 ms; blue line in Figure 4c). The large emission lifetime of EuIII(Rc(1+)/Rc(1+)) for [Eu(hfa)3(Rc+PO)2] is directly linked to the increase of kr and enhancement of the electric dipole transition (613 nm, 5 D0−7F2; 700 nm, 5D0−7F4) in the emission spectra. We propose that the geometrical symmetry of EuIII(Rc(1+)/ Rc(1+)) for [Eu(hfa)3(Rc+PO)2] under potentiostatic polarization is lower than that of the original EuIII(Rc(0)/Rc(0)) for [Eu(hfa)3(RcPO)2]. DFT and TD-DFT Calculations of Polarizability and Atomic Charge for Phosphorylruthenocene Ligands. The electronic transition probability for the 4f−4f transition is explained by the dynamic coupling and static coupling models.45,46 According to the dynamic coupling model for lanthanide complexes, Hatanaka reported that the 4f−4f transition probability is affected by the mixing of the excited orbital of the organic ligand, which is estimated using polarizability.47 To analyze the effective change in the emission properties of [Eu(hfa)3(Rc+PO)2] under potentiostatic polarization, we first performed time dependent DFT (TD-DFT) calculations for the phosphorylruthenocene ligands based on single crystal X-ray analysis data. The polarizabilities of the excited states (TD-DFT calculations) are summarized in Table 4. The charge differences ΔP were observed between the original and oxidized Eu(III) complexes for the changeable [Eu(hfa)3(RcPO)2] with two RcPO ligands (ΔP = +3.86 D) and nonchangeable [Eu(hfa)3(RcBPO)] with one RcBPO

Figure 4. Emission spectra for (a) [Eu(hfa)3(RcPO)2] in acetonitrile under potentiostatic polarization at 1.4 V from 0 to 480 s and for (b) [Eu(hfa)3(RcBPO)] in acetonitrile under potentiostatic polarization at 1.0 V from 0 to 480 s under excitation at 350 nm. Time-resolved emission decay profiles for (c) [Eu(hfa)3(RcPO)2] and (d) [Eu(hfa) 3 (RcBPO)] in acetonitrile (blue lines, original [Eu(hfa)3(RcPO)2] (c) and [Eu(hfa)3(RcBPO)] (d); red lines, oxidized [Eu(hfa)3(Rc+PO)2] (c) and [Eu(hfa)3(Rc+BPO)] (d) under potentiostatic polarization). Excitation at 365 nm using Nd:YAG laser (3ω, 5 ns pulse width).

the luminescence performance previously reported for the [Eu(hfa)3(BIPHEPO)] complex (Table 3).42 The emission Table 3. Photophysical Properties of Eu(III) Complexes at Room Temperature complex [Eu(hfa)3(RcPO)2] [Eu(hfa)3(RcBPO)] [Eu(hfa)3(BIPHEPO)2]e

Φa/% 17 76 60

τobsdb/ms

krc/s−1

knrd/s−1

0.65 1.3 1.3

2.6 × 10 5.8 × 102 4.6 × 102 2

1.3 × 103 2.0 × 102 3.1 × 102

a

Emission quantum yields for Eu(III) complexes were determined by comparison with the integrated emission signal (550−750 nm) of [Eu (hfa)3(biphepo)] as Φ = 0.60. Excitation at 465 nm. bEmission lifetimes (τobsd’s) of the lanthanide complexes were measured by excitation at 355 nm (Nd:YAG 3ω). cRadiative rate constant kr = Φ/ τobsd. dNonradiative rate constant knr = 1/τobsd − kr. eSee ref 40.

quantum yield of [Eu(hfa)3(RcBPO)] was 76%, which is similar to the highly luminescent [Eu(hfa)3(BIPHEPO)] lanthanide complex with the bidentate phosphine oxide. The emission quantum yield of [Eu(hfa)3(RcPO)2] was estimated to be 17%. The small emission quantum yield of [Eu(hfa) 3 (RcPO) 2 ] may be due to free rotation of the ruthenocenyl units, which would increase knr via vibrational relaxation. The time-resolved emission profiles of the [Eu(hfa)3(RcPO)2] and [Eu(hfa)3(RcBPO)] complexes in acetone-d6 revealed single-exponential decays with millisecondscale lifetimes (blue lines in Figure 4c,d). The emission lifetimes were determined from the slopes of logarithmic plots of the decay profiles. The kr and knr rate constants estimated using the emission lifetimes and the emission quantum yields are also summarized in Table 3. The nonradiative rate constant for [Eu(hfa)3(RcPO)2] (knr = 1.3 × 103 s−1) was much larger

Table 4. Polarizability (P) and Atomic Charge (O Atoms; ρ) for Phosphorylruthenocene Ligands

4830

complex

P/D

ρ

[Eu(hfa)3(RcPO)2] [Eu(hfa)3(Rc+PO)2] [Eu(hfa)3(RcBPO)] [Eu(hfa)3(Rc+BPO)]

4.18 8.04 6.33 8.11

−0.58 −0.55 −0.56 −0.52

DOI: 10.1021/acs.jpca.5b01809 J. Phys. Chem. A 2015, 119, 4825−4833

Article

The Journal of Physical Chemistry A switching related to open up chemistry,

ligand (ΔP = +1.78 D) under potentiostatic polarization. Larger differential polarizability ΔP based on the dynamic coupling model may be partially linked to a change of the emission properties of [Eu(hfa)3(Rc+PO)2] in our experiments, because no significant change of [Eu(hfa)3(RcBPO)] was observed. The static coupling model for the lanthanide complexes indicates that changes of geometrical positions and the atomic charge of coordinated oxygen atoms in the ground state influence the emission properties. The average atomic charges of oxygen atoms (DFT calculations) in the phosphorylruthenocenes are also summarized in Table 4. The average atomic charges of oxygen atoms in the original EuIII(Rc(0)/Rc(0)) for [Eu(hfa)3(RcPO)2] (−0.58) and EuIII(Rc(0)) for [Eu(hfa)3(RcBPO)] (−0.56) ligands are similar to those of the corresponding oxidized EuIII(Rc(+1)/Rc(+1)) for [Eu(hfa) 3 (Rc + PO) 2 ] (−0.55) and Eu III (Rc(+)) for [Eu(hfa)3(Rc+BPO)] (−0.52) ligands. Potentiostatic polarization does not indicate a change in the atomic charges of coordinated oxygen atoms. We consider that the significant change in the emission properties of EuIII(Rc(+1)/Rc(+1)) for [Eu(hfa)3(Rc+PO)2] may be based not only on the effective change of the polarizability for enhancement of transition probability, but also on changes of the geometrical structure under potentiostatic polarization (postulated model, Figure 1b). Under potentiostatic polarization at +1.0 V, the original phosphorylruthenocene RcPO unit becomes a phosphorylruthenocene cation, Rc+PO unit. Two cationic Rc+PO units could move with cationic charge repulsion. The molecular motion of the cationic Rc+PO units under potentiostatic polarization induces a geometrical change of the Eu(III) coordination sphere. The geometrical structure of the eight coordinated square antiprism of [Eu(hfa)3(RcPO)2] has no inverted center. The angle of P OEuOP in [Eu(hfa)3(RcPO)2] was found to be 88°. We think that cationic repulsion in EuIII(Rc(+1)/Rc(+1)) for [Eu(hfa)3(Rc+PO)2] might promote formation of a more asymmetric structure with approximately 180° for PO EuOP in [Eu(hfa)3(RcPO)2]. In contrast, the emission properties of EuIII(Rc(+1)) for [Eu(hfa)3(Rc+BPO)2] may not be affected by a small change of potentiostatic polarization and coordination geometry (see Supporting Information, Figure S2). We consider that the two independent units of redox active Rc+PO ligands lead to an effective change in the dynamic polarization of the excited state and the coordination structure, which results in an enhancement of the emission properties of the Eu(III) luminescent center.

■

phenomenon of the photophysical properties is mechanical molecular motion and is expected to a new frontier in the fields of supramolecular molecular science, and engineering.

ASSOCIATED CONTENT

S Supporting Information *

Selected lengths and angles of [Eu(hfa)3(RcPO)2] and [Eu(hfa)3(RcBPO)] (data for shape-measure calculations); image of dynamic coupling model and changeable coordination structure of Eu(III) complex based on the molecular motions of redox active RcPO units; cyclic voltammograms of Eu(III) complexes with RcPO and RcBPO ligands. Crystallographic data in CIF format. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b01809.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +81 11 706 7114. Fax: +81 11 706 7114. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was partly supported by Grants-in-Aid for Scientific Research on Innovative Areas of “New Polymeric Materials Based on Element-Blocks (No. 2401)” (24102012) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

■

REFERENCES

(1) Review on luminescence behaviors: Blasse, G.; Grabmaier, B. C. Luminescent Materials; Springer-Verlag: New York, 1994. (2) Gan, F. Laser Materials; World Scientific: Singapore, 1995; p 70. (3) Lunstroot, K.; Nockemann, P.; Van Hecke, K.; Van Meervelt, L.; Görller-Walrand, C.; Binnemans, K.; Driesen, K. Visible and NearInfrared Emission by Samarium(III)-Containing Ionic Liquid Mixtures. Inorg. Chem. 2009, 48, 3018−3026. (4) Petoud, S.; Cohen, S. M.; Bünzli, J.-C. G.; Raymond, K. N. Stable Lanthanide Luminescence Agents Highly Emissive in Aqueous Solution: Multidentate 2-Hydroxyisophthalamide Complexes of Sm3+, Eu3+, Tb3+, Dy3+. J. Am. Chem. Soc. 2003, 125, 13324−13325. (5) Sastri, V. S.; Bünzli, J.-C. G.; Rao, V. R.; Rayudu, G. V. S.; Perumareddi, J. R. Modern Aspects of Rare Earth and Their Complexes; Elsevier: New York, 2003. (6) Xu, J.; Radkov, E.; Ziegler, M.; Raymond, K. N. Plutonium(IV) Sequestration: Structural and Thermodynamic Evaluation of the Extraordinarily Stable Cerium(IV) Hydroxypyridinonate Complexes. Inorg. Chem. 2000, 39, 4156−4164. (7) Hasegawa, Y.; Wada, Y.; Yanagida, S. Strategies for the Design of Luminescent Lanthanide(III) Complexes and their Photonic Applications. J. Photochem. Photobiol. C: Photochem. Rev. 2004, 5, 183−202. (8) Varlan, M.; Blight, B. A.; Wang, S. Selective Activation of Lanthanide Luminescence with Triarylboron-functionalized Ligands and Visual Fluoride Indicators. Chem. Commun. 2012, 48, 12059− 12061. (9) Smith, D. G.; McMahon, B. K.; Pal, R.; Parker, D. Live Cell Imaging of Lysosomal pH Changes with pH Responsive Ratiometric Lanthanide Probes. Chem. Commun. 2012, 48, 8520−8522. (10) Edkins, R. M.; Sykes, D.; Beeby, A.; Ward, M. D. Combined Two-Photon Excitation and d - f Energy-transfer in Ir/lanthanide Dyads with Time-gated Selection from a Two-component Emission Spectrum. Chem. Commun. 2012, 48, 9977−9979.

■

SUMMARY AND CONCLUSIONS Enhancement of the 4f−4f emission from a Eu(III) complex using a redox switching unit, phosphorylruthenocene, was successfully observed. The luminescent switching phenomenon was dependent on the structure of the phosphorylruthenocene unit under potentiostatic polarization, where a significant change of the 4f−4f transition was observed for a Eu(III) complex attached with monodentate phosphorylruthenocene ligands. The detailed switching mechanism for the enhanced 4f−4f transitions is not clear at the present stage. We now propose that the switching phenomenon may be based on the dynamic polarization of the excited state and molecular motion of the phosphorylruthenocene unit under cationic repulsion. In this study, ruthenocene was introduced as a dynamic molecular unit in a luminescent lanthanide complex for the first time. The 4831

DOI: 10.1021/acs.jpca.5b01809 J. Phys. Chem. A 2015, 119, 4825−4833

Article

The Journal of Physical Chemistry A

(27) Zhang, X.; Jiao, Y.; Jing, X.; Wu, H.; He, G.; Duan, C. pHSensitive Fluorescent Sensors Based on Europium(III) Complexes. Dalton Trans. 2011, 40, 2522−2527. (28) Nakagawa, T.; Atsumi, K.; Nakashima, T.; Hasegawa, Y.; Kawai, T. Reversible Luminescence Modulation in Photochromic Europium(III) Complex Having Triangle Terthiazole Ligands. Chem. Lett. 2007, 36, 372−373. (29) Nakagawa, T.; Hasegawa, Y.; Kawai, T. Photoresponsive Europium(III) Complex Based on Photochromic Reaction. J. Phys. Chem. A 2008, 112, 5096−5103. (30) Greb, L.; Lehn, J.-M. Light-Driven Molecular Motors: Imines as Four-Step or Two-Step Unidirectional Rotors. J. Am. Chem. Soc. 2014, 136, 13114−13117. (31) Kundu, P. K.; Lerner, A.; Kučanda, K.; Leitus, G.; Klajn, R. Cyclic Kinetics During Thermal Equilibration of an Axially Chiral BisSpiropyran. J. Am. Chem. Soc. 2014, 136, 11276−11279. (32) Baroncini, M.; Gao, C.; Carboni, V.; Credi, A.; Previtera, E.; Semeraro, M.; Venturi, M.; Silvi, S. Light Control of Stoichiometry and Motion in Pseudorotaxanes Comprising a Cucurbit[7]uril Wheel and an Azobenzene-Bipyridinium Axle. Chem.Eur. J. 2014, 20, 10737− 10744. (33) Zhang, X.; Zhao, H.; Tian, D.; Deng, H.; Li, H. A Photoresponsive Wettability Switch Based on a Dimethylamino Calix[4]arene. Chem.Eur. J. 2014, 20, 9367−9371. (34) Bredenkötter, B.; Grzywa, M.; Alaghemandi, M.; Schmid, R.; Herrebout, W.; Bultinck, P.; Volkmer, D. Tribenzotriquinacene Receptors for C60 Fullerene Rotors: Towards C3 Symmetrical Chiral Stators for Unidirectionally Operating Nanoratchets. Chem.Eur. J. 2014, 20, 9100−9110. (35) Otón, F.; Espinosa, A.; Tárraga, A.; Molina, P. Synthesis of Multifunctional Aza-Substituted Ruthenocene Derivatives Displaying Charge-Transfer Transitions and Selective Zn(II) Ions Sensing Properties. Organometallics 2007, 26, 6234−6242. (36) Yuasa, Y.; Ueno, H.; Kawai, T. Sign Reversal of a Large Circularly Polarized Luminescence Signal by the Twisting Motion of a Bidentate Ligand. Chem.Eur. J. 2014, 20, 8621−8627. (37) Nishikawa, M.; Nomoto, K.; Kume, S.; Inoue, K.; Sakai, M.; Fujii, M.; Nishihara, H. Dual Emission Caused by Ring Inversion Isomerization of a 4-Methyl-2-pyridyl-pyrimidine Copper(I) Complex. J. Am. Chem. Soc. 2010, 132, 9579−9581. (38) Kowalski, K.; Skiba, J.; Oehninger, L.; Ott, I.; Solecka, J.; Rajnisz, A.; Therrien, B. Metallocene-Modified Uracils: Synthesis, Structure, and Biological Activity. Organometallics 2013, 32, 5766− 5773. (39) Yamaguchi, Y.; Ding, W.; Sanderson, C. T.; Borden, M. L.; Morgan, M. J.; Kutal, C. Electronic Structure, Spectroscopy, and Photochemistry of Group 8 Metallocenes. Coord. Chem. Rev. 2007, 251, 515−524. (40) Hasegawa, Y.; Kimura, Y.; Murakoshi, K.; Wada, Y.; Kim, J.; Nakashima, N.; Yamanaka, T.; Yanagida, S. Enhanced Emission of Deuterated Tris(hexafluoroacetylacetonato)neodymium(III) Complex in Solution by Suppression of Radiationless Transition via Vibrational Excitation. J. Phys. Chem. 1996, 100, 10201−10205. (41) Hasegawa, Y.; Yamamuro, M.; Wada, Y.; Kanehisa, K.; Kai, K.; Yanagida, S. Luminescent Polymer Containing the Eu(III) Complex Having Fast Radiation Rate and High Emission Quantum Efficiency. J. Phys. Chem. A 2003, 107, 1697−1702. (42) Miyata, K.; Nakagawa, T.; Kawakami, R.; Kita, Y.; Sugimoto, K.; Nakashima, T.; Harada, T.; Kawai, T.; Hasegawa, Y. Remarkable Luminescence Properties of Lanthanide Complexes with Asymmetric Dodecahedron Structures. Chem.Eur. J. 2011, 17, 521−528. (43) Lee, C. T.; Yang, W. T.; Parr, R. G. Development of the ColleSalvetti Correlation-energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (44) Becke, A. D. Density-functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (45) Stephens, E. M.; Davis, S.; Reid, M. F.; Richardson, F. S. Empirical Intensity Parameters for the 4f → 4f Absorption Spectra of

(11) Nonat, A.; Regueiro-Figueroa, M.; Esteban-Gomez, D.; de Blas, A.; Rodríguez-Blas, T.; Platas-Iglesias, C.; Charbonnière, L. J. Definition of an Intramolecular Eu-to-Eu Energy Transfer within a Discrete [Eu2L] Complex in Solution. Chem.Eur. J. 2012, 18, 8163− 8173. (12) Smith, D. G.; Pal, R.; Parker, D. Measuring Equilibrium Bicarbonate Concentrations Directly in Cellular Mitochondria and in Human Serum Using Europium/Terbium Emission Intensity Ratios. Chem.Eur. J. 2012, 18, 11604−11613. (13) Andreiadis, E. S.; Imbert, D.; Pécaut, J.; Demadrille, R.; Mazzanti, M. Self-assembly of Highly Luminescent Lanthanide Complexes Promoted by Pyridine-tetrazolate Ligands. Dalton Trans. 2012, 41, 1268−1277. (14) Lewis, D. J. F.; Moretta, F.; Holloway, A. T.; Pikramenou, Z. Evaluation of Quinoline as a Remote Sensitiser for Red and Nearinfrared Emissive Lanthanide(III) Ions in Solution and the Solid State. Dalton Trans. 2012, 41, 13138−13146. (15) Li, Y.-A.; Ren, S.-K.; Liu, Q.-K.; Ma, J.-P.; Chen, X.; Zhu, H.; Dong, Y.-B. Encapsulation and Sensitization of UV−vis and Near Infrared Lanthanide Hydrate Emitters for Dual- and BimodalEmissions in Both Air and Aqueous Media Based on a Porous Heteroatom-Rich Cd(II)-Framework. Inorg. Chem. 2012, 51, 9629− 9635. (16) Mohapatra, S.; Adhikari, S.; Riju, H.; Maji, T. K. Terbium(III), Europium(III), and Mixed Terbium(III)−Europium(III) Mucicate Frameworks: Hydrophilicity and Stoichiometry-Dependent Color Tunability. Inorg. Chem. 2012, 51, 4891−4893. (17) Ramya, A. R.; Sharma, D.; Natarajan, S.; Reddy, M. L. P. Highly Luminescent and Thermally Stable Lanthanide Coordination Polymers Designed from 4-(Dipyridin-2-yl)aminobenzoate: Efficient Energy Transfer from Tb3+ to Eu3+ in a Mixed Lanthanide Coordination Compound. Inorg. Chem. 2012, 51, 8818−8826. (18) de Bettencourt-Dias, A.; Barber, P. S.; Bauer, S. J. Am. Chem. Soc. 2012, 134, 6987−6994. (19) Rodrigues, M. O.; Dutra, J. D. L.; Nunes, L. A. O.; de Sá, G. F.; de Azevedo, W. M.; Silva, P.; Paz, F. A. A.; Freire, R. O.; Júnior, S. A. Tb3+→Eu3+ Energy Transfer in Mixed-Lanthanide-Organic Frameworks. J. Phys. Chem. C 2012, 116, 19951−19957. (20) Hua, K.-N. T.; Xu, J.; Quiroz, E. E.; Lopez, S.; Ingram, A. J.; Johnson, V. A.; Tisch, A. R.; de Bettencourt-Dias, A.; Straus, D. A.; Muller, G. Structural and Photophysical Properties of Visible- and Near-IR-Emitting Tris Lanthanide(III) Complexes Formed with the Enantiomers of N,N′-Bis(1-phenylethyl)-2,6-pyridinedicarboxamide. Inorg. Chem. 2012, 51, 647−660. (21) Ablet, A.; Li, S.-M.; Cao, W.; Zheng, X.-J.; Wong, W.-T.; Jin, L.P. Luminescence Tuning and White-Light Emission of Co-doped Ln− Cd−Organic Frameworks. Chem.Asian J. 2013, 8, 95−100. (22) Xu, J.; Jia, L. N.; Jin, N.; Ma, Y.; Liu, X.; Wu, W.; Liu, W.; Tang, Y.; Zhou, F. Fixed-Component Lanthanide-Hybrid-Fabricated FullColor Photoluminescent Films as Vapoluminescent Sensors. Chem. Eur. J. 2013, 19, 4556−4562. (23) Wartenberg, N.; Raccurt, O.; Bourgeat-Lami, E.; Imbert, D.; Mazzanti, M. Multicolour Optical Coding from a Series of Luminescent Lanthanide Complexes with a Unique Antenna. Chem.Eur. J. 2013, 19, 3477−3482. (24) Nakanishi, T.; Suzuki, Y.; Doi, Y.; Seki, T.; Koizumi, H.; Fushimi, K.; Fujita, K.; Hinatsu, Y.; Ito, H.; Tanaka, K.; Hasegawa, Y. Enhancement of Optical Faraday Effect of Nonanuclear Tb(III) Complexes. Inorg. Chem. 2014, 53, 7635−7641. (25) Tropiano, M.; Kilah, N. L.; Morten, M.; Rahman, H.; Savis, J. J.; Beer, P. D.; Faulkner, S. Reversible Luminescence Switching of a Redox-Active Ferrocene−Europium Dyad. J. Am. Chem. Soc. 2011, 133, 11847−11849. (26) Gunnlaugsson, T.; Leonard, J. P.; Sénéchal, K.; Harte, A. J. pH Responsive Eu(III)−Phenanthroline Supramolecular Conjugate: Novel “Of f−On−Of f ” Luminescent Signaling in the Physiological pH Range. J. Am. Chem. Soc. 2003, 125, 12062−12063. 4832

DOI: 10.1021/acs.jpca.5b01809 J. Phys. Chem. A 2015, 119, 4825−4833

Article

The Journal of Physical Chemistry A Nine-coordinate Neodymium(III) and Holmium(III) Complexes in Aqueous Solution. Inorg. Chem. 1984, 23, 4607−4611. (46) Reid, M. F.; Richardson, F. S. Lanthanide 4f → 4f Electric Dipole Intensity Theory. J. Phys. Chem. 1984, 88, 3579−3586. (47) Hatanaka, M.; Yabushita, S. Theoretical Study on the f−f Transition Intensities of Lanthanide Trihalide Systems. J. Phys. Chem. A 2009, 113, 12615−12625. (48) Hashidzume, K.; Tobita, H.; Ogino, H. Electrochemistry of Octamethyl[3]ruthenocenophane: Synthesis and Structure of the First Dicationic Derivative of Ruthenocene, [{η5:η5-C5Me4(CH2)3C5Me4}Ru(NCMe)](PF6)2. Organometallics 1995, 14, 1187−1194.

4833

DOI: 10.1021/acs.jpca.5b01809 J. Phys. Chem. A 2015, 119, 4825−4833