Kris Driesen

KU Leuven, Leuven, VLG, Belgium

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Publications (60)130.46 Total impact

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    ABSTRACT: Flexible luminescent polymer films were obtained by doping europium(III) complexes in blends of poly(methyl methacrylate) (PMMA) and the ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C(6)mim][Tf(2)N]. Different europium(III) complexes have been incorporated in the polymer/ionic liquid matrix: [C(6)mim][Eu(nta)(4)], [C(6)mim][Eu(tta)(4)], [Eu(tta)(3)(phen)] and [choline](3)[Eu(dpa)(3)], where nta is 2-naphthoyltrifluoroacetonate, tta is 2-thenoyltrifluoroacetonate, phen is 1,10-phenanthroline, dpa is 2,6-pyridinedicarboxylate (dipicolinate) and choline is the 2-hydroxyethyltrimethyl ammonium cation. Bright red photoluminescence was observed for all the films upon irradiation with ultraviolet radiation. The luminescent films have been investigated by high-resolution steady-state luminescence spectroscopy and by time-resolved measurements. The polymer films doped with beta-diketonate complexes are characterized by a very intense (5)D(0)-->(7)F(2) transition (up to 15 times more intense than the (5)D(0)-->(7)F(1)) transition, whereas a marked feature of the PMMA films doped with [choline](3)[Eu(dpa)(3)] is the long lifetime of the (5)D(0) excited state (1.8 ms).
    Physical Chemistry Chemical Physics 02/2010; 12(8):1879-85. · 3.83 Impact Factor
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    ABSTRACT: Luminescence spectra of aqueous solutions containing a fixed concentration of tetracycline (TC) and increasing concentrations of Eu3+ were recorded both in the absence and presence of hydrogen peroxide (H2O2). In H2O2-free solutions in which the Eu/TC molar ratio was varied from 1 : 1 to 8 : 1, the 5D0→7F0 transition consisted of only one peak at 580 nm. In the presence of H2O2, an extra peak appeared in the spectrum at 578 nm when the Eu/TC molar ratios were above 2.5. A detailed analysis of this spectral region revealed that at lower Eu/TC molar ratios (up to 2 : 1), the 5D0→7F0 transition experienced a slight blue shift. This indicates that at low Eu/TC molar ratios, the presence of H2O2 leads to two different environments of the trivalent europium ions, which most likely form bridged peroxide complexes with hydrogen peroxide (μ-H2O2 ligand). Luminescence spectra measured in the presence of molybdate ions, which catalytically decompose H2O2, led to the disappearance of the extra europium(III) site that was formed in the presence of H2O2. The intensity of the hypersensitive 5D0→7F2 transition did not linearly depend on the H2O2/TC molar ratio. For H2O2/TC ratios up to 10, a sharp linear increase of the peak intensity was observed, but with further increase of the H2O2 concentration, the intensity remained nearly constant. For H2O2/TC ratios above 100, the intensity of this transition even started to decrease, which limits the use of the (tetracycline)europium(III) system to quantify hydrogen peroxide in solution.
    Helvetica Chimica Acta 11/2009; 92(11):2387 - 2397. · 1.38 Impact Factor
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    ABSTRACT: Yb(3+)-Er(3+) co-doped fluoride nanoparticles have been prepared. When pumped by 975 nm laser diode into absorption band of Yb(3+), the laser-induced temperature rise up to 800 degrees C has been detected in the nanoparticles by measuring the ratio of the intensities of the thermalised up-conversion luminescence bands (2)H(11/2)-->(4)I(15/2) and (4)S(3/2)-->(4)I(15/2) of Er(3+). These results show that a controlled optical heating of the nanoparticles and their surrounding nano-volumes can be realised, while the location and temperature rise of the nanoparticles and heated nano-volumes can be detected distantly by means of luminescence.
    Optics Express 08/2009; 17(14):11794-8. · 3.55 Impact Factor
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    ABSTRACT: The luminescence spectra of europium(III) complexes dissolved in the liquid crystal 4'-pentyl-4-cyanobiphenyl (5CB) were investigated. Upon alignment of the europium(III)-doped nematic liquid-crystal host in a liquid-crystal cell with alignment layers, polarization effects were observed in the emission spectra. These polarization effects were visible as differences in the relative intensities of the crystal-field components of the transitions. Although the europium(III) complexes do not need to be liquid-crystalline themselves, some structural anisotropy is required for good alignment in the liquid crystal host and for the generation of linearly polarized light.
    The Journal of Physical Chemistry B 08/2009; 113(31):10575-9. · 3.61 Impact Factor
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    ABSTRACT: 4,4-Difluoro-3,5-bis(phenylamino)-8-(4-tolyl)-4-bora-3a,4a-diaza-s-indacene (1), a symmetric fluorescent difluoroboron dipyrromethene dye, has been synthesized by nucleophilic substitution of 3,5-dichloro-4,4-difluoro-8-(4-tolyl)-4-bora-3a,4a-diaza-s-indacene with aniline. The solvent-dependent spectroscopic and photophysical properties have been investigated by means of UV−vis spectrophotometry and steady-state and time-resolved fluorometry and are compared to those of the nonsymmetrically substituted 5-chloro-4,4-difluoro-3-phenylamino-8-(4-tolyl)-4-bora-3a,4a-diaza-s-indacene (2). A new, generalized treatment of the solvent effect, proposed by Catalán (J. Phys. Chem. B 2009, 113, 5951−5960) and based on a set of four mutually independent, empirical scales (dipolarity, polarizability, acidity, and basicity of the medium) is the most appropriate for describing the solvatochromic shifts of the UV−vis absorption and fluorescence emission of 1 and 2. In contrast to the nonsymmetric compound 2, the symmetric dye 1 has higher fluorescence quantum yields (0.45−0.86) and longer fluorescence lifetimes (3.36−4.03 ns) in all solvents studied and its emission maxima are shifted bathochromically by 45 nm. Large differences are also evident in the redox electrochemistry of the two dyes, with the symmetric analogue 1 possessing a much lower oxidation potential (600 mV) than the nonsymmetric 2. The absorption bandwidths and Stokes shifts of symmetric 1 are much smaller than those for nonsymmetric 2. For both dyes, the value of the fluorescence rate constant, [kf = (1.7 ± 0.3) × 108 s−1], does not depend much on the solvent tested. X-ray diffraction analysis of 1 shows that the BODIPY core possesses a planar structure. Quantum-chemical calculations support the different photophysical behavior of the symmetric derivative 1 vis-à-vis the nonsymmetric 2.
    Journal of Physical Chemistry C - J PHYS CHEM C. 07/2009; 113(27).
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    ABSTRACT: Imidazo[4,5-f]-1,10-phenanthroline and pyrazino[2,3-f]-1,10-phenanthroline substituted with long alkyl chains are versatile ligands for the design of metallomesogens because of the ease of ligand substitution. Whereas the ligands and the corresponding rhenium(I) complexes were not liquid-crystalline, mesomorphism was observed for the corresponding ionic ruthenium(II) complexes with chloride, hexafluorophosphate, and bistriflimide counterions. The mesophases were identified as smectic A phases by high-temperature small-angle X-ray scattering (SAXS) using synchrotron radiation. The transition temperatures depend on the anion, the highest temperatures being observed for the chloride salts and the lowest for the bistriflimide salts. The ruthenium(II) complexes are examples of luminescent ionic liquid crystals.
    Inorganic Chemistry 03/2009; 48(6):2490-9. · 4.59 Impact Factor
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    ABSTRACT: Highly luminescent anionic samarium(III) beta-diketonate and dipicolinate complexes were dissolved in the imidazolium ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C(6)mim][Tf(2)N]. The solubility of the complexes in the ionic liquid was ensured by a careful choice of the countercation of the samarium(III) complex. The samarium(III) complexes that were considered are [C(6)mim][Sm(tta)(4)], where tta is 2-thenoyltrifluoroacetonate; [C(6)mim][Sm(nta)(4)], where nta is 2-naphthoyltrifluoroacetonate; [C(6)mim][Sm(hfa)(4)], where hfa is hexafluoroacetylacetonate; and [choline](3)[Sm(dpa)(3)], where dpa is pyridine-2,6-dicarboxylate (dipicolinate) and [choline](+) is (2-hydroxyethyl)trimethyl ammonium. The crystal structures of the tetrakis samarium(III) beta-diketonate complexes revealed a distorted square antiprismatic coordination for the samarium(III) ion in all three cases. Luminescence spectra were recorded for the samarium(III) complexes dissolved in the imidazolium ionic liquid as well as in a conventional solvent, that is, acetonitrile or water for the beta-diketonate and dipicolinate complexes, respectively. These experiments demonstrate that [C(6)mim][Tf(2)N] is a suitable spectroscopic solvent for studying samarium(III) luminescence. High-luminescence quantum yields were observed for the samarium(III) beta-diketonate complexes in solution.
    Inorganic Chemistry 03/2009; 48(7):3018-26. · 4.59 Impact Factor
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    ABSTRACT: Ionogels are solid oxide host networks confining at a meso-scale ionic liquids, and retaining their liquid nature. Ionogels were obtained by dissolving lanthanide(III) complexes in the ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C6mim][Tf2N], followed by confinement of the lanthanide-doped ionic liquid mixtures in the pores of a nano-porous silica network. [C6mim][Ln(tta)4], where tta is 2-thenoyltrifluoroacetonate and Ln=Nd, Sm, Eu, Ho, Er, Yb, and [choline]3[Tb(dpa)3], where dpa=pyridine-2,6-dicarboxylate (dipicolinate), were chosen as the lanthanide complexes. The ionogels are luminescent, ion-conductive inorganic-organic hybrid materials. Depending on the lanthanide(III) ion, emission in the visible or the near-infrared regions of the electromagnetic spectrum was observed. The work presented herein highlights that the confinement did not disturb the first coordination sphere of the lanthanide ions and also showed the excellent luminescence performance of the lanthanide tetrakis beta-diketonate complexes. The crystal structures of the complexes [C6mim][Yb(tta)4] and [choline]3[Tb(dpa)3] are reported.
    Dalton Transactions 02/2009; · 3.81 Impact Factor
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    ABSTRACT: An asymmetrically substituted fluorescent difluoroboron dipyrromethene (BODIPY) dye, with a phenylamino group at the 3-position of the BODIPY chromophore, has been synthesized by nucleophilic substitution of 3,5-dichloro-8-(4-tolyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene. The solvent-dependent spectroscopic and photophysical properties have been investigated by means of UV-vis spectrophotometry and steady-state and time-resolved fluorometry and reflect the large effect of the anilino substituent on the fluorescence characteristics. The compound has a low fluorescence quantum yield in all but the apolar solvents cyclohexane, toluene, and chloroform. Its emission maxima in a series of solvents from cyclohexane to methanol are red-shifted by approximately 50 nm in comparison to classic BODIPY derivatives. Its oxidation potential in dichloromethane is at ca. 1.14 V versus Ag/AgCl. The absorption bandwidths and Stokes shifts are much larger than those of typical, symmetric difluoroboron dipyrromethene dyes. The values of the fluorescence rate constant are in the (1.4-1.7) x 10(8) s(-1) range and do not vary much between the solvents studied. X-ray diffraction analysis shows that the BODIPY core is planar. Molecular dynamics simulations show that there is no clear indication for aggregates in solution.
    The Journal of Physical Chemistry A 01/2009; 113(2):439-47. · 2.77 Impact Factor
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    ABSTRACT: Ionic liquids were used as solvents for dispersing luminescent lanthanide-doped LaF(3):Ln(3+) nanocrystals (Ln(3+) = Eu(3+) and Nd(3+)). To increase the solubility of the inorganic nanoparticles in the ionic liquids, the nanocrystals were prepared with different stabilizing ligands, i.e., citrate, N,N,N-trimethylglycine (betaine), and lauryldimethylglycine (lauryl betaine). LaF(3):5%Ln(3+) :betaine could successfully be dispersed in 1-butyl-1-methylpyrrolidinium bis(tiifluoromethylsulfonyl)imide [C(4)mpyr][Tf(2)N], 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate [C(4)mpyr][TfO], and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C(4)mim][Tf(2)N] but only in limited amounts. Red photoluminescence was observed for the europium(III)-containing nanoparticles and near-infrared luminescence for the neodymium(III)-containing systems
    Journal of Physical Chemistry C. 01/2009; 113(31):13532-13538.
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    ABSTRACT: We report a non-linear blue shift of the zero-phonon emission line 5D0→7F0 of Eu3+-dopant in Eu3+-doped nano-glass-ceramics in magnetic field up to 50T. The shift is significantly larger in nano-glass-ceramics compared to its precursor glass, suggesting that the nanoceraming of the precursor glass decreases the effective mass of the f-electrons bands of Eu3+ resulting in their enhanced magnetic confinement. Moreover, the non-linear character of the magnetic field dependence of this blue energy shift denotes a spatial confinement of f-electrons wave-functions of Eu3+ dopants in the PbF2-based nanocrystals of the nano-glass-ceramics where up to 90% of the dopants partition. The spatial confinement seems to be due to an admixture to the f-states of Eu3+ of its higher lying states of opposite parity which fall in the quantum well comprising of Eu3+-doped PbF2 crystalline nanoparticles embedded in the surrounding glass network.
    Journal of Luminescence 01/2009; 129(12):1575-1577. · 2.14 Impact Factor
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    ABSTRACT: 4,4-Difluoro-3,5-bis(phenylamino)-8-(4-tolyl)-4-bora-3a,4a-diaza-s-indac ene (1), a symmetric fluorescent difluoroboron dipyrromethene dye, has been synthesized by nucleophilic substitution of 3,5-dichloro-4,4-ifluoro-8-(4-tolyl)-4-bora-3a,4a-diaza-s-indacene with aniline. The solvent-dependent spectroscopic and photophysical properties have been investigated by means of UV-vis spectrophotometry and steady-state and time-resolved fluorometry and are compared to those of the nonsymmetrically substituted 5-chloro-4,4-ifluoro-3-phenylamino-8-(4-tolyl)-4-bora-3a,4a-diaza-s-inda cene (2). A new, generalized treatment of the solvent effect, proposed by Catalan (J. Phys. Chem. B 2009, 113, 5951-5960) and based on a set of four mutually independent, empirical scales (dipolarity, polarizability, acidity, and basicity of the medium) is the most appropriate for describing the solvatochromic shifts of the UV-vis absorption and fluorescence emission of 1 and 2. In contrast to the nonsymmetric compound 2, the symmetric dye 1 has higher fluorescence quantum yields (0.45-0.86) and longer fluorescence lifetimes (3.36-4.03 ns) in all solvents studied and its emission maxima are shifted bathochromically by similar to 45 rim. Large differences are also evident in the redox electrochemistry of the two dyes, with the symmetric analogue 1 possessing a much lower oxidation potential (similar to 600 mV) than the nonsymmetric 2. The absorption bandwidths and Stokes shifts of symmetric 1 are much smaller than those for nonsymmetric 2. For both dyes, the value of the fluorescence rate constant, [k(f) = (1.7 +/- 0.3) x 10(8) s(-1)], does not depend much on the solvent tested. X-ray diffraction analysis of 1 shows that the BODIPY core possesses a planar structure. Quantum-chemical calculations support the different photophysical behavior of the symmetric derivative 1 vis-a-vis the nonsymmetric 2
    Journal of Physical Chemistry C. 01/2009; 113(27):11731-11740.
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    ABSTRACT: N-alkyl-N-methylpyrrolidinium cations have been used for the design of ionic liquid crystals, including a new type of uranium-containing metallomesogen. Pyrrolidinium salts with bromide, bis(trifluoromethylsulfonyl)imide, tetrafluoroborate, hexafluorophosphate, thiocyanate, tetrakis(2- thenoyltrifluoroacetonato)europate(III) and tetrabromouranyl counteranions were prepared. For the bromide salts and tetrabromouranyl compounds, the chain length of the alkyl group C(n)H(2n+1) was varied from eight to twenty carbon atoms (n = 8, 10-20). The compounds show rich mesomorphic behaviour: highly ordered smectic phases (the crystal smectic E phase and the uncommon crystal smectic T phase), smectic A phases, and hexagonal columnar phases were observed, depending on chain length and anion. This work gives better insight into the nature and formation of the crystal smectic T phase, and the molecular requirements for the appearance of this highly ordered phase. This uncommon tetragonal mesophase is thoroughly discussed on the basis of detailed powder X-ray diffraction experiments and in relation to the existing literature. Structural models are proposed for self-assembly of the molecules within the smectic layers. In addition, the photophysical properties of the compounds containing a metal complex anion were investigated. For the uranium-containing mesogens, luminescence can be induced by dissolving them in an ionic liquid matrix. The europium-containing compound shows intense red photoluminescence with high colour purity.
    Chemistry 12/2008; 15(3):656-74. · 5.93 Impact Factor
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    ABSTRACT: A zero-phonon component of the 1.5 μm emission band of Er3+ in the nano-glass-ceramics 32(SiO2)9(AlO1.5)31.5(CdF2)18.5(PbF2)5.5(ZnF2):3.5(ErF3) mol %, shifts to higher energy and splits into Zeeman doublet in external magnetic field. These effects make it possible to flatten the amplification band of the Er3+-doped amplifier. The blue photoluminescence shift can be ascribed to quantum confinement effect on the 1.5 μm emission band of Er3+ in nano-glass-ceramics, where most of the Er3+ dopants are located in the PbF2 nanocrystals of about 8 nm in diameter.
    Applied Physics Letters 04/2008; 92(17):171101-171101-3. · 3.79 Impact Factor
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    ABSTRACT: The photophysical properties of lanthanide complexes have been studied extensively; however, fundamental parameters such as the intrinsic quantum yield as well as radiative and nonradiative decay rates are difficult or even impossible to measure experimentally. Herein, a photoacoustic (PA) method is proposed to determine the intrinsic quantum yield of lanthanide complexes with lifetimes in the order of milliseconds. This method is used to determine the intrinsic quantum yields for europium(III)-containing metallomesogens as well as terbium(III) complexes. The results show that the PA signal is sensitive to both the lifetime and the ratio of the fast-to-slow heat component of the samples. It is found that there is an efficient ligand sensitization and a moderate intrinsic quantum yield for the complexes. The intrinsic quantum yield of Eu(3+) in the metallomesogens exhibits an obvious increase upon the isotropic liquid to smectic A transition. The proposed PA method is quite simple, and can contribute to a clearer understanding of the photophysical processes in luminescent lanthanide complexes.
    ChemPhysChem 04/2008; 9(4):600-6. · 3.35 Impact Factor
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    ABSTRACT: The thermal behavior of the metallomesogens [Ln(C12C16dbm)3(phen)] (Ln = La, Nd, Eu, Gd, Tb, Dy, Er, Yb; C12C16dbm = 1-(4-dodecyloxyphenyl)-3-(4-hexadecyloxyphenyl)propane-1,3-dionate; phen = 1,10-phenanthroline) was investigated. The complexes of the light lanthanides (Ln = La, Nd) are not mesomorphic, whereas the complexes of the heavy lanthanides (Ln = Eu, Gd, Tb, Dy, Er, Yb) exhibit a monotropic smectic A phase (SmA). A model for themolecular organization of the molecules in the smectic A phase is proposed. The luminescence properties of the complexes [Eu(C12C16dbm)3(phen)], [Er(C12C16dbm)3(phen)], and [Yb(C12C16dbm)3(phen)] were investigated. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
    Berichte der deutschen chemischen Gesellschaft 02/2008; 2008(5). · 2.94 Impact Factor
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    K Goossens, P Nockemann, K Driesen
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    ABSTRACT: Ionic liquid crystals were obtained by coupling one or two mesogenic units (cholesterol or cyanobiphenyl) to an imidazolium cation. Anions are bromide, bis(trifluoromethylsulfonyl)imide, and tetrakis(2-thenoyltrifluoroacetonato)europate(III). ...
    01/2008;
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    ABSTRACT: We summarise the experimental data and provide a general theoretical basis for efficient infrared emission in the rare-earth doped transparent oxyfluoride nano-glass-ceramics. In these glass-ceramics, more than 90% fraction of the rare-earth dopant, such as Ho3+, Dy3+, Eu3+, Tm3+, Er3+, dissolve in the cubic β-PbF2 nano-crystals with a certain diameter of the order of 10nm, whilst these nano-crystals are embedded in a robust aluminosilicate glass network. A remarkably low maximal phonon energy coupled to the rare-earth dopant in the β-PbF2 (at about 250cm−1) permits the efficient infrared emission of the dopants from the levels, which are non-radiatively quenched in other glassy hosts. Further advantage of this nano-glass-ceramic material is in its robustness typical of the aluminosilicate glass, which offers fabrication of durable waveguide devices. The site of the rare-earth dopant is proposed to be nearly cubic with 8 fluorine ligands around the dopant resulting in prolonged lifetimes of the lasing levels and admixture of the vibronic coupling to the strength of some electric dipole transitions.
    Journal of Alloys and Compounds 01/2008; 451(1):542-544. · 2.73 Impact Factor
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    ABSTRACT: Lanthanide beta-diketonate complexes are found to exhibit a smectic A and a nematic phase over a broad temperature range. Polarized luminescence is observed for the europium(III) complex dissolved in the nematic liquid-crystal mixture E7 and for the aligned vitrified mesophase
    Advanced Materials. 01/2008; 20(2):252-+.
  • Chemistry of Materials - CHEM MATER. 01/2008; 20(4):1278-1291.