Synthesis and solid-state, solution, and luminescence properties of near-infrared-emitting neodymium(3+) complexes formed with ligands derived from salophen

Article abstract of DOI:10.1002/hlca.200900162

A series of free ligands, H₂L¹, H₂L ², H₂L³, and H₂L⁴, designed for the coordination and sensitization of near-infrared(NIR)-emitting Nd³⁺ were synthesized

Full text of DOI:10.1002/hlca.200900162

Helvetica Chimica Acta – Vol. 92 (2009)
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Synthesis and Solid-State, Solution, and Luminescence Properties of Near-
Infrared-Emitting Neodymium(3þ) Complexes Formed with Ligands
Derived from Salophen
by Hyounsoo Uha), Paul D. Badgera), Steven J. Geiba), and Ste´phane Petoud*a)b)
a) Department of Chemistry, University of Pittsburgh, 219 Parkman Ave., Pittsburgh, PA 15260, USA
(phone: þ 1-412-624-8210; fax: þ 1-412-624-8611; e-mail: spetoud@pitt.edu)
b
´
´
) Centre de Biophysique Moleculaire, CNRS, Rue Charles-Sadron, F-45071 Orleans Cedex 2
(phone: þ 33(0)238255652; fax: þ 33(0)238631517; e-mail: stephane.petoud@cnrs-orleans.fr)
Dedicated to Professor Jean-Claude Bꢀnzli on the occasion of his 65th birthday
A series of free ligands, H2L1, H2L2, H2L3, and H2L4, designed for the coordination and sensitization
of near-infrared(NIR)-emitting Nd3þ were synthesized by modifying the salophen Schiff base with
different numbers and locations of Br-substituents. The nature of the Nd3þ complexes in solution was
determined to be [ML2]ꢀ by spectrophotometric titrations as an indication that the different substituents
do not affect significantly the nature of the formed species. The structures were determined in the solid
phase from X-ray diffraction experiments. The stoichiometries and structures in the solid state are
different from those observed in solution. We established that the structures in the solid state can be
partially controlled by the crystallization conditions. The ligands L1 – L4 have the ability to sensitize Nd3þ
through intramolecular energy transfer from the ligand to the metal ion. We quantified that the numbers
and locations of Br-substituents control the emitted luminescence intensity of the complex by the heavy-
atom effect.
Introduction. – There is a continuously growing interest for the properties of near-
infrared(NIR)-luminescent lanthanide complexes [1 – 44]. As examples of applica-
tions, NIR lanthanide luminescence can be used in NIR organic light-emitting diode
technology [45] or in telecommunication where the electronic structure of lanthanide
ions such as Er3þ, Nd3þ, and Ho3þ can be used as the active material for optical
amplification of NIR signals [46]. The use of NIR luminescence is also a promising
approach for biological imaging because NIR photons have less interference with
biological materials: most of biological molecules located in the skin, blood, and tissues
have lower absorption [47][48] in the NIR region, allowing photons to penetrate
deeply into tissues for noninvasive detection. Since the autofluorescence from
biological tissue is mainly located in the violet – blue region [49], imaging in the NIR
region also has an added advantage for the detection sensitivity due to an improved
signal-to-noise ratio, the background emission in the NIR being lower [48]. An
additional advantage of NIR imaging is the limited scattering of NIR photons in
comparison to VIS photons that result in improved image resolution. As the intensity of
scattered light is proportional to 1/l4, the longer wavelength of NIR light has reduced
scattering, which results in the higher resolution of the obtained image [50].
ꢀ 2009 Verlag Helvetica Chimica Acta AG, Zꢁrich

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NIR-Luminescent lanthanide complexes also have the advantages that are common
to all lanthanides, including photostability (allowing the use of the luminescence for
long experiments) [51], sharp emission bands (allowing spectral discrimination) [52],
and long luminescent lifetimes (allowing temporal discrimination) [53].
Due to their electronic structure, free lanthanide cations have low absorption
coefficients and long intrinsic luminescence lifetime [2][3]. To generate a sufficient
number of photons allowing sensitive detection, lanthanide cations must be sensitized
with a suitable antenna that possesses the appropriate electronic structure [54] and that
can sufficiently protect the cation from high-energy vibrations to prevent the
nonradiative deactivation of the lanthanide luminescence.
To achieve adequate emission intensity from lanthanide ions, the sensitizers or
antennae should fulfill several requirements: 1) the sensitizers have to form a complex
with lanthanide ions that does not dissociate easily, which typically have a high
coordination number comprised between 8 to 10; 2) the energy levels of the triplet
states of the sensitizers must match the accepting energy level of the lanthanide ions to
achieve a good energy transfer; 3) the sensitizer-ligand has to provide good protection
for lanthanide ions from nonradiative deactivation through solvent-molecule vibra-
tions. In addition, the sensitizer should be easy to modify to control chemical and/or
photophysical properties of the resulting luminescent lanthanide complex.
Several sensitizers or antennae have been described for the sensitization of NIR-
emitting lanthanide cations [1 – 44]. The number of antennae for NIR-emitting
lanthanides is still moderate, and it is interesting to test additional systems to broaden
our understanding of the relationship between electronic structure of the antenna and
luminescence properties of the resulting complexes.
In this work, we investigated a system based on the ligand salophen (¼ 2,2’-[1,2-
phenylenebis(nitrilomethylidyne)bis[phenolato]2ꢀ) where different substituents can
be rapidly attached to evaluate their effect on the solution behavior, solid-state
structure, and luminescence properties of these complexes.
Metal complexes formed with Schiff ligands have occupied an important role in
coordination chemistry for over half a century [55]. Salen, ethylenediamine-bridged
modified salicylaldehydes, and salophen, 1,2-phenylenediamine-bridged modified
salicylaldehydes, have been extensively employed because of their stability resulting
from the conjugated structure, the pre-organized tetradentate coordination geometry,
the simplicity of synthesis and the facile modification of their structures. Salophen
derivatives can act as good sensitizer-ligands for luminescent lanthanide cations
because this family of ligands can fulfill the specific requirements for lanthanide
luminescence. The planar and rigid ligands have tetradentate coordination geometry
which can be hypothesized to lead to the formation of [ML2]ꢀ complexes with
lanthanide ions to fulfill their coordination-number requirements. The energy of the
triplet state of the unsubstituted salophen chromophoric ligand has been reported to be
located at 17510 cmꢀ1 [56]. This energy of electronic level is suitable for a rapid and
irreversible intramolecular energy transfer to NIR-emitting lanthanide ions such as
Nd3þ that has one main accepting level located at 11300 cmꢀ1. Another advantage of the
salophen moiety is the easy and versatile modification of its structure. A large number
of derived ligands possessing different substituents can be rapidly synthesized in
few steps. A family of ligands possessing a variety of substituents is important for

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the systematic study and control of the photophysical properties of lanthanide com-
plexes.
Results and Discussion. – Synthesis of Ligands and Complexes. The salophen-type
ligands H2L1, H2L2, H2L3, and H2L4 can be easily synthesized by condensation of two
equiv. of a salicylaldehyde and one equiv. of benzene-1,2-diamine in refluxing EtOH
(Scheme,a). This reaction is fast and, in most cases, quantitative, with the heating of the
reaction mixture being essential. Without heating, the major product that was obtained
was a Schiff base with only one amine group condensed with the salicylaldehyde
(bidentate system; Scheme,b). In addition to the effect of the temperature, the
formation and structure of Schiff bases is also controlled by the number of equivalents
of aldehydes and amines. MacLachlan and co-workers reported the syntheses of Schiff
bases with various ratios of aldehyde/amine from the same dialdehyde and diamine
[57]. All the diamines and aldehydes used in this work were commercially available,
except for 3-bromosalicylaldehyde. The latter was synthesized from bromophenol and
paraformaldehyde in the presence of magnesium chloride [58].
Scheme. Condensation Reaction of One Benzene-1,2-diamine with Two Salicylaldehydes and Structures
of the Chromophoric Ligands Used in This Work
The syntheses of the different [NdL2]ꢀ complexes were carried out by the reaction
of 2 equivalents of salophen-type ligand H2L and 1 equiv. of Nd(OTf)3 · 6 H2O.
Depending on the solubility of the ligand, MeCN or a mixture THF/MeCN was used as
solvent. One and a half equiv. of NaOH in MeOH was used as a base to ensure
complete deprotonation of the ligand. The Nd complexes were isolated by solvent
evaporation followed by precipitation.
Crystal Structure of the Complexes. To grow X-ray-diffraction-quality samples of
[Nd(Lx)] (x ¼ 1 – 4), the following method was used: To a suspension of a ligand H2Lx

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dissolved in MeOH, an excess (ca. 500 – 600 equiv.) of Et3N was added. A half
equivalent of Nd(OTf)3 · 6 H2O was then added to the ligand solution, and hexane was
diffused slowly into this solution. Crystals of [Nd2(L1)3] and [Nd(L4)2]ꢀ were obtained
with this method and analyzed with X-ray diffraction.
The crystal of the complex with L4 has the formula Et3NHþ[Nd(L4)2]ꢀ 1). As shown
in Fig. 1,a, the molecular structure of the obtained crystals can be described as a
double-decker sandwich (for selected bond lengths and angles, see Table 1). There is
one previous example of a lanthanide complex with a salophen-derived ligand which
adopts such a double-decker sandwich conformation [59]. This complex is formed by
the coordination of one Ce4þ as metal cation with two salophen as ligand. It is
interesting to note that the lanthanide cation coordinated in this complex has a
different oxidation state and, therefore, cannot be considered as similar. In addition to
the difference in charge, Ce4þ has a significantly different effective ionic radius: 0.97 ꢂ
for Ce4þ vs. 1.109 ꢂ for Nd3þ, both calculated according to Shannon for eight
coordinated cations) [60].
Fig. 1. a) Molecular structure of [Nd(L4)2]ꢀ with Et3NHþ as counter cation. b) Et3NHþ in the crystal-
packing structure of Et3NHþ[Nd(L4)2]ꢀ. Arbitrary atom numbering.
Table 1. Selected Bond Lengths [ꢂ] and Angles [8] in Et3NH[Nd(L4)2]. For atom numbering, see Fig. 1.
NdꢀO(1)
NdꢀO(2)
NdꢀO(3)
NdꢀO(4)
NdꢀN(1)
NdꢀN(2)
NdꢀN(3)
NdꢀN(4)
2.327(6)
2.353(6)
2.367(6)
2.345(6)
2.629(7)
2.672(8)
2.678(7)
2.689(8)
O(1)ꢀNdꢀO(2)
O(2)ꢀNdꢀN(2)
N(2)ꢀNdꢀN(1)
N(1)ꢀNdꢀO(1)
O(3)ꢀNdꢀO(4)
O(4)ꢀNdꢀN(4)
N(4)ꢀNdꢀN(3)
N(3)ꢀNdꢀO(3)
94.9(2)
68.0(2)
60.6(2)
69.6(2)
91.3(2)
66.8(2)
60.4(2)
68.6(2)
Two salophen-type ligands coordinate each to the Nd3þ center in a tetradentate
fashion. The salicylidene arms are bent toward the exterior of the molecule. For each
salophen-type ligand coordinated to Nd3þ, one 5-membered ring, NdꢀN(1)ꢀC(8)ꢀ
C(13)ꢀN(2) and two six-membered rings, NdꢀN(1)ꢀC(7)ꢀC(6)ꢀC(1)ꢀO(1) and
1
)
CCDC-733475 and -733476 contain the supplementary crystallographic data for this work. These
data can be obtained free of charge via http://www.ccdc.cam.ac.uk/data_request/cif.

Helvetica Chimica Acta – Vol. 92 (2009)
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NdꢀN(2)ꢀC(14)ꢀC(15)ꢀC(20)ꢀO(2), are formed (Fig. 2,a). The salicylidene arms
are bent to make these three rings as planar as possible. Since the [Nd(L4)2]ꢀ complex
has a net ꢀ 1 charge, Et3NHþ acts as a counter cation. In the crystal-packing structure
(Fig. 1,b), Et3NHþ is located in the space between two molecules of [Nd(L4)2]ꢀ. The
presence of Et3NHþ may be another factor contributing to the bending of the salophen-
type ligands. Molecules and ions are closely packed together to have minimal energy in
a crystal. The bent salicylidene arms can provide the space to accommodate the
relatively larger Et3NHþ cation in the crystal-packing structure of Et3NHþ[Nd(L4)2] ꢀ.
Fig. 2. a) Five- and six-membered rings formed by coordination of the ligand L4 to Nd3þ and
b) coordination polyhedron around Nd3þ in Et3NHþ[Nd(L4)2]ꢀ. Arbitrary atom numbering.
When viewed from the orthogonal point of view with respect to the salophen plane,
the two salophen-type ligands are oriented in a staggered fashion. The coordination
environment around the Nd3þ cation is close to a square antiprism geometry (Fig. 2,b).
Atoms O(1), O(2), N(2), and N(1) form a nearly planar coordination ring, and atoms
O(3), O(4), N(4), and N(3) also form a nearly planar coordination ring. However,
these coordination rings are trapezoids: the OꢀO segments (3.37 – 3.45 ꢂ) are longer
than the NꢀN segments (2.68 – 2.70 ꢂ). Because two N-atoms N(1) and N(2) (or N(3)
and N(4), resp.) are attached to the same phenylene moiety, there is limited flexibility
around the N-atoms, but the relatively more flexible salicylidene arms allow the OꢀO
distances to be increased. The NdꢀO bonds are shorter than the NdꢀN bonds. This
situation induces the two planes O(1)ꢀO(2)ꢀN(2)ꢀN(1) and O(3)ꢀO(4)ꢀN(4)ꢀ
N(3) being not perfectly parallel.
The structure of the [Nd2(L1)3] complex is shown in Fig. 3. Thus an [M2L3]
structure, i.e., [Nd2(L1)3] · MeOH was isolated (for selected bond lengths and angles,
see Table 2). Similar types of triple-decker and tetra-decker Tb3þ complexes of
salophen derivatives have been reported previously [61].
It is interesting to note that the three ligands are not located on the same axis. L1(1)
and Nd(1) are out of the line formed by the (center of L1(2)) – Nd(2) – (center of
L1(3)). The two N-atoms of L1(2) (N(3) and N(4)) are only coordinated to Nd(2), and
have no coordination to Nd(1). Because an N-atom has only one lone pair to
coordinate to the metal ion, while an O-atom has two or three lone pairs, an N-atom can
bind to only one metal ion. Thus Nd(1) cannot be located on the center of L1(2), and it
is coordinated by the two O-atoms of L1(2) (O(3) and O(4)) and the two O-atoms
(O(1) and O(2)) and the two N-atoms (N(1) and N(2)) of L1(1). To fill the remaining
coordination site, solvent MeOH is bound to Nd(1) in the first sphere of coordination.

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Fig. 3. Molecular structure of [Nd2L1 ] · MeOH. Arbitrary atom numbering.
3
Table 2. Selected Bond Lengths [ꢂ] and Angles [8] in [Nd2(L1)3] · MeOH. For atom numbering, see
Fig. 3.
Nd(1)ꢀO(1)
Nd(1)ꢀO(2)
Nd(1)ꢀN(1)
Nd(1)ꢀN(2)
Nd(1)ꢀO(3)
Nd(1)ꢀO(4)
Nd(1)ꢀO(7)
Nd(2)ꢀO(3)
Nd(2)ꢀO(4)
Nd(2)ꢀN(3)
Nd(2)ꢀN(4)
Nd(2)ꢀO(5)
Nd(2)ꢀO(6)
Nd(2)ꢀN(5)
Nd(2)ꢀN(6)
2.271(8)
2.253(8)
2.545(9)
2.552(10)
2.476(8)
2.409(8)
2.459(10)
2.409(8)
2.410(8)
2.598(10)
2.679(10)
2.313(8)
2.365(8)
2.573(11)
2.575(10)
O(1)ꢀNd(1)ꢀO(2)
O(2)ꢀNd(1)ꢀN(2)
N(2)ꢀNd(1)ꢀN(1)
N(1)ꢀNd(1)ꢀO(1)
O(3)ꢀNd(1)ꢀO(4)
O(3)ꢀNd(1)ꢀO(7)
O(7)ꢀNd(1)ꢀO(4)
O(3)ꢀNd(2)ꢀO(4)
O(4)ꢀNd(2)ꢀN(4)
N(4)ꢀNd(2)ꢀN(3)
N(3)ꢀNd(2)ꢀO(3)
O(5)ꢀNd(2)ꢀO(6)
O(6)ꢀNd(2)ꢀN(6)
N(6)ꢀNd(2)ꢀN(5)
N(5)ꢀNd(2)ꢀO(5)
97.2(3)
72.1(3)
63.4(3)
71.6(3)
69.7(3)
75.8(3)
75.8(3)
70.8(3)
68.1(3)
60.4(3)
69.4(3)
90.9(3)
71.2(3)
62.3(3)
70.0(4)
The same coordination pattern (two salophen-type ligands form an eight-coordinated
metal complex and additional metal ions and salophen-type ligands are located out of
the axis providing 7- or 8-coordination sites with one or two solvent molecules) are
observed in reported triple-decker or tetra-decker structures of Ln3þ complexes [61].
Determination of the Nature and Stability Constants of the Nd3þ Complexes by Batch
Spectrophotometric Titrations. To confirm the nature of the complexes formed between
Nd3þ and the different ligands in solution, batch spectrophotometric titrations were
performed in DMSO. Two methods were employed to fully deprotonate the ligands
prior to titration to ensure the complete formation of the complexes during the titration
process. In the first method, 2 equiv. of KOH per equiv. of ligand were added to a
suspension of ligand in MeOH. Upon addition, the solution became clear suggesting
the deprotonation of the ligand. After evaporation of the solvent, the potassium salt of
the ligand was used as starting material for the creation of the solutions for the batch

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titration. The other method was the addition of an organic base such as triethylamine
(Et3N) into the titration solutions. The solution can then be used as such and
corresponds to a deprotonation of the ligand in situ. This second method has an
advantage since the instability of the potassium salt of the ligand and the undesirable
re-protonation of the ligand can be avoided.
Batch titrations of three salophen-type ligands, H2L1, H2L3, and H2L4, with Nd3þ
were carried out varying metal/ligand ratios from 0 :1 to 2 :1 equiv. For H2L1, KOH was
used as a base and deprotonation was done prior to the titration. For H2L3 and H2L4,
Et3N was used as a base and deprotonation was achieved in situ. The titration of H2L1
with Nd3þ in the presence of Et3N as a base was also tested, but no meaningful results
were obtained. This negative result was attributed to Et3N being not basic enough to
fully deprotonate H2L1 (vide infra). This situation can be explained since H2L1 does not
incorporate electron-withdrawing Br groups that stabilize the basic form of the ligand.
The plots of results are summarized in Fig. 4, where the absorbance of the solutions at
chosen wavelengths are plotted against the metal/ligand ratios. In all three cases, a
breaking point appears at a metal/ligand ratio of ca. 1:2, indicating the formation of the
[ML2] complex for all three ligands.
The stability constants corresponding to the formation of the complexes between
the ligands L1, L3, and L4 with Nd3þ were calculated from these titrations with the data-
analysis software SPECFIT [62]. The best fits were obtained with a model
corresponding to the successive formation of [ML1] and [ML2] complexes. Conver-
gence of the fitting process for the calculation of the logK2 value was possible when the
values of logK1 were fixed to values comprised between 9 and 12, and the obtained
values of logK2 remained systematically unchanged. This observation can be explained
by the very high stability of the [ML1] species formed in solution and has been observed
for other NIR-emitting lanthanide complexes [43]. LogK2 stability-constant values of
6.4 ꢁ 0.3, 6.8 ꢁ 0.4, and 7.2 ꢁ 0.3 were obtained after fitting the data obtained from M/
(L1)2, M/(L3)2, and M/(L4)2 titrations, respectively.
From these data, we can observe that the presence of the Br-substituents on the
salophen moiety do not have a major contribution on the stability of the different
[ML2] complexes formed in solution. With these titration experiments, we were able to
confirm that all the studied salophen-type ligands form [ML2] complexes when treated
with Nd3þ cation in solution.
It is interesting to note that the stoichiometry of this complex which is 1:2 in
solution is different in the solid phase. Since the net charge of the [ML2] complex is ꢀ 1,
a counter cation must be present for the formation of the [ML2] complex. No counter
cation is present in the structure of [Nd2(L1)3], while Et3NHþ acts as counter cation in
the structure of [Nd(L4)2]ꢀ. In the absence of a counter cation, the reaction between
the trivalent Nd3þ and divalent L12ꢀ results in the formation of a complex with a [M2L3]
formula. This result indicates that Et3N cannot act as a base for the deprotonation of
the ligand H2L1 when present in solution with Nd3þ during the crystal-growth process.
Investigation of the acidities of the salophen-type ligands provided more insightful
information of whether Et3N is basic enough to fully deprotonate the ligands described
in this work. A UV/VIS absorbance analysis was carried out to compare the relative
acidities of the salophen-type ligands H2L1, H2L2, H2L3, and H2L4 in DMSO, prior and
after addition of an excess of Et3N (Fig. 5). The absorbance spectra of H2L3 and H2L4

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Fig. 4. a) c) e) Change of absorption spectra and b) d) f) absorbance recorded at selected wavelength vs.
metal/ligand ratio of the batch titration of ligands with Nd3þ in DMSO at 258. a) and b): L1, c ¼ 3.00 ·
10ꢀ5 m, 31 batches; c) and d): L3, c ¼ 2.50 · 10ꢀ5 m, 22 batches; e) and f): L4, c ¼ 2.50 · 10ꢀ5 m, 22 batches.
showed a significant change after the addition of Et3N as an indication of a strong effect
of the deprotonation of the ligands on the electronic structure of the molecule, while
those of H2L1 and H2L2 do not change significantly after addition of Et3N. From this
result, it can be concluded that Et3N can deprotonate the more acidic H2L3 and H2L4
ligands, but it is not sufficiently basic to deprotonate the less acidic H2L1 and H2L2
ligands. This trend of the acidities of salophen derivatives is consistent with that of
acidities of phenol derivatives (Table 3), and it explains the need for a stronger base
such as KOH for the spectrophotometric titration of H2L1.
From these investigations, we can conclude that these structures in the solid state
can be different from those in solution. The structure along with the stoichiometry can

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Fig. 5. UV/VIS Absorbance of salophen-type ligands in DMSO without Et3N (solid) and with excess
Et3N (dashed). a) H2L1, c ¼ 3.77 · 10ꢀ5 m; b) H2L2, c ¼ 4.08 · 10ꢀ5 m; c) H2L3, c ¼ 3.92 · 10ꢀ5 m; d) H2L4,
c ¼ 3.84 · 10ꢀ5 m.
Table 3. pKa Values of Phenol Derivatives [63]
Phenol
9.994
4-Bromophenol
9.366
2-Bromophenol
8.452
2,4-Dibromophenol
7.790
pKa
be controlled by the crystallization conditions. A proper choice of base is the key factor
for the stoichiometry of the complex in the solid state. The deprotonation of the ligand
can be controlled by the strength of the base and the acidity of the ligand which itself is
controlled by the electron-withdrawing effect of the substituents. The protonation state
of the ligand is important since it controls the presence or absence of the counter cation
for the complex. With a counter cation, an [ML2]ꢀ complex is formed because it has a
net ꢀ 1 charge. Without a counter cation, the Nd3þ metal act as counter cation and the
[M2L3] complex is formed.
Luminescence Properties of Nd3þ Complexes in Solution. The absorption, excita-
tion, and emission spectra of [Nd(Lx)2]ꢀ (x ¼ 1 – 4) were recorded in DMSO solution.

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For the deprotonation of the ligands, NaOH was used as a base for the creation of these
solutions.
The absorption, excitation, and emission spectra of [Nd(L1)2]ꢀ are depicted in
Fig. 6,a. The apparent maximum of the absorption bands appears around 385 nm. The
energy positions of the bands present in the excitation spectra recorded under Nd3þ
signal match closely the position of the bands of the absorption spectra. This result
suggests that the NIR-emitting lanthanide cations are sensitized through the electronic
levels of the chromophoric ligand and that the ligand is providing an antenna effect. On
the basis of their epsilon, we can assess the nature of the electronic bands resulting in
the sensitization of Nd3þ as being p – p*. The NIR-emission spectra reveal the presence
of three typical sharp bands arising from the electronic states of Nd3þ. These bands are
4
located at 900, 1060, and 1330 nm in the NIR region and are attributed to the 4F3/2 ! I9/2
,
4
4
4
4F3/2 ! I11/2, and F3/2 ! I13/2 f – f transition, respectively.
The other three complexes, [Nd(L2)2]ꢀ, [Nd(L3)2]ꢀ, and [Nd(L4)2]ꢀ, also show
similar absorption, excitation, and emission spectra, which indicate that all these
ligands sensitize Nd3þ (Fig. 6,b – d). However, when the absorption spectra of the four
complexes are compared to one another, some red shifts of the maxima of the
electronic bands were observed, in the following order: [Nd(L1)2]ꢀ ! [Nd(L3)2]ꢀ !
[Nd(L2)2]ꢀ ! [Nd(L4)2]ꢀ (Fig. 7). The excitation spectra also show the same trend of
red shift. These results indicate that the more the ligand is substituted by Br-atoms, the
more that electronic state is shifted toward lower energy.
To quantify the effect of substituents and to evaluate the efficiency of energy
transfer from the sensitizer-ligand to lanthanide and the presence of quenching
processes deactivating the Nd3þ excited state, the quantum yields of the Nd3þ
complexes formed with L1, L2, L3, and L4 were measured in DMSO with K[Nd(tro-
polonato)4] (F ¼ 2.1 · 10ꢀ3) [40] as a reference. The emission signal of Nd3þ was
monitored, and the excitation was performed upon sensitizer bands. The results are
summarized in Table 4. These quantum yields are relatively close to the value obtained
for other Nd3þ complex in DMSO, which is 2.1 · 10ꢀ3 [40].
Table 4. Quantum Yield (F) of Nd3þ Complexes in DMSO at r.t.
Na[Nd(L1)2]a)
Na[Nd(L2)2]b)
1.23 ꢁ 0.08 · 10ꢀ3
Na[Nd(L3)2]c)
1.6 ꢁ 0.1 · 10ꢀ3
Na[Nd(L4)2]d)
2.01 ꢁ 0.07 · 10ꢀ3
Absolute Ftot
1.18 ꢁ 0.04 · 10ꢀ3
b
c
a) c ¼ 4.95 · 10ꢀ5 m, lex 385 nm. ) c ¼ 8.70 · 10ꢀ5 m, lex 392 nm. ) c ¼ 4.89 · 10ꢀ5 m, lex 388 nm. d) c ¼ 4.99 ·
10ꢀ5 m, lex 395 nm.
When the quantum yields of the studied complexes are compared, we can notice
that the presence of the Br-atoms at the salophen-type ligands enhances the efficiency
of the energy transfer from the sensitizer moiety to the Nd3þ: the greater the number of
Br-atoms, the higher the quantum yield. This result can be attributed to an increase in
the population of the triplet state due to the heavy-atom effect [64]. Na[Nd(L3)2] has a
higher quantum yield than Na[Nd(L2)2], though they have the same number of Br-
atoms. Being located in ortho-position to the coordinating Oꢀ, the Br-atom: 1) has
removed a H-atom from that position, i.e., removed a CꢀH which may cause a

Helvetica Chimica Acta – Vol. 92 (2009)
2323
Fig. 6. Absorption (solid), excitation (dotted), and emission (dashed) spectra of Naþ[Nd(Lx)2]ꢀ (x ¼ 1 –
4) complexes in DMSO at r.t. (lex 395 nm for the emission spectrum, lem 1058 nm for the excitation
spectrum). a) Naþ[Nd(L1)2]ꢀ, c ¼ 4.64 · 10ꢀ5 m; b) Naþ[Nd(L2)2]ꢀ, c ¼ 4.65 · 10ꢀ5 m; c) Naþ[Nd(L3)2]ꢀ,
c ¼ 4.54 · 10ꢀ5 m; d) Naþ[Nd(L4)2]ꢀ, c ¼ 3.51 · 10ꢀ5 m.

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Helvetica Chimica Acta – Vol. 92 (2009)
Fig. 7. Comparison of the absorption spectra of the four complexes Naþ[Nd(Lx)2]ꢀ (x ¼ 1 – 4)
nonradiative deactivation of lanthanide luminescence, and 2) has an increased heavy-
atom effect by the proximity factor. This conclusion is reinforced by the fact that, since
we have not observed important changes of the electronic structures of the
chromophores possessing different substituents, the major parameter that explains
the different quantum-yield values must be related to the number of Br-substituents
through the heavy-atom effect.
The NIR luminescence lifetimes of Nd3þ in Na[Nd(L1)2], Na[Nd(L2)2], and
Na[Nd(L4)2] were measured in DMSO upon ligand excitation (Table 5). The values
obtained are all close to a 1.0 ms value. These close lifetime values indicate that the Nd3þ
are all in fairly similar coordination environments in the three different complexes: a
similar degree of protection of Nd3þ is therefore provided by the organization of the
ligand around the lanthanide ion. These lifetimes compare well with the luminescence
lifetimes previously reported for Nd3þ complexes (Table 6) [18][24][40]. From this
experiment, we got quantitative data showing that the differences in quantum yields
cannot be explained by differences in the protection of the NIR-emitting cations.
Table 5. Luminescence Lifetimes (t) of Nd3þ Complexes (c ¼ 3.7 · 10ꢀ5 m) in DMSO at r.t. The sample was
excited at 337 nm with a nitrogen laser, and the signal was collected at 1060 nm.
Na[Nd(L1)2]
1.00 ꢁ 0.01
Na[Nd(L2)2]
1.01 ꢁ 0.01
Na[Nd(L4)2]
0.998 ꢁ 0.001
t [ms]
Conclusions. – The coordination chemistry and photophysical properties of the
Nd3þ complexes formed with a series of ligands derived from salophen were
systematically studied. All these pre-organized rigid tetradentate ligands form well-
defined [ML2]ꢀ complexes in solution. We demonstrated that the structures of the
complexes in the solid phase can be significantly different from the structures in
solution. The morphology of the crystal structure can be controlled by the proper
choice of base which itself controls the presence or absence of the counter cations. The
spectroscopy studies of the properties of the chromophoric groups of these systems
indicate that the different substituents at the ligand do not affect significantly the
energies of the different electronic levels. It was established that all four chromophoric

Helvetica Chimica Acta – Vol. 92 (2009)
2325
Table 6. Quantum Yields (Ftot) and Luminescence Lifetimes (t) of Relevant Examples of NIR-Emitting
Nd3þ Complexesa) in DMSO
Ftot
t [ms]
[Nd(m-terp1)] [18]
[Nd(m-terp2)] [18]
[Nd(phn)3] [24]
[Nd(phn)4] [24]
K[Nd(trp)4] [40]
N/A
N/A
N/A
N/A
0.0021
1.2
1.2
1.43
1.34
1.10
a) m-terp1 ¼ tri-tert-butyl 2,2’,2’’-[(3,3’’-bis{[(1-butoxypropyl)(phenylcarbonyl)amino]methyl}-5,5’,5’’-
trimethyl-1,1’:3’,1’’-terphenyl-2,2’,2’’-triyl)tris(oxy)]triacetate; m-terp2 ¼ tri-tert-butyl 2,2’,2’’-{[3,3’’-
bis({(1-butoxypropyl)[(4-methylphenyl)sulfonyl]amino}methyl)-5,5’,5’’-trimethyl-1,1’:3’,1’’-terphenyl-
2,2’,2’’-triyl]tris(oxy)}triacetate; phn ¼ 9-hydroxyphenalen-1-onato; trp ¼ tropolonato.
ligand systems are able to sensitize Nd3þ, a NIR-emitting cation, through intra-
molecular energy transfer from the electronic states of the ligand to the metal ion. The
luminescence properties of the complexes can be enhanced by modification of the
ligand with Br-substituents.
Funding was provided through the University of Pittsburgh and through the National Science
Foundation (award DBI0352346). S. P. gratefully acknowledges Le Studium (Agency for research and
´
international hosting associate researchers in ꢃRegion Centreꢄ), Orleans, France, for financial support.
Experimental Part
General. All reagents and solvents were purchased from the following suppliers and used as
received: paraformaldehyde from Acros Organics; 5-bromosalicylaldehyde, 4,5-dimethylbenzene-1,2-
diamine, and MgSO4 from Aldrich; 3,5-dibromosalicylaldehyde and trifluoroacetic acid from Alfa Aesar;
CH2Cl2, MeCN, AcOEt, MeOH, THF, and Na2CO3 from EMD Chemicals Inc.; Et3N from Fisher
Scientific; AcOH, DMSO, Et2O, HCl, hexanes, NaOH, and MgSO4 from Mallinckordt Baker, Inc.; abs.
EtOH from Pharmaco Products, Inc.; Nd(OTf)3 · 6 H2O from Strem Chemicals; 2-bromophenol from
TCI; all deuterated NMR solvents from Cambridge Isotope Labs. M.p.: Fisher-Johns melting-point
apparatus; uncorrected. IR Spectra: Perkin-Elmer-Spectrum BX FT-IR spectrometer; polystyrene film as
1
external standard (1601 cmꢀ1 peak); ˜n in cmꢀ1. H-NMR Spectra: Bruker DPX-300 at 300 MHz; d in
ppm, J in Hz. EI-MS and ESI-MS: Micromass-Autospec and Agilent-HP-1100 LC-MSD, respectively; in
m/z. Elemental analyses were performed by Atlantic Microlab, Inc.
Luminescence Studies. Absorption spectra: Perkin-Elmer-Lambda-9 spectrophotometer. Metal-
cation luminescence emission and excitation spectra: modified Jobin-Yvon-Spex Fluorolog-322 spectro-
fluorometer equipped for both r.t. and 77 K measurements. Luminescence and excitation spectra were
corrected for the instrumental function. Metal-cation luminescence quantum yields were measured for
4
the 4F3/2 ! I11/2 transition with K[Nd(tropolonato)4] (F ¼ 2.1 · 10ꢀ3 in DMSO) as reference [40]. The use
of a Nd3þ complex allows for a simple method of cross calibrating the VIS detector with the NIR detector
of the Fluorolog-322. The quantum yields were calculated by Eqn. 1, where subscript r stands for the
reference and x for the sample; A is the absorbance at the excitation wavelength, I is the intensity of the
excitation light at the same wavelength, h is the refractive index (h ¼ 1.479 in DMSO), and D is the
measured integrated luminescence intensity.
ꢀ
ꢁ
2
Fx ArðlrÞ IðlxÞ hx Dx
¼
ꢂ
ꢂ
(1)
Fr AxðlxÞ IðlrÞ hr Dr

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Helvetica Chimica Acta – Vol. 92 (2009)
The luminescence-lifetime measurements were performed by excitation of solns. in 1 cm quartz cells
with a nitrogen laser (Oriel model 79110, wavelength 337.1 nm, pulse width at half-height 15 ns, 5 – 30 Hz
repetition rate). Emission from the sample was collected at a right angle to the excitation beam by a 3’’
plano-convex lens. Emission wavelengths were selected by means of quartz filters. The signal was
monitored by a cooled photomultiplier (Hamamatsu R316) coupled to a 500 MHz bandpass digital
oscilloscope (Tektronix TDS 754D). The signals (15000 points each trace) from at least 500 flashes were
collected and averaged. Background signals were similarly collected and subtracted from sample signals.
Lifetimes were averages of at least three independent determinations. Data were fitted with exponential
decay curves by OriginPro (V7 SP4) data analysis and linefitting software. Ligand-centered triplet-state
lifetimes were performed by excitation of solid samples in a quartz tube at 77 K with the nitrogen laser
described above. Emission from the samples was collected at a right angle to the excitation beam, and the
emission wavelengths were selected by means of a Spex-FL-1005 double monochromator. The signal was
monitored by a Hamamatsu-R928 photomultiplier coupled to a 500 MHz bandpass digital oscilloscope
(Tektronics TDS 620B). The signals (15000 points each trace) from at least 500 flashes were collected
and averaged. Background signals were similarly collected and subtracted from sample signals.
X-Ray Crystallography2). Crystals suitable for X-ray crystallographic study were coated with
Fluorolubeꢅ, then mounted on a glass fiber, and coated with epoxy cement. X-Ray data were collected
with a Bruker-Apex diffractometer and graphite-monochromatized MoKa radiation (l 0.71073 ꢂ). Data
collection was controlled with the Bruker SMART program, data processing was done with the
SHELXTL program package [65], and the graphics were done with Ortep-3 [66], Mercury 1.2.1 [67], and
Ortex [68]. All H-atoms were calculated and placed in idealized positions (d(CꢀH) ¼ 0.96 ꢂ).
3-Bromosalicylaldehyde (¼ 3-Bromo-2-hydroxybenzaldehyde). To a soln. of MgCl2 (2.38 g,
25.0 mmol), paraformaldehyde (2.61 g, 86.8 mmol) and Et3N (5.00 ml, 3.63 g, 35.9 mmol) in MeCN
(50 ml), 2-bromophenol (1.60 ml, 2.39 g, 13.8 mmol) was added. The mixture was refluxed for 2.5 days
and then allowed to cool. The soln. was diluted with 1m HCl (80 ml) and extracted with Et2O (3 ꢃ 50 ml).
The org. layer was washed with brine (50 ml), dried (MgSO4), and concentrated, and the crude product
purified by column chromatography (silica gel, 20% AcOEt/hexanes): 3-bromosalicylaldehyde (0.961 g,
35%). 1H-NMR (300 MHz, CDCl3): 11.62 (s, OH); 9.88 (s, C(¼O)H); 7.79 (dd, J ¼ 1.5, 7.8, 1 arom. H);
7.55 (dd, J ¼ 1.4, 7.7, 1 arom. H); 6.96 (t, J ¼ 7.7, 1 arom. H). EI-MS: 200 (Mþ, C7H5BrO2þ ; calc. 199.95).
2,2’-[(4,5-Dimethyl-1,2-phenylene)bis(nitrilomethylidyne)]bis[phenol] (H2L1). To a soln. of salicyl-
aldehyde (2.05 g, 16.8 mmol) in EtOH (20 ml), 4,5-dimethylbenzene-1,2-diamine (1.14 g, 8.40 mol) was
added. The mixture was heated and sonicated for 15 min (! yellow precipitate). The mixture was filtered
and the solid washed with hexanes and dried under vacuum: H2L1 (2.54 g, 88%). M.p. 144 – 1468. IR
(selected absorbances only; KBr): 1617 (C¼N), 1278 (ArꢀO). 1H-NMR (300 MHz, CD3CN): 13.22 (s,
2 OH); 8.76 (s, 2 N¼CH); 7.50 (dd, J ¼ 1.6, 7.6, 2 arom. H); 7.47 – 7.36 (m, 2 arom. H); 7.19 (s, 2 arom. H);
6.99 – 6.92 (m, 4 arom. H); 2.32 (s, 2 Me).
2,2’-[(4,5-Dimethyl-1,2-phenylene)bis(nitrilomethylidyne)]bis[4-bromophenol] (H2L2). A soln. of
4,5-dimethylbenzene-1,2-diamine (0.160 g, 1.18 mmol) in EtOH (4 ml) was added to a soln. of 5-
bromosalicylaldehyde (0.486 g, 2.42 mmol) in EtOH (7 ml). The mixture was refluxed overnight
(!orange-yellow precipitate). The mixture was filtered and the solid washed with EtOH and Et2O and
dried under vacuum: H2L2 (0.506 g, 85%). M.p. 241 – 2458. IR (KBr): 1616 (C¼N), 1277 (ArꢀO).
1H-NMR (300 MHz, (D6)DMSO): 8.90 (s, 2 N¼CH); 7.86 (d, J ¼ 2.5, 2 arom. H); 7.52 (dd, J ¼ 2.5, 8.8, 2
arom. H); 7.28 (s, 2 arom. H); 6.92 (d, J ¼ 8.8, 2 arom. H); 2.29 (s, 2 Me).
2,2’-[(4,5-Dimethyl-1,2-phenylene)bis(nitrilomethylidyne)]bis[6-bromophenol] (H2L3). To a soln. of
3-bromosalicylaldehyde (0.197 g, 0.981 mmol) in EtOH (5 ml), 4,5-dimethylbenzene-1,2-diamine
(0.0654 g, 0.480 mmol) was added. The mixture was refluxed for 12 h (!orange precipitate). The
mixture was filtered and the solid washed with Et2O and dried under vacuum: H2L3 (0.124 g, 51%).
1H-NMR (300 MHz, CD3CN): 8.75 (s, 2 N¼CH); 7.67 (dd, J ¼ 1.6, 7.9, 2 arom. H); 7.53 (dd, J ¼ 1.5, 7.7, 2
arom. H); 7.21 (s, 1 arom. H); 6.91 (t, J ¼ 7.8, 2 arom. H); 2.33 (s, 2 Me). EI-MS: 499.974113 (Mþ,
C22H18Br2N2Oþ2 ; calc. 499.973500).
2
)
The diffraction studies were carried out by Dr. Steven Geib, Department of Chemistry, University of
Pittsburgh.

Helvetica Chimica Acta – Vol. 92 (2009)
2327
2,2’-[(4,5-Dimethyl-1,2-phenylene)bis(nitrilomethylidyne)]bis[4,6-dibromophenol] (H2L4). To a
soln. of 3,5-dibromosalicylaldehyde (1.02 g, 3.66 mmol) in EtOH (15 ml), 4,5-dimethylbenzene-1,2-
diamine (0.242 g, 1.78 mmol) was added. The mixture was heated and sonicated for 30 min (!orange
precipitate). The mixture was filtered and the solid washed with hexanes and dried under vacuum: H2L4
(1.02 g, 87%). 1H-NMR (300 MHz, (D6)DMSO): 8.97 (s, 2 N¼CH); 7.93 – 7.91 (m, 4 arom. H); 7.36 (s, 2
arom. H); 2.31 (s, 2 Me). EI-MS: 660 (Mþ, C22H16Br4N2O2þ ; calc. 659.79).
Na[Nd(L1)2]. To a soln. of H2L1 (0.115 g, 0.333 mmol) in MeCN (7 ml), a soln. of Nd(OTf) · 6 H2O
(0.117 g, 0.167 mmol) in MeCN (5 ml) was slowly added. To the resulting soln., 0.1078m NaOH in MeOH
(4.6 ml, 0.496 mmol) was then slowly added under stirring. The mixture was refluxed for 2 h. The yellow
precipitate was centrifuged, washed with Et2O, and dried under vacuum: Na[Nd(L1)2] (0.103 g, 72%). IR
(selected absorbances only, KBr): 1618 (C¼N), 1544 (C¼C), 1385 (ArꢀO).
Na[Nd(L2)2]. As described for Na[Nd(L1)2], with H2L2 (0.0699 g, 0.139 mmol) in THF (15 ml),
Nd(OTf) · 6 H2O (0.0487 g, 0.0696 mmol) in MeCN (7.5 ml), and 0.1078m NaOH in MeOH (1.9 ml,
0.209 mmol); reflux for 6 h. The solvents were evaporated, and the residue was triturated with Et2O,
centrifuged, and dried under vacuum: Na[Nd(L2)2] (0.064 g, 78%). IR (selected absorbances only, KBr):
1616 (C¼N), 1522 (C¼C), 1373 (ArꢀO).
Na[Nd(L3)2]. As described for Na[Nd(L1)2], with H2L3 (0.0504 g, 0.100 mmol) in THF (20 ml),
Nd(OTf) · 6 H2O (0.0351 g, 0.0502 mmol) in MeCN (7 ml), and 0.1078m NaOH in MeOH (1.5 ml,
0.151 mmol); reflux overnight. The solvents were evaporated, and the yellow solid was centrifuged,
washed with Et2O and dried under vacuum: Na[Nd(L3)2] (0.038 g, 65%).
Na[Nd(L4)2]. As described for Na[Nd(L1)2], with H2L4 (0.0534 g; 0.0809 mmol) in THF/MeCN 2 :1
(v/v; 12 ml), Nd(OTf) · 6 H2O (0.0283 g, 0.0405 mmol) in MeCN (3 ml), and 0.1078m NaOH in MeOH
(0.13 ml, 0.121 mmol); reflux overnight. The solvents were evaporated, and the yellow solid was
centrifuged, washed with Et2O, and dried under vacuum: Na[Nd(L4)2] (0.0346 g, 58%).
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Received May 4, 2009