Ytterbium(III) porpholactones: β-lactonization of porphyrin ligands enhances sensitization efficiency of lanthanide near-infrared luminescence

Article abstract of DOI:10.1002/chem.201303972

The near-infrared (NIR) luminescence efficiency of lanthanide complexes is largely dependent on the electronic and photophysical properties of antenna ligands. Although porphyrin ligands are efficient sensitizers of lanthanide NIR luminescence, non-pyrrolic porphyrin analogues, which have unusual symmetry and electronic states, have been much less studied. In this work, we used porpholactones, a class of β-pyrrolic-modified porphyrins, as ligands and investigated the photophysical properties of lanthanide porpholactones Yb-1 a-5 a. Compared with Yb porphyrin complexes, the porpholactone complexes displayed remarkable enhancement of NIR emission (50-120 %). Estimating the triplet-state levels of porphyrin and porpholactone in Gd complexes revealed that β-lactonization of porphyrinic ligands lowers the ligand T₁ state and results in a narrow energy gap between this state and the lowest excited state of Yb³⁺. Transient absorption spectra showed that Yb^III porpholactone has a longer transient decay lifetime at the Soret band than the porphyrin analogue (30.8 versus 17.0 μs). Thus, the narrower energy gap and longer lifetime arising from β-lactonization are assumed to enhance NIR emission of Yb porpholactones. To demonstrate the potential applications of Yb porpholactone, a water-soluble Yb bioprobe was constructed by conjugating glucose to Yb-1 a. Interestingly, the NIR emission of this Yb porpholactone could be specifically switched on in the presence of glucose oxidase and then switched off by addition of glucose. This is the first demonstration that non-pyrrolic porphyrin ligands enhance the sensitization efficiency of lanthanide luminescence and also display switchable NIR emission in the region of biological analytes (800-1400 nm). Lanthanide porphyrinoids: Ytterbium porpholactones show enhanced sensitization efficiency for near-infrared luminescence of Yb³⁺, narrower energy gaps between the porphyrinoid T₁ state and the lowest excited states of Yb ³⁺, and longer transient decay lifetimes than their porphyrin analogues (see figure). Furthermore, a glucose-conjugated water-soluble Yb porpholactone has potential applications as a switchable NIR-emissive probe.

Full text of DOI:10.1002/chem.201303972

DOI: 10.1002/chem.201303972
Full Paper
&
Porphyrinoids
Ytterbium(III) Porpholactones: b-Lactonization of Porphyrin
Ligands Enhances Sensitization Efficiency of Lanthanide
Near-Infrared Luminescence
Xian-Sheng Ke,[a] Bo-Yan Yang,[a] Xin Cheng,[a] Sharon Lai-Fung Chan,[b] and Jun-
Long Zhang*[a]
Abstract: The near-infrared (NIR) luminescence efficiency of
lanthanide complexes is largely dependent on the electronic
and photophysical properties of antenna ligands. Although
porphyrin ligands are efficient sensitizers of lanthanide NIR
luminescence, non-pyrrolic porphyrin analogues, which have
unusual symmetry and electronic states, have been much
less studied. In this work, we used porpholactones, a class of
b-pyrrolic-modified porphyrins, as ligands and investigated
the photophysical properties of lanthanide porpholactones
Yb-1a–5a. Compared with Yb porphyrin complexes, the
porpholactone complexes displayed remarkable enhance-
ment of NIR emission (50–120%). Estimating the triplet-state
levels of porphyrin and porpholactone in Gd complexes re-
vealed that b-lactonization of porphyrinic ligands lowers the
ligand T1 state and results in a narrow energy gap between
this state and the lowest excited state of Yb3+. Transient ab-
sorption spectra showed that YbIII porpholactone has
a longer transient decay lifetime at the Soret band than the
porphyrin analogue (30.8 versus 17.0 ms). Thus, the narrower
energy gap and longer lifetime arising from b-lactonization
are assumed to enhance NIR emission of Yb porpholactones.
To demonstrate the potential applications of Yb porpholac-
tone, a water-soluble Yb bioprobe was constructed by con-
jugating glucose to Yb-1a. Interestingly, the NIR emission of
this Yb porpholactone could be specifically switched on in
the presence of glucose oxidase and then switched off by
addition of glucose. This is the first demonstration that non-
pyrrolic porphyrin ligands enhance the sensitization efficien-
cy of lanthanide luminescence and also display switchable
NIR emission in the region of biological analytes (800–
1400 nm).
Introduction
promising antenna ligands for sensitizing near-infrared (NIR)-lu-
minescent lanthanide complexes, because they absorb strongly
both in the UV and visible regions.[3] However, the NIR emission
efficiency has yet to be improved. Given that the energy levels
of lanthanide ions are hardly affected by their environment,
modulation of the electronic structures of porphyrinoids by re-
placing traditional pyrrolic moieties would be a viable way to
tune NIR emission. To the best of our knowledge, the only ex-
amples thereof are the lanthanide complexes of N-confused
porphyrin,[4] which display weaker NIR emission than their por-
phyrin congeners. This inspired us to explore the effect of pyr-
rolic replacement on lanthanide sensitization, and thus extend
the chemistry of porphyrin derivatives containing non-pyrrolic
moieties to lanthanides.
Porphyrin analogues in which one pyrrolic subunit is replaced
by a non-pyrrolic moiety have recently attracted much atten-
tion for their unusual electronic states and photophysical prop-
erties.[1] These replacements at the porphyrin periphery are
strongly electronically coupled to the chromophores and thus
make them attractive ligands. Despite the tremendous prog-
ress made in transition metal complexes of non-pyrrolic por-
phyrins,[1c,2] related studies on lanthanide (Ln) complexes have
rarely been reported. It is well known that porphyrins are
[a] X.-S. Ke, B.-Y. Yang, X. Cheng, Prof. Dr. J.-L. Zhang
Beijing National Laboratory for Molecular Sciences
State Key Laboratory of Rare-Earth Materials Chemistryand Applications
College of Chemistry and Molecular Engineering
Peking University
Porpholactones are a class of non-pyrrolic porphyrinoids in
which a pyrrole moiety is replaced by an oxazolone group (or
one porphyrin b,b’-double bond is replaced by a lactone
moiety).[5] Their optical and photophysical properties are inter-
mediate between those of porphyrin and chlorin[5b] when the
oxa and oxo atoms are involved in the p conjugation of the
system. This makes them suitable for optical application such
as chemosensors for oxygen sensing in pressure-sensing mate-
rials,[6] pH indicators,[7] and reactive oxygen species.[8] Plati-
num(II) porpholactones show redshifted phosphorescence
Beijing 100871 (P. R. China)
Fax: (+86)1062767034
E-mail: zhangjunlong@pku.edu.cn
[b] Dr. S. L.-F. Chan
Department of Applied Biology and Chemical Technology
The Hong Kong Polytechnic University
Hung Hom, Kowloon, Hong Kong (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303972.
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compared to porphyrin analogues, which indicates that b-lac-
3
tonization of porphyrinic ligands affects the MLCT triplet excit-
ed state.[9]
Herein, we report the first synthesis of lanthanide porpholac-
tone complexes. We found that b-lactonization of porphyrinic
ligands leads to enhanced Yb NIR luminescence, longer life-
times of Yb porpholactone complexes, and a smaller energy
gap between the ligand T1 state (based on estimates of the
levels of the ligand T1 states in Gd porphyrin and porpholac-
tone) and the lowest excited states of Yb3+.
To demonstrate potential applications, we were interested in
the NIR emission window of 800–1400 nm, which has less in-
terference from light scattering and autofluorescence of bio-
logical matter.[10] The Yb3+ ion was chosen because its electron-
ic structure (f13 configuration) is relatively simple, the first excit-
ed state (2F5/2) is located in the NIR region, and Yb porphyrins
have higher quantum yields than Er and Nd analogues.[3c] In
addition, Yb porphyrins exhibit higher stability than Er and Nd
porphyrin analogues because of the smaller size of the Yb3+
ion (Shannon effective ionic radius: 0.985 ꢁ). Recently, some
water-soluble Yb porphyrin derivatives were found to display
strong affinity towards DNA and to cleave DNA on photoacti-
vation;[11] ytterbium porphyrin complexes conjugated with rho-
damine B have been reported for imaging mitochondria;[12]
and recently, Che and co-workers reported their potent anti-
cancer activities with cytotoxic IC50 values down to the sub-
micromolar range.[13] Despite the importance of the application
of Yb porphyrinoids as NIR-emissive bioprobes for signaling
and sensing the biological events in the NIR region, no exam-
ple thereof has been reported in the literature. This is due to
the fact that NIR emission from Yb porphyrins is well known to
be quenched by small molecules with high-energy oscillators
such as OꢀH bonds in water. We envisioned that, through ap-
propriate molecular design, a certain analyte might protect
Ln3+ cations against nonradiative deactivation induced by
water and thus switch on the NIR emission. To demonstrate
this, we prepared a water-soluble Yb porpholactone by conju-
gation with glucose and found the NIR emission was specifical-
ly switched on in the presence of glucose oxidase (GOx). More-
over, this switched-on emission could be quenched by the ad-
dition of glucose, and this suggests that the switched-on emis-
sion is due to selective binding of glucose conjugates to GOx.
Scheme 1. Representative porphyrins and porpholactones and synthetic
route to Ln complexes. acac=acetylacetonate.
alytically pure lanthanide complexes after purification by
column chromatography and recrystallization. Gadolinium
complexes Gd-1a and Gd-1b were synthesized by a similar
procedure.
The products were characterized by IR and UV/Vis spectros-
copy and ESI mass spectrometry, and their purities were con-
firmed by elemental analysis. Data from ESI MS analysis in
CH2Cl2/CH3OH suggested a monomeric structure for all com-
plexes with molecular formula of [Ln(Por or Porp)(acac)]. The
FTIR spectra of Yb-1a–5a and Gd-1a showed a C=O stretching
band at 1778, 1759, 1772, 1769, 1761, and 1770 cmꢀ1, respec-
tively, indicating that the lactone moiety of porpholactone[5a]
remains intact during metalation with the Yb3+ ion.
The photophysical properties of the Yb porpholactones
were characterized by absorption and photoluminescence
spectroscopy in dichloromethane (Table 1). Although Yb por-
phyrins were previously characterized, we measured their pho-
tophysical properties for comparison purposes. Figure 1a
shows the absorption spectra of free bases 1a and 1b, Yb-1a,
and Yb-1b in dichloromethane. Compared with the free bases,
Yb-1a and Yb-1b displayed a moderate redshift (10 nm) of the
Soret band and two moderately intense Q bands in the range
of 550–700 nm, which are characteristic features of metallopor-
phyrins and metalloporpholactones. The major differences in
absorption between Yb-1a and Yb-1b are a redshifted Q(0,0)
band (15 nm) and the increased extinction coefficient (ca. 8-
fold) for Yb-1a. Figure 1b shows NIR emission spectra of Yb-
1a and Yb-1b excited at 420 nm and the excitation spectra of
Yb-1a and Yb-1b monitored at 972 nm in dichloromethane.
Typical NIR emission centered at 972 nm arising from the
Results and Discussion
Ytterbium(III) complexes Yb-1a–5a (1a–5a=meso-tetrakis-
(pentafluororophenyl)porpholactone, meso-tetraphenylporpho-
lactone, meso-tetrakis(p-chlorophenyl) porpholactone, meso-
tetrakis(p-trifluoromethylphenyl)porpholactone, and meso-tet-
ramesitylporpholactone, respectively) and Yb-1b–5b (1b–
5b=meso-tetrakis(pentafluorophenyl)porphyrin,
meso-tetra-
phenylporphyrin, meso-tetrakis(p-chlorophenyl)porphyrin,
meso-tetrakis(p-trifluoromethylphenyl)porphyrin, and meso-tet-
ramesitylporphyrin, respectively) were synthesized according
to the method of Wong[3a] with modifications (Scheme 1). In
general, a mixture of Ln(acac)3 and the corresponding free
ligand was heated in 1,2,4-trichlorobenzene for 8 h to give an-
2
2F5/2! F7/2 transition for Yb3+ was observed, along with weak
fluorescence in the visible region (600–720 nm). At the same
concentration (4ꢂ10ꢀ6 m), Yb-1a showed higher emission in-
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Table 1. Absorption and luminescence data of Yb porphyrinoids (Yb-1a–5b).[a]
[d]
Compound
Absorption lmax [nm]
[lg(e/mꢀ1 cmꢀ1)]
Vis emission
lmax [nm] (t [ns])
NIR emission
lmax [nm]
Ftot [%][b]
tobs [ms][c]
hYb-a/hYb-b
Yb-1a
Yb-1b
Yb-2a
Yb-2b
Yb-3a
Yb-3b
Yb-4a
Yb-4b
Yb-5a
Yb-5b
419 [5.51], 564 [4.09], 608 [4.63]
416 [5.62], 547 [4.29], 582 [3.71]
425 [5.51], 561 [4.06], 606 [4.33]
422 [5.62], 552 [4.24], 589 [3.62]
426 [5.55], 563 [4.12], 607 [4.46]
423 [5.62], 552 [4.27], 590 [3.71]
424 [5.52], 563 [4.10], 607 [4.47]
421 [5.66], 553 [4.30], 592 [3.58]
425 [5.53], 561 [4.14], 606 [4.56]
423 [5.67], 553 [4.33], 589 [3.47]
612 (3.21), 668 (3.45)
588 (2.95), 641 (3.22)
609 (3.27), 667 (3.51)
607 (3.63), 650 (4.60)
612 (2.59), 666 (3.67)
605 (2.63), 647 (4.45)
615 (3.33), 670 (3.46)
611 (3.15), 648 (3.80)
612 (4.02), 664 (4.02)
608 (3.88), 647 (4.35)
972, 1004, 1021
973, 1004, 1022
973, 1006, 1022
974, 1002, 1023
972, 1007, 1025
973, 1006, 1025
972, 1007, 1024
973, 1006, 1025
972, 1006, 1022
973, 1004, 1024
3.3
2.2
2.8
1.6
3.1
1.5
4.1
2.5
2.4
1.1
26.5
23.5
27.0
24.5
29.9
26.3
25.5
26.5
25.7
23.4
1.33
1.59
1.82
1.70
1.99
[a] All the photophysical data were determined in anhydrous CH2Cl2 at room temperature. [b] Quantum yields were measured by using YbTPP(Tp) as refer-
ence; the estimated error is ꢁ15%. [c] NIR luminescence lifetimes were monitored at 975 nm, and all decay curves were fitted to a double-exponential
decay function. [d] hYb-a/hYb-b is the ratio of sensitization efficiencies of porpholactone Yb-a and porphyrin Yb-b.
sorptions for Yb-1a–5a reveals that the B and Q bands [Q(0,1),
Q(0,0)] are systematically redshifted by 2–3, 9–17, and 15–
17 nm, respectively, compared to the corresponding Yb por-
phyrins. More importantly, for Yb porpholactones Yb-1a–5a,
the absorption intensity of the Q(0,0) band centered at 606–
608 nm is about 8–12 times higher than that of Yb porphyrins
Yb-1b–5b, whereas the Q(0,1) band showed lower intensity
(ca. 0.6–0.7-fold) than those of the Yb porphyrins. The system-
atic redshifts and increasing extinction coefficients of the
Q(0,0) band of Yb porpholactones Yb-1a–5a suggests pertur-
bation of the energies of the frontier molecular orbitals of the
porphyrin rings by the oxazolone moiety. All Yb complexes ex-
hibit the typical near-IR emission centered at 972 nm arising
2
from the 2F5/2! F7/2 transition of Yb3+, which is split into multi-
ple bands (1002, 1021 nm) by the ligand field. Weak visible
emission was observed, and the emission band maxima are
tabulated in Table 1. The emission at 600–720 nm originating
from the singlet states of free bases is strongly reduced (by ca.
99%) in Yb complexes, due to intersystem crossing (ISC),
1
which conveys the singlet excitation from the lowest p!p*
3
state into the ligand triplet p!p* state.
To further illustrate the ligand effects on the NIR emission of
Yb complexes, the quantum yields of NIR emission were mea-
sured by comparative methods[12,14] with YbTPP(Tp) (TPP=
5,10,15,20-tetraphenylporphyrinate, Tp=hydridotris(1-pyrazo-
lyl)borate) as standard. The estimated error for the quantum
yields is ꢁ15%.[14e] The quantum yields of the Yb porpholac-
Figure 1. a) Normalized absorption of free bases 1a, 1b and Yb complexes
Yb-1a and Yb-1b in CH2Cl2. b) NIR emission spectra of Yb-1a and Yb-1b
(lex =420 nm) at 4ꢂ10ꢀ6 m and excitation spectra monitored at 972 nm in
CH2Cl2.
tones (2.4–4.1%) are 50–120% higher than those of the corre-
sponding Yb porphyrins (Table 1). Besides b-lactonization of
the porphyrins, the quantum yields of the Yb3+ complex are
also dependent on the meso substituents; for example, Yb-1a
(3.3%) and Yb-4a (4.1%) have higher quantum yields than the
Yb porpholactones with other substituents. However, no sys-
tematic trend in electronic or steric effects on quantum yields
was observed.
tensity than Yb-1b. The excitation bands at 422, 560, and
608 nm for Yb-1a and those at 419, 564, and 608 nm for Yb-
1b are identical to the respective UV/Vis absorptions, and this
suggests that the excitation of the Yb3+ ion originates from
the p!p* transitions of the porphyrinoid antennas.
Luminescence lifetimes were recorded in order to evaluate
the coordination environment around the Yb3+ cations in the
complexes. The luminescence decays of the Yb3+ complexes in
CH2Cl2 were measured and fitted as double-exponential decays
with lifetimes of 23.4–26.5 ms (Table 1). This indicates that
Table 1 summarizes the important UV/Vis absorption and
NIR photoluminescence data. All Yb complexes display an in-
tense B (Soret) band in the 419–426 nm region and two Q
bands in the 561–610 nm region. Close inspection of the ab-
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a unique and consistent coordination environment is present
around the Yb3+ cation in the complexes. Interestingly, we
found that, except for Yb-4a and Yb-4b, the lifetimes of Yb
porpholactones are longer than those of the corresponding Yb
porphyrins. The reason for the increased NIR emission lifetimes
of Yb porpholactones due to introduction of the lactone
moiety is not clear. Possible reasons are 1) the presence of an
oxazolone moiety reduces the strength of the CꢀH oscillator in
pyrrole moieties; 2) faster ISC after substitution with an oxazo-
lone moiety results in increased population of the Yb excited
state and longer NIR emission lifetimes.
Phosphorescence in Gd complexes: comparison of triplet
states of porphyrin and porpholactone
Since Yb porpholactones exhibited stronger NIR emission than
the corresponding porphyrins, it is important to understand
the mechanism of increased sensitization efficiency in Yb por-
pholactones. Assuming
a
porphyrin S1!porphyrinT1!Ln*
energy flow, the overall NIR emission efficiency Ftot is regulat-
ed by the ISC efficiency (S1!T1, FISC), the energy-transfer effi-
ciency (T1!Ln*, FET), and the intrinsic Ln quantum yield
(Ln*!Ln, FYb =tobs/tYb). The overall luminescence quantum
yield Ftot is the product of the yields of the three steps in-
volved in producing photoluminescence [Eq. (1)]. The sensitiza-
tion efficiency of porpholactone and porphyrin could be calcu-
lated according to Equation (2).
Figure 2. Low-temperature (77 K) emission spectra (lex =420 nm) of a) Gd-
1a and b) Gd-1b in MeOH/EtOH (1/1), with cumulative fit function and indi-
vidual Gaussians in gray. The asterisk indicates a second-order transmission
of the 420 nm excitation.
Ftot ¼ FISCFETFYb
hsen ¼ FISCFET
ð1Þ
ð2Þ
Due to the difficulty of obtaining the radiative (or “natural”)
lifetimes tYb in Yb porphyrin complexes, we assumed that the
tYb values of porphyrins and porpholactones are similar be-
cause of their similar coordination spheres. Thus, the relative
sensitization efficiency of porpholactone and porphyrin
hYb-a/hYb-b was estimated according to Equations (1) and (2).
From the lifetimes of NIR emission collected in Table 1, we
found that the longer lifetimes of Yb porpholactones may in-
crease the metal-based quantum yields FYb (tobs/tYb). Thus, the
relative sensitization ability could be deduced from Equa-
tion (2). Systematic increases (33–99%) in sensitization ability
of Yb porpholactones compared with their corresponding Yb
porphyrins were calculated (Table 1). It is noteworthy that the
relative sensitization efficiency is also affected by the substitu-
ents of the meso-phenyl groups, but no systematic trend was
observed.
Gd-1a emits at 810 nm, and Gd-1b at 723, 770, and 809 nm.
The triplet experimental decay times of 51.8 ms for Gd-1a and
130.1 ms for Gd-1b confirm the observation of phosphores-
cence. The redshifted phosphorescence and shorter lifetime for
Gd-1a are consistent with those of previously reported Pd and
Pt porpholactones.[9] The lowest-energy zero-phonon (n0–0
)
band of the triplet state was estimated by spectral deconvolu-
tion of the luminescence signal into a series of overlapping
Gaussian functions, which yielded values of 12346 and
13831 cmꢀ1 for the lowest-energy T1 states of 1a and 1b, re-
spectively. This clearly suggests that replacement of an oxazo-
lone moiety markedly lowers the excited T1 state (DE=
1485 cmꢀ1). According to the energy-gap law, the probability
of intramolecular energy transfer between two electronic
states is inversely proportional to their energy difference.[16]
Thus, as shown in Scheme 2, Yb-1a has a narrower energy gap
(DE=2546 cmꢀ1) between the porphyrinoid T1 state and the
lowest excited states of Yb3+ than Yb-1b (DE=4031 cmꢀ1),
which indicates more efficient energy transfer (FET) from 1a to
To investigate the role of the triplet state in sensitization effi-
ciency,[15] we estimated the ligand triplet levels in Gd porphyrin
and porpholactone. Because the emitting levels of the Gd3+
ion are of higher energy than the ligand singlet and triplet
states, the lack of energy transfer would prevent depletion of
the ligand triplet state and allow the emission of phosphores-
cence. Figure 2 shows the emission from Gd-1a and Gd-1b on
excitation at 420 nm, which is masked at room temperature, at
77 K in methanol/ethanol (1/1). In the range of 730–900 nm,
Yb3+
.
b-Lactonization of porphyrins affects the electronic structure.
Iterative extended Hꢃckel calculations by Gouterman and co-
workers suggested a porphyrinic electronic structure with the
p orbitals on both the oxa and oxo oxygen atoms present in
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Scheme 2. Proposed energy-transfer diagram between porphyrinoids 1a, 1b
and YbIII ion.
three of the four porphyrin orbitals and a resulting symmetry
change.[5b,d] This perturbation of the molecular orbitals of the
lactone moiety would result in changes in the HOMO, LUMO,
and LUMO+1 of the porphyrin, which correspond to S0, S1,
and S2 levels, respectively. Since the electron-withdrawing lac-
tone moiety would stabilize HOMO or HOMOꢀ1 more than
LUMO and LUMO+1, it is not surprising that in Yb-1a the
Q(0,0) band, which is assigned to S0–S1 transitions, is redshifted
by 26 nm and the T1 state is lower in energy (DE=2546 cmꢀ1
)
compared to Yb-1b. This indicates a smaller energy gap be-
Figure 3. a) Transient absorption spectra of Yb-1a in CH2Cl2 solution. Spectra
were obtained at several delay times following the 420 nm laser excitation
pulse (10 Hz, 2 mJ per pulse). b) Transient decay curves of Yb-1a and Yb-1b
in CH2Cl2, monitored at 408 nm.
tween the T1 state of the porphyrinic ligand and the lowest
excited state of Yb3+
.
Transient absorption spectra of Yb-1a and Yb-1b
electronic influence on the p-electron system of the porphy-
rinoids, which would account for a shift in the absorption of
the excited lanthanide complexes. According to Equation (1),
enhanced lifetime due to b-lactonization of porphyrinic ligands
To provide additional information on the electronic structure
and lifetime of the emitting states, transient absorption (TA)
spectroscopy was carried out on Yb-1a and Yb-1b. TA spectra
for Yb-1a and Yb-1b in CH2Cl2 were recorded at various delay
times following excitation by a 420 nm laser pulse (10 Hz, 2 mJ
per pulse). As shown in Figure 3a, the TA difference spectra for
Yb-1a and Yb-1b (see Figure S28 in the Supporting Informa-
tion) over the 400–650 nm region coincide with the Soret and
Q-band ground state. Yb-1a and Yb-1b show a bleaching of
the Soret band at 421 and 418 nm and net absorption around
409 and 406 nm. The transient lifetimes for the YbIII porphyrin
complexes at the Soret band are t=30.8 and 17.0 ms. The Q-
band region shows a derivative-shaped transient absorption.
Weak transient absorption (bleaching) was observed at the Q
bands for Yb-1a and Yb-1b, and the transient lifetimes were
difficult to obtain owing to the low absorption intensity. Transi-
ent absorptions of Yb-1a and Yb-1b are also similar in profile
and lifetime to those of YbIII porphyrin analogues.[3c] In all
cases, coordination of a lanthanide to porphyrin or porpholac-
tone results in rapid ISC of the S0!T1 state followed by energy
transfer to the lanthanide. For Yb-1a and Yb-1b, Yb3+ in their
respective excited states may simply be exerting a different
may increase the intrinsic Ln quantum yield (Ln*!Ln, FYb
=
tobs/tYb), which is an important factor in the overall NIR emis-
sion efficiency (Ftot).
Yb porpholactone as a NIR-emissive bioprobe in aqueous
media
To demonstrate the potential applications of Yb porphyrinoids,
we investigated the ability to detect biological analytes in the
NIR emission window of 900–1100 nm, which has less interfer-
ence from both light scattering and autofluorescence of bio-
logical matter. Because Yb porpholactones display higher NIR
luminescence than the Yb porphyrin analogues, we chose Yb-
1a and attached a glucose moiety through substitution of
para-F atoms to make it water-soluble. As shown in Scheme 3,
direct reaction of Yb-1a with 2,3,4,6-tetra-O-acetylglucosyl
thioacetate followed by subsequent hydrolysis of the acetyl
group by NaOMe afforded glycosylated ytterbium porpholac-
tone 3 in 40% yield. Complex 3 was characterized by matrix-
Chem. Eur. J. 2014, 20, 4324 – 4333
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D2O (0.5% DMSO, 12.1 ms) can be used to quantify the number
of solvent molecules coordinated to the lanthanide cations.
The hydration/solvation number was obtained by using empiri-
cal Equation (3), which was developed by Beeby et al.[17]
q ¼ AðkHꢀkDꢀBÞ A ¼ 1:0 ms, B ¼ 0:2 msꢀ1
ð3Þ
where q is the number of water molecules bound to the Yb3+
ion in the first coordination sphere, kH and kD are the rate con-
stants of excited states of lanthanide ion in H2O and D2O, re-
spectively, A is a proportionality constant related to the sensi-
tivity of the Yb3+ ion to vibrational quenching by OꢀH oscilla-
tors, and B is a correction factor for outer-sphere water mole-
cules.
Scheme 3. Synthetic route for 3. i) 2,3,4,6-tetra-O-acetylglucosyl thioacetate,
DMF/HNEt2 (10/1); ii) CH3ONa, CH2Cl2/MeOH (1/1).
By applying this formula, q values of ꢀ0.03 were calculated
for the YbIII complexes. These near-zero values indicate that no
water molecules are bound to the lanthanide ion in the first
coordination sphere of the complexes in solution. We assumed
that the vacant site of the Yb3+ cation might be occupied by
DMSO. Therefore, we can conclude that the nonradiative deac-
tivation of the NIR emission of 3 was due to outer-sphere
water molecules. This was confirmed by a Stern–Volmer (SV)
plot of the NIR emission from 3 quenched with H2O (0–2.78m).
As shown in Figure 4b, the overall quenching constant KSV was
0.24mꢀ1. Considering that KSV =kqt, where t is the lifetime of
the excited chromophore in the absence of quencher (23.3 ms),
we calculated the bimolecular quenching constant kq to be
1.0ꢂ104 mꢀ1 ꢀ1, which is much lower than the corresponding
s
value of diffusion-limited quenching (1010 mꢀ1 sꢀ1). Thus, the
quenching process could be assigned to a dynamic quenching
process,[18] which indicates that the binding affinity of water to-
wards the Yb center was very weak. Therefore, quenching of
the NIR luminescence process may be caused by collision be-
tween the outer-sphere water molecules and the Yb center.
Glucose conjugate 3 was then applied in detecting glucose-
binding proteins such as glucose oxidase (GOx) by means of
fluorescence spectroscopic measurements. Solutions of GOx
with concentrations increasing in steps of about 0.08 mm were
mixed with a 4 mm solution of 3 in Tris·HCl buffer (50 mm,
pH 7.4). The NIR emission at 900–1100 nm increased with in-
creasing GOx concentration (Figure 5a) up to a GOx concentra-
tion of 1.25 mm, above which the emission intensity remained
constant at approximately 18 times the initial emission intensi-
ty in the absence of GOx. The lifetime of NIR emission was
8.1 ms, which is twice that of 3 in aqueous medium. This sug-
gested that the NIR emission of 3 could be switched on by the
glucose-binding protein GOx in aqueous medium. To examine
the specificity of switched-on NIR emission, we chose 12 other
proteins: avidin, cytochrome c, concanavalin A (con A), deoxyri-
bonuclease I (DNase I), ribonuclease A (RNase A), lysozyme, im-
munoglobulin G (Ig G), papain, trypsin, bovine serum albumin
(BSA), human serum albumin (HSA) and phytohemagglutinin
(PHA). As shown in Figure 5b, only GOx showed an obvious in-
crease in emission intensity compared to the other 12 proteins
at the same concentration. These results suggested that the
switched-on emission is a specific response to GOx and may
Figure 4. a) NIR emission spectral changes of 3 versus concentration of H2O
in DMSO. b) Stern–Volmer plot of NIR emission intensity at 975 nm for
quenching of 3 with H2O.
assisted laser desorption ionization time-of-flight mass spec-
trometry (MALDI-TOF MS) and UV/Vis and IR spectroscopy.
We then examined the effect of water on the NIR emission
of 3. On addition of water, the NIR emission at 900–1100 nm
was quenched gradually, as shown in Figure 4a. Comparison of
the luminescence lifetimes in H2O (0.5% DMSO, 3.9 ms) and
Chem. Eur. J. 2014, 20, 4324 – 4333
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Figure 6. Relative NIR emission intensity at 975 nm of 3 (4 mm) versus con-
centration of glucose in the presence of GOx (1.25 mm) in Tris·HCl buffer
(50 mm, pH 7.4) at 258C. Inset: Relative NIR emission intensity monitored at
975 nm of 3 (4 mm) treated with seven different sugars (2 mm) in the pres-
ence of GOx (1.25 mm) in Tris·HCl buffer (50 mm, pH 7.4) at 258C.
ing of 3 with GOx (Figure S34 in the Supporting Information),
and for the addition of glucose to a solution of 3 and GOx
(Figure S36 in the Supporting Information). Although the reso-
lution of the spectra was not good, transient decay lifetimes of
0.3, 2.9 and 1.4 ms, respectively were measured at 430 nm (Fig-
ures S33, S35, and S37 in the Supporting Information); this
suggests that changes in GOx and glucose affect the photo-
physical properties of 3 in Tris·HCl buffer. These results strongly
suggest that complex 3 binds to the glucose-binding site of
GOx and thus enhances the NIR emission by protecting Yb3+
cations against nonradiative deactivation induced by water.
Figure 5. a) NIR emission spectra of 3 (4 mm) at various concentrations [mm]
of GOx in Tris·HCl buffer (50 mm, pH 7.4) at 258C. b) NIR emission intensity
per mole of protein for 3 (4 mm) treated with 13 different proteins
(1 mgmLꢀ1) in Tris·HCl buffer (50 mm, pH 7.4) at 258C; intensity monitored
at 975 nm.
Conclusion
be due to binding of the glucose conjugate to the glucose-
binding site of GOx.
We have described the synthesis and photophysical properties
of Yb porpholactones. Through comparative studies with Yb
porphyrins, we found that b-lactonization of porphyrin ligands
enhances the sensitization efficiency of the NIR luminescence
of Yb3+ by narrowing the energy gap between the porphy-
rinoid T1 state and the lowest excited states of Yb3+ and in-
creasing the NIR emission lifetime. Thus, this work has demon-
strated the importance of b-modification of the porphyrin pe-
riphery or tuning the molecular symmetry in increasing the
NIR sensitization efficiency of lanthanides and may aid in the
design of further efficient NIR-emissive lanthanide complexes.
Moreover, by glycosylation of ytterbium complex 3, we dem-
onstrated its potential application as a switchable probe emit-
ting in the NIR region of 900–1100 nm. The emission could be
switched on and off in the sequence “off (in water)!on (with
GOx)!off (with glucose)”, which provides access to further de-
velopment of Ln porphyrinates as NIR-emissive probes in the
region in which biological tissues and fluids are relatively trans-
parent.
To verify this, we added free glucose progressively to the
mixed solution of 3 and GOx in Tris·HCl buffer (50 mm, pH 7.4,
containing 1.25 mm GOx). If the glucose conjugate of 3 occu-
pied the glucose binding site, the competition between 3 and
free glucose would result dissociation of 3 from GOx and thus
quenching of the NIR emission. As shown in Figure 6, we ob-
served decreased NIR emission with increasing amount of glu-
cose. To further demonstrate the specificity for glucose, we in-
vestigated the sugars fructose, galactose, mannose, xylose, lac-
tose, and sucrose in competition with 3 and found that only
addition of glucose efficiently quenched the NIR emission of 3
(Figure 6, inset). Although the recognition of glucose at the
side of the molecule bearing the anomeric carbon atom is well
known,[19] in this work, the GOx-recognition mechanism of the
modified glucose moiety is not clear. In addition, we carried
out transient absorption measurements on 3 in aqueous
medium (Figure S32 in the Supporting Information), on bind-
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Synthesis of Yb-3a: Yb-3a was prepared according to a procedure
similar to that of Yb-1a as a blue-purple solid.ꢀ1M.p. >3008C
(ESI+): m/z calcd for C48H31Cl4KN4O5Yb [M+H2O+K]+: 1098.0045;
Experimental Section
Unless otherwise stated, all reactions were performed under an
inert atmosphere of nitrogen by using standard Schlenk tech-
niques and flame-dried flasks. UV/Vis spectra were recorded on an
Agilent 8453 UV/Vis spectrometer equipped with an Agilent
89090A thermostat (ꢁ0.18C). The emission spectrum and lifetime
were recorded on an Edinburgh Analytical Instruments FLS920 life-
time and steady-state spectrometer (450 W Xe lamp/microsecond
flash lamp, PMT R928 for visible emission spectrum, Ge detector
for NIR emission spectrum, Hamamatsu R5509-73 PMT with C9940-
02 cooler for NIR luminescence lifetime). Nanosecond transient ab-
sorption measurements were performed on an LP-920 laser flash
photolysis setup (Edinburgh) with a computer-controlled Nd:YAG
laser/OPO system from Opotek (Vibrant 355 II). Mass spectra were
recorded on Bruker APEX IV FT-ICR mass spectrometer (ESI-MS) or
Biosystems 4700 Proteomics Analyzer 283 (MALDI-TOF MS). Ele-
mental analyses (C, H, N) were recorded on Elementar Analysensys-
~
(decomp); yield: 41% (21 mg); IR (KBr): n=1772 cm (C=O); HRMS
found:
1098.0110;
elemental
analysis
calcd
(%)
for
C48H29Cl4N4O4Yb+0.5H2O+1.5CH3OH: C 54.16, H 3.31, N 5.10;
found: C 54.21, H 3.09, N 4.86.
Synthesis of Yb-3b: Yb-3b was prepared according to a procedure
similar to that of Yb-1a as red-purple solid. M.p. >3008C
(decomp); yield: 58% (30 mg); HRMS (ESI+): m/z calcd for
C49H33Cl4KN4O3Yb [M+H2O+K]+: 1080.0303; found: 1080.0392; ele-
mental analysis calcd (%) for C49H31Cl4N4O2Yb+2H2O: C 55.59, H
3.33, N 5.29; found: C 55.67, H 3.38, N 5.15.
Synthesis of Yb-4a: Yb-4a was prepared according to a procedure
similar to that of Yb-1a as a blue-purple solid.ꢀ1M.p. >3008C
~
(decomp); yield: 80% (47 mg); IR (KBr): n=1769 cm (C=O); HRMS
(ESI+): m/z calcd for C52H31F12KN4O5Yb [M+H2O+K]+: 1232.1128;
1
found:
1232.1194;
elemental
analysis
calcd
(%)
for
teme GmbH vario EL Elemental Analyzer. H NMR spectra were re-
C52H29F12N4O4Yb+0.8CH3OH+C6H14 (n-hexane): C 54.97, H 3.47, N
4.36; found: C 54.24, H 3.24, N 4.14.
corded on a Varian Mercury Plus 300 MHz spectrophotometer. IR
spectra were recorded on a Bruker VECTOR22 FTIR spectrometer as
KBr pellets. For the optical measurements in liquid solution, spec-
troscopic-grade DMSO and CH2Cl2 were used as purchased from
Alfa-Aesar and anhydrous CH2Cl2 was distilled from calcium hy-
dride. Optical measurements at 77 K were performed in methanol/
ethanol (1/1) glass-forming solvent mixture.
Synthesis of Yb-4b: Yb-4b was prepared according to a procedure
similar to that of Yb-1a as a red-purple solid. M.p. >3008C
(decomp); yield: 85% (49 mg); HRMS (ESI+): m/z calcd for
C53H33F12KN4O3Yb [M+H2O+K]+: 1214.1387; found: 1214.1482; ele-
mental analysis calcd (%) for C53H31F12N4O2Yb+0.8CH3OH+C6H14
(n-hexane): C 56.70, H 3.68, N 4.42; found: C 56.80, H 3.67, N 4.11.
Synthesis of Yb-1a: Porpholactones were prepared according to
our modified procedure.[5f] Yb-1a was prepared according to a pro-
cedure similar to that described in the literature.[3a] 5,10,15,20-Tet-
ra(pentafluorophenyl)porpholactone (1a; 50 mg, 0.05 mmol) and
Yb(acac)3·3H2O (80 mg, 0.15 mmol) were added to a Schlenk tube,
and then 5 mL of 1,2,4-trichlorobenzene (TCB) was added. The mix-
ture was heated at reflux for 8 h. The color of the reaction mixture
changed from red-purple to blue-purple. After cooling to room
temperature, the reaction mixture was transferred to a silica
column. TCB was first eluted with petroleum ether, then uncon-
sumed free-base porpholactone with CH2Cl2, and finally the prod-
uct with CH2Cl2/MeOH (5/1). The product was recrystallized from n-
Synthesis of Yb-5a: Yb-5a was prepared according to the proce-
dure for Yb-1a with modifications. 5,10,15,20-Tetramesitylporpho-
lactone (5a; 40 mg, 0.05 mmol), Yb(acac)3·3H2O (80 mg,
0.15 mmol), and NaH (12 mg, 0.5 mmol) were added to a Schlenk
tube, and then TCB (5 mL) was added. The mixture was heated at
reflux for 12 h. The color of the reaction mixture changed from
red-purple to blue-purple. After cooling to room temperature, the
reaction mixture was transferred to a silica column. TCB was first
eluted with petroleum ether, then unconsumed free-base porpho-
lactone with CH2Cl2, and finally the product with CH2Cl2/MeOH (5/
1). The product was recrystallized from n-hexane/CH2Cl2 as a blue-
hexane/CH2Cl2 as a blue-purple solid. M.p. >3008C (decomp);
purple solid. M.p. >3008C (decomp); yield: 45% (24 mg); IR (KBr):
ꢀ1
yield: 86% (54 mg); IR (KBr): n=1778 cm (C=O); HRMS (ESI+): m/
~
ꢀ1
n=1761 cm (C=O); HRMS (ESI+): m/z calcd for C60H58N4O4Yb
~
z calcd for C48H15F20KN4O5Yb [M+H2O+K]+: 1319.9749; found:
1319.9863; elemental analysis calcd (%) for C48H13F20N4O4Yb+
CH3OH+2H2O: C 44.23, H 1.59, N 4.21; found: C 44.29, H 1.62, N:
4.24.
[M+H]+: 1072.3847; found: 1072.3796; elemental analysis calcd (%)
for C60H57N4O4Yb+H2O+CH3OH+C6H14 (n-hexane): C 66.76, H
6.27, N 4.65; found: C 66.71, H 6.53, N 4.59.
Synthesis of Yb-5b: Yb-5b was prepared according to a procedure
similar to that of Yb-5a as a red-purple solid. M.p. >3008C
(decomp); yield: 58% (31 mg); HRMS (ESI+): m/z calcd for
C61H60N4O2Yb [M+H]+: 1054.4105; found: 1054.4010. elemental
analysis calcd (%) for C61H59N4O2Yb+H2O+CH3OH+C6H12 (n-
hexane): C 68.78, H 6.54, N 4.72; found: C 68.43, H 6.07, N 4.93.
Synthesis of Yb-1b: Yb-1b was prepared according to a procedure
similar to that of Yb-1a as a red-purple solid. M.p. >3008C
(decomp); yield: 78% (49 mg); HRMS (ESI+): m/z calcd for
C50H24F20N4O5Yb [M+CH3OH+2H2O+H]+: 1314.0824; found:
1314.1098; elemental analysis calcd (%) for C49H15F20N4O2Yb+
CH3OH+H2O: C 46.38, H 1.63, N 4.33; found: C 47.07, H 1.83, N:
4.72.
Synthesis of YbTPP(Tp): YbTPP(Tp) was synthesized according to
the literature.[20] HRMS (ESI+): m/z calcd for C53H39BN10Yb [M+H]+:
1000.2841; found: 1000.2843; elemental analysis calcd (%) for
C53H39BN10Yb: C 63.73, H 3.83, N 14.02; found: C 63.54, H 3.87, N
Synthesis of Yb-2a: Yb-2a was prepared according to a procedure
similar to that of Yb-1a as a blue purple solid. M.p. >3008C
ꢀ1
~
(decomp); yield: 33% (15 mg); IR (KBr): n=1759 cm (C=O); HRMS
1
(ESI+): m/z calcd for C48H35KN4O5Yb[M+H2O+K]+: 960.1633; found:
960.1742; elemental analysis calcd (%) for C48H33N4O4Yb +CH3OH+
H2O: C 61.76, H 4.13, N 5.88; found: C 61.76, H 4.12, N 5.59.
13.72. For H NMR spectrum, see Figure S45 in the Supporting In-
formation.
Synthesis of Gd-1a: Gd-1a was prepared according to a procedure
similar to that of Yb-1a as a blue-purple solid.ꢀ1M.p. >3008C
Synthesis of Yb-2b: Yb-2b was prepared according to a procedure
similar to that of Yb-1a as a red purple solid. M.p. >3008C
(decomp); yield: 35% (15 mg); HRMS (ESI+): m/z calcd for
C45H32N4OYb 818.1973: [Mꢀacac+CH3OH]+; found: 818.1923; ele-
mental analysis calcd (%) for C49H35N4O2Yb +2H2O: C 63.91, H 4.27,
N 6.08; found: C 63.69, H 4.17, N 6.18.
~
(decomp); yield: 85% (53 mg), IR (KBr): n=1770 cm (C=O). HRMS
(ESI+): m/z calcd for C48H15F20KN4O5Gd [M+H2O+K]+: 1303.9601;
found:
1303.9626;
elemental
analysis
calcd
(%)
for
C48H13F20GdN4O4 +2.5CH3OH: C 45.71, H 1.75, N 4.22; found: C
46.22, H 1.82, N 4.20.
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pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Synthesis of Gd-1b: Gd-1b was prepared according to a procedure
similar to that of Yb-1a as a red-purple solid. M.p. >3008C
(decomp); yield: 82% (50 mg); HRMS (ESI+): m/z calcd for
C49H17F20KN4O3Gd [M+H2O+K]+: 1285.9859; found: 1285.9901; ele-
mental analysis calcd (%) for C49H15F20GdN4O2 +3CH3OH: C 47.14, H
2.05, N 4.23; found: C 49.83, H 2.10, N 4.81.
u ¼ D2r þ Ds2, where Dr is the relative uncertainty of Gr and Ds
the relative uncertainty of Gs. For example, the relative uncertainty
for the quantum-yield determination of Yb-1a can be calculated as
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
0:082 þ 0:052 ¼ 9%. The relative uncertainties of other Yb com-
plexes were obtained by the same method and are listed in
Table S1 of the Supporting Information. The absorbances of all
samples and references were less than 0.1 and were corrected for
the background subtracting the average value over the range of
800–820 nm. The integrated emission intensity from 900 to
1100 nm was corrected by subtracting the blank value (integrated
emission intensity of pure CH2Cl2 under identical conditions).
Synthesis of 3: Complex 3 was prepared according to a procedure
similar to that described in the literature.[21] Yb-1a (33.0 mg,
25 mmol) and 2,3,4,6-tetra-O-acetylglucosyl thioacetate (87.0 mg,
200 mmol) were dissolved in 10 mL of DMF and 1 mL of diethyl-
amine. The solution was stirred for 8 h at room temperature. The
solvent was evaporated under reduced pressure and the product
purified by column chromatography with CH2Cl2/MeOH (16/1) as
eluent to give a deep purple product. The product was dissolved
in CH2Cl2/MeOH (1/1) without characterization and treated with
16 equivalents of CH3ONa at room temperature for 4 h. The mix-
ture was filtered and the product purified by column chromatogra-
phy on silica to give theꢀp1 roduct as a blue purple solid. Yield:
Water titration experiment: An 80 mm stock solution of 3 in
DMSO was prepared, and then a series of solutions with different
concentrations of H2O (0, 0.278, 0.566, 0.833, 1.39, 1.94, 2.78, 5.56,
8.33, 13.9, 19.4, 27.8m) was prepared in a 2 mL volumetric flask.
Then, 10 mL of the stock solution of 3 was added to the volumetric
flask, and the final concentration of 3 was 4 mm. NIR emissions
were recorded on an Edinburgh Analytical Instruments FLS920 life-
time and steady-state spectrometer with a liquid-N2-cooled Ge de-
tector at an excitation wavelength of 425 nm under identical con-
ditions at 258C.
40%; IR (KBr): n=1760 cm (C=O); MS (MALDI+, DMF): m/z calcd
~
for C75H64F16KN5O25S4Yb [M+DMF+K]+: 2079.2; found: 2079.1.
Transient absorption: Excitation at 420 nm with a power of 2.0 mJ
per pulsefrom a computer-controlled Nd:YAG laser/OPO system
operating at 10 Hz was directed to the sample with an optical ab-
sorbance of 0.6 at the excitation wavelength. The laser and analyz-
ing light beam passed perpendicularly through a 1 cm quartz cell.
The complete time-resolved spectra were obtained by using
a gated CCD camera (Andor iSTAR); the kinetic traces were detect-
ed by a Tektronix TDS 3012B oscilloscope and a R928P photomulti-
plier and analyzed by Edinburgh analytical software (LP920).
Glucose oxidase (GOx) titration experiment: A series of solutions
with different concentrations of GOx (0, 0.0781, 0.156, 0.234, 0.312,
0.469, 0.625, 0.781, 0.938 1.25, 1.56, 1.87 mm) was prepared by
using stock solutions of 3 (80 mm in DMSO) and GOx (1 mgmLꢀ1
in 50 mm of pH 7.4 Tris·HCl buffer), and the concentration of 3 was
4 mm in each solution. NIR emissions were recorded on an Edin-
burgh Analytical Instruments FLS920 lifetime and steady-state
spectrometer with a liquid-N2-cooled Ge detector at an excitation
wavelength of 425 nm under identical conditions at 258C. The
emission intensity monitored at 975 nm was corrected by subtract-
ing the blank value (emission intensity at 975 nm with only pure
Tris·HCl buffer under an identical condition).
Determination of quantum yields: Quantum yields in solution
were determined by comparative method[12,14] and the equation:
ꢁ
ꢀ
ꢂ
Fs=Fr ¼ ðGs=GrÞ h2s hr2 , where the subscripts r and s denote ref-
erence and sample, respectively, F is the quantum yield, G the
slope of the plot of integrated emission intensity versus absorb-
ance, and h the refractive index of the solvent. The reference was
YbTPP(Tp) in CH2Cl2 (F=0.032,[3c,12] lex =425 nm). Because of the
low absorption of Yb-1a and Yb-1b at 425 nm, we also performed
measurements at 420 nm. YbTPP(Tp) solutions with five different
concentrations were first prepared in anhydrous CH2Cl2, the ab-
sorbance of the solutions at 420 and 425 nm were recorded on an
Agilent 8453 UV/Vis spectrometer equipped with an Agilent
89090A thermostat (ꢁ0.18C), and NIR emissions were recorded on
an Edinburgh Analytical Instruments FLS920 lifetime and steady-
state spectrometer with a liquid-N2-cooled Ge detector with excita-
Glucose titration experiment:
A stock solution of glucose
(400 mm) and 2 mL of a solution containing 3 (4 mm) and glucose
oxidase (1.25 mm) in Tris·HCl buffer (50 mm, pH 7.4) were prepared.
Then the amount of glucose added to the 2 mL solution was cu-
mulatively increased (0, 1, 2, 3, 7, 10 mm). The volume of total
extra solvent was less than 50 mL to keep the total volume nearly
unchanged. Sufficient mixing was performed after each addition of
glucose. NIR emissions were recorded on an Edinburgh Analytical
Instruments FLS920 lifetime and steady-state spectrometer with
a liquid-N2-cooled Ge detector with excitation wavelength at
425 nm under an identical condition at 258C. The emission intensi-
ty monitored at 975 nm was corrected by subtracting the blank
value (emission intensity at 975 nm of pure Tris·HCl buffer under
identical conditions).
tion wavelength of 420 and 425 nm. According to the ratio of the
ꢀ
slope Gl
Gl
(see Figure S1 in the Supporting Infor-
¼420 nm
¼425 nm
ex
ex
mation), the quantum yield of YbTPP(Tp) can be reasonably adjust-
ed to 0.043 (lex =420 nm), which was used as a reference value for
determination of quantum yields of Yb-1a–5b. For the quantum-
yield determination of Yb-1a and Yb-1b, solutions of Yb-1a, Yb-
1b, and YbTPP(Tp) with three different concentrations were pre-
pared in anhydrous CH2Cl2. The absorbance of all the solutions at
420 nm were recorded on an Agilent 8453 UV/Vis spectrometer
equipped with an Agilent 89090A thermostat (ꢁ0.18C), and NIR
emissions were recorded on an Edinburgh Analytical Instruments
FLS920 lifetime and steady-state spectrometer with a liquid-N2-
cooled Ge detector at an excitation wavelength at 420 nm under
an identical condition. According to the ratio of the slope GYb-1a/Gr
and GYb-1b/Gr (see Figure S2 in the Supporting Information), the rel-
ative quantum yields of Yb-1a and Yb-1b could be obtained ac-
cording to the above equation. Other Yb porpholactones (Yb-2a–
5a) and Yb porphyrins (Yb-2b–5b) were handled in the same way.
The relative uncertainty of the determination can be calculated as
Acknowledgements
Support from NSFC (grant no. 20971007, 21271013 to J.-L.Z.) is
gratefully acknowledged.
Keywords: biosensors
·
lanthanides
·
luminescence
·
porphyrinoids · ytterbium
[1] a) M. Pawlicki and L. Latos-Grazynski, in Handbook of Porphyrin Science
Vol. 2 (Eds: K. M. Kadish, K. M. Smith and R. Guilard), World Scientific,
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Received: October 10, 2013
Revised: December 20, 2013
Published online on March 3, 2014
Chem. Eur. J. 2014, 20, 4324 – 4333
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