Delayed lanthanide luminescent Tb(III) complexes formed from lower rim amide functionalised calix[4]arenes

Article abstract of DOI:10.1080/10610278.2013.810339

The synthesis and the photophysical studies of a new generation of time resolved luminescent systems based on calix[4] arenes alkylated at the lower rim, capable of hosting lanthanide (III) ions such as terbium and sensitising its emission, are described. Two series of ligands were designed to provide an ideal cavity to host terbium (Tb(III)) and were synthesised in high yields following two novel approaches. The tetra-alkylation, which was achieved in one step using with piperidino- and morpholino-acetamide pendant arms, provides eight donor atoms forming a binding 'pocket' at an ideal distance from the metal core to favour the sensitisation via the antenna effect. Of the two ligand series developed, compounds 3 and 4 possess a short spacer between the calix and the amide receptor site. The second series of ligands 6-7, designed with longer pendant amide arms, was synthesised from 2 in two steps through the ester analogue 5. The crystal structure of 3 (and 6 as shown in Supporting Information, available online) is presented. The synthesis and the photophysical studies of the four resulting complexes 3.Tb, 4.Tb, 6.Tb and 7.Tb are described in detail and in each case, successful sensitisation of the terbium emission occurred upon excitation of the phenolic scaffold of the calixarene.

Full text of DOI:10.1080/10610278.2013.810339

Supramolecular Chemistry, 2013
Vol. 25, No. 12, 869–880, http://dx.doi.org/10.1080/10610278.2013.810339
Delayed lanthanide luminescent Tb(III) complexes formed from lower rim amide
functionalised calix[4]arenes
a†
Christophe Lincheneau , Eoin Quinlan , Jonathan A. Kitchen , Thomas McCabe , Susan E. Matthews * and
a
a††
a
b
a
Thorfinnur Gunnlaugsson *
a
b
School of Chemistry, Trinity Biomedical Science Institute, Trinity College Dublin, Dublin 2, Ireland; School of Pharmacy,
University of East Anglia, Norwich, UK
(Received 29 April 2013; final version received 15 May 2013)
The synthesis and the photophysical studies of a new generation of time resolved luminescent systems based on calix[4]
arenes alkylated at the lower rim, capable of hosting lanthanide (III) ions such as terbium and sensitising its emission, are
described. Two series of ligands were designed to provide an ideal cavity to host terbium (Tb(III)) and were synthesised in
high yields following two novel approaches. The tetra-alkylation, which was achieved in one step using with piperidino- and
morpholino-acetamide pendant arms, provides eight donor atoms forming a binding ‘pocket’ at an ideal distance from the
metal core to favour the sensitisation via the antenna effect. Of the two ligand series developed, compounds 3 and 4 possess a
short spacer between the calix and the amide receptor site. The second series of ligands 6–7, designed with longer pendant
amide arms, was synthesised from 2 in two steps through the ester analogue 5. The crystal structure of 3 (and 6 as shown in
Supporting Information, available online) is presented. The synthesis and the photophysical studies of the four resulting
complexes 3.Tb, 4.Tb, 6.Tb and 7.Tb are described in detail and in each case, successful sensitisation of the terbium
emission occurred upon excitation of the phenolic scaffold of the calixarene.
Keywords: supramolecular chemistry; calix[4]arenes; lanthanide; terbium complexes; solution studies; luminescence
Introduction
and demonstrated high-quantum yields which, in some
cases, could be tailored by introduction of a sensitising
unit. Subsequently, both simple carboxylates (12) and
amines (13) have also been investigated. In an alternative
approach, known lanthanide ligands such as heterocyclic
Over the past few decades, the use of lanthanide ions has
becomean important strategy for the development ofsensors
and probes for use in in vivo applications, such as in time-
delayed luminescent chemosensing and imaging, as well as
in the development of targeted magnetic resonance imaging
0
N-oxides (14), 2,2 -bipyridines (15) and dipicolinates (16)
have been incorporated to afford efficient binding and
energy transfer. More recently, the exploitability of
calixarene-based ligands has been increased by the use
of water-soluble calixarene scaffolds (17) and through
direct incorporation in polymers for the synthesis of hybrid
materials (18) and through the development of near-
infrared luminescent sensors (19). In their original work,
Shinkai and coworkers (10) proposed that sensitisation of
the lanthanide excited state could occur via two different
pathways, depending on the nature of the lanthanide ion,
as shown in Figure 1. Path A, used in the case of Tb(III),
involves excitation of the phenolic scaffold followed by an
(
MRI) contrast agents (1–3). A wide variety of ligand
platforms including cyclen (4) and cryptands (5) have been
investigated along with acyclic podands (6), and more
recently, self-assembly, where lanthanides have been
employed to direct the synthesis of various luminescent
architectures, has been exploited (7).
Calixarenes are highly attractive platforms due to their
ease of functionalisation and spatial pre-organisation.
These platforms have proved highly effective in the
development of lanthanide extractants particularly for
nuclear waste remediation (8) and in the development of
MRI contracts agents (9). The earliest examples of
luminescent complexes, from the groups of Shinkai (10)
and Ungaro (11), featured amide residues at the lower-rim
3
intersystem crossing towards the p–p * of the calixar-
enes, with subsequent population of the D excited states
5
4
*Corresponding authors. Email: Susan.Matthews@uea.ac.uk; gunnlaut@tcd.ie
†
Present address: Christophe Lincheneau, Photochemical Nanosciences Laboratory, Dipartimento di Chimica “G. Ciamician”, Universita`
di Bologna, Via Selmi 2, 40126 Bologna, Italy.
††
Present address: Jonathan A. Kitchen, Department of Chemistry, Faculty of Natural & Environmental Sciences, University of Southampton,
Highfield, Southampton SO17 1BJ, UK.
q 2013 Taylor & Francis

8
70
C. Lincheneau et al.
(a)
(b)
1
3
π–π∗
π–π∗
hv
ex
CT level
Path A
3
2
0
0
5
5
D3
Antenna*
5
E.T
D2
O
O
O
O
D4
O
5
D1
5
III
Ln
E.T
D0
O
O
O
R
R
E
R
Antenn
Path
1
hv
em
hv
ex
7
F5
7
5
F
F
2
6
7
F0
TbIII
EuIII
Figure 1. (a) Lanthanide emission sensitisation process. Path A in the case of Tb(III) and Path B for Eu(III). (b) Jablonski diagram for
the sensitisation of lanthanide luminescence using the Path A (in red) and Path B (in orange).
of Tb(III) (12c,14) giving rise to quantum yields of
0–20% in water. However, in the case of Eu(III), the
carbonyl CvO binding to Eu(III) was found to give rise to
Synthesis
5,26,27,28-Tetrakis[((N-piperidinyl)carbonyl)methoxy]
calix[4]arene (3)
1
2
charge transfer levels, which were populated from either
Thecalix[4]arene (2) (500 mg, 1.18 mmol)was reactedwith
2-bromoacetyl piperidine (1.1 g, 5.34 mmol) and K CO
1
3
the pp * or the pp * excited states of the calix-ligand,
and this gave rise to deactivation of the Eu(III) emission
via a non-radiative process and hence quenching of the
luminescence emission. Although these difficulties can be
overcome through inclusion of another antenna possessing
lower excited triplet states, an alternative approach is to
provide an extended complexation cavity. In this paper, we
report the synthesis of a new family of lower-rim
calixarene derivatives, featuring an eight electron donor
cavity with optimised geometry to host Tb(III), and the
investigation of their luminescent properties.
2 3
(1.0 g, 7 mmol) in CH CN (30 mL). Heating at reflux
temperature overnight gave a coffee-coloured suspension.
3
Workup followed by crystallisation from CH OH gave the
3
title compound as a cream solid (523 mg, 48%) (m.p.:
2
1
918C) (dec.). H NMR (400 MHz, CDCl , d ): 6.64–6.60
3
H
(
m, 12H, ArZH), 5.11 (d, J ¼ 14.1 Hz, 4H, ArZCH ZAr),
2
4
.90 (s, 8H, OCH C(O)), 3.52 (t, J ¼ 5.0 Hz, 8H, N
2
[
CH CH ]CH ), 3.43 (t, J ¼ 5.0 Hz, 8H, N[CH CH ]CH ),
2
2
2
2
2
2
3
.26 (d, J ¼ 13.8 Hz, 4H, ArZCH ZAr), 1.64 (m, 8H, N
2
1
3
[
CH CH ]CH ), 1.56 (bs, 16H, N[CH CH ]CH ).
C
NMR (100 MHz, CDCl , d ): 167.3, 155.9, 134.3, 128.0,
2
2
2
2
2
2
3
C
21
1
2
21.8, 71.2, 45.2, 42.1, 31.4, 25.9, 25.1, 24.2. IR (n, cm ):
919, 2848, 1663, 1464, 1204, 1008, 761, 752. ES-MS
Experimental section
(
m/z): 947.46 (M þ Na). Required for C H N O : C,
5
6 68 4 8
General procedures
1
7
2.70; H, 7.41; N, 6.06. Found: C, 72.49; H, 7.43; N, 5.99.
H NMR spectra were recorded using 400 MHz instrument
1
3
or 600.1 MHz; C NMR were recorded at 100 or
50.2 MHz. Mass spectrometry was carried out using
1
25,26,27,28-Tetrakis[((N-morpholino)carbonyl)methoxy]
calix[4]arene (4)
HPLC grade solvents using an electrospray spectrometer in
a positive mode. High-resolution mass spectra were
determined relative to a standard of leucine enkephaline.
Crystal for X-ray diffraction was analysed and solved in-
house. High-grade solvents (methanol and acetonitrile)
were used for the synthesis of the ligand and complex.
Analytical thin-layer chromatography was performed using
silica gel plates. Visualisation was by UV light (254 nm), by
exposure to iodine vapour, immersion aqueous alkaline
To a suspension of calix[4]arene (2) (1.0 g, 2.36 mmol) in
acetonitrile (50 mL) was added K CO (1.95 g, 14 mmol)
2
3
and 2-bromo-1-morpholinoethanone (2.94 g, 14.1 mmol).
The suspension was stirred at reflux overnight, resulting in
a brown suspension. Solvents were removed, and the
residue was partitioned between water (100 mL) and
CH Cl (100 mL). The organic layer was removed and the
2
2
KMnO solution or with a 2% ninhydrin in ethanol spray
4
water layer was extracted again with CH Cl (50 mL). The
2 2
reagent. Columns were run using Silica gel 60 (230–400
mesh). All lanthanide luminescent spectra were in
phosphorescent mode with a gate time of 5 ms and slit
widths of 10 or 20 nm.
combined organic layers were then further washed with
water (100 mL) and dried (Na SO ), and the solvent was
2
4
removed under reduced pressure. The resulting solid was
recrystallised from ethanol to give the title compound

Supramolecular Chemistry
871
1
1.44 g, 66%) (m.p.: 2188C). H NMR (400 MHz, CDCl ,
(
upon the addition of diethylether. Melting point decom-
posed above 2808C. Calculated for C H Cl N O Tb: C,
48.65; H, 4.96; N, 4.05. Found: C, 48.43; H, 4.85; N, 3.95.
3
d ): 6.60–6.67 (m, 12H, ArZH), 4.98 (d, J ¼ 14 Hz, 4H,
H
56 68
3 4 20
ArZCH ZAr), 4.91 (s, 4H, ArOZCH ZC(O)), 3.49–3.69
2
2
3
þ
(
ZAr). C NMR (100 MHz, CDCl , d ): 167.6, 155.4,
m, 32H, N(CH CH ) O), 3.26 (d, J ¼ 14 Hz, ArZCH
HR-MS calculated for C H Cl N O Tb [M
56 68
þ
H
2
2 2
2
2
4
16
1
3
2
þ
2ClO ] (m/z) ¼ 1281.3261. Found: 1281.8023; d
3
C
4
1
34.1, 128.1, 122.2, 70.8, 66.4, 66.2, 44.4, 41.2, 31.1. IR
(CD CN; 400 MHz): 7.28, 6.61, 5.11, 4.89, 3.51, 3.42,
3
2
n, cm ): 3445, 2967, 2912, 2863, 1676, 1658, 1466,
1
(
3.27; IR nmax: 2945, 2857, 1645, 1616, 1500, 1440, 1370,
1278, 1177, 1082, 1001, 952, 901, 856, 783.
1
1
9
6
448, 1466, 1358, 1299, 1274, 1213, 1236, 1196, 1112,
095, 1068, 1026, 998, 911, 852, 778, 754. ES-MS (m/z):
55.38 (M þ Na). Required for C H N O ·H O: C,
5
2
60
4
12
2
5.67; H, 6.57; N, 5.89. Found: C, 65.88; H, 6.49; N, 5.65.
Complex 4.Tb
Complex 4.Tb was synthesised by refluxing 4 (8.9 mg, 0.9
25
2
5
£
10 mol) with Tb(ClO ) (7.38 mg, 1.5 £ 10 mol) in
4
3
25,26,27,28-Tetrakis-N-(piperidinyl-2-aminoethyl)
carbamoyloxymethoxycalix[4]arene (6)
methanol (10 ml) yielding 4.Tb as a white solid (11.4 mg,
9%) after precipitation from methanol using diethylether.
Melting point decomposed above 2808C. Calculated for
9
1
Yield: 556 mg (78%), white powder (m.p.: 2018C). H
NMR (400 MHz, CDCl , d ): 7.63 (bs, 4H, NH), 6.61 (m,
C H Cl N O Tb: C, 44.92; H, 4.35; N, 4.03. Found: C,
52 60 3 4 24
3
H
1
4
2H, ArZH), 4.49 (d, J ¼ 13.8 Hz, 4H, ArZCH ZAr),
44.85; H, 4.28; N, 4.01. HR-MS calculated for C H Cl -
52 60 2
2
3
þ
2
.46 (s, 8H, ArOZCH ZCO), 3.47 (q, J ¼ 6.5 Hz, 8H,
N O Tb (M þ 2ClO ) (m/z1289.2431. Found: 1289.8
þ þ
H 3
2
4
20
4
3
2
4
ZNHZCH ZCH ), 3.26 (d, J ¼ 13.8 Hz, 4H, ArZCH
[M þ 2ClO ] ; d (CD CN; 400 MHz): 7.28, 6.80,
2
2
2
ZAr), 2.50 (t, J ¼ 6.5 Hz, 8H, NHZCH ZCH ), 2.39 (bs,
6.67, 4.81, 3.61, 3.37, 3.26; IR nmax: 2965, 2930, 2853,
1680, 1673, 1486, 1370, 1320, 1280, 1220, 1200, 1000, 920,
780, 750.
2
2
1
6H, N(CH CH )CH ), 1.52 (m, 16H, N(CH CH ) CH ),
2 2 2 2 2 2 2
1
.43 (m, 8H, N(CH CH ) CH ). C NMR (100 MHz,
3
1
CDCl , d ): 169.0, 133.8, 128.4, 122.7, 73.7, 57.2, 54.0,
3
2
1
2
2 2
2
3
C
2
1
5.9, 30.6, 25.5, 23.9. IR (n, cm ): 3284, 2930, 2949,
773, 1657, 1550, 1440, 1350, 1319, 1304, 1245, 1198,
159, 1130, 1087, 1053, 1036, 1011, 831, 770, 758.
Complex 5.Tb
Complex 5.Tb was synthesised by refluxing ligand 5
25
Accurate MS (m/z) – calculated: 1098.6876 (M/2) þ
H; found: 1098.6882. Analysis (%) – required for
C H N O 1/2Et O: C, 69.87; H, 8.26; N, 9.88. Found:
(
20.9 mg, 1.8 £ 10 mol) with Tb(ClO ) (14.9 mg, 1.8
4
3
25
10 mol) in methanol (10 mL) yielding 5.Tb as a
£
6
4
88
8
8
2
hygroscopic white solid (27.4 mg, 97%) after precipitation
from methanol upon the addition of diethylether. Melting
point decomposed above 2808C. Calculated for C H -
C, 69.90; H, 8.08; N, 10.24.
6
4 88
Cl N O Tb: C, 49.44; H, 5.71; N, 7.21. Found: C, 48.38;
3 8 20
25,26,27,28-Tetrakis-N-((4-piperazinyl)-2-aminoethyl)
carbamoyloxymethoxy calix[4]arene (7)
H, 5.74; N, 7.18. HR-MS calculated for C H Cl KN -
8
6
4
88
3
3þ
2
O Tb [M þ 3Cl þ K ] (m/z) ¼ 1399.4531. Found:
þ þ
8
Yield: 314 mg (44%), hygroscopic white powder; m.p.:
9
1399.3651 d (CD CN; 400 MHz): 7.29, 6.62, 4.53, 3.55,
H
3
1
88C. H NMR (400 MHz, CDCl , d ): 7.64 (s, 4H, NH),
2.52, 2.12; IR nmax: 3290, 2940, 2860, 1660, 1634, 1579,
1459, 1443, 1390, 1280, 1250, 1177, 1056, 943, 903, 856,
821, 777, 716, 691.
3
H
6
.63 (m, 12H, ArZH), 4.51 (d, 4H, ArZCH ZAr), 4.48
2
(
s, 8H, ArOZCH ZC(ZO)N), 3.48 (q, J ¼ 6 Hz, H
2
[
NCH CH ]N), 3.46 (d, J ¼ 14 Hz, 4H, ArZCH ZAr),
2
2
2
2
.90 (m, 16H, N[CH CH ]N), 2.54 (t, J ¼ 6 Hz, HN
2 2 2 2
2
2
1
CH CH ]N), 2.42 (m, 16H N[CH CH ]N). C NMR
3
[
Complex 6.Tb
(
100 MHz, CDCl ) d :169.1, 133.7, 128.4, 122.8, 73.6,
3 c
Complex 6.Tb was synthesised by refluxing ligand 6
25
2
1
5
3
1
–
7.1, 53.9, 45.6, 45.4, 35.6, 30.6. IR (n, cm ): 3301,
062, 2912, 2850, 2809, 1659, 1538, 1439, 1244, 1194,
115, 1094, 1009, 867, 852, 778, 761. Accurate MS (m/z)
calculated: 1102.6730 (M/2) þ H; found: 1102.6692.
(
28.2 mg, 2.6 £ 10 mol) with Tb(ClO ) (20.1 mg, 2.6
4
3
25
10 mol) in methanol (10 mL) yielding 6.Tb as a
£
hygroscopic white solid (37.9 mg, 95%) after precipitation
from methanol upon the addition of diethylether. Melting
point decomposed above 2808C. Calculated for C H -
6
0 84
Cl N O Tb: C, 46.23; H, 5.43; N, 10.78. Found: C, 46.18;
3 12 20
Complex 3.Tb
H, 5.40; N, 10.81. HR-MS: calculated for C H Cl KN -
6
0
84
3
12
3
þ
2
þ þ
Complex 3.Tb was synthesised by refluxing ligand 3
5
O Tb [M þ 3 Cl þ K ] (m/z) ¼ 1403.4297. Found
8
2
14.1 mg, 1.5 £ 10 mol) with Tb(ClO ) (11.8 mg, 1.5
(
1403.2623; d (CDCl ; 400 MHz): 7.31, 6.56, 4.45, 3.40;
H
4
3
3
2
10 mol) in methanol (10 mL) to yield 3.Tb as a white
5
£
3.22; 2.85 2.46; IR nmax: 3320, 3080, 2945, 2880, 2830,
1681, 1560, 1450, 1205, 880, 795, 760.
solid (14.2 mg, 69%) after precipitation from methanol

8
72
C. Lincheneau et al.
a longer chain was employed giving the ethylene-
Crystallographic analysis
piperidine (6) and ethylene-piperazyl (7) structures. The
calixarene 2 (20) was synthesised from the commercially
available cyclic tetramer 1 in a conventional way. Ligands
3 and 4 were designed with shorter arms containing amido-
piperidine and amido-morpholine terminal moieties close
from the lanthanide binding pockets. Employing William-
son ether-synthesis, using the appropriate a-bromoaceta-
mides [For examples of the use of such synthesis, see Ref.
X-ray data for ligand 3 (CCDC no. 916512) were collected
on a Bruker SMART APEX CCD diffractometer
using graphite-monochromated Mo Ka radiation
˚
(
l ¼ 0.71073 A). The data sets were processed and
corrected for Lorentz and polarisation effects using
SMART and SAINT-PLUS software. The structures were
solved using direct methods with the SHELXTL program
2
package and refined against all F data. All H-atoms were
positioned geometrically and refined using a riding model
(21).] gave 3 and 4 in 48% and 66% yields, respectively,
after trituration using diethylether. In both cases, the
successful tetra functionalisation and the resulting C4v
1
symmetry was observed from the H NMR analysis as
˚
with d(CHaro) ¼ 0.95 A, U ¼ 1.2U (C) for aromatic,
iso
eq
˚
and 0.99 A, U ¼ 1.2U (C) for CH .
iso
eq
2
shown in Figure 3(a),(b). The resonances of the aromatic
protons coalesce into a broad multiplet at 6.60–6.50 ppm
in both cases. The protons in the a-position of the amido
moiety, which are subjected to strong shielding due to the
presence of the phenolic ether and the amido moiety, were
observed as a singlet at 4.90 and 4.92 ppm for 3 and 4,
Results and discussion
Synthesis and characterisation of the ligands 3–7
With the aim of developing a new generation of
luminescent probes for time-resolved imaging, the two
families of calix[4]arenes 3, 4 and 5–7 shown in Figure 2
were designed to provide an octadentate binding pocket
capable of hosting terbium below the rim of the calixarene
moiety. Here, the tetra-alkylation of the lower rim of the
calixarene provides eight donor atoms for both types of
receptors; the first receptor via the four phenolic oxygens
and the four amido moieties, chosen as amido-piperidine
respectively. The CH methylene bridges of the calixarene
2
2
resonated as two doublets ( J ¼ 14 Hz) at 5.11 and
2
3.26 ppm (d
¼ 4.98, d
¼ 3.26 ppm and J ¼ 14 Hz
H
H
for 4) which are indicative of exo- and endo-environments
for each of these protons. This confirmed the cone
conformation of calix[4]arenes 3 and 4.
The purity of both ligands was also confirmed
using traditional characterisation methods; furthermore,
(
3) and amido-morpholine (4), and for the second systems,
Figure 2. (a) Synthesis of the calix[4]arene 2 in two steps. (b) Synthesis of the calix[4]arene derivatives 3–7 from the calixarene 2.

Supramolecular Chemistry
873
1
Figure 3. (Colour online) The H NMR (CDCl ; 400 MHz) spectra of ligands 3 (a) and 4 (b). (c) Perspective view of 3 with thermal
elipsoids set at 30% probability. Hydrogen atoms and minor components of disorder are omitted for clarity.
3
colourless plate-shaped crystals of 3 suitable for single
X-ray crystal structure analysis were obtained via slow
evaporation at room temperature from a mixture of
CH Cl /MeOH solution. Compound 3 crystallised in the
synthesised as shown in Figure 2(b) from the known tetra
ester 5 (22). The nucleophilic substitution of the ester
moiety, using the pure amine, allowed the formation of the
calix[4]arenes 6–7 in respectively 44% and 78% yields.
The H NMR analysis of each ligand gave similar results
as for the shorter analogues 3 and 4. As shown in Figure 4
2
2
1
monoclinic space group C2/c and contained half of one
molecule in the asymmetric unit with the other half
generated by a twofold rotation axis (Figure 3(c)). The
crystal structure of 3 contains disorder at the piperidine
ring of N(1) where two of the carbon atoms are each
disordered over two sites, C(6)/C(9) and C(66)/C(99) with
relative occupancies of 0.65 and 0.35, respectively (see
Supporting Information, available online for figure). In
addition, the carbonyl oxygen atom on the other
piperidine-containing arm is also disordered over two
sites where O(3) and O(33) have relative occupancies of
(a),(b), the simplicity of the NMR reflects the C symmetry
4
of these structures with the phenolic protons coalescing
within the 6.60–6.50 ppm range, whereas the CH of the
2
calixarene scaffold occurred as a set of doublets at 5.11
and 3.26 ppm for 6, and at 4.53 and 3.26 ppm for 7. In the
case of compound 6, small poor-quality crystals were
obtained by slow vapour diffusion of di-isopropylether
into CH Cl ; however, the data obtained from diffraction
2
2
studies were limited and only the connectivity could be
0
.5 and 0.5, respectively (see Supporting Information,
available online for figure). The calix component of 3
adopts a pronounced ‘boat’ configuration where two of
opposite aromatic rings are ‘flattened’ with inter-centroid
˚
distances determined as 5.15 A apart between the
‘
pinched’ rings, whereas the ‘flattened’ ring centroids are
˚
.67 A apart. The packing observed when viewed along the
7
crystallographic c-axis (see Supporting Information,
available online) showed the formation of a ribbon-like
packing array, whereby the calix[4]arene aromatic core
and the aliphatic piperidine moieties alternate in a head-to-
tail fashion. The solid-state structure showed a cavity of
˚
ca. 6.4 A (measured diagonally between the coordinating
amido oxygens), which should allow for the inclusion of a
cationic guest such as terbium. However, the piperidine
moieties, which are in close proximity to the binding site,
may impart strain on the system lowering the affinity
towards the lanthanide guest.
The second series of ligands 6–7 was designed with an
ethylene spacer between the binding pocket and the
terminal moiety in order to add more flexibility to the
pendant arms and minimise the strain due to the presence
of the terminal moieties. The calixarenes 6 and 7 were
1
Figure 4. (Colour online) The H NMR (CDCl ; 400 MHz)
spectra of ligands 6 (a) and 7 (b).
3

8
74
C. Lincheneau et al.
Figure 5. (Colour online) The synthesis of the complexes 3.Tb, 4.Tb, 6.Tb and 7.Tb from ligands 3, 4, 6 and 7, respectively.
determined from the data. Although no detailed structural
information is discussed, it is clear that the calixarene
portion of 6 also adopted a boat-like configuration in the
solid state (the structure is shown in Supporting
Information, available online).
Furthermore, for each of these complexes, the observed
ES-MS showed the characteristic isotopic distribution
pattern of monometallic Tb(III) complexes which matched
that of the calculated isotopic distribution pattern,
indicating the successful formation of the complex in 1:1
stoichiometry, as shown in Figure 6 for 3.Tb–7.Tb.
The conformation ofL.Tbwasfurther investigated using
H NMR spectroscopy (CDCl , 400 MHz). In all cases
1
Synthesis and characterisation of L.Tb (L 5 3, 4, 6, 7)
3
1
Figure 7), the H NMR spectrum was broad, but not greatly
(
shifted, and each showed a species of high symmetry,
The synthesis of the complexes L.Tb was carried out in
MeOH as described in Figure 5. The complexes were
isolated via precipitation from diethylether in high yield of
reflecting the C symmetrical nature of these structures, and
4
hence, the retention ofthe coneconformationobserved inthe
1
H NMR of the ligands. Having established the successful
formation of these Tb(III) complexes, we next evaluated
their photophysical properties in solution.
9
0–95%, except in the case of 4.Tb, which precipitated in
solution after 5 min stirring in 67% yield. The character-
isation of the complexes L.Tb (L ¼ 3, 4, 6 and 7) was
achieved using conventional spectroscopic methods. Using
MS analyses (ESI þ /ToF mode in MeOH), the complexes
3
.Tb and 4.Tb were detected as a mono- and di-charged
3
þ
2
Photophysical studies of the complexes L.Tb (L 5 3, 4,
6, 7)
fragments, corresponding to
3
M
þ 2ClO
and
4
þ
2
4
M
Tb, containing the longer arms, were found as a mono-
þ ClO ions, respectively, whereas the 6.Tb and 7.
The photophysical properties of the ligands and the Tb(III)
L.Tb (L ¼ 3, 6 and 7) complexes were investigated in
charged ions, corresponding to M þ 3Cl þ Kþ.
3
þ
2
oF fi gt uh re e d6 e. sir Ce da l 1c :u1 la ct oe md ap nl ed x oe bs .s erved isotopic distribution pattern of the complexes L.Tb (L ¼ 3, 4, 6 and 7), demonstrating the formation

Supramolecular Chemistry
875
Figure 7. (Colour online) The proton NMR of the Tb(III) complexes developed in this study.
MeOH solution, whereas for 4 and 4.Tb, the analysis was
performed in a 4:1 diemthyl sulfoxide (DMSO):H O
hypochromically effected upon complexation to Tb(III). In
comparison, the UV–vis absorption spectra of 4.Tb, 6.Tb
and 7.Tb were structurally similar to that seen above for
their respective ligands. The excitation of the calix[4]arene
scaffold at 271 nm, for each of the complexes L.Tb, gave
2
mixture, due to precipitation of the latter structure from
MeOH. For the ligands, the UV–vis absorption spectrum
showed a broad absorption band centred at lmax ca.
2
70 nm, which was the characteristic of the p ! p *
rise to typical Tb(III) emission demonstrating the
5
successful population of the Tb(III) D excited state,
transitions of the aromatic scaffold of the calixarenes. The
molar extinction coefficients 1271 were determined for
each of the ligands, being 1 ¼ 2039, 2006, 1458 and
4
with emission bands occurring at 487, 543, 581 and
5
7
614 nm due to the deactivation of D to F (J ¼ 6–3)
4
J
2
1
21
1
388 M cm for 3, 4, 6 and 7, respectively, in the above
(Figure 8). On all occasions, the recorded fluorescence
excitation spectra matched the UV–vis absorption spectra
of the corresponding ligands, confirming the successful
solvent systems. These results show that 6 and 7 possessed
a similar, and at the same time, a significantly smaller
extinction coefficient than that determined for 3 and 4. The
excitation of the calixarene scaffold of 3, 4, 6 and 7 gave
rise, on all occasions, to a weak and broad fluorescence
emission that were centred at ca. 416 nm.
5
sensitisation of the Tb(III) D excited state by these
4
ligands. In each case, the Tb(III) emission was found to be
long-lived; the lifetimes recoded for the complexes 3.Tb
and 4.Tb excited states in MeOD were longer (t ¼ 2.3–
2.6 ms) than that recoded for 6.Tb and 7.Tb (t ¼ 1.5–
1.7 ms). It can be concluded from these results that the
The UV–vis absorption spectra of 3.Tb was
significantly different to that seen for 3, being ca. 70%
Figure 8. (Colour online) The absorption and the lanthanide luminescence emission spectra of 3.Tb, 6.Tb and 7.Tb when recorded in
MeOH and the same for 4.Tb when recorded in DMSO:H O mixture.
2

8
76
C. Lincheneau et al.
Table 1. Summary of the photophysical properties of the
complexes L.Tb (L ¼ 3, 4, 6, 7) in MeOH.
80:20 DMSO/H O for 6. This involved measuring the
2
phosphorescence intensity of the metal centre following
excitationof the phenyl platform of the calixarene at 271 nm.
The changes in the absorption spectra of 3 are shown
in Supporting Information (available online), where the
absorption band centred at l ¼ 271 nm experienced a
hypochromic effect, which reached a plateau after the
additionof0.7equivalents ofterbium. This canbe indicative
Complex
3.Tb
4.Tb
6.Tb
7.Tb
1271
f (%)
627.2
19.0
1891.9
18.6
1289.2
0.5
1206.1
0.2
t
tMeOH
MeOD
2.325
1.566
2.593
2.500
1.759
1.322
1.457
1.337
of the formation of the 3 .Tb at a low concentration of Tb
2
(
III). This effect was also seen in the results obtained in the
non-radiative deactivation of the structure’s excited states
is more efficient for the complexes 6.Tb and 7.Tb, due to
their longer pendant arms. This behaviour has been also
illustrated by the luminescence quantum yield measure-
ments, which in the case of 3.Tb and 4.Tb resulted in a
highly luminescent terbium-centred emission with quan-
tum yields of 19% and 18.6%, respectively, whereas in the
case of 6.Tb and 7.Tb, the complexes were significantly
less luminescent with less than 1% quantum yield as
shown in Table 1.
luminescence titrations as shown in Figure 9(a), where the
terbium luminescence of the forming complex became
apparent up to the addition of 0.7 equivalents of terbium
perchlorate. This confirmed the initial formation of a 1:2
(M/L) stoichiometric species followed by a displacement of
the equilibrium in solution towards the 1:1 species.
The changes obtained in the spectroscopic properties
of 4 upon the variation of terbium concentration (see the
UV–vis changes in Supporting Information, available
online) are shown in Figure 9(b), and were shown to be
different to that seen above for 3. Firstly, in the 4:1 DMSO/
As a consequence, the systems formed have shown the
unique properties of lanthanide complexes such as an
intense long-lived and narrow-shaped emission in the
visible region. It was also shown that the terbium ion is
hosted within the cavity formed by the four pendant arms
providing eight of the nine coordination required by Tb
H O solvent system, no major changes were seen in the
2
UV–vis absorption spectra of 4 upon terbium complexation
(see Supporting Information, available online). However,
as in the case of ligand 3, the lanthanide luminescence is
successfully switched on within the addition of 0 ! 25
equivalents of terbium. Initially, the luminescence
intensity increases efficiently upon the addition of 1
equivalent of Tb(III), which can be explained by the
formation of the 1:1 L/Tb stoichiometry species, also
observed in the case of the titrations of ligand 3. The
changes were then less pronounced upon the addition of
Tb(III) ranging from 1 ! 2 equivalents where the
luminescence evolution adopted a linear increase pro-
portional to the quantity of Tb(III) added. This is most
likely due to the residual luminescence of Tb(III)
perchlorate in DMSO solution after the formation of the
desired 1:1 complex; but this can also be indicative of the
(
III). Moreover, these results suggest that the architectures
L.Tb are good candidates for the development of sensors
or time-resolved contrast agents. Having analysed the
photophysical properties of these complexes, we next
investigated their formation in situ.
Studies of the formation of the complexes L.Tb in situ
The formation of the complexes L.Tb was investigated
in situ following spectroscopic methods by UV–vis
absorption and lanthanide emission. The titrations were
carried out by adding Tb(III) (delivered as its non-
coordinating, perchlorate salt) to a solution of ligand in
methanol, in the case of 3, 6 and 7 or in a solvent mixture of
formation of the 4.Tb at higher lanthanide concentration.
2
The complexation studies of terbium by ligands 6 and 7
Figure 9. (Colour online) (a) Changes observed in the luminescence emission of 3 upon the addition of Tb(ClO
b) Changes observed during the titration of 4 in 4:1 H O:DMSO mixture.
4 3
) in methanol.
(
2

Supramolecular Chemistry
877
Figure 10. (Colour online) (a) Changes observed in the luminescence emission of 6 upon the addition of Tb(ClO
(
4
)
3
in methanol.
b) Changes observed during the titration of 7.
have also been investigated in MeOH. In each case, the
UV–vis absorption spectrum of the ligand remained
unchanged (see Supporting Information, available online).
As previously shown, the terbium luminescence was
switched on in the presence of lanthanide ion, indicative
of Tb(III) complexation, signifying the formation of the
complex in solution. In the case of compound 6, the
emission intensity increases upon the addition of terbium
salt and reaches a plateau in the presence of one equivalent
of metal suggesting the main formation of the 1:1 species
Determination of the binding constants for the formation
of L.Tb in solution
In order to highlight the complexation process, the data
obtained from the titration above were further investigated
w
using the non-linear regression analysis SPECFIT . In
each case, the luminescence data recorded above were
fitted successfully, using various stoichiometries, with
good convergence factors and the binding constants
resulting from these fits are summarised in Table 2. It can
be concluded from these results that in each case, the
formation of the desired 1:1 stoichiometric species occurs.
However, the speciation and the stability of the complexes
formed seem to be influenced significantly by the nature of
the pendant arms. For example, the fitting of the data
obtained from the titrations of 3 and 4 with Tb(III) showed
in both cases the major formation of the 1:1 stoichiometric
species with Log K11 ¼ 8.2 and 5.5, respectively. The
6
.Tb in solution (Figure 10(a)). However, at high
concentrations of Tb(III), the emission intensity was
weakly enhanced in proportion to the quantity of metal
added, which is due to the luminescence of terbium
directly excited on its UV–vis absorption band at 271 nm.
In the case of 7, the behaviour of the luminescence
emission followed a different pattern as is evident in
Figure 10(b) than to that seen with the other analogues.
Here, the luminescence intensity remained constant upon
the addition of 0.3 equivalent of Tb(III), suggesting the
formation of preliminary species resulting from the self-
assembly of several ligands with Tb(III). Indeed, ligand 7
contains long and flexible pendant arms bearing four
nitrogen atoms through its terminal moieties, which can
easily form extra-cavities or allow the interpenetration of
two ligands around the same metal centre. However, upon
further addition of Tb(III), the displacement of the
complexation equilibrium towards the stable formation of
the host-guest system 7.Tb was observed by the large
increase in the terbium emission upon the addition of 0.9
equivalent of lanthanide ion. Finally, the formation of
other minor species was detected via a quenching of the
luminescence intensity within the addition of 1 ! 5
equivalent of Tb(III). Despite the minor influence that the
terminal moiety seemed to have on the photophysical
properties of the complexes L.Tb (L ¼ 3, 4, 5, 6, 7), the
binding process was highly affected by the nature of the
four pendant arms; hence, these spectroscopic titration
studies have demonstrated the presence of the species L.
Tb in solution and the potential formation of other
different topologies at higher concentration of Tb(III).
stable formation of the species 3 .Tb was also detected
2
with Log K12 ¼ 5.2; however, this indicates that the
complex 3.Tb is the most stable species present in
solution.
The speciation distribution plot derived from these
calculations, shown in Figure 11(a), indicated that the
formation of the species 3 .Tb is maximal in the presence
2
of 0.5 equivalents of Tb(III) in solution (66.4%) and that
the equilibrium was displaced to the exclusive formation
of 3.Tb (89.5%) upon the addition of an equimolar ratio of
lanthanide salt.
In the case of 4, the convergence of the mathematical
model could be reached only with the introduction of the
2:1 M/L species, for which the formation at high terbium
Table 2. Binding constant calculated from the fitting of the
changes observed during the Tb(III) emission titrations of the
calixarenes derivatives.
Log bML
3
4
6
7
Log b1:1
Log b2:1
Log b1:2
Log b2:3
8.2 ^ 0.8
5.5 ^ 0.2
6.8 ^ 0.3
7.8 ^ 0.3
5.3 ^ 0.1
–
13.4 ^ 0.9
–
–
13.5 ^ 0.5
–
–
–
–
–
18.4 ^ 0.1

8
78
C. Lincheneau et al.
Figure 11. (Colour online) (a) Speciation distribution diagram describing the binding process of 3 with Tb(III). Inset: fits resulting from
the fitting. (b) Speciation distribution diagram obtained for the complexation of Tb(III) with 4. Inset: fits resulting from the fitting of the
luminescence titration data.
2
4
concentration ([Tb(III)] . 2 £ 10 M) has been calcu-
their structural similarities showed three different
behaviours. In the case of ligand 6, the results obtained
were similar to those obtained for ligand 3, which bears the
same terminal moieties. The formation of two species 62.
Tb and 6.Tb was detected. Moreover, the 1:2 stoichio-
lated to be very weak with Log K ¼ 1.4. The speciation
2
1
distribution plot describing the binding process of 4 with
Tb(III) (as shown in Figure 11(b)) confirmed the expected
behaviour, showing the unique formation of 4.Tb in the
presence of 1 equivalent of Tb(III), which is followed by
the slow formation of the second less stable species 4.Tb2.
The difference in behaviour between 3 and 4 can possibly
be explained by the nature of the terminal amine moieties.
The self-assembly of two ligands of 3 around the same
terbium ion can potentially occur via tail-to-tail arrange-
ment after rotation of one or several pendant arms.
Nevertheless, the formation of the 1:2 M/L has not been
observed in the case of 4, which could be due to the
configuration of the morpholinyl group, shielding the
access to the metal centre and preventing the formation of
multi-ligand species. Finally, the weak formation of the di-
metallic species can also potentially occur through the
complexation of a second terbium ion with the terminal
oxygen atoms present on the morpholinyl group of 4.
The influence of the nature of the pendant arms on the
binding process was further confirmed by the fitting of the
luminescence titration of ligands 6 and 7, which despite
metric complexes 6 .Tb were observed to be more
2
favoured (Log K12 ¼ 5.60) than in the case of the
complexation of 3, and reached a maximum formation of
ca. 85.4% in the presence of 0.5 equivalents of terbium
salt. This can be explained by the higher flexibility of the
pendant arms, which can stabilise the self-assembly of
ligands to fulfil the coordination requirement of the
terbium ion. However, the formation of the 6.Tb species
was found to be predominant in solution upon the addition
of an equimolar ratio of lanthanide ion (.95% formation)
as shown in Figure 12(a).
The calculations resulting from the luminescence
emission titrations of 7 could also be fitted accurately
by taking into account the formation of the secondary 2:3
M/L species, which does not correspond to the probable
binding mode but more likely reflects the weak formation
of minor secondary species, such as the formation of 1:2 or
3:2 M/L. Nevertheless, these results also showed the
Figure 12. (Colour online) (a) Speciation distribution diagram describing the binding process of 6 with Tb(III). Inset: fits resulting from
the fitting. (b) Speciation distribution diagram obtained for the complexation of Tb(III) with 7. Inset: fits resulting from the fitting of the
luminescence titration data.

Supramolecular Chemistry
879
predominant formation of the host–guest system 7.Tb
during the titration, with a binding constant of
Log K11 ¼ 5.3, which was in the same range as the one
obtained for the formation of 4.Tb.
It was clear from these results that the nature of the
terminal moiety has a considerable influence on the
binding process. Despite the difference of the length of
their pendant arms, the calix[4]arenes derivatives 3 and 5,
which possess the same terminal piperidine terminal
moieties, showed similar behaviour upon the addition of
emission of which can be sensitised by excitation of the
calix[4]arene aromatic core. We are in the process of
modifying these structures further with the view of
developing novel sensors and imaging agents based on
modulation in lanthanide luminescence; particularly with
the view of developing near infrared emitting complexes
(18), and functional complexes for biological applications
(19, 20).
Tb(III). In both cases, the same two species L .Tb and L.
2
Supplemental data
Tb were formed with similar binding constant. Con-
versely, the Tb(III) titration of 4 and 7 showed that the
nature of the terminal moiety can have a dramatic
influence on the binding process. Even if in each case, the
major formation of the 1:1 species has been observed, the
binding constant associated with the formation of the L.Tb
Supplemental data for this article can be accessed here at
http://dx.doi.org/10.1080/10610278.2013.810339.
(
L ¼ 4, 7) complexes were found to be lower by three log
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complexation equilibrium towards species containing
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