1-methyl-3-hydroxy-pyridin-2-one complexes of near infra-red emitting lanthanides: Efficient sensitization of Yb(III) and Nd(III) in aqueous solution

Article abstract of DOI:10.1021/ic902219t

The synthesis, X-ray structure, solution stability, and photophysical properties of several trivalent lanthanide complexes of Yb(III) and Nd(III) using both tetradentate and octadentate ligand design strategies and incorporating the 1-methyl-3-hydroxy-pyr

Full text of DOI:10.1021/ic902219t

4156 Inorg. Chem. 2010, 49, 4156–4166
DOI: 10.1021/ic902219t
1-Methyl-3-hydroxy-pyridin-2-one Complexes of Near Infra-Red Emitting
Lanthanides: Efficient Sensitization of Yb(III) and Nd(III) in Aqueous Solution
ꢀ
Evan G. Moore, Jide Xu, Sheel C. Dodani, Christoph J. Jocher, Anthony D’Aleo, Michael Seitz, and
Kenneth N. Raymond*
Department of Chemistry, University of California, Berkeley, California 94720-1460.
Received November 10, 2009
The synthesis, X-ray structure, solution stability, and photophysical properties of several trivalent lanthanide complexes of
Yb(III) and Nd(III) using both tetradentate and octadentate ligand design strategies and incorporating the 1-methyl-3-
hydroxy-pyridin-2-one(Me-3,2-HOPO) chelategroupare reported. BoththeYb(III) andNd(III) complexes have emission
Yb
bands in the Near Infra-Red (NIR) region, and this luminescence is retained in aqueous solution (Φ ∼ 0.09-0.22%).
tot
Furthermore, the complexes demonstrate very high stability (pYb ∼ 18.8-21.9) in aqueous solution, making them good
candidates for further development as probes for NIR imaging. Analysis of the low temperature (77 K) photophysical
measurements for a model Gd(III) complex were used to gain an insight into the electronic structure, and were found
to agree well with corresponding time-dependent density functional theory (TD-DFT) calculations at the B3LYP/6-
311Gþþ(d,p) level of theory for a simplified model monovalent sodium complex.
Introduction
latter methodology has been successfully applied in lumines-
cence microscopy, using complexes of Eu(III), and the on-
going development of time-resolved imaging equipment with
higher sensitivity in the NIR region will allow its direct
application at these longer wavelengths.
There is a growing interest in organic complexes of Ln(III)
metal ions with emission in the Near Infra-Red (NIR) region
(e.g., Ln = Nd, Yb).1,2 While their usage as the gain medium
in solid state lasertechnologies (e.g., Nd:YAG) has been well-
known for some time, organic complexes of these metals have
more recently attracted considerable interest, a major im-
petus being the development of NIR emitting probes for
bioimaging.3,4 This interest can be traced to the fact that
biological tissues are considerably more transparent at these
low energy wavelengths, which facilitates deeper penetration
of excitation and emitted radiation. Moreover, the lack of
endogenous NIR emissive fluorophores allows for much
greater sensitivity when compared to visible emitters, by
virtually eliminating autofluorescence.5 When compared to
NIR emission from organic molecules such as indocyanine
green, an additional benefit of Ln(III) complexes are their
significantly longer luminescence lifetimes (e.g., Yb(III),
τrad ∼ 1.2-1.3 ms),6 which can be used with time-gated
techniques to remove background emission and scattered
excitation, further improving signal-to-noise ratios. This
As a result of their weak (Laporte forbidden) f-f absorption,
the direct excitation of Ln(III) cations is not efficient. Instead,
sensitization of the metal emission is more effectively achieved
using the so-called “antennae” effect, wherein electronic
excitation is initially absorbed by a strongly absorbing proxi-
mal chromophore, and this excitation is then transferred to the
metal center via an energy transfer process.7 Transition metal
complexes, in particular with Ru(II) and Re(I), have been used
with great success to sensitize emission from NIR emitting
Ln(III) cation, as reviewed recently by Ward.8 Alternately,
several other groups have also used organic chromophores,
such as 8-hydroxyquinoline derivatives, as the light absorbing
antennae.9,10 We have recently examined several tetradentate
and octadentate complexes of Eu(III), featuring the organic
1-hydroxypyridin-2-one (1,2-HOPO) chelate, which also acts
as an antennae chromophore.11,12 While the 1,2-HOPO isomer
(7) Moore, E. G.; Samuel, A. P. S.; Raymond, K. N. Acc. Chem. Res.
2009, 42, 542–552.
(8) Ward, M. D. Coord. Chem. Rev. 2007, 251, 1663–1677.
(9) Comby, S.; Imbert, D.; Chauvin, A.-S.; Bunzli, J.-C. G. Inorg. Chem.
2006, 45, 732–743.
(10) Nonat, A.; Imbert, D.; Pecaut, J.; Giraud, M.; Mazzanti., M. Inorg.
Chem. 2009, 48, 4207–4218.
(11) Moore, E. G.; Xu, J.; Jocher, C. J.; Castro-Rodriguez, I.; Raymond,
K. N. Inorg. Chem. 2008, 47, 3105–3118.
*To whom correspondence should be addressed. E-mail: raymond@
socrates.berkeley.edu. Phone: þ1 510 642 7219. Fax: þ1 510 486 5283.
€
(1) Bunzli, J.-C. G. Acc. Chem. Res. 2006, 39, 53–61.
(2) Faulkner, S.; Beeby, A.; Dickins, R. S.; Parker, D.; Williams, J. A. G.
J. Fluorescence. 1999, 9, 45–49.
(3) Weissleder, R.; Ntziachristos, V. Nat. Med. 2003, 9, 123–128.
(4) Faulkner, S.; Pope, S. J. A.; Burton-Pye, B. P. Appl. Spectrosc. Rev.
2005, 40, 1–31.
(5) Frangioni, J. V. Curr. Opin. Chem. Biol. 2003, 7, 626–634.
(6) Aebischer, A.; Gumy, F.; Buenzli, J.-C. G. Phys. Chem. Chem. Phys.
2009, 11, 1346–1353.
(12) Moore, E. G.; Jocher, C. J.; Xu, J.; Werner, E. J.; Raymond, K. N.
Inorg. Chem. 2007, 46, 5468–5470.
r
pubs.acs.org/IC
Published on Web 04/05/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 4157
Chart 1. Chemical Structure of Tetradentate 5LIO-Me-3,2-HOPO
(left) and Octadentate H(2,2)-Me-3,2-HOPO (right) Ligands for Sensi-
tization of Yb(III) and Nd(III)
appropriate stoichiometric amount of the 5LIO-Me-3,2-HOPO
or H(2,2)-Me-3,2-HOPO free ligands, and diluting with 0.1 M
aqueous TRIS buffer or 1:1 EtOH/MeOH (v/v).
H(2,2)-Me-3,2-HOPO-Bn. To a solution of the H(2,2)-amine
(0.152 g, 0.65 mmol) and five molar equivalents of Me-3,2-
HOPO-Bn-thiaz in CH2Cl2 (50 mL) was added NEt3 (0.274 g,
2.70 mmol). The reaction mixture was stirred in the dark under
N2 for 5 days, then extracted (3 ꢀ 50 mL CH2Cl2) and washed
with 0.1 M KOH. The organic extracts were combined, dried
over MgSO4, and filtered. The solvent was removed in vacuo,
and the resulting residue was purified by column chromato-
graphy on silica gel, initially by eluting with 3% MeOH in
CH2Cl2, and then with increasing polarity to 5-7% MeOH in
CH2Cl2 to elute the crude product. This crude residue was
subsequently repurified by column chromatography on silica
gel eluting with 5-7% MeOH in CH2Cl2, yielding 0.47 g (60%)
of the product as an off-white solid after removal of the solvent.
1H NMR (400.1 MHz, CDCl3): δ 2.27 (bt, 12H, CH2CH2NH
and NCH2CH2N), 3.13 (q, 8H, CH2CH2NH), 3.56 (s, 12H,
HOPO CH3), 5.30 (s, 8H, HOPO Bn CH2), 6.71 (d, 4H, HOPO
Ar-H), 7.09 (d, 4H, HOPO Ar-H), 7.25-7.37 (m, 20H, Bn Ar-
H), 7.89 (t, 4H, CH2NHCO). 13C NMR (100.6 MHz, CDCl3):
δ163.3, 159.5, 146.3, 136.3, 132.1, 130.8, 128.9, 128.7, 128.6,
104.8, 74.6, 52.3, 50.7, 37.7, 37.5.
was found to strongly sensitize Eu(III) emission, the analogous
Me-3,2-HOPO isomer (see Chart 1) does not, which has
prompted our further recent13,14 investigations of the photo-
physical properties for this chromophore. Herein, we describe
the synthesis, structure, aqueous stabilities and photophysical
properties of two bis(tetradentate) and octadentate Me-3,2-
HOPO based ligands, in complex with Yb(III) and Nd(III),
which display strong NIR emission.
H(2,2)-Me-3,2-HOPO. The protected ligand, H(2,2)-Me-3,2-
HOPO-Bn (0.47 g, 0.39 mmol), was dissolved in a mixture of
concentrated HCl (10 mL) and glacial acetic acid (10 mL). The
solution was stirred under N2 for 2 days at which point the
volatiles were removed in vacuo by co-evaporation with MeOH
(5 ꢀ 10 mL) to yield the hydrochloride salt of the product (0.41
g, 92%) as an off-white solid. 1H NMR (400.1 MHz, DMSO-d6):
δ 3.43 (br m, 20H, CH2CH2NH), 3.73 (s, 12H, HOPO CH3),
6.50 (d, 4H, HOPO Ar-H), 7.13 (d, 4H, HOPO Ar-H), 8.73 (br
m, 4H, CH2NHCO). 13CNMR (100.6 MHz, DMSO-d6):
δ166.4, 158.4, 147.9, 128.1, 117.3, 103.2, 52.0, 37.3, 34.5. Anal.
Calcd (Found) for C38H48N10O12 9H2O 4HCl (1144.84
Experimental Section
General Procedures. All solvents for reactions were dried
using standard methodologies. Thin-layer chromatography
(TLC) was performed using precoated Kieselgel 60 F254 plates.
Flash chromatography was performed using EM Science Silica
Gel 60 (230-400 mesh). NMR spectra were obtained using a
Bruker AV-400 spectrometer operating at 400.1 (or 100.6) MHz
for 1H (or 13C) respectively. 1H (or 13C) chemical shifts are
reported in parts per million (ppm) relative to the solvent
resonances, taken as δ = 7.26 ppm (δ = 77.0 ppm) and δ =
2.49 ppm (δ = 39.5 ppm) respectively for CDCl3 and (CD3)2SO
while coupling constants (J) are reported in hertz (Hz). Fast-
atom bombardment mass spectra (FABMS) were performed
using 3-nitrobenzyl alcohol (NBA) or thioglycerol/glycerol
(TG/G) as the matrix. Elemental analyses were performed
by the Microanalytical Laboratory, University of California,
Berkeley, CA.
3
3
g mol-1): C, 39.87 (39.96); H, 6.16 (6.00); N, 12.23 (11.95).
3
[Yb(5LIO-Me-3,2-HOPO)2]. To a suspension of the 5LIO-
Me-3,2-HOPO ligand (22.0 mg, 54.1 μmol) in MeOH (5 mL)
was added 100 μL of pyridine, and this was heated gently to
assist dissolution of the solid. To the resulting solution was
added 0.53 equiv of YbCl3 6H2O (11.0 mg, 28.4 μmol) in MeOH
3
(1 mL). This mixture was refluxed for 4 h, then left to cool
to room temperature. Addition of diethyl ether (ca. 10 mL)
induced precipitation of a white solid, which was collected by
vacuum filtration, washed with diethyl ether, and air-dried to
yield the desired product (17.0 mg, 30%). Anal. Calcd (Found)
for [Yb(C18H20N4O7)(C18H21N4O7))] 3.5H2O(1045.86g mol-1):
Synthesis. Detailed synthesis of relevant precursors including
the important 2-mercaptothiazoline activated carboxyl deriva-
tive, 1-methyl-3-benzyloxy-4-(2-thioxo-thiazolidine-3-carbonyl)-
pyridin-2-one (Me-3,2-HOPO-Bn-thiaz) and the free base
hexamine backbone, N,N,N0,N0-tetrakis-(2-aminoethyl)-ethane-
1,2-diamine (H(2,2)-amine), have been previously described.15,16
Similarly, a detailed synthesis of 5LIO-Me-3,2-HOPO (Chart 1)
has been previously reported.17 Herein, the detailed synthesis of
the H(2,2)-Me-3,2-HOPO ligand (Chart 1) and synthesis of the
[Ln(5LIO-Me-3,2-HOPO)2] and [Ln(H(2,2)-Me-3,2-HOPO)]
complexes isolated as the charge neutral complex and the dica-
tionic chloride salts respectively with Ln = Yb(III) and Nd(III)
are described. Complexes with Gd(III) were prepared in situ by
3
3
C, 41.34 (41.36); H, 4.63 (4.93); N, 10.71 (10.55). Single crystals
suitable for X-ray analysis of this material were recrystallized by
slow diffusion of diethyl ether into a solution of the complex in 5%
aqueous 1:1 (v/v) DMF/MeOH.
[Nd(5LIO-Me-3,2-HOPO)2]. This complex was prepared
using an analogous procedure to that for the Yb(III) complex,
starting with the 5LIO-Me-3,2-HOPO ligand (27.0 mg,
66.4 μmol), substituting 0.53 equiv of NdCl3 6H2O (12.60 mg,
3
35.1 μmol) where appropriate. The desired complex was obtained
as a very faintly pinkish/purple solid (12.0 mg, 34.0%). Anal.
Calcd (Found) for [Nd(C18H20N4O7)(C18H21N4O7)] 6H2O
3
mixing a standardized aqueous solution of GdCl3 6H2O with the
3
(1062.09 g mol-1): C, 40.71 (40.88); H, 5.03 (4.53); N, 10.55
(10.40).
3
[Yb(H(2,2)-Me-3,2-HOPO)]. To a suspension of the H(2,2)-
Me-3,2-HOPO ligand (24.1 mg, 21.1 μmol) in MeOH (10 mL)
was added 100 μL of pyridine. To this mixture was added a clear
(13) Moore, E. G.; Seitz, M.; Raymond, K. N. Inorg. Chem. 2008, 47,
8571–8573.
(14) Moore, E. G.; Szigethy, G.; Xu, J.; Palsson, L.-O.; Beeby, A.;
Raymond, K. N. Angew. Chem., Int. Ed. 2008, 47, 9500–9503.
(15) Xu, J.; Franklin, S. J.; Whisenhunt, D. W., Jr.; Raymond, K. N.
J. Am. Chem. Soc. 1995, 117, 7245–7246.
(16) Wagnon, B. K.; Jackels, S. C. Inorg. Chem. 1989, 28, 1923–1927.
(17) Xu, J.; Kullgren, B.; Durbin, P. W.; Raymond, K. N. J. Med. Chem.
1995, 38, 2606–2614.
solution of YbCl3 6H2O (9.5 mg, 24.5 μmol) in MeOH (1 mL).
3
The solution was heated at reflux overnight during which time a
white solid precipitated. Upon cooling to room temperature, the
desired complex was collected by vacuum filtration and dried
overnight resulting in a faint beige colored solid (10.0 mg,

4158 Inorganic Chemistry, Vol. 49, No. 9, 2010
Moore et al.
Table 1. Summary of Crystal Data for the 5LIO-Me-3,2-HOPO Ligand and Corresponding Yb(III) Complex
5LIO-Me-3,2-HOPO
[Yb(5LIO-Me-3,2-HOPO)2]
empirical formula
C18H26N4O9
442.43
orthorhombic
Pca21
24.438(11)
9.722(6)
8.925(4)
90
C39H40N8O19Yb
1105.88
monoclinic
P21/b
13.451(2)
20.757(3)
16.969(3)
90
101.126(2)
90
4649.0(13)
4
1.58
mol. wt. [g mol-1
crystal system
space group
]
˚
a [A]
˚
b [A]
˚
c [A]
R [deg]
β [deg]
γ [deg]
90
90
3
˚
volume [A ]
Z
2120.5(19)
4
1.386
0.112
936
density, F [g cm-3
]
absorption coefficient, μ [mm-1
F(000)
]
2.094
2232
crystal size [mm3]
Temperature [K]
Wavelength
0.35 ꢀ 0.12 ꢀ 0.07
0.3 ꢀ 0.2 ꢀ 0.1
135(2)
0.71073
104(2)
0.71073
θ range for data collection [deg]
limiting indices
3.52 to 26.01
-25 < h < 25
-1 < k < 11
-10 < l < 10
1892
3.24 to 25.68
-16 < h < 16
-24 < k < 25
-11 < l < 20
15656
measured reflections
independent reflections
completeness to θ
1285 (Rint = 0.0245)
0.85a
15036 (Rint = 0.0122)
0.979
refinement method
full matrix least-squares on F2
1892/2/292
1.001
full matrix least-squares on F2
15656/1/1259
1.035
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2F(I)]
R indices (all data)
R1 = 0.0404, wR2 = 0.0886
R1 = 0.0711, wR2 = 0.1007
0.126 and -0.146
R1 = 0.0253; wR2 = 0.0642
R1 = 0.0271; wR2 = 0.0651
0.816 and -0.698
-3
˚
largest diff. peak and hole [e A
]
a Data set incomplete because of crystal decomposition.
39.9%). Anal. Calcd (Found) for [Yb(C38H47N10O12)]Cl2
The structures were solved within the WinGX22 package by
direct methods using SIR9223 and expanded using full-matrix
least-squares techniques with SHELXL-97.24 Hydrogen atoms
3
6H2O (1187.88 g mol-1): C, 38.42 (38.29); H, 5.01 (4.92); N,
11.79 (11.43).
3
˚
were positioned geometrically, with C-H = 0.93 A for CH
[Nd(H(2,2)-Me-3,2-HOPO)]. This complex was prepared using
an analogous procedure to that for the Yb(III) complex, starting with
the H(2,2)-Me-3,2-HOPO ligand (26.0 mg, 22.7 μmol), substituting
˚
˚
aromatic, C-H = 0.97 A for CH2 methylene, N-H = 0.89 A,
˚
and C-H = 0.96 A for CH3 methyl. Hydrogen atoms were
constrained to ride on their parent atoms, with Uiso(H) values set
at 1.2 times Ueq(C) for all H atoms (Table 1). Resulting drawings
of molecules were produced with ORTEP-3.25
NdCl3 6H2O(8.7mg,24.3μmol) where appropriate. In this case, the
3
desired complex did not precipitate from solution. Instead, the
dropwise addition of diethyl ether was required to induce precipita-
tion of the solid, which was isolated by vacuum filtration and air-
dried to yield a faint purple colored solid (17.0 mg, 60.9%). Anal.
Solution Thermodynamics. The general procedure used to
compare aqueous stabilities of the Ln(III) complexes herein was
that of competition batch titration as described previously26,27
using DTPA as the known competitor. Briefly, varying volumes
of standardized DTPA stock solution were added to aqueous
0.1 M TRIS (or 0.1 M HEPES) buffer (pH 7.4) containing 0.1 M
KCl electrolyte and a fixed amount of the appropriate ligand and
Ln(III) cation delivered by Eppendorf pipet (either 2:1 or 1:1
stoichiometry for the 5LIO-Me-3,2-HOPO and H(2,2)-Me-3,2-
HOPO ligands respectively). If necessary, the pH of the solutions
was readjusted to 7.4 with HCl and/or KOH, and the solutions
were diluted to identical volumes with additional 0.1 M TRIS (or
0.1 M HEPES) buffer (pH 7.4) containing 0.1 M KCl. The initial
concentrations of DTPA relative to the Ln(III) complex ranged
from about 1:10 to 3000:1. After standing for 24 h to ensure
thermodynamic equilibrium, the concentration of the free and
complexed ligand were evaluated. For 5LIO-Me-3,2-HOPO, this
Calcd (Found) for [Nd(C38H47N10O12)]Cl2 10H2O (1231.14
g mol-1): C, 37.07 (37.08); H, 5.49 (4.99); N, 11.38 (10.94).
3
3
Physical Methods. Crystallography. Crystals suitable
for X-ray analysis were mounted on a Kapton loop using
Paratone N hydrocarbon oil and measured at low temperature
using a Siemens SMART CCD18 area detector with graphite
monochromated Mo-KR radiation. Cell constants and orien-
tation matrices were obtained from least-squares refinement.
The resulting data were integrated by the program SAINT19
and corrected for Lorentz and polarization effects. Data
were also analyzed for agreement and possible absorption
using XPREP.20 An empirical absorption correction based on
the comparison of redundant and equivalent reflections was
applied using SADABS.21 Equivalent reflections where appro-
priate were merged, and no decay correction was applied.
(18) SMART, Version 5.059: Area-Detector Software Package; Bruker
Analytical X-ray Systems, Inc.: Madison, WI, 1999.
(19) SAINT, Version 7.07B: SAX Area-Detector Integration Program;
Siemens Industrial Automation, Inc.: Madison, WI, 2005.
(23) SIR92; Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.
J. Appl. Crystallogr. 1993, 26, 343–350.
(24) Sheldrick, G. M. SHELX97, Programs for Crystal Structure Analy-
€
sis; Institut fur Anorganische Chemie der Universitat:Gottingen, Germany, 1998.
(20) XPREP, Version 6.12: Part of the SHELXTL Crystal Structure
Determination Package; Bruker AXS Inc.: Madison, WI, 1995.
(21) SADABS, Version 2.10: Siemens Area Detector Absorption Correc-
tion Program; University of Gottingen: Gottingen, Germany, 2005.
(22) WinGX, 1.70.01; Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837–
838.
(25) ORTEP3 for Windows; Farrugia, L. J. J. Appl. Crystallogr. 1997, 30,
565.
(26) Pierre, V. C.; Botta, M.; Aime, S.; Raymond, K. N. Inorg. Chem.
2006, 45, 8355–8364.
(27) Doble, D. M. J.; Melchior, M.; O0Sullivan, B.; Siering, C.; Xu, J.;
Pierre, V. C.; Raymond, K. N. Inorg. Chem. 2003, 42, 4930–4937.

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 4159
was accomplished by absorbance measurements, using the inte-
gral of the spectra in the wavelength range between two isosbestic
points at 290-370 nm, and the spectra of the free and fully
complexed 5LIO-Me-3,2-HOPO as references. For H(2,2)-Me-
3,2-HOPO, the integrated intensity of the observed NIR emission
spectra at about 980 nm after excitation at 345 nm was used to
determine the equilibrium concentration of the [Yb(H(2,2)-Me-
3,2-HOPO)] complex directly, since the free ligand is not lumi-
nescent at this wavelength and the spectra of [Yb(H(2,2)-Me-3,2-
HOPO)] in the absence of DTPA competitor was used as the
reference.
versus absorbance at 345 nm (i.e., Ar(λr) or Ax(λx)) yield linear
2
plots with slopes equal to Φr/ηr2 or Φx/ηx . Values of η = 1.33 and
η = 1.49 were used for solutions in water and toluene respectively.
The emission of [Yb(TTA)3(H2O)2] in toluene (Φr = 0.35%) was
used as a reference29,30 for the [Yb(5LIO-Me-3,2-HOPO)2] quan-
tum yield determination, which was then used as a relative standard
for the [Yb(H(2,2)-Me-3,2-HOPO)] complex, and the estimated
error on these values is (25%.
Luminescence lifetimes were determined with the same HOR-
IBA Jobin Yvon IBH FluoroLog-3 spectrofluorimeter, adapted
for time-resolved measurements. An N2 laser (LN1000, Laser
Photonics, Inc.) was used as the light source (λex = 337.1 nm),
coupled to the entrance port of the FluoroLog-3. The laser
output pulse energy was about 1.5 mJ/pulse, with an optical
pulse duration of less than 800 ps fwhm. A portion of this
excitation was sampled with a quartz beam sampling plate,
which was focused onto the entrance of a UV-sensitive photo-
diode (DET210, Thor Laboratories). The small amplitude
analogue output from the photodiode was processed into a
TAC Start signal (NIM) using a TB-01 pulse converter module
from IBH. A Hamamatsu H9170-75 NIR fast rise time PMT
operating at -800 V and -60 °C was used as the detector, and
the output signal from the PMT was processed using a TB-02
0.5 GHz preamplifier module from IBH, and a 100 MHz
Constant Fraction Discriminator (CFD) (Model 6915, Phillips
Scientific), yielding appropriate TAC Stop signals (NIM). These
were acquired using a 2 ns PCI Multi Channel Scaling (MCS)
card (Model P7888-1E, FAST ComTec GmbH). Data analysis
was performed using the commercially available DAS 6 decay
analysis software package from HORIBA Jobin Yvon IBH.
Goodness of fit was assessed by minimizing the reduced chi
squared function, χ2, and a visual inspection of the weighted
residuals. Each trace contained at least 5,000 points, and the
reported lifetime values result from three independent measure-
ments. The estimated error on these values is (10%.
Computational Studies. Ground state density functional theory
(DFT) and time-dependent DFT (TD-DFT) calculations were
performed at the Molecular Graphics and Computational Facil-
ity, College of Chemistry, University of California, Berkeley. In
both cases, the B3LYP/6-311Gþþ(d,p) basis set provided in
Gaussian’0328 was used, with simplified input structures derived
from a previously reported15 crystal structure. All calculations
were done in the gas phase, and geometry optimizations were
performed with no symmetry restraints.
Photophysics. Typical sample concentrations for absorption
and fluorescence measurements were about 10-5 M. UV-visible
absorption spectra were recorded on Varian Cary 300 double
beam absorption spectrometer using quartz cells of 1.0 cm path
length. Steady state emission spectra were acquired on a HOR-
IBA Jobin Yvon IBH FluoroLog-3 (FL-3) spectrofluorimeter.
The excitation light source was a 450 W Xe arc lamp, and spectral
selection was achieved by passage through a double grating
excitation monochromator (1200 grooves/mm) blazed at 330 nm.
Spectra were reference corrected for the excitation light source
variation (lamp and grating). Emission spectra were collected
at 90° to the excitation, and emission was collected using a
Hamamatsu H9170-75 NIR PMT as the detector. Spectral
selection of the emission was achieved by passage through a double
grating emission monochromator (600 grooves/mm) blazed
at 1 μm. The emission spectral response of the Hamamatsu
H9170-75 NIR detector is almost linear across the 950-1700
nm wavelength range, and since the Ln(III) emission is essentially
monochromatic and quite narrow (ca. 60 nm at fwhm), the
observed emission signals were not corrected for the efficiency of
the grating. Quantum yields were determined using the following
equation;
Results and Discussion
Synthesis and Structure. The 5LIO-Me-3,2-HOPO and
H(2,2)-Me-3,2-HOPO ligands (Chart 1) were previously
reported as potential actinide sequestering agents.17,31 As a
result of our renewed interest, however, we have now ob-
tained X-ray quality crystals of the former ligand. Although
the collected data set was incomplete (completeness to θof ca.
85%) because of crystal decomposition, a tenable solution
was able to be obtained from the frames collected, and two
views of the resulting structure are shown in Figure 1. The two
Me-3,2-HOPO moieties are slightly offset but remain parallel
to each other, and the two amide functional groups are
coplanar with the aromatic ring system. The aromatic C-C
bonds of the ring system show no evidence of localization,
with all bond lengths within the typical expected range of
"
#
ꢀ
ꢁꢀ
ꢁ
ꢀ
ꢁ
Φx
Φr
ArðλrÞ IðλrÞ η2x Dx
AxðλxÞ IðλxÞ η2r Dr
¼
where A is the absorbance at the excitation wavelength (λ), I is the
intensity of the excitation light at the same wavelength, η is the
refractive index, and Dis the integrated luminescence intensity. The
subscripts ‘x’ and ‘r’ refer to the sample and reference, respectively.
By using a common excitation wavelength of 345 nm in the present
case, the I(λr)/I(λx) term is removed. Hence, a plot of integrated
emission intensities for both sample and reference (i.e., Dr or Dx)
˚
1.3 to 1.4 A. The resonance form for the Me-3,2-HOPO
chelate group, as drawn in Chart 1, was confirmed in the solid
(28) Frisch, M. J., Trucks, G. W., Schlegel, H. B., Scuseria, G. E., Robb;
M. A., C., J. R., Montgomery, J. A., Jr., Vreven, T.; Kudin; K. N., B., J. C.,
Millam, J. M., Iyengar, S. S., Tomasi, J., Barone; V., M., B., Cossi, M.,
Scalmani, G., Rega, N., Petersson, G. A.; Nakatsuji, H., Hada, M., Ehara,
M., Toyota, K., Fukuda, R., Hasegawa; J., I., M., Nakajima, T., Honda, Y.,
Kitao, O., Nakai, H., Klene; M., L., X., Knox, J. E., Hratchian, H. P., Cross,
J. B., Bakken, V.; Adamo, C., Jaramillo, J., Gomperts, R., Stratmann, R. E.,
Yazyev; O., A., A. J., Cammi, R., Pomelli, C., Ochterski, J. W., Ayala; P. Y.,
M., K., Voth, G. A., Salvador, P., Dannenberg, J. J.; Zakrzewski, V. G.,
Dapprich, S., Daniels, A. D., Strain, M. C., Farkas; O., M., D. K., Rabuck,
A. D., Raghavachari, K., Foresman, J. B.; Ortiz, J. V., Cui, Q., Baboul,
A. G., Clifford, S., Cioslowski, J.; Stefanov, B. B., Liu, G., Liashenko, A.,
Piskorz, P., Komaromi, I.; Martin, R. L., Fox, D. J., Keith, T., Al-Laham,
M. A., Peng, C. Y.; Nanayakkara, A., Challacombe, M., Gill, P. M. W.,
Johnson, B., Chen; W., W., M. W., Gonzalez, C., Pople, J. A. Gaussian 03,
revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
state structure, typified by a much shorter C-O bond length
in the 2-oxo position at about 1.24 A, compared to the longer
3-hydroxyl C-O bond length of about 1.35 A. Each of the
hydroxyl protons are hydrogen bonded with the adjacent
˚
keto oxygen, with a separation of about 2.26 A. An additional
˚
˚
strong hydrogen bonding interaction is apparent between the
amide proton and the hydroxyl group oxygen, with an
(29) Tsvirko, M. P.; Meshkova, S. B.; Venchikov, V. Y.; Bol’shoi, D. V.
Opt. Spectrosc. (Engl. Transl.) 1999, 87, 866–870.
(30) Meshkova, S. B.; Topilova, Z. M.; Bolshoy, D. V.; Beltyukova, S. V.;
Tsvirko, M. P.; Venchikov, V. Ya. Acta Phys. Pol., A 1999, 95, 983–990.
(31) Xu, J.; Radkov, E.; Ziegler, M.; Raymond, K. N. Inorg. Chem. 2000,
39, 4156–4164.

4160 Inorganic Chemistry, Vol. 49, No. 9, 2010
Moore et al.
as discussed elsewhere33,34 indicated that Yb(1) and Yb(2)
were best described as idealized bicapped trigonal prism
(C2v) and square antiprism (D4d), respectively. As with our
previous reports using analogous ligands,11 the least-
squares faces of the ‘top’ and ‘bottom’ ligands for both
complexes are offset and rotated by about 123° relative to
one another. Importantly, a consideration of the charge
requirements for the isolated complex reveals it must be
either protonated or anionic with an associated cation to
maintain the required charge balance. However, other
than isolated solvent H2O and CH3OH molecules, no
other peaks were observed in the difference map that
we could attribute to a cation such as Naþ or Kþ as we
have observed previously.12,14 As a result, we conclude the
crystallized complex is protonated, which is in accordance
with the results from elemental analysis of the bulk
material. Unfortunately, despite the excellent qualityof the
X-ray data, the position of this proton on the complex
could not be unequivocally located from the difference
map.
Figure 1. Top-down view (left) and side-on view (right) of the X-ray
crystal structure for 5LIO-Me-3,2-HOPO. Thermal ellipsoids of non-H
atoms are shown at the 50% probability level (C = black, O = red, N =
blue).
˚
average N-H---O separation of 1.95 A. Lastly, two solvent
Solution Thermodynamics. The aqueous stability of
the tetradentate 5LIO-Me-3,2-HOPO and octadentate
H(2,2)-Me-3,2-HOPO ligands in complex with Yb(III)
were assessed in terms of their pM, which by analogy to
pH, is defined as the negative log of the concentration of
free metal in solution (pM = -log10[M]free) at a specified
set of standard conditions (typically [M]total = 1 μM,
[L]total = 10 μM, and pH = 7.4). Competition batch
titrations using DTPA as the competitor with a known35
pYb of 19.4 were performed. For 5LIO-Me-3,2-HOPO,
the absorption spectra in Figure 3 show two isosbestic
points at about 289 and 375 nm, confirming the presence
of only the ligand and fully formed [Yb(5LIO-Me-3,2-
HOPO)2]- complex under the conditions of the experi-
ment. From the analysis, a conditional stability constant
of pYb = 18.8(1) was obtained, which is comparable to
the reported31 value of pCe = 16.3 for the same ligand
with Ce(III). Using the known31 pKa values for the 5LIO-
Me-3,2-HOPO ligand of 7.14(3) and 5.91(4), we can also
evaluate the formation constant for the Yb(III) complex
to be log β120 ∼ 23.4(1), which is similarly comparable
with the log β120 reported31 for Ce(III) of 20.9(2). The
corresponding values for DTPA demonstrate that such
an increase in stability over the lanthanide series is
plausible. For example, the determined35 log β110 values
of DTPA rise from 20.3 with Ce(III) to 22.6 with Yb(III)
representing a similar difference in stability as observed
here, with the increase due to the decreasing size and
increased Lewis acidity of the metal cation.
water molecules complete the unit cell, each of which are
associated with the amide carbonyl oxygen atom via an
intramolecular hydrogen bond.
The corresponding Ln(III) complexes of both ligands
with the NIR emitting metal ions (Ln = Yb, Nd) were
prepared using well established methodologies, as described
in the Experimental Section. The desired complexes with
5LIO-Me-3,2-HOPO were obtained in analytically pure
form as the monoprotonated charge neutral LnL2H com-
plexes, while the complexes with H(2,2)-Me-3,2-HOPO
were isolated as chloride salts of the triply protonated
dicationic LnLH3 complex. This behavior is somewhat
different to the results obtained32 with analogous 1,2-
HOPO ligands, such as 5LIO-1,2-HOPO, which yielded
anionic Ln(III) complexes isolated as pyridinium or alkyl-
ammonium salts, but can be rationalized in terms of the
greater basicity of the Me-3,2-HOPO chelate (vide infra)
and the pKa of the base used to deprotonate the phenol
group (i.e., pyridine, pKa ∼ 5.2).
A single crystal suitable for X-ray analysis was grown
from a 5% aqueous 1:1 (v/v) DMF/MeOH solution of the
[Yb(5LIO-Me-3,2-HOPO)2] complex diffused with diethyl
ether, and the resulting structure is shown in Figure 2. The
compound crystallized in the asymmetric space group P21,
and includes two independent complex molecules in the
asymmetric unit, together with several molecules of co-
crystallized solvent water and methanol. Notably, the
crystal structure with Yb(III) is very similar to that re-
ported31 for the sameligand withCe(IV), and other Ln(III)
complexes formed with the isomeric 5LIO-1,2-HOPO
ligand. As expected, both ytterbium atoms are coordinated
by a total of eight oxygen atoms from the two 5LIO-Me-
3,2-HOPO ligands, with a sandwich-like structure. The
average Yb-O bond lengths, considering either complex,
For the H(2,2)-Me-3,2-HOPO ligand, the correspond-
ing conditional stability constant was evaluated using
fluorescence techniques, and the Yb(III) emission spectra
in the NIR region upon increasing addition of DTPA are
shown in Figure 4. From the analysis of these spectra, a
pYb of 21.9(1) was determined, which is significantly
improved when compared to the corresponding value
for 5LIO-Me-3,2-HOPO, as we would expect because of
the improved chelate effect of the octadentate versus
bis(tetradentate) ligand topology. The magnitude of the
differences are also similar to those observed for the
˚
were identical within experimental error at about 2.33 A
whereas the geometries of the two coordination polyhedra
for both Yb(III) ions were slightly different. Shape analysis
(32) Moore, E. G.; Xu, J.; Jocher, C. J.; Werner, E. J.; Raymond, K. N.
J. Am. Chem. Soc. 2006, 128, 10648–10649.
(33) Porai-Koshits, M. A.; Aslanov, L. A. Zhurnal Strukturnoi Khimii.
1972, 13, 266–276.
(34) Kepert, D. L. Prog. Inorg. Chem. 1978, 24, 179–249.
(35) Smith, R. M., Martell, A. E. Critical Stability Constants; Plenum
Press: New York, 1976.

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 4161
Figure 2. X-ray crystal structures showing views of the two independent Yb(1) (left) and Yb(2) (right) complexes found in the asymmetric unit for the
[Yb(5LIO-Me-3,2-HOPO)2] complex. Co-crystallized solvent molecules, and selected H atoms have been omitted for clarity, and non-H atoms are drawn at
the 50% probabilitylevel(C =black, O=red, N =blue, Yb= green). Only the majorcomponentofthe disorderedligandbackboneisshown fortheYb(2)
complex. The position of the proton necessary for charge neutrality could not be located from the difference map (see text).
Figure 3. (a) Absorption spectra (left) for solutions of [Yb(5LIO-
Figure 4. (a) Luminescence spectra (left) for solutions of [Yb(H(2,2)-
Me-3,2-HOPO)2]- (26 μM in 0.1 M KCl; 0.01 M HEPES at pH 7.4; black
line) andafter addition of uptoabout 500 equiv ofDTPA(red). (b) Plotof
equilibrium concentration (right) determined from absorption spectra
versus equilibrium concentration of log DTPA (mM-1) and correspond-
ing fit used to evaluate equal speciation between 5LIO-Me-3,2-HOPO
and DTPA (i.e., 50% [Yb(DTPA)] at about 0.013 mM, which corres-
ponds to a pYb = 18.8 (1) for the [Yb(5LIO-Me-3,2-HOPO)2 complex).
Me-3,2-HOPO)]- (ca. 1.5 μM in 0.1 M TRIS at pH 7.4; black line) and
after addition of up to about 500 equiv of DTPA (red). (b) Double log
plot (right) of equilibrium metal complex concentrations log[[Yb-
(DTPA)]/[Yb(H(2,2)-Me-3,2-HOPO)]] determined from luminescence
spectra versus equilibrium concentrations of log[[DTPA]/[H(2,2)-Me-
3,2-HOPO]] and corresponding fit used to evaluate equal speciation
between H(2,2)-Me-3,2-HOPO and DTPA (i.e., 50% [Yb(DTPA)] at
about 0.013 mM, which corresponds to a pYb = 21.9 (1) for the
[Yb(H(2,2)-Me-3,2-HOPO)] complex).
structurally analogous ligands using the identical octa-
dentate (H(2,2)-) and tetradentate (5LIO-) ligand back-
bones but with the isomeric 1,2-HOPO chelating unit,
where an increase of ΔpEu = 2.6 log units was seen,
comparable to the ΔpYb = 3.1 log units observed herein.
Electronic Structure Calculations. Density Functional
Theory (DFT) calculations were performed using Gauss-
ian’0328 to elucidate the ground state geometry and
electronic structure of the Me-3,2-HOPO chromophore.
As a simplified model, only the 6-methyl amide of Me-3,2-
HOPO was used as the input, and this was first geometry
optimized with no symmetry constraints to give the
relaxed output geometry shown in Figure 5. The results
obtained show all non-H atoms lying essentially in a single
plane, and the calculatedstructure reproduces the expected
strong H-bonding interactions between the hydroxyl pro-
reveals the nature of the first excited singlet state of the
Me-3,2-HOPO chromophore can be described principally
by a highest occupied molecular orbital (HOMO) f lowest
unoccupied molecular orbital (LUMO) excitation, with a
smaller contribution from the HOMO-3 f LUMOþ2
excitation. An inspection of the relevant Kohn-Sham
molecular orbitals, also shown in Figure 5, reveals these
transitions to be of 1π-π* character. Similarly, the nature of
the lowest energy triplet state (T1) can be described princi-
3
pally by a HOMO f LUMO excitation, that is, a ππ*
transition.
Additional static and time dependent DFT calculations
using the same methodologies were also undertaken with
Gaussian’03 to characterize the excited state of the anio-
nic Me-3,2-HOPO chromophore, in complex with a metal
cation. We have used an identical approach to that
reported11 previously for the 1,2-HOPO chromophore,
wherein the ionizable hydroxyl proton of the Me-3,2-
HOPO chromophore has been replaced with a mono-
valent Naþ cation, chelated in a bidentate mode between
˚
ton and adjacent keto oxygen (O1-H---O2 = 1.99 A) and,
similarly, between the amide proton and the hydroxyl
˚
oxygen atom (N2-H---O1 = 1.98 A). Using time-dependent
techniques (TD-DFT) and employing the same basis set,
the electronic structure was calculated, and the results
are summarized in Table 2. A consideration of this data

4162 Inorganic Chemistry, Vol. 49, No. 9, 2010
Moore et al.
Figure 5. (a) Calculated output geometries obtained from static B3LYP/6-311Gþþ (d,p) geometry optimization and relevant molecular orbital diagrams
from TD-DFT electronic structure calculations for 6-MeAmide-Me-3,2-HOPO (top) and (b) a model [Na(6-MeAmide-Me-3,2-HOPO)] complex (bottom).
The metal centered character of the LUMO in the latter is an artifact of the B3LYP functional calculation (see text).
Table 2. Summary of Electronic Structure Calculations for Me-3,2-HOPO Chromophore and a Model [Na(6-MeAmide-Me-3,2-HOPO)] Complex
excited state
multiplicity
energy (eV)
wavelength (nm)
oscillator strength
orbital(s) involved (n f n0)
transition coefficient
6-MeAmide-Me-3,2-HOPO
1
2
T1
T2
2.580
3.756
480.49
0.0000
0.0000
45 f 49
48 f 49
48 f 51
45 f 49
47 f 49
48 f 49
48 f 51
47 f 49
47 f 51
47 f 56
47 f 60
48 f 51
46 f 49
46 f 60
48 f 49
45 f 51
0.1076
0.7776
-0.1226
-0.2497
0.1471
0.1136
0.6922
0.6870
0.1157
0.1041
0.1958
-0.1724
0.6999
0.1276
0.6286
0.1374
330.06
320.90
3
T3
3.864
0.0000
4
5
T4
S1
3.942
4.045
314.56
306.53
0.0000
0.1043
[Na(6-MeAmide-Me-3,2-HOPO)]
1
2
3
4
T1
T2
S1
T3
2.369
2.831
2.839
3.586
523.42
438.02
436.77
345.79
0.0000
0.0000
0.0039
0.0000
53 f 55
53 f 54
53 f 54
50 f 55
50 f 61
53 f 61
53 f 63
53 f 67
50 f 61
53 f 55
0.7897
0.7066
0.7065
-0.1738
-0.1228
0.7052
-0.1172
-0.1337
0.1299
5
S2
3.844
322.58
0.1816
0.6199
the oxygen atoms, as shown in Figure 5. As we have
noted, while this is a somewhat crude approximation, this
simplification avoids computationally expensive calcula-
tions involving a lanthanide cation, and can nonetheless
be a useful tool for understanding the ligand’s electronic
structure. It must be stressed that the goal of this “calcu-
lation” is not to determine the electronic states of the
Ln(III) cation but rather to approximate the energy for
the lowest excited singlet and triplet states of the ligand.
Similarly, use of the same basis set allows a direct
comparison of the theoretical results obtained for the
1,2-HOPO and Me-3,2-HOPO chromophores. The re-
sulting output geometry is shown in Figure 5, where it is
evident that all non-H atoms are lying essentially in the

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 4163
plane of the aromatic ring, and the energy minimized
geometry of the chelate group is similar to that observed
in the [Yb(5LIO-Me-3,2-HOPO)2] structure by X-ray
crystallography. The bidentate coordination mode of
the monovalent Naþ cation is retained with Na-O
˚
˚
bondlengths of 2.15 A and 2.18 A to the phenolic and
keto oxygen atoms, respectively, mirroring the binding
mode of a Ln(III) cation, and a strong intramolecular
H-bond between the amide proton and the phenolic
˚
oxygen atom is also readily apparent at 1.84 A. The
results from the corresponding TD-DFT electronic struc-
ture calculations for the model [Na(6-MeAmide-Me-3,2-
HOPO)] complex are also summarized in Table 2. A
consideration of this data reveals the first excited singlet
state of the Me-3,2-HOPO chromophore can be described
by a HOMO f LUMO excitation. However, an inspec-
tion of the relevant Kohn-Sham molecular orbitals, also
shown in Figure 5, shows this transition to be of 1LMCT
(ligand to metal charge transfer) character. These results
are analogous to our previous reported11 work using
TD-DFT analysis of a simplified Naþ complex of the
1,2-HOPO chromphore, wherein we noted these LMCT
transitions are typically underestimated by the B3LYP/6-
311Gþþ(d,p) basis set because of self-interaction errors
with the simple exchange correlational (xc) functional
used,36 and a similar argument is applicable in the present
instance. Hence, the real location of this 1LMCT transi-
tion is likely at significantly higher energy, while the
predicted energy of ligand centered (LC) transitions such
as that of the S0 f S2 transition at 322.6 nm, which can be
described principally by a HOMO f LUMOþ1 excita-
tion, is in good agreement with experimentally observed
electronic spectra maxima of about 340 nm (vide infra).
Similarly, the nature of the lowest energy triplet state (T1)
can also be described by a HOMO f LUMO þ1 excita-
tion, and an inspection of the relevant molecular orbitals
reveals this to be of distinctly ligand centered π f π*
character (i.e., a 3ππ* transition) with a predicted energy
of 523.4 nm (19,105 cm-1).
Photophysics. The absorption spectra of the 5LIO-
Me-3,2-HOPO and H(2,2)-Me-3,2-HOPO ligands are
essentially identical (data not shown), displaying a broad
electronic envelope in the UV, which is red-shifted upon
deprotonation and is centered at about 340 nm in aqueous
solution at pH 7.4. From corresponding TD-DFT ana-
lysis (vide infra), we assign this band to the lowest energy
singlet transitions with essentially π-π* character.
Upon complexation with Ln(III) cations, the vibra-
tional fine structure of this band is improved slightly, and
a comparison of the spectra of both ligands with Yb(III)
as examples are shown in Figure 6. We can attribute this
sharpening to a loss of the homogeneous line broadening
normally present because of variable hydrogen bonding
interactions involving the phenolate in aqueous solution,
which is replaced by a more well-defined geometry for the
chromophore upon Ln(III) binding. Notably, the Ln(III)
complexes of Me-3,2-HOPO containing ligands also dis-
play improved vibrational structure when compared with
the corresponding 1,2-HOPO complexes. This may be a
reflection of the stronger aromaticity and π-π* character
Figure 6. (a) Absorption spectra of the [Yb(5LIO-Me-3,2-HOPO)2]
(red) and [Yb(H(2,2)-Me-3,2-HOPO)] (black) complexes in 0.1 M aqu-
eous TRIS buffer, pH 7.4. (b) Emission spectra (λex = 345 nm) at room
temperature (red, ꢀ10) and 77 K (black) of the [Gd(5LIO-Me-3,2-
HOPO)2]- complex measured in a 1:1 (v/v) EtOH/MeOH glassing
solvent, and corresponding fit (blue) to a series of overlapping gaussians
to evaluate the zero phonon transition.
of this transition in the former. Previously, the 1,2-HOPO
chelate has been considered as a cyclic hydroxamic acid37
rather than an aromatic system, and the lowest energy
transition has been assigned to have mixed n-π* and
π-π* parentage.11 Upon closer inspection of the electronic
spectra, we also note a slight shift in λmax between the two
differing ligands in complex with Yb(III). Hence, for
[Yb(5LIO-Me-3,2-HOPO)2], the apparent λmax is located
at 345.6 nm (28,940 cm-1) whereas for [Yb(H(2,2)-Me-3,2-
HOPO)], this value is blue-shifted to 344.7 nm (29,010
cm-1). While this difference is only slight (Δ ≈ 75 cm-1),
it is nonetheless reproduced in the spectra of the corres-
ponding Nd(III) complexes which are otherwise indistin-
guishable from their Yb(III) counterparts, and we attribute
the difference to subtle changes in the coordination geome-
tries supported by the bis(tetradentate) versus octa-
dentate ligand topologies.
A comparison of the room temperature and 77 K emis-
sion spectra for [Gd(5LIO-Me-3,2-HOPO)2] in 1:1 EtOH/
MeOH (v/v) glassing solvent is also shown in Figure 6. Since
Gd(III) lacks an appropriately positioned metal centered
energy accepting level, its complexes may be used to assess
the energetic position of the ligand centered lowest energy
triplet state. At room temperature, the emission of the
complex is quite weak, with an apparent maximum at about
400 nm which we attribute to residual singlet emission from
the Me-3,2-HOPO chromophore. However, upon cooling
to 77 K, a much more intense and vibrationally structured
band appears, with peak maxima at about 536, 579, and
624 nm. We can confidently attribute this emission to
phosphorescence from the ligand centered lowest energy
triplet excited state. Concomitant with this new band,
we also observe an increase in intensity and consider-
able sharpening of the residual singlet excited state emis-
sion band in the UV region, with new peaks at 376 and 393
nm, and a weaker shoulder at about 415 nm. It is worth
noting that the apparent zero phonon energy of the
phosphorescence, when taken as the first peak maxima
of the vibrational progression (ca. 18,657 cm-1), is in good
agreement with that predicted value for the Me-3,2-HOPO
chromophore of 19,105 cm-1 as obtained from TD-DFT
(36) Dreuw, A.; Head-Gordon, M. J. Am. Chem. Soc. 2004, 126, 4007–
4016.
(37) Cunningham, K. G.; Newbold, G. T.; Spring, F. S.; Stark, J. J. Chem.
Soc. 1949, 2091–2094.

4164 Inorganic Chemistry, Vol. 49, No. 9, 2010
Moore et al.
Figure 7. (a) Observed emission spectra (left) of [Yb(5LIO-Me-3,2-
HOPO)2]- (black) and[Yb(H(2,2)-Me-3,2-HOPO)]- (red) in 0.1 M TRIS
buffered aqueous solution at pH 7.4. (b) Corresponding emission spectra
(right) for [Nd(5LIO-Me-3,2-HOPO)2]- (black) and [Nd(H(2,2)-Me-3,2-
HOPO)]- (red) in 0.1 M TRIS buffered aqueous solution at pH 7.4.
Spectra of the 5LIO-Me-3,2-HOPO complexes are horizontally offset for
clarity.
Figure 8. (a) Observed emission spectra (left) of [Yb(5LIO-Me-3,2-
HOPO)2] (black circles) in 1:1 (v/v) MeOH/EtOH at 77 K and fitted
curve (blue) to a series of three overlapping Voigt functions (red) and (b)
corresponding emission spectra (right) for [Yb(H(2,2)-Me-3,2-HOPO)]
(blackcircles) in 1:1(v/v)MeOH/EtOH at77K and fittedcurve(blue) toa
series of four overlapping Voigt functions (red).
line broadening effects. It is pertinent that we have
previously11 assigned local D2d symmetry to the structu-
rally related Eu(III) complex with 5LIO-1,2-HOPO on
the basis of the observed crystal field splitting of the metal
centered 5D0 f 7F0,1 transitions in solution. By contrast,
further splitting of the spectra for the monoprotonated
[Yb(H(2,2)-Me-3,2-HOPO)] complex suggests a lower
symmetry at the metal center. It has been shown that
the splitting of Eu(III) emission bands can be useful to
determine the site symmetry of this metal cation,38 and we
have previously assigned local C2 symmetry to the related
[Eu(H(2,2)-1,2-HOPO)] complex via detailed analysis of
its emission spectrum.12 The results with the Yb(III)
cation and the H(2,2)-Me-3,2-HOPO ligand in this case
are also consistent with this point group, since under C2
analysis of the model [Na(6-MeAmide-Me-3,2-HOPO)]
complex.
The luminescence spectra in the NIR region for the Yb(III)
and Nd(III) complexes with both the 5LIO-Me-3,2-HOPO
and H(2,2)-Me-3,2-HOPO ligands in 0.1 M TRIS buf-
fered aqueous solution at pH 7.4 are shown in Figure 7.
The spectra of both Nd(III) complexes are remarkably
similar, each displaying the characteristic 4F3/2 f 4I11/2
and 4F3/2 f 4I13/2 emission bands at 1064 and 1332 nm,
respectively. By contrast, the spectra of the two Yb(III)
complexes show distinct differences in the character-
2
2
istic F5/2 f F7/2 electronic transition at about 1 μm.
For the [Yb(5LIO-Me-3,2-HOPO)2] complex, the emis-
sion band is highly structured and relatively symme-
trical, with several minor peaks evident because of
crystal field splitting of the single electronic transition.
For the corresponding [Yb(H(2,2)-Me-3,2-HOPO)]
2
symmetry, the F7/2 ground state will be split into four
doubly degenerate levels.39 Under the assumption that at
77 K there is insufficient thermalization to populate the
higher Stark levels of the emitting 2F5/2 state, we should
thus anticipate four lines in the NIR emission spectrum,
as are observed experimentally (Figure 8). Upon the
addition of piperidine acting as a base, we note a sig-
nificant change to the line shape of the observed lumine-
scence (see Supporting Information, Figure S2). None-
theless, the shape of the observed NIR emission can still
be well approximated by four overlapping Voigt func-
tions, suggesting the proposed C2 geometry in alkaline
solution is retained. The change in the splitting pattern
may be correlated to structural perturbations because of
the loss of H-bonding interactions previously noted40 for
the H(2,2)-backbone, in going from the monoprotonated
to doubly deprotonated state, which are translocated to
the attached Me-3,2-HOPO chelates and results in a
subtle change in the crystal field of the Yb(III) cation.
Our initial results reported here hint that, by analogy to
the case with Eu(III), a wealth of information concerning
the first coordination sphere of the Yb(III) cation may be
obtained from more detailed analysis of their NIR emis-
sion spectra. Analyses of this type will be enhanced in
2
2
complex, the splitting of the F5/2 f F7/2 transition
clearly differs, with the appearance of a significantly
more intense shoulder at about 1060 nm. The lack of a
corresponding peak at 1332 nm can be used as evidence
to exclude the possibility of cross contamination with
Nd(III), and, moreover, we have found that the inten-
sity of this 1060 nm peak is also pH dependent (see
Supporting Information, Figure S1), implicating the
associated protonation equilibria of the adjacent amine
backbone as an important factor. It is intuitive that the
presence or absence of a formal positive charge in close
proximity to the Yb(III) cation will have a dramatic
effect on the subsequent crystal field splitting.
To further examine this phenomenon, we have per-
formed low temperature (77 K) measurements for these
two complexes in a 1:1 (v/v) EtOH/MeOH glassing
solvent, with resulting spectra that are remarkably dif-
ferent (Figure 8) and also behave very differently to the
presence and absence of 1% piperidine acting as a strong
base (pKa ∼ 11.2). For the [Yb(5LIO-Me-3,2-HOPO)2]
complex, the spectra with and without added base present
are indistinguishable as was the case in aqueous solution
at differing pH values. Moreover, the resulting emission
spectra are significantly sharper at 77 K, and the spectrum
can be successfully approximated by three overlapping
Voigt functions (see Figure 8) to account for both homo-
geneous (Lorentzian) and inhomogeneous (Gaussian)
(38) Murray, G. M.; Sarrio, R. M.; Peterson, J. R. Inorg. Chim. Acta 1990,
176, 233–240.
(39) Antic-Fidancev, E.; Jorma, H.; Lastusaari, M. J. Phys.: Condens.
Matter 2003, 15, 863–876.
(40) Samuel, A. P. S.; Moore, E. G.; Melchior, M.; Xu, J.; Raymond,
K. N. Inorg. Chem. 2008, 47, 7535–7544.

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 4165
Table 3. Summary of Photophysical Data for Yb(III) and Nd(III) Complexes of Me-3,2-HOPO Ligands
complex
λ
max (nm)
ε
max (M-1cm-1)
τ
H O (μs)
τ
D O (μs)
Φ (%)
q
2
2
[Yb(5LIO-Me-3,2-HOPO)2]
[Yb(H(2,2)-Me-3,2-HOPO)]
[Nd(5LIO-Me-3,2-HOPO)2]
[Nd(H(2,2)-Me-3,2-HOPO)]
345.6
344.7
345.7
344.7
28,940
29,010
28,280
28,850
1.55
1.18
0.082
0.098
25.3
16.1
0.655
0.471
0.22
0.09
N/A
N/A
0.4
0.6
1.0
0.7
the NIR region since, while the magnitude of crystal
field splitting for Eu(III) compared to Yb(III) remain
similar (e.g., ∼100-1000 cm-1), the non-linearity of the
wavelength scale with energy necessitates a vast improve-
ment in the spectral window. The reason why this sensi-
tivity to crystal field splitting is not evidenced with the
corresponding [Nd(H(2,2)-Me-3,2-HOPO)] complex is
unclear, but our investigations into this unusual effect
are ongoing.
Corresponding lifetimes measured in D2O (pD ∼ 7.4)
for all complexes were significantly longer, as summarized
in Table 3, because of the well-known reduced non-radia-
tive quenching effect of vOD versus vOH solvent vibra-
tions. From these luminescence lifetimes in deuterated
solvent, we were able to estimate the number of bound
inner sphere water molecules, q, using the empirical rela-
tionships proposed by Beeby42 and Faulkner;43
To quantify the luminescence efficiency of the observed
NIR emission, the overall quantum yields, Φtot, were
determined for the Yb(III) complexes in 0.1 M TRIS
buffered aqueous solution at pH 7.4, using [Yb(TTA)3-
(H2O)2] in toluene as a reference29,30 (Φref = 0.35%) (see
Supporting Information, Figures S3 and S4). The result-
ing values we obtain for [Yb(5LIO-Me-3,2-HOPO)2] and
q ¼ AðΔkobs - BÞ
determined for Yb(III) and Nd(III), respectively, where
q ¼ ðAΔkobsÞ - C
A = 1 μs for Yb(III) or 130 ns for Nd(III), B = 0.2 μs-1
and C = 0.4. The Δkobs term is evaluated as Δkobs
,
=
kobs(H2O) - kobs(D2O) with kobs = 1/τobs, using units of ns
and μs for the Nd(III) and Yb(III) lifetimes, respectively.
Hence, for [Yb(5LIO-Me-3,2-HOPO)2], we obtain a
q value of about 0.4. We take this value as a slight over-
estimate, indicating an absence of water molecules in the
first coordination sphere in accordance with the observed
solid state structure, and the residual is most likely due to
second sphere contributions such as vC-H and vN-H
vibrations of the ligand which are not correctly accounted
for by the B term. For the [Yb(H(2,2)-Me-3,2-HOPO)]
complex, the q value we obtain of about 0.6 is slightly
higher. However, since almost identical second sphere
contributions should exist with this octadentate ligand
topology, it is most likely that this value is similarly an
overestimate, rather than an equilibrium involving a
hydrated species, and we propose an eight coordinate
q = 0 coordination geometry for the [Yb(H(2,2)-Me-3,2-
HOPO)] complex. Notably, this result differs to that
obtained12 for the isomeric H(2,2)-1,2-HOPO complex
with Eu(III), where lifetime data clearly indicated a resi-
dual inner sphere water molecule (q = 1). The reason for
the proposed differences can be rationalized as a combina-
tion of two factors; primarily because of the presence of
four methyl groups in the 10 position of the Me-3,2-HOPO
chromophores, which introduces additional steric crowd-
ing at the ‘open face’ of the tetrapodal complex, and also
the smaller effective ionic radius of the Yb(III) cation
[Yb(H(2,2)-Me-3,2-HOPO)] were Φtot = 0.22% and Φtot
=
0.09%, respectively. The absolute magnitude of these values
remains modest because of highly competitive non-radiative
deactivation in the NIR region. Also, it is quite likely that
energy transfer from the ligand to metal cation in these
systems is not optimized, since the energy difference between
the lowest energy ligand triplet T1 stateandthemetalislarge,
at about 7500 cm-1. Nonetheless, they still compare very
well with some of the more emissive complexes appearing in
the literature, such as the modified ‘fluorexon’ derivatives
from Werts et al.,41 with Φtot = 0.45% in D2O and the more
recent 8-hydroxyquinoline derived podands reported by
€
9
Bunzli et al., with Φtot = 0.37% in H2O. For the Nd(III)
Me-3,2-HOPO based complexes, given the inherently
weaker signals involved, we made no effort to measure
the quantum yields, although, qualitatively, their emission
was approximately an order of magnitude weaker than the
corresponding Yb(III) complexes.
Another indicator of the luminescence efficiencies are
the luminescence lifetimes of the Ln(III) cations, which
are summarized in Table 3. In 0.1 M TRIS buffered
aqueous solution at pH 7.4, the values for Yb(III) are
comparable with recently reported Yb(III) complexes,
with observed lifetimes in the microsecond (μs) time
domain.41,9 For the Nd(III) complexes, the decay times
are significantly shorter with observed lifetimes in the
nanosecond region because of additional non-radiative
˚
˚
(CN = 8, 0.985 A) compared to Eu(III) (CN = 8, 1.066 A)
may play a role.
For the Nd(III) complexes, the metal center also has a
4
deactivation pathways for the F3/2 excited state invol-
˚
much larger effective ionic radius (CN = 8, 1.109 A)
ving intervening electronic states. Interestingly, the mag-
nitudes of the decay time are reversed in aqueous solution
when comparing the Yb(III) and Nd(III) complexes, such
that while [Yb(H(2,2)-Me-3,2-HOPO)] has a shorter life-
time than [Yb(5LIO-Me-3,2-HOPO)2] in aqueous solu-
tion, the situation is reversed for the Nd(III) complexes.
These changes are likely a result of differing solvent
accessibilities for the differing metal cations, and will be
further discussed below (vide infra).
when compared with Yb(III). Using Faulkner’s equation,
we determine q values of about 1.0 and 0.7 for the 5LIO-
Me-3,2-HOPO and H(2,2)-Me-3,2-HOPO Nd(III) com-
plexes, respectively, both of which are larger than the
corresponding values with Yb(III). While these values
should be interpreted with some caution, they no doubt
(42) 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, 493–503.
(41) Werts, M. H. V.; Verhoeven, J. W.; Hofstraat, J. W. J. Chem. Soc.,
Perkin Trans. 2 2000, 433–439.
(43) Faulkner, S.; Beeby, A.; Carrie, M.-C.; Dadabhoy, A.; Kenwright,
A. M.; Sammes, P. G. Inorg. Chem. Commun. 2001, 4, 187–190.

4166 Inorganic Chemistry, Vol. 49, No. 9, 2010
Moore et al.
reflect an improved solvent accessibility toward the larger
metal. Moreover, while the increase in q for [Nd(H(2,2)-
Me-3,2-HOPO)] is only marginal, the obtained q for
[Nd(5LIO-Me-3,2-HOPO)2] is more than doubled, and
this is accompanied by the previously mentioned conco-
mitant switch in the order of luminescence lifetimes in
aqueous solution. Hence, while [Yb(H(2,2)-Me-3,2-HOPO)]
has a shorter lifetime than [Yb(5LIO-Me-3,2-HOPO)2] in
aqueous solution, the opposite is true of the Nd(III) com-
plexes. Evidently, the steric demands and rigidity of the
octadentate [Yb(H(2,2)-Me-3,2-HOPO)] and [Nd(H(2,2)-
Me-3,2-HOPO)] complexes can provide more adequate
protection from non-radiative solvent deactivation, and
effectively exclude the entry of water molecules to the inner
coordination sphere of the metal. For the bis-tetradentate
complexes, it would appear that the more flexible ligands in
combination with the larger Nd(III) metal ion can accom-
modate an equilibrium involving a hydrated [Nd(5LIO-Me-
3,2-HOPO)2(H2O)] species.
NIR emission spectra from Yb(III) can also be useful in
determining the site symmetry of the metal, as a result of
readily observed crystal field splitting effects. In addition to
their efficient NIR luminescence, these Me-3,2-HOPO based
systems also demonstrate exceptional aqueous stabilities
which will be of paramount importance in their further
development should complexes such as these be used for in
vivoapplications, with pM valueson the order of pYb∼ 21.9,
no doubt reflecting the use of an exclusively oxygen contain-
ing donor set.
Acknowledgment. This work was partially supported by
the NIH (Grant HL69832) and supported by the Direc-
tor, Office of Science, Office of Basic Energy Sciences,
and the Division of Chemical Sciences, Geosciences, and
Biosciences of the U.S. Department of Energy at LBNL
under Contract No. DE-AC02-05CH11231. This tech-
nology is licensed to Lumiphore, Inc. in which some of the
authors have a financial interest. Financial support was
provided to C.J.J. and to M.S. by the German Research
Foundation (DFG). The authors thank Dr. Kathleen
Durkin for assistance with the TD-DFT calculations.
Conclusions
The 1-methyl-3-hydroxy-pyridin-2-one (Me-3,2-HOPO)
chelate group has proven to be a remarkably efficient
sensitizer with the NIR emitting Ln(III) cations (Ln = Yb,
Nd), displaying exceptional photophysical performance in
terms of their overall quantum yield and luminescence life-
times in aqueous solution. Furthermore, we have shown that,
by analogy to the case for Eu(III), a detailed analysis of the
Supporting Information Available: X-ray crystallographic
files (in CIF format), additional variable pH Yb(III) lumine-
scence spectra (Figure S1) and symmetry fits (Figure S2), and
quantum yield determinations (Figure S3 and S4). This material
is available free of charge via the Internet at http://pubs.acs.org.