Lanthanide compounds with fluorinated aryloxide ligands: Near-infrared emission from Nd, Tm, and Er

Article abstract of DOI:10.1021/ic8020639

Ln(OC₆F₅)₃ form stable, isolable compounds with 1,2-dimethoxyethane (DME). Monomeric (DME) ₂Ln(OC ₆F₅)₃ (Ln = Nd, Er, Tm) adopt seven coordinate structures with two chelating DME

Full text of DOI:10.1021/ic8020639

Inorg. Chem. 2009, 48, 3573-3580
Lanthanide Compounds with Fluorinated Aryloxide Ligands:
Near-Infrared Emission from Nd, Tm, and Er
Kieran Norton,† G. A. Kumar,‡ Jennifer L. Dilks,† Thomas J. Emge,† Richard E. Riman,*,‡
Mikhail G. Brik,§ and John G. Brennan*,†
Department of Chemistry and Chemical Biology, Rutgers, The State UniVersity of New Jersey, 610
Taylor Road, Piscataway New Jersey 08854-8087, Department of Materials Science &
Engineering, The State UniVersity of New Jersey, 607 Taylor Road, Piscataway, New Jersey
08854-8065, and Institute of Physics, UniVersity of Tartu, Riia 142, Tartu 51014, Estonia
Received October 27, 2008
Ln(OC6F5)3 form stable, isolable compounds with 1,2-dimethoxyethane (DME). Monomeric (DME)2Ln(OC6F5)3 (Ln
) Nd, Er, Tm) adopt seven coordinate structures with two chelating DME and three terminal phenoxide ligands.
Both (py)4Er(OC6F5)3 and (THF)3Yb(OC6F5)3 were also prepared and structurally characterized, with the latter being
a mer-octahedral compound with bond lengths that are geometry dependent. Emission experiments on crystalline
powders of the Nd(III), Tm(III), and Er(III) DME derivatives show that these compounds are highly emissive near-
infrared sources.
Introduction
waveguide fiber amplification to infrared detection and
imaging. Fluorination is useful in these applications as a way
to reduce the number of C-H functional groups that
vibrationally quench NIR emissions.10
Fluorinated ligands impart unique chemical and physical
properties to metal compounds,1 including superior solubility/
volatility properties, unusual crystal packing motifs, and the
stabilization of elements in high oxidation states. In lan-
thanide chemistry, fluorination of organic ligands also leads
to improved near-infrared (NIR) emission characteristics2-4
that are potentially useful5-9 in fields ranging from planar
Lanthanide compounds with a number of fluorinated ligand
systems have been described, including acetates,11 acetonyl-
acetates,12 t-butoxides,13 amidos,14 and thiolates.2-4,15 Of
these, only the fluorinated thiolate compounds have been
probed photophysically, with stimulating results. Compounds
of Er3+, Nd3+, and Tm3+ with SC6F5 ligands have been
prepared in high yield, and in each case these compounds
had NIR quantum efficiencies that were significantly greater
than the efficiencies of previously reported molecular
compounds. Both the absence of CH bonds in the anionic
ligand and the low Ln-S phonon energies are thought to
* To whom correspondence should be addressed. E-mail: bren@
rci.rutgers.edu (J.G.B.), riman@rci.rutgers.edu (R.E.R.).
†
Department of Chemistry and Chemical Biology, Rutgers, The State
University of New Jersey.
‡ Department of Materials Science & Engineering, The State University
of New Jersey.
§
University of Tartu.
(1) (a) Chan, M. C. W. Macromol. Chem. Phys. 2007, 208, 1845. (b)
Dias, H. V. R.; Fianchini, M. Comments Inorg. Chem. 2007, 28, 73.
(c) Plenio, H. ChemBioChem 2004, 5, 650. (d) Witt, M.; Roesky, H. W.
Prog. Inorg. Chem. 1992, 40, 353.
(2) Kumar, G. A.; Riman, R. E.; Diaz Torrez, L. A.; Garcia, O. B.;
Banerjee, S.; Kornienko, A.; Brennan, J. G. Chem. Mater. 2005, 17,
5130.
(3) Banerjee, S.; Huebner, L.; Romanelli, M. D.; Kumar, G. A.; Riman,
R. E.; Emge, T. J.; Brennan, J. G. J. Am. Chem. Soc. 2005, 127, 15900.
(4) Banerjee, S.; Kumar, G. A.; Emge, T. J.; Riman, R. E.; Brennan, J. G.
Chem. Mater. 2008, 20, 4367.
(5) Rare Earth Doped Fiber Lasers and Amplifiers; Digonnet, M. J. F.,
Ed.; Marcel Dekker Inc: New York, 1993.
(9) Becker, P. C.; Olsson, N. A.; Simpson, J. R. Erbium doped fiber
amplifiers- Fundamentals and Technology; Academic Press: New
York, 1999.
(10) (a) Riman, R. E.; Dejneka, M.; Ballato, J.; Snitzer, E. Eur. J. Solid
State Inorg. Chem. 1995, 32, 873. (b) Ballato, J; Dejneka, M.; Snitzer,
E.; Riman, R. E.; Zhou, W. J. Mat. Res. 1996, 11, 841. (c) Quochi,
F.; Orru, R.; Cordella, F.; Mura, A.; Bongiovanni, G.; Artizzu, F.;
Deplano, P.; Mercuri, M. L.; Pilia, L.; Serpe, A. J. Appl. Phys. 2006,
99, 053520/1–053520/4.
(11) (a) Fujihara, S.; Kato, T.; Kimura, T. J. Sol-Gel Sci. Technol. 2003,
26, 953. (b) Bombieri, G.; Benetollo, F.; Del Pra, A.; Oliveira, V. da
Silva; Melo, D. M. A.; Zinner, L. B.; Vicentini, G. J. Alloys Cmpd.
2001, 323, 181. (c) Kang, S. J.; Jung, Y. S.; Sohn, Y. S. Bull. Korean
Chem. Soc. 1997, 18, 75. (d) Bravo-Vasquez, J. P.; Ching, L. W. C.;
Law, W. L.; Hill, R. H. J. Photopolym. Sci. Technol. 1998, 11, 589.
(6) Jha, A. Proc. SPIE 2006, 6409, 640918.
(7) Soko´lska, I.; Ryba-Romanowski, W.; Golab, S.; Baba, M.; Wirkowiczc,
M.; Ukasiewicz, T. J. Phys. Chem. Solids 2000, 61, 1573.
(8) de Camargo, A. S. S.; Nunes, L. A. O.; Botero, E. R.; Garcia, D.;
Eiras, J. A. Chem. Phys. Lett. 2005, 410, 156.
10.1021/ic8020639 CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 8, 2009 3573
Published on Web 03/24/2009

Norton et al.
MA) were purified with a dual-column Solv-Tek solvent purification
system (Solv-Tek Inc., Berryville, VA). Ln chips (Nd, Tm, Yb),
Er powder, Hg (Strem Chemicals), PhSSPh (Acros), and HOC6F5
(Aldrich) were purchased and used as received. Melting points were
recorded in sealed capillaries and are uncorrected. IR spectra were
taken on a Thermo Nicolet Avatar 360 FTIR spectrometer, and
recorded from 4000-600 cm-1 as a Nujol mull on NaCl plates.
Elemental analyses were performed by Quantitative Technologies,
Inc. (Whitehouse Station, NJ).
minimize competitive vibrational relaxation pathways, and
this in turn results in the formation of exceptionally emissive
molecules.
The fluorinated phenoxide OC6F5 has been used frequently
in main group16 and transition metal17 systems because it is
a water stable anion,18 it gives compounds that are soluble
in organic solvents, and it is commercially available. Lan-
thanide complexes with OC6F5 anions have been described
only for Eu(II), Eu(III), and a heterovalent Eu(II)/Eu(III)
dimer, with structural characterization of the products
revealing interesting π-π stacking interactions and dative
Eu(II)-F bonds.19 Application of the OC6F5 ligand to NIR
emissive Ln is clearly warranted because the added air
stability of this ligand relative to SC6F5 would be useful in
materials fabrication, as long as higher energy Ln-O
vibrations do not quench excited states. In this work, we
extend the lanthanide chemistry of the OC6F5 ligand to the
synthesis and characterization of thermally stable compounds
of Nd, Er, and Tm, and investigate the NIR emission
properties of powdered samples.
Synthesis of (DME)2Nd(OC6F5)3 (1). Nd (145 mg, 1.01 mmol),
diphenyl disulfide (330 mg, 1.51 mmol), and Hg (20 mg, 0.1 mmol)
were combined in DME (20 mL) and stirred for 48 h to give a
light blue solution with blue precipitate. Pentafluorophenol (540
mg, 2.93 mmol) was added, and the solution was stirred for 5 days,
resulting in a pale blue solution with a dark brown precipitate. The
solution was filtered, concentrated to 5 mL, and layered with
hexanes (10 mL) to give blue crystals (524 mg, 62%) that melted
and turned purple at 104-107 °C. Anal. Calcd for C26H20F15O7Nd:
C, 35.7; H, 2.29. Found: C, 35.7; H, 2.42. IR: 2960 (m), 2849 (w),
2668 (w), 2478 (w), 1650 (m), 1621 (w), 1509 (s), 1370 (w), 1309
(m), 1260 (m), 1172 (m), 1094 (s), 1018 (s), 858 (m), 799 (m),
721 (w), 635 (m) cm-1
.
Synthesis of (DME)2Er(OC6F5)3 (2). Er (165 mg, 0.988 mmol),
diphenyl disulfide (326 mg, 1.50 mmol), and Hg (10 mg, 0.05
mmol) were combined in DME (20 mL) and stirred for 4 days to
give a pink solution. Pentafluorophenol (550 mg, 2.99 mmol) was
added, and the solution was stirred for 2 days to give a pink solution
with a black precipitate. The solution was filtered, concentrated to
5 mL, and layered with hexanes (20 mL) to give pink crystals (529
mg, 60%) that melted at 122-129 °C. Anal. Calcd for
C26H20F15O7Er: C, 34.8; H, 2.23. Found: C, 34.7; H, 2.42. IR: 2959
(m), 1652 (w), 1505 (s), 1475 (m), 1309 (w), 1246 (m), 1179 (m),
1098 (m), 1050 (s), 1022 (s), 988 (s), 873 (m), 862 (s), 800 (w),
Experimental Section
General Methods. All syntheses were carried out under ultra
pure nitrogen (WELCO CGI, Pine Brook, NJ), using conventional
dry box or Schlenk techniques. Solvents (Fisher Scientific, Agawam,
(12) (a) Malandrino, G.; Incontro, O.; Castelli, F.; Fragalà, I. L.; Benelli,
C. Chem. Mater. 1996, 8, 1292. (b) Plakatouras, J. C.; Baxter, I.;
Hursthouse, M. B.; Abdul Malik, K. M.; McAleese, J.; Drake, S. R.
J. Chem. Soc., Chem. Commun. 1994, 2455. (c) Rogachev, A. Y.;
Minacheva, L. K.; Sergienko, V. S.; Malkerova, I. P.; Alikhanyan,
A. S.; Stryapan, V. V.; Kuzmina, N. P Polyhedron 2005, 24, 723. (d)
Condorelli, G. G.; Gennaro, S.; Fragalà, I. L. Chem. Vapor Dep. 2001,
7, 151. (e) Condorelli, G. G.; Gennaro, S.; Fragalà, I. L. Chem. Vapor
Dep. 2000, 6, 185. (f) Condorelli, G. G.; Anastasi, G.; Fragalà, I. L.
Chem. Vapor Dep. 2005, 11, 324. (g) Beach, D. B.; Collins, R. T.;
Legoues, F. K.; Chu, J. O. MRS Symp. Proc. 1993, 282, 397. (h) Hirata,
G. A.; McKittrick, J.; Yi, J.; Pattillo, S. G.; Salazar, K. V.; Trkula,
M. MRS Symp. Proc. 1998, 495, 39.
(13) Bradley, D. C.; Chudzynska, H.; Hursthouse, M. B.; Motevalli, M.;
Wu, R. Polyhedron 1994, 13, 1.
(14) Click, D. R.; Scott, B. L.; Watkin, J. G. Chem. Commun. 1999, 633.
(15) (a) Melman, J.; Rhode, C.; Emge, T. J.; Brennan, J. G. Inorg. Chem.
2002, 41, 28. (b) Melman, J.; Emge, T. J.; Brennan, J. G. Inorg. Chem.
2001, 40, 1078.
636 (m) cm-1
.
Synthesis of (DME)2Tm(OC6F5)3 (3). Tm (167 mg, 0.998
mmol), diphenyl disulfide (328 mg, 1.50 mmol), and Hg (10 mg,
0.05 mmol) were combined in DME (20 mL) and stirred for 4 days
to give a tan solution. Pentafluorophenol (550 mg, 2.99 mmol) was
added, and the solution was stirred for 2 days resulting in a pale
tan solution with a black precipitate. The solution was filtered,
concentrated to 5 mL, and layered with hexanes (20 mL) to give
colorless crystals (589 mg, 65%) that melted at 114-121 °C. Anal.
Calcd for C26H20F15O7Tm: C, 34.7; H, 2.23. Found: C, 34.3; H,
2.15. IR: 2961 (m), 1650 (m), 1620 (w), 1503 (s), 1370 (w), 1309
(m), 1260 (m), 1177 (m), 1092 (s), 1048 (m), 1018 (s), 988 (s),
862 (m), 800 (m) cm-1. Unit cell at 100 K from single crystal X-ray
diffraction data: P21/n, a ) 10.3977(7) Å, b ) 12.7197(8) Å, c )
23.4256(15) Å, ꢀ ) 96.483(1), V ) 3078.4(3) Å3.
(16) (a) Britovsek, G. J. P.; Ugolotti, J.; White, A. J. P. Organometallics
2005, 24, 1685. (b) Metz, M. V.; Sun, Y.; Stern, C. L.; Marks, T. J.
Organometallics 2002, 21, 3691. (c) Whitmire, K. H.; Hoppe, S.;
Sydora, O.; Jolas, J. L.; Jones, C. M. Inorg. Chem. 2000, 39, 85. (d)
Jolas, J. L.; Hoppe, S.; Whitmire, K. H. Inorg. Chem. 1997, 36, 3335.
(e) Jones, C. M.; Burkart, M. D.; Bachman, R. E.; Serra, D. L.; Hwu,
S. J.; Whitmire, K. H. Inorg. Chem. 1993, 32, 5136.
Synthesis of (py)4Er(OC6F5)3 (4). Er (165 mg, 0.988 mmol),
diphenyl disulfide (325 mg, 1.49 mmol), and Hg (13 mg, 0.05
mmol) were combined in pyridine (30 mL), and the mixture was
stirred for 5 days to give a pink solution. Pentafluorophenol (562
mg, 3.05 mmol) was added, and the solution was stirred for 7 days
resulting in a pink solution with a black precipitate. The solution
was filtered, concentrated to 5 mL, and layered with hexanes (5
mL) to give pink crystals (0.31 g, 30%) that melted at 127-130
°C. Anal. Calcd for C38H20F15N4O3Er: C, 44.2; H, 1.95; N, 5.42.
Found: C, 43.7; H, 2.00; N, 5.46. IR: 2962 (w), 1650 (m), 1602
(m), 1504 (s), 1444 (m), 1306 (m), 1260 (m), 1220 (w), 1172 (m),
1093 (w), 1068 (m), 1040 (w), 1016 (s), 988 (s), 799 (m), 752
(17) (a) Tremblay, T. L.; Ewart, S. W.; Sarsfield, M. J.; Baird, M. C. Chem.
Commun. 1997, 831. (b) Campbell, C.; Bott, S. G.; Larsen, R.; Van
Der Sluys, W. G. Inorg. Chem. 1994, 33, 4950. (c) Amor, J. I.; Burton,
N. C.; Cuenca, T.; Gomez-Sal, P.; Royo, P. J. Organomet. Chem.
1995, 485, 153. (d) Abbott, R. G.; Cotton, F. A.; Falvello, L. R. Inorg.
Chem. 1990, 29, 514. (e) Dilworth, J. R.; Gibson, V. C.; Redshaw,
C.; White, A. J. P.; Williams, D. J. J. Chem. Soc., Dalton Trans.:
Inorg. Chem. 1999, 2701. (f) Metz, M. V.; Sun, Y.; Stern, C. L.; Marks,
T. J. Organometallics 2002, 21, 3691. (g) Ferreira Lima, G. L.; Araujo
Melo, D. M.; Isolani, P. C.; Thompson, L. C.; Zinner, L. B.; Vicentini,
G. Anais Assoc. Brasil. Quim. 2000, 49, 153. (h) Kim, M.; Zakharov,
L. N.; Rheingold, A. L.; Doerrer, L. H. Polyhedron 2005, 24, 1803.
(i) Buzzeo, M. C.; Iqbal, A. H.; Long, C. M.; Millar, D.; Patel, S.;
Pellow, M. A.; Saddoughi, S. A.; Smenton, A. L.; Turner, J. F. C.;
Wadhawan, J. D.; Compton, R. G.; Golen, J. A.; Rheingold, A. L.;
Doerrer, L. H. Inorg. Chem. 2004, 43, 7709.
(m), 701 (m), 625 (m) cm-1
.
(18) (a) Ramirez, F.; Marecek, J. F. Synthesis 1979, 71. (b) Kresge, A. J.
Chem. Soc. ReV. 1973, 475.
(19) Norton, K.; Emge, T. J.; Brennan, J. G. Inorg. Chem. 2007, 46, 4060.
Synthesis of (THF)3Yb(OC6F5)3 (5). Yb (0.193 g, 1.12 mmol),
PhSSPh (0.370 g, 1.70 mmol), and Hg (0.016 g, 0.080 mmol) were
3574 Inorganic Chemistry, Vol. 48, No. 8, 2009

Lanthanide Compounds with Fluorinated Aryloxide Ligands
Table 1. Summary of Crystallographic Details for 1, 2, 4, and 5a
compound
1
2
4
5
empirical formula
fw
C26H20F15NdO7
873.66
C26H20ErF15O7
896.68
P21/n
10.3977(7)
12.7197(7)
23.426(2)
96.483(1)
3078.4(3)
4
1.935
2.860
0.0319
0.0779
C38H20ErF15N4O3
1032.84
P21/c
11.9793(6)
15.5793(8)
20.030(1)
90.471(1)
3738.1(3)
4
1.835
2.365
0.0201
0.0475
C30H24F15O6Yb
938.53
C2/c
11.423(1)
17.907(2)
16.435(1)
92.225(1)
3359.2(5)
4
1.856
2.909
0.0258
0.0640
space group
a (Å)
b (Å)
c (Å)
ꢀ (deg)
V (Å3)
Z
P212121
10.3931(2)
13.1499(3)
44.970(1)
90.00
6146.0(2)
8
1.888
D(calcd) (g/cm-3
)
abs coeff (mm-1
)
1.824
0.0353
0.0765
R(F)b [I > 2 σ(I)]
Rw(F2)c [I > 2 σ(I)]
a All data were collected at 100(2)°K using Mo KR radiation (λ ) 0.71073 Å). b R(F) ) ∑||Fo| - |Fc||/∑|Fo|. c Rw(F2) ) {∑[w(Fo2 - Fc2)2]/∑w(Fo )2}1/2
.
2
Additional crystallographic details are given in the Supporting Information.
added to THF (20 mL), and the mixture was stirred for 6 days at
room temperature to give an opaque dark red solution. C6F5OH
(0.635 g, 3.45 mmol) was added, and the reaction was stirred for
8 days and then filtered to separate trace gray precipitate. The
colorless solution was concentrated to about 5 mL and layered with
hexanes (∼10 mL) to give translucent colorless crystals that turn
white at 179 °C and melt/turn yellow at 195-199 °C. Anal. Calcd
for C30H24YbF15O6: C, 38.4; H, 2.58. Found: C, 37.9; H, 2.38. The
UV-vis (THF) spectrum contained one peak at 313 nm (ε ) 1 ×
102 L mol-1 cm-1). IR: 2963 (w), 2283 (w), 1655 (w), 1509 (s),
1384 (s), 1260 (m), 1173 (w), 1019 (m), 992 (m), 800 (m) cm-1
.
Unlike the analogous fluorinated thiolate derivative that reacts
instantly with water to form a lightly colored precipitate, this
compound does not appear to react with water at room temperature.
X-ray Structure Determination. Data for compounds 1, 2, 4,
and 5 were collected on a Bruker Smart APEX CCD diffracto-
meter with graphite monochromatized Mo KR radiation (λ )
0.71073Å) at 100 K. Crystals were immersed in Paratone oil
and examined at low temperatures. The data were corrected for
Lorenz effects and polarization, and absorption, the latter by a
multiscan (SADABS)20 method. The structures were solved by
direct methods (SHELXS86).21 All non-hydrogen atoms were
refined (SHELXL97)22 based upon Fobs2. All hydrogen atom
coordinateswerecalculatedwithidealizedgeometries(SHELXL97).
Scattering factors (fo, f′, f′′) are as described in SHELXL97.
Crystallographic data and final R indices for 1, 2, 4, and 5 are
given in Table 1. POV-ray diagrams23 for 1, 2, 4, and 5 are
shown in Figures 1-4 respectively. Complete crystallographic
details are given in the Supporting Information.
Figure 1. POVRAY diagram of the two crystallographically independent
molecules within the unit cell of (DME)2Nd(OC6F5)3, with green F, white
C, and the hydrogen atoms removed for clarity. Significant distances
(Å) and angles (deg): Nd(1)-O(1), 2.238(2); Nd(1)-O(2), 2.272(2);
Nd(1)-O(3), 2.287(2); Nd(1)-O(7), 2.511(2); Nd(1)-O(4), 2.512(2);
Nd(1)-O(6), 2.544(2); Nd(1)-O(5), 2.546(2); C(1)-O(1)-Nd(1),
166.9(2); C(7)-O(2)-Nd(1), 152.7(2); C(13)-O(3)-Nd(1), 129.4(2);
Nd(2)-O(8), 2.241(2); Nd(2)-O(10), 2.271(2); Nd(2)-O(9), 2.273(2);
Nd(2)-O(11), 2.488(2); Nd(2)-O(12), 2.532(2); Nd(2)-O(13), 2.539(2);
Nd(2)-O(14),2.547(2);C(27)-O(8)-Nd(2),163.2(2);C(33)-O(9)-Nd(2),
152.1(2); C(39)-O(10)-Nd(2), 129.5(2).
Spectroscopy. Absorption measurements were carried out with
crystalline powder using an integrating sphere of a double beam
spectrophotometer (Perkin-Elmer Lambda 9, Wellesley, MA).The
emission spectra of the powdered samples were recorded by exciting
the sample with the 800 nm band of a Ti-Sapphire laser (for Nd
and Tm) and with a 980 nm diode laser for the Er sample. The
emission from the sample was focused onto a 0.55 m monochro-
mator (Jobin Yvon, Triax 550, Edison, NJ) and detected by a
thermoelectrically cooled InGaAs detector. The signal was intensi-
fied with a lock-in amplifier (SR 850 DSP, Stanford Research
Figure 2. POVRAY diagram of (DME)2Er(OC6F5)3, with green fluorine,
white carbon, and the hydrogen atoms removed for clarity.Significant
distances (Å) and angles (deg): Er(1)-O(1), 2.150(2); Er(1)-O(2), 2.180(2);
Er(1)-O(3), 2.183(2); Er(1)-O(4), 2.390(2); Er(1)-O(6), 2.413(2);
Er(1)-O(5), 2.423(2); Er(1)-O(7), 2.426(2); C(1)-O(1)-Er(1), 164.7(2);
C(7)-O(2)-Er(1), 149.2(2); C(13)-O(3)-Er(1), 132.7(2).
(20) SADABS, Bruker Nonius area detector scaling and absorption
correction, v2.05; Bruker-AXS Inc.: Madison, WI, 2003.
(21) Sheldrick, G. M. SHELXS86, Program for the Solution of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1986.
(22) Sheldrick, G. M. SHELXL97, Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(23) Persistence of Vision Raytracer, Version 3.6; Persistence of Vision
Pty. Ltd.: 2004; [Computer software retrieved from http://www.
povray.org/download/].
System, Sunnyvale, CA) and processed with a computer controlled
by SynerJY commercial software. To measure the decay time, the
laser beam was modulated by a chopper, and the signal was
collected on a digital oscilloscope (TDS 220, 200 MHz, Tektronix,
Beavetron, OR). The emission data analysis follows our earlier
work.24
Inorganic Chemistry, Vol. 48, No. 8, 2009 3575

Norton et al.
Table 2. Experimentally Observed Band Positions, Their Integrated
Absorbance, Experimental and Calculated Oscillator Strengths for
(DME)2Nd(OC6F5)3
energy
(cm-1
∫R(λ) dλ
f(exp)
af(cal)
transitions
)
(10-7
)
(10-6
)
(10-6
)
2P3/2
25189
24450
23095
21322
19531
17889
16367
13831
12937
11848
2.14
1.14
1.09
0.98
1.83
18.4
58.2
1.82
20.2
22.1
0.119
0.059
0.051
0.039
0.061
0.516
1.363
0.031
0.295
0.272
excluded
0.0030
0.0720
0.0470
0.3236
0.3264
1.3605
0.0509
0.2556
0.2134
2D5/2 +2P3/2
2P1/2
4G11/2 + G9/2 + K15/2
2
2
4G7/2 + G9/2
4
2K13/2 + G7/2
2
4G5/2 + H11/2
2
4F7/2 + S3/2
4
4F5/2 + H9/2
2
4F3/2
Figure 3. POVRAY diagram of (py)4Er(OC6F5)3, with green fluorine, white
carbon, and the hydrogen atoms removed for clarity.Significant distances
(Å) and angles (deg): Er(1)-O(3), 2.164(1); Er(1)-O(2), 2.182(1);
Er(1)-O(1), 2.186(1); Er(1)-N(1), 2.4919(13); Er(1)-N(4), 2.5175(14);
Er(1)-N(2), 2.5446(13); Er(1)-N(3), 2.5464(13); C(13)-O(3)-Er(1),
168.8(1); C(7)-O(2)-Er(1), 160.0(1); C(1)-O(1)-Er(1), 149.1(1).
a Ω2 ) 0.388 × 10-20 cm2, Ω4 ) 0.539 × 10-20 cm2, and Ω6 ) 0.327
× 10-22 cm2; RMS )1.37 × 10-7
.
Table 3. Experimentally Observed Band Positions, Their Integrated
Absorbance, Experimental and Calculated Oscillator Strengths for
(DME)2Er(OC6F5)3
energy
(cm-1
∫R(λ) dλ
f(exp)
af(cal)
band
4G11/2
)
(10-7cm)
(10-6
)
(10-6
)
24631
23810
22321
20202
18832
17123
14409
11905
2.91
12.0
1.36
1.44
3.27
1.05
4.38
1.01
6.82
26.2
2.62
2.27
4.49
1.19
3.52
7.13
excluded
1.58
3.20
3.93
0.76
3.23
0.328
4F3/2
4
4F5/2 + F3/2
4F7/2
2H11/2
4S3/2
4F9/2
4I9/2
0.554
a Ω2 ) 1.54 × 10-20 cm2, Ω4 ) 2.29 × 10-20 cm2 and Ω6 ) 2.05 ×
10-20 cm2; RMS ) 0.81 ×10-6; Fluorescence decay time )1 ms, Radiative
decay time ) 6.17 ms, QE ) 16%.
Figure 4. POVRAY diagram of (THF)3Yb(OC6F5)3, with green fluorine,
white carbon, and the hydrogen atoms removed for clarity.Significant
distances (Å) and angles (deg): Yb(1)-O(1), 2.084(2); Yb(1)-O(2),
2.111(2); Yb(1)-O(2)′, 2.111(2); Yb(1)-O(4)′, 2.296(2); Yb(1)-O(4),
2.296(2);Yb(1)-O(3),2.332(2);O(1)-Yb(1)-O(2),95.76(4);O(1)-Yb(1)-O(2)′,
95.76(4); O(2)-Yb(1)-O(2)′, 168.48(9); O(1)-Yb(1)-O(4)′, 97.35(4);
O(2)-Yb(1)-O(4)′,88.07(7);O(2)′-Yb(1)-O(4)′,90.46(7);O(1)-Yb(1)-O(4),
97.35(4); O(2)-Yb(1)-O(4), 90.46(7); O(2)′-Yb(1)-O(4), 88.07(7);
O(4)′-Yb(1)-O(4), 165.30(9); O(1)-Yb(1)-O(3), 180.0; O(2)-Yb(1)-O(3),
84.24(4); O(2)′-Yb(1)-O(3), 84.24(4); O(4)′-Yb(1)-O(3), 82.65(4);
O(4)-Yb(1)-O(3), 82.65(4); C(1)-O(1)-Yb(1), 180.0; C(5)-O(2)-Yb(1),
162.3(1).
(py)4Er(OC6F5)3 (4) was isolated. Figure 3 contains a
POVRAY diagram of 4, with significant distances and angles
given in the figure caption. This seven coordinate molecule
is best described as a pentagonal bipyramid, and as found
in the DME compounds, there are only terminal phenoxide
ligands with no significant Ln-F interactions. The THF
compound (THF)3Yb(OC6F5)3 was also isolated, and low
temperature diffraction experiments showed that the molecule
adopts a meridional conformation. Figure 4 contains a
POVRAY diagram of 5, with significant distances and angles
given in the figure caption. In this molecule, there are
Ln-O(C6F5) bonds trans to both phenolate and neutral THF
donors, and there appears to be a slight but statistically
significant dependence of Ln-O bond length upon the
identity of the trans ligands. The two crystallographically
related Ln-O(C6F5) bonds that are trans to phenolates
(Ln-O(2) ) 2.111(2) Å) are longer by 0.027 Å than is the
Ln-O phenlolate bond trans to THF (2.084(2) Å). The same
trend is noted in the bonds to THF, where the Ln-O(C4H8)
bond trans to the phenolate 2.332(2) Å is 0.036 Å longer
than the Ln-O bonds trans to THF (2.296(2) Å).
The room temperature absorption spectra for the NIR
active Nd, Er, and Tm compounds (1, 2, and 3, respectively)
are shown in Figures 5a, 6a, and 7a, respectively. The
absorption bands for 1, 2, and 3 are identified using the
standard notations summarized in Tables 2, 3, and 4,
respectively. All absorption profiles were numerically inte-
grated to evaluate the experimental oscillator strengths. The
measured oscillator strengths were fitted with theoretical
oscillator strength values to obtain the three phenomenologi-
Results
Fluorinated phenoxides of the NIR emissive lanthanides
are prepared at room temperature in high yields by meta-
thetical reactions of in situ prepared Ln(SPh)3 with HOC6F5.
Thermally stable DME derivatives (DME)2Ln(OC6F5)3 were
isolated for Ln ) Nd(1), Er(2), and Tm(3) and characterized
by low temperature single crystal X-ray diffraction. All three
compounds are isomorphous, with the Nd compound crys-
tallographically distinct from the isostructural Er and Tm
compounds. Figures 1 and 2 contain POVRAY diagrams for
the Nd and Er/Tm compounds, respectively, with significant
bond geometries given in the figure captions. In both 1 and
2, the seven coordinate Ln bond to three terminal aryloxides
and four neutral oxygen donors from two chelating DME
ligands. There are no significant Ln-F dative interactions.
THF and pyridine derivatives were also prepared in an
attempt to isolate octahedral compounds that might show
anomalous bond length distributions. From pyridine,
(24) Kumar, G. A.; Riman, R. E.; Torres, L. A. D.; Banerjee, S.; Romanelli,
M. D.; Emge, T. J.; Brennan, J. G Chem. Mater. 2007, 19, 2937.
3576 Inorganic Chemistry, Vol. 48, No. 8, 2009

Lanthanide Compounds with Fluorinated Aryloxide Ligands
Figure 6. Absorption (a), emission (b), and fluorescence decay (c) curves
of (DME)2Er(OC6F5)3.
experimental decay time (τeff), together with the calculated
Judd-Ofelt25 radiative decay time of 8.6 ms for 1, results
in a calculated quantum efficiency of nearly 2% for 1. For
2 an efficiency of 16% was obtained (1552 nm with a
radiative decay time of 6.16 ms), and for 3 the efficiencies
of the two bands are 1.9% (1458 nm) and 4.5% (1759 nm).
Assuming a Gaussian line shape the stimulated emission
cross sections of the emission transitions can be evaluated
using the Füchtbauer-Ladenburg equation26
Figure 5. Absorption (a), emission (b), and fluorescence decay (c) curves
of (DME)2Nd(OC6F5)3.
cal intensity parameters. The parameters obtained for 1 are
Ω2 ) 0.388 × 10-20 cm2, Ω4 ) 0.539 × 10-20 cm2, Ω6 )
0.327 × 10-22 cm2, for 2 are Ω2 ) 15.5 × 10-20 cm2, Ω4)
3.28 × 10-20 cm2, Ω6 ) 4.62 × 10-20 cm2, and for 3 are Ω2
)1.54 × 10-20 cm2, Ω4)2.29 × 10-20 cm2, and Ω6 )2.05
× 10-20 cm2 with rms values of 0.137 × 10-6 (Nd) and 2.04
× 10-6 (Tm) and 0.81 × 10-6 (Er) between the experimental
and theoretical oscillator strengths.
λ4A
8πcn2∆λeff
σem
)
NIR Emission spectra of the three compounds are shown
in Figures 5b (Nd), 6b (Er), and 7b (Tm). All three
compounds show emission with characteristic band positions,
that is, Nd3+ at 928 (4F3/2 f 4I9/2), 1059 (4F3/2 f 4I11/2), and
where ∆λeff is the effective line-width of the emission band
obtained by integrating over the entire emission band and
dividing by the peak fluorescence intensity. According to
the above equation the calculated emission cross sections
are 0.061 × 10-20 cm2 (928 nm), 0.082 × 10-20 cm2 (1059
nm) for 1, (1552 nm) 0.54 × 10-20cm2 and 0.9 × 10-20 cm2
(1458 nm) for 2, and 6.5 × 10-20cm2 (1759 nm) for 3. Tables
5 and 6 summarize the radiative spectral properties of 1 and
3. The emission cross sections are greater than the values
reported for either crystalline or amorphous inorganic materi-
4
3
1327 nm (4F3/2 f I13/2); Tm3+ at 1458 (3H4 f H5), 1759
(3F4 f H6), and Er3+ at 1550 nm (4I13/2 f I15/2). Relative
intensities are not comparable because of differences in
sample concentrations.
3
4
To measure the quantum efficiency of the main emission
transition the effective fluorescence decay times (τeff) for 1-3
were extracted from the measured decay curves shown in
Figures 5c, 6c, and 7c, respectively. Decay curves were fitted
with the exponential curve fit to yield decay times of 170
µs for the 1059 nm emission from 1, 1 ms for the 1552 nm
emission band of 2, and 145 and 127 µs, respectively for
the 1458 and 1759 nm bands from 3. The effective
(25) (a) Judd, B. R. Phys. ReV. B 1962, 127, 750. (b) Ofelt, G. S. J. Chem.
Phys. 1962, 37, 511.
(26) Kaminskii, A. A. Laser Crystals: Their Physics and Properties;
Springer Verlag: New York, 1981.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3577

Norton et al.
first solubilizing Ln using the in situ preparation of Ln(SPh)3,
followed by a proton transfer reaction with HOC6F5 to give
Ln(OC6F5)3 [reactions 1 and 2]. Structural characterization
of the DME adducts
3
2
Ln + PhSSPh f “Ln(SPh)3”
(1)
“Ln(SPh)3” + HOC6F5 f Ln(OC6F5)3 + 3HSPh (2)
revealed essentially the same seven coordinate structures as
found in the fluorinated thiolates, with chelating DME ligands
and terminal aryloxides. The pyridine compound 4 similarly
contained four neutral donor atoms saturating the Er(III)
coordination sphere with only terminal phenoxide ligands
being observed.
An octahedral derivative was isolated only when the
sterically more demanding THF ligand was coupled with the
smallest of the Ln to generate mer-octahedral 5. This
geometry was targeted to examine whether bond lengths in
these fluorinated phenoxides would exhibit any directional
dependence. The possibility that lanthanide compounds might
exhibit structural trans-influence was noted at least as early
as 1995 by Deacon,28 who compared a pair of isomorphous
aryloxide compounds with different neutral donor ligands,
and by our laboratory,29 in the description of mer-
(py)3Yb(SPh)3. At that time, these observations conflicted
with the standard view of structure and bonding in lanthanide
systems,30 namely that bond lengths could be predicted and
rationalized by summing the ionic radii of the lanthanide
ion and the donor ligand,31 with the metal ionic radius
dependent only on charge and coordination number. In
subsequent years, fac-octahedral halide coordination com-
pounds,32 chalcogenolate complexes,33 and chalcogenido
based cluster compounds with octahedral Ln geometries34
have been described. In every case, an analysis of the Ln-
anion bond length distribution revealed that bonds trans to
anions are consistently longer than bonds trans to neutral
donors. At first glance, compound 5 also exhibits this
directional bonding effect, with the Ln-O(C6F5) bonds trans
to phenoxides being significantly longer than the Ln-O bond
trans to the neutral THF donor.
Figure 7. Absorption (a), emission (b), and fluorescence decay (c) curves
of (DME)2Tm(OC6F5)3.
Table 4. Experimentally Observed Band Positions, Their Integrated
Absorbance, Experimental and Calculated Oscillator Strengths for
(DME)2Tm(OC6F5)3
energy
(cm-1
∫R(λ) dλ
f(exp)
af(cal)
transitions
)
(10-7
)
(10-6
)
(10-6
)
1D2
28169
21598
14619
12722
7235
1.10
5.41
6.60
10.9
14.5
28.1
5.60
16.1
9.02
11.3
4.85
5.49
4.63
excluded
9.97
9.57
4.34
1G4
3F2 +3F3
3H4
3H5
3F4
5530
7.32
a Ω2 )15.5 × 10-20 cm2, Ω4 ) 3.28 × 10-20 cm2 and Ω6 ) 4.62 ×
10-20 cm2; RMS ) 2.04 ×10-6
.
als.27 The higher stimulated emission cross section results
from the lower effective bandwidths, compared to amorphous
materials that have a range of 60-80 nm. The narrow
bandwidth and high stimulated emission cross sections
facilitates the application of these materials as high gain
optical amplifiers.
Unfortunately, the tendency of these phenoxides to engage
in π-π stacking interactions makes it more difficult to
unambiguously interpret Ln-O distances. In all of the seven
coordinate structures 1, 2, and 4, there are distributions of
(28) Deacon, G. B.; Feng, T.; Skelton, B. W.; White, A. H. Aust. J. Chem.
1995, 48, 741.
Discussion
(29) Lee, J.; Brewer, M.; Berardini, M.; Brennan, J. Inorg. Chem. 1995,
34, 3215.
Stable adducts of the trivalent fluorinated phenoxides
Ln(OC6F5)3 can be isolated in high yields as either DME,
THF, or pyridine complexes. The metals react incompletely
with HOC6F5, but reactions can be driven to completion by
(30) Raymond, K. N.; Eigenbrot, C. W. Acc. Chem. Res. 1980, 13, 276.
(31) Shannon, R. D. Acta Crystallogr. A 1976, 32, 751.
(32) (a) Deacon, G. B.; Feng, T.; Junk, P. C.; Skelton, B. W.; Sobolev,
A. N.; White, A. H. Aust. J. Chem. 1998, 51, 75. (b) Deacon, G. B.;
Feng, T.; Junk, P. C.; Meyer, G.; Scott, N. M.; Skelton, B. W.; White,
A. H. Aust. J. Chem. 2000, 53, 853.
(33) (a) Aspinall, H. C.; Cunningham, S. A.; Maestro, P.; Macaudiere, P.
Inorg. Chem. 1998, 37, 5396. (b) Mashima, K.; Nakayama, Y.;
Fukumoto, H.; Kanehisa, N.; Kai, Y.; Nakamura, A. J. Chem. Soc.,
Chem. Commun. 1994, 2523.
(27) (a) Yang, Z.; Luo, L.; Chen, W. J. Appl. Phys. 2006, 99, 076107. (b)
Truong, V. G.; Jurdyc, A. M.; Jacquier, B.; Ham, B. S.; Quang,
A. Q. L.; Leperson, J.; Nazabal, V.; Adam, J. L. J. Opt. Soc. Am. B.
2006, 23, 2588. (c) CanoTorres, J. M.; Serrano, M. D.; Zaldo, C.;
Rico, M.; Mateos, X.; Liu, J.; Griebner, U.; Petrov, V.; Valle, F. J.;
Galan, M.; Viera, G. J. Opt. Soc. Am. B 2006, 23, 2594. (d) Balda,
R.; Lacha, L. M.; Fernandez, J. M.; Fernandez-Navarro, J. M. Proc.
SPIE 2005, 55, 5723.
(34) (a) Freedman, D.; Melman, J. H.; Emge, T. J.; Brennan, J. G. Inorg.
Chem. 1998, 37, 4162. (b) Kornienko, A.; Emge, T.; Hall, G.; Brennan,
J. G. Inorg. Chem. 2002, 41, 121.
3578 Inorganic Chemistry, Vol. 48, No. 8, 2009

Lanthanide Compounds with Fluorinated Aryloxide Ligands
Table 5. Radiative Properties of (DME)2Nd(OC6F5)3
transition
from F3/2 to
wavelength
(nm)
Sed (10-20
)
4
(cm2)
A (s-1
)
∆λeff (nm)
τrad (ms)
τfl (ms)
σe (10-20) (cm2)
4I15/2
4I13/2
4I11/2
4I9/2
1843
1327
a1059
928
0.000092
0.0007
0.078
0.00775
0.15718
81.71141
34.49011
8.6
8.6
8.6
8.6
26.2
27.2
51
0.17
0.00092
0.082
0.061
0.124
a η) 2%
Table 6. Radiative Properties of (DME)2Tm(OC6F5)3
were noted in divalent Eu-OC6F5 structures (including a
heterovalent compound for which Eu(II)-F but not Eu(III)
interactions were observed) can be explained by noting that
electrostatic bond strengths depend strongly on metal oxida-
tion state, and so for Ln(II) compounds the entropy gain
associated with displacing a neutral Lewis base ligand with
a chelating Ln-F interaction is more likely to compensate
for the relative weakness of the Ln-F bond.
wavelength
(nm)
∆λeff
(nm)
σe(10-20
)
τrad
(µs)
τfl
η
transition
(cm2)
(µs)
(%)
3H4f3F4
3F4f3H6
1458
1759
34
27
0.9
6.5
7518
2776
145
127
1.9
4.5
Ln-O(C6F5) distances with a range of separations (ca. 0.03
Å) that are identical to the difference between the long and
short Ln-O bonds in octahedral structure 5. In each of the
seven coordinate structures, the two phenoxides with the
longer Ln-O bond lengths are the ligands that are interacting
through π-π stacking interactions.
This is not to suggest that a covalent interaction would
represent anything more than a small contribution to complex
stability, because the remaining structural features can all
be discussed in electrostatic terms. For example, while it has
been noted in the past that there appears to be no significant
correlation of bond length with Ln-O-C angle, in com-
pounds 1-5 the phenoxide with the longest Ln-O bond
length usually has the most obtuse Ln-O-C angle (which
range from 129-180° in all compounds). The trend is present
in the structures of 1, 2, and 5, with the exception being the
structure of compound 4, for which two of the phenoxides
have statistically indistinguishable Ln-O bond lengths but
dramatically different Ln-O-C angles. An electrostatic
model to explain this behavior has been proposed35 and is
consistent with the observations here.
Similarly, electrostatic effects also presumably account for
the fact that all of the Ln(III)-OR are terminally bound,
even when crystallized from solvents as weakly basic as
DME. Alkoxide compounds are relatively insoluble materials
because the concentration of electron density on the small,
highly electronegative oxide donor atom increases the
tendency for these ligands to bridge metal centers by
displacing neutral donors from Ln coordination spheres.
However, fluorination of the ring reduces electron density
at the phenolate oxygen, rendering it less able to displace
neutral donors from Ln coordination spheres.
Finally, the consistent absence of Ln-F dative interactions
for the trivalent compounds contrasts with the frequent
observation of these “bonds” in analogous fluorinated thi-
olates. This difference can be rationalized by noting that the
more electronegative oxygen ligand has less electron density
delocalized throughout the arene, rendering the fluorides less
electron rich. An alternative explanation for the absence of
dative Ln-F bonds would be that the electrostatic opening
of these Ln-O-C angles relative to Ln-S-C results in a
displacement of the entire fluorinated ring away from the
Ln coordination sphere. The fact that dative Ln-F bonds
The three NIR emissive Ln ions investigated in this work
show different emission properties and efficiencies that can
be interpreted in terms of their structure and coordination
environments. A comparison of these (DME)2Ln(OC6F5)3
compounds with their thiolate analogues (DME)2Ln(SC6F5)3
offers a unique opportunity to evaluate how S replacing O
influences molecular emission properties. The NIR quantum
efficiencies of (DME)2Nd(SC6F5)3 (9%), (DME)2Tm(SC6F5)3
(2% for 3H4
f f
3F4 and 0.1% for 3F4 3H6), and
(DME)2Er(SC6F5)3 (75%) were found to be considerably
greater than previous literature values for more traditional
Nd(III),36 Tm(III),37 and Er(III)38 coordination compounds.
The lower quantum efficiencies of 1 (2%), 2 (19%), and 3
(1.9% and 4.5%) relative to their fluorinated thiolate
counterparts can be understood by noting that non-radiative
losses due to the relatively high frequency vibronic coupling
of the coordinated alkoxide decreases the relative intensity
of the phenolate emission. An additional potential contributor
to non-radiative quenching is de-excitation to the triplet states
of the arene ligands that are close to the excitation levels,
but unfortunately a quantitative assessment of this process
for both SR and OR compounds, as well as alternative
processes, namely, excitation migration or cross relaxation,
and so forth, can only be obtained from detailed energy
(36) (a) Hasegawa, Y.; Ohkubo, T.; Gogabe, K.; Kawamura, Y.; Wada,
Y.; Nakashima, N.; Yanagida, S. Angew. Chem., Int. Ed. 2000, 39,
357. (b) Klink, S. I.; Hebbink, G. A.; Grave, L.; van Veggel,
F. C. J. M.; Reinhoudt, D. N.; Slooff, L. H.; Polman, A.; Hofstraat,
J. W. J. Appl. Phys. 1999, 86, 1181. (c) Hebbink, G. A.; van Veggel,
F. C. J. M.; Reinhoudt, D. N. Eur. J. Org. Chem. 2001, 4101. (d)
Hasegawa, Y.; Murakoshi, K.; Wada, Y.; Kim, J. H.; Nakashima, N.;
Yamanaka, T.; Yanagida, S. Chem. Phys. Lett. 1996, 248, 8. (e) Wada,
Y.; Okubo, T.; Ryo, M.; Nakazawa, T.; Hasegawa, Y.; Yanagida, S.
J. Am. Chem. Soc. 2000, 122, 8583.
(37) (a) Alexey, V. T.; Carpenter, A. V.; Gorokhovsky, A. A.; Alfano,
R. R.; Chu, T. Y.; Okamoto, Y. Proc. SPIE 1998, 165, 3468. (b)
Kaizaki, S.; Shirotani, D.; Tsukahara, Y.; Nakata, H. Eur. J. Inorg.
Chem. 2005, 1, 3503. (c) Zang, F. X.; Hong, Z. R.; Li, W. L.; Li,
M. T.; Sun, X. Y. Appl. Phys. Lett. 2004, 84, 2679.
(38) (a) Hebbink, G. A.; Reinboudt, D. N.; van Veggel, F. C. J. M. Eur. J.
Org. Chem. 2001, 4101. (b) Wolbers, M. P. O.; van Veggel,
F. C. J. M.; Snellink-Ruel, B. H. M.; Hofstraat, J. W.; Geurts, F. A. J.;
Reinhoudt, D. N. J. Am. Chem. Soc. 1997, 119, 138. (c) Wolbers,
M. P. O.; van Veggel, F. C. J. M.; Peters, F. G. A.; E.van Beelen,
E. S.; Hofstraat, J. W.; Geurts, F. A. J.; Reinhoudt, D. N. Chem.sEur.
J. 1998, 4, 772. (d) Hasegawa, Y.; Okhubo, T.; Sogabe, K.; Kawamura,
Y.; Wada, Y.; Nakashima, N.; Yanagida, S. Angew. Chem., Int. Ed.
2000, 39, 357.
(35) Russo, M. R.; Kaltsoyannis, N.; Sella, A. Chem. Commun. 2002, 2458.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3579

Norton et al.
Tm3+ compound 3 are least affected by these vibronic
couplings.
With direct coordination of Ln to heavier sulfur atoms,
the coupling between the energy levels is less efficient and
the Franck-Condon factor for the relaxation process is
reduced, effectively increasing the lifetime of the thiolate
compounds relative to the phenoxides. Unfortunately, in
terms of device fabrication, the thiolate compounds suffer
from a tendency to react with water. For example,
(py)3Yb(SC6F5)3 reacts instantly with water in pyridine; the
red color that has been assigned as a S-to-Yb charge transfer
absorption39 disappears, and a light yellow solid precipitates.
Identical experiments with the phenoxide derivatives do not
give hydrolysis products. The phenoxides, while not as
emissive as their thiolate counterparts, are considerably more
NIR emissive than most (if not all) prior molecular NIR
sources. If quantum efficiencies can be increased further by
eliminating the CH functional groups on the DME ligands
(i.e., displacement by donor functional groups of a polymer),
then Ln complexes with these relatively air stable OC6F5
anions will be attractive candidates for polymer waveguide
amplifiers.
Figure 8. FTIR spectrum of (DME)2Nd(OC6F5)3; the spectra of the
analogous Er and Tm compounds are indistinguishable.
Figure 9. Energy level diagram for comparing the relative energy levels
of the three emissive Ln ions and the fluorescence quenching C-H and
C-C overtones.
Conclusion
transfer calculations, which are currently in progress.
The other structural aspect that impacts upon emission
intensity is the presence of ligands with C-H and CdC
bonds, because of the well documented quenching through
coupling to ligand CdC and C-H vibrations. The FTIR
spectrum of 1 given in Figure 8 shows the presence of several
such vibronic groups in these compounds, particularly CdC
stretching at 1530 cm-1, C-H deformation at 1020 cm-1,
and C-H asymmetric stretching at 2960 cm-1. The vibronic
energy transfer coupling can be interpreted with the help of
the energy level diagram shown in Figure 9. From the figure
it can be seen that the fourth order CdC vibration is almost
in resonance with the Er3+ emission with a difference of
energy of 278 cm-1. Similarly the second order vibration of
C-H is almost in resonance with Er3+ emission with an
energy difference of 603 cm-1. Because of these vibronic
couplings, the CdC and C-H functional groups are likely
to be significant quenchers of Er emission in complex 2.
Similarly, in the Nd3+ complex 1 the third order C-H
vibrational band is in resonance with the 1060 nm emission
with an energy difference of 316 cm-1 and is likely to be
the major cause of the reduced efficiency of 1060 nm
emission in complex 1. The two NIR emission bands of the
Trivalent lanthanide complexes with OC6F5 ligands can
be prepared in high yield and isolated as DME, THF, or
pyridine adducts. The DME and py compounds adopt seven
coordinate pentagonal bipyramidal structures, while the THF
derivative is a mer-octahedral structure with inequivalent
Ln-O bond lengths. The DME compounds are all NIR
emissive, although not as bright as their thiolate counterparts,
but any decrease in emission intensity is partially compen-
sated for by the greater air stability of the phenoxide anion.
Acknowledgment. This material is based upon work
supported by the National Science Foundation under CHE-
0747165. R.E.R. and G.A.K. acknowledge ONR-DARPA-
ONR under Grant 428833.
Supporting Information Available: Crystal data and structure
refinement, atomic coordinated and equivalent isotropic displace-
ment parameters, bond lengths and angles, anisotropic displacement
parameters, and torsion angles for 1-3 and 5. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC8020639
(39) Lee, J.; Freedman, D.; Melman, J. H.; Brewer, M.; Sun, L.; Emge,
T. J.; Long, F. H.; Brennan, J. G. Inorg. Chem. 1998, 37, 2512.
3580 Inorganic Chemistry, Vol. 48, No. 8, 2009