Highly luminescent, triple- and quadruple-stranded, dinuclear Eu, Nd, and Sm(III) lanthanide complexes based on bis-diketonate ligands

Article abstract of DOI:10.1021/ja048022z

The bis(β-diketone) ligands 1,3-bis(3-phenyl-3-oxopropanoyl)benzene, H₂L¹ and 1,3-bis(3-phenyl-3-oxopropanoyl) 5-ethoxy-benzene, H₂L², have been prepared for the examination of dinuclear lanthanide complex forma

Full text of DOI:10.1021/ja048022z

Published on Web 07/08/2004
Highly Luminescent, Triple- and Quadruple-Stranded,
Dinuclear Eu, Nd, and Sm(III) Lanthanide Complexes Based
on Bis-Diketonate Ligands
Andrew P. Bassett,† Steven W. Magennis,‡ Peter B. Glover,† David J. Lewis,†
Neil Spencer,† Simon Parsons,‡ Rene´ M. Williams,§ Luisa De Cola,§ and
Zoe Pikramenou*,†
Contribution from the School of Chemistry, The UniVersity of Birmingham,
Edgbaston, B15 2TT, United Kingdom, School of Chemistry, The UniVersity of Edinburgh,
King’s Buildings, West Mains Road, Edinburgh, EH9 3JJ, United Kingdom, and Molecular
Photonic Materials, Institute of Molecular Chemistry, Faculty of Science, UniVersity of
Amsterdam, Nieuwe Achtergracht 166, 1018 WS Amsterdam, The Netherlands
Received April 6, 2004; E-mail: z.pikramenou@bham.ac.uk
Abstract: The bis(â-diketone) ligands 1,3-bis(3-phenyl-3-oxopropanoyl)benzene, H2L1 and 1,3-bis(3-phenyl-
3-oxopropanoyl) 5-ethoxy-benzene, H2L2, have been prepared for the examination of dinuclear lanthanide
complex formation and investigation of their properties as sensitizers for lanthanide luminescence. The
ligands bear two conjugated diketonate binding sites linked by a 1,3-phenylene spacer. The ligands bind
to lanthanide(III) or yttrium(III) ions to form neutral homodimetallic triple stranded complexes [M2L1 ] where
3
M ) Eu, Nd, Sm, Y, Gd and [M2L2 ], where M ) Eu, Nd or anionic quadruple-stranded dinuclear lanthanide
3
units, [Eu2L1 ]2- . The crystal structure of the free ligand H2L1 has been determined and shows a twisted
4
arrangement of the two binding sites around the 1,3-phenylene spacer. The dinuclear complexes have
been isolated and fully characterized. Detailed NMR investigations of the complexes confirm the formation
of a single complex species, with high symmetry; the complexes show clear proton patterns with chemical
shifts of a wide range due to the lanthanide paramagnetism. Addition of Pirkle’s reagent to solutions of the
complexes leads to splitting of the peaks, confirming the chiral nature of the complexes. Electrospray and
MALDI mass spectrometry have been used to identify complex formulation and characteristic isotope patterns
for the different lanthanide complexes have been obtained. The complexes have high molar absorption
coefficients (around 13 × 104 M-1cm-1) and display strong visible (red or pink) or NIR luminescence upon
irradiation at the ligand band around 350 nm, depending on the choice of the lanthanide. Emission quantum
yield experiments have been performed and the luminescence signals of the dinuclear complexes have
been found to be up to 11 times more intense than the luminescence signals of the mononuclear analogues.
The emission quantum yields and the luminescence lifetimes are determined to be 5% and 220 µs for
[Eu2L1 ], 0.16% and 13 µs for [Sm2L1 ], and 0.6% and 1.5 µs for [Nd2L1 ]. The energy level of the ligand
3
3
3
triplet state was determined from the 77 K spectrum of [Gd2L1 ]. The bis-diketonate ligand is shown to be
3
an efficient sensitizer, particularly for Sm and Nd. Photophysical studies of the europium complexes at
room temperature and 77 K show the presence of a thermally activated deactivation pathway, which we
attribute to ligand-to-metal charge transfer (LMCT). Quenching of the luminescence from this level seems
to be operational for the Eu(III) complex but not for complexes of Sm(III) and Nd(III), which exhibit long
lifetimes. The quadruple-stranded europium complex has been isolated and characterized as the piperidinium
salt of [Eu2L1 ]2-. Compared with the triple-stranded Eu(III) complex in the solid state, the quadruple-
4
stranded complex displays a more intense emission signal with a distinct emission pattern indicating the
higher symmetry of the quadruple-stranded complex.
structures,1 liquid crystalline materials,2 sensors3 or luminescent
label design4 for specific biomolecule interactions. The lack of
Introduction
The design of molecular lanthanide complexes with defined
architecture is important in order to investigate structure vs
properties relationships in lanthanide solid-state framework
coordination control around the lanthanide imposes a challenge
to the formation of lanthanide supramolecular architectures.
Ligand design has been based mainly on structures that
encapsulate the lanthanide such as macrocycles and cryptands,5
podands6 or shell-type ligands,7 imposing bulkiness around the
metal. In this context, the great majority of lanthanide complexes
† School of Chemistry, The University of Birmingham.
‡ School of Chemistry, The University of Edinburgh.
§ Molecular Photonic Materials, Institute of Molecular Chemistry, Faculty
of Science, University of Amsterdam.
9
10.1021/ja048022z CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 9413-9424
9413

A R T I C L E S
Bassett et al.
Scheme 1. Structures of the Dinucleating Ligands, H2L1 and H2L2
studied are monometallic. There are few reports of macrocyclic
ligand preorganization for recognition of two lanthanide ions.8
The breakthrough in bimetallic lanthanide assembly came with
the formation of dinuclear lanthanide helicates based on
benzimidazole pyridyl ligands.9
We have been interested in employing self-assembly strategies
for lanthanide complexes,4 and have recently used â-diketonates
as sensitizers for lanthanide luminescence.10 Diketonate ligands
have the advantage of a negatively charged binding site that
leads to neutral, 3:1 ligand:lanthanide luminescent complexes.11
The complexes are stable in aqueous solutions and indeed
lanthanide diketonate complexes have found many applications
from chiral sensing12 to antibody labels in DELFIA immuno-
assays,13 and more recently in the development of new materials
based on sol-gel glasses,14 liquid crystals,15 near-IR LEDs16
or polymers.17 One approach to increase the luminescence output
signal is to introduce ligands with multiple binding sites. We
have therefore chosen to examine a bis-(â-diketonate) ligand
for the formation of dinuclear lanthanide complexes and report
our studies on such systems herein.
Ligand H2L1 bears two benzoyl â-diketonate sites joined by
a 1,3-phenylene spacer unit (Scheme 1). The 1,3-phenylene
spacer is ideal for formation of helicate metal complexes.18
Indeed, this ligand has been shown to form triple helical
complexes with Ti(III), V(III), Mn(III), and Fe(III)19 and a
modified ligand has been reported recently to lead to trinuclear
triple stranded helical Mn(II) complexes.20 Dinuclear lanthanide
diketonate complexes have received scant attention, with an
exception of a recent report during the course of this work.21
We report herein our studies to fully characterize H2L1,
determine the formation of dinuclear lanthanide complexes of
H2L1 and its properties as a sensitizer ligand for lanthanide ions.
The Eu(III), Sm(III) and Nd(III) complexes of H2L1 are reported.
To determine the helical nature of the complexes we extended
our approach to synthesize H2L2, a derivative of H2L1 with an
ethoxy substituent as a handle for NMR spectroscopic investiga-
tion in the characterization of the lanthanide complexes. A
quadruple-stranded dinuclear europium complex of H2L1 is also
reported herein, which displays strong luminescence signal with
a pattern distinct from that of the triple stranded europium
complex.
(1) Pan, L.; Adams, K. M.; Hernandez, H. E.; Wang, X. T.; Zheng, C.; Hattori,
Y.; Kaneko, K. J. Am. Chem. Soc. 2003, 125, 3062; Klonkowski, A. M.;
Lis, S.; Pietraszkiewicz, M.; Hnatejko, Z.; Czarnobaj, K.; Elbanowski, M.
Chem. Mater. 2003, 15, 656; Piggot, P. M. T.; Hall, L. A.; White, A. J. P.;
Williams, D. J. Inorg. Chem. 2003, 42, 8344; Thirumurugan, A.; Pati, S.
K.; Green, M. A.; Natarajan, S. J. Mater. Chem. 2003, 13, 2937; Thompson,
D. S.; Thompson, D. W.; Southward, R. E. Chem. Mater. 2002, 14, 30;
Shi, J. M.; Xu, W.; Liu, Q. Y.; Liu, F. L.; Huang, Z. L.; Lei, H.; Yu, W.
T.; Fang, Q. Chem. Commun. 2002, 756; Long, D. L.; Blake, A. J.;
Champness, N. R.; Wilson, C.; Schroder, M. Angew. Chem., Int. Ed. 2001,
40, 2444.
(2) Binnemans, K.; Goerller-Walrand, C. Chem. ReV. 2002, 102, 2302; Nozary,
H.; Piguet, C.; Rivera, J. P.; Tissot, P.; Morgantini, P. Y.; Weber, J.;
Bernardinelli, G.; Bunzli, J. C. G.; Deschenaux, R.; Donnio, B.; Guillon,
D. Chem. Mater. 2002, 14, 1075.
(3) Rudzinski, C. M.; Young, A. M.; Nocera, D. G. J. Am. Chem. Soc. 2002,
124, 1723; Parker, D. Coord. Chem. ReV. 2000, 205, 109.
(4) Glover, P. B.; Rodger, A.; Ashton, P. R.; Kercher, M.; Williams, R. M.;
De Cola, L.; Pikramenou, Z. J. Am. Chem. Soc. 2003, 125, 9918;
Charbonniere, L.; Ziessel, R.; Guardigli, M.; Roda, A.; Sabbatini, N.;
Cesario, M. J. Am. Chem. Soc. 2001, 123, 2436; Vazquez-Ibar, J. L.;
Weinglass, A. B.; Kaback, H. R. Proc. Natl. Acad. Sci. U.S.A. 2002, 99,
3487; Bobba, G.; Kean, S. D.; Parker, D.; Beeby, A.; Baker, G. Perkin
Trans. 2 2001, 1738; Horrocks, W. D.; Bolender, J. P.; Smith, W. D.;
Supkowski, R. M. J. Am. Chem. Soc. 1997, 119, 5972.
(5) Parker, D.; Dickins, R. S.; Puschmann, H.; Crossland, C.; Howard, J. A.
K. Chem. ReV. 2002, 102, 1977; Zucchi, G.; Ferrand, A. C.; Scopelliti, R.;
Bunzli, J. C. G. Inorg. Chem. 2002, 41, 2459; Gawryszewska, P.;
Jerzykiewicz, L.; Pietraszkiewicz, M.; Legendziewicz, J.; Riehl, J. P. Inorg.
Chem. 2000, 39, 5365; Balzani, V.; Scandola, F. In ComprehensiVe
Supramolecular Chemistry; Elselvier Science,: Oxford, UK, 1996; Vol.
10, pp 687; Sabbatini, N.; Guardigli, M.; Lehn, J.-M. Coord. Chem. ReV.
1993, 123, 201.
(6) Marques, N.; Sella, A.; Takats, J. Chem. ReV. 2002, 102, 2137; Charbon-
niere, L. J.; Ziessel, R. HelV. Chim. Acta 2003, 86, 3402; Motson, G. R.;
Mamula, O.; Jeffery, J. C.; McCleverty, J. A.; Ward, M. D.; von Zelewsky,
A. J. Chem. Soc., Dalton Trans. 2001, 1389; Bretonnie`re, Y.; Wietzke, R.;
Lebrun, C.; Mazzanti, M.; Pe´caut, J. Inorg. Chem. 2000, 39, 3499;
Lessmann, J. J.; DeW. Horrocks, W. J. Inorg. Chem. 2000, 39, 3114.
(7) Magennis, S. W.; Parsons, S.; Pikramenou, Z. Chem. Eur. J. 2002, 8, 5761;
Lowe, M. P.; Caravan, P.; Rettig, S. J.; Orvig, C. Inorg. Chem. 1998, 37,
1637.
(8) Setyawati, I. A.; Liu, S.; Rettig, S. J.; Orvig, C. Inorg. Chem. 2000, 39,
496; Avecilla, F.; de Blas, A.; Bastida, R.; Fenton, D. E.; Mah´ıa, J.; Mac´ıas,
A.; Platas, C.; Rodr´ıguez, A.; Rodr´ıguez-Blas, T. Chem. Commun. 1999,
125; Ziessel, R.; Maestri, M.; Prodi, L.; Balzani, V.; Van Dorsselaer, A.
Inorg. Chem. 1993, 32, 1237.
Experimental Section
Materials and Methods. Starting materials were of reagent grade
and used without further purification, unless otherwise stated. Solvents
were of HPLC grade. LnCl3‚6H2O Ln ) Eu, Sm, Nd, and YCl3‚6H2O
were obtained from Aldrich (99.9%); acetophenone from BDH,
dimethyl isophthalate from Acros, NaH as 60% dispersion in mineral
oil from Aldrich (washed several times with hexane to remove oil).
Spectroscopic grade DMF (Aldrich) and deuterated solvents (Goss
Scientific) were used as received. Anhydrous THF and acetonitrile were
freshly distilled over sodium-benzopheneone and P2O5, respectively,
under dinitrogen. Ligand synthesis was performed under dinitrogen
using standard Schlenk and vacuum line techniques. Subsequent workup
of products was carried out without precautions to exclude air.
1H- and 13C{1H} NMR spectra were recorded on Bruker AC-300,
1
360, 500 MHz spectrometers. H and 13C{1H} shifts were referenced
to external SiMe4. Positive ion FAB and MALDI-TOF mass spectra
(9) Bunzli, J.-C. G.; Piguet, C. Chem. ReV. 2002, 102, 1897.
(10) Magennis, S. W.; Craig, J.; Gardner, A.; Fucassi, F.; Cragg, P. J.; Robertson,
N.; Parsons, S.; Pikramenou, Z. Polyhedron 2003, 745.
(17) Bender, J. L.; Corbin, P. S.; Fraser, C. L.; Metcalf, D. H.; Richardson, F.
S.; Thomas, E. L.; Urbas, A. M. J. Am. Chem. Soc. 2002, 124, 8526.
(18) Constable, E. C.; Hannon, M. J.; Tocher, D. A. J. Chem. Soc., Dalton Trans.
1993, 1883; Dietrich-Buchecker, C.; Rapenne, G.; Sauvage, J. P.; De Cian,
A.; Fischer, J. Chem. Eur. J. 1999, 5, 1432.
(19) Grillo, V. A.; Seddon, E. J.; Grant, C. M.; Arom´ı, G.; Bollinger, J. C.;
Folting, K.; Christou, G. Chem. Commun. 1997, 1561.
(20) AromIÅ, G.; Berzal, P. C.; Gamez, P.; Roubeau, O.; Kooijman, H.; Spek,
A. L.; Driessen, W. L.; Reedijk, J. Angew. Chem., Int. Ed. 2001, 40, 3444.
(21) Yuan, J.; Sueda, S.; Somazawa, R.; Matsumoto, K.; Matsumoto, K. Chem.
Lett. 2003, 32, 492.
(11) Weissman, S. I. J. Chem. Phys. 1942, 10, 214.
(12) Tsukube, H.; Shinoda, S. Chem. ReV. 2002, 102, 2389.
(13) Hemmila¨, I. A. Applications of Fluorescence in Immunoassays; J. Wiley
& Sons: New York, 1991; Vol. 117 Chemical Analysis Series.
(14) Binnemans, K.; Lenaerts, P.; Driesen, K.; Gorller-Walrand, C. J. Mater.
Chem. 2004, 14, 191.
(15) Van Deun, R.; Moors, D.; De Fre, B.; Binnemans, K. J. Mater. Chem.
2003, 13, 1520.
(16) Kang, T.-S.; Harrison, B. S.; Bouguettaya, M.; Foley, T. J.; Boncella, J.
M.; Schanze, K. S.; Reynolds, J. R. AdV. Funct. Mater. 2003, 13, 205.
9
9414 J. AM. CHEM. SOC. VOL. 126, NO. 30, 2004

Bis-Diketonate Ligands
A R T I C L E S
were recorded on Kratos MS-50 and Kratos III mass spectrometers,
respectively. Elemental analyses were performed using a Perkin-Elmer
2400 CHN elemental analyzer. UV absorption spectra for luminescence
studies were recorded on Perkin-Elmer Lambda 9 and Lambda 17
spectrometers. All measurements were made at room temperature (ca.
20 °C) in aerated solutions, unless otherwise stated. Luminescence
emission and excitation spectra for Eu(III) and Sm(III) were recorded
on a Photon Technology International (PTI) QM-1 emission spectrom-
eter.22 Emission spectra were corrected for photomultiplier tube
response. Lifetime measurements were carried out using a nanosecond
Nd:YAG laser (Continuum, Surelite II) as the excitation source in an
experimental setup previously described.7a The obtained, unweighted,
data were analyzed by a nonlinear, least-squares, iterative technique
(Marquardt-Levenberg algorithm) using Kaleidagraph software. The
luminescence decay curves were fitted to a single exponential of the
form I(t) ) I(0)exp(-t/τ), where I(t) is the intensity at time t after the
excitation flash, I(0) is the initial intensity at t ) 0 and τ is the
luminescence lifetime. High Pearson’s correlation coefficients (g0.999)
were observed in all cases. Lifetimes were reproducible to ( 5%.
Luminescence quantum yields for Eu(III) and Sm(III) complexes were
measured by an optically dilute relative method23 using [Ru(2,2′-
bipyridyl)3]Cl2 (Φ ) 0.028 in aerated H2O)24 as a standard. The NIR
emission spectra were recorded on a PTI Alphascan fluorimeter, in
which a 75 W quartz-tungsten-halogen lamp is focused through a
SPEX 1680 double grating monochromator onto the sample. The
excitation light was modulated by a mechanical chopper at 35-70 Hz.
The emission was detected under a right angle through a 830 nm cutoff
filter using a single grating monochromator. A North Coast EO 817L
(250 V) liquid nitrogen cooled germanium detector was used, connected
to a Stanford Research SR 530 Lock-in-Amplifier. The lifetimes of
NIR emission were determined using an Edinburgh instruments LP 900,
consisting of a 337 nm nitrogen laser (Laser Technik Berlin, MSG405-
TD with pulses of 20 µJ, 0.5 ns fwhm), an Edinburgh Instruments single
grating monochromator and a North Coast EO 817P liquid nitrogen
cooled germanium detector. The signal was recorded with a Tektronix
digitizing oscilloscope, which was triggered by the laser.
H, 4.9%. Crystallographic data for H2L1: C24H18O4, M ) 370.38,
orthorhombic, space group P212121, a ) 5.2013(5), b ) 16.6336(15),
c ) 20.8923(18) Å, V ) 1807.5(3) Å3, Z ) 4, Fcalcd ) 1.361 g cm-3
,
F(000) ) 776, synchrotron radiation (λ ) 0.68750 Å), T ) 160 K, µ
) 0.092 mm-1, R1 ) 0.0793 [θmax ) 27°, 2798 data F > 4σ (F)], wR2
) 0.2499 for 3580 independent reflections, GOF ) 1.102.
General Synthetic Method for Y2L13 and Ln2L13 where Ln ) Eu,
Sm, Nd, Gd. A solution of LnCl3‚6H2O or YCl3‚6H2O (0.17 mmol) in
MeOH (2 cm3) was added dropwise to a solution of H2L1 (0.0915 g,
0.25 mmol) in CHCl3 (8 cm3), resulting in a pale yellow solution. A
solution of NEt3 (0.052 g, 0.51 mmol) in MeOH (2 cm3) was added
dropwise to this solution, producing a deeper yellow color. A pale
yellow precipitate formed after a few seconds. The mixture was stirred
for 1 h, filtered, and washed with CHCl3 (2 × 2 cm3), methanol (4 ×
2 cm3), H2O (2 × 2 cm3) and ether (2 × 2 cm3) and dried under vacuum
to give the desired product (yields 55-60%).
[Y2L1 ] δH (300 MHz, DMF-d7): 9.64 (3H, Hf), 8.40 (6H, dd, 3,4J
3
) 8, 2 Hz, Hg), 8.27 (12H, dd, 3,4J ) 8, 2 Hz, Hc), 7.47-7.55 (21H,
m, Ha, Hb, Hh), 7.16 (6H, s, He); δC{1H}: (90.6 MHz, DMF-d7) 184.5
(CO), 183.5 (CO), 140.5, 140.1, 131.4, 130.5, 128.6, 128.4, 128.1,
127.9, 94.3; FAB-MS m/z 1283 [M + H]+; λmax/nm (DMF) 357 (ꢀ )
13 × 104 dm3‚mol-1‚cm-1). (Found: C, 67.6; H, 3.9%. Y2C72H48O12
requires: C, 67.4; H, 3.8%).
3
[Eu2L1 ] δH (500 MHz, DMF-d7): 6.5 (6H, t, J ) 6.75 Hz, Ha),
3
3
3
6.4 (15H, br, Hb, Hf), 6.3 (3H, t, J ) 6.24 Hz, Hh), 6.2 (6H, d, J )
6.24 Hz, Hg), 4.6 (12H, br, Hc), 3.8 (6H, s, He); FAB-MS m/z 1409 [M
+ H]+; λmax/nm (DMF) 357 (ꢀ ) 13 × 104 dm3‚mol-1‚cm-1). (Found:
C, 60.7; H, 3.31%. C72H48O12Eu2‚H2O requires C, 60.6; H, 3.53%).
[Sm2L1 ] δH (300 MHz, DMF-d7): 8.53-8.51 (12H, m, Hc), 8.4
3
3
(6H, d, J ) 7 Hz, Hg), 7.9 (6H, s, He), 7.56-7.50 (21H, m, Ha, Hb,
Hf), 6.75 (3H, br, Hh); MALDI-MS m/z 1407 [M + H]+, 1429 [M +
Na]+; λmax/nm (DMF) 357 (ꢀ ) 13 × 104 dm3‚mol-1‚cm-1); (Found:
C, 61.3; H, 3.2%. C72H48O12Sm2 requires C, 61.5; H, 3.4%).
[Nd2L1 ] δH (300 MHz, DMF-d7): 12.0 (18H, m, Hc, He), 10.0 (6H,
3
d, 3J ) 7.5 Hz, Hg), 8.46-8.53 (21H, m, Ha, Hb, Hf), 8.28 (3H, t, 3J )
7.5 Hz, Hh); MALDI-MS m/z 1393 [M + H]+, 1415 [M + Na]+; λmax
/
Synthesis of 1,3-bis(3-phenyl-3-oxopropanoyl)benzene, H2L1. A
nm (DMF) 357 (ꢀ ) 13 × 104 dm3‚mol-1‚cm-1); (Found: C, 62.0; H,
3.6%. C72H48O12Nd2 requires C, 62.1; H, 3.5%).
[Gd2L1 ] MALDI-MS m/z 1421 [M + H]+ 1443 [M + Na]+;,
3
Found: C, 61.0; H, 3.2%. C72H48O12Gd2 requires C, 60.9; H, 3.4%.
Synthesis of (Hpip)2[Eu2L1 ]. A solution of EuCl3‚6H2O (0.020 g,
4
0.055 mmol) in MeOH (6 cm3) was added dropwise to a solution of
H2L1 (0.041 g, 0.11 mmol) in CHCl3 (20 cm3), giving a pale yellow
solution. An excess amount of piperidine (pip) was added to this
solution, resulting in the immediate formation of a pale yellow
precipitate, which was filtered and dried under vacuum.
Found: C, 65.1; H, 4.4; N, 1.3%. C106H88O16N2Eu2 requires C, 65.3;
H, 4.6; N, 1.4%).
solution of dimethyl isophthalate (3.27 g, 0.017 mol) and acetophenone
(5 g, 0.083 mol) in THF (50 cm3) was added to NaH (2.0 g, 0.05 mol)
at 0-5 °C forming an orange precipitate. The mixture was stirred at
this temperature for 2 h and for a further 2 h at room temperature. The
NaH was quenched with water and the mixture filtered. Addition of
dilute HCl to the yellow/orange solution yielded a pale yellow solid,
which was identified as the desired product (2.3 g, 37%).
ESI-MS(-) m/z 888 [M-2(Hpip)]2-, 1799 [M-2(Hpip) + Na]-, 1777
[M-2(Hpip) + H]-.
Synthesis of dimethyl 5-ethoxyisophthalate
δ
H (300 MHz, CDCl3): 15.2 (2H, br s, Hd), 8.58 (1H, t, 4J ) 2 Hz,
Hf), 8.17 (2H, dd, 3,4J ) 8, 2 Hz, Hg), 8.01-8.04 (4H, m, Hc), 7.49-
7.66 (7H, m, Ha, Hb, Hh), 6.94 (2H, s, He); δH (300 MHz, d7-DMF):
4
15.3 (br s, Hd), 8.92 (1H, t, J ) 2 Hz, Hf), 8.50 (2H, dd, 3,4J ) 8, 2
Hz, Hg), 8.26-8.29 (4H, m, Hc), 7.82 (1H, t, Hh), 7.60-7.74 (8H, m,
Ha, Hb, He); δC {1H} (75 MHz, CDCl3): 186.1, 184.8, 136.2, 135.2,
132.7, 130.7, 129.1, 128.7, 127.3, 125.8, 93.4; FAB-MS m/z 371[M +
H]+, 266 [M - COPh]+; λmax/nm (DMF) 357 (ꢀ ) 4 × 104
dm3‚mol-1‚cm-1); C24H18O4 found: C, 77.7; H, 4.9%, requires C, 77.8;
Bromoethane (1.04 g, 1.48 cm3, 9.52 mmol), was added to a solution
of dimethyl 5-hydoxyisophthalate (2.00 g, 9.52 mmol), K2CO3 (0.638
g, 4.76 mmol) and tetrabutylammonium chloride (20 mg), in acetonitrile
(120 cm3). The solution was refluxed for 24 h under N2, to yield an
off-white precipitate. The solvent was removed and the product
extracted with the following: CHCl3 (100 cm3), and H2O (100 cm3).
(22) Haider, J. M.; Chavarot, M.; Weidner, S.; Sadler, I.; Williams, R. M.; De
Cola, L.; Pikramenou, Z. Inorg. Chem. 2001, 40, 3912.
(23) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(24) Nakamura, K. Bull. Chem. Soc. Jpn. 1982, 55, 2697.
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A R T I C L E S
Bassett et al.
3
3
The organic layer was evaporated to produce a white solid, which was
washed with MeOH (2 × 2 cm3), H2O (2 × 2 cm3) and diethyl ether
(2 × 2 cm3), to yield the desired product (2.14 g, 95%) as a white
crystalline solid.
He), 4.30 (2H, q, J ) 6.9 Hz, Hk), 1.45 (3H, t, J ) 6.9 Hz, Hm); δC
{1H} (75 MHz, CDCl3): 185.7, 185.0, 159.5, 137.5, 135.2, 132.7, 128.7,
127.2, 118.1, 116.8, 93.5, 64.2, 14.7; ES-MS (+) m/z 437 [M + Na]+;
λ
max/nm (DMF) 360 (ꢀ ) 4 × 104 dm3 mol-1 cm-1); (Found: C, 75.63;
δ
H (300 MHz, CDCl3); 8.26 (1H, t, 4J ) 2 Hz, H2), 7.74 (2H, d, 4J
H, 5.41%. C26H22O5 requires C, 75.34; H, 5.35%).
) 2 Hz, H4), 4.12 (2H, q, 3J ) 7 Hz, Hk), 3.93 (6H, s, CH3), 1.44 (3H,
t, 3J ) 7 Hz, Hm). δC{1H} (75 MHz, CDCl3); 166.2, 159.0, 131.7, 122.8,
120.8, 119.8, 64.1, 52.4, 14.6. ES+ MS m/z 261 [M + Na]+
Synthesis of 1,3-bis(3-phenyl-3-oxopropanoyl) 5-ethoxy benzene,
H2L2
General Synthetic Method for [Ln2L2 ], where Ln ) Eu, Nd. A
3
solution of LnCl3‚6H2O (0.26 mmol) in MeOH (4 cm3) was added
dropwise to a solution of H2L2 (0.166 g, 0.40 mmol) in CHCl3 (16
cm3), resulting in a pale yellow solution. A solution of NEt3 (0.084 g,
0.82 mmol) in MeOH (4 cm3) was added dropwise to this solution,
producing a deeper yellow color. A pale yellow precipitate formed after
a few minutes. The mixture was stirred for 2 h, filtered, and washed
with CHCl3 (2 × 2 cm3), methanol (4 × 2 cm3), H2O (2 × 2 cm3) and
ether (2 × 2 cm3) and dried to give the desired product (yields 50-
60%).
[Eu2L2 ] δH (500 MHz, DMF-d7): 6.5 (6H, t, 3J ) 7.0 Hz, Ha), 6.4
3
(15H, m, Hb, Hf), 5.6 (6H, s, Hg), 4.5 (12H, br, Hc), 3.7 (2H, s, He), 3.5
3
3
(6H, q, J ) 7.0 Hz, Hk), 0.9 (9H, t, J ) 7.0 Hz, Hm); MALDI-MS
m/z 1542 [M + H]+; λmax/nm (DMF) 360 (ꢀ ) 1 × 105 dm3‚mol-1‚cm-1);
(Found: C, 60.7; H, 4.0%. C78H60O15Eu2 requires C, 60.7; H, 3.9%).
[Nd2L2 ] δH (300 MHz, DMF-d7): 12.0 (18H, m, Hc, He), 9.6 (6H,
3
s, Hg), 8.5 (21H, m, Ha, Hb, Hf), 4.9 (6H, q, 3J ) 6.7 Hz, Hk), 1.9 (9H,
t, 3J ) 6.7 Hz, Hm); MALDI-MS m/z 1547 [M + Na]+; λmax/nm (DMF)
358 (ꢀ ) 1.1 × 105 dm3‚mol-1‚cm-1). (Found: C, 61.6; H, 3.9%.
C78H60O15Nd2 requires C, 61.6; H, 4.0%).
A solution of dimethyl 5-ethoxyisophthalate (2.00 g, 8.4 mmol) and
acetophenone (2.22 g, 2.2 mL, 18 mmol) in THF (30 cm3) was added
to NaH (0.5 g, 21 mmol) at 0-5 °C. The mixture was stirred at this
temperature for 2 h and for a further 2 h at room temperature. The
remaining unreacted NaH was quenched with water (5 cm3) and the
Results and Discussion
mixture filtered. Acidification with dilute HCl (8 cm3, 2 mol dm-3
)
Preparation and Characterization of Ligands. The ligand
H2L1 was obtained by the Claisen condensation of dimethyl
isophthalate and acetophenone following a modification of the
reported preparation.25 In that report, only IR and melting point
characterization of H2L1 was reported. We have fully character-
ized H2L1 and have prepared a new ligand H2L2 based on
condensation of dimethyl 5-ethoxyisophthalate with acetophe-
none. This ligand bears an ethoxy chain at the back of the central
phenyl ring to provide a spectroscopic handle for NMR
investigation of the chirality of the corresponding metal
complexes.
turned the solution a yellow/orange color. The solvent was completely
removed under vacuum and the product extracted using CHCl3/H2O.
The organic layer was reduced in volume to yield a dark brown oil,
which was dissolved in the minimum amount of EtOH. To this, hexane
was added dropwise, causing a light orange precipitate to form. This
was filtered and washed with ice-cold EtOH (2 × 2 cm3), hexane (2 ×
4 cm3), and diethyl ether (2 × 2 cm3). The desired product was isolated
as a light yellow powder (1.19 g, 34%).
δ
H (300 MHz, CDCl3): 15.3 (br s, Hd), 8.13 (1H, t, 4J ) 2 Hz, Hf),
8.02 (4H, dd, 4J ) 1 Hz, 3J ) 7 Hz, Hc), 7.68 (2H, d, 4J ) 2 Hz, Hg),
3
7.48-7.61 (6H, m, Ha, Hb), 6.90 (2H, s, He), 4.19 (2H, q, J ) 7 Hz,
Hk), 1.49 (3H, t, 3J ) 7 Hz, Hm); δH (300 MHz, DMF-d7): 15.2 (br s,
Single crystals of H2L1 suitable for X-ray diffraction analysis
were grown by slow evaporation of a methanol solution. The
4
Hd), 8.53 (1H, t, J ) 1.5 Hz, Hf), 8.26-8.29 (4H, m, Hc), 7.97 (2H,
4
d, J ) 1.5 Hz, Hg), 7.68-7.73 (2H, m, Ha), 7.58-7.64 (6H, m, Hb,
Figure 1. X-ray crystal structure of H2L1 showing (a) numbering scheme (b) a stacking arrangement.
9
9416 J. AM. CHEM. SOC. VOL. 126, NO. 30, 2004

Bis-Diketonate Ligands
A R T I C L E S
1
Figure 2. 300 MHz H NMR spectrum of H2L1 in CDCl3. The dotted peak is due to chloroform.
crystal structure of H2L1 (Figure 1 and Supporting Information)
shows that the ligand adopts a planar structure as expected to
maximize conjugation with the two diketonate units arranged
asymmetrically about the 1,3-phenylene spacer, i.e., one dike-
tonate is oriented in the opposite direction from the other,
pointing toward different protons around the 1,3-central phenyl
ring. The ligands are π-stacked on top of each other to afford
columns of ligands with extensive stacking between the central
portion of one ligand with the end of an adjacent ligand. In
each case, the stacking occurs between a phenyl from one strand
and a diketonate from the other. The columns are packed
Figure 3. UV-vis absorption spectrum of H2L1 in MeOH.
together forming a series of CH‚‚‚π contacts between their
terminal phenyl rings which are arranged in a herringbone-type
motif.
2) a singlet at δ 6.94 is assigned to the two equivalent methine
protons (He). The rest of the protons are assigned based on
coupling constants and correlations obtained in a 1H-1H COSY
spectrum (Supporting Information).
The bond lengths of this structure can be compared with the
reported structure of dibenzoylmethane (HDBM).26 For H2L1,
the carbonyl bond lengths for C(7)-O(7) and C(9)-O(9), are
of similar value 1.290(4) and 1.297(4), respectively, whereas
there is a difference in value of the C(16)-O(16) and C(18)-
O(18) bond lengths which are 1.300(5) and 1.283 (5) Å,
respectively. These data suggest that in the O(7)/O(9) ring the
enolic proton is shared symmetrically with the carbonyl oxygen,
whereas in the O(16)/O(18) ring it is not. In HDBM, the C-O
bonds are not equidimensional (1.292 and 1.317 Å).27 The
structural data indicate that H2L1 exists in the enol form in the
solid state.
1
In the H NMR spectrum of H2L2 (Supporting Information)
the only observed differences include the appearances of a
quartet and a triplet, assigned to Hk and Hm respectively and
the different pattern for Hg and Hc due to the presence of the
ethoxy group. The 13C NMR spectra display 11 signals for
H2L1 and 13 distinct signals for H2L2, as expected based on the
ligand symmetry (Supporting Information).
The UV-vis absorption spectra of both H2L1 (Figure 3) and
H2L2 exhibited an intense band with λmax ) 357 nm, attributed
to singlet-singlet π f π* enol absorption, characteristic of the
enol form of â-diketones.
1
The H NMR spectra of H2L1 and H2L2 in CDCl3 and d7-
DMF, show the characteristic enolic proton absorptions at
around δ 15.2. This is an indication that the enol tautomer for
each ligand is the most dominant form in both solvents. In most
organic solvents, â-diketones are predominately (> 90%)
enolized.27 In the 1H NMR spectrum of H2L1 in CDCl3 (Figure
Synthesis and Characterization of Triple-Stranded Di-
nuclear Lanthanide Complexes of Bis-Diketonate Ligands
L1 and L2. Reaction of H2L1 or H2L2 with a metal chloride
hexahydrate in a 3:2 ratio in the presence of triethylamine lead
to the formation of [M2L1 ‚nH2O], M ) Eu, Sm, Nd, Y or
3
[M2L2 ‚nH2O], M ) Eu, Nd. The solids are soluble in DMF
(25) Martin, D. F.; Shamma, M.; Fernelius, W. C. J. Am. Chem. Soc. 1958, 80,
4891.
(26) Williams, D. E. Acta Crystallogr. 1966, 21, 340.
(27) Emsley, J. Struct. Bonding (Berlin) 1984, 57, 147.
3
and slightly soluble in alcoholic solvents. Although attempts to
grow crystals of the complexes were unsuccessful, NMR and
9
J. AM. CHEM. SOC. VOL. 126, NO. 30, 2004 9417

A R T I C L E S
Bassett et al.
Hd, present in the free ligands. Spectra of all the complexes
show that there is only one species present in solution. The
presence of one singlet for the He methine protons at δ 7.14 is
an indication that the two methine protons on a single ligand
are C2-symmetric and that all three ligands are equivalent. The
1H NMR spectrum of [Y2L1 ] in d7-DMF is shown in Figure 5.
3
The Y3+ ion was chosen as a diamagnetic replacement for Eu3+
.
The methine protons, He, and the aromatic protons Ha, Hb, Hc,
Hg, and Hh shift to lower frequencies with respect to their
chemical shifts in the d7-DMF spectrum of the free ligand; most
notably, He shifts by 0.45 ppm. This shift is consistent with the
ligands binding to the metal ion. In contrast, Hf shows a
relatively large shift of 0.69 ppm to higher frequency compared
to that of the free ligand. This downfield shift was also observed
in previously reported dinuclear helical metal complexes bearing
a 1,3-phenylene a spacer between the binding units.18 The 13C-
Figure 4. Expanded regions of the ES(+) mass spectrum of [Eu2L1 ] in
3
DMF/CH3CN.
mass spectrometric analyses were employed and enabled us to
identify the molecular species.
{1H} NMR spectrum of [Y2L1 ] confirms the complex’s high
3
Several mass spectrometry ionization methods have been used
interchangeably to detect the complexes (Supporting Informa-
symmetry, showing only 11 distinct resonances (Supporting
Information).
tion). The FAB(+)-MS spectrum of the isolated [Eu2L1 ]
3
1
product exhibited intense peaks with mass distribution m/z )
The H NMR spectra of the complexes of the paramagnetic
1407-1414, corresponding to [Eu2L1 + H]+. These peaks in
ions Eu3+, Nd3+, and Sm3+ are fully assigned as described below
(Figure 6 and Supplementary Info). Although line broadening
of certain peaks close to the paramagnetic metal ion was
observed, full assignments have been achieved using high-
resolution instruments to avoid signal overlap. The shifts due
to paramagnetism have their origin in the sum of two contribu-
tions: the contact, or Fermi shift, δc, (which operates through
bonds), and the pseudo-contact shift, δpc, (which operates
through space).28 The pseudo-contact shift is related to the
magnetic anisotropy of the complex and the position of the
proton in question, with respect to the central paramagnetic ion.29
Both the distance of the proton from the metal center and the
angle between the proton and the main molecular symmetry
axis of the molecule are important for the shift variation with
3
the accurate FAB(+)-MS spectrum of the sample show a
pattern in agreement with the theoretical isotope pattern for the
dinuclear complex (Supporting Information). Electrospray mass
spectrometry provided an insight of the solution formulation
of the complex (Figure 4). The peak centered at m/z 1410 is
assigned to [Eu2L13 + H]+ for the dinuclear complex. MALDI-
(+)-S was also used for the complexes. The isotope distribu-
tions illustrated in the peaks for the parent ions in all complexes
unambiguously confirm the dinuclear formation of the com-
plexes.
NMR spectroscopy was used for the elucidation of complex
formation. A clear indication of complex formation of H2L1
and H2L2 is given by the absence of the enolic proton peak,
1
Figure 5. 300 MHz H NMR spectrum of [Y2L1 ] in d7-DMF.
3
9
9418 J. AM. CHEM. SOC. VOL. 126, NO. 30, 2004

Bis-Diketonate Ligands
A R T I C L E S
1
Figure 6. 500 MHz H NMR spectrum of [Eu2L1 ] in d7-DMF. The dotted peak is due to the presence of methanol.
3
the paramagnetic metal ion. The reliance of distance on the
observed shift is generally demonstrated in the 1H NMR spectra
of the paramagnetic complexes in our case, where protons closer
to the metal centers shift by a much larger value in comparison
to protons further away. However, the magnitude and sign of
the observed shift is also dependent on the angular position of
the proton in question which is more difficult to define in the
absence of crystal structures. In the following section, the NMR
spectra are analyzed in detail in order to elucidate how these
assignments have been reached.
1
The H NMR spectrum of [Eu2L1 ] (Figure 6) exhibits a
3
triplet at δ 6.53, which integrates for six protons and is assigned
as Ha. Protons Hb and Hf appear as an unresolved broad peak
at δ 6.43, with the expected integral value. The triplet at δ 6.33
is assigned as Hh and integrates for three protons, its multiplicity
arises from coupling to Hg (3J ) 6.24 Hz). Hg appears as a
broad doublet at δ 6.18 with a 3J ) 6.24 Hz indicating coupling
to Hh, whereas four-bond coupling to Hf is masked by the
broadness of the peak. A broad singlet at δ 4.57 is assigned to
Hc, integrating for twelve protons. The methine protons, He,
appear as a broad singlet at δ 3.78 and correctly integrate for
six protons. He shows the largest shift with respect to the free
ligand, as a consequence of the closeness of these protons to
the binding site of the ligand and the paramagnetic metal.
Figure 7. Molecular model of the [Eu2L1 ] complex.
3
racemic mixture in solution. The required excess of the Pirkle’s
reagent is probably due to the weak formation of the diastero-
meric complexes due to the polar nature of the solvent. These
results lead us to the conclusion that the solution structure of
the complexes is indeed helical, in accordance with the
The 1H NMR spectrum of [Eu2L2 ] in d7-DMF showed
3
similar chemical shifts and multiplicities for protons Ha, Hb,
previously reported dinuclear transition metal structures.19
Hc, He, and Hf, as for [Eu2L1 ], whereas Hg appears as a singlet,
3
1
In the H NMR spectrum of [Nd2L1 ] in d7-DMF a doublet
3
as it no longer couples with the absent Hh. The signals for Hk
and Hm appear at δ 3.51 (quartet), and at 0.90 (triplet),
respectively. It was anticipated from the design of the H2L2
ligand that the Hk protons would be diastereotopic in the
dinuclear complex and that the signal would be split, confirming
the helicate nature of the complex formed. However, no
diastereotopic splitting of the Hk signal was observed, indicating
that the ethoxy substitution is too distant from the binding sites
for the chirality to have an effect that leads to observable
changes of the NMR signal.
at δ 10.0 with a coupling constant of J ) 7.5 Hz is assigned as
Hg, based on the coupling to Hh and the integration for six
protons. The triplet at δ 8.28 integrates to three protons and
3
has a J of 7.5 Hz, which lead to the assignment of Hh. This
resonance is not present in the spectrum of [Nd2L2 ] which
3
confirms the assignment. The remaining multiplets were as-
signed on the basis of integration and their likely chemical
shifts: the resonance at δ 12.0 integrates for eighteen protons
and is assigned to Hc and He. The larger multiplet at δ 8.46-
8.53 integrates for twenty-one protons and is therefore assigned
To confirm helix formation, excess of a chiral reagent was
as Ha, Hb, and Hf. In the 1H NMR spectrum of [Nd2L2 ] in d7-
3
added to a solution of the [Eu2L2 ] complex. Pirkle’s reagent,
3
DMF the ethoxy peaks, Hk and Hm, are assigned unambiguously
{(S)-(+)-2,2,2-trifluoro-1-(9-anthryl)ethanol}, was chosen as
the resolving reagent and has been previously employed for
NMR characterization of chiral complexes.30 Addition of
Pirkle’s reagent to a solution of Eu2L23 in d7-DMF/ d6-acetone
led to splitting of several peaks, indicating the presence of a
due to their coupling patterns.
1
The H NMR spectrum of [Sm2L1 ] in d7- DMF exhibits a
3
multiplet at δ 8.53-8.51, assigned to Hc and integrates correctly
to twelve protons. Hg appears at δ 8.38 and the methine protons,
He at δ 7.90. A broad singlet at δ 6.75 that integrates to three
protons is assigned as Hh, although the peak is not sufficiently
resolved to illustrate coupling to Hg. The remaining protons,
Ha, Hb, and Hf, are accounted for by the multiplet at δ 7.56-
7.50, which integrates to the correct value of twenty-one protons.
The splitting pattern of this multiplet is similar to that of the
(28) Bu¨nzli, J.-C. G., Chopin, G. R., Eds.; Lanthanide Probes in Life and Earth
Sciences, Elsevier: Amsterdam, 1989.
(29) McConnell, H.-M.; Robertson, R. E. J. Chem Phys. 1958, 29, 1361.
(30) Chambron, J.-C.; Sauvage, J.-P.; Mislow, K.; De Cian, A.; Fischer, J. Chem.
Eur. J. 2001, 7, 4086; Geraci, C.; Piatelli, M.; Neri, P. Tetrahedron Lett.
1996, 37, 7627; Zarges, W.; Hall, J.; Lehn, J. M.; Bolm, C. HelV. Chim.
Acta 1991, 74, 1843.
9
J. AM. CHEM. SOC. VOL. 126, NO. 30, 2004 9419

A R T I C L E S
Bassett et al.
photophysical properties of the energy donor ππ* state of the
ligand we examined the luminescence properties of the Y(III)
and Gd(III) complexes. The room-temperature emission spec-
trum of a degassed DMF solution of [Y2L1 ] exhibits a broad
3
emission band with λmax ) 436 nm, attributable to a ligand π*
f π transition (Supplementary Info). The ligand phosphores-
cence is clearly observed in the 77 K emission spectrum of the
[Gd2L1 ] complex (Figure 8).
3
As the energy of the ligand-centered triplet state does not
depend significantly on the metal, measurement of the short-
wavelength, 0-0 transition of [Gd2L1 ] gives the energy of the
3
3ππ* level for all the lanthanide chelates of L1. The 0-0
transition in this case is the band at 490 nm (20408 cm-1). The
vibrational progression of 1300 cm-1 may be assigned to either
C-C or C-O stretching frequencies.31,32 The energy of this
state is higher than the emitting excited states of Eu(III), Sm(III),
and Nd(III) confirming the suitability of the ligand as a sensitizer
for those lanthanides (Figure 9). However, this ligand is not
suitable for Tb(III) and Dy(III) which possess higher energy
excited states.
Figure 8. 77 K emission spectrum of [Gd2L1 ] in DMF:MeOH, 1:4
λexc ) 368 nm.
3
1
multiplets observed in the H NMR spectra of [Nd2L1 ] and
3
[Nd2L2 ], which are, unambiguously assigned also, to protons
3
Ha, Hb, and Hf. This fact provides further validation of our
assignment.
In light of the mass spectral and NMR data, the lanthanide
Red and Pink Luminescent Complexes Based on Eu(III)
and Sm(III). Light excitation into the ligand 1ππ* state at 350
complexes [M2L1 ] and [M2L2 ] may be represented with mo-
3
3
nm of [Eu2L1 ] in DMF or MeOH is followed by strong red
lecular models as neutral triple stranded dinuclear lanthanide
helices (Figure 7). The construction of the models was per-
formed using MM2 calculations based on the H2L1 ligand crystal
structure and the reported structures for the transition metal
complexes of H2L1.19 In the models, no solvent molecules were
considered. Many of the tris-diketonate complexes of lanthanides
are 8-coordinate in the solid state, with solvent molecules filling
the six-coordinate lanthanide coordination sphere.31 The number
of coordinated solvent molecules can be determined, under
certain conditions, using time-resolved luminescence spectros-
copy and this approach is used in a later section.
3
luminescence, characteristic of the 5D0
f
7FJ (J ) 0-4)
emission bands of Eu3+ (Figure 10). Although the resolution
of these spectra precludes a detailed analysis of the symmetry
of the complexes, the similarity of the structured emission
patterns in both solvents suggests that there are no solvent-
dependent structural changes.
The excitation spectrum of [Eu2L1 ] has a band maximum at
3
around 360 nm, which matches the corresponding absorption
spectrum, confirming that energy transfer takes place from the
ligand to the Eu(III) ion. (Figure 11). The [Eu2L2 ] complex
3
Luminescence Properties of Triple Stranded Dinuclear
Complexes of L1 and L2. Properties of L1 and L2 as
Sensitizers for Lanthanide Emission. To investigate the
showed similar emission and excitation spectra as the L1
analogue. The π f π* bands at λmax ) 357 nm in the absorption
spectra of all complexes have extinction coefficients of ꢀ ) 13
Figure 9. Simplified energy diagram showing the lowest lanthanide excited states and the estimated triplet state of the sensitizer L1 in the [Gd2L1 ] complex.
3
9
9420 J. AM. CHEM. SOC. VOL. 126, NO. 30, 2004

Bis-Diketonate Ligands
A R T I C L E S
Table 1. Luminescence Lifetimes of the Eu3+ (5D0) Level in
[Eu2L13] (λexc ) 355 nm)
solvent
τ (RT)/ms
τ (77 K)/ms
DMF
MeOH
MeOD
0.22
0.20
0.30
0.46
0.35
0.65
partly rationalized in terms of the vibrational quenching effect
of coordinated solvent molecules. The lifetime is longer in
MeOD than in MeOH due to the replacement of the high-energy
O-H oscillators with low-energy O-D oscillators. In DMF,
the measured lifetimes are probably affected by traces of water,
which coordinate to the metal ion since DMF was not dried
prior to use. The measurement of lifetimes in both MeOH and
MeOD allows an estimation of the number of coordinated
solvent molecules (q) (equation 1)33
Figure 10. Emission spectrum of [Eu2L1 ] in DMF, λexc ) 350 nm,
3
corrected for PMT response.
q ) A(1/τMeOH - 1/τMeOD - a)
(1)
where A is a proportionality constant for a given ion a is a
correction factor for outer sphere molecules and the lifetimes
are given in ms. In this equation, we have modified the
previously used methods for coordinated methanol molecules
rather than coordinated water.33a Application of eq 1 to the
values given in Table 1, gives the values q ) 3.7 and 2.9 at RT
and 77 K respectively when a ) 0.125 and A ) 2.4 for Eu3+ in
MeOH.33b However, this method is optimized for q ) 1 and if
we use the latest modified approach33c that takes into account
q > 1 with a ) 0.15 and A ) 2.2 in eq 1 this leads to q values
of 3.4 and 2.6 at RT and 77 K, respectively. Within experimental
error, this is good evidence that for [Eu2L1 ] in methanol there
3
are three solvent molecules coordinated to each Eu3+ ion, giving
the Eu3+ ions the common solution coordination number of nine.
In addition to a solvent-dependence, the 5D0 luminescence
lifetime increases dramatically with a decrease in temperature
from room temperature to 77 K in each of the three solvents.
The overall luminescence decay rate constant, k, can be
expressed by eq 2
k ) kr + knr(T) + knr(OH) + knr(other vibr.)
(2)
Figure 11. Absorption (a) and excitation (b) spectra of [Eu2L1 ] in DMF.
3
The excitation spectrum was corrected for lamp and instrument response;
the luminescence signal was monitored at λem ) 610 nm.
where kr is the radiative rate constant, knr(T) is the nonradiative
temperature-dependent rate constant, knr(OH) is the nonradiative
temperature-independent rate constant due to O-H oscillators
and knr(other vibr.) is the same rate constant for vibrations other
than O-H.5e The values of these constants can be obtained for
× 104 dm3 mol-1 cm-1 that confirm the presence of three
ligands per complex in solution.
The luminescence lifetimes of [Eu2L1 ] in DMF, MeOH, and
3
[Eu2L1 ] from the measurements made in MeOH and MeOD if
3
MeOD were measured at room temperature (RT) and 77 K
(Table 1), following excitation into ligand-centered bands (λexc
) 355 nm).
Each of the measured luminescence decays is described by
monoexponential kinetics, even at 77 K, and this suggests that
both Eu3+ ions have the same environment in the complex. This
agrees with the high symmetry observed in the NMR spectra
of all the complexes.
the following assumptions are made: knr(other vibr.) is negli-
gible (there is only one other nearby C-H oscillator per ligand,
and the effect of C-H is small33 in distances of >4 Å), coupling
with O-D oscillators is inefficient and at 77 K thermally
activated processes do not operate. It follows that
kr ) 1/τ(77 K, OD)
(3)
(4)
(5)
knr(RT) ) 1/τ(RT, OD) - 1/τ(RT, OD)
knr(OH) ) 1 / τ(RT, OH) - 1/τ(RT, OD)
It is clear from Table 1 that there is a pronounced variation
5
in the D0 luminescence decay with solvent. This trend can be
(31) Whan, R. E.; Crosby, G. A. J. Mol. Spectroscopy 1962, 8, 315.
(32) Sager, W. F.; Filipesen, N.; Serafin, F. A. J. Phys. Chem. 1965, 69, 1092.
(33) (a) Holz, R. C.; Chang, C. A.; Horrocks, W. D., Jr. Inorg. Chem. 1991,
30, 3270. (b) Beeby, A.; Clarkson, I. M.; Dickins, R. S.; Faulkner, S.;
Parker, D.; Royle, L.; De Sousa, A. S.; Williams, J. A. G.; Woods, M. J.
Chem. Soc., Perkin Trans. 2 1999, 493. (c) Supkowski, R. M.; Horrocks,
W. D., Jr. Inorg. Chem. 1999, 38, 5616.
Calculations for [Eu2L1 ] using eqs 3-5 give values of 1500
3
s-1, 1800 s-1 and 1700 s-1 for kr, knr(RT) and knr(OH),
respectively. The term knr(OH) is the same as that used to
calculate the number of coordinated solvent molecules in eq 1,
9
J. AM. CHEM. SOC. VOL. 126, NO. 30, 2004 9421

A R T I C L E S
Bassett et al.
Figure 13. Emission spectrum of [Nd2L1 ] in d7-DMF, λexc ) 358 nm.
Figure 12. Emission spectra of isoabsorptive solutions (A ) 0.1) of
3
[Sm2L1 ] (solid line) and [Sm(DBM)3] (dotted line) in DMF, λexc ) 353
3
nm.
be 0.16% and its lifetime in DMF was found to be 13 µs. The
value of quantum yield indicates that the ligand acts as a good
sensitizer for Sm(III). Sm(III) complexes are known to have
weak luminescence with the main source for nonradiative loss
attributed to multiphonon emission.36 Ligand to metal charge-
transfer transitions are higher in energy than those present in
the relevant Eu(III) complexes, hence they do not lead to
significant energy loss via radiationless quenching. The observed
luminescence lifetime is longer than the reported value of [Sm
⊂ 2.2.1]3+ in water (5 µs) where the Sm(III) is hexacoordinate.
The value is slightly shorter than the reported values of
nonacoordinated complexes (around 40 µs)37 possibly due to
the fact that there may be up to three water molecules
coordinated to Sm(III), since no special care was taken for
drying the DMF solvent. Indeed, most of the studies for Sm-
(III) are performed in dry or deuteurated solvents to minimize
the effect of O-H and C-H deactivation pathways.7a
and the value of 1700 s-1 is indicative of an efficient
nonradiative pathway.
The temperature-dependent rate constant, knr(T), plays a role
when short-lived, higher-energy states are thermally accessible,
and in this instance, the calculated value of 1800 s-1 for knr-
(RT) is significant. A common deactivation process encountered
in lanthanide complexes is the back transfer of energy from the
metal excited state to the ligand triplet state. From the
phosphorescence data, the triplet state is around 3000 cm-1
above the 5D0 level of Eu3+. This gap is too large, however, to
allow the reverse process. This is confirmed by the lack of any
changes in lifetime following deaeration of a DMF solution of
[Eu2L1 ] with oxygen-free dinitrogen. We suggest, therefore,
3
that the thermal quenching of the 5D0 level of Eu3+ in [Eu2L1 ]
3
occurs by a LMCT process. It is not uncommon for the
luminescence decay of europium â-diketonates to be strongly
temperature dependent and this has been previously attributed
to a ligand to metal charge transfer state.34
To examine the effect of the second lanthanide ion we
performed a luminescence quantum yield experiment to compare
the luminescence signal intensity between [Sm(DBM)3] and
In addition to the time-resolved measurements, the lumines-
[Sm2L1 ]. The luminescence signals of isoabsorptive solutions
3
cence quantum yield of [Eu2L1 ] in DMF at room temperature
3
of the two complexes were examined under quantum yield
conditions (Figure 12). Comparison of the integrated lumines-
cence signals of the two complexes shows that the dinuclear
complex displayed 11 times higher signal intensity than the
mononuclear complex. This confirms the effect of the second
lumophore in the increase of the luminescence signal, and it is
in agreement with the results of the europium complex. This
increase in quantum yield is particularly important for samarium
complexes that are weaker emitters.
was measured to be 5%. The lifetime and quantum yield of
[Eu2L2 ] are similar to that of the L1 complex, which indicates
3
that the ligand substitution in L2 does not affect the lanthanide
coordination sphere. The quantum yields of [Eu2L1 ] and
3
[Eu2L1 ] are higher than the ones reported for [Eu(DBM)3],35
3
which we attribute to the effect of an additional Eu3+ lumophore
in the dinuclear complexes, since the triplet state differences
are not significant. The energy of the L1 state is around 200
cm-1 lower than the one of HDBM, which is not significantly
different to affect the energy transfer.
Near-IR Emitting Nd(III) Complexes. The [Nd2L1 ] com-
3
plex is found to be luminescent, as expected based on the energy
level of the triplet state which is higher than the Nd luminescent
The suitability of the ligand as a sensitizer for Sm(III)
luminescence was examined. Excitation of [Sm2L1 ] solutions
4
excited state, F3/2 (11 527 cm-1). Excitation of the ligand
3
4
at 350 nm leads to strong pink emission of Sm3+ due to G5/2
absorption band of [Nd2L1 ] at 358 nm resulted in narrow band
3
6
f HJ (J ) 5/2, 7/2, 9/2, 11/2) transitions; the most intense
emission at 890, 1054, and 1325 nm (Figure 13). The emission
bands were assigned to 4F3/2 f4I9/2, 4I11/2 and 4I13/2 transitions,
respectively. The experiment was performed in d7-DMF to avoid
4
6
peak is the hypersensitive transition G5/2 f H9/2 at 647 nm
(Figure 12). The quantum yield of [Sm2L1 ] was measured to
3
(34) Berry, M. T.; May, P. S.; Xu, H. J. Phys. Chem. 1996, 100, 9216.
(35) Dawson, W. R.; Kropp, J. L.; Windsor, M. W. J. Chem Phys. 1966, 45,
2410.
(36) Sabbatini, N.; Dellonte, S.; Blasse, G. Chem. Phys. Lett. 1986, 129, 541.
(37) e Silva, F. R. G.; Malta, O. L.; Reinhard, C.; Gudel, H. U.; Piguet, C.;
Moser, J. E.; Bunzli, J. C. G. J. Phys. Chem. A 2002, 106, 1670.
9
9422 J. AM. CHEM. SOC. VOL. 126, NO. 30, 2004

Bis-Diketonate Ligands
A R T I C L E S
quenching by C-H vibration (5900 cm-1, ν ) 2) of the lowest
4
in energy Nd(III) band F3/2 f4I15/2 (5400 cm-1).
Time-resolved experiments were also performed by excitation
of a d7-DMF solution of [Nd2L1 ], and monitoring the decay
3
of the emission of the strongest emission band at 1054 nm.
Analysis of the monoexponential decay results in a lifetime of
1.5 µs. This value compares well with the reported lifetime of
the tris(hexafluoroacetylacetonato) Nd(III) complex in DMF,38
indicating that the bond vibrations in the L1 ligand do not
efficiently quench the excited state of Nd(III). The reasons for
this are 3-fold; (a) in Nd2L13 there are few C-H bonds in close
proximity to the metal ion; (b) the closest o-protons of the phenyl
rings are estimated to be located at distances of over 4 Å
(estimated from model, Figure 7) with the exception of the Hf
proton on the central 1,3-substituted phenyl ring; (c) the C-H
aromatic vibration is usually slightly higher (3050-3100 cm-1
)
than the aliphatic C-H vibration leading to a poorer match of
the vibrational levels responsible for luminescence signal
deactivation.
Figure 14. Expanded region of the ES-MS(-) spectrum of (Hpip)2-
[Eu2L1 ] displaying the peak of the doubly charged species [Eu2L1 ]2-
.
In other reported Nd complexes, the ligand design to
maximize luminescence lifetime is based on either podand,39
macrocylic40 or fluorinated diketonate structures41 which lead
to complexes with similar lifetime values as in our case. An
exception is the perfluoro sulfonylamine ligands that yield
complexes with longer Nd(III) lifetimes.42
4
4
diketonate complexes has been previously examined in the solid
state and their X-ray crystal structures have been determined,45
tetrastranded dinuclear lanthanide complexes have not previously
been investigated. The tetrakis- L1 complex of Eu3+ was
prepared by altering the ligand-to-metal ratio to 2:1 and using
piperidine to act as counterion. Electrospray mass spectrometric
An estimated quantum yield of Nd(III) luminescence of a
certain complex may be calculated by comparison of the
luminescence lifetime of the complexed ion43 with the natural
lifetime of Nd, τ0. By using eq 6 a value of 0.6% for the
quantum yield of the complex is calculated, given a value, τ0,
for the natural lifetime of Nd(III) ) 270 µs.44
analysis of freshly prepared solutions of (Hpip)2[Eu2L1 ] showed
4
a peak for the doubly charged species [Eu2L1 ]2-, confirming
4
the formulation of the complex (Figure 14).
The luminescence spectrum of the powder of (Hpip)2[Eu2L1 ]
4
showed some notably different features when compared with
the spectrum of the [Eu2L1 ] powder (Figure 15). First, the Eu3+
Φ ) τ/τ0
(6)
3
luminescence, following UV irradiation, is much stronger for
the tetrakis complex, as expected due to the absence of any
coordinated solvent molecules. Second, the fine structure of the
This method of quantum yield estimation does not take into
account other factors such as intersystem crossing efficiency
of absolute quantum yield methods using standards. The
obtained quantum yield value compares very well with other
Nd(III) complexes where all the protons of the ligands are either
deuteurated or fluorinated, e.g., the Nd(III) complex of hexaflu-
oroacetylacetonate41 is reported to have a quantum yield of
0.3%. These results indicate again that most of the C-H
vibrations are not in close proximity to the lanthanide center,
which indicates that these bis diketonate ligands are ideal
sensitizers for near-IR luminescence.
5
7
hypersensitive D0 f F2 emission band is clearly different in
both cases. This indicates that the symmetry around the
europium ions in the two complexes is not the same, as
anticipated for a tris- vs tetrakis-complex.46 The higher the
symmetry the less splitting is observed for the 5D0 f 7F2 band.
Taking into account the resolution limits of our instrument, we
can conclude that the tetrakis complex appears to have higher
symmetry than the tris, attributed to the lack of solvent
molecules coordinated to the Eu(III) ion and the additional
ligand. The Y(III) tetrakis-complex was also prepared to
examine the NMR spectroscopic features. The NMR spectrum
showed overlapping peaks rather than the simple pattern
observed in the tris complexes, indicating the presence of an
equilibrium between the two species in solution. This is not
surprising since ligand dissociation in solution can readily take
place leading to formation of the tris-species which may be
affected by the polarity of the solvent and the concentration of
the sample. Attempts to crystallize dinuclear tetrakis complexes
were unsuccessful.
Quadruple-Stranded Dinuclear Complexes of L1, (Hpip)2-
[Eu2L1 ]. Although the formation of mononuclear tetrakis
4
(38) Hasegawa, Y.; Kimura, H.; Murakoshi, K.; Wada, Y.; Kim, J. H.;
Nakashima, N.; Yamanaka, T.; Yanagida, S. J. Phys. Chem. 1996, 100,
10201.
(39) Oude Wolbers, M. P.; van Veggel, F. C. J. M.; Peters, F. G. A.; van Beelen,
E. S. E.; Hofstraat, J. W.; Geurts, F. A. J.; Reinhoudt, D. N. Chem. Eur.
J. 1998, 4, 773.
(40) Faulkner, S.; Beeby, A.; Carrie, M.-C.; Dadabhoy, A.; Kenwright, A. M.;
Sammes, P. G. Inorg. Chem. Commun. 2001, 4, 187; Faulkner, S.; Beeby,
A.; Dickins, R. S.; Parker, D.; Williams, J. A. G. J. Fluoresc. 1999, 9, 45.
(41) Yanagida, S.; Hasegawa, Y.; Murakoshi, K.; Wada, Y.; Nakashima, N.;
Yamanaka, T. Coord. Chem. ReV. 1998, 171, 461.
(42) Hasegawa, Y.; Ohkubo, T.; Sogabe, K.; Kawamura, Y.; Wada, Y.;
Nakashima, N.; Yanagida, S. Angew. Chem., Int. Ed. 2000, 112, 365.
(43) Klink, S. I.; Hebbink, G. A.; Grave, L.; Peters, F. G. A.; Van Veggel, F.
C. J. M.; Reinhoudt, D. N.; Hofstraat, J. W. Eur. J. Org. Chem. 2000,
1923.
(45) Sweeting, L. M.; Rheingold, A. L. J. Am. Chem. Soc. 1987, 109, 2652;
Rheingold, A. L.; King, W. Inorg. Chem. 1989, 28, 1715.
(46) Brecher, C.; Samelson, H.; Lempicki, A. J. Chem. Phys. 1965, 42, 1081;
Bauer, H.; Blanc, J.; Ross, D. L. 1965, 42, 1599; Horrocks, W. D., Jr.;
Albin, M. Progr. Inorg. Chem. 1983, 31, 1.
(44) Weber Phys. ReV. 1968, 171, 283.
9
J. AM. CHEM. SOC. VOL. 126, NO. 30, 2004 9423

A R T I C L E S
Bassett et al.
and high luminescence quantum yields, even in the case of the
Eu3+ complex where it is shown that the presence of a thermally
activated LMCT pathway acts to deactivate the 5D0 state. These
results indicate that the energy transfer process is efficient.
Internuclear cross relaxation and energy migration pathways may
be operational in di- or poly- nuclear lanthanide complexes.
However, in our systems the magnitude of the lifetimes and
the luminescence efficiency of the dinuclear complexes as
compared with the mononuclear species indicate that such
processes do not contribute significantly to luminescence
quenching. The ligand does not possess well-matched C-H
vibrations that can quench the lanthanide excited state. This
ligand feature is particularly important in the design of the near-
IR emitting lanthanide complexes. The only closely spaced
oscillators for vibrational quenching are the O-H oscillators
attributed to the three water or methanol molecules bound to
each lanthanide due to the hexacoordinate nature of the metal
environment. These oscillators are excluded in the formation
of the quadruple-stranded complexes where a fourth ligand
replaces the coordinated solvent molecules. Quadruple-stranded
dinuclear europium complexes have been isolated in the solid
state and they display strong luminescent properties. The tetrakis
complexes display stronger luminescent signals than the tris-
complexes and have a distinct luminescence pattern. The
synthetic versatility of these binucleating ligands allows an
interesting and diverse array of dinuclear complexes to be
synthesized to give strong emitters in the visible and near-
infrared.
Figure 15. Emission spectra of powder samples of (a) (Hpip)2[Eu2L1 ]
4
(b) [Eu2L1 ], λexc ) 350 nm, 2 nm band-pass (em.), corrected for PMT
3
response.
Conclusions
Acknowledgment. We acknowledge the support of EPSRC,
BBSRC, and the Royal Society exchange program for funding
as well as CCLRC and Simon Teat for time in the synchrotron
radiation laboratory.
The results of this work represent a novel ligand design based
on diketonate binding units for the assembly of neutral tris- and
quadruple-stranded dinuclear lanthanide complexes with inter-
esting luminescent properties. NMR spectroscopy has been used
to fully assign the proton environments for paramagnetic
lanthanide and diamagnetic analogue complexes. Addition of
Pirkle’s reagent to solutions of the tris stranded complexes
allows the identification of complex chirality. The sensitizing
properties of the ligands have been investigated by luminescence
Supporting Information Available: The crystal structure of
H2L1; 1H-1H COSY spectrum of H2L1; MALDI-MS and
accurate FABMS spectra of the complexes; 1H NMR spectrum
of H2L2; 13C NMR spectra; 1H NMR spectra of all complexes.
This material is available free of charge via the Internet at
http://pubs.acs.org.
3
spectroscopy. The ligand ππ* state is well placed to allow
energy transfer to Eu3+, Sm3+ and Nd3+ excited states, following
UV absorption. The complexes have long luminescence lifetimes
JA048022Z
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9424 J. AM. CHEM. SOC. VOL. 126, NO. 30, 2004