Effects of Ligand Geometry on the Photophysical Properties of Photoluminescent Eu(III) and Sm(III) 1-Hydroxypyridin-2-one Complexes in Aqueous Solution

Article abstract of DOI:10.1021/acs.inorgchem.5b01927

A series of 10 tetradentate 1-hydroxy-pyridin-2-one (1,2-HOPO) ligands and corresponding eight-coordinated photoluminescent Eu(III) and Sm(III) complexes were prepared. Generally, the ligands differ by the linear (nLI) aliphatic linker length, from 2 to 8

Full text of DOI:10.1021/acs.inorgchem.5b01927

Article
pubs.acs.org/IC
Effects of Ligand Geometry on the Photophysical Properties
of Photoluminescent Eu(III) and Sm(III) 1‑Hydroxypyridin-2-one
Complexes in Aqueous Solution
Lena J. Daumann, David S. Tatum, Christopher M. Andolina, Joseph I. Pacold, Anthony D’Aleo,
́
Ga-lai Law, Jide Xu, and Kenneth N. Raymond*
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
Department of Chemistry, University of California, Berkeley, California 94720-1460, United States
S
* Supporting Information
ABSTRACT: A series of 10 tetradentate 1-hydroxy-pyridin-2-
one (1,2-HOPO) ligands and corresponding eight-coordinated
photoluminescent Eu(III) and Sm(III) complexes were
prepared. Generally, the ligands differ by the linear (nLI)
aliphatic linker length, from 2 to 8 methylene units between
the bidentate 1,2-HOPO chelator units. The photolumines-
cent quantum yields (Φtot) were found to vary with the linker
length, and the same trend was observed for the Eu(III) and
Sm(III) complexes. The 2LI and 5LI bridged complexes are
the brightest (Φtotxε). The change in ligand wrapping pattern
between 2LI and 5LI complexes observed by X-ray diffraction (XRD) is further supported by density functional theory (DFT)
calculations. The bimodal Φtot trends of the Eu(III) and Sm(III) complexes are rationalized by the change in ligand wrapping
pattern as the bridge (nLI) is increased in length.
closely spaced excited states, which favor nonradiative decay
processes.24,32−35 The energy gap between the lowest lying
excited state and highest lying ground state of Sm(III) is
relatively small (∼7400 cm−1), which reduces the lifetime of the
Sm(III) excited state, which is a result of more efficient
nonradiative decay via vibronic coupling of the nearby X−H
oscillators. These processes lead to lower photoluminescence
Φtot, which are typically <1% for Sm(III) complexes in aqueous
buffered solution. However, note that larger Sm(III) quantum
yields have been reported in organic solvents (e.g., 2.7% in
pyridine for Sm(hfa)3(phen)2).1,36−41 Sm(III) complexes can
be used for near-infrared bioimaging,42 and its emission can be
used in conjunction with Tb(III) and Eu(III) for multiplexing
assays.43,44 Understanding the underlying energy transfer
pathways and the influence of coordination geometry on photo-
luminescent parameters is of key interest when designing new
photoluminescent probes.45,46 Yan et al. have recently demon-
strated that controlling the symmetry around the Eu(III) ion
with different bis-β-diketonate ligands can help in designing
more photoluminescent complexes.47 Hasegawa and co-workers
reported that a reduction of geometrical symmetry in photolumi-
nescent Eu(III) complexes leads to a larger krad value.48 They
suggest that mixed hfa/phosphine oxide ligands, which speci-
fically encourage dodecahedral structures, yield complexes with
high quantum yields. The same authors have also considered the
INTRODUCTION
■
Photoluminescent lanthanide(III) complexes have attracted
attention because of their use in a wide variety of applications,
such as biofluoroimmunoassays,1,2 as sensors,3,4 in light-emitting
diodes,5,6 and other materials.7−18 Interest in lanthanide photo-
luminescence is spurred by typically long-lived excited state
lifetimes, large pseudo-Stokes shifts, and insensitivity to
quenching by molecular oxygen compared to organic fluo-
rophores.19 The f−f transitions of the lanthanides are Laporte-
forbidden, resulting in small molar absorption coefficients
(<10 M−1 cm−1) for the Ln(III) cations. Therefore, practical
applications require indirect excitation via an organic chro-
mophore with a large molar absorptivity, the so-called “antenna
effect”.20 However, predicting which antenna will result in the
greatest overall quantum yield (Φtot) for a given Ln(III)
complex still represents a significant challenge. Tuning the
antenna chromophore’s triplet excited state energy is known
to affect rates of energy transfer to the Ln(III) center.21−23
In addition, shielding of the Ln(III) center from nearby X−H
(O−H and N−H) oscillators is critical to developing highly
photoluminescent Ln(III) complexes.24,25 The effects of
differing coordination geometry have been less frequently
studied.26,27 For Eu(III), Φtot values of >90% in organic
solvents have been reported.28−30 Biological assays require large
Φtot in aqueous solution, and recently complexes have been
reported in this medium with remarkable Φtot as high as
37%.15,16,31 For Sm(III) compounds, enhancing the photo-
luminescent output is more difficult to achieve, because of the
Received: August 20, 2015
© XXXX American Chemical Society
A
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Figure 1. Tetradentate ligands used to form ML2 complexes with Ln(III). nLI indicates the aliphatic chain length for nLI-1,2-HOPO.
1
spectrometer operating at 600 MHz for H (or 151 MHz for 13C,
respectively). 1H (or 13C) chemical shifts are reported in ppm, relative
to the solvent resonances, taken as δ 7.26 (77.0) and δ 2.50 (39.5),
respectively, for CDCl3 and (CD3)2SO, while coupling constants (J)
are reported in Hertz. The following standard abbreviations are used
effect of ligand polarization in similar mixed ligand systems. They
conclude that the more photoluminescent complexes are due to
the enhancement of the electric dipole transition and suppression
of vibrational relaxation. Destri et al. investigated the photo-
physical properties of mixed β-diketonate ligands and found that,
in more photoluminescent complexes, the angle of the molecular
plane involving the O−Eu−O and β-diketonate residues is
smaller, perhaps leading to better orbital overlap between Eu and
the oxygen donors.49 This is also consistent with what we have
recently found using magnetic circular dichroism (MCD) with
the octadentate 3,4,3-LI-1,2-HOPO ligand system.46 We have
also recently reported a series of octadentate “all-1,2-HOPO”
complexes, where a change in linker length between two bis-
bidentate 1,2-HOPO units affected the photoluminescent
properties of Eu(III) in aqueous solution.50 Here, we report
the effect of changing the linker backbone length (nLI-) between
two bidentate 1-hydroxypyridin-2-one (1,2-HOPO) chelate
chromophores and the photophysical properties of the resulting
Eu(III) and Sm(III) complexes. These tetradentate 1,2-HOPO
ligands bind to Ln(III) in a 2:1 stoichiometry, yielding a [LnL2]−
complex. The alkyl linker length of the ligands presented here
range from ethyl- (2LI) to octyl- (8LI) chains (Figure 1).
In addition, more-rigid linkers (3LIdiMe and the previously
published 4LIoPhen), as well as the previously reported 5LI and
5LIO linkers, are included here.39,51,52
1
for characterization of H NMR signals: s = singlet, d = doublet, t =
triplet, q = quartet, quin = quintet, m = multiplet, dd = doublet of
doublets. Fourier transform infrared (FT-IR) spectroscopy was
conducted using a spectrometer (Nicolet Model 380) from Thermo
Electron (ZnSe crystal). High-resolution electrospray ionization
(HR-ESI)−mass spectrometry and C,H,N elemental analyses were
performed at the QB3/Chemistry Mass Spectrometry Facility and
the Microanalytical Facility (College of Chemistry, University of
California, Berkeley). The content of samarium and europium metal
was determined with inductively coupled plasma−optical emission
spectrometry (ICP-OES), using an ICP Optima 7000 DV instrument.
Silicon (ICP standard, purchased from Ricca Chemical Company) was
used as an internal standard. Samples and standards were diluted using
3% nitric acid (TraceSelect Nitric Acid, purchased from Fluka
Analytical). Note that the complexes were dissolved in dimethylsulf-
oxide (DMSO) first (1 mg/mL) and then diluted into 3% nitric acid,
which yielded the same results as when the complexes were digested in
hot concentrated nitric acid and then subsequently diluted with water.
The europium ICP standard was purchased from Fluka (TraceCert),
and the samarium was purchased from Merck (Certipur). Calibration
curves and raw data can be found in the Supporting Information
(Figure S1 and Table S1). The overall error of the measurement was
estimated to be 20%. Thin-layer chromatography (TLC) was performed
using precoated Kieselgel 60 F254 plates. Flash chromatography was
performed using the automated Combiflash Rf system from Tele-
dyneIsco using normal-phase (silica) disposable RediSepRf columns.
X-ray Crystallographic Characterization. X-ray-quality crystals
of [Eu(2LI-1,2-HOPO)2]PyH and [Sm(2LI-1,2-HOPO)2]PyH were
EXPERIMENTAL SECTION
■
Materials and Methods. Nuclear magnetic resonance (NMR)
spectra were obtained using (unless otherwise noted) a Bruker AV600
B
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obtained by slow diffusion of methanol into a DMSO solution of the
complexes. Crystals of [Eu(3LIdiMe-1,2-HOPO)2]NMe4 and [Sm-
(3LIdiMe-1,2-HOPO)2]NMe4 were obtained by slow diffusion of
diethyl ether into a dimethylformamide solution of the complexes.
A suitable crystal was selected and mounted on a captan loop with
Paratone oil and cooled under a controlled temperature stream of
liquid nitrogen. X-ray data were collected on a Bruker APEX-I CCD
diffractometer with Mo Kα radiation (λ = 0.71073 Å) except of
[Sm(3LIdiMe-1,2-HOPO)2]NMe4 which was collected on a Bruker
SMART 1000 CCD instrument at the UC Berkeley X-ray crystallo-
graphic facility. Absorption corrections were applied using SADABS.53
The structure was solved using direct methods and refined by the full-
matrix least-squares procedure in SHELXL.54−56 All non-hydrogen
atoms were refined anisotropically. All hydrogen atoms were placed at
calculated positions and included in the refinement using a riding
model. The refinement tool SQUEEZE57 was used to remove electron
density due to disordered solvent from the .hkl file used for final
refinement.
for Sm(III), the NIR transitions were not measured/integrated;
thus, the reported values provide a low estimate of the total Φ.
Excitation spectra were recorded on the same instrument and recorded
from 220 nm to 450 nm with an excitation wavelength of λex
=
612 nm, as well as excitation and emission slit widths of 5 and 1 nm
respectively.
Time-resolved photoluminescence lifetimes were determined on a
Horiba Jobin Yvon IBH FluoroLog-3 spectrofluorometer, adapted for
time-correlated single photon counting (TCSPC) and multichannel
scaling (MCS) measurements, as previously reported.50 A submicro-
second xenon flash lamp (Jobin Yvon, Model 5000XeF) was used as
the light source, coupled to a double-grating excitation monochroma-
tor for spectral selection. Goodness of fit was assessed by minimizing
the reduced chi-squared function (χ2) (see the Supporting Information),
and a visual inspection of the weighted residuals. The number of water
molecules (q) for the Sm(III) complexes was calculated using eq 2 by
Kimura and Kato, which solely relies on the lifetime of samarium in
water:34,35
Ground-State Density Functional Theory (DFT) Calculations.
DFT calculations were performed at the Molecular Graphics and
Computational Facility (College of Chemistry, University of
California, Berkeley), using Gaussian 09.58 The ground-state geo-
metries of the complexes were optimized using the B3LYP functional,
treating the light atoms with the 6-31G(d,p) basis set and the Eu(III)
with the effective core potential MWB52.59,60 No solvent, symmetry
constraints, or counterions were included. Shape analysis of the
calculated structures based on interatomic dihedral angles was
performed using an updated algorithm.61,62
qSm = 0.26kobs − 1.6
(2)
1
kobs (μs−1) =
τ
H2O
(3)
For the Eu(III) complexes, the hydration state was calculated using the
formalism of Horrocks and Sudnick:33
qEu = 1.11(Δkobs − 0.31)
(4)
Photophysical Characterization. Sample preparation for ab-
sorption, Φtot, ε, and τ measurements was conducted as follows. The
complexes were dissolved in DMSO (1 mg/mL) and then diluted into
tris(2-aminoethyl)amine (Tris) buffered saline (TBS) (20 mM buffer,
pH 7.4, 100 mM NaCl) to a final concentration of ∼0.5 × 10−5 M.
Ultraviolet−visible (UV-vis) absorption spectra were recorded with a
diode array UV-vis absorption spectrometer (Model HP 8453).
Relative quantum yields were determined by the optically dilute
method using the same excitation wavelength for both the reference
(Quinine Sulfate, purchased from Sigma−Aldrich, meets USP testing
specifications) and samples (λex = 330 nm) following a previously
described protocol using eq 1.46,63 Quinine sulfate (QS) in 0.5 M
H2SO4 (Φ = 0.546) was used as the aqueous fluorescence quantum
yield standard.63
1
1
Δkobs (ms−1) = =
−
τ
τ
H2O
D2O
(5)
Note that the samples for lifetime measurements in D2O were not
buffered. Concentrations for lifetime measurements were 1 × 10−5
M
with the samples initially dissolved in DMSO (3 mg/mL) and then
diluted into TBS or D2O. To rule out the possibility of species with
higher molecular weight (M2L4, M3L6 complexes, etc.) an experiment
with complex concentrations of 3 × 10−6 M and 1 × 10−9 M was also
conducted. The lifetimes did not change significantly over this
concentration range and thus the presence of higher-molecular-weight
species was ruled out. Further evaluation of photophysical parameters
of the Eu(III) complexes was conducted after previously reported
methods by Beeby et al. and Werts et al.,64,65 where the overall quantum
yield Φtot is a product of the quantum efficiency of europium (ηEu)
(sometimes reported as ΦEu) and the efficiency of ligand sensitiza-
tion (ηsens):
2
⎡
⎢
⎤
⎥
⎡
⎢
⎤
⎥
⎡
⎢
⎣
⎤⎡
⎥⎢
⎦⎣
⎤
⎥
⎦
A
QS(λQS
)
I(λQS
)
Φ
ΦQS
nx
n
Ex
x
=
2
⎢
⎥
⎦
Ax(λx)
I(λ )
⎢
⎣
⎥ E
⎣
⎦
x
QS
QS
(1)
Φ
= ηEuη
(6)
tot
where A is the absorbance at the excitation wavelength (λ), I the
intensity of the excitation light at the same wavelength, n the refractive
index, and E the integrated photoluminescence intensity. The
subscripts “x” and “QS” refer to the sample and reference, respectively.
The same excitation wavelength of 330 nm was used for both the
reference and sample; hence, the I(λQS)/I(λx) term is removed.
Similarly, the refractive indices term, nx2/nQS2, was taken to be identical
for the aqueous reference and sample solutions. The plot of integrated
emission intensity (i.e., EQS) vs absorbance at 330 nm (i.e., AQS) yields
sens
H2O
τ
ηEu
=
τrad
(7)
⎛
⎜
⎝
⎞
⎟
⎠
Itot
I
MD
1
τrad
(s−1) = A(MD,0)n3
(8)
AMD,0 is the spontaneous emission probability of the magnetic dipole
transition (5D0 → 7F1), which is a constant equal to 14.65 s−1, n refers to
the refractive index of water (1.334) and Itot/IMD is the ratio of
integrated emission intensity of the total emission (5D0 → 7FJ, J = 0−6)
a linear plot with a slope that can be equated to the quinine sulfate
63
quantum yield ΦQS
.
In order to increase the absorption signal for
these dilute solutions, absorption measurements (Absmax < 0.25) were
made using a 5 cm quartz cuvette (from Starna Cells, Inc., Spectrosil
Quartz) for both the sample and standard dilutions. Photolumi-
nescence measurements were made using a 1 cm quartz fluorescence
cuvette (from Starna Cells, Inc., Spectrosil Quartz).
65−68
5
7
divided by the integrated D0 → F1 emission (580−600 nm).
The 5D0 → 7F5 and 5D0 → 7F6 transitions were not recorded, because of
their weak intensity and large amount of detector noise in those regions.
Finally, the rate of nonradiative decay (knonrad) can be calculated using
Emission spectra were acquired on a Horiba Jobin Yvon IBH
FluoroLog-3 spectrofluorometer. All measurements were done in
triplicate and a total of six dilutions were used for each measurement.
Spectra were reference-corrected for both the excitation light source
variation (lamp and grating) and the emission spectral response
(detector and grating). For 7LI and 8LI complexes, because of
decreased solubility and increased signal-to-noise (S/N) ratio, the
measurements were repeated five times. It should be noted that,
the experimentally determined lifetime in water (τH O and the calculated
2
radiative lifetime constant (krad), according to the following equation.
1
1
τrad
knonrad (s−1) = kobs − krad
=
−
τ
(9)
obs
The brightness was determined from the product of the molar ex-
tinction coefficient (ε) and Φtot. Time-gated phosphorescence spectra
C
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1
of the in situ-generated gadolinium complexes were measured in
methanol:ethanol (1:4 v/v) at 77 K (λex = 330 nm, delay 0.1 ms),
using the Horiba Jobin Yvon IBH FluoroLog-3 spectrofluorimeter as
described above. The Gd-nLI-1,2-HOPO triplet phosphorescence
spectra were fitted using Gaussian functions with OriginPro 8.5
software.69,70 Spectra were baseline-corrected and fit to nine Gaussian
functions each. The full widths at half maximum (fwhm) were shared
among all Gaussian peaks for each spectrum. The resulting Gaussian
peaks were separated by 700−1100 cm−1. The Gaussian of highest
energy was determined to be the T0−0 of the triplet state of the
chelating 1,2-HOPO and are reported in Table S8 in the Supporting
Information.
3LIdiMe-1,2-HOPOBn. Yield: 73%, beige oil. H NMR (CDCl3) δ:
0.83 (s, 6H, CH3); 3.01 (d, 4H, CH2); 5.30 (s, 4H, CH2); 6.31 (dd,
2H, arH); 6.66 (dd, 4H, arH); 7.27 (m, 2H, arH); 7.30 (m, 6H, arH);
7.44 (m, 4H, arH); 7.61 (m, 2H, NH). 13C NMR (CDCl3) δ: 23.6,
36.9, 46.4, 50.8, 79.3, 105.8, 124.1, 128.5, 129.4, 130.1, 133.2, 138.1,
142.8, 158.6, 161.2.
4LI-1,2-HOPOBn. Yield: 48.2%, beige oil. 1H NMR (CDCl3) δ: 1.39
(bs/m, 4H, CH2); 3.14 (bs/m, 4H, CH2); 5.29 (s, 4H, CH2); 6.37
(d, 2H, arH, J = 6.60 Hz); 6.68 (m, 4H, arH, J = 9.60 Hz); 7.30 (t, 2H,
arH, J = 7.20 Hz); 7.36 (m, 6H, arH); 7.47 (m, 4H, NH, arH).
13C NMR (CDCl3) δ: 26.2, 39.7, 79.7, 106.2, 124.2, 128.7, 129.7,
130.3, 138.0, 142.9, 160.3 (two signals overlapping).
Syntheses. The syntheses of the [Sm(5LI-1,2-HOPO)2]− and
[Eu(5LI-1,2-HOPO)2]−, [Sm(5LIO-1,2-HOPO)2]−, and [Eu(5LIO-
1,2-HOPO)2]−, and [Eu(4LIoPhen-1,2-HOPO)2]− complexes have
been reported previously.39,51,52 The synthesis of 1,2-HOPOBn and its
acid chloride was conducted after a recently optimized published
procedure.46
1
6LI-1,2-HOPOBn. Yield, 82%, beige oil. H NMR (CDCl3) δ: 1.18
(m, 4H, CH2); 1.38 (m, 4H, CH2); 3.19 (m, 4H, CH2, J = 6.66 Hz);
5.28 (s, 4H, CH2); 6.44 (dd, 2H, arH, J = 6.82, 1.72 Hz); 6.69 (dd,
2H, arH, J = 9.26, 1.72); 6.74 (t, 2H, arH, J = 5.61 Hz); 7.30 (dd, 2H,
arH, J = 9.25, 6.82 Hz); 7.32−7.38 (m, 6H, arH); 7.46−7.50 (m, 2H,
arH, NH). 13C NMR (CDCl3) δ: 26.1, 28.6, 39.8, 79.2, 106.2, 123.6,
128.5, 129.3, 129.9, 133.2, 138.2, 142.7, 158.5, 160.1.
1,2-HOPOBn Thiazolide.
1
7LI-1,2-HOPOBn. Yield: 35%, beige oil. H NMR (CDCl3) δ: 1.18
(m, 6H, CH2); 1.39 (m, 4H, CH2); 3.25 (q, 4H, CH2, J = 6.90 Hz);
5.27 (s, 4H, CH2); 6.46 (dd, 2H, arH, J = 6.90, 1.80 Hz); 6.72
(dd, 2H, arH, J = 9.00, 1.50 Hz); 6.77 (t, 2H, arH); 7.30 (dd, 2H, arH,
J = 9.30, 6.90 Hz); 7.33−7.36 (m, 12H, arH, NH). 13C NMR (CDCl3)
δ: 25.9, 28.6, 38.9, 103.5, 119.1, 137.3, 142.4, 157.4, 160.1.
8LI-1,2-HOPOBn. Yield: 93%, beige oil. 1H NMR (500 MHz,
CDCl3) δ: 7.51−7.46 (m, 4H); 7.40−7.35 (m, 6H); 7.30 (dd, J = 9.3,
6.8 Hz, 2H); 6.80 (t, J = 5.8 Hz, 2H); 6.69 (dd, J = 9.2, 1.7 Hz, 2H);
6.48 (dd, J = 6.8, 1.7 Hz, 2H); 5.28 (s, 4H); 3.27 (q, J = 6.7 Hz, 4H);
1.44−1.34 (m, 4H); 1.21−1.12 (m, 8H). 13C NMR (125 MHz,
CDCl3) δ: 26.6, 28.8, 29.1, 40.4, 79.7, 106.8, 124.5, 128.9, 129.8, 130.6,
133.2, 138.3, 142.3, 158.7, 160.1.
1,2-HOPOBn (7.74 g, 0.0315 mol) was converted to the acid chloride
as previously described.46 The 1,2-HOPOBn acid chloride was dis-
solved in dichloromethane (300 mL) and added dropwise to a
vigorously stirred solution of mercaptothiazolide (0.038 mol, 4.5 g)
and potassium carbonate (0.16 mol, 21.8 g) in water (100 mL) during
a period of 2 h while cooling in an ice bath. After complete addition,
the bright yellow mixture was warmed to room temperature and
stirred overnight. Subsequently, the layers where separated and the
aqueous layer extracted with dichloromethane (2 × 100 mL). The
combined organic phases were evaporated using reduced pressure
and the crude yellow oil was purified using column chromatography
(100% dichloromethane). The fractions containing product were
combined and the dichloromethane subsequently evaporated to a
volume of ∼1 mL. The addition of isopropanol (300 mL) and cooling
at 4 °C caused the pure product to crystallize within 12 h and yielded
5.25 g (48.1%) of a yellow powder. 1H NMR (CDCl3) δ: 3.15 (t, 2H,
CH2, J = 7.36 Hz); 4.45 (t, 2H, CH2, J = 7.37 Hz); 5.32 (s, 2H, CH2);
6.18 (dd, 1H, arH, J = 6.82, 1.61 Hz); 6.79 (dd, 1H, arH, J = 9.28,
1.60 Hz); 7.29 (dd, 1H, arH, J = 9.27, 6.82 Hz); 7.38−47 (m, 5H,
arH). 13C NMR (CDCl3) δ: 28.9, 54.4, 79.0, 104.9, 124.4, 128.6,
129.3, 129.8, 134.1, 137.8, 142.1, 158.2, 159.8, 200.1. IR (ν, cm−1)
1696, 1659 (s, CO); 793, 739 (m, arH). p-ESI HRMS (MeOH)
347.0521 (calculated: 347.05 [C16H15N2O3S2]+).
General Procedure for the Deprotection of the LI Ligands.
The ligands were deprotected using a 1:1 mixture of concentrated
hydrochloric acid and glacial acetic acid (10 mL per 100 mg protected
ligand) and stirring the solution for 72 h at room temperature, after
which time the solvent was removed in vacuo. The ligand was co-
evaporated twice with water and once with methanol to yield an off-
white foam. The use of EDTA-washed glassware for this and the
following complexation step is advised.
2LI-1,2-HOPO. Yield, quantitative 1H NMR (DMSO) δ: 3.35
(d, 4H, CH2, J = 6.0 Hz); 6.34 (dd, 2H, arH, J = 6.6, 1.8 Hz); 6.58
(dd, 2H, arH, J = 9.0, 1.8 Hz), 7.40 (dd, 2H, arH, J = 9.6, 6.6 Hz); 8.83
(d, 2H, NH, J = 5.4 Hz). 13C NMR (DMSO) δ: 38.5, 103.9, 119.9,
137.5, 142.5, 157.5, 160.5. p-ESI HRMS (MeOH): 333.0837
(calculated: 333.0841 [C14H13N4O6]− [2LI-1,2-HOPO-H]−).
IR (ν, cm−1): 3336 (b, O−H); 3084 (m, C−H); 1652, 1524
(s, CO); 789 (m, arH). CHN found: C, 49.58; H, 4.12; N, 16.50;
calculated: C, 50.30; H, 4.22; N, 16.70.
General Procedure for the Synthesis of the LI Ligands.
The 1,2-HOPOBn thiazolide (6.3 mmol) was dissolved in dichloro-
methane (100 mL) and the amine (3 mmol) was added. The yellow
solution was stirred at room temperature for 24 h and, subsequently,
the solvent was removed in vacuo and the crude ligand was purified
with column chromatography (5−10% MeOH in dichloromethane,
detection with UV).
3LI-1,2-HOPO. Yield, quantitative 1H NMR (DMSO) δ: 1.71 (quin,
2H, CH2, J = 7.00 Hz); 3.38 (m, 4H, CH2, J = 6.65 Hz); 6.30 (d, 2H,
arH, J = 6.88 Hz); 6.56 (d, 2H, arH, J = 9.08 Hz); 7.39 (dd, 2H, arH,
J = 9.08, 6.95 Hz); 8.79 (t, 2H, NH, J = 5.62 Hz). 13C NMR (DMSO)
δ: 28.8, 37.1, 106.0, 119.8, 137.7, 142.7, 157.9, 160.6. p-ESI HRMS
(MeOH): 349.1142 (calculated: 349.11, [C15H17N4O6]+, [3LI-1,2-
HOPO+H]+); 371.0961 (calculated: 371.10 [C15H16N4O6Na]+, [3LI-
1,2-HOPO+Na]+). IR (ν, cm−1) 3282 (b, O−H); 3088 (m, C−H);
1640, 1519 (s, CO); 793 (m, arH).
1
2LI-1,2-HOPOBn. Yield: 76%, beige oil. H NMR (CDCl3) δ: 3.47
(d, 4H, CH2); 5.19 (s, 4H, CH2); 6.25 (dd, 2H, CH2); 6.54 (dd, 2H,
CH); 7.15 (dd, 2H, CH); 7.24−7.40 (m, 10H, ArH); 7.47 (t, 2H,
NH). 13C NMR (CDCl3) δ: 40.6, 79.3, 106.32, 123.8, 129.4, 130.1,
133.1, 138.1, 142.5, 158.6, 160.3.
1
3LIdiMe-1,2-HOPO. Yield, 55% H NMR (DMSO) δ: 0.90 (s, 6H,
1
CH3); 3.12 (d, 4H, CH2, J = 6.93 Hz); 6.31 (dd, 2H, arH, J = 6.90,
1.65 Hz); 6.58 (dd, 2H, arH, J = 9.08, 1.64 Hz); 7.40 (dd, 2H, arH, J =
9.08, 6.91 Hz); 8.73 (t, 2H, NH, J = 6.23 Hz). 13C NMR (DMSO) δ:
23.7, 37.0, 46.4, 104.1, 119.9, 137.7, 142.6, 157.9, 160.9. p-ESI HRMS
(MeOH): 377.1454 (calculated: 377.15, [C17H20N4O6]+, [3LI22Me-
1,2-HOPO+H]+); 399.1272 (calculated: 399.13 [C17H20N4O6Na]+,
3LI-1,2-HOPOBn. Yield: 89%, beige oil. H NMR (DMSO) δ: 1.66
(quin, 2H, CH2, J = 5.94 Hz); 3.20 (q, 4H, CH2, J = 6.06 Hz); 5.21
(s, 4H, CH2); 6.25 (dd, 2H, arH, J = 6.78, 1.57 Hz); 6.57 (dd, 2H,
arH, J = 9.24, 1.66 Hz); 7.20 (m, 2H, arH, J = 9.23, 6.87 Hz); 7.25−
7.34 (m, 12H, arH, NH). 13C NMR (DMSO) δ: 28.3, 36.6, 52.2,
103.5, 119.3, 127.9, 128.9, 130.1, 130.7, 137.1, 142.1, 157.3, 160.1.
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[3LI22Me+Na]+). IR (ν, cm−1) 3319 (b, O−H); 3084, 2939 (m,
C−H); 1635, 1527 (s, CO); 805, 748 (m, arH).
[Sm(2LI-1,2-HOPO)2]PyH. Yield: 81%. n-ESI HRMS (MeOH):
816.0697 (calculated: 816.0716 [C28H24N8O12Sm]−, [Sm(2-LI-1,2-
HOPO)2]−). CHN found: C, 44.38; H, 3.65; N, 14.11; Sm, 15.71;
calculated: C, 44.48; H, 3.62; N, 14.28; Sm, 16.14. Molecular weight:
931.56 g/mol.
[Eu(3LI-1,2-HOPO)2]PyH·3H20·0.5Eu(OH)3. Yield: 56%. n-ESI
HRMS (MeOH): 845.1028 (calculated: 845.10 [C30H28N8O12Eu]−,
[Eu(3-LI-1,2-HOPO)2]−). CHN found: C, 38.67; H, 3.42; N, 11.65;
Eu, 20.48; calculated: C, 38.92; H, 3.87; N, 11.67; Eu, 21.10.
Molecular weight: 1080.21 g/mol.
[Sm(3LI-1,2-HOPO)2]PyH·3H2O·0.5Sm(OH)3. Yield: 58%; n-ESI
HRMS (MeOH): 844.1023 (calculated: 844.10 [C30H28N8O12Sm]−,
[Sm(3-LI-1,2-HOPO)2]−). CHN found: C, 38.94; H, 3.25; N, 11.66;
Sm, 20.60; calculated: C, 38.94; H, 3.88; N, 11.70; Sm, 20.93.
Molecular weight: 1077.81 g/mol.
[Eu(3LIdiMe-1,2-HOPO)2]PyH·5H2O·0.5 Eu(OH)3. Yield: 53%. n-ESI
HRMS (MeOH): 901.1652 (calculated: 901.17 [C34H36N8O12Eu]−,
[Eu(3-LI-2,2Me-1,2-HOPO)2]−). CHN found: C, 40.85; H, 4.11; N,
11.02; Eu, 19.22; calculated: C, 39.96; H, 4.60; N, 10.75; Eu, 19.44.
Molecular weight: 1172.35 g/mol.
[Sm(3LIdiMe-1,2-HOPO)2]PyH·5H2O·0.5 Sm(OH)3. Yield: 57%. n-
ESI HRMS (MeOH): 900.1640 (calculated: 900.17
[C34H36N8O12Sm]−, [Sm(3-LI-2,2Me-1,2-HOPO)2]−). CHN found:
C, 39.91; H, 4.21; N, 10.78; Sm, 19.09; calculated: C, 40.04; H, 4.61;
N, 10.78; Sm, 19.28. Molecular weight: 1169.94 g/mol.
[Eu(4LI-1,2-HOPO)2]PyH·6H2O·0.5 Eu(OH)3. Yield: 55%. n-ESI
HRMS (MeOH): 873.1337 (calculated: 873.14 [C32H32N8O12Eu]−,
[Eu(4-LI-1,2-HOPO)2]−). CHN found: C, 37.23; H, 3.58; N, 10.70;
Eu, 21.66; calculated: C, 38.23; H, 4.47; N, 10.85; Eu, 19.61.
Molecular weight: 1162.31 g/mol.
[Sm(4LI-1,2-HOPO)2]PyH·6H2O·0.6Sm(OH)3. Yield: 55%. n-ESI
HRMS (MeOH): 872.1339 (calculated: 872.1333 [C32H32N8O12Sm]−,
[Sm(4-LI-1,2-HOPO)2]−). CHN found: C, 38.03; H, 3.72; N, 10.95;
Sm, 18.14; calculated: C, 38.40; H, 4.18; N, 10.89; Sm, 21.65. Molecular
weight: 1157.43 g/mol.
[Eu(6LI-1,2-HOPO)2]PyH·2H2O·0.5Eu(OH)3. Yield: 45%. n-ESI
HRMS (MeOH): 927.1961 (calculated: 929.1958 [C36H40N8O12Eu]−,
[Eu(6-LI-1,2-HOPO)2]−). CHN found: C, 42.26; H, 4.21; N, 10.84;
Eu, 19.65; calculated: C, 42.96; H, 4.53; N, 11.00; Eu, 19.88.
Molecular weight: 1146.35 g/mol.
[Sm(6LI-1,2-HOPO)2]PyH·2H2O·0.5Sm(OH)3. Yield: 45%. n-ESI
HRMS (MeOH): 928.1939 (calculated: 928.20 [C36H40N8O12Sm]−,
[Sm(6-LI-1,2-HOPO)2]−). CHN found: C, 42.49; H, 4.32; N, 10.91;
Sm, 19.26; calculated: C, 43.05; H, 4.54; N, 11.02; Sm, 19.72.
Molecular weight: 1143.95 g/mol.
[Eu(7LI-1,2-HOPO)2]PyH·2H2O·0.5Eu(OH)3. Yield: 70%. n-ESI
HRMS (MeOH): 957.2275 (calculated: 957.23 [C38H44N8O12Eu]−,
[Eu(7-LI-1,2-HOPO)2]−). CHN found: C, 44.13; H, 4.65; N, 10.67;
Eu, 18.49; calculated: C, 43.98; H, 4.76; N, 10.73; Eu, 19.41.
Molecular weight: 1174.41 g/mol.
[Sm(7LI-1,2-HOPO)2]PyH·H2O·0.5Sm(OH)3. Yield: 70%. n-ESI
HRMS (MeOH): 956.2265 (calculated: 956.23 [C38H44N8O12Sm]−,
[Sm(7-LI-1,2-HOPO)2]−). CHN found: C, 44.02; H, 4.54; N, 10.67;
Sm, 19.07; calculated: C, 44.76; H, 4.76; N, 10.92; Sm, 19.54.
Molecular weight: 1153.99 g/mol.
[Eu(8LI-1,2-HOPO)2]PyH·6H2O·0.3Eu(OH)3. Yield: 63%. n-ESI
HRMS (MeOH): 983.2611 (calculated: 938.2596 [C40H48N8O12Eu]−).
CHN found: C, 44.86; H, 5.29; N, 10.33; Eu, 16.67; calculated: C,
44.87; H, 5.27; N, 10.46; Eu, 16.82. Molecular weight: 1204.66 g/mol.
[Sm(8LI-1,2-HOPO)2]PyH·3H2O·0.5Sm(OH)3. Yield: 58%. n-ESI
HRMS (MeOH): 984.2590 (calculated: 984.2594 [C40H48N8O12Sm]−).
CHN found: C, 44.96; H, 5.36; N, 10.30; Sm, 20.04; calculated: C,
44.37; H, 5.09; N, 10.35; Sm, 18.52. Molecular weight: 1218.07 g/mol.
1
4LI-1,2-HOPO. Yield, quantitative H NMR (DMSO) δ: 1.53 (m,
4H, CH2); 3.21 (m, 4H, CH2, J = 5.48 Hz); 6.26 (dd, 2H, arH, J =
6.87, 1.41 Hz); 6.56 (dd, 2H, arH, J = 9.08, 1.38 Hz), 7.39 (dd, 2H,
arH, J = 9.01, 6.94 Hz); 8.76 (t, 2H, NH, J = 5.06 Hz). 13C NMR
(DMSO) δ: 26.4, 39.1, 103.9, 119.7, 137.8, 142.8, 158.1, 160.8. p-ESI
HRMS (MeOH): 363.1296 (calculated: 363.13 [C16H19N4O6]+, [4LI-
1,2-HOPO+H]+); 385.1117 (calculated: 385.11 [C16H18N4O6Na]+,
[4LI-1,2-HOPO + Na]+). IR (ν, cm−1) 3306 (b, O−H); 3088, 2944
(m, C−H); 1651, 1520 (s, CO); 795 (m, arH).
6LI-1,2-HOPO. Yield, 49% 1H NMR (DMSO) δ: 1.32 (m, 4H,
CH2); 1.47 (quin, 4H, CH2, J = 5.97 Hz); 3.19 (q, 4H, CH2, J = 12.76,
6.51 Hz); 6.26 (d, 2H, arH, J = 6.79 Hz); 6.56 (d, 2H, arH, J =
9.03 Hz); 7.38 (dd, 2H, arH, J = 8.85, 7.46 Hz); 8.74 (t, 2H, NH, J =
5.33 Hz). 13C NMR (DMSO) δ: 25.9, 28.6, 38.9, 103.5, 119.1, 137.3,
142.4, 157.4, 160.1. p-ESI HRMS (MeOH): 391.1612 (calculated:
391.16 [C18H23N4O6]+, [6LI-1,2-HOPO+H]+); 413.1413 (calculated:
413.13 [C18H22N4O6Na]+, [6LI-1,2-HOPO+Na]+). IR (ν, cm−1):
3319 (b, O−H); 3084, 2939 (m, C−H); 1635, 1558 (s, CO); 805,
748 (m, arH).
1
7LI-1,2-HOPO. Yield, quantitative H NMR (DMSO) δ: 1.29 (m,
6H, CH2); 1.46 (d, 4H, CH2, J = 6.00 Hz); 3.18 (q, 4H, CH2, J =
6.60 Hz); 6.24 (d, 2H, arH, J = 6.60 Hz); 6.55 (d, 2H, arH, J =
9.00 Hz); 7.38 (t, 2H, arH, J = 9.00 Hz); 8.76 (t, 2H, NH). 13C NMR
(DMSO) δ: 26.5, 28.6, 28.9, 39.2, 103.9, 119.6, 137.7, 143.1, 157.9,
160.4. p-ESI HRMS (MeOH): 405.1773 (calculated: 405.18
[C19H25N4O6]+, [7LI-1,2-HOPO+H]+).
8LI-1,2-HOPO. Yield, 97%. 1H NMR (500 MHz, DMSO-d6) δ: 8.74
(t, J = 5.6 Hz, 2H), 7.39 (dd, J = 9.1, 6.9 Hz, 2H), 6.56 (dd, J = 9.0,
1.7 Hz, 2H), 6.25 (dd, J = 6.9, 1.7 Hz, 2H), 3.18 (dd, J = 13.1,
6.8 Hz, 4H), 1.47 (p, J = 6.9 Hz, 4H), 1.35−1.23 (m, 8H).
13C NMR (125 MHz, DMSO) δ: 26.3, 28.6, 28.7, 38.9, 103.5, 119.2,
137.3, 142.5, 157.5, 160.2. p-ESI HRMS (MeOH): 419.1922
(calculated: 419.1922 [C20H27N4O6]+, [8LI-1,2-HOPO+H]+). CHN
found: C, 57.02; H, 6.50; N, 13.04; calculated: C, 57.41; H, 6.26; N,
13.39.
General nLi Ligand Complex Synthesis. The complexes were
synthesized by combining 2.5 equiv of nLI ligand and the respective
LnCl3·6H2O salt (1 equiv, EuCl3·6H2O from Alfa AESAR, 99.99%
purity and SmCl3·6H2O from Aldrich, 99.99% purity, respectively) in
methanol (∼0.25 M). After dissolution of the lanthanide, pyridine
(5 equiv) was added and the mixture was refluxed for 24 h. After this
time, the precipitated complexes were collected on a glass frit and
washed with methanol (2 mL). The complexes were obtained as off-
white powders. Note: if the complexes did not precipitate out of the
methanol solution, the volume was reduced by 50% and isopropanol
(1 mL) was added. Gd(III) complexes were prepared in situ. Note: the
elemental analyses for the complexes that could not be readily
recrystallized did show that CHN was too low. Initially, this was
thought to be due to the presence of Cl− anions; however, this was
ruled out by a negative test with AgNO3 and the absence of a Cl
signature in the X-ray photoelectron spectroscopy spectrum of a
representative sample. Hence, the complexes were analyzed with ICP-
OES for Sm and Eu content. (See the Supporting Information.) This
analysis suggested that, in the cases where CHN was too low, the Eu
and Sm content was elevated and the elemental analyses could be best
fit with some portion of Sm(OH)3 or Eu(OH)3. The co-crystallization
or coprecipitation of lanthanide salts with Ln(III) complexes has been
reported previously in the literature.71 Despite considerable efforts to
remove this impurity by crystallization or through washing procedures,
the excess lanthanide species could not be removed. Since this
impurity is unlikely to affect the photoluminescent properties (as, for
instance, excess ligand would), we proceeded with the photophysical
characterization.
RESULTS AND DISCUSSION
■
[Eu(2LI-1,2-HOPO)2]PyH. Yield: 84%. n-ESI HRMS (MeOH):
817.0710 (100%), 815.0696 (calculated: 815.0718 [C28H24N8O12Eu],
[Eu(2-LI-1,2-HOPO)2]−). CHN found: C, 44.56; H, 3.69; N, 14.37;
Eu, 13.14; calculated: C, 44.59; H, 3.85; N, 14.44; Eu, 15.67.
Molecular weight: 969.71 g/mol.
The ligands were synthesized by combining the different
diamine precursors with 1,2-HOPOBn thiazolide. After
purification with column chromatography, the ligands were
deprotected, using a mixture of concentrated hydrochloric acid
E
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and glacial acetic acid, yielding the ligands in good yields.
The complexes were synthesized by refluxing Sm(III) or Eu(III)
chloride hydrate with a slight excess of ligand (2.5 equiv) and
pyridine as a base in methanol. They were characterized by
mass spectrometry, elemental analysis (including ICP), and,
where possible, with X-ray analysis. Crystals were obtained for
the [Eu(2LI-1,2-HOPO)2]PyH, [Eu(3LIdiMe-1,2-HOPO)2]-
NMe4, [Sm(2LI-1,2-HOPO)2]PyH, and [Sm(3LIdiMe-1,2-
HOPO)2]NMe4 complexes (see Figure 2, in addition to
Figure 2. ORTEP diagrams of [Eu(2LI-1,2-HOPO)2]PyH and
[Eu(3LIdiMe-1,2-HOPO)2]NMe4. Counterions, solvent, second com-
plex molecule, and hydrogen atoms have been omitted for the sake of
clarity; 50% ellipsoid probability.
Figures S2 and S3, as well as Table S2, in the Supporting Infor-
mation). Complexes with the 2LI-1,2-HOPO ligand crystallized
in the Pna21 space group with two individual complex entities
in the asymmetric unit, displaying only marginal structural dif-
ferences. Structures of 4LIoPhen-, 5LI-, and 5LIO-1,2-HOPO
Eu(III) complexes and 5LI- and 5LIO-1,2-HOPO Sm(III)
complexes have been previously reported.39,51,52 Three low-
energy, high-symmetry coordination geometries are possible for
octadentate complexes: D2d, C2v, and D4d (see Figure S4 in the
Supporting Information).72 Crystal structures presented herein
and of previous 1,2-HOPO complexes have shown that the
identity of the linker backbone alters the overall structure and
local site geometry of the Ln(III) complexes. Based on crystal
structures reported herein and previously reported struc-
tures for 5LI-1,2-HOPO, 5LIO-1,2-HOPO, and 5LINMe-1,2-
HOPO73 Eu(III) and Sm(III) complexes, we find two different
possible coordination modes (a-edge or g-edge bridged) for the
tetradentate ligands.
The 1,2-HOPO chelators always occupy the m-edges of a
dodecahedron. The nLI bridges either occupy the a-edges of a
dodecahedron or the g-edges. The a-edge bridge structures can
adopt either an S4 or D2 overall symmetry, while the g-edge
bridges can be either cis or trans to each other (Figure 3).
The crystal structures of the 3LIdiMe-1,2-HOPO complexes
reported herein correspond to the g-edge cis structure. For the
same ligand (5LIO-1,2-HOPO), both conformations have been
observed for different crystal structures of Sm(III) and Eu(III)
complexes, highlighting the low energy barrier of the two
conformations in the 5LIO-bridged complexes.39 We have
calculated the four possible geometries for our LI-ligand
complex series (see Figure 3 and Figures S5−S34 in the
Supporting Information) and the differences in energies
between the lowest-energy structures are shown in Table 1
(absolute energies can be found in Tables S3 and S4 in the
Supporting Information). For the long bridges, the g-edge trans
structure is clearly favored, because of steric interactions,
whereas, for the short bridges, the a-edge or g-edge cis structure
has the lowest energy. The previously reported crystal structure
Figure 3. DFT structures demonstrating the four possible
coordination modes. (The dodecahedron structure has been taken
from ref 74.)
a
Table 1. Relative Energies of the Four Possible Structures
Relative Energy, ΔE (kcal/mol)
a-edge S4
a-edge D2
g-edge cis
g-edge trans
2LI
0.35
0.00
0.00
0.00
6.14
9.02
5.55
b
0.00
0.03
0.08
3.49
6.51
6.18
4.26
b
8.08
1.95
4.32
1.53
0.00
2.60
0.70
7.61
c
9.71
2.55
5.39
1.83
0.62
0.00
0.00
0.00
0.00
0.00
3LI
3LIdiMe
4LI
4LIoPhen
5LI
5LIO
6LI
7LI
b
b
8LI
b
b
c
a
b
Bold font signifies data found for the XRD structures. Structure did
c
not converge. The cis structure converged to trans during the
geometry optimization.
for [Eu(4LIoPhen-1,2-HOPO)2]+ is in agreement with the
findings from the DFT optimizations (g-edge cis structure).52
Based on the DFT geometry optimizations and crystal struc-
tures, we conclude that the structures most prevalent in solution
are a-edge for 2LI-1,2-HOPO; a-edge and g-edge-cis for 3LI-1,2-
HOPO, 3LIdiMe-1,2-HOPO, and 4LI-1,2-HOPO; g-edge-cis for
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4LIoPhen-1,2-HOPO; and g-edge-trans for 5LI-1,2-HOPO to
8LI-1,2-HOPO. There are two distinct changes in structural
preference within the series: one that occurs after 2LI-1,2-
HOPO and then one that occurs again after 5LI-1,2-HOPO.
Following the assignment of the complex to one of the four
possible ligand structures, we conducted shape analysis for the
local site geometry around the Eu(III) ion of the optimized
geometries, using the shape analysis program developed
earlier.61,62 The results can be found in Tables S5−S7 in the
Supporting Information. In most cases, the lowest-energy
structure determined along the series matches most closely D2d
symmetry around Eu(III).
Photophysical Properties. The triplet excited state
energies of the ligands were determined by preparing the respec-
tive Gd nLI-1,2-HOPO complexes in situ (4:1 ethanol:methanol)
and flash-freezing to 77 K to form a glass (see Figure S35 in the
Supporting Information) and measuring the corresponding
phosphorescence spectra.52,75−77 The determined lowest triplet
excited state energies (T0−0) for the 1,2-HOPO Gd(III) com-
plexes do not vary significantly with linker length. The average
triplet excited state energy of the Gd(III) complexes was
determined to be 21 200 100 cm−1. These values are similar
to those reported prior for 1,2-HOPO ligands (T0−0 = 19 500−
21 500 cm−1).39,78,79 The molar extinction coefficients (ε) for the
Eu(III), Sm(III), and Gd(III) complexes were found to be similar
across the series (see Tables 2 and 3, as well as Table S8 in the
Figure 4. Change in Sm(III) and Eu(III) Φtot with linker length (nLI)
for nLI-1,2-HOPO ML2 complexes in TBS (pH 7.4), λex = 330 nm.
Estimated standard error in Φtot: 20%.
values observed 23.9% (0.4%), 20.7% (0.4%), and 21.5%
(0.4%) for 2LI-1,2-HOPO, 5LI-1,2-HOPO, and 5LIO-1,2-
HOPO Eu(III) (and Sm(III)) complexes, respectively, Φtot
decreases to 11.8% (0.2%), 10.5% (0.2%), and 13.0% (0.3%) in
the “intermediate length” 3LI-1,2-HOPO, 3LIdiMe-1,2-HOPO,
and 4LI-1,2-HOPO complexes, respectively. The decrease in
quantum yield from 2LI to 4LI can be rationalized by
considering the change in phi angle (average value of the S4
and D2 symmetric forms). Calculations using the LUMPAC
software package80 yield significantly higher expected quantum
yields for the D2-symmetric forms of the 2LI-1,2-HOPO,
3LI-1,2-HOPO, and 3LIdiMe-1,2-HOPO Eu(III) complexes
than those for the S4-symmetric forms (details are given in the
Supporting Information). The phi angle is a metric of planarity
of the opposing 1,2-HOPO units, since the 1,2-HOPO chelate
resides on the m-edges. As the bridge length is increased, the
phi angle is also increased, meaning the 1,2-HOPO units are
increasingly less planar. The decrease in planarity may result in
decreased orbital overlap with the metal, attenuating the
transfer of energy assuming a Dexter-type mechanism. The Φtot
of 5LI and 5LIO complexes would be expected to be quite low,
if not for the change in geometry to g-edge bridged structures.
The higher flexibility in the backbone of 6LI-1,2-HOPO−8LI-
1,2-HOPO also leads to a decrease in Φtot in long linker Eu(III)
(and Sm(III)) complexes (12.2% (0.3%), 10.5% (0.2%), and
9.1% (0.3%) respectively; see Tables 3 and 4). Within error, the
trends in LI series are similar for Eu(III) and Sm(III), which
leads us to the conclusion that the ligand geometry is the main
factor contributing to increased or lowered Φtot. The observed
trends also suggest a common sensitization mechanism for
these two lanthanide ions.
Next, we examined the lifetimes and emission spectra of the
Sm(III) complexes. The lifetimes for Sm(III) measured in TBS
are similar to those previously reported in the literature for
1,2-HOPO complexes. The number of water molecules
(denoted as the q number) for the Sm(III) complexes were
calculated using the empirical relationship developed by Kimura
and Kato (see eq 2).34,35 The slightly larger noninteger q
number (up to 0.29) obtained for some of the Sm(III) com-
plexes indicates the presence of q = 1 species that are in rapid
exchange with the q = 0 population (Table 3), specifically for
the 3LI-1,2-HOPO, 3LIdiMe-1,2-HOPO, 4LI-1,2-HOPO, 6LI-
1,2-HOPO, and 7LI-1,2-HOPO complexes. It should be noted
that, for some Sm(III) complexes, the measured photolumi-
nescence lifetimes could be better fitted to a biexponential decay,
suggesting the presence of a second short-lived component in
solution (also see Figures S36−42 in the Supporting Information).
Table 2. Average Phi Angle for a-Edge Structures (S4, D2)
ligand
phi angle
a
ref
0.0
7.7
2LI
3LI
9.2
4LI
11.5
18.5
17.7
5LI
5LIO
a
The term “ref” refers to an idealized reference structure with four
independent 1,2-HOPO units (see Figure 3).
Table 3. Photoluminescent Measurements of nLI-1,2-HOPO
a
ML2 Complexes with Sm(III)
τ
τ
D2O
(μs)
H2O
λMAX
(nm)
ε330
ΦTot
(M−1 cm−1
)
(%)
(μs)
q
2LI
334
334
335
332
331
333
334
337
340
21 600
23 400
22 200
22 500
19 400
19 200
21 700
19 900
22 300
0.4
0.2
0.2
0.2
0.4
0.4
0.3
0.2
0.3
17
15
14
14
15
13
15
15
16
144
120
154
106
138
138
107
106
93
0.03
0.14
0.29
0.27
0.14
0.28
0.21
0.19
−0.01
3LI
3LIdiMe
4LI
b
5LI
c
5LIO
6LI
7LI
8LI
a
Estimated error in ε330, τ, and ΦTot are 10%, 10%, and 20%,
respectively. Complexes in 20 mM TRIS buffer pH 7.4, I = 0.10 M
b
c
NaCl at 25.0 °C. Data taken from ref 39. Data taken from ref 51.
Supporting Information). However, as the number of methylene
units in the linker backbone increases (nLI) from 2 to 4 and
from 5 to 8, a decrease in the photoluminescence quantum
yield in Tris buffered saline (20 mM Tris, pH 7.4, 100 mM
NaCl) for both Eu(III) and Sm(III) complexes is observed
(see Figure 4, as well as Tables 3 and 4). From the highest
G
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Inorganic Chemistry
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a
Table 4. Photoluminescent Measurements of nLI-1,2-HOPO ML2 Complexes with Eu(III)
λMAX (nm)
εMAX (M−1 cm−1
)
ΦTot (%)
τ
H2O
(μs)
τ
D2O
(μs)
q
krad (ms−1
)
knonrad (ms−1
)
τrad (μs)
ηsens
ηEu
2LI
338
334
334
332
331
333
333
335
339
21 600
21 600
20 900
23 600
18 800
19 250
21 700
21 300
21 500
23.9
11.8
10.5
13.0
20.7
21.5
12.2
10.5
9.1
578
767
0.13
0.19
0.21
0.24
0.05
0.09
0.14
0.12
0.13
0.733
0.931
0.907
0.760
0.620
0.609
0.735
0.800
0.705
0.996
0.896
0.894
0.899
0.740
0.807
0.767
0.768
1.012
1363
1074
1102
1314
1476
1640
1359
1249
1418
0.51
0.23
0.21
0.22
0.42
0.49
0.25
0.17
0.22
0.42
0.51
0.50
0.45
0.49
0.43
0.48
0.51
0.41
3LI
547
555
596
737
727
665
637
582
741
770
867
996
1012
938
870
775
3LIdiMe
4LI
b
5LI
c
5LIO
6LI
7LI
8LI
a
Estimated error in ε330, τ, and ΦTot are 10%, 10%, and 20%, respectively. Complexes in 20 mM Tris buffer pH 7.4, I = 0.10 M NaCl at 25.0 °C.
b
c
Data taken from ref 39. Data taken from ref 51.
Examination of the emission spectra for the nLI-1,2-HOPO
Sm(III) complexes show a change in the emission peak shapes
(Figure 5). Notably, the Sm(III) 2LI-1,2-HOPO, 5LIO-1,2-
Figure 5. Normalized emission spectra of the Sm(III) LI series measured
in buffer (20 mM Tris, 100 mM NaCl, pH 7.4, λex = 330 nm); inset
4
6
shows an enlarged view of the hypersensitive G5/2 → H9/2 transition.
Figure 6. Normalized emission spectra of the Eu(III) LI series
measured in buffer (20 mM Tris, 100 mM NaCl, pH 7.4, λex = 330 nm).
Bottom panels show enlarged views of the D0 → F1, and D0 → F4
HOPO, and 5LI-1,2-HOPO emission spectra have the most
5
4
5
4
well-resolved peaks of the hypersensitive transition (4G5/2
→
transitions.
6H9/2). The rigidity of the lanthanide complexes have been
related to the emission spectra peak shapes where the rigidity
correlates to the sharpness of the peak.81 For clarity, separate
plots of the Sm(III) emission spectra for the g-edge trans (6LI-
1,2-HOPO−8LI-1,2-HOPO) and g-edge cis (3LI-1,2-HOPO,
3LIdiMe-1,2-HOPO, 4LI-1,2-HOPO) complexes can be found
in the Supporting Information (Figures S43−S45), along
with a comparison between selected complexes of different
structural types. Since Eu(III) can provide more-detailed
insight when used as a photoluminescent probe, we turned
to the Eu(III) LI series to further rationalize the trends for
this ligand series. Eu(III) lifetimes range from ∼550−740 μs
in TBS with the highest lifetimes observed for the 5LI-1,2-
HOPO and 5LIO-1,2-HOPO complexes (Table 3; also see
Figures S36−42).
The hydration states were calculated using eq 4. The q-values
are in agreement with, although somewhat lower than, the
values obtained for the Sm(III) complexes. Examination of the
emission spectra for the nLI-1,2-HOPO Eu(III) complexes also
show a change in the emission peak shape (Figure 6), as ob-
served for the Sm(III) complexes. For europium, the 5D0 → 7F1
transition can be examined for splitting patterns and linked to
the symmetry of the system.82 What is apparent is that, for 2LI,
3LI, 3LIdiMe, and 8LI, the splitting seems to be similar (one
broad transition, either highly symmetric or not resolved well
enough), whereas, for the remaining 4LI−7LI complexes, a
splitting in at least two peaks is observed. As for the hyper-
sensitive europium transition (5D0 → 7F2, see Figure 6), there is
a change in band shape after 3LIdiMe and the again after 7LI.
Again, for the sake of clarity, separate plots of the Eu(III)
emission spectra for the proposed g-edge trans (6LI-1,2-
HOPO−8LI-1,2-HOPO) and g-edge cis (3LI-1,2-HOPO,
3LIdiMe-1,2-HOPO, 4LI-1,2-HOPO) structures have been
included in the Supporting Information (Figures S46−S48),
along with a comparison between selected complexes of
different structural types. While not resolved quite as clearly,
it appears that the “borderline” structures have similar emission
structures while complexes with clearly one preference for one
structure, e.g., 8LI-1,2-HOPO or 2LI-1,2-HOPO, have well-
resolved distinct spectral signatures. The splitting of the
5
7
magnetic dipole (MD) D0 → F1 transition directly relates
the crystal-field splitting of the 7F1 level.82 Figure 6 (and
Figures S46−S48) demonstrate the change in splitting pattern
upon elongating the bridge between two 1,2-HOPO units.
While the MD transition is considered to be, in a first
approximation, a constant, with an intensity uninfluenced by
the environment of the Eu(III) ion, the intensity of the
5
7
hypersensitive D0 → F2 transition can be directly related to
H
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the local symmetry of Eu(III). This transition is also
responsible for the bright red photoluminescence observed
for the complexes. The 5D0 → 7F3 transition is very weak for all
complexes and of similar intensity; thus, this does not suggest
a significant difference in J-mixing for the complexes within
this series.82 The electric dipole (ED) transition 5D0 → 7F4 also
changes throughout the series (Figure 6). Further examination
of the photophysical parameters for Eu(III) LI series re-
between the Ln(III) accepting state and the chromophore
triplet excited state are effectively the same for all complexes
investigated here and are furthermore similar to the 1,2-HOPO
complexes reported previously. The effect of the chromophore
geometry, altered by the length of the aliphatic linker around
the Ln(II) center, is thus proposed to be the causal factor
causing a dramatic change in the brightness of these com-
pounds. We show that the 2LI and 5LI backbones place the
four 1,2-HOPO chelator/chromophores in a position that
yields the greatest Sm(III) and Eu(III) photoluminescence
(Φtot) in aqueous buffer. However, the direct link between the
coordination geometries and the observed photoluminescent
efficiency (i.e., sensitization efficiency) is not yet fully under-
stood. A theoretical model that connects geometry and energy
transfer remains an important, but elusive, goal. For chelate
chromophore systems, such as the 1,2-HOPO complexes, it is
difficult to sort out which sensitization mechanisms are most
important, since the ligand−metal donor−acceptor distances
are fixed by the metal−ligand bonds. However, the large Φtot
changes brought about by 1,2-HOPO ligand structural modifi-
cations are more consistent with a Dexter-type transfer (or a
combination of different exchange processes) rather than a
vealed that the ligand-centered sensitization efficiency (ηsens
)
follows the same trend along the LI series as Φtot (Figure 7).
Forster-type mechanism, since the donor−acceptor distances
̈
do not change significantly from complex to complex.85,86
This study represents another piece in the puzzle to elucidate
underlying mechanisms of energy transfer in photoluminescent
1-hydroxypyridin-2-one complexes.45,46,85,86
Figure 7. Trends in Φtot and ηsens along the nLI series.
Therefore, the ηsens parameter is primarily dependent on the
ligand backbone (and the hereby enforced structure) and
ranges from 0.17 for 7LI-1,2-HOPO up to 0.51 for 2LI-1,2-
HOPO. The Eu(III)-centered efficiency (ηEu) (also often
reported as ΦEu64) ranges between 0.42 and 0.51 and did not
change significantly from sample to sample. This is a rather
unexpected result and highlights that the trends within this
series are mainly based on ligand geometry and the energy
transfer step from the antenna to the Ln cation, rather than
differences in nonradiative relaxation.
The excitation spectra of the complexes were also recorded
and are shown together with the absorbance and emission
spectra for each Eu(III) complex in the Supporting Information
(Figures S49−S58); however, they did not provide any obvious
information as to why some complexes were more photolumi-
nescent than others. Given that the chromophore (1,2-HOPO)
is the same in all complexes, this is not surprising. Interestingly,
a higher τrad did not correlate with high Φtot, as usually ob-
served for Eu(III) complexes64,82,83 and, here, the main factor
to influence high Φtot is the sensitization efficiency (ηsens) of the
ligand, not the Eu-centered efficiency (ηEu) (see Figure 7).
Shortening τrad has been one successful strategy to enhance Φtot
in photoluminescent lanthanides; however, it often involves
changing chromophores rather than geometry around the metal
ions.84 Recently, it has also been proposed, in mixed-ligand
systems, that the more asymmetric a complex, the less
forbidden the f−f transitions and, thus, the greater the photo-
luminescence.30 Also note that the brightness followed the
same trend as ηsens in the series, which is not surprising, given
the similarities in molar absorptivities.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.5b01927.
■
S
ICP results, crystal structure data, ORTEP plots of
samarium complexes, low-energy structures of octaden-
tate polyhedra, DFT-optimized structures and energies of
the complexes, shape analysis details, calculations of
expected quantum yields using the LUMPAC software,
emission, excitation and absorption spectra (PDF)
Crystallographic data for C33H30EuN9O12 (CIF)
Crystallographic data for C42H58EuN9O13 (CIF)
Crystallographic data for C33H30N9O12Sm (CIF)
Crystallographic data for C42H58N9O13Sm (CIF)
Zip file with pdb files of the DFT optimized structures
AUTHOR INFORMATION
Corresponding Author
*E-mail: raymond@socrates.berkeley.edu.
■
Notes
The authors declare the following competing financial
interest(s): Some of this technology is licensed to Lumiphore,
Inc., in which some of the authors have a financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Director, 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.
L.J.D. is grateful for a fellowship from the Alexander von
Humboldt Foundation. The Small Molecule X-ray Crystallog-
raphy Facility is supported by the NIH Shared Instrumentation
(through Grant No. S10-RR027172), and the Molecular
CONCLUSION
■
The results presented herein show the importance of
optimizing the ligand geometry about the lanthanide center
to maximize the efficiency of energy transfer from the organic
chromophore to the lanthanide center in order to enhance Φtot.
The triplet excited state energy, and thus the energy gap
I
DOI: 10.1021/acs.inorgchem.5b01927
Inorg. Chem. XXXX, XXX, XXX−XXX

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Article
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Graphics and Computation Facility wishes to acknowledge the
NSF Grant Nos. CHE-0233882 and CHE-0840505.
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DOI: 10.1021/acs.inorgchem.5b01927
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