Luminescent Carbazole-Based EuIII and YbIII Complexes with a High Two-Photon Absorption Cross-Section Enable Viscosity Sensing in the Visible and near IR with One- And Two-Photon Excitation

Article abstract of DOI:10.1021/acs.inorgchem.9b03561

The newly synthesized Eu^III and Yb^III complexes with the new carbazole-based ligands CPAD^2- and CPAP^4- display the characteristic long-lived metal-centered emission upon one- and two-photon excitation. The Eu^III complexes show the expected narrow emission bands in the red region, with emission lifetimes between 0.382 and 1.464 ms and quantum yields between 2.7% and 35.8%, while the Yb^III complexes show the expected emission in the NIR region, with emission lifetimes between 0.52 and 37.86 μs and quantum yields between 0.028% and 1.12%. Two-photon absorption cross sections (σ_2PA) as high as 857 GM were measured for the two ligands. The complexes showed a strong dependence of the one- and two-photon sensitized emission intensity on solvent viscosity in the range of 0.5-200 cP in the visible and NIR region.

Full text of DOI:10.1021/acs.inorgchem.9b03561

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Article
Luminescent Carbazole-Based EuIII and YbIII Complexes with a High
Two-Photon Absorption Cross-Section Enable Viscosity Sensing in
the Visible and Near IR with One- and Two-Photon Excitation
Jorge H. S. K. Monteiro, Natalie R. Fetto, Matthew J. Tucker, and Ana de Bettencourt-Dias*
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ABSTRACT: The newly synthesized EuIII and YbIII complexes with the new carbazole-based
ligands CPAD2− and CPAP4− display the characteristic long-lived metal-centered emission
upon one- and two-photon excitation. The EuIII complexes show the expected narrow emission
bands in the red region, with emission lifetimes between 0.382 and 1.464 ms and quantum
yields between 2.7% and 35.8%, while the YbIII complexes show the expected emission in the
NIR region, with emission lifetimes between 0.52 and 37.86 μs and quantum yields between
0.028% and 1.12%. Two-photon absorption cross sections (σ2PA) as high as 857 GM were
measured for the two ligands. The complexes showed a strong dependence of the one- and
two-photon sensitized emission intensity on solvent viscosity in the range of 0.5−200 cP in the
visible and NIR region.
particles also have been frequently studied for this purpose,
although their synthesis and purification are not always
straightforward and they might suffer from solubility issues.34
LnIII ion complexes do not suffer from these limitations, as
indicated above, and are therefore great candidates for 2PA
imaging.
INTRODUCTION
■
Lanthanide (LnIII) ions are interesting as luminescent labels
and sensors for a variety of applications.1−15 The luminescence
is color pure, due to the core nature of the f orbitals involved in
the emission, and is long-lived, due to the forbidden nature of
the f−f transitions, which enables time-delayed emission
spectroscopy with increased signal-to-noise ratio.16−18 The
forbidden transitions render direct excitation inefficient, and
thus the emission is sensitized through an appended ligand
chromophore (Figure 1). This is referred to as the antenna
effect and leads to a large Stokes shift of sensitized emission,
which is advantageous due to separation of excitation and
emission wavelengths. In addition, ligands are easily derivat-
ized, which enables tuning the chemical and spectroscopic
properties of the complexes. Typically, chromophores absorb
in the high-energy region of the spectrum,19,20 and these
wavelengths have low tissue penetration and result in radiation
damage, thus making them undesirable for bioimaging
applications.21−24 Excitation at lower nondamaging energies
is achieved using ligands with extended aromatic systems that
absorb in the visible; however, the low energy singlet (S) and
triplet (T) states cannot sensitize visible emitting LnIII
ions.16,25 An alternative is to use two-photon absorption
(2PA),21,22,26−29 a nonlinear process where two photons with
half the energy required by the one-photon excitation (1PA)
are absorbed simultaneously (Figure 1).30,31 Organic dyes32
and transition-metal complexes33 can be efficient photo-
luminescent labels for 2PA imaging, but they are often affected
by photobleaching and have short emission lifetimes and
narrow Stokes shifts, which limit their usefulness. Nano-
Lakowicz and co-workers pioneered the sensitization of EuIII
emission using 2PA.35,36 Since then, the use of luminescent
LnIII complexes as 2PA labels and sensors has flourished, and
examples of both visible and NIR emitters are known.37−49
Andraud, Maury, and co-workers studied 2PA-sensitization of
EuIII complexes with dipicolinato- and cyclononane-based
ligands and concluded that ligands with charge-transfer (CT)
states have improved 2PA cross sections (σ2PA).49−51
Physicochemical properties of biological systems provide
important information regarding physiological processes. For
example, viscosity plays an important role in the intra- and
extracellular mass transport,52,53 and abnormalities in blood
and plasma viscosity are indicative of diabetes and hyper-
tension.52,54,55 Viscosity of macroscopic systems is easily
determined, yet the same methods cannot be applied at
cellular level.52 Organic dyes, such as molecular rotors, have
been used as luminescent viscosity sensors.56−58 To date, only
one LnIII-based viscosity sensor has been reported, by Zhang
Received: December 6, 2019
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2F7/2 transitions in the emission spectra (Figure 3, right).
Different splitting patterns in the emission spectra are a result
of different coordination environments around the metal ion,
due to the different ligand binding.17 The excitation spectra
(Figures 2B and 3 left) closely resemble the absorption spectra
of the ligands (Figure S22), indicating that the ligands sensitize
the metal-centered emission.
The excited state lifetimes (τ) and emission efficiencies
(ΦELu) of the complexes are summarized in Table 1. The τ
values of 1.464 ms for [Eu(CPAD)3]3− and 1.702 ms for
[Eu(CPAP)(DMSO)2]− were measured in DMSO. In water/
DMSO (9:1), the latter complex shows an emission lifetime of
382 μs, consistent with exchange of the coordinated DMSO
molecules for water (vide infra). The long excited state
lifetimes are accompanied by high intrinsic quantum yields
Eu
Eu
(Φ ) in DMSO and a lower value for the complex in water.
ΦELu values of 31.7%, 35.8%, and 2.7% were observed for
[Eu(CPAD)3]3−, [Eu(CPAP)(DMSO)2]−, and [Eu(CPAP)-
(H2O)2]−, respectively. Compared with Cs3[Eu(dpa)3] (dpa =
dipicolinato) in DMSO (Table S1), the lower emission
efficiencies are consistent with an increase in the donor−
acceptor distances RL (Table 1)61 and quenching caused by
O−H vibrational coupling in the case of [Eu(CPAP)(H2O)2]−
(vide infra). The favorable energies of S and T states of the
ligands result in high ηsens for the EuIII complexes (Table 1).
ΦLYb values of 0.75% and 0.028% were observed for
[Yb(CPAP)(DMSO)2]− and [Yb(CPAP)(H2O)2]−, respec-
tively (Table 1). These values are similar to related
dipicolinato-based complexes and compare favorably with
known compounds.45,47,48,67,68 The emission lifetimes for the
[Yb(CPAP)(L)2]− complex (L = H2O or DMSO) show a
similar behavior. The τ is longer for L = DMSO at 8.91 μs than
for L = H2O at 0.52 μs, as is to be expected due to quenching
through the O−H oscillators.
Figure 1. (A) Energy level diagram illustrating the antenna effect for
LnIII. 2PA and 1PA, the two- and one-photon absorption, respectively;
F, fluorescence; P, phosphorescence; ISC, intersystem crossing; ET,
energy transfer; BT, back transfer; L, luminescence; NR, nonradiative
pathways; S and T, states with singlet and with triplet multiplicity,
respectively. (B) Molecules studied in this work.
and co-workers. Their YbIII porphyrinato complexes with a
̈
Klaui ligand displayed emission lifetimes τ that varied by a
factor of ∼2.8 in the viscosity range of 0.52−1200 cP.59 Since
viscosity alters the population of intramolecular charge transfer
states, molecules with such states should provide a unique
opportunity to sense it.60
De Bettencourt-Dias and co-workers reported a carbazole-
substituted dipicolinato as sensitizer of EuIII and TbIII
emission.61,62 Carbazole is known for its 2PA properties.63−65
Thus, with the goal of increasing our knowledge of 2PA-
sensitized LnIII emission and expanding the availability of LnIII-
based viscosity sensors, we isolated two new carbazole-based
ligands with potential CT states,50,62 CPAD2− (4-((4-(9H-
carbazol-9-yl)phenyl)ethynyl)pyridine-2,6,dicarboxylate) and
CPAP4− (2,2′-(4-((4-(9H-carbazol-9-yl)phenyl)(pyridine-2,6-
diyl)bis(methylene)dimalonate) and their LnIII complexes
(LnIII = EuIII, GdIII, and YbIII).
In the case of the LnIII complexes with the ligand CPAP4−, a
decrease in the emission efficiency and lower τ in TRIS/HCl
buffered aqueous system (pH ∼ 7.4, 10% DMSO), compared
with the DMSO solution, are observed. This decrease is a
result of the nonradiative deactivation of the LnIII excited state
by the O−H vibrations of coordinated water molecules. The
presence of two coordinated water molecules q was confirmed
for the EuIII complex by comparing τ in water (τH2O) and D2O
(τD2O) (eq S5 and Table 1).
Two-Photon Spectroscopy Studies of the K3[Ln-
(CPAD)3] and K[Ln(CPAP)(L)2] (Ln = EuIII or YbIII and L
= DMSO or H2O) Complexes. All EuIII complexes can be
excited with a wide range of low energy photons in a 2PA
process (Figure S32). By exciting at 720 or 750 nm, with the
latter chosen for comparison with the 2PA standard rhodamine
B,69 the characteristic metal-centered emission pattern is
observed (Figures 2B and S29a−S31a). The quadratic
dependence of the emission intensity I on the laser power P
(Figures 2C and S29b−S31b, Tables 2 and S2) confirmed the
2PA process. The emission spectra obtained by one- or two-
photon excitation are the same (Figures 2 and S29−S31),
indicating that the same excited states are involved in the
process.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the H2CPAD and
H4CPAP Ligands and of the LnIII Complexes. H2CPAD
and H4CPAP (Figure 2A) were synthesized through modified
literature procedures (Scheme S1) and characterized using
standard techniques (Figures S2, S4, S6, S8, S10, and S12).
The LnIII (LnIII = EuIII, GdIII, YbIII) complexes were obtained
as yellow solids in ∼70% yield by reacting LnCl3 with K2CPAD
in a 1:3 metal-to-ligand ratio and with K4CPAD in a 1:1 metal-
to-ligand ratio in water/DMF (1:20). They were characterized
by mass spectrometry (Figures S13−S18) and shown to be
photostable (Figure S19).
One-Photon Spectroscopy Studies of the K3[Ln-
(CPAD)3] and K[Ln(CPAP)(L)2] (Ln = EuIII or YbIII and L
= DMSO or H2O) Complexes. K3[Ln(CPAD)3] and
K[Ln(CPAP)(L)2] (Ln = EuIII or YbIII and L = DMSO or
The two-photon absorption cross sections (σ2PA) are
summarized in Table 2. A value of 857 GM was obtained
for K3[Eu(CPAD)3]. This is among the highest reported for
EuIII complexes in solution, such as 775 GM for [NBu4]3[Eu-
(dpa5)3] described by Maury, Andraud, and co-workers.45
H2O) in solution display the characteristic EuIII-centered D0
5
→ 7FJ (J = 0−4) (Figure 2A, right) and YbIII-centered 2F5/2
→
B
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Figure 2. (A) 1PA excitation (black lines, left) and emission spectra (red lines, right) of EuIII complexes (λexc = 380 nm). (B) 2PA emission spectra
of EuIII complexes. (C) Plot of the log of the emission intensity I at 615 nm upon 2PA excitation as a function of the log of the laser power P. From
top to bottom in each figure: (a) K3[Eu(CPAD)3] in DMSO, (b) K[Eu(CPAP)(DMSO)2] in DMSO, and (c) K[Eu(CPAP)(H2O)2] in TRIS/
HCl buffered aqueous system (pH ∼ 7.4, 10% DMSO); λexc = 720 nm; [complex] = 1 × 10−4 M.
Sensitization of EuIII using 2PA is not commonly reported
for complexes with coordinated solvent molecules because the
latter often quench the emission. Parker and co-workers found
a σ2PA of 1.7 and 0.4 GM for aqueous solutions of
[Eu(do2ax1)(H2O)](OTf)3 and [Eu(do2ax2)(H2O)](OTf)3,
respectively (do2ax1 and do2ax2 are dota derivatives with
appended 5H-thiocromeno[2,3-b]pyridine-5-one and methyl
5-oxo-5H-thiocromeno[2,3-b]pyridine-7-carboxylate, respec-
tively).43 We were pleasantly surprised to determine a σ2PA
of 228 GM for the K[Eu(CPAP)(H2O)2] in TRIS/HCl
buffered aqueous system (pH ∼ 7.4, 10% DMSO), similar to
the value in DMSO, consistent with the fact that σ2PA is
intrinsic to the ligand and not affected by the solvent.70 This
value also indicates that CPAP4−, which yields complexes
soluble in water/DMSO, is viable for 2PA bioimaging,
although this application is not shown here.
Viscosity-Sensing Studies using the K3[Ln(CPAD)3]
(Ln = EuIII or YbIII) Complexes. The temperature-dependent
Figure 3. Excitation (black lines, left) and emission spectra (pink
lines, right) of YbIII complexes: (a) K3[Yb(CPAD)3] in DMSO (λexc
370 nm), (b) K[Yb(CPAP)(DMSO)2] in DMSO (λexc = 350 nm),
and (c) K[Yb(CPAP)(H2O)2] in TRIS/HCl buffered aqueous system
(pH ∼ 7.4, 10% DMSO) (λexc = 350 nm) [complex] = 1 × 10−4 M.
=
phosphorescence spectra of K3[Gd(CPAD)3], with maxima at
510 nm at 298 K and 435 nm at 77 K (Figure S33), indicate
that a twisted intramolecular charge-transfer (TICT) state is
present in CPAD2−, involving the carbazole and phenyl
rings.62,71 As mentioned above, changes in the viscosity often
C
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Ln
Ln
Table 1. Singlet (S) and Triplet (T) State Energies of the Ligands, Lifetime (τ), Intrinsic Emission Efficiency (Φ ), Quantum
Yield (ΦLLn) of Sensitized Efficiency, Sensitization Efficiency (ηsens), and the Number of Coordinated Water Molecules (q) for
the Complexes at λexc = 360 nm
a
a
S [cm−1
]
T [cm−1
]
τ
Φ
[%]
ΦLLn [%]
ηsens [%] RL [Å]
q
Ln
complex
solvent
DMSO
DMSO
DMSO
DMSO
Ln
b
b
b
K3[Eu(CPAD)3]
K3[Yb(CPAD)3]
K[Eu(CPAP)(DMSO)2]
K[Yb(CPAP)(DMSO)2]
K[Eu(CPAP)(H2O)2]
K[Yb(CPAP)(H2O)2]
23520 460
19690 300
1.464 0.003
37.86 0.06
1.702 0.037
8.91 0.16
0.382 0.011
0.52 0.01
69
31.7 4.0
1.12 0.12
35.8 3.7
0.75 0.02
2.7 0.4
46
−
8.8883
−
−
−
−
1.9
−
c
−
63
−
11
−
25570 350
20490 270
57
9.2466
c
−
25
−
d
d
water/DMSO
water/DMSO
c
0.028 0.002
a
b
c
d
Obtained using the GdIII complex at 77 K.66 In milliseconds (ms). In microseconds (μs). TRIS/HCl buffered aqueous solution (pH ∼ 7.4,
10% DMSO).
Table 2. Two-Photon Absorption Cross Sections (σ2PA) of
the EuIII Complexes
a
b
complex
solvent
DMSO
DMSO
σ2PA [GM]
K3[Eu(CPAD)3]
K[Eu(CPAP)(DMSO)2]
K[Eu(CPAP)(H2O)2]
857 24
266 34
228 36
c
water/DMSO
a
b
c
λexc = 750 nm. 1 GM = 10−50 cm4 s photon−1 molecule−1. TRIS/
HCl buffered aqueous solution (pH ∼ 7.4, 10% DMSO).
alter the population CT states.71 Thus, we prepared 0.2 mM
solutions of K3[Ln(CPAD)3] (Ln = EuIII or YbIII) with varying
amounts of methanol and glycerol to obtain a wide range of
viscosities. This allowed us to observe changes in the emission
intensity as a function of the viscosity using one- and two-
photon excitation for the K3[Eu(CPAD)3] complex, the first
example of viscosity sensing using EuIII-centered emission in
the visible region of the spectrum using two-photon excitation.
Upon excitation through the ligand at 400 or 800 nm, the
emission intensities increase by about 2-fold or 5.8-fold,
respectively, as the viscosity increases (Figures 4(A) and 4(B))
in the range of 0−200 cP. This is in the same order of
magnitude as the increase observed for the YbIII complex
described by Ning and co-workers for the same viscosity
range.59
Viscosity-dependent emission intensity is also observed for
K3[Yb(CPAD)3] complex, as shown in Figure 5. An
approximate 2-fold increase over the 0−200 cP range is seen,
on the same order of magnitude of the only other example of
YbIII-centered emission used for viscosity sensing.59
In addition to viscosity, the emission intensity can be
influenced by the solvent oxygen content, as the interaction
between oxygen and the triplet level of the ligand adds a
nonradiative deactivation pathway that decreases the EuIII
emission lifetime.72 Methanol and glycerol have different
amounts of dissolved oxygen; the mole fractions at 298 K and
101.3 kPa are 4.15 × 10−4 for methanol and 4.8 × 10−6-5.5 ×
10−6 for glycerol.73,74 To ensure that emission behavior is due
to solvent viscosity, and not quenching through oxygen,72,75
the emission lifetimes of the complexes were determined. At
1.318 0.052 ms in methanol and at 1.298 0.014 ms in 1:9
methanol/glycerol (Figures S26 and S27), these are equivalent.
The emission intensity of both EuIII and YbIII complexes is
independent of the amount of dissolved oxygen in solution as
well (Figure S34). This confirms that there is no quenching,
and thus the intensity changes are due to the viscosity of the
medium. Another solvent property, namely its polarity, can
influence the emission intensity of the LnIII as well, as high
polarity solvents can stabilize ligand CT states,49 and thus the
Figure 4. (A) Emission spectra of K3[Eu(CPAD)3] (λexc = 400 nm)
upon one-photon excitation with varying viscosities. (B) Emission
spectra of K3[Eu(CPAD)3] (λexc = 720 nm) upon two-photon
excitation with varying viscosities. The insets show the plot of the
emission intensity at 615.5 nm upon one-photon or two-photon
excitation as a function of viscosity. [complex] = 1 × 10−4 M.
excited state energy of the ligand, which results in changes to
the ligand → LnIII energy transfer rates. To ensure that the
energy of the ligand excited state is not affected by solvent
polarity, the phosphorescence spectra of the [Gd(CPAD)3]
complex obtained in 100% methanol and mixture of methanol/
glycerol (1:9) (Figure S35). The lack of significant change of
the phosphorescence emission bands confirms that the polarity
does not affect the energy levels of the ligand, and thus
D
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Agilent model G6230A with a QTOF analyzer in the high-resolution
mode for the metal complexes. The samples were prepared by diluting
acetonitrile solutions to a concentration of ∼1 mg/mL and passing
them through a 0.2 mm microfilter.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03561.
■
sı
Details of the synthesis and characterization, as well as
spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Ana de Bettencourt-Dias − Department of Chemistry,
University of Nevada, Reno, Nevada 89557, United States;
orcid.org/0000-0001-5162-2393; Email: abd@unr.edu
Figure 5. Emission spectra of K3[Yb(CPAD)3] (λexc = 400 nm) upon
one-photon excitation with varying viscosities. The inset shows the
plot of the emission intensity ratios at 976 and 1020 nm upon one-
photon excitation as a function of viscosity. Mixtures of MeOH/
glycerol at various ratios were used as solvents with variable viscosity.
[complex] = 1 × 10−4 M.
Authors
Jorge H. S. K. Monteiro − Department of Chemistry, University
of Nevada, Reno, Nevada 89557, United States
Natalie R. Fetto − Department of Chemistry, University of
Nevada, Reno, Nevada 89557, United States
Matthew J. Tucker − Department of Chemistry, University of
Nevada, Reno, Nevada 89557, United States; orcid.org/
0000-0001-9724-4455
emission intensity changes are solely due to the viscosity of the
medium.
CONCLUSIONS
■
A series of new LnIII complexes containing new carbazole-
based ligands were synthesized and their photophysical
properties were measured. EuIII emission efficiencies up to
35.8% were observed using one-photon excitation. EuIII
emission using 2PA was observed for all the complexes. The
σ2PA of 857 GM for K3[Eu(CPAD)3] is, to date, the highest
determined for EuIII complexes in solution. A substantial
change in emission intensity as a function of the viscosity was
observed for K3[Eu(CPAD)3] upon excitation with one-
photon and, for the first time, two-photon excitation.
Viscosity-dependent emission intensity is observed as well
for the analogous YbIII complex, which is only the second
example of NIR LnIII-based emission viscosity sensing. These
complexes are thus effective sensors of the viscosity of the
medium, which can be monitored through the intensity of the
emitted light in the visible and NIR excited by either one or
two photons. These results establish this carbazole-based
family of ligands as a foundation for improved 2PA dyes and as
viscosity sensors. This will contribute to the fields of imaging,
sensing of physicochemical properties, and diagnosis in
microscale systems.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b03561
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the NSF and NIH for
financial support of this work (CHE-1800392 to A.d.B.D. and
R15GM1224597 to M.J.T., respectively).
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EXPERIMENTAL SECTION
■
All commercially obtained reagents were of analytical grade and were
used as received. Solvents were dried and purified by standard
methods unless otherwise noted. All synthetic steps were completed
under N2 unless otherwise specified. The detailed synthetic
procedures of the ligands and their LnIII complexes are provided in
the Supporting Information. Details of the one- and two-photon
photophysical characterization and viscosity measurements are
provided in the Supporting Information as well.
NMR Spectroscopy. All NMR spectra were recorded on Varian
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ppm) in deuterated chloroform (CDCl3) against tetramethylsilane
(TMS, 0.00 ppm) at 25.0 0.1 °C.
Mass Spectrometry. Electrospray ionization mass spectra (ESI-
MS) were collected in positive ion mode on a Waters Micromass ZQ
quadrupole in the low-resolution mode for the ligands and on an
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