BODIPY-functionalized 1,10-phenanthroline as a long wavelength sensitizer for near-infrared emission of the ytterbium(iii) ion

Article abstract of DOI:10.1039/c9dt02850a

Two BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) moieties were chemically appended to the 4,7-positions of 1,10-phenanhtroline resulting in two new ligands (BODIPY-Phen and 4I-BODIPY-Phen) with strong absorption at 507 nm and 540 nm, respectively. BODIPY-Phen emits fluorescence strongly centered at 507 nm, whereas the fluorescence of 4I-BODIPY-Phen was completely quenched due to the introduction of four I atoms at its 2,6 positions. The two ligands reacted readily with tris(1,1,1,5,5,5-hexafluoro-2,4-pentanedionate) ytterbium(iii) dihydrate through substitution reactions forming eight-coordinate complexes that emit strongly at 976 nm upon excitation at their absorption maximal positions. Both complexes exhibited a lifetime of ～11 μs in dichloromethane at room temperature.

Full text of DOI:10.1039/c9dt02850a

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DOI: 10.1039/C9DT02850A
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ARTICLE
BODIPY-functionalized 1,10-phenanthroline as long wavelength
sensitizers for near-infrared emission of ytterbium (III) ion
Adedayo Kukoyi, a Eric A. Micheli,a Beibei Liu,a Hongshan He,*a and P. Stanley May b
Received 00th January 20xx,
Accepted 00th January 20xx
Two BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) moieties were chemically appended to the 4,7-positions of 1,10-
phenanhtroline resulting in two new ligands (BODIPY-Phen and 4I-BODIPY-Phen) with strong absorption at 507 nm and 540
nm, respectively. BODIPY-Phen emits strongly centered at 507 nm, whereas the fluorescence of 4I-BODIPY-Phen was
completely quenched due to the introduction of four I atoms at its 2,6 positions. Two ligands reacted readily with
tris(1,1,1,5,5,5-hexafluoro-2,4-pentanedionate) ytterbium (III) dihydrate through substitution reactions forming eight-
coordinate complexes that emits strongly at 976 nm upon excitation at their absorption maximal positions. Both complexes
exhibited a lifetime of ~ 11 µs in dichloromethane at room temperature.
DOI: 10.1039/x0xx00000x
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can be partially overcome by the formation of complexes with
high coordination numbers.13-19 This provides a barrier for the
access of solvent molecules to the central lanthanide ion. The
Introduction
The fluorescence-based diagnosis has emerged as a powerful
tool for gaining a greater understanding of structural and
functional properties in living systems.1, The information
second challenge is to broaden the absorption window of ligands
toward the red light region.7 This will enable the use of long
wavelength light sources to reduce the photodegradation, which
is significant when a UV or a near UV light source is used. This
can be addressed by appending chromophore to a ligand-
binding-group (LBG). During the last two decades, numerous
attempts have been made and promising results have been
obtained.13 Among all lanthanide ions, ytterbium (III) exhibits
the strongest emission in the NIR region; therefore is a potential
candidate for diagnostic applications. However, it is quite
challenging to sensitize the Yb (III) ion under long wavelength
excitation due to the weak absorption of the ligands that are
normally used to form coordination compounds.
BODIPY dyes have been explored as potential sensitizers for
Yb(III)-centered NIR emission. BODIPY, abbreviated from 4,4-
difluoro-4-bora-3a,4a-diaza-s-indacene, is a family of boron-
dipyrromethene compounds that have high absorptivity and
tunable spectral characteristics in the visible region.20-25 Though
the BODIPY core lacks the binding capability to the Yb(III) ion,
it can be modified by a lanthanide-binding-group (LBG) to bring
the BODIPY close to Yb(III) for sensitizing.26 In 2006, Jean-
Claude G. Bünzli et al27 linked a terpyridine to a BODIPY core
and prepared its Yb(III) complex, which emitted moderately at
980 nm. We attached an 8-hydroxyquinoline to a BODIPY core
and found the resulting Yb(III) complex also emitted at 980 nm
upon excitation at 507 nm.28, 29 Its iodinated analog sensitizes
effectively with a much longer lifetime upon excitation at 535
nm.28 The enhancement is attributed to an efficient intersystem
crossing for a higher triplet yield due to a "heavy atom" effect.
Recently, we linked two BODIPY moieties to 1,10-
phenanthroline and found it can sensitize the ytterbium emission
after the coordination to the metal.30 To further our work in this
direction, we designed two new BODIPY-modified 1,10-
2
provided is crucial to the visualization of biological events,
development of new medicines, and assessment of the
effectiveness of medical treatments.3-5 The key to the diagnosis
is the sensitivity of the probe. Currently available clinical probes
emit in the visible region and their fluorescence spectra overlap
with autofluorescence from substrates, which significantly limits
test sensitivity.5, 6 Most state-of-the-the-art probes also require
excitation at a short wavelength, which often results in severe
photobleaching of biological substrates. Therefore, it is desirable
to develop novel fluorescent probes that emit strongly in the
near-infrared (NIR) region under long wavelengths excitation.
NIR probes will not only increase sensitivity by eliminating
background signals but also provide deep penetration capability
for in vivo imaging. Certain lanthanide ions are potential NIR
probes.7-10 For example, ytterbium (III) displays emission at 980
nm, which is usually realized by a sensitization process (the
“antenna effect”). In this process, energy from the excited state
(usually a triplet state) of a sensitizer transfers to the excited state
of Yb(III) ion. After relaxation to its ground state, NIR emission
is produced.11-13
Two challenges have to be addressed in regards to biological
applications of NIR emitting lanthanide complexes. The first one
is to suppress the fluorescence quenching from vibrations of X-
H (X = N, O, C) in the vicinity of the coordination sphere, which
a.Department of Chemistry and Biochemistry, Eastern Illinois University, Charleston,
IL 61920 Email: hhe@eiu.edu. Tel: 1-217-581-6231
b.Department of Chemistry, University of South Dakota, Vermillion, SD 57069.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Journal Name
phenanthroline ligands and their interaction with tris(1,1,1,5,5,5- BF3·OEt2 (~ 0.05 mL) was added and the resultingViseowlAurttiicolenOwnlianes
DOI: 10.1039/C9DT02850A
hexafluoro-2,4-pentanedionate) ytterbium (III) dihydrate, stirred magnetically at room temperature for 12h. To this
[Yb(HFA)3(H2O)2] as shown in Scheme 1. The resulting solution 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) (0.45 g,
complexes are eight-coordinate with strong NIR emission in the 2 mmol) was added. After stirring for 2h, triethylamine (4 mL)
near-infrared region. In this report, the synthesis, and BF3·OEt2 (4 mL) were added sequentially. The solution
characterization of ligands and their interactions with ytterbium became fluorescent immediately under a UV lamp, indicating the
(III) will be discussed.
formation of the final product. The solution was magnetically
stirred for another 6h and the solvent was removed on a rotary
evaporator. The residue was re-dissolved in about 5 mL of
dichloromethane and was then loaded on column (silica)
chromatography and eluted with chloroform. The major band
was collected. Yield: 1.05 g, 77%. Anal Found (Calcd.) for
C38H34B2F4N6·CH3OH·H2O: C, 65.03 (64.84); H, 5.20 (5.58); N,
11.99 (11.63). 1H NMR (CDCl3, ppm) 9.34 (s, 2H), 7.82 (s, 2H),
7.66 (d, 2H), 5.95 (s, 4H), 2.56 (s, 12H), 1.06 (s, 12H). ESI-HR
MS: 673.3055 (M+H) (calcd. 673.3045)
I
I
F
F
F
F
N
N
N
N
H
H
B
B
B
B
F
F
F
F
N
N
N
N
O
O
N
N
N
N
N
N
N
N
(c)
(c)
(b)
(b)
(a)
(a)
I
I
I
I
N
N
N
N
N
N
N
N
O
O
N
N
N
N
H
H
B
B
F
F
B
B
F
F
N
N
N
N
F
F
F
F
Synthesis of L2.
R
I
I
R
To L1 (0.473 g, 2.0 mmol) in absolute ethanol was added I2 (1.06
g, 10 mmol). Then HIO3 (1.4 g, 8.0 mmol) in water (1.0 mL) was
added slowly during 10 minutes. The temperature of the solution
was then increased to 60˚C and the solution was stirred
magnetically for 2h. During the course of the reaction, the
fluorescence weakened gradually. After the completion of the
reaction, the solvent was removed on a rotary evaporator. The
crude product was dissolved in chloroform and purified on a
silica column using chloroform as the elute. The major band was
collected. The final product was crystallized from CHCl3/MeOH
(v/v: 1:20). Yield: 0.66 g, 80 %. Anal Found (Calcd.) for
C38H30B2F4I4N6: C, 38.51 (38.81); H, 2.99 (2.57); N, 6.98 (7.15).
1H NMR (CDCl3, ppm). 9.39 (d, 2H), 7.78 (s, 2H), 7.66 (d, 2H),
2.65 (s, 12H), 1.08 (s, 12H). ESI MS:1176.9 (M+H) (Calcd.
1176.9)
F
F
N
N
F
F
B
B
N
N
F
F
B
B
F
F
N
N
N
N
N
N
F
C
C
(d)
(d)
F33
R
R
R
R
N
N
O
O
Yb
Yb
N
N
O
O
N
N
N
N
F
C
F33C
3
3
N
N
B
B
F
F
N
N
B
B
F
F
F
F
N
N
F
F
R
R
=
L2-Yb
I,
R =H, L1-Yb; R = I, L2-Yb
R =H, L1-Yb
R
=
R
L2
L2
R =H, L1
I,
R =H, L1; R = I,
;
;
Scheme
1 Synthesis of BODIPY-modified 1,10-phenanthroline ligands and their
ytterbium (Ⅲ) complexes. (a) SeO2, dioxane, reflux; (b), CH2Cl2, N2, TFA; DDQ; Et3N;
BF3OEt2; (c) I2, HIO3, EtOH; (d) [(HFA)3Yb(H2O)2], toluene, r.t.
Synthesis of complexes
Experimental Section
Reagent and General procedures
Two complexes, L1-Yb and L2-Yb, were prepared in a similar
way and a typical one is given for L1-Yb. To an ethanol (25 mL)
solution of [Yb(HFA)3(H2O)2] (18 mg, 0.022 mmol) was added
L1 (15 mg, 0.022 mmol) in dichloromethane (10 mL). The
solution became reddish in colour. The resulting solution was
magnetically stirred for 2h at room temperature. The solid was
collected, washed with ethanol and dried at 80°C for 12h. L1-
4,7-dimethyl-1,10-phenanthroline
(4,7-Me-Phen),
SeO2,
BF3·OEt2 (48%), HIO3, I2, 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ), 2,4-dimethylpyrrole and triethylamine
(Et3N) were obtained commercially and used without further
purification.
Tris(1,1,1,5,5,5-hexafluoro-2,4-pentanedionate)
Yb
.
Yield: 27 mg, 85%. Anal Calcd. (Found) for
ytterbium (III) dihydrate ([(HFA)3Yb(H2O)2]) was purchased
from the Strem Chemicals, Inc. and was used directly for
reactions. Solvents for photophysical studies were ACS grade
and ultra-dry in Aeroseal container and used as received. CDCl3
was from the Cambridge Isotope Laboratories with 99.8% D in
ampule and used without further purification. Elemental
compositions were determined on a Perkin Elmer Series II-2400
CHNS analyser. All other chemicals and solvents for synthesis
and purification were analytical grade and used as received. 4,7-
diformyl-1,10-phenathroline was prepared according to the
C53H37B2F22N6O3Yb, C, 43.31 (43.20); H, 2.54 (2.62), N 5.73
(5.53) ESI MS: 1260.2 (M-HFA) (Calcd. 1260.2); L2-Yb, Yield:
37 mg, 87%. Anal Calcd. (Found) for C53H33B2F22I4N6O6Yb, C,
32.31 (32.54); H 1.69 (1.87), N 4.27 (4.35). ESI MS: 1791.8 (M-
HFA+CO) (Calcd.1791.9)
X-ray crystallographic analysis
Single crystals of L1 were obtained by slow evaporation of
solvent from
a 1:1 (v:v) mixtures of methanol and
dichloromethane at room temperature. The crystals were
mounted on glass fibres for data collection. Diffraction
measurements were made on a CCD-based commercial X-ray
diffractometer using Mo Kα radiation (λ= 0.71073 Å). The
frames were collected at 125K with a scan width 0.3° in ω and
integrated with Bruker SAINT Software package using narrow-
frame integration algorithm. The unit cell was determined and
refined by least squares upon the refinement of XYZ-centroids
literature method31 and confirmed by the H NMR and MS
(Figure S1 –S4).
Synthesis of L1
1
To a dry dichloromethane (150 mL) solution of 4,7-diformyl-
1,10-phenanthroline (346 mg, 2 mmol) was added 2, 4-
dimethylpyrrole (0.462 ml, 4 mmol) under nitrogen. A drop of
2 | J. Name., 2012, 00, 1-3
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of reflections above 20σ(I). The data were corrected for Synthesis and Characterization
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absorption using the SADABS program.32 The structures were
Ligands (L1 and L2) and complexes (LD1O-YI: b10.a10n3d9/LC29D-YT0b2)85a0rAe
prepared according to a procedure as outlined in Scheme 1. The
4,7-dicarbonyl-1,-10-phenanthroline precursor was prepared by
oxidation of 4,7-dimethyl-1,10-phenanthroline with SeO2 in 1,4-
dioxane with a moderate yield.36 The L1 was prepared by the
reaction between 4,7-dicarbonyl-1,10-phenanthroline and 2,4-
dimethylpyrrole in CH2Cl2 under N2 for 12h in the presence of
one drop of BF3OEt2, followed by oxidation using 2,3-dichloro-
5,6-dicyano-1,4-benzoquinoline (DDQ) and chelating to BF2
unit in the presence of triethylamine (Et3N) and BF3·OEt2.37 The
resulting compound L1 was slightly yellowish. The L1 was then
reacted with iodine and HIO3 in ethanol at 60ºC to L2 with ~95%
yield. The L2 was purplish. The compositions of the two
compounds were confirmed by elemental analysis and 1H NMR
refined on F2 using the Bruker SHELXTL (version 5.1) software
package.33 Crystal data for L1: C40H42B2F4N6O2, MW = 736.42,
triclinic, space group = P-1, a = 8.5972(5), b =10.6737(6), c =
21.2322(13) Å, α = 97.6450 (10), β = 97.9850 (10)°, γ =
105.6490 (10), V = 1828.00 (19) Å3, Z = 2, ρcalcd. = 1.338 Mgm–
3, μ(Mo-Kα) = 0.097 mm–1, F(000) = 772, T = 100 (2) K. 18013
reflections were measured, of which 6419 were unique (Rint =
0.0345). Final R1 = 0.0524 and wR2 = 0.1344 values were
obtained for 4724 observed reflections with I>2σ(I), 500
parameters, and GOF = 1.023. CCDC1861312.
Photophysical measurements
Absorption spectra were obtained on a Cary500 UV-visible
spectrophotometer at room temperature. Steady-state and time-
resolved spectroscopy studies were performed on an FS5
fluorimeter (Edinburg Instruments) with a Xenon arc lamp as a
light source. For the visible emission measurements, the slit
width for emission and excitation arms was 1 nm. For the NIR
emission, the slit width for both emission and excitation was 8
nm. The fluorescence quantum yield, Φ, in the visible region was
measured using the following equation:
(
Figures S1- S4). Both ligands were very soluble in CHCl3 and
CH2Cl2 and slightly soluble in CH3OH. Samples are not soluble
in water, which could be a barrier to medical diagnosis and could
be overcome by introducing carboxylic groups to the BODIPY.
We are currently working on this direction. The color of the L1
solution was yellowish, whereas the L2 solution was purplish.
The structure of L1 was further confirmed by single-crystal X-
ray diffraction analysis. Its ORTEP diagram is shown in Figure
2
�
ꢀꢁꢂꢃꢄ
Φx =
∅
ST
�
��ꢇꢇꢄ
ꢀꢁꢂꢃꢅꢆ
ꢅꢆ
1
. The two BODIPY units, each having a B center with a
where Grad the gradient from the plot of integrated fluorescence
intensity vs absorbance of five samples with different
concentrations, and n is the refractive index of the solvents.
Rhodamine 6G in ethanol (ΦST = 0.95, λex = 480 nm) was used
as a reference. The decay curves of the samples were also
measured on the FS5 (Edinburg Instruments) spectrometer. A
laser diode EPL 375 (Edinburg Instruments) with the wavelength
of 375 nm was used as light source. The NIR decay curves were
acquired using an optical parametric oscillator (OPOTEK
Opolette) as an excitation source. NIR emission was detected
using a 0.3 m flat-field monochromator (Jobin Yvon TRIAX
320) equipped with a NIR-sensitive photomultiplier tube
(Hamamatsu R2658P) in a cooled housing (Products for
Research). All spectra were corrected for instrument response.
The output from the photomultiplier was pre-amplified (Stanford
Research SR 445A) and fed to a multichannel scaler (Stanford
Research SR 430) for time-resolved photon counting. The entire
system was PC controlled using LabView software.
tetrahedral geometry, are almost perpendicular to the 1,10-
phenanthroline plan with torsion angles 86.98 (6) and 84.77 (4),
respectively. The two BODIPY units are pointing away from two
nitrogen atoms (N1 and N2) of the 1,10-phenanthroline unit with
a distance of 10.927Å between B1 and B2. There are two
methanol molecules in each asymmetric unit, both of which form
an intermolecular hydrogen-bond with N1 and N2 from the
neighboring unit. The crystals of the L2 diffracted weakly,
therefore no X-ray diffraction data were obtained. Synthesis of
the complexes, L1-Yb and L2-Yb, was achieved by mixing one
equivalent
of
ligands
with
[Yb(HFA)3(H2O)2]
in
dichloromethane. The simple replacement of two H2O molecules
by the bidentate ligands afforded the desired complexes. The
complexes are soluble in most organic solvents including
chloroform, dichloromethane, and toluene and are slightly
soluble in methanol and hexane. The compositions of the
complexes were confirmed by elemental analysis.
Theoretical calculation
Calculations for ligands were performed at a density functional
theory (DFT) level using Gaussian 09 software.34 The initial
input structures were built based upon the crystal structure of a
similar complex. The ground state geometries of compounds
were optimized using 6-31G(d) as a basis set for C, H, N, B, and
F atoms and Midix for I atom and MWB28 for Yb. All other
parameters were default set. No negative frequency was found in
the final optimized structures. All calculations were carried out
in SMD model for mimicking the solvent effect. The calculated
absorption data were analyzed by GaussSum software.35
a.
Figure 1. ORTEP diagram of L1 with 50% thermal ellipsoid probability. Hydrogen atoms
and two methanol molecules were omitted for clarity.
Results and Discussion
Photophysical Properties of Ligands
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Absorption and fluorescence properties of the ligands in solution for a proton on the BODIPY ring. After different amount of
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DOI: 10.1039/C9DT02850A
at room temperature were studied. As shown in Figure 2, the L1 [Yb(HFA)3(H2O)2] were added, all these peaks except those
exhibits strong absorption in the visible region with a peak corresponding to CH3 were broadened and shifted to low field
position at 507 nm in CH2Cl2, which is quite typical for BODIPY region due to impact from paramagnetic Yb3+ ion. The spectrum
dyes. With iodination at C2 and C6 positions in L2, the colour of did not change after the molar ratio was above 1.0. This result is
the solution changed from yellow to pinkish and a strong consistent with emission spectral changes.
absorption at 540 nm was observed, and the absorption
coefficients are also enhanced. The fluorescence properties of L1
and L2 are different. The L1 gives strong fluorescence at 524 nm
[Yb(HFA)3(H2O)2] + L2 → [Yb(HFA)3(L2)] + 2 H2O
with a quantum yield of 84.6% in CH2Cl2. The emission The isolated products showed the expected formula of
wavelength does not change too much in different solvents; [Yb(HFA)3(L1)] (L1-Yb) and [Yb(HFA)3(L2)] (L2-Yb). Their
however, the quantum yields vary significantly (Table S1) from compositions were ascertained by elemental analysis. Single
the lowest quantum yield in DMF (8.7%) to the highest quantum crystals obtained from a solution of [Yb(HFA)3(L2)] gave poor
yields in CHCl3 (88.9%) and CH2Cl2 (84.6%). The L2 emits at X-ray diffraction; therefore, no reliable structural information
569 nm with a quantum yield of about 0.4%, which is was obtained. However, the geometry-optimized structure of L2-
significantly lower than that of the L1. This is most likely due to Yb, as shown in Figure 4, revealed an eight-coordination
the “heavy atom” effect from introduced iodine atoms as geometry around Yb(III) ion. The phenanthroline unit binds to
previously observed. The heavy atom facilitated the intersystem the Yb(III) through two N atoms with two BODIPY units aligned
crossing for an increased quantum yield of triplet state, which almost vertically to the phenanthroline unit with dihedral angles
will benefit the sensitization of near-infrared emission of 87.27 and 87.60°, respectively. The Yb-O bond length varied
ytterbium (III).
from 2.482 to 2.526Å. The two Yb-N bond lengths are 2.700 Å
and 2.690Å.
Figure 2. The normalized absorption and fluorescence of L1 and L2 in CH2Cl2 at room
temperature. The concentration of the all samples is 1 × 10-6 mol/L in CH2Cl2.
Figure 3. The increase Yb(III) emission (λex = 545 nm) intensity of [Yb(HFA)3(H2O)2] in
CH2Cl2 upon addition of L2 at room temperature. The incubation time was five minutes
after addition of L2. The concentration of [Yb(HFA)3(H2O)2] was 2 × 10-4 mol/L in CH2Cl2.
Interaction of Ligands with Yb(III)
The interaction of ligands with Yb(III) was first examined from
fluorescence titrations of [Yb(HFA)3(H2O)2] solutions with
ligands L1 and L2 at room temperature. A typical spectral
change of [Yb(HFA)3(H2O)2]/L2 system is shown in Figure 3
.
The CH2Cl2 solution of [Yb(HFA)3(H2O)2] did not give any
emission between 900 and 1200 nm upon excitation at 545 nm
since [Yb(HFA)3(H2O)2] has no absorption at 545 nm. As L2
was added to the solution, the characteristic emission of Yb (III)
in the NIR region was readily observed. The emission intensity
increased with the continued addition of L2 until the molar ratio
of L2/Yb3+ reached 1.0, indicating the stoichiometric
replacement of two water molecules by one L2 ligand as shown
in the following equation. Similar NIR emission changes were
also observed for the L1/Yb3+ system. In addition, the reaction
between L1 and [Yb(HFA)3(H2O)2] in CDCl3 was also
monitored by 1H NMR spectroscopy as shown in Figure S5. The
L1 gave well-resolved peaks in CDCl3 at 9.34, 7.82, and 7.89
ppm for protons on the phenanthroline fragment, and 5.95 ppm
Figure 4. Side-view of geometry-optimized structure of L2-Yb.
Photophysical properties of the complexes
4 | J. Name., 2012, 00, 1-3
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Figure 6. Normalized NIR emission spectra of L1-Yb (λex = 507 nm) and L2-Yb (λex = 547
The complexes L1-Yb and L2-Yb exhibit absorption spectra in
the visible region that are similar to the corresponding ligands as
shown in Figure 5. After forming a complex, the emission
wavelength shifted from 524 nm in L1 to 550 nm in L1-Yb and
569 nm in L2 to 584 nm in L2-Yb. The fluorescence of L1 and
L2 decreased 95% and 15% after forming the complex with
Yb(III) ion, respectively. The characteristic NIR emission
spectra that are quite similar to the final spectrum during the
titration are also observed. As shown in Figure 5, upon visible
excitation, the two complexes exhibit a broad emission feature
centered at 1003 nm, which contains three distinct peaks at 976,
1003 and 1019 nm, respectively. These are typical 2F5/2 → 2F7/12
emission peaks of the ytterbium (III) ion. The emission lifetimes
of L1-Yb and L2-Yb from the exponential fitting of their decay
curves at 976 nm were 11.2 and 11.3 µs, respectively. The decay
curve for L2-Yb is also shown in Figure 6 and Table 1. The
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nm) in CH2Cl2 at room temperature. The decay was monitored at 978 upon excitation at
DOI: 10.1039/C9DT02850A
the 375 nm.
Table 1. Photophysical properties of L1, L2, L1-Yb and L2-Yb in CH2Cl2 at
room temperature.
Abs
(nm)
507
Compound
Emission (λ/ nm, τ)a
QY(%)
L1
524 (10 ns)
84.6b
-
0.93c
545
507
545
L2
569 (<1 ns)
L1-Yb
L2-Yb
550, 976 (11.2 µs), 1003, 1019
584, 976 (11.3 µs), 1003, 1019
0.94 c
aλex = 375 nm for the VIS emission; bin CH2Cl2; ccalculated from QYb
obs/τrad, τrad = 1.2 ms
=
τ
calculated intrinsic quantum efficiencies ΦYb (= τobs/τrad, τrad
=
1.2 ms)38, 39 are 0.93% and 0.94%, respectively. To verify the
origin of sensitization, we recorded the excitation spectra of L1-
Yb and L2-Yb (λem = 976 nm). As shown in Figure 7, the
excitation spectra match the absorption spectra very well,
indicating clearly that the BODIPY moiety is responsible for the
sensitization of Yb3+emission.
Figure 7. Excitation and absorption spectra of L2-Yb in CH2Cl2 at room temperature. The
emission wavelength was 976 nm. The excitation spectrum of L1-Yb can be found in
Figure S6.
The NIR emission of ytterbium (III) is sensitized by ligand-to-
Yb(III) energy transfer. The energy transfer process occurs either
from the triplet state of ligand to the excited state of lanthanide
(III) (Dexter mechanism) or through dipole-dipole interaction
(Förster mechanism).13 An early study from Jean-Claude G.
Bünzli et al.26, 27 revealed that the energy level of the triplet state
of BODIPY functionalized terpyridine ligand was ~ 17450 cm-1,
which is high enough for energy transfer to Yb(III). The
sensitization was proposed to occur by a Dexter electron
exchange mechanism. To probe if such a mechanism is also valid
Figure 5. Absorption spectra of L1-Yb and L2-Yb in CH2Cl2 at room temperature. The
concentration is5.18 ×10-6 mol/L.
for our complexes, we added
a
slight excess of
[Yb(HFA)3(H2O)2] to solutions of ligands L1 and L2 with the
same concentration (5.18 × 10-6 M) in CH2Cl2 in order to
generate the L1-Yb and L2-Yb and to ensure that there are no
free ligands remaining in solution. The intrinsic fluorescence of
ligand L1 decreased 97%, whereas that of ligand L2 decreased
14% (Figures S7 and S8). Such a drastic decrease is a stark
contrast to terpyridine and benzoic acid functionalized BODIPY-
27
Yb complexes.26,
Figure 8 shows that the NIR emission
intensities from the L1-Yb and L2-Yb solutions are very similar
upon excitation at 514 nm, at which wavelength the two solutions
exhibit the same absorbance. Similar emission intensities from
the two solutions are also observed when the solutions are
excited at their respective absorption peak positions (507 nm and
547 nm), at which the absorbance was adjusted to 0.316 (Figure
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S9). These results demonstrate clearly that the efficient intrinsic
intersystem crossing in ligand L2, induced by the I atoms, does
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DOI: 10.1039/C9DT02850A
not affect its ability to sensitize Yb(III) relative to ligand L1
.
Also, the fact that the sensitization efficiency is essentially
identical in L1-Yb and L2-Yb would suggest that the
mechanism of sensitization in both complexes is the same. The
most straightforward explanation of these observations is that the
Yb(III) ion can also induce near the quantitative intersystem
crossing in the two complexes, after which, sensitization occurs
from the triplet state of the ligand via a Dexter mechanism.
Figure 9. Electron density distribution profiles of ligand L2 and its Yb(III) complex.
Conclusions
In conclusion, near-infrared emission of ytterbium (III) ion was
sensitized successfully by an iodized BODIPY chromophore
under excitation at 540 nm. Luminescence lifetimes as long as ~
11 µs was observed for 975 nm emission in the iodinated and
non-iodinated complexes. The results provide a new strategy for
novel optical probes that emit in the NIR region under long
wavelength excitation for sensitive biomedical imaging and
detection.
Figure 8. NIR emission spectra of L1-Yb and L2-Yb upon excitation at 514 nm. The two
solutions have equal absorbance at 514 nm and show nearly equal emission intensities,
indicating that the internal quantum efficiencies of sensitized emission from 1-Yb and 2-
Yb are similar.
We performed DFT calculations for ligand L1 and ligand L2 and
their complexes, L1-Y and L2-Y, respectively. For the free
ligands, the HOMO and LUMO are both located on the BODIPY
moieties (Table S2 and S3). The predicted absorption spectra
Conflicts of interest
There are no conflicts to declare
show a red-shift in compound L2 relative to compound L1
,
which is consistent in trend with the measured absorption
spectra. It should be mentioned that BODIPY dyes are notorious
in spectral predicting due to the lack of accurate functional and
bases sets for calculation.40, 41 The triplet level for ligand L1 and
L2 are respectively at 782 and 804 nm, which are quite consistent
with the measured data by Zhao et al.42 The energy levels are
positioned at higher energy than the emitting energy level of
Yb(III). For the complexes, the electron density at the HOMO
(H) and LUMO (L) are high at BODIPY units as shown in
Figure 9. Significant electron densities at Yb center are in L+2
and L+3, indicating the energy transfer from BODIPY to the Yb
upon excitation.
Acknowledgements
HH thanks National Science Foundation (Award # 1507871) and
The Extreme Science and Engineering Discovery Environment
(XSEDE) (CHE130116) for financial support of this work. Authors
are grateful to a crystallographer for structure refinements
during the review of this article.
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