Efficient luminescence from fluorene- and spirobifluorene-based lanthanide complexes upon near-visible irradiation

Article abstract of DOI:10.1002/chem.201305029

We describe herein the synthesis and photophysical characterization of new lanthanide complexes that consist of a (9,9-dimethylfluoren-2-yl)-2-oxoethyl or a (9,9-spirobifluoren-2-yl)-2-oxoethyl unit as the antenna, covalently linked to a 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A) unit as the Ln³⁺ (Gd³⁺, Eu³⁺, Sm³⁺, Tb ³⁺, Dy³⁺) coordination site. We were able to translate the spectroscopic properties of the innovative bipartite ligands into the formation of highly luminescent europium complexes that exhibit efficient emission (φ_se>0.1) upon sensitization in the near-visible region, that is, with an excitation wavelength above 350 nm. The luminescence of the Eu ³⁺complexes is clearly detectable at concentrations as low as 10 pM. Furthermore, the structural organization of these bipartite ligands makes the complexes highly soluble in aqueous solutions and chemically stable over time. Lighting up: Innovative fluorene- and spirobifluorene-based lanthanide complexes show efficient luminescence upon sensitization in the near-visible region. Lanthanide emission is detectable at concentration as low as 10 pM in aqueous solution (see figure).

Full text of DOI:10.1002/chem.201305029

DOI: 10.1002/chem.201305029
Full Paper
&
Luminescence
Efficient Luminescence from Fluorene- and Spirobifluorene-Based
Lanthanide Complexes upon Near-Visible Irradiation
Gregorio Bottaro,[a] Fabio Rizzo,[b, c] Marco Cavazzini,[b] Lidia Armelao,*[a] and Silvio Quici*[b, c]
Abstract: We describe herein the synthesis and photophysi-
cal characterization of new lanthanide complexes that con-
sist of a (9,9-dimethylfluoren-2-yl)-2-oxoethyl or a (9,9’-spiro-
bifluoren-2-yl)-2-oxoethyl unit as the antenna, covalently
linked to a 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid
(DO3A) unit as the Ln3+ (Gd3+, Eu3+, Sm3+, Tb3+, Dy3+) co-
ordination site. We were able to translate the spectroscopic
properties of the innovative bipartite ligands into the forma-
tion of highly luminescent europium complexes that exhibit
efficient emission (fse >0.1) upon sensitization in the near-
visible region, that is, with an excitation wavelength above
350 nm. The luminescence of the Eu3+complexes is clearly
detectable at concentrations as low as 10 pm. Furthermore,
the structural organization of these bipartite ligands makes
the complexes highly soluble in aqueous solutions and
chemically stable over time.
Introduction
Over the past years, we have synthesized a variety of lantha-
nide complexes[5–9] (mainly Eu3+ and Tb3+) with different cyclic
and acyclic polydendate polyaminopolycarboxylates coordina-
tion sites, by following the so-called pendant chromophore
strategy.[1,7] This strategy implies the use of conceptually
simple two-component ligands that consist of a single light-
harvesting unit flexibly connected to a multidentate ligand
through a methylene bridge.
The use of lanthanide complexes as luminescent probes in bio-
chemical and biomedical applications represents a cutting-
edge research field in which a steady increase in activity has
been observed.[1,2] Ideal candidates for practical applications in
this field are represented by complexes that bear sensitizing
antennae endowed with high solubility in water over a physio-
logically relevant pH range (typically 4–8) and excellent chemi-
cal stability over time and at sub-nanomolar concentrations.[1–3]
The preparation of highly luminescent complexes that bear
suitable coordination sites and sensitization units is a mandato-
ry requisite to fully exploit the unique emission properties of
lanthanide ions, that is, large Stokes shifts, narrow emission
bands characteristics for each element of the rare-earth family,
and long lifetimes (microsecond–millisecond range). Whereas
enhanced measurement sensitivity can be easily obtained by
exploiting the long-lived emission of the lanthanide elements
for time-resolved homogeneous immunoassays,[1,3,4] bright lu-
minescence and excellent stability over a wide concentration
range can be achieved only by properly designed complexes.
Phenanthroline or acetophenone derivatives were used as
efficient antenna systems owing to their high molar absorption
coefficients (e), high intersystem crossing efficiency, and favora-
ble energy separation (ꢀ2000 cmꢁ1) between their excited
triplet states and the Ln3+ emitting levels.[10,11] The complexes
showed high solubility and stability in aqueous medium, and
high overall photoluminescence quantum yields (up to 0.45) in
aqueous solution and in silica thin films upon UV-light excita-
tion.
Although high efficiency has been attained, the possibility
to shift the wavelength of excitation from medium UV towards
the visible region by using chromophores that absorb light
above 350 nm should result in beneficial practical implications.
In fact, especially in immunoassay applications, this should
facilitate the use of easily available excitation sources and
avoid the use of expensive quartz optics, limit the degradation
of biomolecules, which is severe upon UV irradiation, and
reduce spectral interference (below 320 nm) owing to the
auto-fluorescence of tissues.[12–14] In this regard, efficient euro-
pium and terbium emission at long excitation wavelength has
been previously reported using N-alkyl acridones, azathiaxan-
thones, and hydroxyisophthalamides as chromophores.[4,15–18]
We are presenting in this study two series of innovative lu-
minescent lanthanide antenna complexes in which the light-
harvesting unit is bonded to the 1,4,7,10-tetraazacyclodode-
cane-1,4,7-triacetic acid (DO3A),[5,19] a macrocyclic multidentate
ligand that acts as a highly effective hosting unit for the Ln3+
[a] Dr. G. Bottaro, Dr. L. Armelao
IENI-CNR and INSTM, Dipartimento di Scienze Chimiche
Universitꢀ di Padova, Via Marzolo 1, 35131 Padova (Italy)
E-mail: lidia.armelao@unipd.it
[b] Dr. F. Rizzo, Dr. M. Cavazzini, Dr. S. Quici
ISTM-CNR, Via Golgi 19, 20133 Milano (Italy)
E-mail: silvio.quici@istm.cnr.it
[c] Dr. F. Rizzo, Dr. S. Quici
ISTM-CNR and Polo Scientifico Tecnologico
Via Fantoli 15/16, 20138 Milano (Italy)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201305029. It includes detailed synthetic
procedures of all new compounds, H and 13C NMR spectra of all new com-
pounds, and spectroscopic characterization of ligands and complexes at
different T and concentration.
1
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cations. 9,9-Dimethylfluorene and 9,9’-spirobifluorene were se-
lected as the chromophores and linked to the DO3A unit
through a short and flexible spacer that contained a ketone
group that allows the binding on lanthanide ions when com-
plexes are formed (Figure 1).[8,19]
the solid state or in solution of organic solvents (DMSO or
CH2Cl2). At variance, the complexes here reported show 1) re-
markable thermodynamic stability and kinetic inertness owing
to the employment of a preorganized polydentate ligand,
2) an effective antenna-to-lanthanide energy transfer thanks to
the favorable energy matching between the chromophore trip-
let states and lanthanide emitting levels, and, 3) most impor-
tantly, efficient luminescence at excitation wavelength higher
than 350 nm and at concentrations as low as the picomolar
level thanks to the peculiar spectroscopic properties of the
sensitizers.
Results and Discussion
Synthesis of the ligands and complexes
Figure 1. Structure of Ln3+ꢂ 1 and Ln3+ꢂ 2 complexes (Ln=Eu, Tb, Sm, Gd,
Dy).
We synthesized bipartite ligands 1 and 2 (see Schemes 1 and
2) that feature a (9,9-dimethylfluoren-2-yl)-2-oxoethyl and
a (9,9’-spirobifluoren-2-yl)-2-oxoethyl unit as the antenna, re-
Fluorene and 9,9’-spirobifluor-
ene derivatives with tailor-made
optical and redox properties are
being actively investigated as or-
ganic light-emitting materials for
optoelectronics and in electro-
generated chemiluminescence
studies.[20–22]
There is a great interest that
surrounds spirobifluorene and
fluorene since they can be
chemically modified to tune the
spectroscopic and electrochemi-
cal properties both in solution
and in the solid state. By merg-
ing the unique properties of lan-
thanides and the versatility of
Scheme 1. Synthesis of ligand 1 and the corresponding Ln3+ꢂ 1 complexes.
fluorene and spirobifluorene
molecules, a family of innovative
complexes, supramolecular ar-
chitectures, and functional materials can be developed. These
aspects are particularly important to rationalize the chemical
design and the preparation of highly efficient light-emitting
systems and materials.
spectively, which are covalently linked to a 1,4,7,10-tetraazacy-
clododecane-1,4,7-triacetic acid (DO3A) unit as the Ln3+ coor-
dination site. The choice of these chromophores as antennae
was mainly on account of their triplet energy levels, which pro-
mote sensitization of lanthanide luminescence in the range of
wavelengths between 350 and 420 nm. Furthermore, similarly
to acetophenone[5] they coordinate the lanthanide cation
through the carbonyl group, thus ensuring efficient energy-
transfer processes from the antenna to the complexed metal.
DO3A was selected as the Ln-chelating subunit because of its
high thermodynamic and kinetic stabilities, which have been
thoroughly assessed for its lanthanide complexes.[26]
Ligand 1 and the corresponding complexes Ln3+ꢂ 1 were
synthesized according to Scheme 1. 9,9-Dimethylfluorene (3)
and 2-acetyl-9,9-dimethylfluorene (4) were prepared according
to literature procedures.[27] Treatment of 4 with N-bromosucci-
nimide (NBS) and p-toluensulfonic acid (TsOH) in CH3CN under
reflux conditions for 2 h produced 2-bromo-1-(9,9-dimethyl-
To the best of our knowledge, very few studies have ap-
peared in the literature that report the use of fluorene and spi-
robifluorene derivatives as sensitizers for lanthanide emission.
Moreover, the ligand-sensitizer system of the literature com-
plexes is substantially different from the one described herein.
In some cases, the coordination of the metal center is achieved
by the assembly of different units[23,24] around the Ln3+ rather
than by a preorganized polydentate unit. Only in one case has
a preformed and organized ligand been adopted, but the large
distance between the antenna and the Eu3+ site, and the lack
of short-range interaction between the donor and the acceptor
result in low photoluminescence quantum yields (0.08 in
CH2Cl2).[25] None of these studies reports the use of water as
solvent, and the luminescence data refer to the compounds in
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lectronic applications. They can
be chemically modified with
a large variety of donor and/or
acceptor groups to tune their
electronic and optical properties.
The high versatility of the
chemistry of these molecules
was recently exploited to synthe-
size molecules that bear fluorene
and spirobifluorene cores that
show nonlinear optical or high
emissive properties and electro-
chemiluminescent behavior.[20–22]
Spirobifluorene is characterized
by two fluorene units connected
through the spiro carbon atom.
This particular spatial configura-
Scheme 2. Synthesis of ligand 2 and the corresponding Ln3+ꢂ 2 complexes.
tion induces strong rigidity and
increases the stability of the
fluoren-2-yl)ethanone (5)[28] in 84% yield after purification by
silica gel column chromatography.
system, thereby hampering the oxidation process that usually
occurs at position 9 of the fluorene ring.[20] For this reason, the
fluorene derivative used as a sensitizer for lanthanide emission
in this study has been substituted with two methyl groups in
position 9.
Alkylation of DO3A tris(tert-butyl) ester hydrobromide 6[29,30]
with 5 was carried out in CH3CN under reflux conditions in the
presence of Na2CO3 as base to afford sodium bromide complex
7 in quantitative yield after column chromatography. Indeed
the complexing ability of alkylated DO3A triesters to coordi-
nate sodium cation has been already assessed,[9,31] and the
presence of the sodium bromide complex simplifies the chro-
matographic purification and allows higher yields of isolated
product. Decomplexation of 7 takes place during deprotection
of the tert-butyl groups, which was achieved by treatment of 7
with CF3COOH at room temperature and afforded the tricar-
boxyl derivative 1 as white solid in 85% yield after purification
through a short Amberlite XAD 1600T column eluting with
pure H2O and H2O/CH3CN (1:1 v/v). The Eu3+ꢂ 1 complex was
prepared by the addition of a solution of EuCl3·6H2O in H2O
into a water solution of the ligand 1 under magnetic stirring
and adjusting the pH at 6.5–7.0 with 2% aqueous NaOH. The
reaction mixture was then heated at reflux for 4 h. Purification
of the residue by a short Amberlite XAD 1600T column eluting
first with pure H2O to remove all inorganic salts and then with
H2O/CH3CN (1:1 v/v) afforded the Eu3+ꢂ 1 in 89% yield as
a white crystalline solid. All the lanthanide complexes—
namely, Tb3+ꢂ 1, Sm3+ꢂ 1, Dy3+ꢂ 1, and Gd3+ꢂ 1—were pre-
Absorption of ligands 1 and 2 and of the corresponding
Eu3+ꢂ 1 and Eu3+ꢂ 2 complexes in water is reported in Fig-
ure 2a. The spectra of both ligand 1 and Eu3+ꢂ 1 are dominat-
ed by a strong single band in the UV region, which shows
a bathochromic shift of approximately 20 nm in the complex.
A very different band profile is observed for ligand 2 and for
the corresponding Eu3+ꢂ 2 derivative. In fact, we observe
a redshift of approximately 25 nm in the onset of the absorp-
tion bands and a new absorption maximum peak at around
325 nm. Spirobifluorene is characterized by two fluorene units
connected through the spiro carbon atom. The presence of
the spiro bridge is expected to only marginally affect the mo-
lecular properties. In fact spiro conjugation is weak relative to
p conjugation because of the 908 twisted orientation of the
molecular fragments involved in the spiro bond, which mini-
mizes mutual electrostatic interactions.[22,34] Owing to this par-
ticular conjugation, the presence of Ln3+ ions represents
a much stronger perturbation for the energy levels of the fluo-
rene moiety to which the DO3A unit is bonded, thus affecting
its absorption properties and leaving the other half of the mol-
ecule only slightly perturbed. To assign the absorption peaks,
a solvatochromism experiment was performed in a series of
solvents characterized by different dielectric constant and hy-
drogen-bonding characteristics (see the Kamlet–Taft parame-
ters in Figure 2).
pared by following a procedure similar to that used for Eu3+
ꢂ
1. Ligand 2 and the corresponding complexes Ln3+ꢂ 2 were
synthesized according to Scheme 2 by following synthetic pro-
cedures similar to those described above (Scheme 1) starting
from 2-bromo-1-(9,9’-spirobifluoren-2-yl)ethanone (9). 2-Acetyl-
9,9’-spirobifluorene (8) was prepared from the commercially
available 9,9’-spirobifluorene by a modified literature proce-
dure.[32,33]
The absorption spectra of spirobifluorene, reported in Fig-
ure 2b, show two narrow peaks, the shape and position of
which are very slightly affected by the nature of the solvent.
The same behavior is observed for fluorene (Figure S17 in
the Supporting Information). From these results, we can attri-
bute the narrow peaks of Ln3+ꢂ 2 complexes (Figure 2c) to
transitions localized on the bare fluorene part. In addition, we
observe (Figure 2c) a series of transitions related to the spirobi-
Absorption properties of bipartite ligands 1 and 2
Fluorene, 9,9’-spirobifluorene, and their derivatives are widely
employed as strong blue or green emitters for organic optoe-
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by the presence of
a single
broad band, which is only slight-
ly different to the position of the
maximum, in analogy to the be-
havior previously evidenced in
the absorption spectra shown in
Figure 2a. To get a deeper in-
sight into the nature of the in-
volved electronic transitions, the
phosphorescence spectra of
Gd3+ꢂ 1 and Gd3+ꢂ 2 were re-
corded at low temperature
(77 K). The emission spectra (Fig-
ure 3a) present a more complex
profile, with a number of bands
and their relative intensities
strongly dependent on the exci-
tation wavelength.
From the spectra reported in
Figure 3b, it clearly appears that
in the Gd3+ꢂ 1 complex there
are two distinct triplet states,
the energy values of which are
T1(1)=22990 cmꢁ1 (435 nm) and
T2(1)=20408 cmꢁ1
(490 nm).
Figure 2. Absorption spectra of a) bipartite ligands 1 and 2, and of the corresponding Eu3+ꢂ 1 and Eu3+ꢂ 2 com-
plexes in aqueous solution (10ꢁ5 m); b) spirobifluorene in different solvents, and c) Ln3+ꢂ 2 complexes in different
solvents. The dielectric constant values and the Kamlet–Taft parameters[35] are reported for the selected solvents.
Upon excitation into the high-
energy electronic transitions
(280 nm), both T1(1) and T2(1)
can be populated, whereas at
fluorene part linked to the DO3A ligand, the profile of which is
heavily influenced by the nature of the medium. The arrows in
Figure 2c indicate a set of transitions that could be related to
the carbonyl moiety of the Ln3+ꢂ 2 complex since they are in-
fluenced by the solvent proticity and hydrogen-bonding ca-
pacity. However, the bands at high wavelengths show the typi-
cal behavior of p!p* transitions that result in a redshift upon
an increase in the solvent dielectric constant.[36–39]
lower excitation energy (340 nm), emission only from the latter
is observed. All the other bands present in Figure 3a have
been assigned to singlet levels since their intensity vanishes if
emission is recorded over a time window delayed by 50 ms
after excitation. In Figure 3, we can also observe a vibronic
progression with energy of 1450 cmꢁ1. To obtain a complete
knowledge of the emission properties of Gd3+ꢂ 1, we estimat-
ed the phosphorescence lifetimes of the two triplet levels. A
difference higher than one order of magnitude was found be-
tween the t value (680 ms) of the high-energy triplet state
(T1(1): lexc =280 nm, lem =435 nm) and the one (15 ms) of the
low-energy triplet state (T2(1): lexc =348 nm; lem =490 nm).
As a general rule, the absorption spectra are generated from
n,p* and p,p* transitions, and the phosphorescence spectra
As previously highlighted, chromophores that absorb light
above 350 nm are highly sought after and represent a chal-
lenge for the design of lanthanide complexes for biological
and biomedical applications. From the spectra reported in Fig-
ure 2a, it is evident that the bipartite ligands 1 and 2 have
transitions well beyond this threshold value. A further necessa-
ry requisite to this end is the favorable energy gap between
the excited triplet states of the chromophores and the Ln3+
-emitting levels. The energies of the triplet states of ligands
1 and 2 involved in the energy-transfer events that promote
the sensitized lanthanide emission[10,11,40] have been experi-
mentally evaluated by measuring the phosphorescence of the
related Gd3+ complexes, as reported below.
3
3
are due to (n,p*) and/or (p,p*) triplet levels. In aryl ketones
that bear strong donating groups, charge-transfer bands from
the substituent to the carbonyl group are also possible.
Owing to the difference in the involved initial and final
states (i.e., 3(n,p*) and/or 3(p,p*)), the nature of the lowest-
lying levels is strongly dependent on the solvent (e.g., polarity,
hydrogen-bonding capability).[41–45] Despite the different solva-
tochromic behavior, the two kinds of triplet levels can be dis-
criminated through their phosphorescence lifetimes being, at
Phosphorescence study of Gd3+ꢂ 1 and Gd3+ꢂ 2 complexes
3
77 K, in the millisecond range for (n,p*) and 10–1000 times
At room temperature, the emission spectra of ligand 1 and
Gd3+ꢂ 1 (Figure S18b in the Supporting Information) excited
at the corresponding absorption maximum (Figure S18a in the
Supporting Information) are quite similar and are characterized
longer for 3(p,p*).[44] On this basis, the low-energy triplet of
Gd3+ꢂ 1 is attributed to the n,p* type that involves the car-
bonyl group directly bonded to the metal ion. Therefore, if this
level is energetically suitable to sensitize lanthanide emission,
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visible region, thereby disclosing
the possibility of sensitizing lan-
thanide emission upon excita-
tion at longer wavelengths. Fi-
nally, the presence of the spiro-
bridge does not seem to have
any influence on the spectro-
scopic properties of both ligand
2 and the corresponding Gd3+
2 complex.
ꢂ
Spectroscopy of Eu3+ꢂ 1 and
Eu3+ꢂ 2 complexes
The excitation and emission
spectra of Eu3+ꢂ 1 are reported
in Figure 4. Interestingly, the ex-
citation and absorption spectra
are completely overlapped; this
allows sensitization of europium
luminescence over a large spec-
tral range. In Eu3+ꢂ 1, fluorene
can act as antenna through both
of its triplet levels (see Figure 3),
but the higher emission intensity
is reached by exploiting the
lower energy one (T2(1)), the ex-
citation maximum of which is at
Figure 3. Emission spectra at 77 K excited at different wavelengths for 10ꢁ5 m solutions of a) Gd3+ꢂ 1 fluorescence
(steady state), b) phosphorescence spectra (50 ms delay after pulse), c) Gd3+ꢂ 2 fluorescence (steady state), and
d) phosphorescence spectra (50 ms delay after pulse). For all solutions, a methanol/ethanol mixture (1:1 v/v) has
been used as the solvent. The energy values of the triplet levels for spirobifluorene and fluorene derivatives are
listed in Table 1.
approximately
350 nm
(Fig-
ure S19 in the Supporting Infor-
mation). Quite remarkably, euro-
the excitation wavelength can be pushed well beyond 350 nm
as demonstrated by the complex excitation spectra (Figure S19
in the Supporting Information).
pium emission can be achieved upon excitation well over
350 nm, up to the boundary with the visible region (Figure 4c).
Between 550 and 750 nm the spectrum is dominated by emis-
sion from the Eu3+centers, which is composed of several
By taking into account the energy match condition for the
involved levels, the T2(1) triplet (20408 cmꢁ1) should be able to
sensitize the emission of Eu3+ (5D0, 17250 cmꢁ1) and Sm3+ (4G5/
5
7
bands associated with D0! FJ transitions (J=0, 1, 2, 3, and 4);
the higher the J value, the lower the transition energy (Fig-
17800 cmꢁ1), the emitting states of which lie more than
ure 4b).[46] We observed an abnormal intensity of the D0! F4
transition that has been reported in literature only in a few
cases.[47,48] Ferreira et al. have theoretically interpreted this be-
havior in terms of the Wl intensity parameters and their de-
pendence on the nature and local symmetry of the chemical
environment around the Eu3+ ion, thus leading to the conclu-
sion that the chemical environment is highly polarizable with
a local symmetry that corresponds to a slightly distorted D4d
coordination geometry.[47] We can hypothesize a similar explan-
ation in our case as well, although the structure of the com-
plexes has not been yet determined owing to the difficulty in
obtaining good crystals.
5
7
2,
2000 cmꢁ1 lower in energy, but not the emission of Tb3+ (5D4,
20430 cmꢁ1) and Dy3+ (4F9/2, 20960 cmꢁ1).[11,40] At variance, it is
worth noting that the energy of T1(1) (22990 cmꢁ1) is suitable
for the sensitization of all lanthanide ions emitting in the visi-
ble region. The behavior of Gd3+ꢂ 2 is very close to the one
observed for Gd3+ꢂ 1; the spectral features have been found
to be dependent on the excitation wavelength, and the phos-
phorescence spectra (50 ms delay time) revealed the presence
of two distinct triplet levels with energy T1(2)=22727 cmꢁ1
(440 nm) and T2(2)=20000 cmꢁ1 (500 nm). The corresponding
lifetimes are comparable to those determined for Gd3+ꢂ 1,
their value being t(T1(2))=638 ms (lexc =280 nm; lem
=
The sensitized lanthanide emission is clearly perceived upon
tuning the excitation wavelength up to 400 nm, at which point
the UV region fades into the visible band (Figure 4c, inset). De-
spite the expected overall reduced intensity owing to the
lower absorbance value of the antenna, the spectrum displays
all europium components with comparable resolution to that
observed upon excitation at 345 nm, that is, into the maximum
of the excitation band. As previously emphasized, the optimal
440 nm) and t(T2(2))=12 ms (lexc =350 nm; lem =500 nm). Ac-
cordingly, the triplet transitions have been assigned as for the
Gd3+ꢂ 1 complex. As it can be observed, the energy of the
Gd3+ꢂ 2 triplet levels is slightly lower than the corresponding
transitions for Gd3+ꢂ 1. Moreover, the absorption and excita-
tion spectra (see Figure S20 in the Supporting Information) of
the spirobifluorene derivative are more extended toward the
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sensitized emission to occur up
to excitation wavelengths of
420 nm, that is, 20 nm longer
relative to Eu3+ꢂ 1 (Figure 5b,c).
Similarly to Eu3+ꢂ 1, Eu3+ꢂ 2
shows chemical stability in
highly diluted aqueous solutions
(ꢀ10 pm); emission spectra of
Eu3+ꢂ 2 at decreasing concen-
trations are shown in Figure S22
of the Supporting Information.
By surveying the light re-
sponse from diluted aqueous
solutions (1 nm) of both europi-
um complexes over time, we no-
ticed a steady behavior upon
aging under ambient conditions.
The emission spectra measured
at regular intervals up to several
months (Figure S23 in the Sup-
porting Information) indicate
a very high stability even at sub-
nanomolar concentrations and
an excellent reproducibility of
color chromaticity.
Figure 4. a) Absorption and excitation spectra monitored at lem =615 nm, and b) emission spectrum of Eu3+
ꢂ
1 (10ꢁ5 m water solution) excited at 345 nm. The details of the spectrum are reported in the inset. c) Emission
spectrum of Eu3+ꢂ 1 (10ꢁ5 m water solution) excited at 400 nm. The water Raman band is highlighted with an as-
terisk in the spectrum. In the inset are reported the high-resolution spectra at two different excitation wave-
lengths: 345 (red curve) and 400 nm (black curve). d) Emission from a Eu3+ꢂ 1 water solution (10ꢁ5 m) in a poly-
methylmethacrylate (PMMA) cuvette illuminated at 380 nm with a Xe lamp.
The photophysical data for
Eu3+ꢂ 1 and Eu3+ꢂ 2 com-
plexes are summarized in
Table 1. Both Eu3+ꢂ 1 and Eu3+
ꢂ 2 show absolute quantum
spectroscopic compatibility of such complexes with low-cost
Table 1. Photophysical data of europium complexes.
polymeric optical materials represents a substantial advantage
for practical applications. As highlighted in Figure 4d, a bright
Eu3+ꢂ 1
Eu3+ꢂ 2
345 (325)
17152 (19792)
22727
20000
0.598
2.134
0.14[a] (0.12)[b]
0.38[a]
red emission is clearly visible with the naked eye from Eu3+
ꢂ
1 aqueous solutions (10ꢁ5 m) in a polymethylmethacrylate
(PMMA) cuvette excited at l=380 nm. A further major qualifi-
cation for fluorescent probes to be used in biochemical and
biomedical applications concerns their chemical stability in the
very low concentration range. The luminescence performances
of Eu3+ꢂ 1 fulfill this requirement as its emission is still detect-
able at concentrations as low as 10 pm (Figure S21 in the Sup-
porting Information). Although the presence of free Eu3+ ions
cannot be completely ruled out, the observed emissions are
unambiguously attributed to the Eu3+ꢂ 1 complex. Indeed,
this is the only species capable of emitting light at 615 nm
upon excitation at 345 nm. For the spirobifluorene derivative
Eu3+ꢂ 2, the whole absorption band (Figure 5a) can be ex-
ploited for the sensitization of europium emission thanks to
the suitability of all transitions to populate the low-energy trip-
let level T2(2) at 20000 cmꢁ1.
lmax [nm]
345
e [mꢁ1 cmꢁ1
]
20312
22990
20408
0.605
T1 [cmꢁ1
T2 [cmꢁ1
]
]
tH O [ms]
2
tD O [ms]
2.097
2
f
se in H2O
se in D2O
q[c]
0.12[a] (0.12)[b]
0.36[a]
f
1ꢃ0.5
1ꢃ0.5
[a] Quantum yield values obtained by using the integrating sphere (lexc
345 nm). [b] Quantum yield values obtained by using [Ru(bpy)3]Cl2 as
standard (lexc =345 nm). [c] Number of water molecules in the first coor-
=
dination shell estimated by the Horrocks formula: qEu =1.11(1/tH O ꢁ1/
2
tD Oꢁ0.31).
2
yields higher than 10%, and the excited-state dynamics for the
7
5D0! F2 transition are characterized by a single exponential
decay (Figure S24 in the Supporting Information).
The number of J multiplets, their position, and their relative
intensity are very similar in the Eu3+ꢂ 1 and Eu3+ꢂ 2 com-
plexes, clearly indicating an equivalent local coordination ge-
ometry for the Eu3+ sites[10,11,49] despite the presence of a bulki-
er chromophore in Eu3+ꢂ 2. It is worth highlighting that in
Eu3+ꢂ 2 the presence of the spirobifluorene unit allows the
The remarkable increase of the photoluminescence yields
and the decay lifetimes determined in D2O for both complexes
indicate the presence of water in the first coordination sphere
of the metal. On the basis of the Horrocks equation,[50–53] we
estimated the presence of one coordinated solvent molecule
at each europium center (Table 1).
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Since the stability of the luminescent systems under contin-
uous irradiation is a key factor for most applications such as
sensing, bio-probing, lighting, and so on, and a high resistance
towards photoinduced damages is required to improve the
service life of molecules, we evaluated the photostability of
the solid Eu3+ꢂ 1 and Eu3+ꢂ 2 complexes under continuous
irradiation at l=345 nm. The integrated intensity was moni-
tored as a function of the exposure time (Figure 6). The emis-
sion intensity decreases to approximately 50% of the initial
value after 8 h irradiation, thereby evidencing a moderate sta-
bility toward photobleaching of the chromophore.
Figure 6. Photodecomposition profiles (lexc =345 nm) for the solid Eu3+
1 and Eu3+ꢂ 2 complexes.
ꢂ
The photodegradation process has been observed in a varie-
ty of lanthanide complexes including b-diketonate chelates,
carboxylates and their derivatives, cryptates, and others. Their
degradation under UV irradiation involves a series of compli-
cated processes, often attributed to photobleaching or
photon-induced chemical damage, although it has not been
yet completely elucidated. Very recently an unprecedented im-
proved photostability for Eu3+ and Gd3+ b-diketonate com-
plexes has been reported for the first time, which is related to
a trans-to-cis photoisomerization process of the photoactive
ligand.[46] A direct comparison of the photostability of different
complexes is difficult since a great variability in experimental
conditions has been reported in the literature. Photostability is
measured over time ranges from a few minutes up to tens of
hours on the pristine complexes or on the compounds dis-
solved in solution or dispersed in solid matrices. Cases that are
very comparable with our results report emission losses that
range from 30 to 50% over a time range from 80 min to
10 h.[55–57]
Figure 5. a) Absorption and excitation spectra monitored at lem =615 nm,
and b) emission spectra excited at 345 nm of 10ꢁ5 m Eu3+ꢂ 2 aqueous solu-
tion. Details of the europium emission spectrum are reported in the inset of
(b). c) Emission spectra of Eu3+ꢂ 2 in water (10ꢁ5 m solution) excited at
420 nm. The Raman band of water is highlighted with an asterisk in the
spectrum. In the inset are reported the high-resolution spectra at two differ-
ent excitation wavelengths: 345 (red curve) and 420 nm (black curve).
Emissive lanthanide complexes for use as tags in bioassays
or as optical probes require both a high emission quantum
yield and large molar absorptivity at an excitation wavelength
above 330 nm to give high brightness, B, in which B=ef.[54] In
aqueous media, no europium complexes have been reported
with a brightness (lexc =337 nm) that exceeds 3000mꢁ1 cmꢁ1.
Our systems work quite well for europium emission. For excita-
Spectroscopy of Ln3+ꢂ 1 and Ln3+ꢂ 2 complexes (Ln=Sm,
Dy, Tb)
Along with europium, three further lanthanide elements con-
sidered to be of great interest for their emission in the visible
region are samarium, dysprosium, and terbium. Therefore, bi-
partite ligands 1 and 2 were used to prepare the correspond-
tion at 345 nm, the brightness is 2440mꢁ1 cmꢁ1 for Eu3+
ꢂ
1 and 2400mꢁ1 cmꢁ1 for Eu3+ꢂ 2. Values of around 25000–
30000mꢁ1 cmꢁ1 are reported for the best systems.[13]
Chem. Eur. J. 2014, 20, 4598 – 4607
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In Tb3+ꢂ 1 and Tb3+ꢂ 2 complexes, the emission can be
sensitized through the high-energy triplet states of ligands
1 (T1(1)) and 2 (T1(2)) upon excitation into the high-energy por-
tion of the corresponding absorption bands (Figure 2a). How-
ever, besides the typical terbium green emission, a relevant
ligand-centered contribution to the total luminescence is ob-
served in the visible region (Figure S25 in the Supporting Infor-
mation). This contribution becomes predominant in Dy3+
ꢂ
1 and Dy3+ꢂ 2, and metal-centered emissions are not ob-
served. These findings point out the scarce sensitization effi-
ciency of ligands 1 and 2 toward TbIII and DyIII emission. The
emission ineffectiveness due to back-energy transfer has been
checked by measuring the spectra of all complexes after dear-
eation. We observed that, whereas the Dy3+ꢂ 1 and Dy3+ꢂ 2
spectra do not present any variation, a sensitive decrease of
the ligand-centered emission is found only for Tb3+ꢂ 2 (Fig-
ure S26 in the Supporting Information).
Conclusion
We have reported the synthesis and photophysical behavior of
lanthanide complexes that feature fluorene and spirobifluorene
chromophores as efficient light-harvesting units in the near-
visible region covalently linked to a preorganized polydentate
macrocycle as a highly effective ligand for the LnIII cations. The
spectroscopic properties of both fluorene and 9,9’-spirobifluor-
ene antennae, that is, the absorption behavior and the energy
position of the triplet states, have been carefully determined
and properly translated into an efficient sensitized europium
emission upon excitation of the complexes well above the
threshold value of 350 nm. The luminescence of Eu3+ꢂ 1 and
Eu3+ꢂ 2 molecules is clearly detectable at concentrations as
low as 10 pm. By exploiting the high versatility of the chemis-
try of fluorene and 9,9’-spirobifluorene molecules, their elec-
tronic and optical properties can be tuned with a large variety
of donor and/or acceptor groups. We also report that the
structural organization of the bipartite ligands renders the
complexes of EuIII, SmIII, DyIII, and TbIII highly soluble in aqueous
solutions and chemically stable over time. The information
gained in this study is currently being applied to the develop-
ment of new lanthanide molecular probes with higher-emis-
sion quantum yields that have the potential to provide new
multifunctional molecules for biochemical and biomedical pur-
poses.
Figure 7. Emission spectra of a) Sm3+ꢂ 1 (lexc =345 nm) and b) Sm3+ꢂ 2
(lexc =325 nm) in aqueous solutions (10ꢁ5 m).
ing Ln3+ꢂ 1 and Ln3+ꢂ 2 complexes. In the case of Sm3+
ꢂ
1 and Sm3+ꢂ 2, the sensitized emission from water solutions
(10ꢁ5 m) is clearly detected, as shown in Figure 7.
The spectrum of both complexes is characterized by four
main emission bands around 560, 600, 650, and 710 nm, which
are assigned to the intraconfigurational 4f transitions from the
6
6
4G5/2 excited state level to the H5/2 6H7/2 6H9/2, and H11/2
, ,
ground levels, respectively.[58] In the spectrum of Sm3+ꢂ 1 (Fig-
ure 7a) we also observe a relevant luminescence contribution
between 400 and 500 nm owing to ligand-centered emission.
This contribution is instead completely absent in Sm3+ꢂ 2
(Figure 7b). The absolute quantum yields evaluated for the
metal-centered emission are comparable for the two com-
plexes (Table 2). Despite the low absolute values, these data
are remarkable for samarium complexes in aqueous solutions.[7]
Higher quantum yields (0.007) have been recently reported for
samarium derivatives excited at 274 nm.[59]
Experimental Section
Materials and methods
All available chemicals and solvents were purchased from commer-
cial sources and were used without any further purification. Thin-
layer chromatography (TLC) was conducted on plates precoated
with silica gel Si 60-F254 (Merck, Darmstadt, Germany). Column
chromatography was conducted by using silica gel Si 60, 230–400
mesh, 0.040–0.063 mm (Merck, Darmstadt, Germany). 1H and
13C NMR spectra were recorded using a Bruker Avance 400 (400
and 100.6 MHz, respectively). Unless otherwise specified, all experi-
ments were made at room temperature. Chemical shifts were re-
Table 2. Room-temperature luminescence data in water solution.
Sm3+ꢂ 1
Sm3+ꢂ 2
lmax [nm]
fse
345
325
0.0014,[a] 0.0020[b]
0.0012,[a] 0.0017[b]
[a] Quantum yield values obtained by using the integrating sphere (lexc
345 nm). [b] Quantum yield values obtained by using [Ru(bpy)3]Cl2 as
standard (lexc =345 nm).
=
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ported in parts per million downfield from SiMe4 by using the re-
sidual proton (CHCl3, d=7.26 ppm; CHD2OD, d=3.30 ppm;
(CHD2)2NCOD, d=2.75, 2.92 ppm; and (CD3)2NCOH, d=8.03 ppm)
and carbon (CDCl3, d=77.0 ppm; CD3OD, d=49.1 ppm;
(CD3)2NCOD, d=29.8, 34.9, and 163.1 ppm) solvent resonances as
internal reference. Protons and carbon atoms assignments are
based on homonuclear (1H,1H double quantum filtered COSY) and
Keywords: antennas · fluorenes · lanthanides · luminescence ·
photophysics
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